Eocene adakitic porphyries in the central-northern Qiangtang Block, central Tibet: partial melting of thickened lower crust and implications for initial surface uplifting of the plateau

The time and mechanism of crustal thickening and initial surface uplift of the Tibetan Plateau remain highly controversial. Here we report on zircon U-Pb age, and mineral, element, and Sr-Nd isotope composition data for Eocene adakitic porphyries in the Laorite Co area of central-northern Qiangtang Block, central Tibet. Laser ablation-inductively coupled plasma-mass spectrometry zircon U-Pb age dating shows that the porphyries were generated at 42-41 Ma. All samples display adakitic geochemical characteristics, such as high SiO2 and Al2O3, and low Y and Yb contents, and high Sr/Y and La/Yb ratios with positive Sr and negligible Eu anomalies and Nb-Ta depletion. They also have low magnesium number (Mg #) (16-45) and high K2O (4.0-4.6 wt %) values, as well as high (Sr-87/Sr-86) i (0.7076-0.7080) and low epsilon Nd(t) (-5.9 to -4.4) values. These adakitic rocks were most probably generated by partial melting of thickened lower crust in the stability field of garnet, amphibole, and rutile but with relatively minor plagioclase. Moreover, such a crustal thickening process beneath this block likely occurred no later than the middle Eocene and was most probably caused by the southward subduction of the Songpan-Ganzi Block in the early Cenozoic. In addition, the occurrences of exposed adakitic porphyries unambiguously indicate that exhumation process occurred after their formation. Thus, these -42 Ma adakitic porphyries indicate the occurrence of significant exhumation since 42 Ma. Integrated with the thermochronological and paleoelevation data, we suggest that the initial surface uplift of the Qiangtang Block, triggered by crustal thickening, was underway by the middle Eocene (-42 Ma).