Abstract A newly discovered suite of Paleoproterozoic/Mesoproterozoic amphibolite facies supracrustals and foliated granitoids between Pernem and Phonda along the northern Konkan coast of western India limits the northern boundary of the Archean Western Dharwar Craton (WDC). Top‐to‐the south transport of the suite is associated with progressively southward tightening of folds with down‐dip stretching lineation, obliteration of pre‐existing fabrics, and widespread stabilization of garnet (lacking in the WDC) along a mid‐crustal counterclockwise pressure‐temperature path. Th‐U‐total Pb ages in metamorphic monazites in the WDC supracrustals are 2.5 Ga; chemical ages of metamorphic monazites (2.3 Ga, 2.2 Ga, 1.9 Ga, and 1.7 Ga) in the WDC‐limiting Pernem‐Phonda tectonic zone decreases northwards. Based on the finding, we suggest that the southern boundary of the Central Indian Tectonic Zone (CITZ)—the accretion zone between the North and the South Indian Blocks—should be relocated ~ 550 km south from its perceived location near the Gulf of Cambay. In spite of the apparent geometric fit, we suggest the CITZ and the Trans North China Orogen may not be continuous as has been suggested in some Paleoproterozoic reconstructions of the Columbia supercontinent. The monazite ages in the footwall schists are similar to 2.5, 2.25, and 2.10 Ga Pb‐Pb ages in detrital zircon in the Sahantaha Group located north of the Paleoarchean Antongil‐Masora Block in Madagascar. We suggest that the Paleoproterozoic/Mesoproterozoic shear zone system extended westward into Madagascar north of the Antongil Block in East Gondwanaland, and this may clarify the unexplained source of zircon in the Sahantaha quartzites.
P–T paths were reconstructed using P–T pseudosection modelling from garnets in a finely laminated mylonitic hornblende‐biotite schist with four sub‐cm wide mineralogically distinct layers separated by poly‐crystalline quartz layers from the northern margin of the Western Dharwar Craton (WDC). The almandine‐grossular‐rich garnets, syn/post‐tectonic with respect to the mylonite foliation, exhibit considerable variations in the zoning patterns of divalent cations in the different layers. By contrast, the compositions of the matrix minerals dominated by biotite, chlorite, hornblende and plagioclase are tightly constrained within and across the layers. The varying abundances of major element oxides in the layers reflect variations in the abundances of the minerals, rather than the mineral compositions. P–T conditions estimated using mineral thermo‐barometry in the garnet‐muscovite‐biotite‐plagioclase assemblages are 520–575°C and 6–8 kbar; somewhat higher P–T values (615–665°C, 7–9 kbar) are obtained from hornblende‐garnet‐plagioclase‐quartz assemblages. The P–T paths obtained from the multi‐mineralic layers using P–T pseudosection modelling yield near‐isothermal loading (4–7 kbar) at 500–550°C in all layers. The near‐isothermal loading experienced by the multi‐layered rock suggests rapid crustal thickening involving overthrusting followed by nucleation of steeply dipping transpressional shear zones due to oblique convergence of the WDC with a crustal domain in the north. U‐Th‐total Pb chemical dating of metamorphic monazites indicates the crustal convergence occurred at 2.2–2.1 Ga. The event does not have known equivalents in the neighbouring crustal domains eastwards in India as well as in Madagascar in the west. This suggests the crustal domain may be an exotic block.
The Great Proterozoic Fold Belt of India (GIPFOB) is the inferred accretion zone between the North India Block (NIB) and the South India Block (SIB). Mesoscopic structures, phase petrology, and U-Th-Pb (total) monazite ages in the Bangriposi Shear Zone (BSZ) located in the eastern syntaxis of the GIPFOB along the fringe of the Paleo/Mesoarchean Singhbhum Craton (SC) are examined to constrain the time of NIB-SIB assembly. The BSZ at the eastern fringe of the Archean Singhbhum Craton is a NNE-trending west-vergent 0.95–1.0 Ga greenschist-facies (<0.6 GPa at 500° ± 30°C) sinistral-reverse shear zone. The BSZ is dominated by Meso/Neoarchean (2.6–3.1 Ga) cratonic lithologies and an overlying suite of intensely to weakly deformed cobble/granule quartz conglomerates interbanded with quartzite, low-grade phyllites and mafic schist, and gabbro-wehrlite complexes. The precursors to the low-grade conglomerate-phyllite-quartzite sequence and the obducted ultrahigh-pressure (5.3–5.9 GPa at 1200°–1230°C) wehrlite body constitute a 0.95–1.0 Ga tectonic mélange interleaved with cratonic (Archean) lithologies of higher metamorphic grade. The lack of Mesoproterozoic (1.5–1.7 Ga) metamorphism in the BSZ and neighboring areas, which are common along the NW/N margin of the Singhbhum Craton and the accreted southern meta-flysch belt of the North Singhbhum Mobile Belt (NSMB) is significant. It is suggested that the northern meta-flysch belt of the NSMB and the Singhbhum Craton were not juxtaposed until ∼1.0 Ga. These crustal domains were finally accreted during the final stages (0.95–1.0 Ga) of Rodinia supercontinent assembly forming a -shaped ∼1.0 Ga tectonic zone in eastern India. The "Greater India" landmass comprising the NIB and the SIB came into existence during this ∼1.0 Ga global tectonic episode.
Abstract At Bangriposi, variable stages in replacement of staurolite by chloritoid – Na–K–Ca mica shimmer aggregates in muscovite schists provides insight into the complex interplay between fluid flow, mass transfer, and dissolution–precipitation during pseudomorph growth. Idioblastic chloritoid growing into mica caps without causing visible deformation, and monomineralic chloritoid veins (up to 300 μ m wide) within shimmer aggregates replacing staurolite attest to chloritoid nucleation in fluid‐filled conduits along staurolite grain boundaries and crystallographic planes. The growth of shimmer aggregates initiated along staurolite margins, and advanced inwards into decomposing staurolite along networks of crystallographically controlled fluid‐filled conduits. Coalescence among alteration zones adjacent to channel fills led to dismemberment and the eventual demise of staurolite. Mass balance calculation within a volume‐fixed, silica‐conserved reference frame indicate the shimmer aggregates grew via precipitation from fluids in response to mass transport that led to the addition of H 2 O, K 2 O, Na 2 O and CaO in the reaction zone, and Al 2 O 3 was transported outward from the inward‐retreating margin of decomposing staurolite. This aided precipitation of chloritoid in veins and in the outer collars, and as disseminated grains in the shimmer aggregates at mid‐crustal condition (~520 ± 20 °C, 5.5 ± 2.0 kbar). Computation using one‐dimensional transport equation suggests that staurolite decomposition involved advection dominating over diffusive transport; the permeation of externally derived H 2 O caused flattening of chemical potential gradients in H 2 O and aqueous species, for example, and , computed using the Gibbs method. This suggests that staurolite decomposition was promoted by the infiltration of a large volume of H 2 O that flattened existing chemical potential gradients. In the initial stages of replacement, chloritoid super‐saturation in fluid caused preferential nucleation and growth of chloritoid at staurolite grain boundaries and in crystallographic planes. As reaction progressed, further chloritoid nucleation was halted, but chloritoid continued to grow as the 3‐mica aggregates continued to replace the remaining staurolite in situ , while the chloritoid‐compatible elements were transported in the water‐rich phase facilitating continued growth of the existing chloritoid grains.
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