Article

High-grade crystalline basement of the northwestern Wilson Terrane at Oates Coast: New petrological and geochronological data and implications for its tectonometamorphic evolution

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  • retired (from Federal Geological Survey of Germany, Hannover)
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Abstract

The high-grade basement of the northwestern Wilson Terrane at Oates Coast is subdivided into three roughly north-south trending zones on the basis of tectonic thrusting and differences in metamorphic petrology. New results of detailed petrological investigations show that metamorphic rocks of the central zone were formed in course of one single, clockwise directed P-T evolution including a medium-pressure and high-temperature granulite-facies stage at about 8 kbar and >800° C, a subsequent isothermal decompression and a final stage with retrograde formation of biotite + muscovite gneisses. In the eastern and western zones the majority of metamorphic rocks experienced clockwise oriented metamorphism at somewhat lower P-T conditions of about 4-5.5 kbar and 700-800°C. While some rocks in both zones did not reach the upper stability limit of muscovite + quartz, granulite-facies rocks detected in parts of the western zone were formed under P-T conditions similar to those of the central zone. New SHRIMP data support an age for the metamorphic peak of 496-500 Ma in the central zone (Henjes-Kunst et al., 2004). 40Ar-39Ar dating of amphiboles and micas indicate a general trend to younger ages from the west to the east of the basement complex, i.e. from 488-486 Ma to 472-469 Ma for amphiboles and from 484-482 Ma to 466 Ma for micas. This is explained by temporal differences in the retrograde metamorphic evolution of the three zones in the course of the late-Ross-orogenic thrust-related uplift of the basement complex, with the western zone being exhumed earlier than the eastern zone.

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... The northernmost part of the Wilson Terrane was basically investigated for the first time during the German Antarctic North Victoria Land Expeditions GANOVEX V and VII in 1988/89 and 1992/93. There, a subdivision of the crystalline basement into three different NNW–SSE trending zones was recognised: a central zone with granulite-facies gneisses and migmatites is flanked by one eastern and one western zone with gneisses which were formed under lower amphibolite-to lowest granulite-facies conditions (Schüssler, 1996; Schüssler et al., 1999 Schüssler et al., , 2004 ). The zones are confined by the prominent Wilson, Exiles, and Lazarev thrust systems ( Kleinschmidt, 1991, 1993; Läufer et al., 2006 ) except the boundary between the eastern and the central zone which is still of unclear character. ...
... The zones are confined by the prominent Wilson, Exiles, and Lazarev thrust systems ( Kleinschmidt, 1991, 1993; Läufer et al., 2006 ) except the boundary between the eastern and the central zone which is still of unclear character. The petrography of the metamorphic rocks in the northern Wilson Terrane and estimates of metamorphic conditions have been detailed in Schüssler et al. (2004): Metamorphic rocks of the central zone were formed in the course of one single clockwise P–T evolution including a medium-pressure/high-temperature granulite-facies stage at about 8 kbar and >800 °C, a subsequent isothermal decompression , and a final amphibolite-facies stage with retrograde formation of biotite and muscovite gneiss. In the eastern and western zones the majority of metamorphic rocks experienced clockwise oriented P–T paths at somewhat lower P–T conditions of about 4–5.5 kbar and 700–800 °C. ...
... Previously, monazite U–Pb isotopic data have been published from four samples collected in the central, granulite-facies zone of the Wilson Terrane. Two of them were analysed by SHRIMP (Henjes-Kunst et al., 2004), the other two by TIMS (Schüssler et al., 1999Schüssler et al., , 2004). The SHRIMP analysed samples are G8-57.4 ...
