Neoproterozoic (942 Ma) calc-alkaline magmatism from Simdega, Chhotanagpur Gneissic Complex and their mineralisation aspects

Chhotanagpur Gneissic Complex (CGC) is a part of the E-W trending Central Indian Tectonic Zone (CITZ), India. The CITZ is a major intercontinental suture which separates the northern Indian and the southern Indian blocks whose subduction polarity is a contentious issue. We present petrography, mineral chemistry, bulk rock geochemistry, Lu-Hf, Re-Os and Pb-Pb isotopes of the Neoproterozic lamprophyre from Simdega. The studied lamprophyre is an unmetamorphosed and undeformed that exhibits a strong porphyritic-panidiomorphic texture imparted by the megacrysts/phenocrysts of mica and amphibole with feldspar, apatite, titanite, zircon and opaques confined to the groundmass. Our lamprophyre shows shoshonitic affinities and is classified to be of calc-alkaline variety (minette). The Mg# values (ranging from 70.7 to 78.2) indicate a primitive melt character while the trace element ratios are consistent with those of subduction-related rocks globally as well as with the calc-alkaline lamprophyres from the Eastern Dharwar Craton (southern India) that suggest no crustal contamination. Our findings demonstrate that the western part of the CGC was less affected by the M3 regional amphibolite-grade metamorphic event (ca. 920-880 Ma) compared to the eastern part. Our study also supports geodynamic models proposing northward subduction of the Southern Indian block beneath the Northern Indian block synchronous to the Rodinia assembly. Scanning Electron Microscopy (SEM) helped to identify some of the mineral phases occurring as inclusions in zircons having abnormally higher Thorium (Th) and Silver (Ag) concentrations.