Triassic diorites and granitoids in the Foping area: Constraints on the conversion from subduction to collision in the Qinling orogen, China

The western Qinling is characterized by Triassic granitic plutons, which are keys tc constrain the timing of subduction and collisional processes between the North China and South China Blocks. This paper presents a set of new LA-ICPMS U-Pb zircon geochronology, major and trace element geochemistry for several representative diorite and granitoid plutons in the Foping area. The diorites are characterized by high MgO, Al2O3, Y and Yb contents, but low SiO2, Sr/Y and La/Yb ratios, as well as depletion of Nb, Ta, P and Ti, suggesting derived from a subduction related setting. LA-ICPMS zircon U-Pb age of 216.9 +/- 1.5 Ma (MSWD = 0.69) from the diorite constrains the time of the subduction event. The medium- and fine-grained monzogranites as well as the biotite granites exhibit relative high SiO2. MgO and Rb, low Y and Yb contents, and low Sr/Y and La/Yb ratios, as well as various depletions of Nb, Ta, P and Ti. These features indicate a derivation from lower continental crust but had been influenced by slab released fluids in a syn-collisional setting. The medium-grained monzogranite and biotite granite yield LA-ICPMS zircon U-Pb ages of 207.3 +/- 2.1 Ma (MSWD = 1.8) and 201.6 +/- 1.2 Ma (MSWD = 1.04), respectively. proposing the syn-collision during ca. 207-201 Ma. Geochemical and geological evidences suggest that the granodiorites intruding in the syn-collisional monzogranites were related to the post-orogenic collapse after the collision between the North China and South China Blocks. The LA-ICPMS U-Pb zircon age of 189.3 +/- 2.9 Ma (MSWD = 5.6) for the granodiorite constrains the time of the post-orogenic collapse. Our geochronological and geochemical data provide important constraints on the Triassic evolution from subduction to collision then up to post-collisional collapse along the Qinling orogen. (C) 2011 Elsevier Ltd. All rights reserved.