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 structure involving orthogneisses and migmatites overlain, on its northern flank, by muscovite bearing marbles. An E-W striking fault bounds the dome to the 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 ⁴⁰Ar-³⁹Ar method using 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 Early 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 nearby Red River Shear Zone using the same method. These data show that the Song Chay Massif was already high in the crust when the high temperature deformation of the Red River Shear Zone took place. The final exhumation of the Song Chay orthogneiss constrained by fission-track analysis on samples along the same transect occurred during the Early Miocene and could be interpreted as the consequence of a first normal sense of motion along the fault which 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 reserved.