Comparative Geochemical and Petrographic Studies of the Various Granitoids between Central and Western Arm, in the Central Part of Ramagiri Schist Belt and Their Petrogenetic Histories

Abstract

Granitoids between the central and western arm of Ramagiri schist belt in its central part, are broadly classified into the migmatite gneiss, grey granodiorite and pink monzogranite, based on field characteristics and petrographic features. These granitoids belong to the Tonalite-Granodiorite-Monzogranite (TGM) suite of PGC-II. All the samples are fresh as per the CIA values, PC1-PC2 binary plot and MFW ternary plot. The granodiorites occupy the expected field in the normative IUGS, TAS, and R 1 -R 2 classification diagrams, but the monzogranites occupy the monzogranite field in the normative IUGS classification diagram and granite to alkali granite field in the rest. The granodiorites exhibit both ferroan to magnesian, alkali-calcic nature with metaluminous I type features and falls in the calc-alkaline to high K calc-alkaline series. They have high ΣREE (an average 327.905 ppm) content, and show LREE enrichment ((La/Sm) N = 3.1 - 6.8) with enriched but relatively flat HREE ((Gd/Yb) N = 1.75 - 5.26) patterns and weak negative to positive Eu anomaly (Eu/Eu* = 0.62 - 1.18). The monzogranites, on the other hand, are peraluminous, alkalic, ferroan, high K calc-alkaline, S-type granites, exhibiting relatively low ΣREE (an average 118.693 ppm) contents, strongly fractionated REE patterns with highly enriched LREE ((La/Sm) N =1.74 - 9.76), depleted HREE ((Gd/Yb) N = 0.43 - 2.21) patterns having concave upward shape, and strong negative Eu anomaly (Eu/Eu* = 0.23 - 0.89). Geothermobarometry revealed the average emplacement temperature and pressure of the granodiorites and monzogranites as 812.5 ℃ , 8.14 ± 0.6 kbar and 775 ℃ , 3.14 kbar, respectively. Based, on the observations, it can be concluded that the granodiorites have formed in volcanic arc setting by partial melting of the lower crust and S-type monzogranites have been produced at a relatively shallower depth in the crust, by continental crust recycling due to hydrothermal influx.