Late Triassic granitoids of the eastern margin of the Tibetan Plateau: Geochronology, petrogenesis and implications for tectonic evolution

Late Triassic granitoids in the Songpan-Garze Fold Belt (SGFB), on the eastern margin of the Tibetan Plateau, formed at 230 to 220 Ma and can be divided into two groups. Group 1 are high-K calc-alkaline rocks with adakitic affinities (K-adakites), with Sr>400 ppm, Y<11 ppm, strongly fractionated REE patterns ((La/Yb)(N)=32-105) and high K2O/Na2O (approximate to 1). Group 2 are ordinary high-K calc-alkaline 1-types with lower Sr (<400 ppm), higher Y (>18 ppm) and weakly fractionated REE patterns ((La/Yb)(N)<20). Rocks of both groups have similar negative Eu anomalies (Eu/Eu* = 0.50 to 0.94) and initial Sr-87/Sr-86 (0.70528 to 0.71086), but group 1 rocks have higher epsilon(Nd)(t) (-1-01 to -4.84) than group 2 (-3.11 to -6.71). Calculated initial Pb isotope ratios for both groups are: Pb-206/Pb-204 = 18.343 to 18.627, Pb-207/Pb-204 = 15.6 10 to 15.705 and Pb-208/Pb-204 = 38.269 to 3759. Group 1 magmas were derived through partial melting of thickened and then delaminated TTG-type, eclogitic lower crust, with some contribution from juvenile enriched mantle melts. Group 2 magmas were generated by partial melting of shallower lower crustal rocks. The inferred magma sources of both groups suggest that the basement of the SGFB was similar to the exposed Kangding Complex, and that the SGFB was formed in a similar manner to the South China basement. Here, passive margin crust was greatly thickened and then delaminated, all within a very short time interval (similar to 20 Myr). Such post-collisional crustal thickening could be the tectonic setting for the generation of many adakitic magmas, especially where there is no spatial and temporal association with subduction. (C) 2006 Elsevier B.V. All rights reserved.