Petrology, geochemistry and tectonic significance of Palaeoproterozoic alkaline lamprophyres from the Jungel Valley, Mahakoshal supracrustal belt, Central India

Abstract

Palaeoproterozoic (~1.6 Ga) lamprophyres occur as dykes, plugs and lava flows within the Mahakoshal supracrustal belt of the Jungel Valley, Central India. Although metamorphosed under greenschist facies conditions, the lamprophyres still retain remnants of the diagnostic igneous textures viz., porphyritic-panidiomorphic habit of mafic phenocrysts, leucocratic (carbonate-rich) ocelli and volatile-rich mineralogy. The studied lamprophyres are mineralogically uniform but texturally variable (porphyritic to aphanitic). Chemically they are silica undersaturated, ultrabasic, dominantly alkaline (some of them have normative nepheline) through mildly alkaline to sub-alkaline, sodic to mildly potassic and their magmas are primitive as well as evolved. The lamprophyres are characterized by Ni and Cr abundances and MgO sufficiently high to indicate a mantle source. The chondrite-normalized REE patterns confirm crystallization from a LREE enriched magma. Combined petrological and geochemical studies classify them as belonging to alkaline lamprophyre category in general and camptonites in particular. Geochemistry also suggests that their magmas are not affected by any crustal contamination. This study establishes the occurrence of lamprophyres in the Jungel area and brings to end the speculations about their mistaken identity. The presence of a wide spectrum of undersaturated alkaline magmatism comprising lamprophyres, syenites, tinguites, and possibly kimberlties, highly carbonated rocks and ultrabasic rocks as intrusives in the Mahakoshal supracrustal belt undoubtedly highlights the presence of a carbonate-rich palaeo- to early Proterozoic alkaline province. The occurrence of greenschist facies metamorphosed lamprophyres of Palaeoproteorozoic age in the eastern central Indian shield is also significant in the context of re-construction of ancient continental fits as similar rocks of comparable age have been reported from the Yilgarn and Pilbara cratons of Western Australia.