Early Cretaceous magmatism in Yunnan traces the tectonic transition from the Tengchong-Baoshan continental collision to Neo-Tethyan slab rollback

Early Cretaceous magmatic rocks of the Tengchong and Baoshan blocks provide insight into the late Mesozoic evolution of southeastern Tibet-Yunnan. We present LA-ICP-MS U-Pb zircon age, zircon eHf(t) isotope, and whole-rock geochemical and Sr-Nd isotopic data for dioritic porphyrite and rhyolite, outcropping in the Eastern Tengchong Belt and the Gaoligong Belt, respectively. The dioritic porphyrite has a 123.0 +/- 1.3 Ma (2 sigma) age, negative whole-rock (epsilon Nd)(t) of 1.99 to 2.16, initial Sr-87/Sr-86 of 0.70690-0.70734, zircon (epsilon Hf)(t) of 1.42 to 4.11, and adakitic geochemical features, suggesting an origin from mixing of mantle- and crust-derived magmas; the rhyolite has a 122.2 +/- 1.0 Ma age, low whole-rock (epsilon Nd)(t) of 9.69 to 8.83, initial Sr-87/Sr-86 of 0.71104-0.71246, and zircon (epsilon Hf)(t) of 8.55 to 2.59, suggesting crustal derivation. Our data-along with the published data-suggest that the Early Cretaceous rocks (140-109 Ma) comprise two magmatic phases with a geochemical and isotopic transition at similar to 123 Ma. The 140-123 Ma phase-mainly outcropping in the Gaoligong Belt-is mainly peraluminous, highly silicic, and has negative zircon (epsilon Hf)(t) values, suggesting derivation from remelting of ancient crust in a postcollisional setting; the 123-109 Ma phase is mainly metaluminous, restricted to the Eastern Tengchong Belt, and formed in a continental-arc setting with mantle melt involved. We propose that the similar to 123 Ma geochemical and isotopic change marks the transition from contraction due to the Tengchong-Baoshan collision to extension resulting from slab rollback of the Neo-Tethyan lithospheric slab.