講演情報
[R7-P-01]Hydrothermal alteration of apatite and associated REE enrichment in syenite: Implications on the Late Tonian alkaline magmatism and subsequent reworking from the northern Eastern Ghats Belt, India
*Kaushik DAS1, Proloy Ganguly2, Aparupa Banerjee3, Yukiyasu Tsutsumi4, Sankar Bose5, Shuvankar Karmakar5, Gautam Ghosh5 (1. Hiroshima University, 2. Durgapur Government College, 3. Shahid Matangini Hazra Government General Degree College for Women, 4. National Museum of Nature and Science, 5. Presidency University)
キーワード：
Syenite、Apatite、Metasomatism and REE enrichment、Tonian magmatism and late metasomatism
The present study is focused on the apatite alteration in clinopyroxene and K-feldspar-dominated deformed syenite and the processes involved in REE enrichment from the northern Eastern Ghats Belt (EGB). A petrographic study reveals that the fluorapatite megacrysts, most likely crystallized from syenitic magma, underwent dissolution and reprecipitation in the presence of hydrothermal fluid of the C-H-O system. Such fluid appears to be of late-magmatic origin, which became fluorine-rich (C-H-O-F fluid) following its interaction with the already formed fluorapatite grains. During such interaction, REEs from the fluorapatite were released to form REE-fluorcarbonate+monazite+allanite+thorite±epidote, resulting in the REE deposition. In-situ U-Pb zircon dating yields the age of emplacement of syenite at ca. 756 Ma. Moreover, from the Th-U-total Pb monazite chemical dating, ca. 735 Ma is interpreted as the age of hydrothermal alteration. Monazite and zircon spot dates younger than ca. 700 Ma, additionally, correspond to younger metamorphic and deformation episodes. The present study provides new evidence of alkaline magmatism at the northern EGB, which has remained less explored in recent times. Such magmatism is possibly related to the fragmentation of the composite Rayner-Eastern Ghats block and marks the location of an otherwise undetected paleosuture resulting from the collision of India and East Antarctica during the late Neoproterozoic time.