Dynamics of Early Neoproterozoic accretion, west-central India: II ~1.65 Ga HT-LP and ~0.95 Ga LT-HP metamorphism in Godhra-Chhota Udepur, and a tectonic model for Early Neoproterozoic accretion
Résumé
The N/NNE-striking Aravalli Delhi Fold Belt (ADFB) and the E-striking Central Indian Tectonic Zone (CITZ) converge at the Godhra-Chhota Udepur (GC) sector, west-central India. Analyses of mesoscale deformation structures and metamorphic phase equilibria in the basement and supracrustal rocks are integrated with geochronological-geochemical data (accompanying article) to address the dynamics of the Early Neoproterozoic CITZ-ADFB accretion. In the GC sector, ~1.65 Ga granulite facies anatectic gneisses, ~2.5 Ga and 1.03–1.02 Ga granitoids, and greenschist to amphibolite facies allochthonous supracrustal rocks constitute a tectonic mélange (D2 deformation). The lithodemic units are traversed by networks of W/WNW-striking steep-dipping transpressional shear zones with sinistral kinematics (D3). The shallow-dipping mélange with top-to-the south kinematics is intruded by post-D2 to syn-D3 0.95–0.93 Ga granitoids. Mn-NCKFMASH P-T pseudosection analyses of the anatectic gneisses with pre-D2 garnet + cordierite-bearing leucosomes suggest the rocks evolved along a clockwise P-T path in the range of 5–6 kbar and 680–720 °C. By contrast, the Early Neoproterozoic (0.95–0.93 Ga) chlorite + phengite (Si up to 3.32 apfu) + clinozoisite + quartz ± biotite ± garnet schists in the mélange attained pre/syn-D2 peak metamorphic conditions (10–12 kbar, 450–500 °C). NCKFMASH pseudosection analyses of the ~0.95 Ga phengite-bearing schists indicate the supracrustal rocks evolved along a high-P, low-T clockwise path; phengite-poor micas defining the D3 fabric (Si up to 3.04 apfu) attest to post-peak decompression in the schists. The ~2.5 Ga ADFB granites accreted with the ~1.65 Ga HT-LP anatectic gneisses, 1.03 Ga granites, and the ~0.95 Ga LT-HP supracrustal rocks of the CITZ during D2 thrusting. The crustal convergence continued with the emplacement of post-D2 0.95–0.93 Ga granitoids that culminated with the nucleation of W/WNW-striking D3 transpressional shear zones. This broad contemporaneity among felsic plutonism, LT-HP metamorphism in the supracrustal rocks, and the D2-D3 shortening are interpreted to be the result of a switch in subduction polarity between 1.03 and 0.93 Ga during oblique ADFB-CITZ convergence.
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