Mantle heterogeneity,plume-lithosphere interaction at rift controlled ocean-continent transition zone:Evidence from trace-PGE geochemistry of Vempalle flows,Cuddapah Basin,India

This study reports major, trace, rare earth and platinum group element compositions of lava flows from the Vempalle Formation of Cuddapah Basin through an integrated petrological and geochemical approach to address mantle conditions, magma generation processes and tectonic regimes involved in their formation. Six flows have been identified on the basis of morphological features and systematic three-tier arrangement of vesicular-entablature-colonnade zones. Petrographically, the studied flows are porphyritic basalts with plagioclase and clinopyroxene representing dominant phenocrystal phases.Major and trace element characteristics reflect moderate magmatic differentiation and fractional crystallization of tholeiitic magmas. Chondrite-normalized REE patterns corroborate pronounced LREE/HREE fractionation with LREE enrichment over MREE and HREE. Primitive mantle normalized trace element abundances are marked by LILE-LREE enrichment with relative HFSE depletion collectively conforming to intraplate magmatism with contributions from sub-continental lithospheric mantle(SCLM) and extensive melt-crust interaction. PGE compositions of Vempalle lavas attest to early sulphur-saturated nature of magmas with pronounced sulphide fractionation, while PPGE enrichment over IPGE and higher Pd/Ir ratios accord to the role of a metasomatized lithospheric mantle in the genesis of the lava flows. HFSEREE-PGE systematics invoke heterogeneous mantle sources comprising depleted asthenospheric MORB type components combined with plume type melts. HFSE-REE variations account for polybaric melting at variable depths ranging from garnet to spinel lherzolite compositional domains of mantle. Intraplate tectonic setting for the Vempalle flows with P-MORB affinity is further substantiated by(i) their origin from a rising mantle plume trapping depleted asthenospheric MORB mantle during ascent,(ii) interaction between plume-derived melts and SCLM,(iii) their rift-controlled intrabasinal emplacement through Archeane Proterozoic cratonic blocks in a subduction-unrelated ocean-continent transition zone(OCTZ). The present study is significant in light of the evolution of Cuddapah basin in the global tectonic framework in terms of its association with Antarctica, plume incubation, lithospheric melting and thinning, asthenospheric infiltration collectively affecting the rifted margin of eastern Dharwar Craton and serving as precursors to supercontinent disintegration.

Detrital zircon U-Pb geochronology of stromatolitic carbonates from the greenstone belts of Dharwar Craton and Cuddapah basin of Peninsular India

Oldest rocks are sparsely distributed within the Dharwar Craton and little is known about their involvement in the sedimentary sequences which are present in the Archean greenstone successions and the Proterozoic Cuddapah basin.Stromatolitic carbonates are well preserved in the Neoarchean greenstone belts of Dharwar Craton and Cuddapah Basin of Peninsular India displaying varied morphological and geochemical characteristics.In this study,we report results from U-Pb geochronology and trace element composition of the detrital zircons from stromatolitic carbonates present within the Dharwar Craton and Cuddapah basin to understand the provenance and time of accretion and deposition.The UPb ages of the detrital zircons from the Bhimasamudra and Marikanve stromatolites of the Chitradurga greenstone belt of Dharwar Craton display ages of 3426±26 Ma to 2650±38 Ma whereas the Sandur stromatolites gave an age of 3508±29 Ma to 2926±36 Ma suggesting Paleo-to Neoarchean provenance.The U-Pb detrital zircons of the Tadpatri stromatolites gave an age of 2761±31 Ma to1672±38 Ma suggesting Neoarchean to Mesoproterozoic provenance.The Rare Earth Element(REE)patterns of the studied detrital zircons from Archean Dharwar Craton and Proterozoic Cuddapah basin display depletion in light rare earth elements(LREE)and enrichment in heavy rare earth elements(HREE)with pronounced positive Ce and negative Eu anomalies,typical of magmatic zircons.The trace element composition and their relationship collectively indicate a mixed granitoid and mafic source for both the Dharwar and Cuddapah stromatolites.The 3508±29 Ma age of the detrital zircons support the existence of 3.5 Ga crust in the Western Dharwar Craton.The overall detrital zircon ages(3.5-2.7 Ga)obtained from the stromatolitic carbonates of Archean greenstone belts and Proterozoic Cuddapah basin(2.7-1.6 Ga)collectively reflect on^800-900 Ma duration for the Precambrian stromatolite deposition in the Dharwar Craton.

Phanerozoic magmatism in the Proterozoic Cuddapah Basin and its connection with the Pangean supercontinent

Magmatic pulses in intraplate sedimentary basins are windows to understand the tectonomagmatic evolution and paleaoposition of the Basin.The present study reports the U-Pb zircon ages of mafic flows from the Cuddapah Basin and link these magmatic events with the Pangean evolution during late Carboniferous-Triassic/Phanerozoic timeframe.Zircon U-Pb geochronology for the basaltic lava flows from Vempalle Formation,Cuddapah Basin suggests two distinct Phanerozoic magmatic events coinciding with the amalgamation and dispersal stages of Pangea at 300 Ma(Late Carboniferous)and 227 Ma(Triassic).Further,these flows are characterized by analogous geochemical and geochronological signatures with Phanerozoic counterparts from Siberian,Panjal Traps,Emeishan and Tarim LIPs possibly suggesting their coeval and cogenetic nature.During the Phanerozoic Eon,the Indian subcontinent including the Cuddapah Basin was juxtaposed with the Pangean LIPs which led to the emplacement of these pulses of magmatism in the Basin coinciding with the assemblage of Pangea and its subsequent breakup between 400 Ma and 200 Ma.