... The present study was performed on high-grade metamorphic and related igneous rocks from the Wilson Hills located in the northwestern coastal area of Oates Land ("Oates Coast") ( Fig. 1) and forming part of the inboard high temperature-dominated metamorphic belt of the WT. In this area, major thrusts activated during final tectonometamorphism of the Ross Orogeny separate units with contrasting metamorphic degrees (Flöttmann & Kleinschmidt, 1991;Schüssler et al., 1999;Schüssler et al., 2004). These structures were interpreted to indicate that blocks from different crustal levels of the WT basement were tectonically exhumed to various degrees along the thrust zones during the late stage of the Ross Orogeny (Flöttmann & Kleinschmidt, 1991;Schüssler, 1996). ...
... Basement rocks of the Wilson Hills consist mostly of psammitic metasediments with local variations to more quartzitic or more pelitic compositions (Henjes-Kunst & Schüssler, 2003). Differences in metamorphic grade distinguish a very-high-grade granulite-facies central zone from high-grade granulite-to high amphibolite-facies eastern and western zones (Schüssler, 1996;Schüssler et al., 1999, Schüssler et al., 2004 (Fig. 2). Whereas metasediments in the latter two zones are extremely monotonous, calc-silicate and amphibolitic intercalations are relatively abundant in the central zone resulting in more variegated series. ...
... Formation of a late-tectonic pegmatite at Ringgold Knoll in the central very-highgrade zone of the Wilson Hills (Fig. 2) was dated at about 490 Ma by Rb-Sr whole-rock and U-Pb monazite dating (Schüssler & Henjes-Kunst, 1994;Schüssler et al., 1999). 40 Ar-39 Ar dating of amphibole and mica from igneous and metamorphic rocks of that area gave ages in the ranges of 486 -472 Ma and 476 -466 Ma, respectively (Schüssler et al., 1999;Schüssler et al., 2004.;Henjes-Kunst, unpubl. ...
Article
High- to very-high-grade migmatitic basement rocks of the Wilson Hills area in northwestern Oates Land (Antarctica) form part of a low-pressure high-temperature belt located at the western inboard side of the Ross-orogenic Wilson Terrane. Zircon, and in part monazite, from four very-high grade migmatites (migmatitic gneisses to diatexites) and zircon from two undeformed granitic dykes from a central granulite-facies zone of the basement complex were dated by the SHRIMP U-Pb method in order to constrain the timing of metamorphic and related igneous processes and to identify possible age inheritance. Monazite from two migmatites yielded within error identical ages of 499 ± 10 Ma and 493 ± 9 Ma. Coexisting zircon gave ages of 500 ± 4 Ma and 484 ± 5 Ma for a metatexite (two age populations) and 475 ± 4 Ma for a diatexite. Zircon populations from a migmatitic gneiss and a posttectonic granitic dyke yielded well-defined ages of 488 ± 6 Ma and 482 ± 4 Ma, respectively. There is only minor evidence of age inheritance in zircons of these four samples. Zircon from two other samples (metatexite, posttectonic granitic dyke) gave scattered 206Pb-238U ages. While there is a component similar in age and in low Th/U ratio to those of the other samples, inherited components with ages up to c. 3 Ga predominate. In the metatexite, a major detrital contribution from 545 - 680 Ma old source rocks can be identified. The new age data support the model that granulite- to high- amphibolite-facies metamorphism and related igneous processes in basement rocks of northwestern Oates Land were confined to a relatively short period of time of Late Cambrian to early Ordovican age. An age of approximately 500 Ma is estimated for the Ross-orogenic granulite-facies metamorphism from consistent ages of monazite from two migmatites and of the older zircon age population in one metatexite. The variably younger zircon ages are interpreted to reflect mineral formation in the course of the post-granulite-facies metamorphic evolution, which led to a widespread high-amphibolite-facies retrogression and in part late-stage formation of ms+bi assemblages in the basement rocks and which lasted until about 465 Ma. The presence of inherited zircon components of latest Neoproterozoic to Cambrian age indicates that the high- to very-grade migmatitic basement in northwestern Oates Land originated from clastic series of Cambrian age and, therefore, may well represent the deeper-crustal equivalent of lower-grade metasedimentary series of the Wilson Terrane.
