Petrography and geochemistry of Permian basalts of the Cam Thuy formation and their relation to Song Da and Emeishan magmatic rocks

Nguyen Hoang1*, Tran Thi Huong1, Masatsugu Ogasawara2, Le Duc Anh3,  Nguyen Thi Mai1, Nguyen Thi Thu1, Cu Sy Thang1, Le Thi Phuong Dung1 
1 Institute of Geological Sciences, Vietnam Academy of Science and Technology (VAST), Hanoi, Vietnam 
2 Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Japan
 3 Institute of Marine Geology and Geophysics (VAST), Hanoi, Vietnam 
Received 20 November 2015. Accepted 15 November 2016 
ABSTRACT Cam Thuy Permian basalts consisting of thick lava flows and pyroclastic layers appear along both sides of the Song Ma fault zone in Thanh Hoa and in Son La and Ninh Binh provinces, NW Vietnam. The magmatism has been thought to have genetic relationship with Permian volcanism in the Song Da rift zone, which is believed to be part of the Emeishan large igneous province, having been extruded along the Red River shear zone following Paleogene India-Eurasian collision. A set of Cam Thuy volcanic samples including olivine and alkaline basalts was collected in the Lam Son area (Tho Xuan, Thanh Hoa province) to analyze for geochemical major, trace element and Sr-Nd-Pb isotopic composition. The Cam Thuy basalts are high-TiO2, CaO, FeO*, moderate MgO and SiO2 that plot between the Song Da and Emeishan high- and low-Ti basalt distribution fields and closely overlap that of Song Da’s high-Ti field. The primitive mantle and chondrite normalized trace element patterns of Cam Thuy basalts are essentially enriched oceanic island basalt (OIB)-like; this feature, together with crustal contamination-free, chondritic Sr, Nd and Pb initial (255Ma) isotopic composition are certainly of asthenospheric origin. These geochemical and isotopic characteristics are closely analogous to those features observed for the Song Da high-Ti basalts, suggesting similarity in their source of origin. Nevertheless, while the Song Da (and Emeishan) magmatism is signified by the presence of both high- and low-Ti basalts, with the latter being derived from heterogeneous and partially crustal-material contaminated sources in the lithospheric mantle, this low-Ti volcanic rock type has yet to be discovered in the Cam Thuy formation. Keywords: Northwest Vietnam, Song Ma fault zone, Song Da Rift zone, Cam Thuy Permian basalt, isotopic geochemistry.