Tran Tuan Anh

U-Pb dating and tectonic implication of ophiolite and metabasite from the Song Ma suture zone, northern Vietnam

The Song-Ma suture zone including the “classical Song Ma suture” and the Nam Co antiform between Indochina and South China blocks in northern Vietnam is a crucial area for understanding the tectonic evolution of Southeast Asia. To explore their natures and evolutionary history, this paper reports new petrological studies and SIMS zircon U-Pb age data for the Song Ma ophiolites, metabasites and fine-grained quartzite. The Song Ma ophiolites consist mainly of peridotite (Fo90-91, En92), basalt and gabbro, and experienced greenschist- to lower amphibolite-facies ocean-floor metamorphism. Lenses of rodingite and altered pyroxenite in serpentinites formed during alternation of peridotite. We identified two types of zircon in the altered pyroxenite: I uniform metamorphic (70 vol.%), and II, zircon having inherited magmatic cores and metamorphic rims (30 vol.%). All U-Pb analyses of both zircon types yield an upper intercept age of 340 ± 29 Ma that we interpret as the protolith age. Type I zircon and Type II metamorphic rims together yield a mean 206Pb/238U age of 280 ± 2 Ma, with rodingite forming broadly coevally at 283 to 282 Ma. Metabasalt (plagioclase-amphibole schist) has a MORB-type geochemical affinity. Magmatic zircon from a plagioclase-amphibole schist yields a protolith age of 315 ± 4 Ma. High-pressure eclogite, garnet-phengite-quartz schist, and garnet-hornblende rock are distributed in the northwestern Nam Co antiform. Zircon crystals from both eclogite and garnet hornblende rock have low Th/U ratios of 0.01 to 0.07 and 0.02 to 0.06, and yield metamorphic ages of 231 ± 8 Ma and 228 ± 3 Ma, respectively. The ages of detrital zircon grains from fine-grained, banded quartzite from the Nam Co antiform range from Paleozoic to Proterozoic with a peak at 1.7 to 1.8 Ga. All available data indicate a three-stage tectonic evolution: (1) 340 to 280 Ma: the Paleotethys that separated the Indochina and South China blocks was characterized by oceanic crust formation at 340 to 315 Ma and ocean-floor metamorphism at 283 to 280 Ma, (2) <280 to 230 Ma: the Paleotethyan oceanic lithosphere was subducted and subjected to HP metamorphism at ca. 230 Ma; closure of the Paleotethys led to collision between the Indochina and south China blocks in the Middle Triassic; and (3) <230 Ma: breakoff of the Paleotethyan oceanic lithosphere with subsequent exhumation of the subducted slabs. The HP metamorphic rocks were tectonically juxtaposed against the Nam Co complex. However the subduction polarity (northward versus southward subduction) is problematic, and requires more investigation.

Mineralogical characteristics of arsenopyrite and pyrite in Bo Va and Nam Quang gold-sulfide deposits (Northeast Vietnam)

Crystal morphology and chemical composition of arsenopyrite and pyrite in the Nam Quang and Bo Va deposits of the Song Hien basin (Northeast Vietnam) are studied in detail. Arsenopyrite crystals often form elongated crystals. Their chemical composition are relatively poor in As, but rich in S with S/As ratios often > 1.1, impurity elements such as Ni, Co, Sb, and sometimes containing gold (up to 0.02 wt.%). Pyrite is characterized by the stoichiometric composition, but contains relatively high arsenic and sometimes contain gold. The correlation between the concentration of As and Au in pyrite has been recorded. The morphology and chemical composition of arsenopyrite and pyrite of Bo Va and Nam Quang deposits can be correlated to those in the Au-sulfide deposits in black shale in Kazastan and southestern China. References Trần Trọng Hoa (chủ bien), 1996: Xây dựng mo hinh điều tra, sử dụng hợp lý, bảo vệ moi trường trong qua trinh khai thac tai nguyen khoang sản quý hiếm tỉnh Cao Bằng. Lưu trữ Viện Địa chất, Ha Nội. Trần Trọng Hoa, Polyakov G.V., Trần Tuấn Anh, Borisenko A.S., Izokh A.E., Balyakin P.A., Ngo Thị Phượng, Phạm Thị Dung, 2011: Hoạt động magma va sinh khoang nội mảng miền Bắc Việt Nam. Nxb. Khoa học Tự nhien va Cong nghệ, 368tr. Trần Trọng Hoa, Nevolko P., Ngo Thị Phượng, Trần Tuấn Anh, Phạm Thị Dung, Bui Ấn Nien, Nguyễn Xuân Huyen, Nguyễn Thuy Dương, Vũ Hoang Ly, Trần Văn Hiếu, 2013: Cac tụ khoang vang-sulfide trong cac thanh tạo lục nguyen - carbonat rift Song Hiến, ÐBVN. Tuyển tập Bao cao KHCN toan quốc năm 2013. ISBN: 978-604-59-0693-4; tr.131-140. Nong Thế Tang (chủ bien), 1996: Bao cao Khảo sat va tim kiếm đanh gia vang gốc Nam Quang, Bảo Lạc, Cao Bằng. Lưu trữ Trung tâm Thong tin lưu trữ Địa chất, Ha Nội. Chen M.H., Mao J.W., Bierlein F.P., Norman T., Uttley P.J., 2011: Structural features and metallogenesis of the Carlin-type Lannigou gold deposit, Guizhou Province, China. OreGeol. Rev.43 (1), 217-234. Fleet, M.E., Mumin, A.H., 1997: Gold-bearing pyrite and arsenopyrite from Carlin Trend gold deposits and laboratory synthesis: American Mineralogist, v. 82, p.182-193. Kovalev K.R., Kalinin Yu.A., Naumov E.A., Kolesnikova M.K., Korolyuk V.N., 2011: Gold-bearing arsenopyrite in eastern Kazakhstan gold-sulfide deposits. Russian Geology and Geophysics 52, 178-192. Kovalev K.R., Kalinin Yu.A., Polynov V.I., Kydyrbekov E.L., Borisenko A.S., Naumov E.A., Netesov M.I., Klimenko A.G., and Kolesnikova M.K., 2012: ISSN 1075-7015, Geology of Ore Deposits, vol. 54, No.4, 254-275. Peters S.G., HuangJ.Z., Li Z.P., JingC.G., 2007: Sedimentary rock-hosted Au deposits of the Dian-Qian-Guiarea, Guizhou, Yunnan Provinces, and Guangxi District, China. Ore Geol. Rev.31,170-204. Volkova A. V. , Genkina A. D. , and Academician of the RAS Goncharovb V. I., 2006. New data on invisible gold in disseminated sulfide ores of the Natalka deposit. Vol. 409A, No 6, pp. 879-883. Zhong H.R., Chao S.W., Wu B.X., Zhi T.G., Hofstra A.H., 2002: Geology and geochemistry of Carlin- type gold deposits in China Mineralium Deposita 37:378-39. Zhou T.H., Goldfarb R.J., Phillips G.N., 2002: Tectonics and metallogeny of gold deposits in China. Mineralium Deposita 37, 249-282.