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

Journal article

Authors/Editors

Rajesh, Hariharan

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Publication Details

Author list: Rajesh HM

Publisher: Elsevier

Place: OXFORD

Publication year: 2006

Journal: Journal of Asian Earth Sciences (1367-9120)

Journal acronym: J ASIAN EARTH SCI

Volume number: 26

Issue number: 5

Start page: 541

End page: 553

Number of pages: 13

ISSN: 1367-9120

eISSN: 1878-5786

Languages: English-Great Britain (EN-GB)

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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 similar to 935 degrees C, with the different lamellar generations formed between similar to 914 and similar to 748 degrees C. Pre-metamorphic anorthositic gabbro crystallization is considered to have taken place at deep crustal levels (similar to 30 km; similar to 9-10 kbar) and later emplacement at shallower crustal levels (similar to 15 km; similar to 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. (C) 2004 Elsevier Ltd. All rights reserved.

Keywords

anorthositic gabbro, clinopyroxene, orthopyroxene, Perinthatta, polybaric crystallization, progressive or continual exsolution

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