Geology and sup(40)Ar/ sup(39)Ar age of the Khopoli olivine gabbro intrusion, Konkan Plain, western Deccan Traps

Abstract

The Khopoli intrusion is a small olivine gabbro intrusion exposed in the Konkan Plain, in the western part of the Deccan Traps continental flood basalt province. It intrudes lavas of the Neral and Thakurvadi formations, which belong to the lower part of the Western Ghats stratigraphic sequence and mainly comprise small-scale compound pahoehoe flows and sheet lobes, respectively. Many of these lavas contain abundant cumulus olivine and clinopyroxene. The Khopoli intrusion is of considerable interest because its olivine gabbros are among the most magnesian Deccan rocks known, with bulk-rock MgO contents reaching 27 wt. percent. Textural, mineralogical and geochemical features indicate that the olivine gabbros are olivine-pyroxene cumulates formed from an evolved tholeiitic basalt melt. Much of the original outcrop of the intrusion (mapped in 1980) is now lost owing to large-scale urban and industrial development. We have remapped the intrusion and obtained a sup(40)Ar/ sup(39)Ar age of 67.3 plus-minus 1.5 Ma (2 Sigma) on fresh intercumulus plagioclase grains separated from one of the olivine gabbros, which is consistent with the age of the host volcanic sequence. Measured true density values of 2.93 to 3.13 g/cm sup(3) for olivine gabbros of the Khopoli intrusion suggest possible shallow causes for at least some of the high gravity anomalies found in the Deccan Traps.