Sequence and petrogenesis of the volcanic rocks from the middle Sanjiang Tethys Orogen, SW China: Implications for the Sanjiang Paleo‐Tethyan evolution

Abstract

The volcano‐sedimentary sequences from the Weixi belt provide significance for the Sanjiang Paleo‐Tethys evolution. This manuscript constrains the ages of volcano‐sedimentary sequences by LA‐ICPMS U–Pb, including bimodal volcanics (245.0–244.0 Ma) from Pantiange Formation, trachyandesite–basaltic andesite (234 ± 2.0 Ma) from Cuiyibi Formation, high silica subalkaline rhyolite (221 ± 1.4 Ma) from Waigucun Formation and Early Jurassic high silica subalkaline rhyolite (184 ± 2.2 Ma). The volcanics show various Hf isotopic composition, which includes Middle Triassic dacite (ε Hf[t] = −15.7 to −8.5, TDM2 = 2,006–1,613 Ma), Late Triassic rhyolite (ε Hf(t) = −9.8 to +1.9, TDM2 = 1,667–1,021 Ma), Late Triassic trachyandesite (ε Hf(t) = +2.6 to +5.7, TDM2 = 990–816 Ma) and Early Jurassic rhyolite (ε Hf(t) = −2.7 to +1.1, TDM2 = 1,250–1,145 Ma). The Middle–Late Triassic volcanics exhibit similar characteristics, the content of SiO2 exerts positive correlation with K2O and negative correlation with oxide of Ti, Fe, Al, Mg, Ca, Mn and P. The Hf isotopic and geochemical features indicate that the Middle Triassic basalt and rhyolite derived from mantle and crust by crystal fractionation, respectively. The absence of Lower Jurassic strata and existence of Triassic bimodal volcanic sequences suggest that the middle Sanjiang Tethys Orogen experienced collision during the Middle Triassic with local extension resulting from oblique convergence. Late Triassic–Early Jurassic rhyolite exhibit high SiO2 (77.12–77.81 wt%), K2O (4.35–6.10 wt%), Na2O (1.91–3.15 wt%) content, and a negative Eu anomaly. Together with the sedimentary sequence, the middle Sanjiang Tethys Orogen evolved into post‐collision phase during Late Triassic – Early Jurassic.