Contrasted granite emplacement modes within an oblique crustal section: the Closepet Granite, South India

Abstract

The Closepet Granite, in South India, is a large, syntectonic Archaean granitic complex. Differential erosion has exposed it from the lower (25 km) to upper crust (5 km). Four main parts are recognized from bottom to top: (i) A root zone, where magmas formed, collected and rose within active shear zones, leaving schlieren behind. The surrounding crust was highly ductile, leading to diffuse deformation. (ii) A transfer zone, where the magma was progressively enriched in K-feldspar phenocrysts during its ascent. In this part, the granite rose as a mush moving as a whole within a less ductile crust. Slow cooling was responsible for a long magma residence time under conditions favoring to fabric enhancement and strain partitioning, leading to horizontal and vertical melt migration. (iii) A “gap” (dyke complex that acted as a filter zone), were the ascent of the mush was stopped, probably due to high phenocryst load and high viscosity contrast with the wall rocks. Only crystal-poor melts could continue their ascent through the dykes. (iv) A zone of shallow intrusions, where the liquids extracted from the mush filled small, elliptical plutons, cooling quickly and developing only very weak fabrics.