Dewashish Upadhyay

Unraveling Sedimentary Provenance and Tectonothermal History of High‐Temperature Metapelites, Using Zircon and Monazite Chemistry: A Case Study from the Eastern Ghats Belt, India

The geochemical behavior of detrital zircon and monazite during granulite facies anatexis in metapelites from the Eastern Ghats Belt (EGB), India, is explored using U‐Pb geochronology, Hf isotopes, and trace elements. In a metapelite from the Ongole Domain, detrital zircon reequilibrated by coupled dissolution‐reprecipitation and diffusion reaction during ultrahigh‐temperature metamorphism at 1.63 Ga. The event completely reset the U‐Pb systems, but Hf isotopes and trace elements were only partially reequilibrated. Overgrowths on the altered cores date migmatization at 1.61 Ga. Monazite yields metamorphic ages similar to those of zircon. In metapelites from the Eastern Ghats Province (EGP), detrital zircon grains give 2.44–1.40‐Ga ages and metamorphic ones 1.2–0.5‐Ga ages. Metamorphic components include detrital grains reequilibrated by coupled dissolution‐reprecipitation in the presence of anatectic melt and newly crystallized overgrowths and grains. In reequilibrated domains, the U‐Pb system was completely reset, but Hf isotope compositions of precursors were often retained. The 176Hf/177Hf of most zircon scatters between 2.7‐ and 1.9‐Ga crust evolution lines, indicating late Archaean to Mesoproterozoic juvenile provenance with major crust formation between 2.7 and 1.9 Ga and only minor perturbation of the Lu‐Hf system during metamorphism. The 1.2–0.92‐ and 0.62–0.50‐Ga metamorphic populations are related to Rodinia and Gondwana assembly, respectively. The 1.63‐, 1.2–0.92‐, and 0.62–0.50‐Ga ages allow correlation of the Ongole Domain and EGP with the Rayner Complex, suggesting that East Antarctica was contiguous with Proto‐India in the Paleoproterozoic. Rifting in this terrane and sedimentation in the resulting basin deposited the EGP metapelites between 1.42 and 1.2 Ga, culminating in reamalgamation of East Antarctica with Proto‐India during Rodinia assembly. The final crustal architecture of the belt was attained during Pan‐African orogenesis when the EGB granulites were thrust westward over the cratons.

142Nd evidence for an enriched Hadean reservoir in cratonic roots

The isotope 146Sm undergoes α-decay to 142Nd, with a half-life of 103 million years. Measurable variations in the 142Nd/144Nd values of rocks resulting from Sm–Nd fractionation could therefore only have been produced within about 400 million years of the Solar System’s formation (that is, when 146Sm was extant). The 142Nd/144Nd compositions of terrestrial rocks are accordingly a sensitive monitor of the main silicate differentiation events that took place in the early Earth. High 142Nd/144Nd values measured in some Archaean rocks from Greenland hint at the existence of an early incompatible-element-depleted mantle. Here we present measurements of low 142Nd/144Nd values in 1.48-gigayear-(Gyr)-old lithospheric mantle-derived alkaline rocks from the Khariar nepheline syenite complex in southeastern India. These data suggest that a reservoir that was relatively enriched in incompatible elements formed at least 4.2 Gyr ago and traces of its isotopic signature persisted within the lithospheric root of the Bastar craton until at least 1.48 Gyr ago. These low 142Nd/144Nd compositions may represent a diluted signature of a Hadean (4 to 4.57 Gyr ago) enriched reservoir that is characterized by even lower values. That no evidence of the early depleted mantle has been observed in rocks younger than 3.6 Gyr (refs 3, 4, 7) implies that such domains had effectively mixed back into the convecting mantle by then. In contrast, some early enriched components apparently escaped this fate. Thus, the mantle sampled by magmatism since 3.6 Gyr ago may be biased towards a depleted composition that would be balanced by relatively more enriched reservoirs that are ‘hidden’ in Hadean crust, the D′′ layer of the lowermost mantle or, as we propose here, also within the roots of old cratons.

