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Neoproterozoic and Cambrian continental rifting, continent-arc collision and post-collisional magmatism


Abstract and Figures

3.1 Introduction
Major advances have been made over the last decade in our understanding of
the distribution, compositions, age and significance of Late Neoproterozoic
and Cambrian rocks in Victoria.These advances have been driven mainly by
the new generation of geological mapping carried out by the Geological
Survey of Victoria, using detailed aeromagnetic, gravity and radiometric
datasets covering much of the state.In addition,detailed work by universities
over certain areas, particularly the Glenelg region in the far west and locally
around Stawell, have made significant contributions to the knowledge of the
Cambrian geology.
In western Victoria, Cambrian rocks comprise all the known
sedimentary and volcanic bedrock as well as numerous granites. Although
there is some possibility of Proterozoic rocks occurring here, no dated rocks
have returned ages older than Cambrian. The rock units and geological
histories of the Glenelg, Grampians–Stavely and Stawell zones, which were
poorly known until recently, are now much better understood.In central and
eastern Victor ia, the Cambrian rocks generally lie at deeper structural levels
and are only exposed in the hanging walls of major faults. Important new
information is available for the Glenelg River Complex in westernmost
Victoria, the exposed and drilled volcanic belts in western Victoria, and the
Cambrian greenstones around Pitfield and on Phillip Island.
3.1.1 Proterozoic
There are few rocks in Victoria that can be confidently assigned to the
Precambrian. The existence of old, almost entirely hidden, subcrust beneath
parts of Victoria has been suggested by previous authors (Scheibner, 1985;
Clemens, 1988; Chappell & White, 1992; Cas, 1983; Powell, 1983; Fergusson
et al., 1986; Gray et al., 1991; McBride et al., 1996). Its possible presence
beneath central Victoria comes from a new interpretation of many geological
features and regional magnetic data (VandenBerg et al., 2000; Cayley et al.,
2002). In this model, the Melbourne Zone and the eastern part of the
Bendigo Zone are underlain by thin Neoproterozoic–Cambrian continental
crust — the Selwyn Block — that forms a norther n extension of Tasmania.
The major definitive characteristic of the Selwyn Block is deformation
during the Cambrian, equivalent in time to the Tyennan Orogeny of
Tasmania. It has been argued (VandenBerg et al., 2000; Cayley et al., 2002)
that slices of the Cambrian cover sequence of the Selwyn Block are exposed
as structural windows eroded through the Mount Useful Fault Zone in
central eastern Victor ia, cropping out as the Licola and Jamieson Volcanics.
3.1.2 Cambrian
Rocks in western Victoria can be assigned to the Delamerian and the
Lachlan Fold belts (Fig. 3.1), with the boundary between these presently
taken as the Moyston Fault (Figs.3.2, 3.3) immediately east of the Grampians
Ranges (Cayley & Taylor, 1996b, 1998a;Cayley et al., 2002).The Delamerian
Fold Belt rocks in western Victoria lie in the Glenelg and Grampians–Stavely
zones (Fig. 3.2) (VandenBerg et al., 2000), which have been affected by the
515 to 490-480 Ma Delamerian Orogeny. The Stawell Zone to the east was
not deformed until much later, at 450-420 Ma, in an event equivalent in time
and effect to the Benambran Orogeny of eastern Victoria (VandenBerg, 1978,
1999; Cayley & McDonald, 1995; Foster et al., 1999).
Chapter 3
Neoproterozoic and Cambrian
continental rifting, continent–arc collision
and post-collisional magmatism
Within-plate MORB
Black Range
Mt Stavely
Glen Creek
Phillip Island
Mt Wellington
Waratah Bay
100 km
Fig. 3.1: Distribution and affinities of Cambrian and Late Neoproterozoic rocks in the Lachlan
and Delamerian Fold belts in Victoria, showing the major greenstone occurrences and inferred
extent based on aeromagnetic data (modified from VandenBerg et al., 2000).
A. J. Crawford (Coordinator), R. A. Cayley, D. H. Taylor, V. J. Morand, C. M. Gray, A. I. S. Kemp,
K. E. Wohlt, A. H. M. VandenBerg, D. H. Moore, S. Maher, N. G. Direen, J. Edwards,
A. G. Donaghy, J. A. Anderson and L. P. Black.
3.2 Delamerian Fold Belt
3.2.1 Introduction
The Glenelg and Grampians–Stavely zones are the most westerly geological
zones in Victoria and represent the easternmost extension of the Delamer ian
Fold Belt (Fig. 3.2).The Glenelg Zone consists of the more deformed and
higher grade westerly portion including the Glenelg River Complex,
whereas the Grampians–Stavely Zone comprises the less deformed and less
metamorphosed eastern portion.
In this review, Delamerian Fold Belt rock sequences are described from
west to east. Detailed information is available for outcropping Cambr ian
sequences of the Glenelg Zone exposed in the catchment of the Glenelg River.
The Cambrian rocks of the Grampians–Stavely Zone are much more poorly
exposed and best known from greenstone-dominated belts outcropping further
east in the Black Range, Mount Stavely and Mount Dr yden regions. The
northward extent and distribution of these and other rocks beneath Murray
Basin cover (Fig.3.1) has been traced using new aeromagnetic and gravity data,
with some drillhole control (e.g. Moore,1996;Maher et al., 1997a).
3.2.2 Glenelg Zone (including the Glenelg
River Complex)
The Glenelg Zone comprises the metamorphic and igneous rocks
underlying the Dundas Tableland of far western Victoria, including those of
the Glenelg River Complex (Wells, 1956; Gibson & Nihill, 1992; Turner et
al., 1993; Anderson & Gray, 1994; Kemp & Gray, 1999b; Gray et al., 2002;
Kemp et al., 2002). The eastern limit of the Glenelg Zone, and of the
igneous–metamorphic complex, is the north-trending Yarramyljup Fault
(Fig. 3.3, 3.4), which juxtaposes high-grade metasedimentary rocks against
slate and metasiltstone of the Grampians–Stavely Zone immediately east of
Balmoral (Gibson & Nihill, 1992).The western extent of the Glenelg Zone is
obscured by Cenozoic cover immediately to the west of the Glenelg River
at Dergholm. The portion of the Delamerian Fold Belt in Victor ia is
separated from that exposed in South Australia by a wide expanse of younger
Murray Basin cover, which also covers the northern extent of the
igneous–metamorphic complex. Migmatites and biotite–muscovite schists
were drilled under the Murray Basin north of the exposed Glenelg River
Complex rocks in VIMP-7, -12 and -13 (Maher et al., 1997a).The Otway
Basin covers the southern extent.
The timing of sedimentation is unknown, but presumed to be Cambrian
or even Late Neoproterozoic. Protoliths of the Glenelg River Complex rocks,
and less deformed and metamorphosed rocks further west, are correlated with
the Moralana Supergroup (Preiss, 1982), which includes the Normanville and
Kanmantoo Group metasediments in eastern South Australia. Many of the
stratigraphic elements of the well-described Cambrian Normanville and
Kanmantoo Groups in South Australia are present in parts of the Glenelg
Zone, but mapping has yet to resolve their distribution. Stratigraphic
continuity can only be established very locally due to the discontinuous
outcrop and complex deformation.Therefore, the metasedimentary rocks have
not been differentiated beyond Moralana Supergroup (Fig. 3.3), which
encompasses the Normanville and Kanmantoo groups (Preiss, 1982).
However, fault slices of tholeiitic to picritic basalt and gabbro that are
extensive under cover nor th of the Glenelg Zone and have continuity into
South Australia may be correlated with the Mount Arrowsmith Volcanics in
westernmost New South Wales (Crawford et al., 1997), the Truro Volcanics of
the Normanville Group in South Australia, and the picrites of King Island,
To gari Group metabasalts and Crimson Creek Formation rift tholeiites of
northwestern Tasmania (Crawford & Berry, 1992).The correlation is on the
basis of their linked distribution, their broadly similar age and the geochemical
Evolution of the Palaeozoic Basement74
Glenelg Zone
Grampians–Stavely Zone
Grampians Group
50 km
Fig. 3.2: Geological domains and major faults of the Delamerian Fold Belt in western Victoria,
showing the Glenelg Zone and Grampians–Stavely Zone, and location of the Glenelg River
Complex west of the Yarramyljup Fault (modified from VandenBerg et al., 2000)
characteristics of basalts erupted in an evolving rift–drift setting (Crawford &
Direen, 1998; Direen, 1999). Furthermore, several phlogopite-bearing
cumulate ultramafic rocks in the western part of the Glenelg Zone are
probably related to the same magmatic episode.
A structural history for the Glenelg Zone is resolved into five
deformational events (see section 2.2.3). The second was the most intense
and was responsible for the development of the most pervasive regional
foliation (S2), mesoscopic isoclinal folds and transposed layering.
Glenelg River Complex
The Glenelg River Complex (Fig. 3.4) has a NW–SE regional strike and is
subdivided into (a) a southwestern metamorphic zone, (b) a northeastern
metamorphic zone and (c) an axial granitic batholith zone. K–Ar mica and
U–Pb zircon magmatic ages for several of the syn- to post-tectonic granites,
and K–Ar metamorphic ages for several high-grade gneisses, all yield ages of
about 520–500 Ma (Richards & Singleton, 1981;Turner et al., 1993; Maher et
al., 1997a).These demonstrate that orogenic activity was part of the Cambro-
Ordovician Delamerian Orogeny. Post-tectonic plutons give slightly younger
K–Ar ages of about 500–480 Ma (Richards & Singleton, 1981;Turner et al.,
1993) and place an Early Ordovician age on the close of regional
deformation.The long-held view of a link with the Delamerian Fold Belt in
the Mount Lofty Ranges of South Australia (e.g.Wells, 1956) is confirmed by
a similar deformation chronology and numerous geological similarities, such
as comparable lithologies and distinctive post-tectonic granitic rocks. In
particular, the metamorphic core of the Glenelg Zone has a similar structural
and intrusive history to the strongly deformed and metamorphosed core of
the southern Delamerian Fold Belt in the Mount Lofty Ranges (Sandiford et
al., 1992;Anderson & Gray, 1994;Foden et al., 1999; Gray et al., 2002).
Southwestern metamorphic zone
The southwestern metamorphic zone outcrops for about 40 km between the
Wando Vale – Coleraine and Dergholm – Burke Bridge areas, prograding for
about 15 km southwest to northeast through biotite, garnet–andalusite,
sillimanite and migmatite zones (Fig. 3.4).VandenBerg et al. (2000) placed the
western boundary of the Glenelg River Complex at the Hummocks Fault.
Sweeping regional curvature of mainly NW-striking zone boundaries reflects
F5folding. Low-grade rocks to the west of this fault are mainly turbiditic
metagreywackes that retain sedimentary textures and structures. A
sedimentary carbonate component is most apparent in minor outcrops of
grey marble in Nolan Creek, and dark dolomitic slate occurs in Steep Bank
Rivulet and the Glenelg River south of Dergholm (Wells, 1956). Rare
dolomitic breccias are also present. Calc-silicates and actinolite–quartz schists
in higher-grade zones appear to be derived from these dolomitic rocks.
In the low-grade part of the sequence there are diverse layer-parallel
metabasites that were originally dolerite to gabbro and diorite (Wells, 1956;
Gray et al., 2002). These are now mainly plagioclase+actinolite+biotite and
have textures that range from intact igneous, to severely deformed and finely
layered, with foliated biotite or actinolite around plagioclase augen. Also
within the low-grade rocks is a minor volcanic component.Rare metabasalts
outcrop in the Dergholm–Nangeela area.They are fine-grained and plagioclase-
phyric with weakly variolitic textures, and appear to be lava flows. Minor
flows of plagioclase-phyric meta-andesite occur at Nolan Creek.
Chapter 3 Neoproterozoic and Cambrian 75
Halls Gap
50 km
Nargoon Group
Mt Stavely Volcanic Complex
Boninite–tholeiite association (Dimboola Igneous Complex)
Late Neoproterozoic Truro Volcanics
Magdala Volcanics
Geological boundary
Zone boundary
Bedding trend
Trend of dominant foliation
Schistosity in high-strain zone
Moralana Supergroup
St Arnaud Group
Middle Devonian
Lower Devonian
Rocklands Volcanics
Grampians Group
Fig. 3.3: Regional geological map of the Grampians area showing the distribution of the Mount Stavely Volcanic Complex rocks east of the Grampians (Mount Dryden Belt), south of the Grampians
(Mount Stavely Volcanic Complex) and between the Grampians and the Yarramyljup Fault (Black Range and Glen Isla belts) (modified from VandenBerg et al., 2000).
At biotite and higher grades, metabasites occur as two main types:
fine-grained laminated amphibolites and coarse-grained metagabbros.
Amphibolites are E-MORB compositions typical of tholeitic magmatism in
extending continental crust (Gibson & Nihill, 1992;Anderson & Gray, 1994).
Local proximity to, and textural gradation with, metagabbro suggest
formation by recrystallisation of early gabbroic sills. Unrelated metagabbros
with distinctively different MORB-type compositions (higher Al, Mg and Sr
contents) form layer-parallel sheets up to 15 m thick at Wando Vale, in Steep
Bank Rivulet and its tributaries (Gibson & Nihill, 1992;Turner et al., 1993;
Anderson & Gray, 1994) and along the Glenelg River between Dergholm
and Burkes Bridge. It is not known if these are dykes or sills. Their
actinolite+plagioclase metamorphic assemblage indicates that they may
represent a distinct intrusive phase emplaced late in D2.
Layer-parallel lenses of serpentinite occur as three large masses such as
the Hummocks Serpentinite (100 x350 m) (Wells,1956;Turner et al., 1993;
VandenBerg et al., 2000) and several 1–20-m thick sheet-like bodies.Their
interiors preserve the textures of cumulate peridotites, and their margins
often have a mylonitic fabric parallel to S2in the host rock. Relict chromites
in the serpentinite have Cr/(Cr+Al) values (0.71–0.93) (Turner et al., 1993),
characteristic of highly depleted boninitic magmas rather than mid-ocean
ridge basalt-type magmas, and are compositionally akin to chromites in
Cambrian boninitic cumulates from further east in Victoria, such as at
Heathcote and Howqua.
In areas showing medium- to high-grade metamorphism, the dominant
rock type of the complex is homogeneous, grey, fine-g rained quartzo-
feldspathic schist with layers 10–100 cm thick (Fig. 3.5). Quartzo-feldspathic
migmatites outcrop in places (Kemp & Gray, 1999b; Gray et al., 2002). Calc-
silicate rocks are similar throughout this grade range and vary from
centimetre thickness to substantial units at least 150 m thick. Rocks are fine-
to medium-grained, green to grey, with 10–60 cm internal layering; quartz-
rich layers (<1 mm to >20 cm thick) are aligned with S2.
There are numerous pre- or syn-D2intrusive rocks in the southwestern
zone (Kemp et al., 2002).A chain of plutons extending from the Wando River
to Wennicott Creek includes, from west to east, the Wando Tonalite and Torah,
Meissen and Deep Creek granodiorites (Anderson & Gray, 1994; Kemp et al.,
2002) and the Wennicott and Warradale tonalites (Bushell, 1996). Where
present, igneous contacts and/or magnetic signatures indicate pluton diameters
of 2–5 km.The S2fabric varies in intensity, with the Torah Granodior ite being
gneissic, the Wennicott Tonalite having a pervasive biotite foliation, and the
Deep Creek Granodiorite having a massive core and marginal foliation.These
are subdivided into lithological types (Kemp et al., 2002; Fig. 3.4).Wando-type
intrusives are pale grey, foliated to gneissic hornblende tonalites with an even,
medium grainsize that contain numerous igneous enclaves. Deep Creek types
have poikilitic K-feldspar, sporadic hornblende and a distinctive high-Na, high-
Sr character. Wennicott types commonly contain hornblende but, unlike
Wa ndo types, evolve towards lower K2O with increasing silica (Kemp, 2002).
