Age and geochemical evolution of granite magmatism in Olkhon region from Caledonian syncollisional ore-free granite to the rare metal granite and pegmatite of Middle Paleozoic intraplate setting

The detailed description of two granite complexes in the Olkhon subterrane is given.The Early Paleozoic Sharanur complex was formed by granitization of gneisses of the Olkhon series.It includes migmatites,granite-gneisses,granites and pegmatites of normal alkalinity;they belong to the type of syncollisional granites.The Middle Paleozoic Aya granite complex includes mother Aya massif of amazonite-bearing granites and several types of rare-metal pegmatites.They have elevated alkalinity,low of Ba,Sr,and high LILE and HFSE elements contents.The Aya pegmatites lie in northwest cracks of stretching and associated with the rise of the territory under the influence of the North Asian plume.These cracks and pegmatites mark the beginning of a new intraplate geodynamic setting.Two geochemical types are distinguished among the pegmatites of this complex.These are amazonite pegmatites of Li-F type with Ta mineralization and complex type pegmatite with Be-Rb-Nb-Ta and Li-F mineralization(the Ilixin vein).The Tashkiney pegmatite vein is similar to Ilixin,but lies in the gneisses of the Olkhon series.It shows high concentrations of Be,Nb,Ta,as well as W,Sn,but lacks Li and F,due to a greater depth and higher temperature of the melt crystallization of this pegmatite.

Middle Paleozoic metamorphic and sedimentary events in the central Korean Peninsula and their geological implications

The Middle Paleozoic tectonic evolution of the central Korean Peninsula(Gyeonggi Massif,Hongseong-Imjingang,and Okcheon zone of Okcheon Belts)remains controversial.Particularly,the occurrence of high-grade metamorphism and sedimentation need further examination.In this study,we conducted zircon U-Pb-rare earth element analyses from in the ultramafic-mafic complex in the central Korean Peninsula(Cheonan and Gapyeong areas)and the Paleozoic metasedimentary rocks in the Okcheon Zone of the Okcheon Belt to constrain the timing of maximum depositional and metamorphic ages.We also examined the metamorphic P-T-t path from garnet-bearing amphibolite in the central Korean Peninsula by pseudosection modeling and geothermobarometer.The results show that(i)some of the ultramafic-mafic complex and metasedimentary rocks in the central Korean Peninsula formed during the Middle Paleozoic(ca.450–374 Ma);(ii)garnet-bearing amphibolite underwent successive metamorphism from amphibolite facies condition(7.5–8.0 kbar and 540–630℃)at pre-peak stage to granulite facies condition(10.9–11.8 kbar and 740–820℃)at peak stage,and then retrograded into amphibolite facies condition(5.7–7.7 kbar and 530–670℃)along the clockwise P–T path during ca.403–362 Ma.This and previous studies suggest that the central Korean Peninsula underwent subduction-related orogenic events during the Middle Paleozoic era,and it is well correlated to those of the orogenic events in the North Qinling belt.