Ar-Ar and fission-track ages in the Song Chay Massif: Early Triassic and Cenozoic tectonics in northern Vietnam

The Song Chay Massif is the northeasternmost metamorphic complex in Vietnam , to the east of the Red River Shear Zone. It shows a large antiformal stru cture involving orthogneisses and migmatites overlain, on its northern Rank , by muscovite bearing marbles. An E-W striking fault bounds the dome to th e South. Kinematic indicators along a S-N section reveal top-to-the-N shear sense along the interface between the orthogneissic core and the overlying metasediments. Radiometric ages were obtained by the Ar-40-Ar-39 method us ing purified mica separates. Across the dome ages range from 236 Ma at the southern edge to 160 Ma in the core, attesting to a strong imprint in the E arly Triassic time. A clear difference is seen between these Mesozoic ages and the Eocene to Miocene ages (from 40 to 24 Ma) that obtained in the near by Red River Shear Zone using the same method. These data show that the Son g Chay Massif was already high in the crust when the high temperature defor mation of the Red River Shear Zone took place. The final exhumation of the Song Chay orthogneiss constrained by fission-track analysis on samples alon g the same transect occurred during the Early Miocene and could be interpre ted as the consequence of a first normal sense of motion along the fault wh ich bounds the massif to the south. Timing is similar to that of exhumation in the Red River Shear zone. (C) 2001 Elsevier Science Ltd. All rights res erved.