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This paper reports the metamorphic texture of cordierite megacrysts and the metamorphic P–T path of a newly exposed section of gneiss in East Antarctica. We used mineral textures and pseudosection modeling to reconstruct the metamorphic P–T path of cordierite- and spinel–garnet-bearing gneisses from Botnnuten, an isolated nunatak located ~ 60 km from the southern edge of Lützow-Holm Bay in East Antarctica. The gneisses underwent low-P granulite-facies metamorphism at 5.0–6.1 kbar and 850 ± 20 °C followed by isobaric cooling. The isobaric cooling path implies long residence in the middle to shallow crustal level without rapid exhumation. This contrasts with the widely recognized clockwise P–T path of basement rocks of the Lützow-Holm Complex. The rocks at Botnnuten have long been considered part of the Lützow-Holm Complex based on their petrographical features and geothermobarometric data. However, the present results, combined with a reevaluation of available data, indicate the metamorphic history of the Botnnuten gneisses is more comparable to that of the Yamato Mountains, located southwest of the study area. [ABSTRACT FROM AUTHOR]

Fluid infiltration into Proterozoic and Early Palaeozoic dry, orthopyroxene-bearing granitoids and gneisses in Dronning Maud Land, Antarctica, has caused changes to rock appearance, mineralogy, and rock chemistry. The main mineralogical changes are the replacement of orthopyroxene by hornblende and biotite, ilmenite by titanite, and various changes in feldspar structure and composition. Geochemically, these processes resulted in general gains of Si, mostly of Al, and marginally of K and Na but losses of Fe, Mg, Ti, Ca, and P. The isotopic oxygen composition (δ18OSMOW = 6.0‰–9.9‰) is in accordance with that of the magmatic precursor, both for the host rock and infiltrating fluid. U-Pb isotopes in zircon of the altered and unaltered syenite to quartz-monzonite indicate a primary crystallization age of 520.2 ± 1.0 Ma, while titanite defines alteration at 485.5 ± 1.4 Ma. Two sets of gneiss samples yield a Rb-Sr age of 517 ± 6 Ma and a Sm-Nd age of 536 ± 23 Ma. The initial Sr and Nd isotopic ratios suggest derivation of the gneisses from a relatively juvenile source but with a very strong metasomatic effect that introduced radiogenic Sr into the system. The granitoid data indicate instead a derivation from Mid-Proterozoic crust, probably with additions of mantle components. [ABSTRACT FROM AUTHOR]

This paper reports geochemical characteristics of mafic gneisses and granulites collected from four localities in central Dronning Maud Land to evaluate the tectonic setting of their precursor rocks. Precursor rocks for the mafic gneisses and granulites in central Dronning Maud Land were formed in different geological backgrounds and tectonic settings. The mafic gneisses and granulites in the Schirmacher Hills were derived from basaltic rocks in a back-arc setting. Published U–Pb zircon ages and the geochemical variation of the basement rocks indicate their emplacement between 800 and 650 Ma, close to peak metamorphism. While similar protoliths were recognized in the inland nunataks of Hochlinfjellet, in Filchnerfjella the mafic gneisses/granulites were derived from basaltic rocks formed in marginal continental arcs or island arcs. Highly disturbed trace-element patterns indicate that the metamorphic process influenced the geochemical composition during the prograde metamorphic stage. Our results imply that the outcrops in central Dronning Maud Land with different metamorphic ages contain mafic gneisses/granulites from precursor rocks formed under different tectonic settings. [ABSTRACT FROM AUTHOR]

Tafoni are a type of cavernous weathering that is found in a variety of rock types and locations around the world. Tafoni have been documented in a number of climatic zones ranging from hot and cold deserts to moist coastal environments. Despite the widespread distribution of tafoni, the major processes controlling tafoni weathering are not well understood and are still a matter of discussion. This study addresses the frequent distribution of well-developed tafoni in the cold, arid environment of the inland mountain range of central Dronning Maud Land, Antarctica. The aim is to document and characterize the nature of tafoni present in Gjelsvikfjella (2°E) eastward to Filchnerfjella (8°E) and to discuss formation processes. The cavities occur in groups and are typically spherical to oval shaped. They range in diameter and depth from 1 dm up to 1.5 m. The cold, arid environment of this region favors mechanical weathering mechanisms such as freeze-thaw actions and wind abrasion. Furthermore, the structural, textural, and mineralogical properties of the parent rock can potentially have a strong control on weathering and cavity development. Observed tafoni are typically formed in massive granitoid intrusives and granitic gneisses and migmatites. Chemical dissolution of pyroxene to iddingsite and radiation from rare earth element–bearing accessory minerals cause microfracturing, which facilitates freeze-thaw actions and accordingly enhances the weathering. [ABSTRACT FROM AUTHOR]

