Petrogenesis of the Mandi Gabbroic Rocks, Lesser Himalaya, India

The gabbroic rocks of Mandi area occur as an oval-shaped, 500 m thick intrusive body within the Mandi granite. The central zone of the body is made up of fresh gabbro and it grades into metagabbro and epidiorite towards the margins. The relatively high abundances of Ti, AI, Ca and Mg in clinopyroxenes of the central part as compared to the clinopyroxenes of the marginal part suggest different rates of cooling within the Mandi gabbroic body. The zoned plagioclases have calcic-rich cores and calcic-poor rims. The Al - (Fe+Ti) - Mg values and incompatible trace element ratios of the rocks indicate tholeiitic nature of the body. The discrimination diagrams based on major elements as well as immobile minor and trace elements suggest that these rocks are of MORB type. However, the chemical characteristics like negative Nb, Ti and P anomalies in the primordial mantle-normalised spiderdiagram together with low TiN and ZrN ratios testify that the Mandi gabbroic rocks are akin to low-Ti continental flood basalts. [Mg]-[Fe] modelling, incompatible versus compatible trace element data and lower Ni abundances for the given Mg contents suggest 10-15 % fractional crystallisation of olivine after the rocks were generated by partial melting under pressure conditions ranging between 25 and 40 kb. The intrusive appears to have formed by low degree of partial melting of a non-pyrolitic metasomatised mantle source that was heterogeneous with respect of Zr and Fe/Mg ratios.