Geochemistry of lamprophyres at the Daping gold deposit, Yunnan Province, China: Constraints on the timing of gold mineralization and evidence for mantle convection in the eastern Tibetan Plateau

摘要

Cenozoic lamprophyre dykes occur widely along the Ailao-Shan-Red-River (ASRR) shear zone related to the Indian-Eurasian collision. Two generations of lamprophyres have been found at the Daping gold deposit in the southern part of the ASRR shear zone and have been investigated by using phlogopite ~(40)Ar/~(39)Ar dating and whole-rock major and trace element as well as Sr and Nd isotope geochemical analyses. The ~(40)Ar/~(39)Ar plateau ages of phlogopite from the two generations of lamprophyres bracket the emplacement of auriferous quartz veins in the Daping deposit between 36.8 ± 0.2 Ma and 29.6 ± 0.2 Ma, consistent with the timing of gold mineralization in other parts of the ASRR shear zone. Geochemical data suggest that these lamprophyres most likely originated from a subduction-modified mantle source consisting of phlogopite-bearing spinel lherzolite, which underwent partial melting with contributions from crust materials. In particular, the second generation lamprophyres are characterized by more primitive geochemical features than the first, suggesting that secular source evolution probably resulted from post-collisional slab break-off mantle convection and remelting from ascending astheno-sphere after subducted lithosphere break-off. Widespread and episodic occurrences of lamprophyres and other potassic volcanism in the eastern Tibetan Plateau were probably related to the onset of trans-tensional tectonics along the ASRR shear zone during Oligocene. A genetic model involving transtensional tectonics has been proposed for lamprophyres and gold mineralization in the ASRR shear zone.