Eastern Ghats Province (India)-Rayner Complex (Antarctica) accretion: Timing the event

There is consensus that, at 1.0–0.9 Ga, the granulites in the Eastern Ghats Province (EGP), Eastern India, and the Rayner Complex, Antarctica, were parts of a coherently evolved crustal block. Paleogeographic reconstructions suggest that in the Neoproterozoic/Early Paleozoic, India and Antarctica were closely positioned at equatorial latitudes in two periods at 1.0–0.9 Ga and 0.6–0.5 Ga. The question is, in which of these periods did the EGP–India vis-à-vis India–Antarctica accretion occur. Top-to-the-west thrusts juxtaposed the EGP with the Bastar Craton, a part of the Greater India landmass. The eastern fringe of the craton underwent anatexis (750–780 °C; 8–9 kbar) and high deformation strain that demonstrably weakened westward. Zircon in the anatectic migmatites at the EGP margin and in the weakly-deformed and non-migmatite granite in the hinterland in the west yields U–Pb upper intercept ages of 2.5–2.4 Ga whereas titanite, hosted in the leucosome of a metatexite and in a granite, has an age of 502 ± 3 Ma coinciding with the lower intercept ages of zircon discordia lines. The lack of 1.0–0.9 Ga dates in the cratonic margin suggests that the EGP accreted with the Bastar Craton and the Greater India landmass at 0.5 Ga during the Gondwanaland assembly, and not in the Early Neoproterozoic. It is within the realms of possibility that the EGP had already separated from the Rayner Complex during the disintegration of Rodinia, and therefore, the 0.5 Ga accretion of the dismembered EGP with Greater India may not be symptomatic of India–Antarctica accretion, in spite of the proximity of the two landmasses inferred from paleogeographic reconstructions

Geochemistry and detrital zircon geochronology of Khammam Schist Belt, Eastern Dharwar Craton: Implication for India – North China Craton –Antarctica connection in Paleo-Mesoproterozoic crustal assembly

This communication reports the results of geochemical investigations and detrital zircon geochronology of metasediments of the Khammam Schist Belt that occur at the trijunction of the Eastern Dharwar Craton–Bastar Craton–Eastern Ghats Belt. Biotite (XMg = 0.46–0.52) and muscovite (Si atom per formula unit (apfu) of 11 O = 3.08–3.17) with alkali-feldspar constitute the mineralogy of studied samples. The Ti content in biotite yields a mean temperature of 652 °C (1σ = 10 °C), and biotite–muscovite pairs yield an average pressure of 0.46 GPa (1σ = 0.06 GPa). Fe-Ti oxides and zircon occur as accessory phases. The Al2O3 exhibits a positive correlation with K2O and TiO2, which implies that mica and biotite control the major element abundances of studied samples. These samples indicate negative Sr and positive Th anomalies in a Post Archean Australian Shale (PASS) normalised spider diagram. Also, these samples show a nearly horizontal trend with (La/Yb) PASS varying between 0.56 and 1.92 with a negative to slightly positive Eu anomaly (Eu/Eu* = 1.20, 1σ = 0.38). LA-ICPMS analysis of detrital zircon grains (number of analyses = 100 from two samples) yields 207Pb/206Pb ages range from 1500 to 2600 Ma. The zircons grains with weighted average ages between 2500 Ma, 2400 Ma, 2200 Ma, 2000 Ma, 1900 and 1800 Ma exhibit magmatic and high-temperature deformation features. The 1604 Ma old zircons exhibit homogeneous domains and overgrowths over older zircons, implying metamorphic origin. The Chemical Index of Alteration (CIA = 66 to 77), Chemical Index of Weathering (CIW = 73 to 95), and Plagioclase Index of Alteration (PIA = 81 to 91) values indicate moderate to intense weathering of the source area. Source and tectonic discrimination plots imply a felsic source and active tectonic setting. Accordingly, 1900–1800 Ma old magmatic zircons in the current samples constrain the maximum depositional age for the Khammam Schist Belt. Compared with the detrital zircon geochronology of the North China Craton and East Antarctica, the current samples exhibit peaks at circa 2500 Ma, 2400 Ma, 2200 Ma, 2000 Ma, 1900–1800 Ma, and 1600 Ma, implying Khammam Schist Belt as part of the South India Cratonic Block shares similar geological history with North China Craton and East Antarctica. Our study suggests that North China Craton and East Antarctica were connected with the South Indian Cratonic Block during the Columbia assembly

Dry and strong quartz during deformation of the lower crust in the presence of melt

Granulite facies migmatitic gneisses from the Seiland Igneous Province (northern Norway) were deformed during deep crustal shearing in the presence of melt, which formed by dehydration melting of biotite. Partial melting and deformation occurred during the intrusion of large gabbroic plutons at the base of the lower crust at 570 to 520 Ma in an intracontinental rift setting. The migmatitic gneisses consist of high-aspect-ratio leucosome-rich domains and a leucosome-poor, restitic domain of quartzitic composition. According to thermodynamic modeling using synkinematic mineral assemblages, deformation occurred at T = 760°C–820°C, P = 0.75–0.95 GPa and in the presence of ≤5 vol % of residual melt. There is direct evidence from microstructural observations, Fourier transform infrared measurements, thermodynamic modeling, and titanium-in-quartz thermometry that dry quartz in the leucosome-poor domain deformed at high differential stress (50–100 MPa) by dislocation creep. High stresses are demonstrated by the small grain size (11–17 μm) of quartz in localized layers of recrystallized grains, where titanium-in-quartz thermometry yields 770°C–815°C. Dry and strong quartz forms a load-bearing framework in the migmatitic gneisses, where ∼5% melt is present, but does not control the mechanical behavior because it is located in isolated pockets. The high stress deformation of quartz overprints an earlier, lower stress deformation, which is preserved particularly in the vicinity of segregated melt pockets. The grain-scale melt distribution, water content and distribution, and the overprinting relationships of quartz microstructures indicate that biotite dehydration melting occurred during deformation by dislocation creep in quartz. The water partitioned into the segregated melt crystallizing in isolated pockets, in the vicinity of which quartz shows a higher intracrystalline water content and a large grain size. On the contrary, the leucosome-poor domain of the rock, from which melt was removed, became dry and thereby mechanically stronger. Melt removal at larger scale will result in a lower crust which is dry enough to be mechanically strong. The application of flow laws derived for wet quartz is not appropriate to estimate the behavior of such granulite facies parts of the lower crust