• The discovery of exceptionally low oxygen values (δ18O < 2.7 ‰) in primary magmatic zircon from 3.75 to 3.50 Ga. • This finding indicates the existence of shallow magmatic-hydrothermal systems driven by meteoric water in the early Archean. • The study provides evidence for the emergence of land by 3.7 Ga. The question as to whether there was emergent land in the Eoarchean (4.0–3.6 Ga) is fundamental not only for understanding Earth's evolution but also for life itself. Magmatic rocks with isotopically light oxygen can indicate interaction of magmas or their sources with surface water (e.g. meteoric water). Consequently, the presence of such rocks in the geological record of the early Earth can provide an indicator for the emergence of land. Zircon from two Eoarchean orthogneisses in the Tula Mountains, Napier Complex, East Antarctica, show such light isotopic signatures. A ca. 3.75 Ga trondhjemitic gneiss and a ca. 3.55 Ga dioritic gneiss, both with high Y-HREE-Nb-Ta that can be ascribed to the melting of shallow sources at < 1.0 GPa, contain zircon with exceptionally low δ18O values of 1.0–2.7 ‰ (normalized to Vienna Standard Mean Ocean Water). The lowest δ18O value in zircon previously reported from Paleoarchean orthogneisses is 3.7 ‰ at 3.56 Ga. We show that although the generation of such isotopically light signatures can be achieved through mixing with seawater during a hydrothermal event, the proportion of oxygen from seawater would have to be exceptionally high. In contrast, only a small and more realistic proportion of water with much lighter δ18O, such as that characteristic of meteoric water would be required. These new results, therefore, are consistent with the presence of shallow magmatic or hydrothermal systems involving meteoric water at 3.73 Ga, providing the earliest known evidence for the emergence of land on Earth. [ABSTRACT FROM AUTHOR]

The gneisses outcropping in the Larsemann Hills of East Antarctica are ideal for studying the composition and evolution of the Prydz tectonic belt. In this study, detailed petrological analyses, phase equilibrium modelling, and zircon analyses were performed on a felsic orthogneiss from the Broknes Peninsula in the Larsemann Hills. The study results show that the protolith of the felsic gneiss is granite, which is further identified to have been an S-type granitoid. The felsic orthogneiss intruded into the basement complex at ~976 Ma. The magmatic zircons in the felsic orthogneiss have lower LREE and higher HREE contents than those of the metamorphic zircons in the same rock. The felsic gneiss consists of the mineral assemblage garnet +ilmenite+ plagioclase +K-feldspar + sillimanite + quartz and underwent granulite facies metamorphism in the Grenvillian period (～899 Ma), with peak conditions of ～870°C and ～9.5 kbar. The Larsemann Hills should be categorized as the orogenic process from arc-continent collision to continent-continent collision during the Grenvillian period, crust thickening(~899 Ma) and subsequent collapse(550–500 Ma). [ABSTRACT FROM AUTHOR]

We have summarized the results of studies of bottom rock material in Quaternary sediments in the polar seas of the Arctic, North Atlantic, and Antarctica, obtained by sieve washing on expeditions of the Shirshov Institute of Oceanology, Russian Academy of Sciences. We have compiled maps of the quantitative distribution of this material over water areas and created pie charts of the petrographic composition for various sites. A wide variety of petrographic types of rocks has been revealed, caused by their transport by glaciers, icebergs, and ice from glaciation centers on continents and islands. We have identified petrographic provinces based on the composition of the rock material and determined its sources. At the same time, great importance has been given to searching for marker rocks, making it possible to correlate the rock material of the seas with their land sources. [ABSTRACT FROM AUTHOR]

The north‐eastern part of the Chotanagpur Granite Gneiss Complex (CGGC) in the East Indian shield contains enclaves of migmatitic pelitic granulites (PG) within felsic orthogneiss (FOG). Field observations, petrology and geochronology (LA–MC–ICP–MS U–Pb dating of zircon and EPMA Th–U–total‐Pb dating of monazite) of the PG suggest two distinct metamorphic events. The earliest event M1, which is characterized by high‐temperature (>850°C) granulite facies metamorphism, occurred in the timespan of ~1680–1580 Ma. Extensive dehydration melting of biotite + sillimanite + quartz‐rich protoliths led to stabilization of the restitic assemblage (garnet + alkali‐feldspar + quartz + sillimanite + ferrian‐ilmenite) together with large volumes of felsic melts (leucosomes). Collisional tectonics followed by delamination and asthenospheric upwelling could have triggered the M1 event. Subsequently, at ~1470–1400 Ma, the igneous protolith of the host FOG intruded and hydrated the PG. Thereafter, a second metamorphic event, M2, accompanied by compressional structures, affected both the rock types. A clockwise P–T path that culminated at ≥10 kbar ~760–850°C and is followed by a steeply decompressive retrograde path characterizes this event. The P–T path and the inferred geothermal gradient (<27°C/km) are compatible with a continent–continent collisional setting. Geochronological findings suggest a protracted orogeny for the M2 event with its major pulse during ~970–950 Ma. When combined with the published information, this study supports the view that a large (if not the entire) portion of the Indian shield and the granulite terranes of east Antarctica share similar tectonothermal events that led to the formation of two supercontinents, Columbia and Rodinia. [ABSTRACT FROM AUTHOR]

