Petrology and geochronology of the volcanic rocks of the central Thailand Volcanic Belt

Volcanic rocks along the Central Thailand Volcanic Belt erupted during the Late Devonian-Carboniferous, Triassic and Tertiary. The isotopic ages of the volcanic rocks from the northern part of the belt (the Loei and Phetchabun areas) are older than the samples from the central part of the belt (the Lop Buri area).;The volcanic rocks in the Loei area include Devonian rhyolites, Middle Devonian-Lower Carboniferous basalts and Permo-Triassic andesites. Trace element and isotopic compositions of the rhyolites suggest that they were generated by partial melting of continental crust at approximately 374 Ma, likely during a subduction event prior to the suturing of Indosinia onto Cathaysia. Continental crustal evolution models based on both the Sr and Nd isotopic compositions suggest that the western edge of the Indosinia craton in the Loei area existed at least 1140 Ma b.p. The ocean floor basalts in this area belong to a younger magmatic episode (ca., 360 Ma) and were generated at a spreading center in the ocean basin between the Sinoburmalaya and Indosinia. The tholeiitic basalts appear to have been generated by a high degree of partial melting of a depleted mantle source, whereas the spilitic basalts were generated by low degrees of partial melting of garnet-free slightly enriched mantle.;Basalts, basaltic andesites and andesites from the Phetchabun area have enriched LILE (Large-Ion Lithophile Element) and depleted HFSE (High Field Strength Element) characteristics typical of arc magmas. Their trace element and Sr, Nd and Pb isotopic signatures suggest that the basaltic and andesitic rocks in the Phetchabun area may have been derived by partial melting of an enriched mantle source similar to OIB (Oceanic Island Basalt)-type mantle. The mantle source may have been metasomatized by fluid released by dehydration of subducted oceanic slab and sediments. The ;Tertiary volcanic rocks in the Lop Buri area of central Thailand are dominated by mildly alkaline olivine tholeiitic basalt, alkaline basalt, trachyandesite and rhyolite. The ;Trace element and isotopic signatures of the Lop Buri basaltic rocks suggest that they were derived by partial melting of a heterogeneous mantle source, probably in the subcontinental lithospheric mantle. The AFC (Assimilation Fractional Crystallization) modelling of trace elements and Sr-Nd isotopic ratios suggests that the evolution of trachyandesitic magma involved 30-35% fractional crystallization, accompanied by 9-14% crustal assimilation of the melt generating from 40% partial melting of basic plutonic components. The Lop Buri rhyolites were derived from partial melting of continental crust followed by plagioclase and ilmenite fractionation. The Sr and Nd model ages of the crustal source rocks for the rhyolitic magmas indicate that the crust in the area is not older than 450 Ma.