T1：アジアの地殻進化と資源開発に関する総合的理解（シンポジウム）

[T1-P-01] Syenitic veining at the northern Eastern Ghats Belt, India: Formation mechanism, fluid-rock interaction and a review of its economic mineral potential

*Kaushik DAS^1,5, Proloy Ganguly², Aparupa Banerjee³, Sankar Bose^4,5 (1. Hiroshima University, 2. Kazi Nazrul University, 3. Shahid Matangini Hazra Government General Degree College, 4. Presidency University, 5. HiPeR, Hiroshima University)

キーワード：Carbo-fracturing, Syenite veining, Trace element mobility, Metasomatism

Extensive development of meter-to-micrometer scale felsic bands and veins at the contact between the calc-silicate granulite (clinopyroxene-plagioclase-scapolite-wollastonite-calcite-grandite garnet-titanite-apatite-quartz), charnockite (plagioclase-K-feldspar-orthopyroxene-ilmenite-quartz±garnet), and mafic granulite (clinopyroxene-orthopyroxene-plagioclase±garnet-hornblende±quartz) is observed at a 50-60 km wide area near the northern boundary (south of Mahanadi shear zone) of the Eastern Ghats Province of the Eastern Ghats Belt, India. The calc-silicate and mafic granulites witnessed HT to UHT metamorphism at ca. 1000 Ma, and two pulses of charnockite magmatism between ca. 970 Ma and 950 Ma. Clinopyroxene-bearing syenitic (clinopyroxene-K-feldspar-titanite-REE-phases-minor quartz) bands and veins occur in these rocks sometimes at the rock interface and even as a vein network inside one of the preexisting rocks. Inside the veins close to their wall clinopyroxene (+titanite) formed at the expense of orthopyroxene (+ilmenite). Anorthitic patches and myrmekite intergrowth replaced the albitic plagioclase of the wall zone of charnockite. All these reactions suggest selective mobility of at least Ca and Si in the vein with a definite gradient of these from the interior to the wall. Primary and secondary fluid inclusions in the vein mineralogy are rich in CO₂. Compositional variation in apatite from the host rock to the vein also shows scavenging of REE and new profuse formation of REE-rich megacrystic allanite, titanite, apatite, and zircon inside the vein. The contact rocks of charnockite show reactions of monazite to allanite, apatite, and zircon.
It seems that veins were formed by carbo-fracturing followed by metasomatism post-dating the charnockite magmatism. The nature of the fluid was possibly slightly saline, but carbonic. From our own data and that in the existing literature, we also review the economic potential of these veins.