Progressive or continual exsolution in pyroxenes: an indicator of polybaric igneous crystallization for the Perinthatta anorthositic gabbro, northern Kerala, southwestern India

Abstract

Clinopyroxene and orthopyroxene from the Perinthatta anorthositic gabbro in southwestern India exhibit exsolution intergrowths of pyroxenes (occurring in five stages involving pigeonite, orthopyroxene and/or augite) and Fe–Ti oxides. The thicker irregular pyroxene lamellae appear to be the result of heterogeneous nucleation, indicating that this was the first unmixing process to begin. This was followed by homogeneous nucleation of regularly spaced, quite uniform pyroxene lamellae and defect-free fine pyroxene platelets between early-formed exsolution lamellae, suggesting progressive or continual nature of exsolution during slow cooling. Pyroxene thermometric calculations indicate that subcalcic augite and pigeonite equilibrated at ∼935 °C, with the different lamellar generations formed between ∼914 and ∼748 °C. Pre-metamorphic anorthositic gabbro crystallization is considered to have taken place at deep crustal levels (∼30 km; ∼9–10 kbar) and later emplacement at shallower crustal levels (∼15 km; ∼5–6 kbar) suggesting polybaric conditions. The apparent difference between the pressure of crystallization and the emplacement of the Perinthatta anorthositic gabbro supports the common Proterozoic anorthosite petrogenetic model, involving initial crystallization at depth, followed by intrusion as a crystal-rich mush. Following emplacement, the igneous assemblage was affected by granulite facies metamorphism on cooling, as indicated by smaller granulite minerals along grain boundaries and the partial re-equilibration of exsolved phases.