In situ analysis of a garnet porphyroblast from a granulite facies gneiss from Sør Rondane Mountains, East Antarctica, reveals discontinuous step‐wise zoning in phosphorus and large δ18O variations from the phosphorus‐rich core to the phosphorus‐poor rim. The gradually decreasing profile of oxygen isotope from the core (δ18O = ~15‰) to the rim (δ18O = ~11‰) suggests that the 18O/16O zoning was originally step‐wise, and modified by diffusion after the garnet rim formation at ~800°C and 0.8 GPa. Fitting of the 18O/16O data to the diffusion equation constrains a duration of the high‐T event (~800°C) to c. 0.5–40 Ma after the garnet rim formation. The low δ18O value of the garnet rim, together with the previously reported low δ18O values in metacarbonates, indicates regional infiltration, probably along a detachment fault, of low δ18O fluid/melt possibly derived from meta‐mafic to ultramafic rocks. [ABSTRACT FROM AUTHOR]

Petrology, geothermobarometry, and phase equilibrium modelling of garnetiferous felsic gneiss from Grovnes peninsula in the Larsemann Hills of Prydz Bay, East Antarctica provide pristine evidence for the preservation of high-grade metamorphic imprint in the area. The metamorphic evolution of the sample is demonstrated by the development of the assemblage Grt+Bt+Melt+Pl+Sill+Kfs+Qtz+Ilm at peak metamorphic conditions of ∼790 °C and ∼7.5 kbar, which subsequently underwent retrogression and cooling to lower P-T conditions along a clockwise path. Texturally constrained chemical dating of monazites constrain the timing of peak metamorphism and garnet formation at ∼575 Ma, whereas the apatite U–Pb ages constrain cooling ages at ∼518 Ma. The clockwise P˗T˗t trajectory of the studied samples, together with the Ediacaran-Cambrian metamorphic/cooling ages demonstrate the long-lived nature of metamorphism in Prydz Bay, which is ascribed to collisional tectonism prevalent during the final stages of the assembly of East Gondwana supercontinent. Similar results from adjacent continental fragments including Sri Lanka, Eastern Ghats Belt, Madagascar, and South India suggest their coeval metamorphic evolution during the East African orogeny. • Clockwise P-T evolution of high-T garnetiferous felsic gneiss from Larsemann Hills. • Timing of peak metamorphism and cooling constrained at ∼575 Ma and ∼518 Ma. • Ediacaran-Cambrian metamorphism in Larsemann Hills and its tectonic significance. [ABSTRACT FROM AUTHOR]

Mesoproterozoic magmatism has contributed extensively to the concept of a Rodinian Supercontinent with Sverdrupfjella, Dronning Maud Land, East Antarctica occupying a central position in a magmatic belt extending from Sri Lanka in the east to Namaqualand in the west. Age constraints based on U Pb zircon data, combined with Sr, Nd and Hf data, from western Sverdrupfjella, are sparse with a new Mesoproterozoic dataset presented here. The Jutulrøra and Roerkulten granites in the Maud Terrane, have ages of ca. 1085 Ma, ca. 55 Myr younger than the ca. 1140 Ma arc-related tonalitic Jutulrøra Gneiss they intrude. Unlike the Jutulrøra Gneiss, which has Paleoproterozoic-Archaean Nd T DM ages, the Jutulrøra and Roerkulten granites have relatively juvenile Nd T DM ages, Hf T DM ages and ε Nd 1080Ma and Sr i 1080Ma characteristics comparable to the on-craton ca. 1108 Ma dolerite sills of the Borgmassivet Suite, correlated with the Umkondo LIP of southern Africa. The Brekkerista Granite, located ca. 100 km E of the Grunehogna Craton in the Maud Terrane, has an Archaean age of ca. 2850 Ma and has Archaean Nd T Dm and Hf 2 stage T Dm zircon ages of ca. 3.2 and 3.4 Ga respectively. The ages and chemistry of the three granites, previously assumed to be Mesoproterozoic in age and related to the continental arc setting of the Maud Belt, confirm Archaean crust underlying the Maud Belt in Sverdrupfjella and also suggest the 1085 Ma granite magmatism in the Maud Belt may be either be related to the intracratonic ca. 1108 Ma Umkondo LIP, or marginally younger ca. 1090 Ma A-type magmatism, rather than the older ca. 1140 Ma arc orogenic accretionary setting previously inferred. The data are discussed with reference recent correlations between the timing and proximity of Laurentia to the Kalahari Craton. In paleogeographic reconstructions, the ca. 2850 Ma Brekkerista Granite is similar to basement gneiss from Pietersburg province, northern Kaapvaal Craton, southern Africa. In contrast the ca. 3100 Ma Annandagstoppane Granite, ca. 200 km WSW of Sverdrupfjella. underlying the Grunehogna Craton, is comparable to Barberton province granites, southeastern Kaapvaal Craton. • Newly discovered Archean basement to the Maud Terrane. • Possible link between the Mkondo LIP Province and granites in the Maud Terrane. • Correllation between Grunehogna and Kaapvaal Cratons [ABSTRACT FROM AUTHOR]

