Petrogenesis of two granites from the Nilgiri and Madurai blocks, southwestern India: Implications for charnockite-calc-alkaline granite and charnockite-alkali (A-type) granite link in high-grade terrains

摘要

Igneous charnockites share some of the characteristics of calc-alkaline and alkali (A-type) granites, and together constitute important rock types in many high-grade terrains. The Neoproterozoic Kalpatta and Munnar biotite-hornblende granites intrude the upper-amphibolite to granulite-facies terrain of southwestern India. Geochemical characteristics of the Kalpatta granite are similar to high-K calc-alkaline magnesian granitoids, whereas those of the Munnar granite are similar to alkali ferroan granitoids. Within the constraints imposed by the high temperature, K-rich nature of the magmas, comparison with experimental studies on various granitoid source compositions, and trace- and rare-earth-element modeling, the distinctive features of both granites reflect a source rock of charnockitic nature. In this context, the northern Kerala (NKM) and Cardamom hill (CM) charnockite massifs, occurring near the granites, were considered as probable source rock compositions. Both the NKM and CM charnockites consists of an intermediate (low SiO2) type and silicic (high SiO2) type, with the intermediate type showing similarities to high-Ba-Sr granitoids with low K2O/Na2O ratios and the silicic type showing similarities to high-Ba-Sr granitoids with high K2O/Na2O ratios. The proposed petrogenetic model involves partial melting of the intermediate NKM charnockite forming the Kalpatta calc-alkaline granite. In contrast, the Munnar alkali (A-type) granite was the product of melting, followed by fractional crystallization of the intermediate CM charnockite. It is suggested that granitoids that formed by melting of intermediate type charnockites show high-K calc-alkaline magnesian geochemical characteristics, whereas those formed by intermediate type charnockite melting-fractional crystallization show alkali ferroan geochemical characteristics.