| Abstract: |
The Loei volcanic province is in the northern part of the Central Thailand Volcanic belt, Loei-Prachinburi. The volcanic rocks are Devonian rhyolite, Middle Devonian-Lower Carboniferous basalt and Permo-Triassic andesite. The Sr isotopic ages provide convincing evidence that the volcanic rocks are not part of a contemporaneous volcanic arc as was previously thought. The age data indicate that the volcanism occurred in two magmatic episodes at approximately 374 and 361 Ma. Trace element and isotopic compositions of the rhyolites suggest that they were generated by partial melting of continental crust at 374 Ma. Furthermore, continental crustal evolution models based on both the Rb-Sr and Sm-Nd isotopic compositions suggest that the western edge of the Indosinia craton in the Loei are existed at least 1140 Ma b.p. The model age supports the westward accretionary growth of the Indosinia craton. The ocean floor basalts in this belong to a younger magmatic episode at 361 Ma. The tholeiitic basalts appear to have been generated by a high degree of partial melting of a depleted mantle source, whereas the splitic basalts were generated by low degrees of partial melting of garnet-free slightly enriched mantle. The geochemical data and the variability in isotopic compositions of Loei basalts also support a heterogenous nature of the source regions which may reflect changes in the mantle composition over significant geologic time.
|
