Differences in isotopic signature of Neoproterozoic crust of NW India - evidence for a lithospheric boundary?
Final Report Abstract
In this project we analysed major and trace elements and Sr and Nd isotopic compositions of felsic and mafic plutonic and volcanic rocks in NW India. A compilation of our new data for the Indian sector along with published data from Nagar Parkar in E-Pakistan and well-studied Neoproterozoic basement inliers in E-Oman led to an evolutionary model for an arc to continent transition during Tonian times. Basement inliers in E Oman represent the fore-arc to arc setting and Nagar Parkar in SE Pakistan part of the volcanic arc. Granodiorites, gabbros and granites in NW India (Harsani-type and Bhinmal-type basement) are emplaced in the back-arc region into thinned and stretched continental crust. The data indicate ca 130 Myr of ongoing subduction (900 to 770 Ma) with magmatic activity identified as episodic comprising distinct periods of intense melt generation in the arc history at ca. 840 Ma, ca. 825 Ma and 770 Ma. Our new model relates the formation of the Malani Igneous suite to slab roll-back in the late-stage of ca. 130 Myr of Cordilleran-type subduction in front of the Neoproterozoic India-South China plate. Remelting of the crust was triggered by underplating of asthenosphere and melt emplacement focused along lithospheric discontinuities in the pre-Malani basement architecture. Mafic dykes with pure asthenospheric composition indicate deep fracturing during crustal extension and formation of rift structures. Our data from an E to W section cover the transition from pre-Malani stable continental crust into the continental margin of the Indian plate and identified a significant influence of the underlying basement in the geochemical and isotopic composition of the MIS. This is especially evident in differences in Nd isotopes, as the Malani rocks mimics the signature of the pre-Malani basement with negative εNd values east of the Jaswantpura Belt and positive εNd values west of this lineament. This change in isotopic signature is related to pre-Malani continental margin evolution, signatures of contribution of continental crust in melt generation is also seen in the Seychelles and E-Oman and does not support a lithospheric boundary within the Trans-Aravalli Block in NW India.
Publications
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Neoproterozoic geodynamics in NW India – evidence from Erinpura granites in the South Delhi Fold Belt. International Geology Review, 64(8), 1051-1080.
De Wall, Helga; Regelous, Anette; Schulz, Bernhard; Hahn, Gregor; Bestmann, Michel & Sharma, Kamal Kant