... The northernmost part of the Wilson Terrane was basically investigated for the first time during the German Antarctic North Victoria Land Expeditions GANOVEX V andVII in 1988/89 and1992/93. There, a subdivision of the crystalline basement into three different NNW-SSE trending zones was recognised: a central zone with granulite-facies gneisses and migmatites is flanked by one eastern and one western zone with gneisses which were formed under lower amphibolite-to lowest granulite-facies conditions (Schüssler, 1996;Schüssler et al., 1999Schüssler et al., , 2004. The zones are confined by the prominent Wilson, Exiles, and Lazarev thrust systems Kleinschmidt, 1991, 1993;Läufer et al., 2006) except the boundary between the eastern and the central zone which is still of unclear character. ...
... The petrography of the metamorphic rocks in the northern Wilson Terrane and estimates of metamorphic conditions have been detailed in Schüssler et al. (2004): Metamorphic rocks of the central zone were formed in the course of one single clockwise P-T evolution including a medium-pressure/high-temperature granulite-facies stage at about 8 kbar and >800°C, a subsequent isothermal decompression, and a final amphibolite-facies stage with retrograde formation of biotite and muscovite gneiss. In the eastern and western zones the majority of metamorphic rocks experienced clockwise oriented P-T paths at somewhat lower P-T conditions of about 4-5.5 kbar and 700-800°C. ...
... Previously, monazite U-Pb isotopic data have been published from four samples collected in the central, granulite-facies zone of the Wilson Terrane. Two of them were analysed by SHRIMP , the other two by TIMS (Schüssler et al., , 2004. The SHRIMP analysed samples are G8-57.4 ...
... The northernmost part of the Wilson Terrane was basically investigated for the first time during the German Antarctic North Victoria Land Expeditions GANOVEX V andVII in 1988/89 and1992/93. There, a subdivision of the crystalline basement into three different NNW-SSE trending zones was recognised: a central zone with granulite-facies gneisses and migmatites is flanked by one eastern and one western zone with gneisses which were formed under lower amphibolite-to lowest granulite-facies conditions (Schüssler, 1996;Schüssler et al., 1999Schüssler et al., , 2004. The zones are confined by the prominent Wilson, Exiles, and Lazarev thrust systems Kleinschmidt, 1991, 1993;Läufer et al., 2006) except the boundary between the eastern and the central zone which is still of unclear character. ...
... The petrography of the metamorphic rocks in the northern Wilson Terrane and estimates of metamorphic conditions have been detailed in Schüssler et al. (2004): Metamorphic rocks of the central zone were formed in the course of one single clockwise P-T evolution including a medium-pressure/high-temperature granulite-facies stage at about 8 kbar and >800°C, a subsequent isothermal decompression, and a final amphibolite-facies stage with retrograde formation of biotite and muscovite gneiss. In the eastern and western zones the majority of metamorphic rocks experienced clockwise oriented P-T paths at somewhat lower P-T conditions of about 4-5.5 kbar and 700-800°C. ...
... Previously, monazite U-Pb isotopic data have been published from four samples collected in the central, granulite-facies zone of the Wilson Terrane. Two of them were analysed by SHRIMP , the other two by TIMS (Schüssler et al., , 2004. The SHRIMP analysed samples are G8-57.4 ...
