Amphibolites in the rare metal- and tin-bearing Bastar–Malkangiri pegmatite belt in Chhattisgarh and Odisha, India

Abstract

Amphibolites and Abukuma-type low-grade and low-pressure meta-sedimentary rocks (MSR) host mineralised rare metal (Nb–Ta, Be and Li)- and Sn-bearing granite pegmatites and their parental, fertile S-type granites in the Bastar–Malkangiri Pegmatite Belt (BMPB) in the SE part of the Bastar craton in parts of Chhattisgarh and Odisha, India. Amphibolites, based on their field relationship (as sills in MSR), mineral assemblage and textural features, are broadly classified into three types: (i) tremolite-, (ii) hornblende–actinolite-, and (iii) hornblende-bearing amphibolites. Petrochemically, amphibolite types (i–iii) show a regular decrease in their Mg content and related trace elements such as Cr (81–1731 ppm), Ni (135–379 ppm) and Co (23–68 ppm) and, hence, are designated, respectively, as high-, intermediate- and low-Mg amphibolites. All the three types show slight enrichment in Si, Fe and P, reflected in their normative quartz, hypersthene and lesser diopside, magnetite, ilmenite and apatite, but no olivine. They are relatively enriched in incompatible elements such as Rb (16–526 ppm), Ba (15–538 ppm), Zr (56–147 ppm) and Nb (3–44 ppm), with relatively a higher concentration of these elements in those near to the contacts with pegmatites, thereby demonstrating some influence of the pegmatites on the chemistry of the host amphibolites. In spite of these, the three types of amphibolites show regular differentiation trends, reflected by their systematic decrease of Ni, Cr and Co with a decrease of Mg, and also show Fe-enrichment in their alkalies $$(\\hbox {Na}_2\\hbox {O} + \\hbox {K}_2\\hbox {O})-(\\hbox {FeO} + \\hbox {Fe}_2\\hbox {O}_3)-\\hbox {MgO}$$(Na2O+K2O)-(FeO+Fe2O3)-MgO (AFM) plot. The field, petrography and major–minor trace elements petrochemistry, including rare earth elements with slight negative to no Eu-anomaly, and their chemical patterns cumulatively point out that these amphibolites are ortho-type and sub-alkaline in nature, with quartz tholeiitic magmatic affinity, and the formation due to more of fractional crystallisation and less of 5–25% partial melting of an enriched primordial mantle magma (enriched mid-oceanic ridge basalt – E-MORB), emplaced at a deduced shallow depth of 10–25 km, with the main controlling phases being olivine, plagioclase, clinopyroxene and apatite.