17:15 〜 19:15
[SMP26-P09] Pressure–temperature–time paths of pelitic schists in the A Vuong Formation, southern Truong Son Belt, Vietnam
キーワード：A Vuong Formation, Metamorphic zonation, P–T–t path, Truong Son Belt, Vietnam
The A Vuong Formation in the southern Truong Son Belt, central Vietnam has been considered as weakly metamorphosed Cambrian to Ordovician sedimentary sequences which seem to be intruded by the Ordovician to Devonian Dai Loc granitoids. Our field and petrological investigation identified the dominant low- to high-grade metapelites (phyllites to schists) in this formation. Two distinct metamorphic zones of biotite–chlorite zone and staurolite zone are defined by indicative mineral assemblages in pelitic schists which show the progressive metamorphic grade from greenschist- to amphibolite-facies. Conventional geothermobarometric estimations constrained on the peak and retrograde metamorphic conditions for garnet–chlorite–biotite–muscovite schist (biotite–chlorite zone) and garnet–biotite–muscovite ± staurolite ± kyanite schists (staurolite zone).
In the biotite–chlorite zone, the peak and retrograde conditions were estimated at approximately 4–6 kbar and 520–600 oC, and 4–6 kbar and 480–540 oC, respectively. Higher peak metamorphic condition of 5.5–8.5 kbar and 590–650 oC and retrograde one of 4.5–7.5 kbar and 530–580 oC were calculated from the staurolite zone. Notably, a porphyroblastic andalusite identified from a pelitic schist in this zone includes kyanite, sillimanite and staurolite + quartz and is rimmed by kyanite with a preferred orientation. It suggests their equilibrium on the prograde stage. The petrological evidences and estimated metamorphic conditions for the current pelitic schists are likely to indicate the anti-clockwise or hairpin-shaped P–T trajectory for the A Vuong Formation. In addition, U–Th–Pb monazite dating of the garnet–staurolite–kyanite–biotite–muscovite schist in the staurolite zone yielded a clustered age of ca. 400 Ma probably corresponding to the timing of peak metamorphism. The peak P–T–t metamorphic condition for the staurolite zone is likely consistent with the kyanite-grade amphibolite-facies one recorded in the Dai Loc pelitic gneisses. It may indicate their juxtaposition during early Devonian. These findings provide critical insights into the tectono-metamorphic history of the A Vuong Formation, enhancing the understanding of regional tectonics in the Truong Son Belt.
In the biotite–chlorite zone, the peak and retrograde conditions were estimated at approximately 4–6 kbar and 520–600 oC, and 4–6 kbar and 480–540 oC, respectively. Higher peak metamorphic condition of 5.5–8.5 kbar and 590–650 oC and retrograde one of 4.5–7.5 kbar and 530–580 oC were calculated from the staurolite zone. Notably, a porphyroblastic andalusite identified from a pelitic schist in this zone includes kyanite, sillimanite and staurolite + quartz and is rimmed by kyanite with a preferred orientation. It suggests their equilibrium on the prograde stage. The petrological evidences and estimated metamorphic conditions for the current pelitic schists are likely to indicate the anti-clockwise or hairpin-shaped P–T trajectory for the A Vuong Formation. In addition, U–Th–Pb monazite dating of the garnet–staurolite–kyanite–biotite–muscovite schist in the staurolite zone yielded a clustered age of ca. 400 Ma probably corresponding to the timing of peak metamorphism. The peak P–T–t metamorphic condition for the staurolite zone is likely consistent with the kyanite-grade amphibolite-facies one recorded in the Dai Loc pelitic gneisses. It may indicate their juxtaposition during early Devonian. These findings provide critical insights into the tectono-metamorphic history of the A Vuong Formation, enhancing the understanding of regional tectonics in the Truong Son Belt.