A note on the origin of Clinopyroxene megacrysts from the Udiripikonda lamprophyre, Eastern Dharwar Craton, southern India

Abstract

Abundant sub-rounded to sub-angular and centimeter-sized clinopyroxene megacrysts constitute a conspicuous feature of the Udiripikonda lamprophyre, located in the Eastern Dharwar craton, Southern India. These clinopyroxene megacrysts, at times, are also associated with minor amounts of biotite. The megacrysts lack reaction-rim or any other disequilibrium textures generally displayed by crustal and mantle xenocrysts/xenoliths entrained in such volatile-rich magmas. Cr2O3-impoverished (< 0.1 wt%) nature of the clinopyroxene megacrysts preclude them from being chrome-diopside, derived from the disaggregation of upper mantle rocks, and commonly found entrained in kimberlites,. The clinopyroxene megacrysts (Wo47.43- 49.20En32.44- 33.64 Fs13.73- 15.03; Ac3.32- 4.69) and associated biotite (Mg#: 0.84- 0.90) are compositionally similar to the clinopyroxene (Wo43.68-47.76; En37.47-44.58; Fs8.36 – 12.31; Ac2.70- 3.38) and biotite (Mg#: 0.84- 0.88) occurring as liquidus phases within the host lamprophyre. Clinopyroxene barometry reveals an overlapping pressure estimates for megacrysts (9.8 to 12.4 kbar) and phenocrysts (8.4 to 10.1 kbar). Likewise, the Tiin-biotite geothermometry also suggests an overlapping temperature range of 957o C to 1097o C and 904o C to 1069o C for megacrystal suite and phenocrysts respectively at pressure of ˜10 kbar. The clinopyroxene ± biotite megacrysts of this study are, thus, inferred to be cognate products which crystallized under high- to medium-pressure conditions during the evolution of lamprophyre magma.