In the Glenelg River valley about 10 km northeast of Dergholm is the small
Caupaul Igneous Complex (Ferguson, 1993), composed of quartz diorite,
diorite and gabbro-pyroxenite.The S2foliation intensity varies from weak to
strong. Of a number of outcrops of gabbronorite and pyroxenite,three exceed
500 m across. Ferromagnesian minerals include orthopyroxene, clinopyroxene
and anhedral hornblende, the latter commonly enclosing pyroxene.The range
in textures, variable plagioclase abundance (0–20 %) and mafic nature of many
rocks are consistent with a fractionating basaltic system. However, the limited
variation in plagioclase composition indicates only part of the fractionation
sequence is exposed. Deformational features are minor and timing of
emplacement is unclear.
Several unfoliated medium- to coarse-grained Loftus Creek-type
hornblende granodiorites with rare mafic igneous enclaves (Loftus Creek,
Cloven Hills) also intrude the southwestern metamorphic zone.
Northeastern metamorphic zone
The northeastern metamorphic zone occupies a NW-striking belt about 15
km wide prograding from the sillimanite zone to migmatite grade from
northeast to southwest. It extends westward from the Yar ramyljup Fault,with
the most complete section in the Glenelg River between Kanagulk and
Harrow. The sillimanite zone is a monotonous sequence of quartzo-
feldspathic and semi-pelitic schist with minor bands of biotite schist.The
only prominent metapelite forms a 120-m wide band of quartz+
plagioclase+biotite+muscovite+garnet+sillimanite schist in the Glenelg
River near Kanagulk. Pegmatites are common in the sillimanite and
migmatite zones as thin layers and irregular bodies. Metasedimentary rocks at
Evolution of the Palaeozoic Basement76
141° 45' E
N141° 15' E
Wando type
Wennicott type
Deep Creek type
Tuloona type
Loftus Creek type
Harrow type
Pre- to syn-D2Post-D2, pre-D5
Caupaul Igneous Complex
Post- D5 plutons
Chetwynd R
Wando R.
Yarramyljup Fault
axial granitic batholith northeastern zone
southwestern zone
Glenelg River Complex
Trend of S2 foliation SSerpentinite outcrop
Fig. 3.4: Simplified Cambro-Ordovician geology of the Glenelg River Complex (adapted from
Kemp et al., 2000). Depicted pluton shapes of the muscovite-bearing Harrow-type granitic
rocks in the northeast are approximate only. Note that much of the area is covered by Permian
and younger sedimentary and igneous rocks.
Fig. 3.5: Garnet–andalusite-zone schists with quartz-rich bands exposed in Corea Creek
Gorge, near Wando Vale, southwestern metamorphic zone, Glenelg River Complex.
Photograph by A. VandenBerg.
migmatite grade are intermittently exposed along the Glenelg River, where
they are intimately associated with varied granitic rocks (Kemp & Gray,
1999b; Kemp et al., 2002). The proportion of leucosome increases westward
leading to nebulitic migmatites, which grade rapidly into structurally
concordant, muscovite-bearing plutons whose leucosome compositions
correlate with those of adjacent migmatites. Comparable high-grade rocks
occur in Bryan and Robson creeks about 30 km to the south along strike.
The northeastern metamorphic zone contains numerous granitic
bodies, both structurally concordant and discordant. Textures are igneous,
and structures in enclaves constrain emplacement to syn- or post-D4to
pre-D5.Structurally concordant plutons have gradational contacts with
enclosing migmatites in which quartzo-feldspathic schist, migmatite,
pegmatite and muscovite-bearing granitic rock are complexly interleaved at
outcrop scale. In the Glenelg River about 1 km west of Scabbing Station
Creek, nebulitic migmatite grades over about 50 m into Dunmore
Leucotonalite. This body is 1.5 km across and is laden with migmatite
enclaves and micaceous selvages, particularly at the margins; diffuse mafic
schlieren define an internal fabric. In Schofield Creek, the Car rigeen
Granodiorite is 750 m across with a core of pale, homogeneous, medium-
grained, equigranular, muscovite g ranodiorite. Towards the margins it
becomes progressively more heterogeneous, littered with micaceous
selvages, microcline megacrysts and quartzo-feldspathic schist slabs. Locally
it has a schlieric fabric defined by biotite+muscovite+ sillimanite.
Ultimately, it grades into nebulitic migmatite at its northern and southern
contacts (Fig. 3.6). Homogeneous, muscovite-bearing granitic rocks are also
intrusive into the northeastern zone. The Harrow Granodiorite, exposed
for about 3.3 km in the Glenelg River valley immediately east of Harrow, is
medium-grained with primary muscovite, biotite,poikilitic microcline and
sillimanite. It crosscuts adjacent units such as the Carrigeen Granodiorite
and has a relatively low density of metasedimentary inclusions compared to
the concordant bodies and a weak annular mica foliation. The Marn
Mering Granodiorite, in the Glenelg River east of Schofield Creek, is 3 km
across. It is a light grey, medium- to coarse-grained, porphyritic rock with
microcline phenocrysts and generally lacks metasedimentary enclaves.
Garnet–muscovite granitic bodies and felsic dykes are also widespread.
Axial granite batholith zone
The central parts of the Glenelg River Complex (Fig. 3.4) consist of a wide
expanse of granites between Harrow and Chetwynd.These extend over 20
km across regional strike and separate the northeastern and southwestern
metamorphic zones. Extrapolation from this transect using additional limited
outcrop and aeromagnetic data implies a batholith at least 40 km long.This
area is dominated by the numerous Tuloona-type granitic rocks (Tuloona,
Chetwynd, Coojar and Patawilya, Glendara), which are unfoliated, grey
granodiorite-granite with mafic igneous enclaves, minor muscovite and
poikilitic microcline. Many of the plutons have marginal, muscovite-rich
felsic phases that merge into migmatite (Fig. 3.7). Elongate quartzo-
feldspathic schist enclaves define a flow str ucture. In addition, ellipsoidal,
mafic, igneous-textured enclaves have plagioclase phenocrysts in a matrix of
plagioclase, quartz and biotite.
Texturally distinctive, Loftus Creek-type hornblende granodiorites are
medium- to coarse-grained with large (5 mm) euhedral biotites, poikilitic
K-rich feldspar, prominent titanite euhedra and rare mafic igneous enclaves.
There are four main plutons, all of which are unfoliated and post-date D2.
The Koolomurt Granodiorite crosscuts the Glendara Adamellite in Pigeon
Ponds Creek. Further south the Cairns Creek Granodiorite is zoned from a
porphyritic, hornblende-bearing margin to a felsic core without hornblende.
The Loftus Creek and Cloven Hills granodiorites (Anderson & Gray, 1994;
Kemp, 2002) intrude the southwestern metamorphic zone.
Undeformed post-tectonic granite occurs in the vicinity of Dergholm,
just to the west of the metamorphic complex. Distinctive common features
are red to buff colour,equidimensional grey to black quartz, highly perthitic
microcline, albitic plagioclase, common accessory fluorite, pleochroic biotite
(very dark brown to black),and evolved chemical compositions.Tur ner et al.
(1993) treated these rocks as a single body, the ‘Dergholm Granite’. However,
three or four textural types are recognised and regarded as forming distinct
plutons, with their shapes deduced from magnetic signatures. Magnetic data
also indicate continuity as a batholith extending subsurface into South
Australia, confirming petrological links to scattered exposures as far distant as
Murray Bridge. The Baileys Rocks Granite outcrops over 8 km in a NNW
direction in the vicinity of the reserve of the same name. It has a lobe-like
magnetic shape about 7 km across extending N–S for at least 15 km. It is
characterised by buff K-rich feldspar phenocrysts and may contain
hornblende and titanite.The Dergholm Granite, equidimensional and 7 km
across, is located in the environs of the Glenelg River around Dergholm. It
has a distinctive medium-grained, equigranular texture with beta-quartz
crystals set in square, buff crystals of both K-rich and plagioclase feldspar. An
oval pluton with low magnetic intensity and diameter of 12 km at Poolaijelo
is defined as the Poolaijelo Granite. Minor exposures in and around Salt
Creek, south of Poolaijelo, are distinct from the Baileys Rocks lithology.The
most abundant type is even-textured and medium- to coarse-grained, having
equidimensional quartz grains combined with feldspar, usually uniformly red,
but sometimes contrasting with cream or pale green. Local variants are finer
grained and grade to aplite.
Age constraints and correlations
Most of the Glenelg Zone is probably correlated with the Late
Neoproterozoic – Early Cambrian Moralana Supergroup and its correlatives
in the Koonenberry belt of westernmost New South Wales.These sequences
are dominated by turbiditic metasediments and shales, with minor carbonates,
Chapter 3 Neoproterozoic and Cambrian 77
Fig. 3.6: Transition zone from schist and migmatite to muscovite granite exposed in Schofield
Creek, near Harrow, northeastern metamorphic zone, Glenelg River Complex. Photograph
by R. Cayley.
Fig. 3.7: Marginal phase of the Tuloona Granodiorite exposed in Schofield Creek, with a
directional fabric defined by elongate metasedimentary enclaves and micaceous schlieren. An
igneous-textured microgranular enclave occurs in the centre of the photograph near the pen.
Photograph by T. Kemp.
with a major pulse of rift-type transitional alkaline to tholeitic magmatism at
about 600–590 Ma (Crawford et al., 1997). Due to a lack of fossils, age
constraints on the rocks of the Glenelg Zone consist of radiometric age
determinations on detrital and metamor phic minerals, and also on the
numerous Late Cambrian granites which intrude the sequence. A relatively
discrete population of detrital zircons at about 590 Ma in biotite gneiss from
drillhole VIMP-12 (Maher et al., 1997a) provides a maximum possible age
for the precursors of the Moralana Supergroup rocks here.A minimum age
constraint is provided by the Early Cambrian Bringalbert Gabbro in the same
drillhole (524±9 Ma; Maher et al., 1997a).This appears to have intruded the
sedimentary sequence, and suffered some subsequent deformation. Thus, the
age of at least part of the pelitic sequence in this region is broadly bracketed
at between 590 and 524 Ma, i.e.Ediacaran to Early Cambrian.
K–Ar cooling ages of granitic rocks in the Glenelg River Complex are
late Middle Cambrian (~500 Ma) for syntectonic pegmatite and Late
Cambrian (~490 Ma) for post-tectonic pegmatite and the post-tectonic
A-type Baileys Rocks Granite (Turner et al., 1993). Cooling of the
metamorphic rocks to about 300ºC (blocking temperature of K–Ar biotite)
also occurred at this time (Turner et al., 1993; Maher, et al., 1997).Two Ar/Ar
dates on metamorphic biotite are also Late Cambrian (Turner et al., 1993).
Thus the sedimentary protoliths are older than Late Cambrian, when the
deformation and accompanying metamorphism appear to have occurred.
3.2.3 Grampians–Stavely Zone
The Grampians–Stavely Zone (Fig. 3.2) is distinguished from the Glenelg
Zone by lower grade, less-deformed rocks, and by the absence of
syn-tectonic granites. The Cambrian rocks of this zone are generally poorly
exposed, largely buried beneath the spectacular younger cover of the
Grampians Group.They consist of a number of belts of calc-alkaline volcanics
(the Mount Stavely Volcanic Complex, made up of the Black Range and
Mount Dryden belts and correlated units) that are structurally intercalated
with black shale and sandstone of the Nargoon Group (Fig. 3.3). In the
western half of the zone, these rocks are of greenschist grade and possess a
cleavage, but east of the Escondida Fault they are of lower grade and lack
cleavage development. There are scattered occurrences of tholeiitic to
boninitic lavas and intrusives throughout the zone, with most occurring as
narrow fault slices discovered by drilling beneath shallow cover. The
Williamsons Road Serpentinite occurs as elongate faulted slivers of boninitic
cumulate-derived serpentinite within calc-alkaline lavas of the Mount Stavely
Vo lcanic Complex. Aeromagnetic data, however, show a major belt of
magnetic rocks (Fig. 3.8) — the Dimboola Igneous Complex (VandenBerg
et al., 2000) — becoming extensive to the north under Murray Basin cover.
Whether this represents massive accumulations of the tholeiite-boninite
sequence (the Dimboola Igneous Complex of VandenBerg et al., 2000), or
seaward-dipping reflector packages forming part of the 600-Ma r ifted margin
of southeastern Australia (Crawford & Direen, 1998;Direen, 1999; Crawford
& Direen, 2001) remains to be established.
Evolution of the Palaeozoic Basement78
0 50 100
Fig. 3.8: Aeromagnetic image of Victoria showing the major magnetic high striking northwest beneath the Murray Basin. Known as the Dimboola Igneous complex of VandenBerg et al. (2000),
the rocks responsible for this feature may be either Late Neoproterozoic metabasalts of the 600-Ma volcanic passive-margin sequence (Crawford & Direen, 2001) or an oceanic forearc-derived
basaltic pile that collided with the passive margin in the early Middle Cambrian. (image courtesy of Geological Survey of Victoria).
Mount Stavely Volcanic Complex
The Mount Stavely Volcanic Complex (Buckland & Ramsay, 1982;Buckland,
1986; Crawford,1982, 1988; Donaghy, 1994; Crawford et al., 1996a,b; Cayley
& Taylor, 1997) forms a series of NW-trending linear volcanic belts in the
Grampians–Stavely Zone (Fig. 3.3).These are the subparallel Mount Stavely
and Mount Dryden belts, respectively south and east of the Grampians.The
two belts outcrop sporadically as a number of low hills with good exposure.
Just west of the Grampians in the Black Range, several belts of volcanics are
much more poorly exposed but have been extensively drilled. These lavas
have the same calc-alkaline geochemistry as the other belts. Although
individual belts show considerable petrographic and geochemical var iation
along strike, the variation between belts strongly overlaps (Buckland, 1986;
Donaghy, 1994; Direen, 1999). The belts of volcanics incorporate rare fault
slices of variably serpentinised boninitic rocks and are intercalated with low-
grade and weakly deformed turbidites and black shale of the Nargoon
Group.Most of the volcanic rocks are essentially undeformed and only partly
altered by prehnite-pumpellyite grade metamorphism, so that or iginal
igneous textures are largely preserved.
The Mount Stavely Volcanic Complex also outcrops south of the
Grampians, where it consists of fault-bounded but internally undefor med blocks
of volcanics.The basal sequence is dominated by medium- to high-K, calc-
alkaline andesite and dacite of the Fairview Andesitic Breccia (Table 3.1). More
felsic compositions in the upper parts of the pile include the Narrapumelap
Road Dacite and the Nanapundah and Towanway Tuffs. Small tonalite-
trondhjemite plutons of the Lalkaldarno Tonalite (Fig. 3.9c) have intruded the
volcanic rocks. Detrital zircons in a volcaniclastic rock, and magmatic zircons in
a meta-dacite of the Mount Stavely Volcanic Complex, have yielded
crystallisation ages of 501±9 Ma and 495±5 Ma respectively (Stuart-Smith &
Black, 1994). Biotite from the Lalkaldarno Tonalite yielded an Ar/Ar age of
500±2 Ma (Foster et al., 1996b).
Mount Dryden Belt
The Mount Dryden Belt outcrops intermittently between the Moyston and
Mehuse Faults, east of the Grampians (Fig. 3.3). Principal sites where volcanics
in the belt outcrop are (from north to south) Mount Dryden, Lake Fyans,
McMurtrie Hill, Jallukar,Barton and much further south at Lake Bolac. Mount
Elliot occurs halfway between the Mount Dryden Belt and the Mount Stavely
Belt as an isolated hill on a NW-trending linking belt. Parts of the Mount
Dryden Belt have previously been described in some detail by Buckland (1986)
and Crawford (1988).The width of the Mount Dryden Belt is g reater than
previously thought, up to 5 km at Mount Dryden. Magnetic data show that the
isolated outcrops expose different levels of a conformable west-facing sequence
within the belt, rather than being direct along-strike extensions. The belt has
been informally subdivided into three major units that are intercalated within it
(Cayley & Taylor, 2001).These are:
1. About 1000 m of relatively low-Ti andesitic to dacitic lavas including
pillow lavas (Table 3.1) and high-level intrusions;
2.About 500 m of volcanic conglomerate (Fig. 3.10);
3. More than 500 m of volcaniclastic sandstone.
It is impossible to determine the orig inal thicknesses of the units because
of faulting; however, the minimum true thickness of the conformable sequence
at Mount Dryden is 2000 m, with a possible further 1500 m of undifferentiated
volcanics to the west, beneath thin alluvial cover.In the vicinity of McMurtrie
Hill, these volcanic and volcaniclastic rocks have been intruded by a diorite sill
with a thickness of 450 m, but it appears to be truncated on its eastern side by
the faulted eastern margin of the belt.
