Geochemistry of Alirajpur basement rocks and relationship to granitic basement rocks underneath the Deccan Traps

Basement rocks, specially of Precambrian origin, are crucial in understanding early Earth processes and crustal evolution.The composition of Precambrian granites of central and western India, which are entirely overlain by the Deccan Traps continue to remain poorly characterized due to lack of exposed outcrops. A recent scientific drilling expedition in the Koyna region is the first report of the geochemistry for these granitic basement rocks underlying the Deccan Traps (GBD). Here we present the first report of major and trace element geochemistry of Precambrian basement granites from the Alirajpur(AB) region of central India. This is an unique region as it records a complete sequence of basement granites,overlain by the marine Turonian Bagh beds which are considered as being infratrappean to the overlying Decaan Traps. These granitic basement rocks are mainly composed of orthoclase, quartz, plagioclase, and biotite as major minerals with accessory minerals comprising of hornblende, zircon, and monazite. Detailed petrography indicates that zircons are zoned and abundant zircon grains are hosted within biotites and hornblendes. The bulk rock geochemistry of these granites show a range from tholeiitic to calc-alkaline magmas associated with adamellite-granodiorite-tonalite suites, this pattern is also observed in the GBD. In contrast to the GBD, the AB have a prevalence of potassium over sodium, although granites from both GBD and AB are peraluminous. The GBD and AB occupy a prominent quartz-rich granitoid field in the silica oversaturated bracket of the QAPF diagram. The previous study from GBD shows fractionated REE patterns with enriched light REE and depleted heavy REE which closely matches the pattern of AB. A detailed petrological-geochemical comparison of GBD and AB, separated by ~500 Kms indicates that they are genetically related and give us clues about the extent of the Precambrian basement underlying the ~5,00,000Km2 Deccan Traps.