Mafic Xenoliths In Proterozoic Kimberlites From Eastern Dharwar Craton, India: Mineralogy And P-t Regime

摘要

Mafic xenoliths of garnet pyroxenite and eclogite from the Wajrakarur, Narayanpet and Raichur kimber-lite fields in the Archaean Eastern Dharwar Craton (EDC) of southern India have been studied. The composition of clinopyroxene shows transition from omphacite (3-6 wt% Na_2O) in eclogites to Ca pyroxene (< 3 wt% Na_2O) in garnet pyroxenites. Some of the xenoliths have additional phases such as kyanite, ensta-tite, chromian spinel or rutile as discrete grains. Clinopyroxene in a rutile eclogite has an X_(Mg) value of 0.70, which is unusually low compared to the X_(Mg) range of 0.91-0.97 for all other samples. Garnet in the rutile eclogite is also highly iron-rich with an end member composition of Prp_(26.5)Alm_(52.5)Grs_(14.7)Adr_(5.1-)TiAdr_(0.3)Sps_(1.0)Uv_(0.1). Garnets in several xenoliths are Cr-rich with up to 8 mol% knorringite component. Geothermobarometric calculations in Cr-rich xenoliths yield different P-T ranges for eclogites and garnet pyroxenites with average P-T conditions of 36 kbar and 1080 ℃, and 27 kbar and 830 ℃, respectively. The calculated P-T ranges approximate to a 45 mW m~(-2) model geotherm, which is on the higher side of the typical range of xenolith/xenocryst geotherms (35-45 mW m~(-2)) for several Archaean cratons in the world. This indicates that the EDC was hotter than many other shield regions of the world in the mid-Proterozoic period when kimberlites intruded the craton. Textural and mineral chemical characteristics of the mafic xenoliths favour a magmatic cumulate process for their origin as opposed to subducted and metamorphosed oceanic crust.