Heavy Mineral and Mineral Magnetic Tracers of Basaltic versus Cratonic Weathering as Indicators of Spatio-temporal Shifts in the Monsoonal Intensity over Central Indian Region

Indian summer monsoon holds its core over Central Indian Region (CIR) through Bay of Bengal and Arabian Sea branches. The majority of CIR is occupied by the Godavari Drainage Basin (GDB) which generates a large and episodic flux of sediments in the Bay of Bengal indicating a major shift in the monsoonal pattern over CIR. The large part of Deccan basalt province of GDB is present in the semi-arid watershed governed by the precipitation mostly from Arabian sea branch. Whereas the cratonic province is present in a high monsoonal zone governed by the precipitation from Bay of Bengal branch. Both these zones also represent Cratonic (non-basaltic) versus Basaltic sources converging at the Pranhita/Godavari confluence before their mixing onwards into delta and finally in the Bengal fan. Based on the heavy mineral and low temperature magnetic mineral characteristics of bed loads from the Godavari River before, at and after the confluence we attempt to characterise these two sources as a function of shift in monsoonal pattern over central India. The quantitative heavy mineral studies show dominance of stable minerals derived from the non-basaltic sources with a downstream trend for stable heavy minerals in a long profile to indicate stronger cratonic sources. The surface textures of garnet, epidote and hornblende indicate rapid transport and low residence time for the sediments derived from Precambrian granites, Proterozoic and Gondwana sediments presently defining the Core Monsoon Zone. The opaque grains represented by discrete ilmenite, magnetite and magnetite inclusions in aggregated quartz characteristic of the Deccan basaltic source show decreasing order downstream. The low temperature magnetic susceptibility for heavy minerals indicate Multi Domain and Ti-rich magnetite from Deccan basalts and magnetic inclusion in siliciclastic grains derived mostly from local sources. More detailed heavy mineral and geochemical analysis of the core sediments would help in documenting the temporal shifts in monsoonal intensity and its core regime over the Indian continent.