Crystallization pressure and radiogenic heat production in A-type granite from the Eastern Dharwar Craton, Peninsular India

Abstract

This paper describes the studies on the crystallization pressure and radiogenic heat production of A-type hornblende biotite granite from Yelabotharam area in the Eastern Dharwar Craton, Southern India. Field studies indicate that the Yelabotharam granite is porphyritic at places. Petrographically, the Yelabotharam granite is essentially composed of quartz, K-feldspar and plagioclase, while hornblende and biotite are the mafic phases. Zircon, apatite and Fe-Ti oxides are the accessory minerals. In the IUGS QAP diagram the rock falls within the granite field. Geochemically, the Yelabotharam granite is calc-alkaline and metaluminous in nature. In the trace element tectonic discrimination diagram the granite exhibits within plate granitoid (WPG) character indicating its anorogenic nature. The average crystallization pressure calculated on the basis of normative quartz and albite + orthoclase is 368 MPa, which is equivalent to ∼10 km emplacement depth. The Al hornblende thermobarometry yielded the pressure values ranging 301 MPa to 620 MPa and temperatures ranging 877 °C to 924 °C respectively. The reason for higher pressure estimates obtained by using the Al-in hornblende barometry, could be due to earlier crystallization of hornblende at deeper depths (∼16 km) compared to quartz and feldspars. The granite comprises of high Th (3.80–39.40 ppm) and low U (1.09–5.54 ppm) and K (3.02 - 9.13 wt%) concentrations respectively. The radiogenetic heat production (HP; 0.94 - 4.81 µWm−3), Radioactive Element Concentration (Ur 10.70–37.85 ppm) and Heat Generation Unit (HGU; an average value of 5.29 HGU) suggest that the Yelabotharam A-type granite belongs to ‘cold crust’ category, and is derived from dehydration melting of calc-alkaline granitoids in the shallow crust.