Utility of tectonic classification diagrams for metamorphosed basic rocks: a case study from a suite of basic rocks from makrohar area, Madhya Pradesh

Abstract

The CITZ is an ensemble of several low to medium-grade supracrustal belts, gneisses, granitoids and a few linear tracts of granulite belts, all of which are intruded by a suite of mafic rocks. We focus on the metaigneous mafic rocks, which occur as dykes and lensoidal bodies that intruded the granitic country rocks. It has a sharp contact with the host felsic rocks, and usually appear fine grained massive rock. The mafic dyke is mainly Gabbro to Troctolite or olivine bearing Gabbro in composition. The rock has been variably altered and metamorphosed. Unaltered samples preserve the magmatic subophitic texture which suggests rock was formed due to rapid cooling at shallow crustal depth. Geochemical study of the rock suggests that Protolith magmas of the mafic dykes emplaced in the northern segment of the CITZ is mainly tholeiitic basalt. Some rock samples are plotted in calc-alkaline field, but this enrichment of alkali elements Na + K is an artefact of alteration during metamorphism of the dykes. This basalt is slightly enriched in LREE & shows flat HREE pattern which resembles the composition of continental arc or flood basalts. Zr depletion, high to moderate TiO2 supports this tholeiitic basalt is generated due to higher degree of partial melting of asthenopheric mantle. By applying tectonic discrimination diagram, the less altered samples do not yield proper constrain for interpreting the tectonic settings. So, these discrimination diagrams can not be used in every cases for interpreting the tectonic settings. Therefore, these diagrams should be used with caution and with proper support from other geological observations.