In Marie Byrd Land, West Antarctica, the Fosdick Mountains migmatite-cored gneiss dome was exhumed from mid- to lower middle crustal depths during the incipient stage of the West Antarctic Rift system in the mid-Cretaceous. Prior to and during exhumation, major crustal melting and deformation included transfer and emplacement of voluminous granitic material and numerous intrusions of mantle-derived diorite in dikes. A succession of melt- and magma-related structures formed at temperatures in excess of 665 ± 50 °C based on Ti-in-zircon thermometry. These record a transition from wrench to oblique extensional deformation that culminated in the development of the oblique South Fosdick Detachment zone. Solid-state fabrics within the detachment zone and overprinting brittle structures record translation of the detachment zone and dome to shallow levels. To determine the duration of exhumation and cooling, we sampled granite and gneisses at high spatial resolution for U–Pb zircon geochronology and 40 Ar/ 39 Ar hornblende and biotite thermochronology. U–Pb zircon crystallization ages for the youngest granites are 102 Ma. Three hornblende ages are 103 to 100 Ma and 12 biotite ages are 101 to 99 Ma. All overlap within uncertainty. The coincidence of zircon crystallization ages with 40 Ar/ 39 Ar cooling ages indicates cooling rates > 100 °C/m.y. that, when considered together with overprinting structures, indicates rapid exhumation of granite and migmatite from deep to shallow crustal levels within a transcurrent setting. Orientations of structures and age-constrained crosscutting relationships indicate counterclockwise rotation of stretching axes from oblique extension into nearly orthogonal extension with respect to the Marie Byrd Land margin. The rotation may be a result of localized extension arising from unroofing and arching of the Fosdick dome, extensional opening within a pull-apart zone, or changes in plate boundary configuration. The rapid tectonic and temperature evolution of the Fosdick Mountains dome lends support to recently developed numerical models of crustal flow and cooling in orogenic crust undergoing extension/transtension, and accords with numerous studies of migmatite-cored gneiss domes in transcurrent settings. [ABSTRACT FROM AUTHOR]

Low or medium spatial resolution satellite images are used for environmental monitoring in remote, extremely cold areas such as Antarctica. However, they cannot provide detailed spatial and spectral information over Antarctic areas. For obtaining this information, we propose an Antarctic land-cover classification method using IKONOS and Hyperion data over Terra Nova Bay, Antarctica. High spatial resolution IKONOS imagery enabled the detection of detailed, accurate boundaries between areas of snow, ice, vegetation, water and rock. Rock types were classified in detail using high spectral resolution Hyperion imagery. Because Antarctic land-cover types exhibit unusual spectral features, a step-by-step classification using various data sets and methods was applied. Five soil types and two rock types were identified by applying spectral linear unmixing to Hyperion data. Our land-cover classification map shows small mossy areas near emperor penguin or skua habitat. Lakes, which are an important factor in environmental monitoring, are also clearly shown. The detailed rock and soil classification map revealed that biotite gneiss and clay soils are dominant. This Antarctic land-cover map is useful for determining a suitable site for an environment-friendly Antarctic station. Among the various land-cover types, the lakes and the vegetation, rock and soil areas are considered to be important environmental factors related to Antarctic living organisms. [ABSTRACT FROM AUTHOR]

Abstract: U–Pb zircon geochronology of 6 samples collected from Fyfe Hills (2 felsic orthogneisses and 1 quartzo-feldspathic gneiss) and Mt. Cronus (1 quartzo-feldspathic gneiss and 2 quartzites) in the western part of the Napier Complex, East Antarctica, revealed an Archaean–Proterozoic history ranging from 3128 to 1824Ma. The absence of ancient evidence >3800–3600Ma indicates that these ages were not always dominant in these two areas. The 3128–2580Ma protolith ages are similar to those of other areas (e.g., Mt. Riiser-Larsen, Tonagh Island), while the magmatic protolith age of 2741Ma obtained for the Fyfe Hills orthogneiss is a new component of the Archaean magmatic protolith in the Napier Complex. Metamorphic overgrowth or recrystallized zircons yielded systematic disconformable ages for both Fyfe Hills (>2518Ma) and Mt. Cronus (~2490Ma). The presence of 2580Ma inherited zircons provides important constraints that place the ultrahigh temperature metamorphic peak later than 2580Ma. The post metamorphic ages suggest local fluid infiltration or local deformation event(s). [Copyright &y& Elsevier]

The article presents a study on the age of charnokitic magmatism from Obruchev Hills, Bunger Hills-Denman Glacier area in the East Antarctica region of Antarctica. It says that the reconstruction of geological history of the Bunger Hills-Denman Glacier area and its correlation with processes in other areas are vital to understand East Antarctic Shield evolution in the Precambrian. It states that the fragment of the Obruchev Hills comprises mainly of foliated felsic orthogneisses with subordinate mafic gneiss layers. It mentions that the foliated mafic and felsic orthogneisses unit is intruded by orthopyroxene-bearing granites known as charnokites. It adds that the Concordia age is interpreted to date the time of the crystallization of charnokites.

