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Title: Magmatic, hydrothermal and subsolidus evolution of the agpaitic nepheline syenites of the Sushina Hill Complex, India: Implications for the metamorphism of peralkaline syenites
Authors: Chakrabarty A.
Mitchell R.H.
Ren M.
Saha P.K.
Pal S.
Pruseth K.L.
Sen A.K.
Published in: Mineralogical Magazine
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). © 2016 by Walter de Gruyter Berlin/Boston 2016.
Citation: Mineralogical Magazine (2016), 80(7): 1161-1193
Issue Date: 2016
Publisher: Mineralogical Society
Keywords: clinopyroxene
metamorphosed agpaitic rocks
subsolidus alteration
Sushina Hill
ISSN: 0026461X
Author Scopus IDs: 25121521500
Author Affiliations: Chakrabarty, A., Department of Geology, Durgapur Government College, Durgapur, West Bengal, 713214, India
Mitchell, R.H., Department of Geology, Lakehead University, Thunder Bay, ON P7B 5E1, Canada
Ren, M., Department of Geoscience, University of Nevada, Las Vegas, NV, United States
Saha, P.K., Department of Geology, Durgapur Government College, Durgapur, West Bengal, 713214, India
Pal, S., Department of Geology, Durgapur Government College, Durgapur, West Bengal, 713214, India
Pruseth, K.L., Department of Geology and Geophysics, Indian Institute of Technology Kharagpur, Kharagpur721302, India
Sen, A.K., Department of Earth Sciences, Indian Institute of Technology Roorkee, Roorkee, Uttarakhand, 247667, India
Funding Details: This work is supported by the Department of Science and Technology (DST), India through a Fast Track Research Scheme (SR/FTP/ES-65/2013) awarded to AC. R. H. Mitchell's work on alkaline rocks is supported by the Natural Sciences and Engineering Research Council of Canada. The manuscript has benefitted from many rounds of discussions with Axel Sj?qvist, Rajib Kar, Supriyo Pal and Sudipta Chakrabarty. We are thankful to the reviewers for their thought provoking and constructive suggestions which helped to enhance the clarity of this communication. The editorial care of Prof. PeterWilliams and Dr. Katharina Pfaff is highly appreciated.
Appears in Collections:Journal Publications [ES]

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