GEOCHEMISTRY AND PETROGENESIS OF GRANITIC ROCKS AROUND HUTTI SCHIST BELT, DHARWAR CRATON

Abstract

The Dharwar craton of south India is one of the major Archean cratons that consists of low to high grade supracrustals within a sea of tonalite-trondhjemite-granodiorite (TTG) suite of rocks. The eastern Dharwar craton consists of several gold mineralized schist belts (e.g. Kolar, Ramagiri, and Hutti) that are essentially composed of pillowed metabasalts, rhyolites, polymictic conglomerates, grits, phyllites and Banded Iron Formation (BIF) at various stratigraphic levels. Of these, the Hutti schist has attained greenschist to amphibolite grade of metamorphism. The typical hook shaped map pattern of the Hutti schist belt is a consequence of two co-axial phases of folding. Geochemically the granoitiods surrounding the Hutti schist belt are divided into four distinct groups: (1) quartz monzodiorite (2) granodiorite, (3) granite, and (4) monzodiorite. The quartz monzodiorites have Si02 content of the order of 62 wt.%. They have higher MgO, CaO and Ba and lower Na20 and K20 content as compared to that of the granodiorites. Their Mg# (0.6-0.66) are much higher than that of any other rock types analyzed from the study area. They show LREE enriched (CeN/YbN = 14.14) and HREE depleted (ErN/YbN = 0.89) rare earth element patterns with slight or no Eu anomaly. On the basis of geochemical modeling, it is argued that the magmas parental to quartz monzodiorites can neither be derived by partial melting of tholeiitic metabasalts occurring in the Hutti schist belt nor they represent the residual magmas formed by fractional crystallization of common mineral phases from tholeiitic magmas. [Mg]-[Fe] modeling suggests that the parental magmas of quartz monzodiorites are derived by partial melting of a composite mantle source that is compositionally similar to komatiite. But the trace element modeling suggests that the sources, formed by mixing of garnet Iherzolite and basaltic magmas, must have been enriched in incompatible trace elements prior to melting. It is argued that partial melting of composite sources in the mantle wedge, similar in composition to that of komatiite enriched in LILEs and LREEs might have given rise to the magmas parental to the quartz monzodiorites. Granodiorites are the most dominant rock types amongst the granitoids around the Hutti schist belt. They are silica saturated to oversaturated rocks (65.25 to 72.49 wt.%). Na20 abundance in these rocks is invariably higher than K20. The rocks are meta- aluminous in nature and define a calc-alkaline trend. In general, these rocks have high concentration of Ba (454 to 918 ppm) and Sr (> 450 ppm). Though the granodiorites occurring to east, west and north of the belt have broadly similar major element chemistry but they differ in their trace element, REE abundance, and chondrite normalized REE patterns. Western granodiorites have chondrite normalized REE patterns parallel to that of quartz monzodiorites with no Eu anomaly. Their major and trace element abundance as well as geochemical modeling suggest that they can be derived by fractional crystallization processes including liquid immiscibility from quartz monzodiorites. The eastern granodiorites (named as Kavital granodiorites) have higher MgO, FeO, Ti02, and higher Mg# than that of western granodiorites. They have highly fractionated chondrite normalized REE patterns with slight negative or no Eu anomaly. It is suggested that they might have been derived by partial melting of intermediate rocks similar in composition to that of sanukitoids, high magnesian andesites or adakites. Granodiorites occurring to the north of the belt are named as Northern granodiorites. Although their major and trace element abundance are comparable to that of Kavital granodiorites, their chondrite normalized REE patterns are less fractionated and show negative Eu anomaly. On the basis of geochemical modeling it is suggested that Northern granodiorites are derived by low pressure partial melting of Hutti metabasalts having island arc affinities. The granites of Hutii schist belt are classified into two categories: high alkali granites and low alkali granites that occur towards northeast and northwest parts of the belt, respectively. In general, the low alkali granites have higher abundance of CaO, Ba and m lower Sr than the high alkali granites. They show fractionated chondrite normalized REE patterns with strong negative Eu anomaly. In contrast, the high alkali granites show more negative Eu anomaly as compared to the low alkali granites. Geochemical modeling suggests that both type of granites could have been derived by low extents partial melting of granodiorites similar in composition to that of Kavital granodiorites under hydrous conditions of melting (PH2o = Ptotai) at pressures > 0.5 GPa. Although the major element abundance in the monzodiorite occurring east and west of the Hutti schist belt are similar, these two monzodiorites differ in their REE patterns. The eastern monzodiorite is characterized by highly fractionated chondrite normalized REE pattern (CeN/YbN = 37.0) with slight negative Eu anomaly (Eu/Eu* = 0.78), whereas, the western monzodiorite is characterized by less fractionated chondrite normalized REE patterns (CeN/YbN = 7.07) and almost no Eu anomaly (Eu/Eu* = 0.97). On the basis of geochemical modeling it implies that the eastern monzodiorite could have been derived by partial melting of LREE enriched basaltic andesites, whereas, the western monzodiorite can be generated by partial melting of Hutti metabasalts. On the basis of field, petrographic and geochemical characteristics it is shown that the granodiorites occurring to east, west and north of the belt have distinct petrogenetic characteristics and they form three disparate granitoid terranes. It is suggested that these unrelated granitoid terranes that are separated by the schist belt might have been emplaced in island arc tectonic settings. The tholeiitic basalts occurring in the Hutti schist belt have evolved in the island arc environment.