The Ross Sea is bordered by the Late Precambrian–Cambrian Ross–Delamerian Orogen of East Antarctica and the more Pacific-ward Ordovician–Silurian Lachlan–Tuhua–Robertson Bay–Swanson Orogen. A calcsilicate gneiss from Deep Sea Drilling Project 270 drill hole in the central Ross Sea, Antarctica, gives a U-Pb titanite age of 437 ± 6 Ma (2σ). This age of high-grade metamorphism is too young for typical Ross Orogen. Based on this age, and on lithology, we propose a provisional correlation with the Early Palaeozoic Lachlan–Tuhua–Robertson Bay–Swanson Orogen, and possibly the Bowers Terrane of northern Victoria Land. A metamorphosed porphyritic rhyolite dredged from the Iselin Bank, northern Ross Sea, gives a U-Pb zircon age of 545 ± 32 Ma (2σ). The U-Pb age, petrochemistry, Ar-Ar K-feldspar dating, and Sr and Nd isotopic ratios indicate a correlation with Late Proterozoic–Cambrian igneous protoliths of the Ross Orogen. If the Iselin Bank rhyolite is not ice-rafted debris, then it represents a further intriguing occurrence of Ross basement found outside the main Ross–Delamerian Orogen. [ABSTRACT FROM AUTHOR]

Abstract: This is the first report of osumilite occurring as fine isolated inclusions within garnet porphyroblasts, as observed in garnet–sillimanite gneiss from Rundvågshetta, Lützow-Holm Complex, East Antarctica. The osumilite is characterized by high Si content (10.60 and 10.95 atoms based on 30 oxygens per formula unit), low Al content (2.99 and 3.82), a high content of M site-occupying cations (2.51 and 3.03), and high X Mg values (about 0.81). We also report a spinel+quartz association found as inclusions within garnet porphyroblasts. Spinel grains, which are in direct contact with quartz and are spatially associated with sillimanite, show extremely high Zn contents (X Zn Spl =0.33−0.46) and high X Mg values (0.45–0.54). The garnet is rimmed by sillimanite, K-feldspar, plagioclase, and quartz. Biotite and cordierite are found only as inclusions within garnet porphyroblasts, where biotite coexists with spinel–quartz or with rutile. Porphyroblastic garnet contains rutile needles and has low X Mg values (about 0.36). The sillimanite contains a high Fe content (about 1.2wt.% Fe2O3). The occurrence of osumilite and spinel+quartz indicates a clockwise pressure–temperature path of ultrahigh-temperature metamorphism, involving the following events: (1) the Rundvågshetta granulites suffered prograde metamorphism within the kyanite and sapphirine+quartz fields; (2) subsequent retrograde metamorphism, involving near-isothermal decompression, occurred in the orthopyroxene+sillimanite+quartz field; (3) the granulites passed through the garnet+cordierite+sillimanite+quartz field during decreasing temperature; (4) the granulites entered the osumilite stability field at around 8kbar and 950°C; and (5) the granulites retain a final record of retrograde metamorphism within the biotite+sillimanite+K-feldspar and quartz field at 6.1kbar and about 830°C. [Copyright &y& Elsevier]

We applied SHRIMP zircon U-Pb age dating to ultrahigh-temperature (UHT) sapphirine-bearing orthopyroxene garnet (SOG) granulite and biotite-hornblende (Bt-Hbl) gneiss in the Schirmacher Hills, East Antarctica. In the Bt-Hbl gneiss, concordant ages of 657 ± 7.1 and 807 ± 9.1 Ma were obtained from zircon overgrowth rims and zircon cores, with oscillatory and irregular zoning, respectively. The zircon overgrowth rims (657 ± 7.1 Ma) with low Th/U ratios from the Bt-Hbl gneiss are interpreted as having a metamorphic origin. Oscillatory-zoned and/or irregularly zoned zircon cores may have crystallized during an igneous event at 807 ± 9.1 Ma; 800-Ma igneous events have not previously been recognized in central Dronning Maud Land (DML) inland nunatak. Zircons in the SOG granulite yielded a concordant age of 643 ± 6.5 Ma, using analyses of sector-zoned and simple-zoned grains. These zircons have relatively high Th/U ratios with a narrow range, and they occur in association with garnet breaking down to form cordierite. The 643 ± 6.5-Ma age obtained from these zircons is interpreted as the timing of crystallization from a high-Th/U partial melt soon after peak metamorphism. The combination of a ca. 800-Ma igneous age and 660-640-Ma metamorphic ages obtained from Schirmacher Hills is different from that of other neighboring parts of central DML. In addition, a metamorphic PT path involving ultrahigh temperatures at early and subsequent isobaric cooling (IBC) stages at around 650 Ma has not previously been known in the central DML nunatak region. The Ca. 650-Ma UHT metamorphic event probably occurred in a back-arc tectonic setting and predates the main collisional event of central DML (ca. 550-500 Ma). [ABSTRACT FROM AUTHOR]

