Antarctica – A window to the far off land

The Antarctic continent and surrounding oceans,which are cold and isolated from human activities,constitute a key region for multidisciplinary investigations.Since the early interest during the International Geophysical Year （IGY） during 1957-1958,numerous scientific studies have so far been carried out in Antarctica by different countries,which have provided important insights into Earth and environmental processes such as regional climate warming and its effect to biodiversity,changes in ice sheet and ocean circulation,ozone depletion,and origin and evolution of continents.The challenge of the next phase of Antarctic research will be to integrate all fields of science into a holistic understanding of Earth and life processes of the Antarctic region.In this thematic issue of Geoscience Frontiers,we assemble a set of scientific papers related to geomorphology,biology,molecular spectroscopy,and geology reflecting the recent research activity in the Antarctic region.     

Metamorphic phase equilibria modelling and zircon U–Pb geochronology of ultrahigh-temperature cordierite granulites from the Madurai Block,India: implications for hot Gondwana crust

The Madurai Block (MB) is the largest Precambrian crustal block in the Southern Granulite Terrane (SGT) of India and hosts rare cordierite- and orthopyroxene-bearing granulites. Investigations based on field study, petrology, metamorphic P–T estimation, and detrital zircon geochronology of these granulites are crucial for understanding the ultrahigh-temperature (UHT) metamorphism and crustal evolution in this block. Here we investigate the petrology and zircon U–Pb geochronology of two new localities of cordierite granulites at Kottayam (southern MB; SMB) and Munnar (central MB; CMB). Petrographic observations and phase equilibria modelling results indicate that these rocks experienced UHT metamorphism with the peak temperature exceeding 950℃ and involving clockwise P–T paths. The prograde mineral assemblages define the P–T conditions of 6.8–8.7 kbar and 750–875℃. The peak conditions are estimated using pseudosection modelling and geothermometry, which yield P–T estimates of 7.1–9.1 kbar and 955–985℃. The retrograde cooling and decompression are inferred at 860–790℃ and <6.5 kbar, respectively. Partial melting played an important role during metamorphism and contributed to the overgrowth around detrital zircons. The melt production process was probably related to biotite dehydration melting, and was mainly triggered by heating, with or without the effect of decompression. Detrital zircons in cordierite granulite samples from the two localities show similar age distributions and have dominantly Neoproterozoic ages (1024–760 Ma). The zircon cores show oscillatory zoning with a wide range of Th/U ratios (0.01–0.96), implying complex protoliths from multiple Neoproterozoic provenances from both southern and central domains of the MBs. Zircon rims and homogeneous bright zircons yield mean ages of 549 ± 5 Ma, 536 ± 6 Ma, and 544 ± 6 Ma, which are interpreted to represent zircon overgrowths during the post-peak cooling and decompression process. The timing of peak UHT metamorphism is constrained as 549–599 Ma, which coincides with the assembly of the Gondwana supercontinent.     

Fluid-induced high-temperature metasomatism at Rundv?gshetta in the Lützow-Holm Complex,East Antarctica: Implications for the role of brine during granulite formation

We report new petrological, phase equilibria modeling, and fluid inclusion data for pelitic and mafic granulites from Rundv?gshetta in the highest-grade region of the Neoproterozoic Lützow-Holm Complex(LHC),East Antarctica, and provide unequivocal evidence for fluid-rock interaction and high-temperature metasomatism in the presence of brine fluid. The studied locality is composed dominantly of well-foliated pelitic granulite(K-feldspar+quartz+sillimanite+garnet+ilmenite) with foliation-parallel bands and/or layers of mafic granulite(plagioclase+orthopyroxene+garnet+ilmenite+quartz+biotite). The boundary between the two lithologies is defined by thin(about 1 -20 cm in thick) garnet-rich layers with a common mineral assemblage of garnet+plagioclase+quartz+ilmenite+biotite ? orthopyroxene. Systematic increase of grossular and decrease of pyrope contents in garnet as well as decreasing Mg/(Fe+Mg) ratio of biotite from the pelitic granulite to garnet-rich rock and mafic granulite suggest that the garnet-rich layer was formed by metasomatic interaction between the two granulite lithologies. Phase equilibria modeling in the system NCKFMASHTO demonstrates that the metasomatism took place at 850 -860℃, which is slightly lower than the peak metamorphism of this region, and the modal abundance of garnet is the highest along the metapeliteemetabasite boundary(up to 40%), which is consistent with the field and thin section observations. The occurrence of brine(7.0 -10.9 wt.% Na Cleqfor ice melting or 25.1 -25.5 wt.% NaC leqfor hydrohalite melting) fluid inclusions as a primary phase trapped within plagioclase in the garnet-rich layer and the occurrence of Cl-rich biotite(Cl = 0.22 -0.60 wt.%) in the metasomatic rock compared to that in pelitic(0.15 -0.24 wt.%) and mafic(0.06-0.13 wt.%) granulites suggest infiltration of brine fluid could have given rise to the high-temperature metasomatism. The fluid might have been derived from external sources possibly related to the formation of major suture zones formed during the Gondwana amalgamation.     

