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SHRIMP Zircon U-Pb Dating of Sapphirine-Bearing Granulite and Biotite-Hornblende Gneiss in the Schirmacher Hills, East Antarctica: Implications for Neoproterozoic Ultrahigh-Temperature Metamorphism Predating the Assembly of Gondwana

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 and Ma were obtained from zircon overgrowth rims and zircon cores, with oscillatory and irregular zoning, respectively. The zircon overgrowth rims ( 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 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 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 -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).

Contrasting metamorphic records and their implications for tectonic process in the central Sør Rondane Mountains, eastern Dronning Maud Land, East Antarctica

Abstract Metamorphic rocks in the central part of Sør Rondane Mountains, eastern Dronning Maud Land, East Antarctica, are classified into three types based on petrological characteristics. (i) The Austkampane area preserves c. 800 °C and 0.5–0.6 GPa peak metamorphic conditions followed by decompression and subsequent isobaric cooling and later hydration (A-type). (ii) The Brattnipene and eastern Menipa area preserve peak P–T conditions of c. 800 °C and 0.7–0.8 GPa with subsequent isobaric cooling and later hydration (B-type). (iii) The area including Lunckeryggen, southern Walnumfjella and western Menipa preserves an amphibolite–facies peak metamorphic condition with signatures of prograde metamorphism (L-type), which are typically unaffected by the retrograde hydration event. Peak granulite–facies metamorphism of A- and B-type rocks are contemporaneous at c. 640–600 Ma, but a difference in the P–T paths between these rocks can be explained by thrusting of the A-type rock unit onto the B-type rock unit. By contrast, the timing of the metamorphism of the L-type rocks is significantly younger at c. 550 Ma, possibly related to the intrusion of pegmatites and granitoids. These metamorphic records in the central part of the Sør Rondane Mountains can be a test ground for the regional tectonic processes proposed for the orogeny related to Gondwana formation. Supplementary material: Representative mineral compositions are listed at www.geolsoc.org.uk/SUP18623

Contrasting metamorphic P – T path between Schirmacher Hills and Mühlig-Hofmannfjella, central Dronning Maud Land, East Antarctica

Abstract Retrograde metamorphic P–T paths of garnet–pyroxene-bearing mafic gneisses from three regions in central Dronning Maud Land (CDML) were examined. No difference in P–T conditions estimated from rocks of the three regions was recognized, and they are within the range of c. 6–8 kbar, 750–830 °C. However, localities in the Mühlig-Hofmann Range (Filchnerfjella and Jutulsessen) preserve rocks with mineral textures that indicate near-isothermal decompressional histories. In the Schirmacher Hills, an isolated exposure on the Princess Astrid Coast, metamorphic texture observed in mafic gneiss is indicative of an isobaric cooling history. Combining their P–T paths and age determinations suggests that the Schirmacher Hills was a separate terrane, together with present-day SE Africa, whereas the Grenvillian-age east–west-trending CDML inland nunatak regions are characterized by an isothermal decompressional metamorphic history related to the final amalgamation of Gondwana.

Geochemistry of post‐kinematic mafic dykes from central to eastern Dronning Maud Land, East Antarctica: evidence for a Pan-African suture in Dronning Maud Land

Abstract The region comprising central to eastern Dronning Maud Land (2°W to 40°E), East Antarctica, is underlain by Mesoproterozoic to Cambrian metamorphic rocks and post-kinematic intrusive rocks with varied compositions. The post-kinematic mafic dykes linked to the Pan-African orogen include various types of lithologies: lamprophyre and lamproite in Mühlig-Hofmannfjella in central Dronning Maud Land and lamprophyre and high-K dolerite in the Sør Rondane Mountains in eastern Dronning Maud Land. Most of the mafic dykes have been weakly affected by low-grade metamorphism, but clearly preserve their igneous textures. The mafic dykes show a high abundance of Rb, Ba, Sr and light rare earth elements with negative anomalies of Nb, Ta and Ti in a multi-element primitive mantle-normalized diagram. The geochemical characteristics of the mafic dykes suggest that they were derived from a metasomatized mantle source leaving phlogopite, rutile and/or titanite as residual phases. Considering Sr and Nd isotopic systematics of the mafic dykes and the host metamorphic rocks and coeval felsic intrusive rocks, a large crustal boundary potentially related to a suture zone of West and East Gondwana should pass between Mühlig-Hofmannfjella and the Sør Rondane Mountains.

Multiple Collisions in the East African–Antarctica Orogen: Constraints from Timing of Metamorphism in the Filchnerfjella and Hochlinfjellet Terranes in Central Dronning Maud Land

Sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon age dating was applied to several types of cordierite-bearing gneisses and orthopyroxene-bearing gneiss from the basement rocks in central Dronning Maud Land, East Antarctica, to clarify the timing of tectonothermal events during the amalgamation of East and West Gondwana. The zircon ages of ca. 522–525 and 598–599 Ma were obtained in Filchnerfjella and Hochlinfjellet, respectively, and the cathodoluminescence images of analyzed zircons suggest that these were formed during high-temperature metamorphism. These two sets of ages are interpreted to represent periods that immediately followed the peak metamorphism. The dating results reveal an age gap of 80 Myr between the two areas, indicating different collisional events. In the cordierite-bearing samples in Filchnerfjella, zircon cores with oscillatory zoning have concordia ages that range from 1800 to 650 Ma. These are considered to represent the ages of detrital zircons derived over a long time from different source materials. In Hochlinfjellet, the inherited detrital zircons with oscillatory zoning have concordia ages of ca. 930, 780, and 730 Ma, indicating sedimentary protoliths derived from differently aged igneous source rocks, possibly formed in a subduction zone as an accretionary prism before the collision. According to previous age results, previously documented shear zones are now interpreted as representing a major tectonic boundary. The obtained age results imply that different allochthonous metamorphic terranes were assembled along the collisional zone of the East African–Antarctica Orogen during the period from 650 to 500 Ma.

Sapphirine — Orthopyroxene — Garnet Granulite from Schirmacher Hills, Central Dronning Maud Land

A third locality of sapphirine granulite was discovered in the Schirmacher Hills, central Dronning Maud Land, East Antarctica; one unique for the association with orthopyroxene and garnet. Sapphirine occurs as inclusions together with orthopyroxene within garnet and as discrete grains together with spinel, secondary orthopyroxene in cordierite coronas. Orthopyroxene porphyroblasts have the highest Al2O3 contents (∼10 wt.-%) so far reported from Dronning Maud Land. The earlier of two metamorphic stages inferred for the sapphirine-bearing granulite is characterized by breakdown of sapphirine and orthopyroxene to form garnet at peak conditions approaching 950–1050°C and 9–10 kbar. The later stage is decompression when spinel-cordierite-orthopyroxene formed at about 950–980°C and 8 kbar. The age of the ultrahigh-temperature event could be ca. 1.15 Ga, somewhat older than the late Mesoproterozoic granulite-facies event elsewhere in central Dronning Maud Land. Metamorphic temperatures estimated to exceed 900°C imply that the tectonothermal regime in the Schirmacher Hills was distinct and could be critical in determining relationships between central Dronning Maud Land and metamorphic belts in southern Africa and in understanding the evolution of Rodinia and Gondwana.