Black Range Belt
In the Black Range about 35 km west of Halls Gap, midway between the
Grampians and the Yarramyljup Fault, calc-alkaline volcanics outcrop in the
25-km long, NW-trending Black Range Belt (Fig. 3.1). A smaller (<10 km
strike length) belt, the Glenisla Belt, occurs to the east of the main belt.A third
belt, the Tyar Belt, occurs beneath laterite to the west of the Black Range and
has been delineated by aeromagnetics and drilling (Spencer-Jones, 1965;
McArthur, 1990; Cayley & Taylor,1997).Mafic to inter mediate volcanics occur
in all three belts and intermediate to felsic volcanics occur on the eastern side of
Chapter 3 Neoproterozoic and Cambrian 79
Fig. 3.9: Photomicrographs of rocks from the Mt Stavely Volcanic Complex. (a) Strongly
altered base of an olivine-rich picritic lava flow showing serpentinised euhedral former
olivine crystals, some with small chromite inclusions, and glassy groundmass now replaced
by low birefringent chlorite. From Yanac South, Western Victoria, prospect drilled by MIM.
(b) Quenched top of a dacitic lava with typical rosettes of rapidly cooled plagioclase, and
an altered and recrystallised, formerly glassy groundmass now replaced by chlorite, quartz
and albite. From McRaes prospect, Black Range, Western Victoria. (c) Shallow intrusive
tonalite of the Lalkaldarno Porphyry from south of Mt Stavely, showing euhedral
hornblende, quartz and albitised plagioclase phenocrysts. All images are c.5 mm across and
are taken with crossed polars.
the main belt (Cayley & Taylor, 1997). Unlike the other belts further east, the
volcanics in the Black Range show variably developed cleavage or schistosity.
Drill holes bored by CRAE at McRaes prospect and elsewhere in the Black
Range Belt intersected diverse volcanics dominated by dacitic lavas (Fig. 3.9b),
but including quartz-phyric rhyolite and subordinate andesitic and basaltic lava
(Direen, 1999).Similar rocks have been drilled at VIMP-3 during the Geological
Survey VIMP drilling program in this region. Analytical data for andesitic lavas
from these drill holes were reported by Maher et al., (1997a) and Direen (1999)
(see Table 3.1).The plagioclase+augite-phyric andesites and dacites of the Black
Range Belt have medium-K calc-alkaline affinities, with characteristically low
TiO2values and other compositional features indicating a possible correlation
with the Mount Dryden Belt (Maher et al., 1997a; Direen, 1999).
Setting and regional correlation of the Mount Stavely
Volcanic Complex.
The Mount Stavely Volcanic Complex shares similar submarine eruptive setting,
geochemistry and age to the Mount Read Volcanics of Tasmania (Crawford et
al., 1996a). In Tasmania, these rocks are interpreted as post-collisional rift
Evolution of the Palaeozoic Basement80
Fig. 3.10: Volcanic conglomerate/breccia, Mount Dryden Volcanics, north slope of Mount
Dryden. Photograph by A. VandenBerg.
298.8 m 216.6 m FP1 FP4 GM47-227 GM47-70.8 GM48-96.2 26226 26222 26203 26205
SiO244.70 46.40 52.50 54.80 62.70 75.00 72.60 56.40 59.60 58.60 61.40
TiO20.05 0.16 0.31 0.26 0.86 0.34 0.60 0.33 0.31 0.50 0.49
Al2O33.91 11.30 13.10 11.30 15.40 12.40 13.40 15.30 14.60 15.30 15.60
FeO* 10.40 9.54 9.96 7.89 6.37 2.63 5.23 10.20 7.95 8.91 9.42
MnO 0.17 0.22 0.16 0.13 0.13 0.04 0.17 0.10 0.11 0.15 0.12
MgO 39.30 22.60 14.30 14.80 4.49 3.21 3.71 7.70 5.34 3.91 2.91
CaO 0.24 7.73 5.66 5.90 3.12 0.32 0.33 4.03 8.76 7.58 5.16
Na2O0.060.89 1.13 0.81 2.12 4.60 1.91 5.52 2.36 4.61 4.03
K2O0.010.02 1.72 3.19 3.99 1.03 1.29 0.37 0.83 0.39 0.70
P2O50.00 0.01 0.07 0.05 0.11 0.07 0.17 0.03 0.08 0.12 0.13
LOI 11.90 6.76 10.10 7.10 4.81 2.18 3.71 1.76 1.43 1.78 1.01
Trace elements (ppm)
Ni 1567 976 429 376 26 15 8 51 45 35 17
Cr 229 186 59 18
V68164 199 178 157 64 90 178 197 222 263
Sc 17 34 36 33 25 8 13 34 27 25 22
Zr 1 7 28 25 131 116 114 48 56 60 69
Nb <1 <1 1.3 1.7 4 2.3 2.4 <2 <2 <2 <2
Y1.38.38 8231412.5 8 10 12 13
Sr 3 21 122 111 81 66 67 74 246 235 258
Rb <1 <1 20 44 64 17 9 15 22 14 18
Ba 4 5 648 859 522 236 382 26 96 121 29
12 13 14 15 16 17 18 19 20 21 22
26229 108.7 m V2S1 V2O2 V2A V2K V2Q V2W2(2) V2I3 V2H3 26233
SiO269.40 63.60 53.30 58.80 59.80 60.80 63.00 65.40 72.20 75.00 65.80
TiO20.43 0.43 0.75 0.73 0.61 0.81 0.97 0.58 0.48 0.46 0.54
Al2O313.90 14.50 15.40 15.60 12.50 16.90 16.20 12.50 13.80 13.50 16.80
FeO* 5.30 6.87 10.90 7.67 8.53 5.76 5.48 6.80 2.99 2.59 2.93
MnO 0.14 0.09 0.16 0.12 0.14 0.09 0.08 0.09 0.05 0.04 0.05
MgO 1.54 3.06 6.03 5.04 7.07 3.55 2.79 4.27 1.21 0.46 3.73
CaO 3.22 4.32 10.05 6.94 7.29 6.36 6.35 5.89 2.89 0.35 4.73
Na2O4.373.25 2.39 3.81 2.87 4.86 4.23 3.47 3.65 4.99 4.89
K2O1.520.49 0.75 0.98 0.96 0.62 0.63 0.73 2.63 2.59 0.49
P2O50.19 0.19 0.29 0.28 0.24 0.26 0.31 0.22 0.12 0.07 0.13
LOI 1.03 3.15 6.20 5.16 7.22 3.64 2.87 4.36 1.26 0.50 3.78
Trace elements (ppm)
Ni 4 12 88
Cr 15 123 99 313 59 4 52 25 3 59
V188 185 349 220 213 161 173 159 87 28 83
Sc 18 22 36 31 35 22 19 23 12 11 10
Zr 85 55 43 137 97 168 109 125 138 171 87
Nb <2 2 2 5 4 4 4 3 7 4
Y22 12 18 14 12 15 16 12 20 26 15
Sr 416 50 430 418 407 525 733 466 263 151 451
Rb 36 11 17 24 25 15 14 22 56 39 12
Ba 330 273 191 149 160 106 86 119 489 499 108
Table 3.1: Whole-rock analyses for Late Neoproterozoic and Cambrian greenstones from the Stavely Greenstone Belt and Glenelg Zone. 1, 2: Dunitic cumulates associated with Late Neoproterozoic
picritic lavas, Yanac South drillholes YANS1 and YANS2 drilled by MIM; depths shown. 3, 4: Boninitic lavas from the early Middle Cambrian allochthon drilled at the Frying Pan prospect by CRA, near
Jallukar, within the Stavely Greenstone Belt. 5–22 are all from the Mt Stavely Volcanic Complex. 5–7: Dacite and rhyolite lavas drilled by CRA at McRaes prospect in Black Range. 8–11: Low-Ti
andesites from Mt Dryden. 12: Dacite from Jallukar. 13: Low-Ti andesite akin to those at Mt Dryden, drilled in VIMP-3 by Geological Survey of Victoria (Maher et al., 1997a). 14–21 are from the Mt
Stavely Volcanic Complex, analyses 12–19 are from andesites and dacites of the Fairview Andesitic Breccia, and 20 and 21 are from the Narrapumelap Road Dacite. 22: Tonalite from the Lalkaldarno
Tonalite intruding the Mt Stavely Volcanic Complex. 1–13 and 22 are from A. J. Crawford (unpublished) and Direen (1999). 14–21 are from Donaghy (1994).
volcanics erupted into rift basins developed in the older crust during the waning
phases of the Delamerian/Tyennan Orogeny (Crawford & Berry, 1992).The
strong temporal and compositional similarities between the post-collisional
volcanics in western Victor ia and better exposed correlatives in western
Tasmania, suggest there is some potential for VHMS- and porphyry-style
mineralisation in the unexposed sequences that have been shown by magnetics
to extend for some considerable distance beneath the Murray Basin.
Nargoon Group
This group incorporates all the sedimentary bedrock of the Grampians–
Stavely Zone, comprising the poorly outcropping black slate and sandstone
in the Black Range area as well as the much better exposed and described
Glenthompson Sandstone further east (Buckland, 1986). The sedimentary
rocks are all interpreted to have originally overlain the Mount Stavely
Vo lcanic Complex, and thus represent the sedimentary fill in the upper parts
of the rift basin occupied by those volcanic rocks.
The Glenthompson Sandstone, originally defined in the vicinity of the
Mount Stavely Volcanic Complex (Buckland, 1986), forms a uniform and
widespread package of micaceous quartz-rich turbidites which compr ise much
of the sedimentary bedrock in the Grampians–Stavely Zone between the
Moyston and Escondida faults (Fig. 3.3).This includes the discontinuous belt
of turbiditic sandstone referred to as the Moyston Sandstone by Watchorn &
Wilson (1989), sandwiched between this fault and the Mount Dryden Belt.
The Glenthompson Sandstone possibly conformably overlies Mount
Stavely Volcanic Complex. A gradational sedimentary contact with the
underlying volcanic rocks is indicated by the presence of interbedded
rhyolitic and andesitic volcaniclastic arenite beds within the Glenthompson
Sandstone (e.g. Stuart-Smith & Black, 1994; Donaghy, 1994).
Between the Escondida and Yarramyljup faults, the Nargoon Group
consists of extremely poorly exposed quartz-rich turbiditic metasandstone,
mudstone, schist and black slate, which separate the fault-emplaced belts of
the Mount Stavely Volcanic Complex (Cayley & Taylor, 1997). Apart from
minor outcrops in the vicinity of the Black Range,south of Balmoral, and in the
vicinity of Chatsworth, most information on these rocks comes from drill holes
intersecting the sequence in the Black Range region. From the limited data
available, it appears that they are predominantly deep-marine terrigenous to
hemipelagic metasedimentary rocks of low greenschist grade, although they
reach biotite grade in the vicinity of Chatsworth (Stuart-Smith & Black,1994).
We ak magnetic striping in the various metasedimentary rocks of the Nargoon
Group may be due to the presence of dykes, or interbedded mafic volcanics,
as interpreted to the west of Mount Stavely (Stuart-Smith & Black, 1994).
Although unfossiliferous, the age of the Nargoon Group is well
constrained to Late Cambrian by the underlying 500 Ma volcanics of the
Mount Stavely Volcanic Complex, and the 489 Ma Bushy Creek Granite
which intrudes the deformed Nargoon Group just west of these volcanics.
The tight timing of deposition, deformation and intrusion of the group
suggests that it was deposited during the waning phases of the Delamerian
Orogeny.This inference is confirmed by the large population of 500–510 Ma
detrital zircons in the unit, which must have been derived from Delamerian
magmatism as recorded in the Glenelg Zone to the west.
Mafic–ultramafic rocks
Scattered occurrences of ultramafic to mafic, boninitic and tholeiitic volcanics
and intrusive rocks occur in the eastern portion of the Grampians–Stavely Zone.
On the basis of their distinctive geochemical signature,they are unambiguously
correlated with the main Cambrian tholeiite–boninite association of the
Heathcote Greenstone Belt in the Lachlan Fold Belt further east, and also with
the Early Cambrian mafic–ultramafic complexes of western Tasmania (Crawford
& Berry, 1992). Ultramafic serpentinite slivers, such as the Williamsons Road
Serpentinite, in-faulted into the Mount Stavely Volcanic Complex are included
in this suite, as are unusual boninitic lavas at the Frying Pan prospect west of
Moyston (Menpes, 1994), and at Wartook west of the Grampians (Stewart,
1993).The presence of these presumed fault slices west of the Moyston Fault-
defined margin of the Lachlan Fold Belt is important. It implies that the
tholeiite–boninite basement of the southeastern Lachlan Fold Belt was either
originally thrust westwards onto Grampians–Stavely Zone elements of the
Delamerian Fold Belt, or was thrust west of the Moyston Fault at some late
stage of the Delamerian deformation event.
Widespread and voluminous magnetic mafic rocks beneath younger
cover rocks north of the Grampians are indicated by aeromagnetics (Fig. 3.5)
and limited drilling. The large magnetic package around Dimboola has been
drilled by North Ltd. and shown to include altered basaltic lapilli tuff and
basalt, with microgabbro and pyroxenite; geochemical data for these rocks are
unavailable (O’Neill,1994).Further south, the VIMP-9 dr ill hole intersected
moderately augite-phyric tholeiitic basalt with hyaloclastite peperitic
interaction with the interlayered red siltstone. (Maher et al., 1997a).A smaller
magnetic high close to the Victoria–South Australia border at Yanac South
was drilled by MIM and intersected distinctive olivine-rich picritic lavas and
cumulates with rift-tholeiite compositions (Direen, 1999) (Fig. 3.9a).These
have been correlated with the Late Neoproterozoic picrites on King Island
and in the Smithton Trough in northwestern Tasmania.
3.2.4 Regional synthesis
A geological synthesis of the Glenelg Zone is hampered by a lack of age control
because the sediments are unfossiliferous and the mafic volcanics lack dateable
minerals. Much of the regional synthesis draws on lithological correlations with
better-understood sequences in South Australia and Tasmania that were also
involved in the Delamerian Orogeny. In the Glenelg Zone,the mainly quartzo-
feldspathic turbidites and associated mafic volcanics represent an outboard
portion of the passive margin sequence of the Delamerian Fold Belt, referred to
as the Stansbury Basin (Belperio et al., 1988). Basalts in easternmost South
Australia and westernmost New South Wales (Crawford et al., 1997) are
compositionally transitional from intraplate basalts to rift tholeiites, and record
magmatism preceding and during breakup of this section of Gondwana, at
about 600 Ma. East of the Glenelg Zone, the large belt of magnetic mafic
igneous rocks may represent the remnants of an arc–forearc complex that lay to
the east of the Stansbury Basin in the Cambrian (VandenBerg et al., 2000).An
alternative interpretation for these rocks is that they represent rift tholeiites
formed orig inally as seaward-dipping reflectors on an east-facing volcanic
passive margin during the 600–590 Ma continental breakup in eastern
Gondwana (Crawford & Direen,1998; Direen, 1999).
