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

Rajesh H.M. (2006) Progressive or continual exsolution in pyroxenes: an indicator of polybaric igneous crystallization for the Perinthatta anorthositic gabbro, northern Kerala, southwestern India. Journal of Asian Earth Sciences, 26 5: 541-553. doi:10.1016/j.jseaes.2004.11.004


Author Rajesh H.M.
Title Progressive or continual exsolution in pyroxenes: an indicator of polybaric igneous crystallization for the Perinthatta anorthositic gabbro, northern Kerala, southwestern India
Journal name Journal of Asian Earth Sciences   Check publisher's open access policy
ISSN 1367-9120
Publication date 2006
Sub-type Article (original research)
DOI 10.1016/j.jseaes.2004.11.004
Volume 26
Issue 5
Start page 541
End page 553
Total pages 13
Subject 1904 Performing Arts and Creative Writing
1907 Geology
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.
Keyword Anorthositic gabbro
Clinopyroxene, Orthopyroxene, Progressive or continual exsolution
Perinthatta
Polybaric crystallization
Q-Index Code C1
Q-Index Status Provisional Code
Institutional Status Unknown

Document type: Journal Article
Sub-type: Article (original research)
Collection: Scopus Import
 
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