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

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 R1-R2 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.

Geochemistry of Late Archaean shaly BIF formed by oxic exogenic processes: an example from Ramagiri schist belt,Dharwar Craton, India

The central block of the auriferous Ramagiri schist belt, in the Eastern Dharwar Craton, India consists of bimodal volcanics(mafic-felsic), shaly BIF and metasedimentary rocks. Geochemical studies of the associated shaly BIF have indicated the enrichment of the major and trace elements such as Si O2, Al2O3, Ti O2, K2 O, Mg O, Fe2O3(T),Zr, Y, Cr, Ni, alkali and alkaline earth elements indicates that the clastic component of the shaly BIF had their contribution from the contemporaneous bimodal volcanics.The concave chondrite normalized REE patterns share ubiquitously anomalous positive cerium anomaly, absence of positive europium anomaly and the overall HREE enrichment. The REE patterns resemble those from the modern day sea water, except for positive Ce anomaly. The data suggests that arc related bimodal volcanism had been the plausible source of Fe, silica, REE and other trace elements. The coherent behaviour of Fe, Ti, Mn and P with the REEs indicates that they got incorporated from Fe–Ti–Mn bearing primary minerals and secondary products like clays. The variability of REE patterns in the BIF formation samples probably results from the differences in scavenging efficiency. The BIF bears signatures of mixing of the contemporaneous clastic and chemical processes, as well as the changes accompanying diagenesis and metamorphism.The precipitation of Fe did not stop during the sedimentation in an island arc related tectonic setting. The BIF strongly lacks the signatures from hydrothermal input. The presence of positive cerium anomalies and the absence of positive europium anomalies in the shaly banded ironformations imply that iron oxidation during BIF deposition took place in shallow waters rather than at depth, at oxicanoxic boundary.