Corundum–leucosome-bearing aluminous gneiss from Ayyarmalai, Southern Granulite Terrain, India: A textbook example of vapor phase-absent muscovite-melting in silica-undersaturated aluminous rocks

Abstract An aluminous gneissic rock associated with high-pressure mafic and felsic granulites in the Palghat- Cauvery Shear Zone of southern India provides a classic example of quartz-absent muscovite melting. The anatectic gneiss shows a conspicuous migmatitic structure defined by closely spaced centimeter to decimeter sized, corundum-bearing leucosomes developed in a weakly foliated mesosome of plagioclase (An21Ab77Or2) and biotite (4.9 wt% TiO2, XMg = 0.51-0.47). The boundaries between leucosome and mesosome domains are sharp, and no melanosome selvages are developed at the interface. Corundum occurs as euhedral crystals up to 2 cm in diameter, typically centered in the leucosome matrix of coarse-grained perthitic alkali feldspar (integrated composition: An2Ab35Or63), minor relict biotite (4.2-5.1 wt% TiO2, XMg = 0.48-0.46) and plagioclase (An21Ab78Or1). In some domains, the mesosomes also contain elongate clusters of similarly oriented smaller corundum plates that are intergrown with perthitic alkali feldspar, presumably replacing former kyanite blades. The textural and mineralogical characteristics and petrogenetic grid considerations indicate breakdown of muscovite through two successive dehydration-melting reactions: (1) formation of corundum+K-feldspar-clusters via the reaction muscovite+aluminosilicate → corundum+liquid at the sites of kyanite/sillimanite, and (2) development of corundum-bearing leucosomes through the reaction muscovite → corundum+K-feldspar+liquid, focused around the sites of nucleation and growth of peritectic corundum. P-T pseudosection modeling in the Na2O-CaO-K2O-FeO-MgO-Al2O3- SiO2-H2O-TiO2 system locates the onset and completion of the muscovite-melting reaction 2 in the steep narrow quadrivariant field Ms+Bt+Pl+Kfs+Crn+Liq, which extends from ~6 kbar, 720 °C to higher pressures. Biotite remained stable and was not involved in the melting reactions. Two-feldspar thermometry gives peak-temperatures of 800 ± 50 °C. Combined with P-T estimates for metapelitic granulites in the area, these P-T constraints appear to be consistent with a clockwise P-T evolution of the eastern Palghat Cauvery shear zone with peak P-T conditions not exceeding ca. 800 °C and 10-12 kbar. The timing of partial melting and HT-metamorphism is constrained at ~529 Ma by U-Pb spot dating of oscillatory zoned individual grains and overgrowths on detrital zircon cores included in peritectic corundum of leucosome domains. The zircon cores indicate a Paleoproterozoic (2.5-2.0 Ga) provenance of the sedimentary protolith

The Basement of the Deccan Traps and Its Madagascar Connection: Constraints from Xenoliths

Paleogeographic reconstructions of India and Madagascar before their late Cretaceous rifting juxtapose the Antongil Block of Madagascar against the Deccan Traps of India, indicating that the Western Dharwar Craton extends below the Deccan lavas. Some recent studies have suggested that the South Maharashtra Shear Zone along the northern Konkan coast of India limits the northern extent of the Western Dharwar Craton, implying that the craton does not extend below the Deccan Traps, raising a question mark on paleogeographic reconstructions of India and Madagascar. The continuity of the Western Dharwar Craton north of the South Maharashtra Shear Zone below the Deccan Traps—or its lack thereof—is critical for validating tectonic models correlating Madagascar with India. In this study, zircons in tonalitic basement xenoliths hosted in Deccan Trap dykes were dated in situ, using the U-Pb isotope system. The data furnish U-Pb ages that define three populations at 2527 ± 6, 2456 ± 6, and 2379 ± 9 Ma. The 2527 ± 6 Ma ages correspond to the igneous crystallization of the tonalites, whereas the 2456 ± 6 and 2379 ± 9 Ma ages date metamorphic overprints. The results help to establish for the first time that the basement is a part of the Neoarchean granitoid suite of the Western Dharwar Craton, which extends northward up to at least Talvade in central and Kihim beach in the western Deccan. By implication, the South Maharashtra Shear Zone cannot be the northern limit of the Western Dharwar Craton. The granitoids are correlated with the Neoarchean felsic intrusions (2.57–2.49) of the Masaola suite in the Antongil Block of Madagascar, supporting the existence of a Neoarchean Greater Dharwar Craton comprising the Western Dharwar Craton and the Antongil-Masora Block.

