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A Barrovian-type overprint on high pressure rocks: PTt-evolution in the Ordovician Corner Brook Complex (W-Newfoundland, Canada)

Authors:
[4] Holland, T.J.B. & Powell, R., 1998: Journal of Metamorphic Geology 16, 309-343.
[2] Cawood, P.A. & Van Gool, J.A.M. (1998): Geological Survey of Canada Bulletin 427,
[1] Castonguay, S. et al. (2014): Geoscience Canada 41, 459-482. [5] Willner, A.P. et al. (2015): Abstracts International Eclogite Conference Rio San Juan 2015, 104.
[3] Connolly, J., 2005: Earth and Planetary Science Letters 236, 524–541.
Introduction
Petrographic characteristics
Age dating
Geodynamic implications
Thermodynamic modeling
References
Mineral compositions
Geological setting
The result of thermodynamic modeling in combination with petrographical observations reflects a geodynamical evolution of the CBC characterized by
interrelated PT paths of different metamorphic zones.
The assignment of rocks to distinct Barrovian-type metamorphic zones on the basis of index minerals is now supported by the PT fields of the observed
mineral assemblages in the corresponding pseudosections.In addition, mineral fabrics (e.g. symplectitic garnet), composition of different minerals (e.g. Si
in white mica), and different shapes of PT paths emphasize that fluid flow plays a significant role in controlling the intensity of metamorphism and
metasomatism.
Consequently, the progress of an overprint of an older high pressure stage likely related a Taconic (?) deformation by a younger metamorphic stage during a
midcrustal deformation (Salinic event) and metasomatic features differ between the metamorphic zones, but also within in the zones, and even on a
handspecimen scale. A geodynamic model which can explain the different interrelated PT-paths is shown on the right for four major stages in the CBC.
By contrast, the Fleur the Lys Complex was overprinted by Salinic deformation to minor extent.
Variations between the metapelitic
samples are observed mainly in
modal abundance, grain size and
fabrics of mineral phases. The
occurence of index minerals was
used to assign samples to meta-
morphic zones.
The metamorphic overprint is mani-
fested in the existence of e.g. two
distinct generations of garnet in one
sample. The older garnet population
displays extensive symplectitic
intergrowth during resorption of
garnet. The younger garnet lacks this
feature. Locally extensive fluid flow,
which provides enhanced mass
transfer, leads to changes in modal
abundance/phase reduction and
formation of large porphyroblasts.
However, both phenomena do not
need to occur in the same rock.
Metapelite with megablasts of
several cm lacks signs of phase
reduction.
PT pseudosections were calculated using the software PERPLE_X (Connolly, 2005) and the thermodynamic
dataset of Holland & Powell (1998). Increasing peak metamorphic temperatures were associated with the observed
mineral assemblages (marked in red) from garnet zone (lowest grade) to grt-bt-ky zone (highest grade). PT
conditions at different stages of garnet growth were derived using the intersections of garnet isopleths. Significantly
differing PT paths result for different zones in the CBC. Maximum Si contents in (relic) white mica were recorded at a
high pressure stage in every metamorphic zone, also where assemblages were completely overprinted at later
intermediate PT conditions. Remnants of first garnet growth occurred at high pressure conditions.
Mineral compositions in
metapelite of the CBC are
restricted in their variability.
Nonetheless, both garnet
populati ons th at were
identified on the basis of
differences in their physical
appearance display distinct
compositional features.
The geological evolution of Newfoundland is characterised
by successive collision of microcontinents during the
Palaeozoic.
The Fleur de Lys Complex (FLC; 1) and the Corner Brook
Complex (CBC; 2) in western Newfoundland, comprise
metasedimentary rocks of the partially subducted Humber
margin of Laurentia, metamorphosed at medium grade and
intermediate to high pressure conditions during the collision
with outboard arcs on a Laurentia-derived Dashwoods
microcontinent. Both complexes represent collisonal wedges
along the Baie Verte Brompton Line, a suture zone that was
closed at 485-479 Ma. First peak high pressure conditions at
461-465 Ma (Castonguay et al. 2014; Willner et al. 2015) are
recorded in eclogite of the FLC during the Taconic event (~500-
450 Ma). Such ages are as yet not known in the CBC.
The Corner Brook Complex (CBC) is
bounded to the west by the Humber River
Fault against Cambrian sedimentary
rocks of the Humber margin of Laurentia
and to the east by the Cabot Fault against
Ordovician volcanosedimentary rocks of
the Dashwood microcontinent. All are
overlain by Carboniferous cover rocks.
Characteristic features of the CBC are
moderately E-dipping thrust sheets,
predominant metapelitic rocks and a
pronounced Barrow-type zonation of six
zones: chlorite, biotite, garnet, staurolite,
Grt-St-Ky and Grt-Bt-Ky zones.
