Geochemistry, zircon U-Pb age and Hf isotopic constraints on the petrogenesis of the Silurian rhyolites in the Loei fold belt and their tectonic implications

Zircon U-Pb dating, Lu-Hf isotopic and geochemical data for the Silurian rhyolites from the Loei fold belt are presented to constrain their petrogenesis and tectonic settings. The rhyolites give a weighted mean 206Pb/238U age of 423.7±2.7 Ma, and are characterized by high SiO2, Al2O3, K2O and low MnO, MgO and P2O5. All samples are enriched in LILEs (e.g., Ba, K, Pb) and LREEs and depleted in HFSEs (e.g., Nb, Ta, Ti) with obvious negative Eu-anomalies (dEu=0.56–0.63). The calc-alkaline rhyolites are typical arc-related rocks. The Loei rhyolites have high A/CNK ratios (1.19–1.34) and positive εHf(t) (4.03–5.38), which can be interpreted as partial melting of juvenile crustal materials followed by multistage melting and differentiation, similar to highly fractional I-type rocks. Combined with regional geological surveys, the Loei rhyolites should be formed in a volcanic arc environment and may be in contact with the Truong Son fold belt during the Early Paleozoic. Moreover, the Simao Block might be in contiguity with the Indochina Block during Silurian.