The upward transfer of partially molten crust and the formation of gneiss domes and metamorphic core complexes commonly take place by localization of normal or oblique extension in the middle and upper crust. In Marie Byrd Land, Antarctica, a transition from wrench to oblique extension occurred during oblique plate divergence along the East Gondwana margin and intracontinental crustal extension associated with the West Antarctic Rift System in mid-Cretaceous time. Migmatites in the Fosdick dome record steep fabrics formed during wrenching, and associated granite networks display crystallization ages of 117-115 Ma. These steep fabrics are overprinted by subhorizontal foliation and leucogranite sheets with crystallization ages in the 109-102 Ma range. Syntectonic emplacement of granite sheets in the South Fosdick detachment zone indicates that detachment tectonics led to rapid exhumation of the terrain by 100 Ma. This study has implications for understanding melt transport, magma accumulation, and the formation of detachments in an oblique tectonic setting. [ABSTRACT FROM AUTHOR]

A diorite from drillhole GDH 53 at Voktipur in the Dinajpur Block in NW Bangladesh records an age of 1686 ± 4 Ma, whereas a tonalite from drillhole GDH 54 at Barapaharpur has an age of 1641 ± 7 Ma. Conversely, charnockite and monzogranite gneiss from drillhole GDH 31 at Gaibandha record considerably younger ages of 1144 ± 10 and 1133 ± 16 Ma, respectively. The diorite-tonalite-granodiorite suite is metaluminous, calc-alkaline, magnesian, with weak to moderate negative Eu anomalies and I-type characteristics. This suite is interpreted to have been derived from the melting of heterogeneous mafic crust, followed by a moderate degree of fractionation in an arc setting, identical to the source for similar rocks reported elsewhere in the basement of Bangladesh. The charnockite-monzogranite gneissic rocks from Gaibandha are ferroan, display weak negative Eu anomalies, belong to the high K calc-alkaline series, and plot as A-type granitoids (A2 subtype). It is considered that asthenospheric upwelling in an extensional/post-collisional setting provided the heat required to generate these rocks at the base of thinned crust. The geochemical data, lithostratigraphy and age of 1.73–1.64 Ga for the diorite-tonalite-granodiorite suite in NW Bangladesh do not match rocks of the Chotanagpur Granite Gneiss Complex and Central Indian Tectonic Zone to the west in India, negating their association with the collision that united the North and South Indian blocks. However, the Shillong-Meghalaya Gneissic Complex in NE India, with ~1.78–1.62 Ga granite gneiss and 1.28–1.08 Ga charnockite, provides a closer analogue. Events at these two times have also been recorded within the east Gondwana fragments in the Bunger and Obruchev Hills of East Antarctica. The similarities with both the Shillong-Meghalaya Gneissic Complex, lying to the east in India, and with East Antarctica, suggest that Proterozoic magmatic events in the Dinajpur Block of NW Bangladesh relate to events in both the Columbia/Nuna and Rodinia supercontinents and support a connection with these supercontinents throughout the Proterozoic. [Display omitted] • Two felsic rock suites are identified at 1.68–1.64 and ~ 1.14 Ga in NW Bangladesh. • The felsic suites cannot be directly linked to either the CGGC or CITZ in India. • Granitoids in the SMGC in NE India offer a closer analogue to those of Bangladesh. • Coeval Proterozoic magmatism in East Antarctica, suggests a link to the Columbia/Nuna and Rodinia supercontinents. [ABSTRACT FROM AUTHOR]

Abstract: The North Mawson Escarpment, in Antarctica’s Prince Charles Mountains, forms part of a Palaeoproterozoic crustal complex which separates Archaean cratonic material to the south from Early Neoproterozoic and Early Palaeozoic metamorphic belts to the north. It consists of nappe-like masses of grey gneiss and metasupracrustal rocks that were subjected to repeated ductile deformation under upper amphibolite to lower granulite facies metamorphic conditions. In this paper, we report zircon U–Pb dating results that constrain the principal tectonothermal events to the periods 2490–2420Ma, 2180–2080Ma, 940–880Ma and 530–490Ma. The magmatic precursors of volumetrically important grey gneiss were produced, and reworked at least partly, during the 2490–2420Ma and 2180–2080Ma events. The superimposition of these events is clearly documented by U–Pb ages from several composite zircons. The formation of associated metasupracrustals, possibly in a passive margin setting, occurred between the 2490–2420Ma and 940–880Ma events. Both grey gneiss and metasupracrustals were subsequently reworked during the 940–880Ma and 530–490Ma events, corresponding to the superposition of both the Rayner and Prydz Belts onto Palaeoproterozoic lithosphere of the Lambert Complex. Transposition and peak metamorphism preceded the intrusion of leucogranite dykes at 905Ma and, based on metamorphic zircon growth at around 930Ma in metapelitic schist, are related to the Rayner orogeny. It is suggested that the North Mawson Escarpment was assembled to most coastal regions of the Antarctic sector between 45° and 80°E by at least 900m.y. ago. [Copyright &y& Elsevier]

