Petrographic characterization of amphibolites and associated rocks from the Gobbagurthi area in Khammam Schist Belt, Telangana state, India

Petrographic characterization of amphibolites and associated rocks from Gobbagurthi area in Khammam Schist Belt (KSB) that covers an area about of 240 sq.km, (Survey of India Toposheet No 65 C/7 and 65 C/8 the latitude 17⁰ 10^’-17⁰ 21^’ N and longitude 80⁰ 15^’-80⁰ 27^’ E). The area has been covered with 200 representative lithological samples and 100 thin sections were made. The amphibolites, metabasalts, metadolerites, anorthosites, granulites, gneisses and schist samples were collected for petrography and modal analyses. The geological setting of the area, field relationship and petrographic characters of the rocks have been investigated.

The mineralogical and textural characteristic of the rocks from the Gobbagurthi area vary from primary magmatic to metamorphic and deformed. The original cumulate characters of mafic to ultramafic intrusives (layered complex), ophitic to sub-ophitic textures from meta-basalts and meta-dolerites have been described. These are modified by later metamorphic imprint and recrystallization under amphibolite to granulite facies metamorphism. Due to recrystallization micro-structures are observed as shown by as coronal and sieved garnets. The KSB depicts retrograde metamorphism from granulite to amphibolite and amphibolite to greenschist facies conditions.

The original mineral grains in each rock-unit are modified by late dynamic deformation in which the coarse-grained minerals (plagioclase, hornblende, clinopyroxene and Fe-Ti oxides) are broken-down to form smaller recrystallized grains by grain boundary migration and recrystallization (mortar texture) under lower temperature conditions. Textural equilibrium between polygonal mineral grains with smooth planar boundaries and triple junctions in the rock-units of KSB suggest solid-state recrystallization due to grain boundary migration during plastic deformation at higher temperatures. This type of textural feature is overprinted on early folded gneissose and schistose rocks, streaky amphibolites and metamorphosed mafic & ultramafic rocks. Hornblende coronas around clinopyroxene and Fe-Ti oxides, epitaxial overgrowth of hornblende over clinopyroxene, scapolite over plagioclase and zoisite/clinozoisite mineral growth over plagioclase and hornblende indicate replacement textures in the rocks of Gobbagurthi area in KSB.