Geochemistry and geochronology of A-type basement granitoids in the north-central Aravalli Craton: Implications on Paleoproterozoic geodynamics of NW Indian Block

The basement granite gneisses from the north-central Aravalli Craton in NW India were investigated for geochemistry and geochronology.In a peneplain terrain,the granite gneiss outcrops are scanty and samples were collected mainly from two small hills and several ground-level exposures in the Sakhun–Ladera region.Wellfoliated granite gneiss is the dominant lithology that also hosts dark,lenticular enclaves,and is in turn,intruded by mafic dykes.The granite gneiss has silica content ranging from 61.37 wt.%to 68.27 wt.%that marks a slight overlap with the enclaves(54.32wt.%to 62.17wt.%).Both groups have a highK2O/Na2O(~2 or higher)ratio.Geochemically,the granite gneiss classify as granite–granodiorite,and enclaves as granodiorite-diorite.The In-situ LA-ICP-MS zircon U–Pb geochronology of granite gneiss has yielded a statistically valid 1721±9 Ma age that we interpret as the emplacement age for the granitic protolith.Geochemical characteristics of granite gneiss underline fractional crystallization of an I-type melt as themain process,and continuity of trends in enclaves underlines their mutual genetic link.The genetic association is further verified by a consistency in the trace element characteristics and REE patterns.The Nd-isotope signatures define a single grouping for both granite gneiss and enclaves,withεNd(t)values ranging from−6.38 to−6.61,further substantiating a common source.The geochemical tectonic discrimination schemes consistently point toward an extensional setting and A-type characteristics for granite gneiss and enclaves.These are analogous to the coeval(1.72–1.75 Ga),A-type granitoids from the Khetri and Alwar basin in the North Delhi Fold Belt,implying a much larger areal extent for the Paleoproterozoic anorogenic magmatism in the northern segment of the Aravalli Craton.The Paleoproterozoic age for the presumed‘Archean’basement in this region offers tacit evidence that the BGC–II is a stratigraphically younger terrane as compared to the Archean age,BGC–I.

Age of the Marwar Supergroup,NW India:A note on the U–Pb geochronology of Jodhpur Group felsic volcanics

The Marwar Supergroup(NW Peninsular India)is thought to be of Ediacaran-Cambrian age,based on previous paleontological and geochronological studies.However,direct constraints on the onset of sedimentation within the Marwar basin are still scarce.In this study,we report U–Pb zircon,LA-ICP-MS,and SIMS ages from the Chhoti Khatu felsic volcanic rocks,interlayered with the Jodhpur Group sandstones(Lower Marwar Supergroup).The cathodoluminescence images of the zircons indicate complex morphologies,and core-rim textures coupled with the wide range of ages indicate that they are likely inherited or in the case of thin poorly indurated ash-beds,detrital in origin.The age spectra of 68 zircon analyses from our sampling display a dominant 800–900 Ma age peak corresponding to the age of basement"Erinpura granite"rocks in the region.The youngest inherited zircon from a felsic ash layer yielded a U–Pb age of651 Ma±18 Ma that,together with previous studies and paleontological evidence,indicates a postCryogenian age for the initiation of Marwar sedimentation following a~125 Ma hiatus between the end of Malani magmatism and Marwar deposition.

The drift history of the Dharwar Craton and India from 2.37 Ga to 1.01 Ga with refinements for an initial Rodinia configuration

Coupled paleomagnetic and geochronologic data derived from mafic dykes provide valuable records of continental movement.To reconstruct the Proterozoic paleogeographic history of Peninsular India,we report paleomagnetic directions and U-Pb zircon ages from twenty-nine mafic dykes in the Eastern Dharwar Craton near Hyderabad.Paleomagnetic analysis yielded clusters of directional data that correspond to dyke swarms at 2.37 Ga,2.22 Ga,2.08 Ga,1.89-1.86 Ga,1.79 Ga,and a previously undated dual polarity magnetization.We report new positive baked contact tests for the 2.08 Ga swarm and the 1.89-1.86 Ga swarm(s),and a new inverse baked contact test for the 2.08 Ga swarm.Our results promote the 2.08 Ga Dharwar Craton paleomagnetic pole(43.1°N,184.5°E;A95=4.3°)to a reliability score of R=7 and suggest a position for the Dharwar Craton at 1.79 Ga based on a virtual geomagnetic pole(VGP)at 33.0°N,347.5°E(a95=16.9°,k=221,N=2).The new VGP for the Dharwar Craton provides support for the union of the Dharwar,Singhbhum,and Bastar Cratons in the Southern India Block by at least 1.79 Ga.Combined new and published northeast-southwest moderate-steep dual polarity directions from Dharwar Craton dykes define a new paleomagnetic pole at 20.6°N,233.1°E(A95=9.2°,N=18;R=5).Two dykes from this group yielded 1.05-1.01 Ga 207Pb/206Pb zircon ages and this range is taken as the age of the new paleomagnetic pole.A comparison of the previously published poles with our new 1.05-1.01 Ga pole shows India shifting from equatorial to higher(southerly)latitudes from 1.08 Ga to 1.01 Ga as a component of Rodinia.

Decoupling between Oxygen and Radiogenic Isotopes: Evidence for Generation of Juvenile Continental Crust by Partial Melting of Subducted Oceanic Crust

There is increasing evidence indicating that melts derived from subducted oceanic crust and sediments may have played a key role in building continental crust. This mechanism predicts that juvenile arc crust should have oxygen isotope characteristics ranging from mantle-like to supracrustal, but consistent mantle-like radiogenic(Nd-Hf) isotopic signatures. Here we present in-situ zircon U-Pb dating, Hf-O isotope analyses, and whole rock major-trace element and Nd isotope analyses of a granitoid from NW India. In-situ secondary ion mass spectrometry(SIMS) zircon U-Pb dating yields a weighted mean ^(207)Pb/^(206)Pb age of 873±6 Ma for the granitoid. It displays mantle-like zircon εHf(εHf(873 Ma)= +9.3 to +10.9) and whole-rock Nd(εNd(873 Ma)= +3.5) values but supracrustal δ^(18)O values, the latter mostly varying between 9‰ and 10‰. The calculated whole-rock δ^(18)O value of 11.3‰±0.6‰ matches well with those of hydrothermally-altered pillow lavas and sheeted dykes from ophiolites. The major and trace element composition of the granitoid is similar to petrological experimental melts derived from a mixture of MORB+sediments. Thus, the granitoid most likely represents the product of partial melting of the uppermost oceanic crust(MORB+sediments). We propose that the decoupling between Hf-Nd and O isotopes as observed in this granitoid can be used as a powerful tool for the identification of slab melting contributing to juvenile continental crustal growth. Such isotopic decoupling can also account for high δ^(18)O values observed in ancient juvenile continental crust, such as Archean tonalitetrondhjemite-granodiorite suites.