Neoarchaean crustal evolution along the eastern flank of Nallamalai Shear Zone, southern India
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The Southern Granulite Terrane (SGT) in India is composed of Archaean to Proterozoic crustal blocks and intervening shear/suture zones. The Nallamalai shear zone lies along the eastern flank of the Shevaroy Block. This study is focused on the eastern part of this shear zone where the basement rocks are dominantly composed of charnockite, quartzo-feldspathic gneiss and metagabbro, all of which preserve original magmatic texture. Zircon U-Pb LA-ICPMS analysis on the charnockite samples yielded an age of 2680 to 2500 Ma for the protolith emplacement followed by metamorphism at 2520 to 2450 Ma. Magmatic zircon grains in the quartzo-feldspathic gneiss and metagabbro yielded weighted mean ages of ca. 2557 Ma and ca. 2583 Ma with metamorphism at ca. 2518 Ma. Zircon Lu-Hf data suggest that the protolith of metagabbro was sourced mainly from the recycled Mesoarchaean crustal component. The ƐHf(t) of charnockite (−0.4 to 4.1) and quartzo-feldspathic gneiss (0.5 to 6.0) represent a combined source of the mantle as well as recycling crust. All the rocks underwent amphibolite to granulite facies metamorphism and petrological studies and pseudosection modelling shows a similar P-T range of 7–8 kbar and 650–830°C. Results from the present study suggest that crustal evolution along the eastern flank of the Nallamalai Shear Zone involved melting of older crustal components (Mesoarchaean age) with significant juvenile input (ƐHf(t) = ~+6.0) within a continental arc setting.Keywords:
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Charnockite
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Indian Shield
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The Rae craton on Melville Peninsula, Nunavut, comprises several lithotectonic domains, including a structurally and lithologically distinct yet poorly known crustal terrane, the Repulse Bay block (RBb). This study presents new lithological and petrographic observations, combined with U–Pb zircon data, to better understand the Archean and Paleoproterozoic crustal evolution of the RBb. The new data demonstrate that the central-eastern RBb consists of the following: (i) upper amphibolite- to granulite-facies, ca. 2.73–2.71 Ga intermediate granitoid gneisses and gabbroic sheets; (ii) ca. 2.69 Ga two-pyroxene charnockite to enderbite intrusions; and (iii) thin slivers of both Archean and Paleoproterozoic supracrustal rocks. Inherited zircon also attests to the presence of a Mesoarchean to Paleoarchean substrate. A semi-pelitic gneiss from one of the Paleoproterozoic supracrustal panels was deposited sometime after ca. 1.89 Ga and shows a similar detrital zircon age profile to a <1.92 Ga semi-pelitic gneiss from the Lyon Inlet boundary zone at the northern extent of the RBb. Zircon in most rocks analyzed record metamorphism related to the Trans-Hudson orogeny between ca. 1.84 and 1.82 Ga. Results from this study are most consistent with the RBb, representing a piece of lower to middle crustal level of the Rae craton, rather than a distinct and separate crustal entity (i.e., an exotic block).
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The Pikwitonei Granulite Domain located at the northwestern margin of the Superior Province is one of the largest Neoarchean high-grade terranes in the world, with well-preserved granulite metamorphic assemblages preserved in a variety of lithologies, including enderbite, opdalite, charnockite, and mafic granulite. U–Pb geochronology has been attempted to unravel the protolith ages and metamorphic history of numerous lithologies at three main localities; Natawahunan Lake, Sipiwesk Lake, and Cauchon Lake. The U–Pb age results indicate that some of the layered enderbite gneisses are Mesoarchean (3.4–3.0 Ga) and the more massive enderbites are Neoarchean. The high-grade metamorphic history of the Pikwitonei Granulite Domain is complex and multistage with at least four episodes of metamorphic zircon growth identified: (1) 2716.1 ± 3.8 Ma, (2) 2694.6 ± 0.6 Ma, (3) 2679.6 ± 0.9 Ma, and (4) 2642.5 ± 0.9 Ma. Metamorphic zircon growth during episodes 2 and 3 are interpreted to be regional in extent, corresponding to M 1 amphibolite- and M 2 granulite-facies events, respectively, consistent with previous field observations. The youngest metamorphic episode at 2642.5 Ma is only recognized at southern Cauchon Lake, where it coincides with granite melt production and possible development of a major northeast-trending deformation zone. The timing and multistage metamorphic history recorded in the Pikwitonei Granulite Domain is similar to most Superior Province high-grade terranes and marks a fundamental break in Archean crustal evolution worldwide at the termination of prolific global Neoarchean greenstone belt formation.
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Granitoid gneisses dominated by tonalitic–trondhjemitic–granodioritic (TTG) compositions, with metamorphic supracrustal rocks consisting of sedimentary and volcanic rocks, are widely exposed in the Eastern Hebei terrane, North China Craton (NCC). This study presents systematic zircon U–Pb geochronological and whole-rock geochemical data of the Neoarchean granitoid gneisses and supracrustal rocks in Eastern Hebei. Zircon U–Pb isotopic dating for the representative samples reveals that magmatic precursors of granitoid gneisses were emplaced between 2524 ± 7 and 2503 ± 12 Ma, and the protoliths of the pelitic granulites were deposited in the Late Neoarchean era. Both of them have been subjected to granulite facies metamorphism during 2508 ± 10 to 2468 ± 33 Ma, coeval with the intrusion of syenogranitic pegmatite (2488 ± 5 Ma). Zircon ages of 2.45–2.01 Ga obtained from the analyzed samples were considered mixed data from 2.53–2.48 Ga and 1.9–1.8 Ga and were chronologically meaningless. Paleoproterozoic metamorphic zircon ages of 1.9–1.8 Ga were usually neglected because of hardly being obtained from TTG gneisses and supracrustal rocks. The tectonic regime during the Neoarchean era was considered to be dominated by vertical tectonism in the Eastern Hebei terrane.
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