Article
The electron microprobe (EMP) Th-U-Pb monazite bulk chemical dating method was applied to granulite-facies rocks of the Wilson Terrane in Antarctica. A combination of this method to isotopic U-Pb-SHRIMP ages for the evaluation of metamorphic processes required the analysis of reference monazites. These can be subdivided into three groups: a) Monazite with variable total Pb at constant Th (e.g. VK-1) is unsuitable for EMP data evaluation; b) Monazite with highly variable total Pb and Th, but with at least some Th/Pb approximating an apparent isochrone (e.g. MPN) is partly useful; and c) Monazite with constant Th/Pb at high Th (e.g. Madmon monazite) is best suitable for the combined approach and can be additionally used to improve the Th calibration for EMP. Study of monazite in grain mounts and in thin sections led to partly different but complementary results: Older monazites with EMP ages up to 680 Ma occur mainly in a grain mount from diatexite and metatexite and are interpreted as detrital relics. Some of these monazites show structures and mineral-chemical zonation trends resembling metasomatism by alkali-bearing fluids. A marked mobility of Th, P, Ce, Si and U is observed. The age of the metasomatic event can be bracketed between 510 and 450 Ma. Furthermore, in the grain mount and in numerous petrographic thin sections of migmatites and gneisses, the EMP Th-U-Pb and SHRIMP U-Pb monazite data uniformly signal a major metamorphic event with a medium-pressure granulite facies peak between 512 and 496 Ma. Subsequent isothermal uplift and then amphibolite-facies conditions between 488 and 466 Ma led to crystallisation of pristine monazite. The high-grade metamorphic event, related to the Ross Orogeny, can be uniformly traced more than 600 km along strike in the Wilson Terrane.
... Eclogite-facies metamorphism in the Lanterman Range took place at ca. 500 Ma, amphibolite-facies regression at 490-486 Ma, and shear deformation under amphibolite to greenschist conditions at ca. 480-460 Ma (Di Vincenzo et al., 1997, 2001Palmeri et al., 2012;Di Vincenzo et al., 2014). A comparable metamorphic history is recorded by migmatites in the northern Wilson Terrane: a major high-grade metamorphic event with a granulite-facies peak between 512 and 496 Ma was followed by subsequent isothermal uplift and amphibolite-facies conditions between 488 and 466 Ma Schüssler et al., 2004;Schulz and Schüssler, 2013). ...
... The Schulz and Schüssler, 2013). It should be considered that high-temperature granulite-facies conditions (N 800°C; Schüssler et al., 2004) could have affected the isotope system of zircons. Adams et al. (2014) report a youngest zircon age population of 545 ± 11 Ma for a sample from the greenschist-facies Berg Group. ...
Article
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The Ross-Delamerian orogenic belt was formed along the eastern side of the Australian-East Antarctic continent during west-directed subduction of the Palaeo-Pacific Ocean in the early Palaeozoic. Northern Victoria Land (NVL) in Antarctica was located at a central position of the Ross-Delamerian system. Its metamorphic basement is formed by three lithotectonic units formerly interpreted as terranes: the Wilson, Bowers and Robertson Bay terranes (from west to east). Dating of detrital zircons from 14 meta¬sedimentary samples of these terranes combined with petrographical and whole-rock geochemical studies give new insights into the stratigraphic and tectonic evolution of NVL. All samples show very similar zircon age spectra with two main intervals, a Ross/Pan-African-age interval (470–700 Ma) and a Grenville-age interval (900–1300 Ma), as well as subordinate craton-related ages dispersed over the range of ca. 1600–3500 Ma. The Ross/Pan-African-age zircon population tends to get more dominant from the Priestley Formation of the Wilson Terrane to the Molar Formation of the Bowers Terrane, and finally to the Robertson Bay Group, whereas the number of craton-related ages diminishes in this direction. A common East Antarctic source area is indicated for all analyzed samples. The Priestley Formation was deposited on the Palaeo-Pacific passive continental margin of East Gondwana in the late Neoproterozoic after Rodinia breakup. The sequence was subsequently metamorphosed and intruded by the Granite Harbour Intrusives during the Ross Orogeny. The Molar Formation of the Bowers Terrane is interpreted as a turbiditic sequence deposited in an accretionary setting on the active continental margin in the Late Cambrian during and after accretion of the Glasgow island arc allochthon. The thick, homogeneous sequence of the Robert¬son Bay Group resulted from continuous turbiditic sedimentation in an accretionary wedge in front of the Ross Orogen after docking and imbrication of the Glasgow island arc in the Early Ordovician.