Episodic tourmaline growth and re-equilibration in mica pegmatite from the Bihar Mica Belt, India: major- and trace-element variations under pegmatitic and hydrothermal conditions

Mica pegmatites from the Bihar Mica Belt contain three distinct generations of tourmaline. The major-element composition, substitution vectors and trajectories within each group are different, which indicates that the three types of tourmalines are not a part of one evolutionary series. Rather, the differences in their chemistries as well their mutual microtextural relations, can be best explained by growth of tourmaline from pegmatitic melts followed by episodic re-equilibration during discrete geological events. The euhedral, coarse-grained brown type I tourmaline cores have relatively high Ca, Mg (X Mg c. 0.37) and Al with correlated variation in Sr, Sc, Ti, Zr, Y, Cr, Pb and Rare Earth elements (REEs). They are inferred to have crystallized from pegmatitic melts. Monazites included within these tourmalines give chemical ages of 1290−1242 Ma interpreted to date the crystallization of the pegmatitic tourmaline. The bluish type II and greyish type III tourmalines with low Ca and Mg contents (X Mg = 0.16−0.27) and high Zn, Sn, Nb, Ta and Na, formed by pseudomorphic partial replacement of the pegmatitic tourmaline via fluid-mediated coupled dissolution–reprecipitation, are ascribed to a hydrothermal origin. The ages obtained from monazites included in these tourmalines indicate two alteration events at c. 1100 Ma and c. 950 Ma. The correlated variation of Ca, Mg and Fe and the trace elements Sr, Sn, Sc, Zn and REE within the tourmalines indicates that the trace-element concentrations of tourmaline are controlled not only by the fluid chemistry but also by coupled substitutions with major-element ions.

The Geological evolution of the Gangpur Schist Belt, eastern India: constraints on the formation of the Greater Indian Landmass in the Proterozoic

Handling Editor: Katy Evans Abstract The Central Indian Tectonic Zone (CITZ) is a Proterozoic suture along which the Northern and Southern Indian Blocks are inferred to have amalgamated forming the Greater Indian Landmass. In this study, we use the metamorphic and geochronological evolution of the Gangpur Schist Belt (GSB) and neighbouring crustal units to constrain crustal accretion processes associated with the amalgamation of the Northern and Southern Indian Blocks. The GSB sandwiched between the Bonai Granite pluton of the Singhbhum craton and granite gneisses of the Chhotanagpur Gneiss Complex (CGC) links the CITZ and the North Singhbhum Mobile Belt. New zircon age data constrain the emplacement of the Bonai Granite at 3,370 ± 10 Ma, while the magmatic protoliths of the Chhotanagpur gneisses were emplaced at c. 1.65 Ga. The sediments in the southern part of the Gangpur basin were derived from the Singhbhum craton, whereas those in the northern part were derived dominantly from the CGC. Sedimentation is estimated to have taken place between c. 1.65 and c. 1.45 Ga. The Upper Bonai/Darjing Group rocks of the basin underwent major metamorphic episodes at c. 1.56 and c. 1.45 Ga, while the Gangpur Group of rocks were metamorphosed at c. 1.45 and c. 0.97 Ga. Based on thermobarometric studies and zircon–monazite geochronology, we infer that the geological history of the GSB is similar to that of the North Singhbhum Mobile Belt with the Upper Bonai/Darjing and the Gangpur Groups being the westward extensions of the southern and northern domains of the North Singhbhum Mobile Belt respectively. We propose a three‐stage model of crustal accretion across the Singhbhum craton—GSB/North Singhbhum Mobile Belt—CGC contact. The magmatic protoliths of the Chhotanagpur Gneisses were emplaced at c. 1.65 Ga in an arc setting. The earliest accretion event at c. 1.56 Ga involved northward subduction and amalgamation of the Upper Bonai Group with the Singhbhum craton followed by accretion of the Gangpur Group with the Singhbhum craton–Upper Bonai Group composite at c. 1.45 Ga. Finally, continent–continent collision at c. 0.96 Ga led to the accretion of the CGC with the Singhbhum craton–Upper Bonai Group–Gangpur Group crustal units, synchronous with emplacement of pegmatitic granites. The geological events recorded in the GSB and other units of the CITZ only partially overlap with those in the Trans North Received: 17 February 2018 | Accepted: 13 September 2018 DOI: 10.1111/jmg.12452