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[SMP25-P01] Coeval magmatism and intermediate depth metamorphism during Archean-Proterozoic boundary at the eastern margin of Bastar Craton, India
キーワード：Archean Bastar Craton, India, Eastern margin, Basement of Ampani basin, Granitic gneiss-Mafic schist, Archean-Proterozoic boundary
Bastar Craton in east-central India forms one of the oldest Archean cratonic blocks in India. The eastern margin of this craton hosts a series of Mesoproterozoic sedimentary basins, namely, Chhattisgarh Basin, Khariar Basin, Ampani Basin. All these basins were formed at the nearly same time, between 1450~1500 Ma. On their eastern sides, all these basins are juxtaposed by a major Meso- to Neoproterozoic orogenic belt, the Eastern Ghats Belt. Quite a good amount of information related to the tectonometamorphic evolution of this orogenic belt, as well as the age, paleoenvironmental reconstruction for the basin-fills, and the related tectonics are available. That means the tectonic history on a regional scale is well-understood till the Mesoproterozoic time. However, the tectonic development of the Archean basement, i.e. the Bastar Craton is yet to be understood due to lack of data.
In the present study we shall focus on the basement of the Ampani Basin to understand the metamorphic and magmatic evolution of the biotite gneiss, metabasalt and deformed granitic pegmatite. This lithosuite occurs as a part of the basement rocks ~ 3 km westward from the craton-EGB boundary shear zone. The biotite gneiss comprises of plagioclage feldspar porphyroclasts within the mylonitic quartzofeldspathic matrix with biotite-rich layers (biotite + apatite) imparting the gneissic foliation. The metabasalt is massive to foliated as portions, with a mineral assemblage of plagioclase, hematite-ilmenite, sphene, actinolite with incipient hornblende. Sphene and low-Al amphibole form thin to thick corona separating hematite-ilmenite from plagioclase. The P-T pseudosection and thermobarometric estimates suggest a greenschist to lower amphibolite facies metamorphism. Felsic pegmatite also has porphyroclasts of feldspar in elongated and recrystallized quartzofeldspathic matrix. LA-ICPMS zircon U-Pb age data of the host biotite gneiss suggests the age of metamorphism as 2543±15 Ma with inherited zircon age ranging from ca. 2600 Ma to ca. 2900 Ma. While, the age of pegmatite crystallization is slightly yonger as the separated zircon grain yield a weighted average mean age of 2491±6 Ma. Direct geochronological data from the metabasalt was indiscernible.
We consider that the rocks suffered shallow- to mid-crustal metamorphism immediately followed by a pulse of felsic magmatism at the cratonic margin during the Neoarchean-Paleoproterozoic time, were later deformed strongly showing mylonitic fabric during the final cratonization of the EGB in the Neoproterozoic time.
In the present study we shall focus on the basement of the Ampani Basin to understand the metamorphic and magmatic evolution of the biotite gneiss, metabasalt and deformed granitic pegmatite. This lithosuite occurs as a part of the basement rocks ~ 3 km westward from the craton-EGB boundary shear zone. The biotite gneiss comprises of plagioclage feldspar porphyroclasts within the mylonitic quartzofeldspathic matrix with biotite-rich layers (biotite + apatite) imparting the gneissic foliation. The metabasalt is massive to foliated as portions, with a mineral assemblage of plagioclase, hematite-ilmenite, sphene, actinolite with incipient hornblende. Sphene and low-Al amphibole form thin to thick corona separating hematite-ilmenite from plagioclase. The P-T pseudosection and thermobarometric estimates suggest a greenschist to lower amphibolite facies metamorphism. Felsic pegmatite also has porphyroclasts of feldspar in elongated and recrystallized quartzofeldspathic matrix. LA-ICPMS zircon U-Pb age data of the host biotite gneiss suggests the age of metamorphism as 2543±15 Ma with inherited zircon age ranging from ca. 2600 Ma to ca. 2900 Ma. While, the age of pegmatite crystallization is slightly yonger as the separated zircon grain yield a weighted average mean age of 2491±6 Ma. Direct geochronological data from the metabasalt was indiscernible.
We consider that the rocks suffered shallow- to mid-crustal metamorphism immediately followed by a pulse of felsic magmatism at the cratonic margin during the Neoarchean-Paleoproterozoic time, were later deformed strongly showing mylonitic fabric during the final cratonization of the EGB in the Neoproterozoic time.