Structural and geochronological constraints on the tectono-thermal evolution of the Danba domal terrane, eastern margin of the Tibetan plateau

摘要

The Songpan-Ganze terrane of the Tibetan plateau is underlain by Neoproterozoic crystalline basement rocks of the Yangtze block. These basement rocks are exposed as a series of extensional tectonic domes that form a nearly north-south trending extensional belt more than 1000 km long in the eastern margin of the Tibetan plateau. In the Danba area, detachment faults separate the basement core complexes (e.g., the Gezong and Gongcai complexes) from the Paleozoic strata which have been thinned or removed completely. The cover sequences have undergone upper greenschist to lower amphibolite facies metamor-phism to form the Danba schist and are overlain by the Triassic Xikang Group, a thick flysch sequence. Both the basement rocks and the Paleozoic rocks have undergone multiple stages of deformation and thus provide an excellent opportunity to study the tectono-thermal evolution of the eastern margin of the Tibetan plateau. Two stages of deformation, corresponding to three generations of foliation (S_1 S_2~1 and S_2~2), have been recognized on the basis of structural and microscopic observations. We selected amphibole and biotite separates associated with distinct generations of foliation for ~(40)Ar/~(39)Ar dating using laser microprobe incremental heating technique to place numerical constraints on the major tectono-thermal events within the Danba area. The geochronogical results reveal an earliest metamorphic event at 258.6 ± 0.5 Ma (S_1 biotite) and 263.6 ± 0.8 Ma (S_1 amphibole), coinciding temporally with the mantle plume that produced the voluminous Emeishan flood basalts. The second event was a progressive extensional deformation first occurred at 159-166 Ma (S_2~1 amphibole) responsible for the earlier tectonic doming of the crystalline basement, and then the final tectono-thermal overprint recorded by S_2~2 foliation and M_2~2 metamorphism locally in the core complexes at 47-58 Ma for the Gezong complex and 64-81 Ma for the Gongcai complex. This major post-orogenic extensional event is believed to be a consequence of collision between the North China and South China blocks. The apparent discrepancy of the ~(40)Ar/~(39)Ar ages observed between localities suggests a slow cooling process associated with progressive uplift.