10:15 〜 10:30
[SMP22-06] Oligocene boron-bearing leucogranite formation evidenced by tourmaline-bearing nanogranite inclusions in garnet, central Nepal Himalaya
キーワード：大陸衝突帯、部分融解、電気石優白質花崗岩、ナノ花崗岩包有物、U-Pbジルコン年代
Tourmaline-bearing nanogranite inclusions (NIs) in garnet are newly found in the pelitic gneiss from the Formation (Fm) I exposed in Central Nepal (Marsyangdi River). The sample locality belongs to the structurally higher-grade side of the sillimanite-in isograd. The NIs are included in the garnet core, and classified into ~ 100 μm-sized “large NIs” and several 10 μm-sized “small NIs”. The large NIs showed mineral assemblages of Bt + Tur + Chl +Pl +Qz and Bt + Pl +Qz, whereas small NIs showed mineral assemblages of Bt + Ms ± Chl +Qz, Bt + Pl ± Qz ± Apt, Ms + Chl + Qz, and Bt + Qz. The tourmaline-bearing NIs are direct evidence of B-bearing melt formed during the incongruent melting of the pelitic gneiss in Formation I.
The U-Pb dates and REE concentrations of zircon were determined by LASS-ICP-MS analyses. The outermost rim of zircon including sillimanite gave ~18 Ma with steeply positive HREE pattern. The inner rim of zircon gave concordia age of 25.2 ±0.7 Ma with nearly flat HREE pattern. These suggest that the garnet with tourmaline-bearing NIs was formed at ca. 25 Ma and partly broke down at ca. 18 Ma. Therefore, B-bearing leucogranite was likely formed during the Oligocene through incongruent melting.
Tourmaline-bearing leucogranite veins are abundant in the structurally higher Formation II exposed along the Marsyangdi river. Multiple timings of the leucogranite vein formation are recognized by the presence of gneissosity-parallel and gneissosity-cutting veins. These veins probably represent the B-bearing melt extracted from the Fm I gneisses. Accumulation of such melt possibly formed the High Himalayan leucogranite plutons.
The U-Pb dates and REE concentrations of zircon were determined by LASS-ICP-MS analyses. The outermost rim of zircon including sillimanite gave ~18 Ma with steeply positive HREE pattern. The inner rim of zircon gave concordia age of 25.2 ±0.7 Ma with nearly flat HREE pattern. These suggest that the garnet with tourmaline-bearing NIs was formed at ca. 25 Ma and partly broke down at ca. 18 Ma. Therefore, B-bearing leucogranite was likely formed during the Oligocene through incongruent melting.
Tourmaline-bearing leucogranite veins are abundant in the structurally higher Formation II exposed along the Marsyangdi river. Multiple timings of the leucogranite vein formation are recognized by the presence of gneissosity-parallel and gneissosity-cutting veins. These veins probably represent the B-bearing melt extracted from the Fm I gneisses. Accumulation of such melt possibly formed the High Himalayan leucogranite plutons.