We dated two samples with a predominating stage III assemblage by Rb-Sr mineral isochrones
at 429±6 and 440±6 Ma. These ages confirm previous ages (U/Pb zircon 434 Ma, monazite 430
Ma, rutile 437 Ma; Ar/Ar white mica 413-430 Ma; Cawood et al. 1994) and are related to the
Salinic orogenic overprint (~440-420 Ma) at midcrustal conditions.
1 1 2 3 1 4
Bißbort, T. , Willner, A.P. , Glodny, J. , van Staal, C.R. , Schertl, H.-P. & Zagorevski, A.
(1) Ruhr-Universität Bochum, Germany (2) Deutsches Geoforschungszentrum Potsdam, Germany, (3) Geological Survey of Canada Vancouver, Canada, (4) Geological Survey of Canada Ottawa, Canada
A Barrovian-type overprint on high pressure rocks:
PTt-evolution in the Ordovician Corner Brook Complex
(W-Newfoundland, Canada)
Grt
zone
St
zone
Grt-Bt-Ky
zone
Pressure [kbar]
Temperature [° C]
7
9
11
6
8
10
12
5
600500 550 650 700
stacking in mid-crustal level
I
II
Exhumation - thermal relaxation
extensive fluid flow
I
P
T
Subduction / Collision
crustal stacking
in midcrustal
level & prograde
dehydration of lower plate
Composite Laurentia
ductile shear
zone
P
T
T
T
P
P
Stage III:
Interrelated PT paths due to midcrustal stacking accompanied
by extensive fluid flow during the Salinic orogenic event
x
y
lower plate y
upper plate x
Humber margin
basement
exhumation
channel
Stage II:
Collision - maximum submergence of Humber margin
sedimentary rocks, return flow in an exhumation channel and
thermal relaxation likely during the Taconic orogenic event
Stage I: Closure of Taconic seaway during Cambrian
Dashwoods
Humber margin
Laurentia
Oceanic crust
W E
CBCC
erosion
c
W E
Zones
Grt|St|Grt-Bt-Ky|Grt-St-Ky|St|Grt
thrust fault
thrusting
W E
b
afolding
W E
Stage IV:
Late deformation and final emplacement of units in an
upper crustal level by large scale folding and brittle thrusting
1000 °C
600 °C
2 cm
homogeneously
distributed garnet
no garnet
band of phase reduction
homogeneously
distributed garnet
increase in number
decrease in grain size
[vol. %]
19(Grt) 21(Pl) 38(Wm)
10(Bt) 10(Ilm) 2(Other)
0(Grt) 15(Pl) 73(Wm)
3(Bt) 6(Ilm) 3(Other)
60(Grt) 22(Pl) 3(Wm)
4(Bt) 9(Ilm) 2(Other)
20(Grt) 20(Pl) 40(Wm)
8(Bt) 11(Ilm) 1(Other)
17(Grt) 22(Pl) 39(Wm)
12(Bt) 8(Ilm) 2(Other)
Faults/Tectonic boundaries
100 km
49°N
47°N
Exploits
Subzone
Notre Dame
Subzone
Dunnage Zone
Humber Zone
53°W
Avalon Zone
Gander
Zone
Silurian and
Devonian plutons
„Iapetus suture“
Gondwana
Laurentian margin
metased. rocks incl. HP rocks
Grenvillian basement
Humber Zone
Laurentian Margin
Notre Dame Subzone
(Dashwoods)
Laurentia
Lower Carboniferous
cover sedimentary rocks
Laurentian margin
sedimentary rocks
BVBL
AVALONIA
GANDERIA
DASHWOODS
59°W 57°W 55°W
Terranes of
Atlantic Canada
Meguma
Avalonia
Laurentia
Ganderia
Peri-Laurentia
200 km
N
West Africa
Armorican Terrane
Assemblages
North America
250 km
Baltica
Peri-Gondwanan realm
(Ganderia, Avalonia, Meguma)
North Atlantic
spreading axis
Gondwanan realm
(West Africa, Armorican Terranes)
Laurentian realm
Baltican realm
250 km
Rheic suture Iapetus suture
BVBL Baie Verte Brompton Line
