R8: Metamorphic rocks and tectonics

[R8P-13] Metasomatic syenite at the interface between charnockite and calc-silicate granulite, Eastern Ghats Belt, India: Mineral-chemical characterization and its implications during orogenesis

*Kaushik DAS^1,4, Proloy Ganguly², Sankar Bose^3,4 (1. Hiroshima University, 2. Durgapur Government College, Department of Geology, Durgapur, India, 3. Presidency University, Kolkata, India, 4. HiPeR, Hiroshima)

Keywords:Syenite, Apatite, Trace elements, Metasomatism

We have recently found occurrences of meter-to-micrometer scale bands and veins of syenite at the contact between the calc-silicate granulite (clinopyroxene-plagioclase-scapolite-wollastonite-calcite-grandite garnet-titanite-apatite-quartz) and the coarse-grained charnockite (plagioclase-K-feldspar-orthopyroxene-quartz) exposed at the Phulbani area of the Proterozoic Eastern Ghats Province (EGP). The calc-silicate granulite witnessed ultrahigh temperature (UHT) metamorphism followed by charnockite intrusion. Syenite (clinopyroxene-K-feldspar-apatite±titanite±quartz) bands and mylonitized veins are roughly parallel to the axial plane of the mesoscopic-scale fold present in the calc-silicate granulite. Anorthitic patches, and myrmekite intergrowth replacing the albite component of plagioclase of the contact zone of charnockite are common. In charnockite, clinopyroxene and K-feldspar-bearing syenitic vein occurs which in turn replaces anorthitic plagioclase (An > 96 mol%) of calc-silicate granulite to scapolite+calcite bearing mineral assemblage. All these indicate a fluid-rock interaction while forming syenite.
During this deep crustal metasomatism, fluor-apatite overgrowths and neoblastic growths occurred in the syenite as well as the leached part of the wall rock. While the early-formed apatite grains are Ce, La, Y, and Si-enriched, the overgrowths are relatively depleted. The present study highlights the interactions of lower crustal rocks with saline, carbonic, and REE-depleted fluid. Similar metasomatic syenite occurrences are reported from other portions of the northern EGP implying its formation on a large scale during orogenic evolution.