Appraisal of crustal contamination in southern Bastar Mafic Dykes in Chattisgarh, India

Abstract

The present work is an attempt to assess the effect of crustal contamination through the Granitoids host rock, within the southern Bastar mafic dykes of Chhattisgarh, India, in the light of geochemical characteristics. Petrographically, these dykes are classified as Amphibolite, Dolerite/Meta-Dolerite and Diorite. Geochemically, all dyke samples have been classified as “high iron sub-alkaline Tholeiites”. On account of overlapping magnesium and iron concentration in Amphibolite and Dolerite dykes, distinctly higher High-Field Strength Element (HFSE), higher Rare-Earth Element (REE) concentrations in the Dolerite dykes than in the Amphibolite dykes, it is inferred that both dyke swarms are fed from two different Tholeiitic magmas. Conclusively, these dykes are recognized as belonging to two different swarms, BD1 and BD2 respectively. This is also corroborated by differences in the LREE patterns i.e. BD2 dykes have relatively enriched LREE pattern than that of BD1 dykes. It is evident from higher LaN/LuN ratio in the BD2 swarm, than in BD1 dyke swarm. These dykes intrude in the coarse-grained leucocratic Bastar Granitoids/Granite Gneisses, which are the host for these dykes. The comparative study of the Primordial mantle-normalized multi-element spider grams, and Chondrite-normalized rare-earth element patterns for the average of BD1 and BD2 dykes and the average of Bastar Granitoids, clearly reflect that the great degree of variation in LIL elements, observed in the Bastar dykes, is either due to metamorphism or due to secondary alteration, and not due to crustal contamination. This is further supported by much higher average “Nb”/“La” ratios in Bastar Mafic Dykes, than in the Bastar Granitoids. Crustal assimilation plays almost no role in the petrogenesis of the Bastar mafic dykes. The Bastar mafic dykes owe their incompatible element characteristics, certainly mantle derived.