Abstract:

SHRIMP U[CD*2]Pb zircon ages are reported for rhyolites in the bimodal volcanic rocks from the Phlaythong area at the Southern of Laos. The zircons with well-defined oscillatory zoning from rhyolite sample yielded mean ²⁰⁶Pb/²³⁸U age of (229.0±2.0) Ma, which represents the eruption age of the volcanic rocks during Middle to Upper Triassic (T_2-3). Geochemically, the basalt shows an average SiO₂ content of 50.70%, enrichment of TiO₂，MgO，CaO，TFeO，LILE(Sr，Rb，Ba，Th) and LREE, and depletion of alkali, P₂O₅ and HFSE. Rhyolites are generally characterized by rich SiO₂(76.33%), alkali(w（K₂O）＞w（Na₂O）), and poor TiO₂, MgO, CaO and TFeO, they have relatively high Th, U and Zr and rather low Ta, Nb, Ba, Sr and Ti, and display obvious negative anomaly of Ba, Sr and Ti, obvious enrichment of LREE, noticeable negative Eu anomalies (δEu=0.31-0.82). According to this research, the basalt was originated from depleted asthenosphere mantle (DM) material upwelling and contaminating a little part of crustal materials, and rhyolite resulted from melting crustal materials heated by basaltic magma and felsic magma. The bimodal volcanic rocks are the products of partial extension in an intracontinental rift.

Key words: bimodal volcanic rocks, SHRIMP zircon U-Pb dating, geochemistry, petrogenesis, Laos