High-grade gneisses from Mt. Riiser-Larsen, East Antarctica, have been dated by whole-rock-mineral Sm-Nd and SHRIMP zircon and monazite U-Pb to help define the thermal history of ultrahigh temperature (UHT) metamorphism in the Napier Complex. Both the monazite and youngest zircon yield a range of apparent ages (∼2.51-2.47 Ga), consistent with crystallization during an extended period of metamorphism. Some zircon also preserves an isotopic record of earlier events, placing an upper limit of a few million years on the duration of peak metamorphic conditions. The similarity of the monazite and zircon U-Pb ages implies rapid initial postpeak cooling to below the blocking temperature of these minerals (∼900°C). Consistently lower Sm-Nd whole-rock-mineral isochron ages (∼2.38 Ga) indicate that cooling slowed before the temperature reached ∼650°C. The history of the UHT metamorphism is interpreted to be (1) protracted high-temperature (≥800°C) conditions ∼2.51-2.47 Ga, (2) peak conditions (up to 1100°C) for at most a few million years, (3) rapid cooling (10°-60°C /m.yr.) immediately after peak metamorphism, and (4) very slow cooling (≤4°C/m.yr.) at midcrustal levels (∼30-km depth) to a steady state geotherm by 2.38 Ga. [ABSTRACT FROM AUTHOR]

Wagnerite-Ma5bc polytype is found in a gneiss associated with cordierite-prismatine-bearing gneiss, northern Stornes Peninsula, Larsemann Hills, East Antarctica. Wagnerite coexists with apatite and both of them are present as rock-forming minerals. Ca depletion is not essential to the formation of wagnerite; sufficient Mg, P and F components, and a relatively low Ca/Mg ratio is critical, otherwise only apatite crystallizes, Strong anatexis of the quartzofeldspathic gneiss of the area results in differentiation and separate enrichment of leuco- and melanocratic components. P and F tend to accumulate in the mafic-rich melanocratic component, and when their abundance reaches certain concentration, wagnerite forms. It is perhaps the chemistry of the setting that controls the appearance of wagnerite, whereas the PT condition is responsible for the variety of wagnerite polytype. A disordered polytype crystallizes at relatively high temperature, then cools down slowly and transforms into one of the ordered polytypes, in this case, the Ma5bc polytype. [ABSTRACT FROM AUTHOR]

Geochronological data, combined with field and petrological evidence, constrain the timing and rate of near-isothermal decompression at granulite facies temperatures in rocks from the Lützow-Holm Complex of East Antarctica. Granulite facies gneisses from Rundvågshetta in Lützow-Holm Bay experienced a peak metamorphic temperature of over 900 °C at c. 11 kbar, as evidenced by primary orthopyroxene–sillimanite-bearing assemblages, and secondary cordierite–sapphirine-bearing assemblages in metapelites. Peak metamorphic assemblages show strong preferred mineral orientation, interpreted to have developed synchronously with pervasive ductile deformation. Zircon from a syndeformational leucosome has a U–Pb age of 517±9 Ma, which is interpreted as a melt crystallization age. This age provides the best estimate of the time of peak metamorphic conditions. The post-peak metamorphic history is characterized by near-isothermal decompression, recorded by mineral textures in a variety of rock compositions. Field and textural relations indicate that decompression post-dated pervasive ductile deformation. K/Ar and 40Ar/39Ar ages from hornblende and biotite represent closure ages during cooling subsequent to decompression, and indicate cooling to temperatures between c. 350 and 300 °C by c. 500 Ma, thus placing a lower time limit on the duration of the high-temperature isothermal decompression episode. The combination of the zircon age from a syndeformational melt with K/Ar and 40Ar/39Ar closure ages indicates that near-isothermal decompression from c. 11 to c. 4 kbar at granulite facies temperatures, followed by cooling to c. 300 °C, took place within a time interval of 20±10 Myr. Simple one-dimensional models for exhumation-controlled cooling indicate that these data require exhumation rates of the order of c. 3 km Myr-1 for several million years, then cessation of exhumation followed by relatively isobaric cooling during thermal re-equilibration. [ABSTRACT FROM AUTHOR]

Proterozoic basement outcrops in the vicinity of Battye Glacier, northern Prince Charles Mountains, are dominated by granulites and gneisses derived from felsic (granitoid) intrusive igneous rocks, and by pegmatites. Felsic orthopyroxene granulites, garnet leucogneisses and garnet pegmatites have major and trace element compositions of highly felsic, but not strongly fractionated, granites. The garnet leucogneisses and garnet pegmatites have S-type characteristics, whereas the felsic granulites are probably I-type, although their high Zr+Nb+Y+Ce abundances suggest possible A-type affinities. Intermediate orthopyroxene ± clinopyroxene granulites mostly resemble I-type quartz diorites, except for a small subgroup of samples (characterised by low Na2O and K2O, and high MgO, Ni, Cr and HREE) of uncertain affinities and significance. Element ratios involving LILE (e.g. K/Rb, Rb/Ba, Rb/Sr, K/La, La/Th) closely match those typical of the inferred granitoid protoliths, suggesting that these rocks have experienced relatively little LILE depletion (except possibly for U) during regional metamorphism. It is therefore inferred that metamorphism was probably broadly isochemical. Because the felsic and intermediate granulites and garnet leucogneisses are Sr-depleted, Y-undepleted and mostly have negative Eu anomalies they are inferred to be the products of partial melting of felsic crustal sources leaving plagioclase-bearing residua. Plagioclase fractionation during crystallisation could also account for these characteristics, but K/Rb, Rb/Ba and Rb/Sr ratios in these rocks are not consistent with strong fractionation of feldspar. Garnet pegmatites differ chemically from garnet leucogneisses mainly in their lower Fe, Ti, Nb, Zn, Zr, Th and REE abundances and positive Eu anomalies, related to lower garnet, ilmenite and zircon contents in the garnet pegmatites. A genetic link between these two rock types, probably involving fractionation of these... [ABSTRACT FROM AUTHOR]

