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

Abstract

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 εHf(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 (2σ) age, negative whole-rock ℇNd(t) of −1.99 to −2.16, initial 87Sr/86Sr of 0.70690–0.70734, zircon ℇHf(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 ℇNd(t) of −9.69 to −8.83, initial 87Sr/86Sr of 0.71104–0.71246, and zircon ℇHf(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 ∼123 Ma. The 140–123 Ma phase—mainly outcropping in the Gaoligong Belt—is mainly peraluminous, highly silicic, and has negative zircon εHf(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 ∼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.