... Metamorphic rocks in the Lanterman and Salamander ranges are also probably correlative with the Wilson Group (Talarico et al., 1998). Regional metamorphism is variable from greenschist to granulite facies, and in many areas these rocks are intimately mixed with granitoids and migmatites of the Granite Harbour batholith (Babcock et al., 1986;Schüssler et al., 1999Schüssler et al., , 2004. Despite variable and locally high-grade metamorphism, available geochronologic evidence shows that the Wilson terrane does not represent primary Archean or Paleoproterozoic crust of the East Antarctic shield (westward, the first such rocks occur in the Terre Adélie craton of Wilkes Land), but rather is a composite metasedimentary assemblage of latest Neoproterozoic to Cambrian age. ...
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The Transantarctic Mountains (TAM) are one of Earth's great mountain belts and are a fundamental physiographic feature of Antarctica. They are continental-scale, traverse a wide range of latitudes, have high relief, contain a significant proportion of exposed rock on the continent, and represent a major arc of environmental and geological transition. Although the modern physiography is largely of Cenozoic origin, this major feature has persisted for hundreds of millions of years since the Neoproterozoic to the modern. Its mere existence as the planet's longest intraplate mountain belt at the transition between a thick stable craton in East Antarctica and a large extensional province in West Antarctica is a continuing enigma. The early and more cryptic tectonic evolution of the TAM includes Mesoarchean and Paleoproterozoic crust formation as part of the Columbia supercontinent, followed by Neoproterozoic rift separation from Laurentia during breakup of Rodinia. Development of an Andean-style Gondwana convergent margin resulted in a long-lived Ross orogenic cycle from the late Neoproterozoic to the early Paleozoic, succeeded by crustal stabilization and widespread denudation during early Gondwana time, and intra-cratonic and foreland-basin sedimentation during late Paleozoic and early Mesozoic development of Pangea. Voluminous mafic volcanism, sill emplacement, and layered igneous intrusion are a primary signature of hotspot-influenced Jurassic extension during Gondwana breakup. The most recent phase of TAM evolution involved tectonic uplift and exhumation related to Cenozoic extension at the inboard edge of the West Antarctic Rift System, accompanied by Neogene to modern glaciation and volcanism related to the McMurdo alkaline volcanic province. Despite the remote location and relative inaccessibility of the TAM, its underlying varied and diachronous geology provides important clues for reconstructing past supercontinents and influences the modern flow patterns of both ice and atmospheric circulation, signifying that the TAM have both continental and global importance through time.
... The Wilson Terrane is made up of variably metamorphosed (greenschist to granulite facies) rocks that are intruded by plutonic and volcanic rocks of the Granite Harbour suite (GREW et al. 1984). Isotopic data points to a Cambrian to Early Ordovician age (520-480 Ma) for the timing of magmatism, high-grade metamorphism and deformation (BORG et al. 1987, SCHÜSSLER et al. 2004). The Lanterman Fault Zone defines the eastern margin of the Wilson Terrane and its contact with the Bowers Terrane ( Fig. 1). ...
Article
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Rocks of the Millen Schists were analysed during GANOVEX X (2009/10) to evaluate the nature of the contact between the Ross-age Bowers and Robertson Bay Terranes in northern Victoria Land. The majority of this work was carried out in proximity to the Millen Thrust System, a major structure that separates the whole Millen Shear Belt into two overlying tec tonic units. The Millen Shear Belt has been widely acknowledged to repre sent the tectonic contact between the two terranes. Lithological similarities between the rocks in the hanging wall and footwall of the Millen Thrust Sys tem and those located in the Bowers and Robertson Bay Terranes support this suggestion. The structural history of the Millen Schists can be divided into three stages: (i) formation of isoclinal folds and pervasive S1 foliation that largely parallels bedding S0; (ii) upright D2 folding along northwest-south east axes and (iii) localised D3 high-strain that was dominantly related to reverse transport along the Millen Thrust System. Interpretations based on field observations and the available geochronological data supports a model where: (i) sub-horizontal northeast-southwest directed pure shear shortened the juxtaposed (by the late Cambrian) Bowers and Robertson Bay terranes; (ii) strain localisation along the Millen Thrust System resulted in the devel opment of a complex finite strain pattern in the Millen Schists, which records evidence of dominant northeast directed reverse transport with minor lateral displacement.
