Mid-Triassic felsic igneous rocks from the southern Lancangjiang Zone, SW China: Petrogenesis and implications for the evolution of Paleo-Tethys

The southern Lancangjiang magmatic belt is an important component of the Triassic San-Jiang igneous zone in Southwestern China and is mainly composed of the Lincang (LC) batholith and a Mesozoic volcanic belt that contains abundant rhyolites. Our new LA-ICP-MS zircon U-Pb results, together with previous SHRIMP and LA-ICP-MS ages, define two emplacement ages for the Lincang batholith: at similar to 230 Ma and similar to 220 Ma, corresponding to the eruption times of volcanic rocks of the Manghuai and Xiaodingxi formations, respectively. Most of the similar to 230 Ma LC granites possess high A/CNK values (>1.1) and display strongly peraluminous characteristics, similar to S-type granite, whereas the Manghuai Formation (MHF) rhyolites show an affinity to A-type granite, especially their elevated FeOt/(FeOt + MgO) and Ga/Al ratios. Both of the granitic and volcanic rocks have strongly fractionated REE patterns ((La/Yb)N = 2.66-33.2 and 13.1-19.7, respectively) and have conspicuous negative Eu anomalies (Eu*/Eu = 0.06-0.65 and 0.52-0.74, respectively) with a similar depletion in HFSE (Nb, Zr, Hf), P, Ba and Sr. The Sr-Nd isotopic data and T-DM2 model ages suggest that the LC granitic magma had a dominantly crustal source, whereas the MHF rhyolites were derived from mixing between middle/upper crustal rocks similar to the source of the LC granitic magma, with a small volume of mantle-derived melt. In combination with regional studies, our new geochemical data and geochronological results show that the mid-Triassic magmatism was generated in a post-collisional tectonic setting. The spatial distribution pattern of the Mesozoic igneous rocks along the Lancangjiang zone favors a slab breakoff model, which resulted in post-collisional extension and asthenospheric upwelling that induced large-scale partial melting of the middle-lower crust to produce voluminous amounts of felsic magma. Therefore, the occurrence of the mid-Triassic post-collisional magmatism clearly indicates that the final continent-continent/arc collision between the Gondwana-derived Sibumasu and Indochina blocks of Paleo-Tethys was completed by the early mid-Triassic. (C) 2013 Elsevier B.V. All rights reserved.