Serpentinised ultramafic cumulates and comagmatic lavas with boninitic
affinity occur as fault-bounded slices referred to as the Dimboola Igneous
Complex by VandenBerg et al. (2000). These appear to be associated in places
with MORB-type tholeiitic lavas.This association is better exposed and well
studied from the Heathcote and Mount Wellington Greenstone Belts of the
Lachlan Fold Belt further east, where it has been interpreted as part of a massive
allochthonous sheet of crust and upper mantle of Early Cambrian age that was
emplaced westward onto the 600-Ma passive margin (Crawford & Berry, 1992).
This arc–continent collision represents the earliest phase of the Delamerian
Orogeny, and subsequent crustal thickening led to uplift of the Glenelg Zone,
which shed sediment into one or more localised extensional submarine rift
basin(s) forming during the uplift. These were initially filled with calc-alkaline
volcanics of the Mount Stavely Volcanic Complex and related suites in the
Mount Dryden and Black Range Belts. As volcanism waned the marine
sandstones and black shale of the Nargoon Group accumulated upon the
volcanics and were only weakly deformed shortly after deposition in the
terminal phases of the orogeny. Based on (1) the age and compositional
similarity of all but the Mount Dryden Belt lavas with the Mount Read
Vo lcanics in Tasmania, which unambiguously post-date emplace-ment of the
forearc-derived ophiolitic allochthons in western Tasmania and Victoria
(Crawford & Berry, 1992), and (2), the occurrence of in-faulted slices of
boninite-derived serpentinised ultramafics, it is assumed here that these calc-
alkaline volcanic suites in western Victor ia are also post-collisional magmatic
suites. The Mount Dryden Belt remains undated, but regional geological
considerations (e.g. it is conformably overlain by Late Cambrian Glenthompson
Sandstone) suggest that these lavas too are a post-collisional suite.
Chapter 3 Neoproterozoic and Cambrian 81
3.3 Lachlan Fold Belt
3.3.1 Stawell and Bendigo zones
In the western Lachlan Fold Belt the Magdala, Pitfield and Heathcote
Vo lcanics occur in the hanging wall of major fault zones (Fig. 3.1).These
three predominantly volcanic packages are inferred to be exposed portions
of a sheet of ocean-floor volcanics that regionally underlie the Cambro-
Ordovician turbidite pile.The major craton-directed Moyston Fault has
thrust the mid-crustal levels of the Lachlan Fold Belt over the Delamer ian
Fold Belt, and brought the Moornambool Metamorphic Complex to the
surface (VandenBerg et al., 2000). This metamorphic complex contains a
variety of mafic and pelitic schists derived from Cambrian volcanic and
sedimentary protoliths during the Late Ordovician – Early Silurian
Benambran Orogeny (see Chapter 2,section 2.3.1).The unfossiliferous and
presumed Cambrian turbidites of the St Arnaud Group overlie the
volcanics in the west, whereas the Goldie Chert and Knowsley East Shale
in the east lie between the Cambrian volcanics and the Ordovician
turbidites of the Castlemaine Group.
Magdala Volcanics
The Magdala Volcanics occur in the Moornambool Metamorphic Complex,
a 15-km wide high strain zone adjacent to the Moyston Fault (Cayley &
Taylor, 1998b).The volcanics consist of lavas and volcaniclastic sediments, best
known from the Stawell Mine (‘Footwall volcanics’ of Watchorn & Wilson,
1989 and ‘Magdala volcanogenics’ of Watchorn, 1986). Other exposures of
this unit occur south of Great Western and southwest of Ararat. The Magdala
Vo lcanics consist of massive and pillowed basaltic lavas, volcaniclastic
interflow sediments and minor chert, attesting to eruption in a submarine
environment.The volcanics are regarded as the protolith of the higher grade
Deenicull Schist and Carrolls Amphibolite, which also outcrop within the
Moornambool Metamorphic Complex, such as in the Moyston area.
The Magdala Volcanics form the lowest and oldest rock unit exposed
in the westernmost portion of the Lachlan Fold Belt.They underlie the St
Arnaud Group but the base is not exposed and their thickness is not
known.The Magdala Antiform has a width of about 1000 m of deformed
basalt in the Stawell gold mine, but there is evidence of considerable
structural thickening here (Watchorn & Wilson, 1989). The thickest
package that is relatively undeformed is approximately 200 m thick in a
fault slice at Sheepfold Hill, southwest of Ararat.At Stawell, a 10–70 m
thick sequence of layered and massive volcaniclastic sediments occurs on
the west flank of the Magdala Antifor m (Willman, 1987; Watchorn &
Wilson, 1989; Phillips et al., 2002). An apparently conformable transition
from lava through volcaniclastic sediments up into the overlying Warrak
Formation of the St Arnaud Group is preserved in some drillholes on each
side of the Magdala Antiform, although disrupted by some faults
(Watchor n, 1986;Willman, 1987; Fredrickson & Gane, 1998).
Unpublished geochemical studies of the Magdala Volcanics metabasalts
show them to be tholeiitic basalts essentially identical in composition to
the better exposed Cambrian tholeiitic basalts in the Heathcote and
Mount Wellington Greenstone Belts (Crawford, 1994). Whole-rock
geochemical analyses (Table 3.2) show a strong Fe enrichment, and
increases in TiO2and V with advancing fractionation, features typical of
strongly differentiated tholeiitic suites (Will, 1990). The geochemical
signature is transitional between ridge-generated basalts and island-arc
tholeiites, similar to rocks erupted in modern backarc basin settings. Some
samples from the Magdala Volcanics have extremely low TiO2contents and
are depleted in Zr and Y (Rowe, 1989), features that are atypical of the
tholeiitic rocks at Stawell, but that are very close to the low-Ti tholeiites
in the Early Cambrian mafic–ultramafic complexes in western Tasmania
(Brown & Jenner, 1989; Crawford & Berry, 1992). Some samples of
strongly deformed Carrolls Amphibolite contain relict Cr-rich and Al-
poor chromite grains, which together with the widespread occurrence of
tremolite and magnesian hornblende in the Deenicull Schist and the
Carrolls Amphibolite (Cayley & Taylor, 2001) suggest a substantial
boninitic component to parts of the Magdala Volcanics sequence. This
tholeiite–boninite association is identical to the better-exposed Heathcote
Vo lcanics and Dookie and Thiele igneous complexes.
The Magdala Volcanics are economically important, because the lavas
form the rig id buttress of rock at Stawell against which quartz-gold
mineralisation has developed.The overlying volcaniclastic sediments host
some of the most mineralised parts of the Central Lode System (Fredrickson
& Gane, 1998). Strongly metamor phosed equivalents of the Magdala
Vo lcanics also host gold mineralisation at Moyston.The Mount Ararat copper
lode may represent a deformed and metamorphosed VHMS deposit related
to the Magdala Volcanics, in the higher-grade parts of the Moornambool
Metamorphic Complex south of Ararat.
Because the Magdala Volcanics underlie the St Arnaud Group, they are
therefore no younger than Cambrian.An Early Cambrian age is inferred by
comparison with similar rocks exposed at Heathcote, where there are
associated Early Cambrian fossils. A Pb/Pb isochron age of 700±30 Ma
Evolution of the Palaeozoic Basement82
12 3 4 5 67 8 9 101112 13
68890 68887 68888 P3 P4 BH7 BH12 BH11 BH6 26255 26263 26265 26266
SiO249.60 51.60 51.30 48.66 49.17 46.70 49.80 49.80 51.50 62.50 64.00 65.00 65.00
TiO21.25 1.56 2.11 1.07 0.98 0.03 0.06 0.12 0.12 0.30 0.26 0.28 0.25
Al2O316.60 14.80 13.80 16.72 15.50 18.00 16.80 16.70 16.10 13.00 15.10 16.30 16.80
FeO* 10.30 9.90 11.30 12.25 11.28 4.68 5.62 7.62 10.62 7.30 6.25 5.33 4.54
MnO 0.17 0.17 0.21 0.17 0.20 0.13 0.16 0.16 0.19 0.10 0.16 0.12 0.06
MgO 6.06 6.43 6.12 7.42 8.61 11.60 11.20 10.10 6.92 7.34 3.82 3.26 2.66
CaO 10.80 9.16 8.14 9.54 12.13 18.20 15.50 14.00 12.90 3.25 4.59 2.17 2.58
Na2O3.39 4.56 4.68 2.25 1.65 0.71 0.84 1.46 1.45 5.53 4.42 5.31 7.95
K2O0.68 0.53 1.04 1.79 0.35 0.03 0.07 0.08 0.07 0.56 1.42 2.20 0.19
P2O50.05 0.11 0.18 0.10 0.10 0.00 0.00 0.00 0.01 0.11 0.08 0.08 0.07
LOI 5.16 2.87 1.85 3.63 2.34 1.53 1.16 2.68 1.00 2.02 1.60 1.16 3.20
Trace elements (ppm)
Ni 66 85 110 97 107 199 116 125 90 76 50 67 24
Cr 148 324 160 145 164 672 551 385 117 300 52 44 31
V330 389 495 89 123 158 278 90 86 99 80
Sc 37 41 55 63 17 17 14 11
Zr 71 91 135 14 3 3 24 84 106 97 90
Nb <1 2 3 2 <1 <1 <1
Y31 38 48 2 1 5 6 17 13 20
Sr 188 153 103 47 43 48 37 232 259 199 163
Rb 17 10 24 2 <1 3 2 86 25 28 11
Ba 33 35 50 400 132 642 151
Table 3.2: Whole-rock analyses for Cambrian greenstones from the Stawell and Bendigo zones. 1–3: Magdala Volcanics: tholeiitic metabasalts from underground drilling, Stawell old mine
(A. J. Crawford, unpublished). 4–5: Pitfield Volcanics: Greenstones from mine dumps at Pitfield (R. W. R. Ramsay, unpublished). 6–9: Ceres Metagabbro near Geelong (V. J. Morand, unpublished).
10–13: Lazy Bar Andesite: low-Ti andesite lavas associated with boninite in the central section of the Heathcote Greenstone Belt (A. J. Crawford, unpublished).
reported from the Magdala Volcanics at Stawell (Wilson et al., 1992) probably
reflects incorporation of old crustal lead. A two-end-member mixing age of
518±52 Ma better represents the age at which the rocks were extruded
(Crawford, 1994; D. Foster, personal communication,1999).
Pitfield Volcanics
Metamorphosed volcanic and intrusive rocks, including ultramafic rocks of
tholeiitic affinity, occur as small fault-bounded slices along the Avoca Fault
Zone from south of Pitfield, north to Burkes Flat (Fig. 3.1).They are known
as the Pitfield Volcanics (Taylor et al., 1996) and their age is assumed to be
Cambrian on the basis of their similarity to Cambr ian tholeiitic rocks at
Heathcote. Rock types are mainly foliated to massive basalt and minor gabbro
to dolerite, with green- schist facies assemblages. In the Pitfield area two mine
dumps have yielded some serpentinised peridotite.The rocks do not outcrop,
and have only been described from drill core and mine dump float, which
includes some rounded cobbles from palaeodrainages (Ramsay et al., 1996;
Morand et al., 1995).
The least-deformed basalts have relic igneous textures typical of
submarine lava flows, varying from aphyric to mildly plagioclase- or
clinopyroxene-phyric. Chlorite-rich layers with amygdules are interpreted as
former glassy selvages that have been deformed and chloritised. Hyaloclastite
occurs in drillcore from Burkes Flat. A few samples of medium-grained
gabbro were probably high-level sills or dykes. Peridotites have cumulate
textures, with original euhedral cumulus olivine now fibrous serpentine, set
in a fine groundmass of serpentine and chlorite which may include unaltered
intercumulus clinopyroxene.The most deformed rocks are intensely foliated
greenschists with the metamorphic assemblage albite–chlorite–titanite–
muscovite–carbonate–quartz±actinolite±epidote. Sulphides, dominantly
pyrite, are also present in many samples.
Compositions of the Pitfield rocks (Table 3.2) are essentially the same as
the tholeiites of the Heathcote and Mount Wellington Belts (Crawford &
Keays, 1987; Ramsay et al.,1996). They have MgO 7.4–8.6%, moderate TiO2
(0.98–1.18) and immobile elemental ratios Ti/Zr, Zr/Y and Ti/V (av. 80,3.3
and 23 respectively), similar to average mid-ocean ridge tholeiites at 7%
MgO (Ti/Zr=110, Zr/Y=2.8 and Ti/V=22).The tholeiitic association of
basalts and subvolcanic dolerite–gabbro sills is typical of low-K tholeiites,
with geochemical signatures similar to depleted mid-ocean ridge basalts
erupted in backarc basin-type settings (Crawford,1988;Ramsay et al., 1996).
St Arnaud Group
Overlying the tholeiitic basalts of the Stawell Zone is a pile of largely
unfossiliferous, marine quartz–mica turbidites and occasional black shales
known as the St Arnaud Group. Detrital zircon and mica populations show
that the turbidites of the western Lachlan Fold Belt were derived from rocks
uplifted during the Delamerian Orogeny at about the end of the Cambrian
(Tur ner et al., 1993).The only places where the base of the St Arnaud Group
may be exposed are in the vicinity of Stawell, where quartz-rich turbidites
appear to lie conformably on the Magdala Volcanics.With the exception of
the complex rocks of the Moornambool Metamorphic Complex, the St
Arnaud Group forms the bedrock across the entire Stawell Zone.
The St Arnaud Group is subdivided into three formations: the Warrak,
Beaufort and Pyrenees formations on the basis of differences in sand to silt
ratio, bed thickness, composition, and facies characteristics (Cayley &
McDonald, 1995). Contacts between the War rak Formation and the
Beaufort and Pyrenees formations are faulted and the stratigraphic
relationship are not known.The Warrak Formation is the oldest unit of the
group, overlying Magdala Volcanics in Stawell Mine boreholes. It can be
distinguished from its correlative in the Delamerian Fold Belt to the west,
the Glenthompson Sandstone, because the latter has a significant
component of coarse detrital mica flakes and is generally more texturally
and compositionally immature. A Late Cambrian age is inferred for the
Wa r rak Formation from the regional geological setting. It has a minimum
thickness of 2–2.5 km.
The Beaufort Formation (Fig. 3.11) is at least 1–1.5 km thick and is
relatively rich in siltstone, with sandstone:mudstone ratios ranging from 0.5:1
to 1:1. Structural relationships suggest that the Pyrenees Formation
conformably overlies the Beaufort Formation. The monotonous lithology
and tight folding of the Pyrenees Formation make it impossible to determine
the thickness; about 2.5 km is a rough estimate.This formation is distinguished
by a much higher sandstone content and by much greater bed thickness than
other formations in the group.
Heathcote Greenstone Belt
The meridional-trending Heathcote Greenstone Belt provides one of the
best-exposed sequences of Cambrian rocks in Victoria. It has been divided
into three geologically distinct segments (Crawford, 1988):
1.A structurally simple southern segment south of the Cobaw Batholith
that contains tholeiitic basalts, minor dolerite sills and sediments
(VandenBerg, 1992);
2. A structurally complex central segment around Heathcote dominated
by andesitic and boninitic volcanics and hemipelag ic sediments and with
intercalated fault slices of Ordovician turbidites and black shale;
3. A northern segment dominated by tholeiitic dolerite, basalt and
hemipelagic sediment, underlain by minor boninite and volcanic sediment.
Revision of the Cambrian stratigraphy has been made possible by a
combination of geochemical work (Crawford, 1982, Crawford et al.,
1984), new detailed mapping (Gray & Willman, 1991a;VandenBerg, 1992;
Edwards et al., 1998; Spaggiari et al., 2002b) and acquisition of new
geophysical data. A new lithostratigraphic group called the Heathcote
Vo lcanics has been defined (Edwards et al., 1998). It incorporates three
newly defined formations, the Sheoak Gully Boninite, Lazy Bar Andesite
and the expanded Mount William Metabasalt. These formations are
overlain by the Knowsley East Shale (Thomas & Singleton, 1956) and
Goldie Chert (VandenBerg, 1992).The base of the sequence is always the
Mount William Fault. The top of the Cambrian sequence is faulted against
Ordovician sediments in the central segment, and is also possibly faulted
in the northern segment. In the southern segment, the Cambrian
sequence passes conformably up into the overlying Ordovician
Castlemaine Group of the Bendigo Zone.