1
2
1 Fleur de Lys Complex
2 Corner Brook Complex
5 km
N
Thrust
Normal fault
a
b
Carboniferous
sedimentary cover
Silurian Granitoids
Ordovician volcano-sedimentary rocks
(Notre Dame Subzone)
Corner Brook Complex
Humber margin sedimentary rocks:
Humber River Fault
Cabot Fault
Cambrian-Ordovician shelf carbonates :
Ediacaran to Lower Cambrian continental slope siliciclastic sedimentary rocks :
Megablast
zone
Biotite-Zone
Garnet-Zone
Corner Brook
Lake
5 km
Chlorite-Zone
Garnet-Zone
b
Staurolite-Zone
Grt-St-Ky-Zone
Grt-Bt-Ky-Zone
Humber River Fault
500 mm
Grt
Wm
Bt
Chl
Ilm
Ap
3.10 Si apfu
3.20 Si apfu
WmPg
ClPl
Am
Ep
Rt
WmPg
ClGt
TtAm
LwRt
WmPgCl
GtPlEp
ImMt
BtWmPg
GtPlEp
MtRt
BtWmGtKy
PlMtRt
BtWmGt
PlMtRt
WmGtPl
MtRt
WmGtKy
PlMtRt
BtWmGtSil
PlImMt
BtWmGtSt
PlImMt
WmPg
ClGtTt
AmEp
WmPgClGt
TtAmEpPl
WmPgCl
TtAbAm
Ep
WmPg
ClTt
PlAm
Ep
WmPg
ClPl
EpMtRt
WmPgCl
GtPlEp
MtRt
WmPgCl
GtPlAm
EpRt
WmPg
ClTt
AmLw
BtWm
ClGt
PlEp
Mt
Rt
BtWm
GtSt
PlMtRt
BtWm
ClGtPl
ImMt
WmCl
GtPlEp
ImMt
bivariant
trivariant
quadrivariant
quintvariant
sixtvariant
+Qz +V
Pressure [kbar]
7
9
11
6
8
10
12
Garnet Zone
5
Temperature [° C]
600
400 500
350 550
450 650 700
500 mm
symplectitic
Grt
Bt
Pl
Wm
Ilm
Grt with pronounced
symplectitic rims
0.03 0.07 0.11 0.15 0.19
0.20
0.10
0.00
X(Ca)
X(Mg)
Grt-Bt-Ky Zone
0.06
0.04
0.02
0.00
X(Mn)
Gt with pronounced
symplectitic rims
Grt-St-Ky Zone
0.03 0.07 0.11 0.15 0.19
0.20
0.10
0.00
X(Mn) | X(Ca)
X(Mg)
Garnet Zone
0.62 0.66 0.70 0.74
0.20
0.10
0.00
X(Mn) | X(Ca)
X(Fe)
Core X(Ca)
Outer Core X(Ca)
Rim X(Mn)
Outer Rim X(Mn)
Rim X(Ca)
Outer Rim X(Ca)
Core X(Mn)
Outer Core X(Mn)
min.0.19 X(An)
0.07 Mn apfu
0.33 Mg apfu
0.39 Ca apfu
2.06 Fe apfu
0.48 Ca apfu
0.38 Mg apfu
2.03 Fe apfu
0.54 Ca apfu
0.38 Mg apfu
1.99 Fe apfu
0.33 Mg apfu
0.60 Ca apfu
0.08 Mn apfu
1.93 Fe apfu
0.20 Mg apfu
0.67 Ca apfu
0.21 Mn apfu
Core
Outer
Core
Inner
Rim
Rim
Rim
Pressure [kbar]
Temperature [° C]
7
9
11
6
8
10
12
5
600
400 500
350 550
450 650 700
200 mm
Mn
Ca
Grt-Bt-Ky Grt-St-Ky Staurolite Garnet
Mn
Mn
Mn
Ca
Ca
Ca
200 mm
200 mm200 mm
200 mm200 mm
500 mm
500 mm
10 cm
1.7
1.5
1.3
1.1
0.9
0.7
0.5
87 86
Sr/ Sr
87 86
Rb/ Sr
data-point error crosses are 2σ
16Ca16a - Grt-Ky-St schist
wm 160-90 μm
0 40 80 120 160
Age = 428.8 ± 6.4 Ma
Initial 87Sr/86Sr = 0.71657 ± 0.00017
n = 4; MSWD = 1.7
biotite
wm 250-160 μm
wm 355-250 μm
0.738
0.734
0.730
0.726
0.722
0.718
87 86
Sr/ Sr
87 86
Rb/ Sr
data-point error crosses are 2σ
16Ca02 - garnet micaschist wm
500-355 μm
wm
160-90 μm
K-fsp
0.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8
Age = 439.3 ± 5.6 Ma
Initial 87Sr/86Sr = 0.719406 ± 0.000059
n = 3; MSWD = 0.86
450 550 650 700 550 650 700 550 650 700 550 650 700 550 650 700 550 650 700
Pressure [kbar]
12
10
8
6
Temperature [° C]
Staurolite Zone Grt-Bt-Ky Zone Grt-St-Ky Zone Zone of Garnet Zone
megablast formation
Pressure [kbar]
metasomatic
affected
?
?
?
?
?
Grt
Zone
St
Zone
Grt-Bt-Ky
Zone
Grt-St-Ky
Zone
St
Zone
Grt
Zone
WNW ESE
500
0
[m a.s.l.]
500 m
schistosity
12
10
8
6
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