Sanjoy Sanyal

Chapter 20 Provenance, timing of sedimentation and metamorphism of metasedimentary rock suites from the Southern Granulite Terrane, India

Abstract The Southern Granulite Terrane of India exposes remnants of an interbanded sequence of orthoquartzite–metapelite–calcareous rocks across the enigmatic Palghat–Cauvery Shear Zone (PCSZ), which has been interpreted as a Pan-African terrane boundary representing the eastward extension of the Betsimisaraka Suture Zone of Madagascar. Zircon U–Pb geochronology of metasedimentary rocks from both sides of the PCSZ shows that the precursor sediments of these rocks were sourced from the Dharwar Craton and the adjoining parts of the Indian shield. The similarity of the provenance and the vestiges of Grenvillian-age orogenesis in some metasedimentary rocks contradict an interpretation that the PCSZ is a Pan-African terrane boundary. The lithological association and the likely basin formation age of the metasedimentary rocks of the Southern Granulite Terrane show remarkable similarity to the rock assemblage and timing of sedimentation of the Palaeoproterozoic to Neoproterozoic shallow-marine deposits of the Purana basins lying several hundred kilometres north of this terrane. Integrating the existing geological information, it is postulated that the shallow-marine sediments were deposited on a unified land-mass consisting of a large part of Madagascar and the Indian shield that existed before Neoproterozoic time, part of which was later involved in the Pan-African orogeny. Supplementary material: Details of the zircon U–Pb LA-MC-ICP-MS analyses of samples are available at http://www.geolsoc.org.uk/SUP18793.

Controls of P-T path and element mobility on the formation of corundum pseudomorphs in Paleoproterozoic high-pressure anorthosite from Sittampundi, Tamil Nadu, India

Abstract The Archaean Sittampundi Layered Magmatic Complex (SLC) of south India is interpreted as a part of the oceanic crust that formed in a suprasubduction zone setting. The assemblage corundum + anorthite + amphibole (magnesiohornblende to tschermakite to pargasite) + clinozoisite (C1PACz) developed in highly calcic anorthosite of the SLC at the culmination of a ca. 2.46 Ga tectonothermal event. Changing physicochemical conditions during this early Paleoproterozoic (Siderian) event produced spinel + anorthite + second generation amphibole through destabilization of corundum + first generation amphibole. Spinel retains the shape of the corundum that it replaces (spinel pseudomorphing corundum, SCP) and is surrounded by a rind of plagioclase that separates spinel from the matrix amphibole. Development of the assemblage chlorite + clinozoisite + secondary corundum after spinel + anorthite + amphibole marks the terminal metamorphic event in this area. Mass-balance calculations on pseudomorphs and modeling of preserved reaction textures show that Na, Mg, Ca, and silica were mobile during the formation of the SCP. Al and Fe were mobile at the grain scale but remained immobile in the scale of a thin section. Activity adjusted partial petrogenetic grid in the systems Na2O-CaO-Al2O3-SiO2-H2O (NCASH) and Na2O-CaO-MgO-Al2O3-SiO2-H2O (NCMASH) along with the mineralogy of the rocks that were co-metamorphosed with the anorthosite show that (1) the assemblage C1PACz was formed during high-pressure metamorphism (11 ± 1 kbar and 725 ± 25 °C), (2) SCP and the plagioclase rind around it were formed during exhumation of these higher pressure rocks to lower pressure (7 ± 1 kbar, 675 ± 50 °C) along a steeply decompressive retrograde P-T path, and (3) post decompression cooling and hydration at near isobaric condition triggered the formation of the chlorite-clinozoisite-secondary corundum assemblage (6-8 kbar, <620 °C). The inferred P-T path is consistent with the view that the studied area that represents Archaean oceanic crust of SSZ affinity was subducted and subsequently exhumed during the early Paleoproterozoic orogeny.

Mineralogical Control on Rheological Inversion of a Suite of Deformed Mafic Dykes from Parts of the Chottanagpur Granite Gneiss Complex of Eastern India

At Deoghar- Jashidihi, Jharkhand, India, a suite of mafic dykes amidst migmatitic felsic gneiss underwent superposed folding, polyphase boudinage, and was accompanied by syntectonic migmatization (in the host felsic gneiss) and emplacement of different generations of pegmatite. Structural features of the deformed and torned-out mafic dykes demonstrate that compared to the enclosing felsic gneiss, these rocks initially behaved as a competent layer. With progressive deformation the different competency between the two lithologies decreased and eventually mafic dykes became less competent than the felsic gneiss. The rheological inversion is manifested by fish-head boudin, flame-folds, and boudins with trapezoidal cross section. Modeling of reaction textures of a mafic boudin identified following infiltration driven reactions that occurred in succession:

Fluid Induced Metamorphism in a Suite of Mafic Dykes from PalaeoProterozoic Mahakoshal Group, Central India

A number of mafic dykes have intruded the tremolite-zone siliceous dolomite of PalaeoProterozoic Mahakoshal Group of rocks of Central India exposed near Jabalpur, MP. While the central part of most of the dykes are massive, being constituted of randomly oriented coarse hornblende laths in a fine grained feldspathic matrix, the marginal parts near the contact of enclosing siliceous dolomite have been completely converted into a talc + chlorite + dolomite ± quartz schist, the pervasive foliation being axial planar to the first generation regional fold (F1). Conventional thermobarometry supplemented by P–T and T–XBulk pseudosections conform that the talc-chlorite association was formed at temperature below 500°C at 4 kbar in presence of a fluid (XCO2 < 0.6). Numerically constructed phase diagram also suggest that the observed mineral assemblages in the dykes didn’t form due to metamorphism in a close system. In contrast, fluid induced mass transfer between the dyke and the enclosing talc-rich band in the adjoining marble seems to explain all the pertrological attributes of the studied dykes. Reactions deduced in T–XCO2 sections in the system CMASV and the constraints from the constructed pseudosections are consistent with pervasive fluid permeation into the marginal portions of the dykes and associated ion mobility rendering the rock depleted in Ca+2 and enriched in Mg+2, thus facilitating development of the observed assemblage. The central part of the dyke, on the other hand, escaped such metasomatic changes and was converted to Hornblende + Plagioclase + Quartz assemblage during post D1 static recrystallization. Quantification of fluid flux suggests that the volume of fluid was ~50–60% the volume of rock with which the fluid has interacted.