Fluid characteristics of retrogressed eclogites and mafic granulites from the Cambrian Gondwana suture zone in southern India

Eclogite-facies rocks and high-pressure granulites provide windows to the deeper parts of subduction zones and the root of mountain chains, carrying potential records of fluids associated with subduction-accretion-collision tectonics. Here, we report petrological and fluid inclusion data on retrogressed eclogite and high-pressure granulite samples from Sittampundi, Kanji Malai and Perundarai in southern India. These rocks occur within the trace of the Cambrian collisional suture which marks the final phase of amalgamation of the Gondwana supercontinent. The garnet–clinopyroxene assemblage in the eclogites preserves relict omphacite, whereas the high-pressure granulites are characterized by an assemblage of garnet and clinopyroxene in the absence of omphacite and with minor plagioclase, orthopyroxene, and quartz. Phase relations computed for the eclogite assemblage yield peak P–T conditions of 19 kbar and 1,010°C. The mafic granulites also preserve the memory of high to ultrahigh-temperature metamorphism followed by an isothermal decompression. Systematic fluid inclusion optical, microthermometric and laser Raman spectroscopic studies were conducted in garnet and plagioclase from the eclogite–high pressure granulite suite. The results suggest that the early fluids were a mixture of CO₂, CH₄ and N₂ probably derived from decarbonation and devolatilization reactions in a subduction setting during the prograde stage. The later generation inclusions, which constitute the dominant category in all the samples studied, are characterized by a near-pure CO₂ composition with moderate to high densities (up to 1.154 g/cm³). The highest density fluid inclusions recorded in this study occur within the mafic granulites from Sittampundi (0.968–1.154 g/cm³) and Kanji Malai (1.092–1.116 g/cm³). In some cases, carbonate minerals such as dolomite and calcite are associated with the CO₂-rich fluid inclusions. The composition and densities of the later generation fluids closely match with those of the CO₂-bearing fluid inclusions reported from ultrahigh-temperature granulites occurring proximal to the eclogite–high pressure granulite suite within this suture zone, and suggest a common tectonic link for the fluid regime. We evaluate the fluid characteristics associated with convergent plate margin processes and propose that the early aqueous fluids probably associated with the eclogites were consumed during the formation of the retrograde hydrous mineral assemblages, whereas the fluid regime of the high-pressure and ultrahigh-temperature granulites was mostly CO₂-dominated. The tectonic setting of the rocks along a collisional suture marking the trace along which crustal blocks were welded through subduction–collision process is in favor of a model involving the derivation of CO₂ from sub-lithospheric sources such as a carbonated tectosphere invaded by hot asthenosphere, or underplated mafic magmas.     

Petrology,geochemistry, and zircon U-Pb geochronology of the Zambezi Belt in Zimbabwe: Implications for terrane assembly in southern Africa

The Zambezi Belt in southern Africa has been regarded as a part of the 570-530 Ma Kuunga Orogen formed by a series of collision of Archean cratons and Proterozoic orogenic belts.Here,we report new petrological,geochemical,and zircon U-Pb geochronological data of various metamorphic rocks(felsic to mafic orthogneiss,pelitic schist,and felsic paragneiss) from the Zambezi Belt in northeastern Zimbabwe,and evaluate the timing and P-T conditions of the collisional event as well as protolith formation.Geochemical data of felsic orthogneiss indicate within-plate granite signature,whereas those of mafic orthogneiss suggest MORB,ocean-island,or within-plate affinities.Metamorphic P-Testimates for orthogneisses indicate significant P-T variation within the study area(700-780 C/6.7-7.2 kbar to 800-875 C/10-11 kbar) suggesting that the Zambezi Belt might correspond to a suture zone with several discrete crustal blocks.Zircon cores from felsic orthogneisses yielded two magmatic ages:2655±21 Ma and 813士5 Ma,which suggests Neoarchean and Early Neoproterozoic crustal growth related to within-plate magmatism.Detrital zircons from metasediments display various ages from Neoarchean to Neoproterozoic(ca.2700-750 Ma).The Neoarchean(ca.2700-2630 Ma) and Paleoproterozoic(ca.2200-1700 Ma) zircons could have been derived from the adjacent Kalahari Craton and the Magondi Belt in Zimbabwe,respectively.The Choma-Kalomo Block and the Lufilian Belt in Zambia might be proximal sources of the Meso-to Neoproterozoic(ca.1500-950 Ma) and early Neoproterozoic(ca.900-750 Ma) detrital zircons,respectively.Such detrital zircons from adjacent terranes possibly deposited during late Neoproterozoic(744-670 Ma),and subsequently underwent highgrade metamorphism at 557-555 Ma possibly related to the collision of the Congo and Kalahari Cratons during the latest Neoproterozoic to Cambrian.In contrast,670-627 Ma metamorphic ages obtained from metasediments are slightly older than previous reports,but consistent with~680-650 Ma metamorphic ages reported from different parts of the Kuunga Orogen,suggesting Cryogenian thermal events before the final collision.