The Sheoak Gully Boninite and the Lazy Bar Andesite outcrop in a
series of fault slices in the central segment of the Heathcote Greenstone
Belt.There are minor outcrops of Sheoak Gully Boninite in the northern
segment. Boninite is also interpreted to occur under cover along the
eastern margin of the norther n segment and to the north of the
outcropping belt, extending beyond the New South Wales border
(Fig. 3.12).The remainder of the outcrops of volcanics in the northern and
central segments and in the entire southern segment are composed of
Mount William Metabasalt. This is conformably overlain by Middle and
Upper Cambrian sediments, but relationships with the other volcanics are
Chapter 3 Neoproterozoic and Cambrian 83
Fig. 3.11: Thin-bedded, silty turbidites typical of the Beaufort Formation, Freemans Road
Creek, east of St Arnaud. The angular folds occur in the hanging wall of the west-dipping
St Arnaud Fault. Photograph by J. Krokowski de Vickerod.
Evolution of the Palaeozoic Basement84
Tholeiite–boninite association
Calc-alkaline association
Kerrie Syncline
Goat Rocks Syncline
Phillip Island
Coopers Creek
Yea Alexandra
Violet Town
Mt Major
Mt Strathbogie
Glen Creek
Trafalgar Morwell
Bonnie Doon
Mt Piper
Tanjil River
50 km
Cambrian greenstones
Zone boundary
Fig. 3.12: Regional geological setting of central Victoria showing distribution of Cambrian greenstones in the Heathcote and Mount Wellington Greenstone belts, with subcrop along the Murray
River inferred from aeromagnetic data (modified from VandenBerg et al., 2000). MWGB = Mt Wellington Greenstone Belt.
invariably faulted.The northern segment of the Heathcote Greenstone Belt
is a 2–4.5 km wide belt that extends northwards from Mount Camel under
Murray Basin cover to just beyond the New South Wales border (Fig. 3.12).
Rock exposure is restricted to a 2-km wide band along the western margin
of the belt between Mount Camel and Rochester, but its complete
distribution is evident in the aeromagnetic data (Fig. 3.5).The outcropping
sequence consists of north-trending, concordant packages of Cambrian
volcanics and pelagic sediments. Large-scale structures are interpreted from
aeromagnetics and include duplexing in the easternmost belt of rocks
(possibly volcanics) and an antiformal stack just north of Rochester. It is
likely that the upper (?)volcanic package is similarly deformed and
separated from the overlying Castlemaine Group by a fault.
Spaggiari et al. (2002b) recognised the central segment of the Heathcote
Greenstone Belt as a fault-bounded mélange zone. It includes an intensely
deformed belt of rocks some 7 km long that consists of fault slices of both
Ordovician and Cambrian rocks.The internal deformation of the fault slices
varies from moderately deformed in the Ordovician rocks with well-preserved
sedimentary structures and fossils, to variably foliated Cambrian volcanics with
patches of preserved igneous textures, and highly contorted Cambrian sediments
(Gray & Willman, 1991a). Small (to 2 m long) blocks of blue schist contain the
blue amphibole winchite and record metamorphic conditions of <450ºC and
between about 500 and 700 Mpa (Spaggiari et al., 1998, 2002b).
The southern segment is much less deformed; no décollement exists
here between the Cambrian rocks and the overlying turbidites, so that a
relatively undisturbed and conformable sequence is preserved. Greenstones,
including lower dolerites and overlying basalts and intercalated pelagic
sediments of the Mount William Metabasalt, are conformably overlain by the
Knowsley East Shale (formerly called Monegeetta Shale, 200–500 m thick).
These shales are, in turn, conformably overlain by the Goldie Chert,
consisting of 190–290 m of chert and siliceous siltstone (VandenBerg, 1992).
Sheoak Gully Boninite
The Sheoak Gully Boninite consists of boninite lava and minor boninitic
volcaniclastics and rhyolitic lava.The largest outcrop area occupies two of
the largest fault slices in the duplex system of the central segment of the
Heathcote Greenstone Belt.All the obser ved contacts of the Sheoak Gully
Boninite are faults. Boninitic lava dominates the formation, mostly as
massive coherent lava flows and minor pillow lavas. These lavas are a
distinctive fine-grained blue–green rock, and igneous textures are best
preserved at Sheoak Gully and Cornella East; elsewhere, the boninites are
extensively altered and schistose. North of Ladys Pass the lavas have been
contact-metamorphosed by the Crosbie Granite.
The boninites are high-Mg lavas containing up to 30% phenocrysts of
low-Ca pyroxene which are mainly pseudomorphed by pale green chlorite
(Crawford, 1982, 1984). The matrix of recrystallised glass contains euhedral,
low- to high-Ca pyroxene as skeletal grains, dendritic aggregates and
spherulites (Fig. 3.14a,b). Minor euhedral chromite grains occur in the
matrix and as inclusions in the pyroxene phenocrysts. Ubiquitous quench
textures indicate that the boninites probably formed thin lava flows.
A small outcrop of rhyolite lava occurs in Sheoak Gully (Edwards et al.,
1998). Patchy flow banding is defined by stretched vesicles up to 5 cm long
which are filled by chalcedony and calcite.The formerly glassy lava displays well-
developed devitrification textures, and spherulites are visible in hand specimen.
The rhyolite contains embayed quartz and minor sericitised plagioclase
phenocrysts in a commonly perlitically fractured and spherulitic groundmass
consisting of devitrified glass, quartz and feldspar with minor opaque minerals.
Calcite and sericite are common alteration minerals.The rhyolite lava has been
partially quench-fragmented to form hyaloclastite and rhyolite breccia.The
hyaloclastite contains fragments (to 5 cm) of rhyolite lava with jigsaw-fit
textures, in a groundmass of devitrified glass.The rhyolite breccia contains large
(to 10 cm), poorly sorted, angular to subrounded fragments of rhyolite in a
matrix of chlorite-altered perlite. Margins of rhyolite fragments are marked by a
thin zone of intense devitrification.The rhyolite in the Sheoak Gully Boninite is
assumed to be a differentiate of the boninitic magma, but further geochemical
work on the rhyolite is required to determine its petrogenesis.
Sediments interbedded within the Heathcote Volcanics near Heathcote
have yielded a single Early Cambrian dolichometopiid trilobite (P.A. Jell,
personal communication in VandenBerg, 1992).The exact location of this
trilobite discovery is unknown.
Lazy Bar Andesite
The Lazy Bar Andesite is a generally poorly exposed and weathered sequence
of andesitic lava, andesitic volcaniclastic sandstone, and minor vitric ash-rich
fine-grained volcaniclastics (Nicholls, 1965; Crawford, 1982; Crawford et al.,
1984; Crawford & Cameron, 1985).The formation is restricted to the central
segment of the Heathcote Greenstone Belt in several fault slices between the
Cobaw Batholith and the Crosbie Granite.The andesite structurally overlies
the Sheoak Gully Boninite with a concordant contact that is probably a fault.
The Lazy Bar Andesite lavas are generally fine-grained and non-vesicular.
Where fresh, they contain euhedral phenocrysts of plagioclase, chloritised
orthopyroxene and augite in a glassy groundmass, with grains of plagioclase,
pyroxene and Fe–Ti oxide.Plagioclase is albitised and commonly over printed
by chlorite and epidote.The chartacteristic lower greenschist metamorphic
assemblage is actinolite–chlorite–albite–epidote–quartz– leucoxene. Near the
Heathcote Fault the andesite is altered to talc-actinolite schist.
The andesitic lavas were erupted into a marine environment. Sedimentary
features indicate that the andesitic volcaniclastic sandstones were deposited by
low-concentration turbidity currents, probably generated on the steep slopes
of the growing andesitic volcanic pile. Ash-r ich beds, including possible
pumice, are interbedded with the lavas and indicate that the andesitic volcanic
pile grew to a shallow enough depth for pyroclastic fragmentation to occur.
Geochemical and isotopic data suggest that these boninitic andesites
were not derived by crystal fractionation from the underlying more mafic
boninites, but rather, appear to have been derived from the same shallow
refractory mantle source as the boninites by lower degrees of partial melting
(Nelson et al., 1984; Crawford & Cameron, 1985). There is little doubt that
the Lazy Bar Andesite and Sheoak Gully Boninite are closely related
temporally, and were produced in the same tectono-magmatic setting.
Mount William Metabasalt
The Mount William Metabasalt encompasses the tholeiitic volcanic
sequence and associated sediments that outcrop along virtually the entire
length of the northern and southern segments of the Heathcote
Greenstone Belt.The formation is composed predominantly of thick sills of
dolerite and basalt flows, some pillowed (Fig. 3.13), with minor bands of
siliceous sediment (chert and jasper). It is overlain by the Knowsley East
Shale in both the northern and southern segments. In the southern
segment this contact is considered to be sharp, concordant and possibly
conformable (VandenBerg, 1992).The Mount William Metabasalt is at least
2.5 km thick in the northern segment and over 1.6 km in the southern
segment. The lower contact of the formation with the Sheoak Gully
Boninite is faulted by the Corop Fault along the northern segment of the
greenstone belt. South of Mount Camel, the lower contact with the Sheoak
Gully Boninite is also faulted. However, as evolved tholeiitic dolerite dykes
are known to cut the boninitic lavas east of Toolleen, the tholeiites clearly
post-date the boninites and presumably overlie them.
The dolerite is a medium- to coarse-grained, green–black rock
consisting of sparse phenocrysts of albitised plagioclase, euhedral to
subhedral crystals of augite and minor opaques. Ophitic textures are
common, and low greenschist metamorphic assemblages (chlorite–
actinolite–albite–sericite-epidote) are widespread. Basalts are dominantly
massive, often aphyric lava with minor pillow lavas. Sparse euhedral
phenocrysts of albitised plagioclase and augite may be present (Crawford
& Keays, 1987) (Fig. 3.14c). Metamorphic assemblages vary from
prehnite–pumpellyite facies, with augite and anorthitic plagioclase
preserved in places, to greenschist facies assemblages with actinolite after
augite, albitised plagioclase, common epidote and chlorite, and leucoxene
after the former Fe–Ti oxides.
Chapter 3 Neoproterozoic and Cambrian 85
These basalts and dolerites are low-K tholeiites with flat REE patterns,
and trace element signatures like MORB-type basalts generated in
extensional zones (backarc basins and forearc extensional zones) above
modern West Pacific-type subduction zones (Crawford & Keays, 1987),
Knowsley East Shale
Overlying the Mount William Metabasalt is the Knowsley East Shale, which
includes the Middle Cambrian black shale and volcaniclastic units in all
segments of the Heathcote Greenstone Belt. The formation includes the
entire Knowsley East beds and a large part of the Goldie Beds of Thomas
(1956), as well as the Goldie Shale sediments in Trilobite Gully described by
Wilkinson (1977). It does not include the Goldie Chert at Lancefield in the
southern segment. Other components of the Knowsley East Shale include
minor interbedded chert, mafic lithic sandstone, polymictic conglomerate,
monomictic chert breccia, and ash.
The formation is exposed along the western margin of the norther n
segment and as fault slices within the central segment, between Tooborac
and Heathcote. It also commonly occurs as fault-bounded blocks which
are juxtaposed against various other units of the Heathcote Volcanics and
the Castlemaine Group. In the southern segment at Lancefield, the
formation is conformably overlain by the Goldie Chert (VandenBerg &
Stewart, 1992), but in the northern and central segments the boundaries
are not so clear, and are probably faulted. A felsic airfall tuff in the
Knowsley East Shale at Lancefield has a population of small equant
zircons with a Middle Cambrian age (503±8 Ma).This age matches well
zircon ages for the Mount Stavely Volcanic Complex (Stuart-Smith &
Black, 1994; Crawford et al., 1996a,b) and the compositionally similar
Mount Read Volcanics in wester n Tasmania, both of which have been
interpreted as post-collisional magmatic suites (Crawford et al., 1992,
1996a,b). Although no similar post-collisional lavas are present in the
Heathcote Greenstone Belt, the existence of zircons of this age suggests
close proximity of the Heathcote Belt to the post-collisional magmatism
now exposed in the Stavely Greenstone Belt and in erosion windows
through the Melbourne Zone rocks further east at Jamieson and Licola.
At Heathcote, the basal portion of the Knowsley East Shale contains a
spectacular upward-thinning package of graded sandstone beds deposited
as turbidites or grain flows and composed of pyroxene crystals with minor
feldspar.Thick beds of polymictic conglomerate at higher levels contain
clasts of black shale, chert, jasper and mafic lava in a sandstone matrix.
The shales and mudstones of the formation accumulated as hemipelagic
mud but the coarser clastic rocks were deposited by various types of gravity
flows.The coarsest rocks all occur at the base of the formation and consist
mainly of mass flows derived from a var iety of igneous and sedimentary
rocks. Such material ceased to be a significant component for the remainder
of the formation, probably because the source region of the volcanic edifice
became buried by pelagic sediments.Translational slide deposits, mainly chert
breccias, show that the pelagic sediments were deposited on a slope which
was sufficiently steep to be unstable.
Goldie Chert
The Goldie Chert is a wholly pelagic sediment, deposited below wave-base
in the marine environment. In the southern segment the Goldie Chert
contains abundant phyllocarid crustacea and a single conodont, either
Cordylodus angulatus or C. rotundatus,indicative of Datsonian age (I.Stewart, in
Va ndenBerg, 1992), very close to the Cambrian–Ordovician boundary. The
depositional process was by grain-by-grain or aggregate settling of sediment
through the water column.The presence of Goldie Chert in the central
segment is uncertain (VandenBerg, 1992). Intensely deformed siliceous shale
and chert at Ladys Pass in the central segment is tentatively assigned to the
Goldie Chert, but distinguishing these sediments from the silicified shale of
the Knowsley East Shale is difficult. At Ladys Pass, these rocks form a
lozenge-shaped fault slice surrounded by Lazy Bar Andesite. The chert is
absent from the northern segment.
Regional synthesis
The boninite–tholeiite association in the Heathcote Greenstone Belt is
best matched on the modern Earth by extensional forearc regions of intra-
oceanic arcs (e.g. Bonin–Mariana, North Tonga). Taking into account the
600-Ma, east-facing passive margin represented by the Glenelg and
Grampians–Stavely zones, the Delamerian Orogeny may have involved
collision of this Cambrian forearc with the leading edge of the passive
margin.Thus, during the Cambrian, the Stawell and Bendigo zones appear
to have been a deep marine ocean basin just outboard of the Delamerian
Orogen as it was accreted to the Australian margin. In the west, the
terrigenous turbidite sequences of the St Arnaud Group are interpreted,
on the basis of provenance, isotope geochemistry and inherited zircon
population, to be the detritus shed from the newly accreted and uplifted
Delamerian Orogeny. The Cambrian chert and shale sequences at
Heathcote, further to the east, appear to be time-equivalent but more
distal deposits, accumulating here until the start of the Ordovician.By this
time the turbidite fan had prograded east to this point, as recorded by the
commencement of Castlemaine Group deposition.
Evolution of the Palaeozoic Basement86
Fig. 3.13: Pillows in Mount William Metabasalt, exposed in the Lake Cooper quarry, north
end of Heathcote Volcanic Belt. Photograph by R. Cayley.
3.3.2 Melbourne Zone
The Melbourne Zone consists of deformed Ordovician to Devonian
sedimentary rocks. Along the eastern margin of the zone, erosion has
carved windows through the highly sheared base of this metasedimentary
sequence to expose a basement of Cambrian volcanic rocks (Gray, 1995;
Va ndenBerg et al., 1995).