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Classic tectonic models interpret the Ross-orogenic structural edifice of northern Victoria Land in terms of three terranes, which formed due to westward subduction of the Palaeo-Pacific Ocean underneath the East Gondwana active continental margin (Kleinschmidt & Tessensohn 1987). These comprise from west to east the Wilson Terrane (low- to high-grade metamorphic rocks and voluminous granitoids), the Bowers Terrane (very low- to low-grade metamorphic island arc-type volcanic and sedimentary rocks), and the Robertson Bay Terrane (very low- to low-grade turbidites). U-Pb ages of detrital zircons separated from meta-sedimentary rocks of these three terranes were dated by LA-ICP-MS in order to get information on provenance and age of the rocks. We analyzed 4 samples from the Priestley Formation (Wilson Terrane), 3 samples from the Molar Formation (Bowers Terrane), 2 samples from the Millen Schists along the Bowers-Robertson Bay terrane boundary, and finally 3 samples from the Robertson Bay turbidites. The youngest concordant zircon ages of the Priestley Formation were around ca. 592 Ma, which is interpreted as a maximum age of deposition of the sediments. Ross-orogenic maximum ages were derived from the Molar Formation (ca. 495 Ma), the Millen Schists (ca. 485 Ma), and the Robertson Bay turbidites (ca. 470 Ma). The late Cambrian maximum age of the Molar Formation does not conflict with the middle Cambrian age of fossils that occur in exotic limestone blocks (Wolfart 1994). The following zircon-age groups are present in the samples with different percentages: a) 3500–1600 Ma: Archean-Palaeoproterozoic cratonic areas; b) 1300–900 Ma: Grenville Orogen; c) 700–580 Ma: Pan-African collision belts or pre-Ross-orogenic rifted-margin of East Gond-wana; d) 530–470 Ma (except Priestley Formation): Ross Orogeny. These results imply: (1) the existence of Grenville-age and Pan-African mobile zones in the ice-covered hinterland of the modern Transantarctic Mountains, and (2) a common East Antarctic source area for all analyzed formations. The Priestley Formation was deposited on the passive continental margin of East Gondwana in late Neopro¬tero¬zoic times, but before the onset of Ross-orogenic subduction of the Palaeo-Pacific Ocean. It was metamorphosed in Ross-orogenic times and intruded by syn- to late-orogenic granitoids (Granite Harbour Intrusives). The Bowers Terrane formed in an intra-oceanic island arc setting in the Cambrian and was accreted to the Ross-age active continental margin not later than ca. 495 Ma. The Robertson Bay turbidites formed in an accretionary setting in front of the now combined Wilson-Bowers terrane active continental margin of East Gondwana due to on-going westward subduction of the Palaeo-Pacific Ocean (e.g., Roland et al. 2004). References: Kleinschmidt, G. & Tessensohn, F. (1987): Early Paleozoic westward directed subduction at the Pacific continental margin of Antarctica. – In: McKenzie, G. (ed.): Gondwana Six. AGU Geophys. Monogr., 40: 89-105. Wolfart, R. (1994): Middle Cambrian Faunas, North Victoria Land, Antarctica. – Geol. Jb., B 84: 1-164; Hannover. Roland, N.W., Läufer, A.L. & Rossetti, F. (2004): Revision of the Terrane Model of Northern Victoria Land (Antarctica). – Terra Antarctica, 11: 55-65; Siena.
