Neoarchean Microblock Amalgamation In Southern India

The Indian Peninsula is composed of five major cratons including the Dharwar Craton(DC),Aravalli Craton(AC),Bundhelkland Craton(BKC),Bastar Craton(BC),Singbhum Craton(SC).As the largest craton in India,Dharwar Craton is bordered by a number of crustal blocks to the in south,including the Coorg Block(CB),Nilgiri Block(NB),Billigiri Rangan Block(BRB),Shevaroy Block(SB),Madras Block(MB),Namakkal Block(NB).Systematic studies on the geological and tectonic evolution of the individual blocks within the major cratons and their assembly with other crustal blocks are lacking.Such studies are important in contributing to our understanding of the Archean to Paleoproterozoic crust evolution on the globe and supercontinent cycles.In this study,three critical regions were selected from different blocks in southern India(Karwar Block in the Western Dharwar Craton,Chitradurga Suture Zone between the Western and Central Dharwar Craton,Nallamalai Suture Zone between Shevaroy and Madras Block)for detailed petrological,geochemical and geochronological studies,and in conjunction with results from previous studies,to evaluate the crustal evolution history and tectonics.Zircon U-Pb coupled with in-situ Rb-Sr dating from the granitic and amphibolite rocks of Karwar Block(KB)show multiple Mesoarchean to Neoarchean magmatism at 3.2-2.9 Ga,2.8-2.6 Ga,2.2 Ga.The upper amphibolite-and granulite-facies meta-igneous and metasedimentary rocks from the Chitradurga Suture Zone(ChSZ)yield similar zircon U-Pb ages of 3.10-3.03 Ga,2.97-2.86 Ga,2.61-2.51 Ga.Zircon grains from the hornblende-biotite gneiss,amphibolite,granodiorite,diorite,charnockite,BIF(banded iron formation)and BMQ from the Nallamalai Suture Zone(NLSZ)show magmatic age clustering around 2.56 to 2.50 Ga,except the meta-monzogranite showing ages up to 3.2 Ga.Zircon U-Pb and monazite Pb-Pb data from Chitradurga Suture Zone and zircon U-Pb data from the Nallamalai Suture Zone reveal regional metamorphism of garnet-amphibolite and garnet-granulite facies at c.2.48-2.44 Ga represent synchronous bracket the timing of microblock accretion to the Archean-Proterozoic boundary.The Mesoproterozoic metamorphism/thermal imprint in Karwar Block(as young as 1.2 Ga)correlates to extension of Betsimisaraka Suture Zone in Madagascar.The results reveal that the Karwar Block and the Western Dharwar Craton(WDC)is one single continent during Mesoarchean,the Block(Western Dharwar Craton)in southern Peninsular India likely represent fragments of the first landmass or supercontinent on our planet.Comparison of geochronology data shows that the WDC and KB experienced(?)a major Mesoarchean continental growth event(c.3.2 to 2.9 Ga),(?)younger Neoarchean and Paleoproterozoic magmatism/migmatization(syn-and post-tectonic granite formation events at c.2.8-2.6 Ga and 2.2 Ga,respectively).The magma emplacement from all these domains(Karwar Block,Chitradurga Suture Zone and Nallamalai Suture Zone)are correlating with long-lived convergent margin magmatism through multiple slab melting episodes,the metamorphic age marking the timing of collision of two continental blocks with consumption of the intervening oceanic lithosphere.Zircon Lu-Hf isotope data show diverse origins.Zircon sHf(t)from the Karwar Block exhibit positive values of+0.4 to+9.8 with TDMC age of 3.54-2.84 Ga,which suggests juvenile crust and depleted mantle source for the magmatism that constructed this crustal block.Zircon ?Hf(t)of magmatic zircon grains from the Nallamalai Suture zone yield both positive and negative values range from-4.1 to+5.7,together with TDMC Hf model ages of 2672-3247 Ma,a dominantly juvenile crust growth is indicated,which initiated around 3.3 Ga and continued to 2.7 Ga,followed by crustal reworking in a continental arc towards the end of Neoarchean.The magma provenance also involved deplete mantle source,which derived by subducted slab melting.In addition,detrital magmatic zircon grains from Chitradurga Suture Zone with ages of c.3.10-3.03 Ga and c.2.97-2.86 Ga imply contamination of these magmas with Mesoarchean material sourced from the Western Dharwar Craton continental nucleus.Thus,the Lu-Hf isotopic data from the Dharwar craton and Nallamalai Suture Zone imply Mesoarchean to Neoarchean crust growth and Neoarchean