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Magmatic, hydrothermal and subsolidus evolution of the agpaitic nepheline syenites of the Sushina Hill Complex, India: implications for the metamorphism of peralkaline syenites

Published online by Cambridge University Press:  02 January 2018

A. Chakrabarty ,
R. H. Mitchell ,
M. Ren ,
P. K. Saha ,
S. Pal ,
K. L. Pruseth  and
A. K. Sen
A. Chakrabarty*
Affiliation:
Department of Geology, Durgapur Government College, Durgapur 713214, West Bengal, India
R. H. Mitchell
Affiliation:
Department of Geology, Lakehead University, Thunder Bay, Ontario, Canada P7B 5E1
M. Ren
Affiliation:
Department of Geoscience, University of Nevada, Las Vegas, Nevada, USA
P. K. Saha
Affiliation:
Department of Geology, Durgapur Government College, Durgapur 713214, West Bengal, India
S. Pal
Affiliation:
Department of Geology, Durgapur Government College, Durgapur 713214, West Bengal, India
K. L. Pruseth
Affiliation:
Department of Geology and Geophysics, Indian Institute of Technology Kharagpur, Kharagpur 721302, India
A. K. Sen
Affiliation:
Department of Earth Sciences, Indian Institute of Technology Roorkee, Roorkee, Uttarakhand 247667, India
*
*E-mail: [email protected]
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Abstract

The Proterozoic Sushina Hill Complex is the only agpaitic complex, reported from India and is characterized by a eudialyte-rinkite-bearing nepheline syenite. The complex is considered a ‘metamorphosed agpaitic complex'. This study describes the mineral assemblages formed during successive stages of evolution from magmatic to hydrothermal stages and low-temperature subsolidus re-equilibration assemblage. The primary-late magmatic assemblage is characterized by albite, orthoclase, unaltered nepheline, zoned diopside-hedenbergite, rinkite, late magmatic eudialyte and magnesio-arfvedsonite formed at ∼700°C with maximum aSiO2 of 0.60. In contrast, a deuteric assemblage (400-348°C) is represented by aegirine-jadeite-rich clinopyroxene, post-magmatic eudialyte, sodalite, analcime and the decomposition assemblages formed after eudialyte with decreasing aSiO2 (0.52-0.48). A further low-temperature subsolidus assemblage (≤250°C) represented by late-forming natrolite could be either related to regressive stages of metamorphism or a continuum of the subsolidus processes. Considering the P/T range of the greenschist - lower-amphibolite facies of metamorphism it is evident that the incorporation of a jadeite component within pyroxene is related to a subsolidus process between ∼400°C and 348°C in a silica deficient environment. We emphasize that the deuteric fluid itself acted as an agent of metamorphism and the decomposition assemblage formed after eudialyte is retained even after metamorphism due to the convergence of subsolidus and metamorphic domains. The formation of jadeite-rich aegirine is not considered to result from metamorphism. Overall it is near-impossible to discern any bona fide metamorphic textures or mineral assemblages in these syenites which appear to preserve a relict mineralogy regardless of their occurrence in country rocks which have experienced greenschist - amphibolite facies metamorphism. The Sushina complex is very similar in this respect to the Norra Kärr complex (Sweden).

Keywords

metamorphosed agpaitic rocksSushina Hilleudialyteclinopyroxenesubsolidus alteration
Type
Research Article
Information
Mineralogical Magazine , Volume 80 , Issue 7 , December 2016 , pp. 1161 - 1193
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2016

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