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The study of the petrogenesis of some magmatic rocks with special geochemical attributes provides effective information for us to explore the deep geodynamic background of their formation. A series of granitic porphyry dykes have been found in the mélange zone of the Asa region in southern Tibet, whose genesis may be closely related to the evolution of the Meso-Tethyan Ocean. Regional geodynamic evolution is investigated by whole-rock geochemical analysis, zircon U–Pb dating and Lu–Hf isotopic analysis of two porphyritic granites. The Asa porphyritic granites have high SiO2 (74.29–78.65 wt %) and alkalis (Na2O + K2O = 6.51–9.35 wt %) contents, and low Al2O3 (11.60–14.51 wt %), CaO (0.04–0.19 wt MgO (0.01–0.10 wt %) contents. They are enriched in Zr, Nb, Ce, Y and Hf and depleted in Ti, Ba, Sr and P, consistent with A-type granites. The samples are relatively rich in LREEs, with LREE/HREE ratios of 1.73–3.04. They display negative Eu anomalies (Eu/Eu* = 0.24–0.28) and obvious Ce anomalies in some samples. Zircon U–Pb analyses show that the porphyritic granites formed in late Early Cretaceous time, 107.4 to 105.5 Ma. Zircon εHf(t) values are in the range of 6.9 to 12.0. These data indicate that the porphyritic granites were sourced from interaction between mantle-derived and juvenile lower crust-derived melts, with the addition of oceanic sediment-derived melts. This occurred when the subducting Bangong–Nujiang oceanic crust split to create a slab window. Rising asthenosphere triggered re-melting of lower crust basalts, resulting in the formation of the late Early Cretaceous A-type granites around Asa. [ABSTRACT FROM AUTHOR]

• The newly discovered Middle Permian granite dikes are adakitic in composition. • The adakitic magma was generated by partial melting of subducted oceanic slab. • The granite dikes were related to the subduction of the Sumdo Paleo-Tethys Ocean. The Sumdo Paleo-Tethys Ocean was the southernmost part of the Paleo-Tethys Ocean, located on the Qinghai–Tibet Plateau. However, the evolution of the Sumdo Paleo-Tethys Ocean and the origin of eclogites in the Sumdo area of Tibet are still matters of debate. Here we report newly discovered adakitic granite dikes from the Sumdo area of the central Lhasa terrane. The rocks have high SiO 2 , Al 2 O 3 , and Sr contents, high Sr/Y and La/Yb ratios, enrichment in light rare-earth elements (REEs), and depletion in Y, Yb, Th, and heavy REEs. They are characterized by low 87Sr/86Sr (0.703678–0.704427) and high 143Nd/144Nd (0.512825–0.512904) ratios, and positive ε Nd (t) (+6.41 to +7.38) and ε Hf (t) (+13.8 to +15.7) values. Zircon U–Pb dating yielded ages of 269.1 ± 4.1 and 266.4 ± 2.0 Ma for the rocks. Our data indicate that the Sumdo adakitic granite dikes were derived from partial melting of subducted oceanic slab, with the magma being influenced by slab-derived fluids and slight interaction with peridotite in the mantle wedge during ascent. By combining our new geochemical data with results of previous studies, we infer a subduction event involving the Sumdo Paleo-Tethys Ocean during the middle Permian, which led to the formation of the Sumdo adakitic granite dikes and the Sumdo, Jilang, and Xindaduo eclogites. [ABSTRACT FROM AUTHOR]