Precambrian mafic magmatism in the Indian shield: an introduction

Abstract

The phases of mafic magmatism, which periodically punctuate the geological record on every continent, represent major thermal events, resulted in extensive mantle melting. Mafic magmatism may also be linked to important tectonic events that disturbed the continent. Out of these, the Precambrian mafic magmatism is an important phenomenon that occurred all over the globe and played an essential role in crustal evolution. At that time the Earth was a hotter body than it is now and more capable of generating higher temperature magmas. Wide varieties of mafic and ultramafic lavas that includes komatiite, komatiitic basalt and sub-alkaline basalts, form a significant proportion of most low-grade Archaean supracrustal successions. One of the most fundamental question concerning Precambrian mafic magmas is whether or not they are geochemically and petrogenetically the same as those produced in equivalent tectonic settings at the present day. The most significant difference between mafic suites of Archaean and those of post-Archaean times is the abundances of Archaean komatiite suites (Hall and Hughes, 1990; Ernst et al. 1995). Although most of the geologists believe that the present is the key to the past but in many cases it does not hold good while considering rocks of Precambrian age, in particular the Archaean rocks. It is now established that the earliest formed rocks cannot have similar tectono-magmatic controls as those formed during post-Precambrian periods. The Archaean earth was very different from the modern day earth, with many tectonic features of the lithosphere being affected by secular change due to the slow cooling of the planet from a liquid magma ocean (~ ca. 4.0 Ga), to the one covered by the same proportion of continental crust by 2.7 Ga (Sylvester et al. 1997). Thus, uniformitarianism - in the strict sense - cannot be applied for the Precambrian tectono-magmatic processes.