Jamieson–Licola Volcanics
This belt consists of four major exposures or ‘windows’ with irregular
outlines within the Mt Useful Fault Zone. New airborne geophysical data
and mapping by the Victorian Geological Survey (VandenBerg et al., 1995),
along with contributions from university researchers (Hendrickx, 1993;
Cherry, 1999), have significantly revised the shape of the greenstone
exposures and clarified their internal stratigraphy. The base of these volcanic
rocks, which have been referred to informally as the ‘Barkly River
Greenstone Belt’ (Turner, 1996; Cherry, 1995),is not exposed.Until recently
these exposures were interpreted as fault slivers incorporated into the basal
parts of the Mount Wellington Fault Zone (Fergusson et al., 1986; Gray,
1995). A more recent appraisal re-interprets them as erosional windows
through the highly sheared base of the Mount Useful Fault Zone into the
underlying Selwyn Block (VandenBerg et al., 1995, 2000) (Fig. 3.15).The
Jamieson Window is defined by aeromagnetic data and the response appears
to continue southwards, linking with the magnetic highs of the Whisky Knob
Window, and implying a subsurface link between the two windows. South of
the Whisky Knob Window are the Fullarton Spur and Licola windows
(Fig. 3.12).These two southern windows have much more subdued magnetic
responses compared with the two northern windows, implying different rock
packages. Much further to the north of all these occurrences is a poorly
known exposure at Glen Creek (Fig.3.12).
Jamieson and Whisky Knob windows
The Jamieson Window has been mapped in detail by Hendrickx (1993) and
summarised by VandenBerg et al., (1995), who subdivided the stratigraphy
within the window into a number of formations.These include the Br issces
Hut Andesite, the Warrambat Andesite Breccia, Wrens Flat Andesite,
Lakelands Flat Andesite Breccia, Hardwicke Creek Rhyolite, and the
Handford Creek Formation. Subsequent mapping by Cherry (1999),
supplemented by geochemical data, suggested that the andesite and andesite
breccia formations erected by VandenBerg et al., (1995) might better be
considered as a single stratigraphic unit, and that bedding mostly trends
almost east–west, rather than NW-trending as reported earlier. Dips are
mainly to the south. Cherry (1999) estimated that coherent lavas form less
than 50% of the volcanic–volcaniclastic package. Coarse sandstone and mass
flow breccia are abundant, with siltstone common. Lavas are mainly
plagioclase+augite-phyric andesite with good textural preservation;
monomictic andesite lava breccias are common. Metamorphic assemblages
are low greenschist facies.At several localities, occasional clasts in polymictic
breccia show a tectonic foliation. High-level intrusive andesitic rocks
probably represent thin sills and dykes but are difficult to distinguish from the
andesitic lavas. Geochemical data indicate that the intrusive rocks and
andesitic lavas are comagmatic. It is very likely that the thick andesitic pile
east of the Hardwicke Creek Rhyolite is fault-repeated, possibly several times.
A felsic sequence constituting the Hardwicke Creek Rhyolite overlies
the andesitic sequence and occupies much of the southern third of the
Jamieson Window. Cherry (1999) reported rhyolitic and dacitic lava and
lava breccia, many of which carry quartz phenocrysts, and common fine-
to medium-grained thinly bedded sandstone. Rare andesites with
hornblende and plagioclase are probably lavas. The Hardwicke Creek
Rhyolite has an interpreted true thickness of about 1100 m, but its base is
marked by a major shear zone several tens of metres wide. Its upper
contact appears to be faulted against the overlying Handford Creek
Formation (Hendrickx, 1993),although Cherry (1999) suggested that this
faulting may only be local and that the contact may be conformable over
much of its length.
Chapter 3 Neoproterozoic and Cambrian 87
Fig. 3.14: Photomicrographs of typical textures in the Heathcote Volcanics (a,b from Sheoak
Gully Boninite). (a) Low-Ca boninite 26278 (north Sheoak Gully) showing chlorite pseudomorphs
after clinoenstatite phenocrysts in a groundmass charged with actinolite-altered pyroxene
microlites and altered glass. Note the euhedral chromite inclusions in the altered clinoenstatite
phenocrysts (bottom left). (width of field ~4mm). (b) Low-Ca boninite 26279 (Sheoak Gully)
showing typical chlorite-altered quenched orthopyroxene microphenocrysts with cruciform
growths extending into formerly glassy hollow centres of crystals, and a groundmass charged with
pyroxene microlites set in glass that has altered to chlorite and quartz. (width of field ~4mm).
(c) Typical interior of thick tholeiitic basalt flow, showing fresh clinopyroxene (augite) plates, partly
fresh and partly sericite-albite-altered plagioclase laths often partially included in the augite,
common fine-grained Fe-Ti oxides and interstitial chlorite. From old rail cutting at Kilmore Pass, Mt
William Metabasalt. All images are about 5 mm across and are taken with crossed polars.
The Handford Creek Formation forms the top few hundred metres of
the Cambrian stratigraphy in the Jamieson Window. It contains mainly
volcaniclastic sedimentary rocks, with no lavas or lava breccias. Massive
poorly sorted sandstones carry common volcanic quar tz detritus and volcanic
lithic clasts. Granule- to cobble-sized conglomerate beds 1–2 m thick occur
within the finer grained sequence. Clasts include cherty rocks,felsic lavas and
possible pumice fragments.
Geochemical data (Crawford, 1982, 1988; Cherry, 1999) show that the
Jamieson rocks are medium-K calc-alkaline andesites (Table 3.3). The
limited range of compositions among the analysed samples collected from
a wide area of the northern part of the Jamieson Window suggests that a
single lithostratigraphic unit is represented among the extensive andesitic
lavas and lava breccias. This supports the suggestion by Cherry (1999) that
the four formations erected by VandenBerg et al., (1995) are better
considered as a single lithostratigraphic unit, albeit probably repeated by
thrust faulting. Further geochemical studies of the felsic volcanics are
required to evaluate their significance.
The stratigraphy of the Whisky Knob Window is less well known, but
rhyolites are common and are petrographically close to those in the southern
section of the Jamieson Window; these remain to be studied geochemically.
Fullarton Spur and Licola windows
The Fullarton Spur and Licola windows are less well known, due largely
to difficult access.The excellent exposure on the Jamieson-Licola Road in
the Licola Window is of columnar high-K hornblende andesite (the
Tobacco Creek Andesite). This andesite is strikingly similar
petrographically and compositionally to the 500-Ma Anthony Road
Andesites of the Mount Read Volcanics in western Tasmania (Crawford
et al., 1992, 1996a,b).
Evolution of the Palaeozoic Basement88
146 15' 146 20' 146 25'
37 20'
37 25'
37 30'
Thomas Fault
Fullarton Fault
Gp 2 structures
Gp 3 structures
Kanimblan structures
Upper Devonian
SilurianLower ? Cambrian L.Ord. U.Ord.
Howqua Chert (sporadic on Fullarton
Fault but too thin to show)
Older Volcanics
Mansfield Group
Delatite Group
Jordan River Group
Snowy Plains Formation
Mount Kent Conglomerate
Wellington Volcanic Group
Kevington Creek Formation
Moroka Glen Formation
Walhalla Group
Snake-Edwards Divide Member
Murderers Hill Sandstone
Serpentine Creek Sandstone
Donnellys Creek Siltstone
Lazarini Siltstone
Mount Easton Shale
Handford Creek Formation
Hardwicke Creek Rhyolite
Lakelands Flat Breccia
Whisky Knob Rhyolite
Cobbs Spur Andesite Breccia
Tobacco Creek Andesite
Licola Volcanics
Jamieson Volcanics
Wrens Flat Andesite
Warrambat Andesite Breccia
Brissces Hut Andesite
5 km
Fig. 3.15: Distribution of fault slices of Licola and Jamieson volcanics in the Mt Useful Fault Zone, eastern part of the Melbourne Zone (modified from VandenBerg et al., 2000).
Glen Creek Window
At Glen Creek south of Mount Strathbogie (Fig. 3.12), andesitic greenstones
of presumed Cambrian age (Sandl, 1989) may be correlatives of the Jamieson
and Licola volcanics. No petrological or geochemical studies of these rocks
have been undertaken.A lower unit consists of porphyritic andesite lavas, in
part vesicular and with good flow textures.An overlying polymict breccia
consists of shale and siltstone fragments, ultramafic material, basaltic andesite
and possibly felsic volcanic clasts, impure quartzite and possible clasts of
granitic origin (Sandl, 1989). Interbedded with the breccia are lithic quartz
sandstones and carbonaceous mudstone and siltstone.The margins of the
Glen Creek Window are presumed to be faulted.
Waratah Bay –— Maitland Beach Volcanics
At Waratah Bay (Fig.3.1), Cambrian igneous rocks outcrop in a broad,NNE-
striking horst block exposed between the Waratah and Bell Point faults.
Although often referred to in the literature, they were not formally named
until very recently (VandenBerg et al., 2000; Cayley et al., 2002). Rock units
include metabasalt and interbedded pelagic sediments, metagabbro, and
strongly altered olivine-rich ultramafic rocks. Most of the belt between the
Waratah and Bell Point Fault zones consists of outcropping Maitland Beach
Vo lcanics, with outcrops of the Corduroy Creek Gabbro and ultramafics
confined to a relatively small area near Digger Island.
The Maitland Beach Volcanics were mapped by Lindner (1953) and
Sandiford (1978). Crawford (1982) reported geochemical data (Table 3.3)
for the tholeiitic basalts that form a major part of the exposed sequence.
These basalts, which include rare pillowed flows and thin dykes, are either
aphyric or sparsely augite+plagioclase-phyric, and show prehnite–
pumpellyite or lowest greenschist facies metamorphic assemblages. On the
basis of pronounced petrographic and geochemical similarities with
Heathcote and Howqua tholeiitic basalts, the Maitland Beach Volcanics are
interpreted to be Cambrian in age.
A major fault along the eastern margin of the Maitland Beach Volcanics is
marked by zones of intense shearing and hydrothermal alteration, slices of
dark recrystallised limestone, the coarse-grained Corduroy Creek Gabbro, and
serpentinised and silicified peridotite. Previous authors have regarded the
gabbro’s age as either Devonian (e.g. Lindner, 1953) or Cambrian (Crawford,
1988). Its pre-Ordovician age is demonstrated by the unconformably
overlying Cambrian Bear Gully Chert and Lancefieldian Digger Island
Formation (Cayley et al., 2002). Metamorphic hor nblende in the Corduroy
Creek Gabbro indicates significantly higher grade metamorphism than in the
adjacent Maitland Beach Volcanics. Petrographic observations of the altered
ultramafic rocks indicate that they were originally dunite. Iridium–osmium
nuggets probably sourced from the serpentinised ultramafics have been found
in Cainozoic placers up to 40 km away.The occur rence of these alloys invites
comparison with the Cambrian mafic–ultramafic complexes of Tasmania, in
which dunite is commonly associated with placer deposits of ‘osmiridium’
(Brown & Jenner, 1989). Geochemical data for the Maitland Beach Volcanics,
together with their close association with gabbro and peridotite, are also
reminiscent of the Cambrian ophiolite sequences in Tasmania (and Victor ia),
rather than the supracrustal Late Neoproterozoic rift tholeiite sequences in
Tasmania, in which gabbro and peridotite are unknown.
The deformed Maitland Beach Volcanics and the Corduroy Creek
Gabbro are both overlain by the Bear Gully Chert, a thin siliciclastic unit
(Cayley et al., 2002), and by the Early Ordovician Digger Island For mation, a
shallow marine limestone unit (see Chapter 4). This contrasts markedly with
other occurrences of presumed Cambrian greenstone in Victoria, for example
along the northern and southern Heathcote Greenstone Belt, and at Howqua,
where Late Cambrian – Early Ordovician deep-water pelagic sediments
conformably follow the basaltic greenstones.The Bear Gully Chert is exposed
immediately above the unconformity approximately 350 m north of Digger
Island. It consists of fine-grained quartz and small lithic clasts, which form a
matrix supporting larger angular to rounded deformed lithic clasts and
occasional large rounded quartz pebbles. No clasts of the underlying meta-
igneous rocks have been recorded.The unit is pyritic, with small pyrite crystals
forming up to 20% of the rock.Although thin (<20 cm),this unit is cr ucial in
fingerprinting the provenance of the deformation of the underlying meta-
igneous rocks, as it indicates that uplifted, deformed continental siliciclastics
were being shed onto the unconformity prior to the Lancefieldian, the age of
the conformably overlying Digger Island Formation.The Bear Gully Chert
can therefore be no younger than Lancefieldian, and a Late Cambrian age is
most likely. This unit is a direct correlate in terms of age and lithology with
the Owen Group of western Tasmania, which unconformably overlies the
Tyennan unconformity there.The pre-Ordovician age of the unconformity at
Waratah Bay therefore presents key evidence for the presence of the Tyennan
or Delamerian Orogeny in central Victoria.
Chapter 3 Neoproterozoic and Cambrian 89
26295 26296 26298 26299 26304 26309 26362 26363 26364 E12498 E12299 E12292
SiO257.70 55.60 60.30 66.20 60.60 62.20 50.20 49.30 50.60 49.20 50.50 48.20
TiO20.48 0.46 0.41 0.42 0.48 0.46 2.18 1.52 1.73 0.86 0.98 0.28
Al2O314.20 14.90 12.80 12.60 15.00 14.70 13.70 14.10 13.10 14.70 14.90 16.00
FeO* 8.96 9.26 7.70 6.03 6.28 6.40 17.30 13.90 14.20 10.10 9.85 9.05
MnO 0.14 0.13 0.10 0.09 0.12 0.10 0.27 0.16 0.22 0.22 0.22 0.20
MgO 5.92 6.82 5.34 2.99 4.66 3.40 5.94 8.21 6.89 10.10 9.65 14.20
CaO 8.30 8.59 8.33 7.35 7.70 5.85 7.41 9.52 9.53 10.50 9.00 8.36
Na2O2.532.83 4.07 1.52 2.78 4.03 2.41 2.89 3.37 3.23 4.40 1.92
K2O1.641.19 0.82 2.65 2.18 2.60 0.47 0.29 0.27 0.25 0.23 1.47
P2O50.17 0.16 0.14 0.19 0.26 0.25 0.19 0.12 0.14 0.10 0.08 0.02
Loss 2.31 3.12 1.27 1.47 2.80 3.68 4.02 2.54 1.64 1.12 0.82 1.25
Trace elements in ppm
Ni 25 16 27 21 53 42 27 64 61 206 120 344
Cr 133 159 173 119 167 150 47 117 74 546 377 2143
V233 237 194 218 164 161 509 340 375 256 265 179
Sc 26 27 25 22 15 17 42 41 41 35 47 34
Zr 108 80 72 102 216 161 94 65 99 56 48 15
Y20 181722282541 2437201917
Sr 831 443 172 830 2096 672 132 127 172
Rb 47 34 26 20 68 87 16 7 10
Ba 476 484 636 112 1431 1533 180 58 65
Table 3.3: Whole rock analyses for Cambrian greenstones from the Mt Wellington Greenstone Belt. 1–4: Andesites from the Jamieson window (A. J. Crawford, unpublished). 5, 6: Andesites from
the Licola Window (A. J. Crawford, unpublished). 7–9: Tholeiitic metabasalts from Cape Liptrap (Maitland Beach Volcanics) (A. J. Crawford, unpublished). 10–12: metabasalts and metadolerite (12)
from Kitty Miller Bay, Phillip Island (Henry & Birch, 1992).
Phillip Island and Barrabool Hills
Greenstone exposed on the southern coast of Phillip Island (Henry & Birch,
1992; Bushby, 2001) (Fig. 3.16) appears to lie on a northward extrapolation
of a major magnetic high which trends across Bass Strait to exposures of
Neoproterozoic rift tholeiites and picrites on the southeast coast of King
Island.Although tentatively correlated with the Neoproterozoic basaltic rocks
of Tasmania and King Island by Cayley et al. (2002), the Phillip Island rocks
include greenschist facies formerly glassy boninitic lavas, dolerite and
cumulate ultramafic rocks with characteristic high-Cr chromites. Also, their
geochemical signature matches better with the low-Ti dolerites and boninitic
lavas and ultramafics of the Cambrian sequences exposed at Howqua and in
western Tasmania, and is atypical of the Late Neoproterozoic r ift
tholeiite–picrite sequences of western Tasmania.
