Review: the charnockite problem, a twenty first century perspective

Abstract

Beginning of the twentieth century was marked by coinage of a new rock name, Charnockite, first described as a hypersthene-bearing granite from Southern India. Since then charnockites have been described from most of the conti-nents and mostly restricted to high-grade belts. Later half of the last century saw a lively debate over an igneous versus metamorphic origin. However, two factors acted as deterrents for the resolution of the debate. First, charnockites and associated rocks occur in a variety of different structural setting and display diverse field rela-tions, attesting to possible different mode of origin. Second and possibly more important is the lack of consensus on the nomenclature of charnockites and associated rocks and this is commonly linked with the metamorphic versus magmatic perspective. Scanning the literature of this period makes one believe that both metamorphic and magmatic hypotheses are valid, but applicable to different field setting only. Before critically evaluating individual cases, it is imperative that a uniform approach in nomenclature should be agreed upon. It is proposed that name charnockite be adopted for any quartzofeldspathic rock with orthopyroxene, irrespective of its mode of occurrence, struc-tural setting and mode of origin. The associated more mafic varieties, be better described as mafic granulite, rather than basic charnockite. For the patchy charnockites of east Gondwana (including parts of Peninsular India, Sri Lanka and Antarctica), metamorphic transformation from amphibolite facies gneiss, by two different mechanisms: CO2 ingress from deep level, and drop in fluid pressure, has been proposed. However, all such patchy occurrence is not amenable to explanation by metamorphic trans- formation. In some instances, migmatisation of older charnockitic rocks is evident. Also pro- gressive charnockitisation relating patchy char-nockite to banded variety could be argued against on two counts: grain-size relation and time-relation. Larger bodies or bands have been explained as magmatic, but in many instances, from geochemical consideration alone. The compositional variation, commonly encoun-tered in many high-grade belts, if not described in terms of field relation, may lead to wrong no-tions of magmatic differentiation of mantle-de- rived melts. Crustal melting of dry granulite fa-cies source rocks has been proposed from geochemical and isotopic data of charnockitic intrusions. This model proposes high-tempera-ture melting of previously dehydrated and dry granulite source rocks. However, tectonic per-turbation subsequent to granulite facies meta-morphism that might have been responsible for such high temperatures, is not well constrained in this model. Finally, with advent of high- pressure dehydration-melting experiments in the nineties, dehydration-melting of mafic to intermediate composition, syn-kinematic with granulite facies metamorphism has been pro-posed.