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The early Palaeozoic Ross Orogen in northern Victoria Land, Antarctica, consists of three major fault-bounded tectonostratigraphic terranes. Their true nature, far-travelled or local part of the accretionary collage, is under discussion. The inboard Wilson terrane (WT) consists mainly of high-grade metamorphic rocks intruded by the calc-alkaline magmatic arc of the Ross Orogen. The terrane nature of the WT is doubtful, as it appears more like the leading edge of the East Antarctic craton. The Bowers terrane (BT) comprises a mixed sedimentary-volcanic succession, beginning with volcanic rocks of island-arc character, followed by turbidites, mudstones, conglomerates and fossiliferous Middle Cambrian shallow-water sediments. The whole sequence is capped by a fluvial to deltaic quartzitic series several kilometres thick, with strong continental affinity. The combination of primitive forearc to back-arc volcanics at the bottom and mature continental sediments at the top poses a problem. The outer Robertson Bay terrane (RBT) is made up of a thick turbidite succession which, in one area, contains allochthonous blocks of fossiliferous Tremadocian limestones. All terrane boundaries appear to be distinct fault zones. The WT/BT boundary forms a deep-reaching continent-ocean suture associated with strongly sheared rock units, ultramafic lenses and high-pressure rocks. Coesite in eclogites of the Lanterman Range indicates a depth of burial of around 90 km. A greenschist-facies schist belt marks the BT/RBT boundary. The terranes contain evidence for subduction at an active margin setting as well as for accretion processes along major faults. The present changes of the Cambrian time-scale, such as younging of the base of the Upper Cambrian by about 30 Ma since the 1980s, allow separation of arc formation and later terrane accretion events.
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A detailed in situ isotopic (U–Pb, Lu–Hf) and geochemical study of zircon populations in a composite sequence of foliated to massive Cambro-Ordovician intrusions in the Deep Freeze Range (North Victoria Land, Antarctica), has highlighted great complexity in zircon systematics. Zircons in deformed granitoids and tonalites display complex internal textures, a wide spread of concordant U–Pb ages (between 522 and 435 Ma) and unusual trace-element compositions (anomalous enrichment of light rare earth elements, U, Th and Y) within single zircon grains. In contrast, zircons from undeformed samples display a limited range of U–Pb ages and trace-element compositions. Zircons from all age and textural populations in most of the deformed and undeformed samples show a relatively narrow range of &epsiv; Hf values, suggesting that the Lu–Hf system remained undisturbed. Inferred emplacement ages cover a time interval of about 30 Myr: from 508 to 493 Ma for the oldest strongly foliated synkinematic Howard Peaks megacrystic monzogranites and high-K calc-alkaline mafic to intermediate rocks of the ‘Corner Tonalite’ unit; from about 489 to 481 Ma for the younger massive shoshonitic mafic dyke suite and the high-K calc-alkaline Keinath granite. The observed isotopic and chemical variations in zircon are attributed to a sub-solidus recrystallization under hydrous conditions and varying temperature, in a setting characterized by a transpressional to extensional stress regime.
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Granulite-facies rocks occur as mappable relics in the Wilson Terrane (northern Victoria Land), the lithotectonic unit of the Cambro-Ordovician Ross orogen closest to the East Antarctic craton. Despite the widespread amphibolite-facies overprinting of Ross age, large-scale low-strain tectonic lozenges preserve a layered unit of felsic and quartz-poor gar-net + orthopyroxene f cordierite metasedimentary granulite, with minor, metre-thick, layers of mafic two-pyroxene granulite and rare lenses of marble. Large bodies of massive enderbite are also present and locally show discordant, intrusive contacts with respect to the layered metasedimentary sequence. Mineral assemblages, reaction textures and geothermobarometric estimates in granulite rocks point to a pre-Ross decompres-sional evolution from higher-pressure (P = 7.7 + 0.7 kbar at T= 820 + 100°C) to lower-pressure (6.3 + 0.4 kbar and 830 & 50°C) granulite-facies conditions. Geological and petrological data suggest that the granulite-facies rocks of the Wilson Terrane form a distinct tectono-metamorphic unit very similar to other Neoproterozoic granulite-facies terrains of the East Antarctic craton. In this aspect, the occurrence of granulite-rocks within the Transantarctic Mountains strongly suggests the reactivation of the palaeo-Pacific margin of East Antarctica during the Ross orogeny.