Ceres Metagabbro
A small, little-known outcrop of metagabbro, the Ceres Gabbro, occurs in the
Barrabool Hills near Geelong (Fig. 3.1).Small outcrops of the same metagabbro
occur a few kilometres to the north at Dog Rocks.The metagabbro has no
contacts with Palaeozoic rocks other than a few tiny granite intrusions of
presumed Late Devonian age.The Ceres outcrops appear to form a thr ust slice
of gabbro that has been metamorphosed to amphibolite facies. The rock is
mainly massive, coarse- to fine-grained, with sporadic subtle layering defined
mainly by changes in grainsize (Morand, 1995;Cayley et al., 2002).This layering
is vertical and strikes northwest. Mg-rich diopside and some bytownite are the
only igneous minerals preserved. Opaque minerals are rare, with Fe–Ti oxides
occurring only in the most Fe-rich samples.The rock has equigranular gabbroic
or rare subophitic textures; cumulate textures have not been obser ved. Analyses
reveal that the rock is a tholeiitic gabbro (Table 3.2), with 47–52% SiO2contents
on an anhydrous basis. Notable features are the moderately high MgO, low
Na2O and the very low TiO2,K
2O and P2O5contents.
The metamorphic assemblage is anorthite–calcic amphibole, with minor
chlorite; most plagioclase is recrystallised into a fine-grained granoblastic
aggregate of anorthite and most clinopyroxene is partly or completely
replaced by Mg-rich amphibole. Many rocks are massive, but foliated to
mylonitic samples with the same amphibolite facies metamorphic assemblage
are common.This amphibolite facies metamorphism was accompanied by
N–S compression, as indicated by a conjugate set of internal shear zones
(Morand, 1995).This trend is at odds with the regional structures developed
in the Bendigo Zone to the north (Morand, 1995).The metamorphism and
deformation affecting the Ceres rocks appear to be regional, with the small
post-tectonic granite plutons intruding the metagabbro appearing to have
had little metamorphic effect.
Correlation of the Ceres Metagabbro with metabasic rocks of
Neoproterozoic or Cambrian age in Victoria and Tasmania is not
straightforward. Is it better correlated with the Late Neoproterozoic greenstone
exposed on King Island and northwestern Tasmania, or with the tholeiitic
gabbros such as those at Howqua? Geochemical data for the Ceres Metagabbro
match well with the Howqua gabbros. In contrast,gabbroic rocks are limited to
a few microgabbro sills and dykes in the Neoproterozoic packages of wester n
Tasmania. Although it remains unproven, the Ceres Metagabbro is suggested to
be a deep-derived, amphibolite-grade thrust-slice of Cambrian gabbros, such as
those in the Heathcote and Mount Wellington Greenstone Belts.
Regional Synthesis
The calc-alkaline exposures along the eastern margin of the Melbourne Zone
(Jamieson and Licola volcanics) show remarkable geochemical and petrographic
similarity to the 500-Ma Mount Read Volcanics of western Tasmania.The
interpretation that these exposures are erosion windows through the Melbourne
Zone into an older basement — the Selwyn Block — which is the northern
extension of Tasmanian crust, reinforces this suggestion (VandenBerg et al.,
2000). In this scenar io, the Jamieson and Licola volcanics might be broadly
interpreted as along-strike continuations of the Mount Read Volcanics.
3.4 Eastern Victoria (Tabberabbera Zone)
3.4.1 Mount Wellington Greenstone Belt
The Mount Wellington Greenstone Belt occurs as a series of discontinuous
fault slices in the hanging wall of the Governor Fault along the western
margin of the Tabberabbera Zone (Fig.3.12). Outcrop areas include Dookie,
Tatong, Howqua and Dolodrook River, with aeromagnetic data suggesting
more greenstone in the north under Murray Basin cover. The rocks are
mainly the same boninite–tholeiite association as exposed along the
Heathcote Greenstone Belt, and include the Dookie Igneous Complex at
Dookie, the Lickhole Volcanic Group on the Howqua River and the Thiele
Igneous Complex on the Wellington River (VandenBerg et al., 2000).
Overlying the Lickhole Volcanic Group in the Howqua section is the
Howqua Chert, approximately 500 m of mostly chert and siliceous shale
with minor lithic sandstone, pebbly sandstone and chert conglomerate
(Crawford, 1988). Only the uppermost portion contains useful fossils, which
are basal Ordovician (Lancefieldian, La2) graptolites and conodonts at
Howqua River. The only other recorded fossils are small inarticulate
brachiopods. Volcaniclastics associated with the chert also occur in the
hanging wall of the Wonnangatta Fault near Crooked River.The same rock
unit overlies the Dookie Igneous Complex, where the chert consists of
quartz and albite (Christie, 1978).The chert is highly pyritic when fresh.
Minor components are graded volcaniclastic sandstone and conglomerate,
and mudstone. Some conglomerates contain gabbroic detritus (Tickell,1989).
Howqua Section
The best-studied volcanic sequence in this greenstone belt is the Lickhole
Vo lcanic Group on the Howqua River (Crawford, 1982; Spaggiari et al.,
2002b).The basal part of the greenstone sequence is separated by a major fault
zone from a 3-km-wide polydeformed mélange zone that includes blueschist
blocks up to 5 m long, in which relict glaucophane and winchite suggest
metamorphic conditions of <450ºC at 700–900 MPa (Spaggiari et al., 2002b).
The basal Mountain Chief Andesite is a thin (100–250 m) formation of
andesitic volcaniclastics and mafic boninitic lava and hyaloclastite.Overlying this
is the Sheepyard Flat Boninite, 1000–1500 m of ultramafic boninitic lava and
volcanic breccia with remarkable textural variations and rare interbeds of finer
volcaniclastics. Two thin flows of tholeiitic basalt occur close to the top of the
pile.Above this is the Malcolm Creek Hyaloclastite, about 750 m of 5–10-m
Evolution of the Palaeozoic Basement90
Fig. 3.16: Greenstone outcrops near Kitty Miller Bay, Phillip Island. Cambrian metavolcanics
form the shore platform in the foreground and the small headland in the middle distance.
Photograph by W. Birch.
thick beds of tholeiitic hyaloclastite with occasional beds of pebbly grit and
volcaniclastic sandstone. The hyaloclasts contain phenocr ysts of fresh augite,
sparse chloritised olivine and albitised plagioclase, and the volcaniclastics contain
clasts of boninite, serpentinite and porphyritic andesite (Crawford, 1982).The
uppermost volcanic unit is the thick Eagle Peaks Basalt, up to 1.5–2 km of
pillowed and massive aphyric tholeiitic basalt with minor amounts of interflow
and interpillow cherty sediment (Crawford & Keays,1987).
Intercalated with the boninitic volcanics are an olivine pyroxenite sill,
comagmatic with the Sheepyard Flat Boninite (Crawford, 1980),and sills and
dykes of dolerite and gabbro comagmatic with the Eagles Peak Basalt
(Crawford & Keays, 1987) (Fig. 3.17a,b).The largest sill is close to 500 m
thick and shows significant compositional layer ing.The tholeiitic basalt and
underlying boninite in the Mount Wellington Greenstone Belt are closely
comparable in their geochemistry and petrography to those in the Heathcote
Greenstone Belt (Crawford & Cameron, 1985;Crawford & Keays,1987).
Dookie and Tatong sections
The sequence and rock relationships of the Cambrian rocks at Dookie are
still not well understood.The exposed sequence is at least 1000 m thick, but
bedding-parallel thrusts have probably been overlooked in previous studies of
the area.There are three main rock types: metabasalt, gabbro and sediments.
The basalts are all tholeiitic flows that appear to lie between two successions
of similar sedimentary rocks.These sediments consist mainly of chert, which
includes beds consisting of quartz and albite that were probably originally
volcanic ash (Christie, 1978), and also rare detrital quartz grains and sponge
spicules. Also present are black shale and siltstone, and sandstone and
conglomerate with grains and clasts of basalt and gabbro. A thick sill of
gabbro occurs in the ‘lower’,southern belt of sediments.
The outcrops at Tatong comprise a sequence of tholeiitic gabbro,
dolerite and basalt,overlain by Howqua Chert and Pinnak Sandstone
(McGoldrick, 1976; Crawford, 1988).
Dolodrook River Inlier
The Dolodrook River Inlier (Thiele Igneous Complex; Fig. 3.12) is an
anticlinal structure cored by serpentinised ultramafic rocks (Fig. 3.18), and
surrounded by incomplete rings of successively younger rocks ranging in
age from early Middle Cambrian to Silurian (Teale, 1920; Duddy, 1974;
Spaggiari, 2003). They form a curved elongate belt about 5 km long,
trending northwest, parallel to the structural trend of the surrounding
Silurian rocks. Ultramafic rocks have been shown (Crawford, 1982) to
include cumulates from both boninitic and low-K tholeiitic magmas, and
are almost certainly directly related to the boninite-tholeiite lava
association exposed elsewhere in the Mount Wellington Greenstone Belt
(e.g. at Howqua).A series of thinly bedded green sandstone and shale, with
minor conglomerate, the Garvey Gully Formation, is at least 200 m thick
and unconformably overlies the serpentinised ultramafics. Channel
structures and rounding of coarse clasts suggest that the Garvey Gully
Formation was deposited rapidly under shallow, moderate- to high-energy
conditions. Intercalated in the upper part of the formation is the
Dolodrook Limestone Member,a shelly algal pelletal limestone containing
late Middle to early Late Cambrian trilobites. Fault-bounded, incomplete
rings of Middle Ordovician sandstone and slate, and Late Ordovician dark
shale and chert, surround the greenstone core.
Cambrian volcaniclastics have recently been discovered in the hanging
wall of the Wonnangatta Fault, underlying the Howqua Chert, near Crooked
River. The section is a few metres thick and consists of thin, graded beds of
what were probably pyroxene sandstones, but are now talc–chlorite slate.The
chert contains conodonts of the Datsonian (latest Cambrian) C. proavus Zone
(VandenBerg et al., 2000).
Chapter 3 Neoproterozoic and Cambrian 91
Fig. 3.18: Serpentinised ultramafic host to chromite deposit, Dolodrook River, Thiele Igneous
Complex (lens cap for scale). Photograph by W. Birch.
Fig. 3.17: Photomicrographs of rocks from the Lickhole Volcanics Group in the Mt Wellington
Greenstone Belt. (a) Former ultramafic cumulate from boninitic magma, with crystals of
clinoenstatite and rare olivine replaced by talc and tremolite-actinolite. From Cold Creek crossing
on the Howqua Track. (b) Tholeiitic gabbro showing fresh clinopyroxene plates and smaller
plagioclase crystals replaced by near-isotropic microcrystalline epidote. From large layered sill, Lower
Howqua track. Both slides are c5 mm across and are taken with crossed polars.
3.5 Summary
The Late Neoproterozoic and Cambrian rocks in Victoria differ from later
Palaeozoic sequences by the predominance of volcanic and volcaniclastic
rocks. It is convenient to consider the volcanic rocks within the framework
of three broad tectono-magmatic associations that have relatively well-
defined temporal constraints.These are:
Association 1
A latest Neoproterozoic rift–drift sequence around 590–600 Ma, dominated
by rift tholeiites and some olivine-rich picritic lavas. This association is
presently thought to be restricted to the Delamerian Fold Belt section of
western Victoria, west of the Moyston Fault.
Association 2
An intra-oceanic arc association, which consists of allochthonous slices of
boninitic lavas and their cumulate counterparts, and overlying backarc basin-
type tholeiitic basalts, largely restricted to the Lachlan Fold Belt. Rocks of
this association dominate the Magdala, Pitfield, Heathcote and Mount
We llington Greenstone Belts, but also occur as limited fault-bounded slices
west of the Moyston Fault at Wartook and west of Moyston. Early Cambrian
trilobites, and analogous sequences in wester n Tasmania dated at 514 Ma,
suggest ages probably between 520 and 510 Ma. By analogy with better
exposed sections of the same sequence in western Tasmania, this association
was probably emplaced during the earliest phase of the Delamerian Orogeny
(510–505 Ma).This occurred during collision of the forearc section of an
intra-oceanic arc with the east-facing, rifted passive margin of the
Delamerian Fold Belt (characterised by Association 1 and its sediment cover).
The leading edge of this collision probably extended to western Victoria,
probably as far as the Moyston Fault. More distal from the collision zone, in
sections typified by the Heathcote and Howqua sections, there is no
structural evidence for this collision, and the volcanics are conformably
overlain by cherts and a deep marine sediment sequence that extends
through to the end of the Ordovician.This is an important difference from
the similar age rocks of western Tasmania, which were all deformed during
the Cambrian Tyennan Orogeny. These are proximal to the collision zone,
since Association 2 rocks sit immediately upon the Association 1 rift
sequences, with amphibolitic mylonite soles recording west-directed
emplacement of Association 2 (Berry & Crawford, 1988).
The best exposures of Association 2 in the Heathcote and Mount
We llington Greenstone Belts are basal duplexes of major east-directed thrust
systems of probable Latest Ordovician to Early Silurian age (Gray & Foster,1997).
Association 3
A post-collisional association is represented by diverse medium- to high-K
calc-alkaline andesites and high-Mg andesites occurring mainly west of
the Moyston Fault upon the recently deformed Delamerian Fold Belt
rocks. It is also exposed along the eastern margin of the Melbourne Zone
at Jamieson and Licola in what are probably erosion windows through to
an older basement of Tasmanian affinity. The best-exposed sequence, in the
Mount Stavely Belt, has been dated at about 500 Ma. In terms of age and
composition, these volcanics resemble the Mount Read Volcanics in
western Tasmania, for which a post-collisional setting can be demonstrated
(Crawford & Berry, 1992).
Assembly of the basement elements across central and western Victoria is
still poorly understood, but in broad terms, rock sequences record a mid-
Cambrian collision between a west-facing intra-oceanic arc and an east-facing
passive margin. Post-collisional extension at about 500 Ma produced widespread
calc-alkaline magmatism, now largely preserved as fault-bounded slices.
Evolution of the Palaeozoic Basement92
Chapter 3 Neoproterozoic and Cambrian 93
... cycle of supercontinent agglomeration and fragmentation (Veevers et al., 1997; Veevers, 2000). A volcanic passive margin is now recognised in southeastern Australia after continental fragmentation at around 600 Ma (Direen and Crawford, 2003a,b). Igneous activity included a mildly alkaline volcanic succession in the Wonominta Block of western New South Wales, and eruption of tholeiitic rocks in western Victoria and western Tasmania (Crawford et al., 2003; Direen and Crawford, 2003a,b; Meffre et al., 2004). ...
... A volcanic passive margin is now recognised in southeastern Australia after continental fragmentation at around 600 Ma (Direen and Crawford, 2003a,b). Igneous activity included a mildly alkaline volcanic succession in the Wonominta Block of western New South Wales, and eruption of tholeiitic rocks in western Victoria and western Tasmania (Crawford et al., 2003; Direen and Crawford, 2003a,b; Meffre et al., 2004). ...
... equivalents of the Anakie Inlier metamorphic succession in the Charter Towers Province of the northern Thomson Fold Belt (Hutton et al., 1997; Withnall et al., 1997, 2002, 2003; Fergusson et al., 2007a,b). We have made a detailed geochemical study of the tholeiitic suite to the west of Clermont in the Anakie Inlier (Fig. 3) that tests the proposed volcanic passive margin setting of East Gondwana inferred from rock assemblages in southeastern Australia (Direen and Crawford, 2003a,b; Crawford et al., 2003). We also explore the implications of these data for the reconstruction of the East Gondwana passive margin. ...