crust recycling in south Peninsular India.Geochemically,the gran itic samples from the Karwar Block display high LREE/HREE and A1 contents suggest variable degrees of mantle melting in multiple phases and at different crustal levels.Low(Nb/La)PM and high(Th/Nb)PM ratios are attributed to(?)reworking of older crustal components and(ii)generation of parent magma from a source of mantle metasomatized by subduction components.Low Sr/Y ratio and Y content for the amphibolite and hornblende gneiss samples also reflects a subduction imprint for their origin and evolution.Tonalite-trondhjemite-granodiorite(TTG)gneisses and potassic granitoids comprise integral manifestations of Precambrian crust building episodes and yield robust evidence for primitive mantle differentiation and crustal evolution in early earth.In Nallamalai Suture Zone,the geochemical data suggest that tholeiitic to calc-alkaline parental melts for the mafic-intermediate-felsic suite were generated by low degree partial melting of a peridotitic mantle wedge metasomatized by subduction-derived fluids and sediments and continuous slab melting ensued by intracrustal fractional crystallization of melts with reworking of older continental crust.The BIF and BMQ samples display positive Eu anomalies,negative to negligible Ce anomalies,and superchondritic Y/Ho ratios suggesting their formation in an oceanic realm with dissolved iron and silica and contributions from hydrothermal fluids of volcanic origin,followed by deposition in an arc related off-shelf marine environment proximal to an active continental margin setting.The tectonic attributes are consistent with eastward oceanic subduction and ocean closure along the NLSZ which is defined here as the trace of a suture welding the Shevaroy and Madras Blocks.Metamorphic P-T conditions were evaluated using a combined petrological and geochronological study of high grade rocks along the Chitradurga Suture Zone that marks the collisional boundary between the Western Dharwar Craton(WDC)and the Central Dharwar Craton(CDC).Monazite Pb-Pb and zircon U-Pb dating reveals regional garnet-amphibolite and garnet-granulite facies metamorphism at c.2.48-2.44 Ga.The data suggest a short-lived metamorphism for the amalgamation and collision of Western Dharwar Craton and Eastern Dharwar Craton.Calculated conventional thermobarometry and petrological phase equilibria modelling from the upper amphibolite-and granulite-facies meta-igneous and metasedimentary rocks reveals metamorphic conditions at?10 kbar with temperatures between-820-875?,indicating equilibration at the base of thickened continental crust.Light rare earth element(LREE)enrichment within the zircon grains indicates magma formation in a suprasubduction zone environment.Comparison of these metamorphic age data with those recorded in the EDC show that the WDC,CDC,and EDC must have accreted synchronously,driven by two separate eastward-dipping ocean-continent convergent plate margins.These data further support a recent abundance of observations from the geological record supporting the hypothesis that subduction-driven plate tectonics had initiated on Earth before c.2.5 Ga,as opposed to a Neoproterozoic onset(c.0.8-0.9 Ga)reported by numerous studies.Based on lithology,geochronology and crust thickness studies,nine crustal blocks are recognized in the southern part of Peninsular India a including the Coorg Block(<32 km),Nilgiri Block(?41 km),Billigiri Rangan Block(-38-33 km),Shevaroy Block(-32 km),Madras Block(-32 km),Namakkal Block(<32 km),Western Dharwar Craton(35 km),Central Dharwar Craton(-34 km),Eastern Dharwar Craton(?34-32 km).Paleoarchean to Mesoarchean magmatism and continent nuclei formation are well illustrated in Western Dharwar Craton,Coorg Block,Billigiri Rangan Block.Mesoarchean to Neoarchean continent growth(2.86-2.48 Ga)is widespread in the Eastern Dharwar Craton,Nilgiri Block,Shevaroy Block and Madras Block.Western Dharwar Craton collided with the Coorg Block during 3.18-3.08 Ga,following by an eastward subduction though Billigiri Rangan Block to the Shevaroy Block until the Shevaroy Block.Multiple subduction and amalgamation of several microblocks occurred in the Dharwar Craton and its southern domains,amalgamating several microblocks during the Archean-Paleoproterozoic transition(2.57-2.40 Ga).