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The high-grade assemblage Cd-Ga-Si-Qz can be thermodynamically modelled from available calorimetric data on the metastable reaction:$$3 MgCd \rightleftarrows 2 Py + 4 Si + 5 Qz$$ (I) naturalK DFe-Mg between garnet and cordierite and experimental results on cordierite hydration. In the system SiO2-Al2O3-MgO-H2O, reaction (I) becomes$$3 MgCd \cdot nH_2 O \rightleftarrows 2 Py + 4 Si + 5 Qz + 3 nH_2 O$$ (II). However, hydrous cordierite is neither a hydrate nor a solid solution between water and anhydrous cordierite and when nH2O (number of moles of H2O in Cd) is plotted against\(P_{H_2 O} \), the resulting isotherms are similar to adsorption isotherms characteristic of zeolitic minerals. Reaction (II) can thus be considered as a combination of reaction (I) with a physical equilibrium of the type nH2O (in Cd)⇆nH2O (in vapor phase). Starting from a point on equilibrium (I), introduction of H2O into anhydrous Mg-cordierite lowers the chemical potential of MgCd and hence stabilizes this mineral to higher pressure relative to the right-hand assemblage in reaction (I). The pressure increment of stabilization,ΔP, above the pressure limit of anhydrous cordierite stability at constantT, has been calculated using the standard thermodynamics of adsorption isotherms. Cordierite is regarded as a mixture of Mg2Al4Si5O18 and H2O. The activity of H2O in the cordierite is evaluated relative to an hypothetical standard state extrapolated from infinite H2O dilution, by using measured hydration data. The activity of Mg2Al4Si5O18 in the cordierite is then obtained by integration of the Gibbs-Duhem equation, and the pressure stabilization increment,ΔP, computed by means of the relation:$$\Delta V_s \Delta P \cong - RT\ln a_{MgCd}^{MgCd \cdot nH2O} \left( {\Delta V indepentdent of P and T} \right)$$. Thus, one may contour theP-T plane in isopleths of nH2O=constant within the area of Mg-cordierite stability allowed by the hydration data for\(P_{H_2 O} = P_{total} \). The present model indicates greater stabilization of cordierite by H2O than the model of Newton and Wood (1979) and the calculated curve for metastable breakdown of hydrous MgCd is consistent with experimental data on cordierite breakdown at\(P_{H_2 O} = P_{total} \). Mg/(Mg+Fe) isopleths have been derived for cordierites of varying nH2O in the SiO2-Al2O3-MgO-FeO-H2O system using the additional assumptions that (Fe, Mg) cordierite and (Fe, Mg) garnet behave as ideal solutions, and that typical values of the distribution coefficient of Fe and Mg between coexisting garnet and cordierite observed in natural parageneses can be applied to the calculations. The calculated stable breakdown curve of Fe-cordierite under conditions of\(P_{H_2 O} = P_{total} \) to almandine, sillimanite, quartz and vapor has a positive slope (dP/dT) apparently in contrast to the experimental negative slope. This may be explained by hydration kinetics, which could have allowed systematic breakdown of cordierites of metastable hydration states in the experiments. The bivariant Cd-Ga fields calibrated from the present model are, potentially, good petrogenetic indicators, as, given the iron-magnesium ratio of garnet and cordierite and the hydration number of cordierite, the temperature, pressure and water fugacity are uniquely determined. As this geothermobarometer is in part based on the water content of cordierite, it can be used only if there is some assurance that the actual hydration is inherited from high-grade metamorphic conditions. Such conditions could be realised if the slope of unloading-cooling retrograde metamorphism is more or less parallel to a cordierite isohydron.