Development of the East Gondwana passive margin and when it occurred are constrained by the composition of low-grade maficschists and U-Pb ages of detrital zircons in psammitic schists from the Bathampton Metamorphics in the Anakie Inlier of central Queensland. These rocks show considerable variation in light lithophile elements due to post-magmatic processes. They have flat heavy rare earth element patterns, low-TiO2 (<2 wt.%) contents and their immobile element Ti, V, Y, La, Nb, Th and Zr values, indicate that they have an N-MORB-like magmatic affinity. However, they differ from N-MORB in that they show light rare earth depleted patterns and lower incompatible trace element contents. Their relative low abundance and association with metasediments suggest they formed in a magma-poor rifted margin setting. They are associated with psammitic rocks with detrital zircon ages indicating probable deposition in the late Neoproterozoic at ca 600 Ma. A magma-poor rifted margin in northeastern Australia differs from the volcanic passive setting that occurred in southeastern Australia at this time. These findings support development of the East Gondwana margin at 600 Ma that may have been related to rifting of a microcontinent off East Gondwana well after the breakup of Rodinia at ca 750 Ma. (c) 2008 Elsevier B.V. All rights reserved.
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The Delamerian Orogen in southeast Australia represents a Proterozoic continental rift margin, overprinted by convergent margin Andean-style subduction in the Cambrian. A detailed 150 km east–west magnetotelluric transect was collected across the orogen to investigate the electrical resistivity structure. The magnetotelluric transect follows an existing full crustal reflection seismic transect, of which interpretations support a westward-dipping Cambrian subduction model as derived from field mapping and geochemistry. A 2D inversion of the data from the 68 station broadband magnetotelluric transect imaged a heterogeneous crust with lateral changes as large as 10,000 Ω m occurring over ~ 15 km. The crust within the western Glenelg Zone is resistive, in contrast to the eastern Glenelg Zone and the Grampians–Stavely Zone (above the paleo-subduction zone), which host three conductive pathways. The main low resistivity regions (~ 1–10 Ω m) reside at mid-lower crustal depths (~ 10–30 km), extending up to the surface with a higher resistivity (~ 300 Ω m), but still much less than surrounding resistivity (mantle ~ 1000 Ω m, crust ~ 10,000 Ω m). Fluids released from the upper mantle during the Cambrian west-dipping subduction are interpreted to have moved up crustal faults to create the observed low resistivity pathways by serpentinisation and magnetite creation in mafic–ultramafic rocks. The electrical conductivity of hand samples of serpentinised mafic–ultramafic rocks in the region was found to be much greater than most other rock types present. In addition to adding insight into the crustal structure, the magnetotelluric data also supports geological surface mapping, as the major Lawloit and Yarramyljup Faults that bound different geological domains also mark domains of different electrical structure.
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Biomineralised arthropods, notably trilobites and agnostids, represent some of the most abundant and diverse constituents of Cambrian shelly faunas worldwide, and have proven to be excellent tools for biostratigraphy, correlation and biogeography. This study presents the first quantitative biogeographic and diversity analyses of middle Cambrian (Series 3) trilobites and agnostids from the palaeoequatorial East Gondwana margin, comprising mainland Australia, Tasmania, New Zealand and eastern Antarctica. Analysis of 224 genera of trilobites and agnostids from 78 fossil sites across three Cambrian Stage 3 time intervals (Stage 5, Drumian, Guzhangian) is presented. Results of the Stage 5 analyses reveal a major grouping of faunas from the Arafura, Georgina and Warburton basins, plus the Gnalta Shelf in New South Wales, typically represented by deep-water, outer shelf assemblages that commonly contain agnostids, oryctocephalids, Pagetia and Xystridura. Faunal exchange between these depocentres was permitted by a transgression that was associated with tectonically induced subsidence of basement blocks in the Georgina Basin, such as the Mt Isa block, during this stage. Drumian faunas are represented by three distinct site groupings: Group D1 is represented exclusively by several Georgina Basin assemblages that occur in shallow marine (intertidal to subtidal) settings, including the common trilobite genera Asthenopsis, Chondranomocare, Fuchouia and Penarosa, plus a range of agnostids, that inhabited an epeiric sea with connections to the open ocean; Groups D2 and D3 are represented by sites along the entire margin, from northern Australia to the Transantarctic Mountains that characterise a range of shallow to deep marine palaeoenvironments. These complex Drumian groupings most likely reflect long-range faunal exchange along the margin permitted by the eustatic transgression taking place at this time, which particularly influenced the distribution of eurytopic agnostid species that are common in these faunas. Two Drumian faunas from the Hodge Slate and Que River Beds in Tasmania (part of Group D3) exemplify a unique biofacies occurring in a deep-water, outer shelf setting, possibly in the lower photic zone, with the former assemblage containing the blind trilobites Meneviella and Holocephalina, and the latter containing only agnostids. Results of the Guzhangian analyses show four obvious faunal groupings, with Groups G1 and G2 being the largest and representing sites along the entire margin, while Group G3 is restricted to some of the Warburton Basin, Tasmanian and Antarctic sites, and Group G4 comprises only Tasmanian sites. Groups G1 and G2 correspond to Boomerangian and Mindyallan faunas, respectively, representing two temporally separated biofacies situated on the outer shelf to slope: Group G1 assemblages typically contain the trilobites Acontheus, Amphoton, Centropleura, Dorypyge, Fuchouia, Huzhuia, Pianaspis and Solenoparia; and Group G2 faunas often contain the trilobites Blackwelderia, Genevievella, Liostracina, Meteoraspis, Metopotropis, Mindycrusta, Palaeadotes, Rhyssometopus and Townleyella; with both groups containing a considerable number of eurytopic agnostid species. Group G4 assemblages also inhabit outer shelf settings, but have lower diversity, with common taxa including the trilobite Nepea and the agnostids Clavagnostus, Oidalagnostus and Valenagmostus. The considerable number of cosmopolitan Guzhangian agnostid species in association with distinct deep-water trilobite-agnostid assemblages along the entire East Gondwana margin strongly reflects the eustatic transgressive event that reached its pinnacle during this stage of Cambrian Series 3 that allowed for greater faunal exchange between areas on the margin and other palaeocontinents and terranes. The East Gondwana margin represents a biodiversity 'hot spot' during Cambrian Series 3, containing almost one-quarter (similar to 23%) of the trilobite and agnostid genera known worldwide. Our data support previous interpretations that Cambrian Series 3 trilobites and agnostid faunas from the East Gondwana margin, particularly those from Australia and Antarctica, have strong biogeographic links with those from Chinese terranes (especially North and South China), the Himalaya, and to a lesser extent, Iran, Kazakhstan, Laurentia and Siberia. Our data also reveal an overall increase in generic diversity throughout Cambrian Series 3, reaching a peak in the Guzhangian, with major diversifications most likely corresponding to eustatic transgressive phases, particularly in the Drumian and Guzhangian. This diversity trend for the East Gondwana margin closely matches that observed for contemporaneous faunas in other parts of the world, especially in China, Kazakhstan and West Gondwana, although diversity in the latter region reaches an acme in Drumian times.
NW-SE trending, transverse lineaments, including the lithospheric-scale Mont-Laurier lineament, are interpreted from regional Bouguer gravity of the Grenville orogen of SW Quebec and adjacent Superior Craton in southeastern Canada. These lineaments, transverse to the ENE trending Grenville orogen, are inferred to correspond to Palaeoproterozoic structures in Archaean basement that have played an important role: (i) in the development of volcano-sedimentary back-arc basins along this segment of the Laurentian margin; (ii) on the geometry of thrust sheets and folds formed during thrusting in the ca. 1.23-1.2 Ga Elzevirian orogeny and incorporation of the basins within the orogen; (iii) on reorientation of early-formed structures in the Central Metasedimentary Belt of Quebec (CMB-Q) during ca. 1.19-1.17 Ga post-Elzevirian orogenic collapse; and (iv) for development of syn-plutonic deformation corridors and shear zones at the onset of the emplacement of the Morin anorthosite-mangerite-charnockite-granite (AMCG) suite. In the CMB-Q, a 100 km wide megakink zone developed during ca. 1.19-1.17 Ga differential, post-Elzevirian orogenic collapse in the upper-most nappe above transverse sinistral shear corridors 10-20 km wide located in an underlying thrust sheet or “lower-deck”. Emplacement of 1.17 Ga Chevreuil intrusive suite preferentially occurred within the megakink zone, starting late in the post-Elzevirian collapse and culminating during a switch to local shortening early in (and in part as a consequence of) the emplacement of the voluminous Morin anorthosite and associated AMCG-suite plutons. The Labelle deformation zone separating the CMB-Q and Morin terrane is interpreted as a post-Shawinigan, reverse shear zone that truncates folded lithological layering in the eastern CMB-Q and western Morin terrane that is either subsequently folded above the Mont-Laurier lineament during its further reactivation, or developed as a curved shear zone stepping across the Mont-Laurier lineament. The Grenville Province of SW Quebec therefore provides an example of strain partitioning and distinct deformation responses at different crustal levels during reactivation of basement structures.
During the last 20 years, seismic tomography has frequently been used to provide information on the structure of the lithosphere beneath the Australian continent. New tomographic models are presented using two complementary seismological techniques in order to illustrate the current state-of-knowledge. Surface wave tomography is the ideal method to obtain information of velocity variations across the whole continent. The latest models use data from over 13 000 source–receiver paths, allowing a higher resolution than in previous studies using the same technique. In Western Australia the results at 100 km depth clearly reveal the contrast in structure between the Pilbara and Yilgarn Cratons and the Capricorn Orogen. At greater depths, the Kimberley Block has a distinct fast velocity anomaly in comparison with the surrounding mobile belts. In the east of the continent, strong horizontal gradients in velocity indicate transitions in lithospheric structure, although the new high resolution models reveal a complexity in the transitions through central Victoria and New South Wales. Complementing the surface wave tomography, we also present the results from the inversion of over 25 000 relative arrival times from body wave phases recorded in southeast Australia and Tasmania. The body wave tomography uses the surface wave model to provide information on long-wavelength structure and absolute velocities that would otherwise be lost. The new results indicate a distinct boundary between the Delamerian and Lachlan orogens within the upper mantle, the location of which is consistent with an east-dipping Moyston Fault, as observed by deep seismic reflection profiling. The new models also confirm a distinct region of fast velocities beneath the central sub province of the Lachlan Orogen. A significant new observation is that the inferred eastern edge of this central sub-province has a strong correlation with the location of copper/gold deposits; a similar relationship is observed at a larger scale in Western Australia where mineral deposits appear to flank the regions of fastest velocity within the West Australian Craton.
The Ross-Delamerian orogenic belt formed along the early Paleozoic active Pacific margin of the newly merged Gondwana supercontinent. In its northernmost segment in the Townsville region of northeastern Australia, we have identified a short contractional phase of the Delamerian orogeny in the Argentine Metamorphics postdating formation of a mafic breccia with a U-Pb zircon age of 500 ± 4 Ma. Contraction was followed by widespread inferred extensional deformation with formation of flat-lying foliation, domal features, and amphibolite grade and greenschist retrograde metamorphism all synchronous with latest Cambrian to Early Ordovician extensional back-arc volcanism, sedimentation, and intrusions. One of these intrusions gives a U-Pb zircon age of 480 ± 4 Ma. Foliation related to the extensional deformation is crosscut by a late granodiorite dike with a U-Pb zircon age of 461 ± 4 Ma. Late east-west contractional deformation affected the higher-grade part of the assemblage. In contrast to the Ross-Delamerian orogenic belt in the Transantarctic Mountains and southeastern Australia, the orogenic belt in northeastern Australia was affected by a short episode of contraction at ∼495 Ma followed by long-lived back-arc extension from ∼490 Ma to 460 Ma with subsequent contractional deformation.
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Seismic data from three separate experiments, a marine active source survey with land-based stations, and two teleseismic arrays deployed to record distant earthquakes, are combined in a joint inversion for the 3D seismic structure of the Tasmanian lithosphere. In total, travel-time information from nearly 14 000 source–receiver paths are used to constrain a detailed model of crustal velocity, Moho geometry and upper mantle velocity beneath the entire island. Synthetic reconstruction tests show good resolution beneath most of Tasmania with the exception of the southwest, where data coverage is sparse. The final model exhibits a number of well-constrained features that have important ramifications for the interpretation of Tasmanian tectonic history. The most prominent of these is a marked easterly transition from lower velocity crust to higher velocity crust which extends from the north coast, northeast of the Tamar River, down to the east coast. Other significant anomalies include elevated crustal velocities beneath the Mt Read Volcanics and Forth Metamorphic Complex; thickened crust beneath the Port Sorell and Badger Head Blocks in central northern Tasmania; and distinctly thinner, higher velocity crust beneath the Rocky Cape Block in northwest Tasmania. Combined with existing evidence from field mapping, potential-field surveys and geochemical data, the new results support the contention that east and west Tasmania were once passively joined as far back as the Ordovician, with the transition from lithosphere of Proterozoic continental origin to Phanerozoic oceanic origin occurring some 50 km east of the Tamar River; that the southeast margin of the Rocky Cape Block may have been a former site of subduction in the Cambrian; and that the Badger Head and Port Sorell Blocks were considerably shortened and thickened during the Cambrian Tyennan and Middle Devonian Tabberabberan Orogenies.
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Palaeogeographical reconstructions of the Australian and Antarctic margins based on matching basement structures are commonly difficult to reconcile with those derived from ocean-floor magnetic anomalies and plate vectors. Following identification of a previously unmapped crustal-scale structure in the southern part of the early Palaeozoic Delamerian orogen (Coorong Shear Zone), a more tightly constrained plate reconstruction for these margins is proposed. This reconstruction places the Coorong Shear Zone opposite the Mertz Shear Zone in Antarctica and lends itself to a revised interpretation of continental rifting along Australia’s southern margin in which rift basin architecture, margin segmentation and the formation of ocean-floor fracture zones are all linked to pre-existing basement structure and the reactivation of a few deep-rooted crustal structures inherited from the Delamerian orogeny in particular. Reactivation of the Coorong Shear Zone and other basement structures (Avoca–Sorell Fault Zone) during the earlier stages of rifting was accompanied by the partitioning of extensional strain and formation of late Jurassic–Early Cretaceous normal faults and half-graben in the Bight and otway basins with opposing NE–SW and NW–SE structural trends. Previously, the Mertz Shear Zone has been correlated with the Proterozoic Kalinjala Mylonite Zone in the Gawler Craton but this positions Australia 300–400 km too far east relative to Antarctica prior to breakup and fails to secure an equally satisfactory match in both basement geology and the superimposed extension-related structures.
Major 440 Ma orogenic-gold deposits in the western Victorian goldfields formed during east – west shortening but have markedly different structural complexity. These deposits occur in: (i) a Cambrian Delamerian basement block that was substantially reworked and reactivated during the Lachlan Orogeny (Stawell); and (ii) Ordovician turbidites deformed solely by Lachlan-aged deformation (Bendigo, Ballarat, Castlemaine). This produced different structural histories prior to mineralisation, although gold deposits have been localised at the top of regional domal culminations. At Stawell, the 440 Ma gold event reactivated a strike-change along a pre-existing Cambrian fault system above a major lithospheric boundary. In the Bendigo Zone, 440 Ma orogenic-gold deposits have a trend oblique to the dominant structural grain and parallel to the western edge of an inferred crystalline basement block (the Selwyn Block). This gold trend is parallel to metamorphic field gradients and pluton age boundaries, which suggests an underlying basement control on the localisation of these orogenic-gold deposits, even though the exact basement architecture is still unresolved. Major variations in the regional stress fields occurred between 425 and 370 Ma, with large gold deposits [>62 t (2 million ounces) endowments] forming at ca 380 – 370 Ma. These events are not deposit-scale structural anomalies as they also regionally affect overlying cover sequences (e.g. the Grampians Group). Gold deposits that formed in the 425 – 400 Ma period have small endowments, but introduce a marked amount of structural and mineralogical complexity to the gold province. The 425 – 400 Ma period preserved at Stawell records sinistral wrenching associated with gold mineralisation, southeast-directed faulting, intrusion-related gold mineralisation and extensive high-level Early Devonian plutonism.
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