Examines the earth history of a sample of high-grade gneiss from the Nimrod Group in the East Antarctic shield. Geologic setting; Core characteristics of zircon crystals; Length of zircon overgrowths; Interpretations of old zircons.

We summarize U–Pb age data for the Lützow–Holm Complex (LHC) in East Antarctica and propose the following geological subdivisions based on protolith ages, along the coast of Dronning Maud Land from southwest to northeast: the Innhovde Suite (INH, 1070–1040 Ma) composed mainly of felsic orthogneiss; the Rundvågshetta Suite (RVG, 2520–2470 Ma), mostly felsic orthogneiss with minor mafic and metasedimentary gneisses; the Skallevikshalsen Suite (SKV, 1830–1790 Ma), felsic to mafic orthogneiss with abundant dolomitic marbles, calc-silicates and other metasediments; the Langhovde Suite (LHV, 1100–1050 Ma), mostly felsic orthogneiss with minor mafic and calc-silicate gneisses; the East Ongul Suite (EOG, 630 Ma), with various orthogneisses and metasediments; and the Akarui Suite (AKR, 970–800 Ma) with diverse orthogneisses and paragneisses. The oldest crustal components of the LHC lie in the southern part of Lützow–Holm Bay, and consist of late Neoarchean and Paleoproterozoic protoliths to charnockites and enderbites that dominate the Rundvågshetta and Skallevikshalsen Suites. This older domain is surrounded by gneisses and granulites with late Mesoproterozoic to early Neoproterozoic protolith ages, including the Innhovde Suite and the Langhovde Suite. The Akarui Suite contains diverse orthogneisses with Neoproterozoic protoloiths, and the youngest unit is the East Ongul Suite with protolith ages of ~630 Ma. Cape Hinode, located geographically within the Akarui Suite, underwent high-grade metamorphism at ~960 Ma that is much older than that which produced the gneisses and granulites of the surrounding LHC (~600–520 Ma). Cape Hinode is therefore exotic, independent of the surrounding LHC, and defined as the "Hinode Block". The boundaries proposed in this paper are largely consistent with those inferred from magnetic anomalies, gravity anomalies, and bedrock topographical data. [ABSTRACT FROM AUTHOR]

• Gneisses from Dronning Maud Land record a prolonged Pan-African evolution characterized by ITD followed by IBC path. • Reaction textures and phase relationships in anatectic paragneisses permit a fine-scale analyses of the metamorphic evolution. • U-Pb ID TIMS geochronology on zircon and monazite are presented. The bedrock of Mühlig-Hofmannfjella, central Dronning Maud Land in eastern Antarctica, is part of the high-grade Maud Belt and comprises a deep-seated metamorphic-plutonic complex. The P-T-t evolution of anatectic supracrustal gneisses has been recovered through a study of mineral assemblages, textural relationships and U-Pb ID TIMS geochronology on zircon and monazite followed by pseudosection modelling. Peak conditions reached granulite facies conditions (T ≥ 810–820 °C) at moderate crustal depths (P = ca. 8 kbar) and resulted in partial melting. Peak-pressure conditions were followed by isothermal decompression at elevated temperatures. After exhumation to crustal levels of about 4–5 kbar, the area underwent a final near-isobaric cooling, which is documented by a secondary growth of garnet. Zircons indicate a period of growth at 570–566 Ma, whereas monazite ages range from 610 to 525 Ma. A likely heat source for the granulite facies metamorphism is decay of radioactive heat-producing elements in the core of the orogen. The combined geochronology and metamorphic data indicate a prolonged, clockwise P-T path, which reflects collision and formation of a long-lived orogenic plateau. [ABSTRACT FROM AUTHOR]

The article focuses on the age and relation between metamorphic gneisses and the Trinity Peninsula Group (TPG) along the Bowman Coast of Graham Land in Antarctica. The TPG, a weakly metamorphosed thick sequence of predominantly quartz- and feldspar-rich greywacke, has tentatively been correlated with metasedimentary rocks exposed along the Bowman Coast of Graham Land. In this study, the base of the TPG is not observed but the Bowman Coasts rocks defined as the Bowman Coast Succession (BCS), is proximal to high-grade gneisses. This article focuses on the relationship of the BCS with the adjacent gneisses.