The extrusive rock samples were collected from outcrops at Khun Dan Prakarn Chon Dam site, Nakhon Nayok Province. They are parts of the Pre-Cretaceous Khao Yai Volcanics located at the southern end of the western Loei-Petchabun-Nakhon Nayok Volcanic Subbelt. Petrological study found that the extrusive rock samples can be classified into four groups: felsic pyroclastic rocks, felsic volcanic lava flows, mafic pyroclastic rocks, and mafic-to-intermediate volcanic/hypabyssal rocks. Geochemical characteristics of the extrusive rock samples, however, can be divided into two groups which are mafic-to-intermediate extrusive rocks and felsic extrusive rocks. Although the major and some trace element signatures of these extrusive rocks are identical to those of typical calc-alkalic magmas, the tectonic discrimination diagrams for least-altered rocks showed that they were different in the magmatic suite and erupting episode; and discriminant diagrams can be successfully used to identify the tectonic environments of extrusive rocks and to evaluate the tectonic setting of a region. Incompatible trace element plot on the tectonic discrimination diagrams for the mafic-to-intermediate extrusive rock samples indicated high affinity with a within-plate tectonic setting, while felsic extrusive rock samples had high affinity with continental volcanic arc tectonic setting. In addition, the tectonic setting of these mafic-to-intermediate extrusive rock samples could be correlated to the Khiao Andesite located in the east of studied area. Dykes could be associated with multiple arc magmatism in Loei Fold Belt and resulted in the subduction in the Late Permian.

The Permo-Triassic Khao Yai Volcanics in Nakhon Nayok Province, Thailand, is a part of the Loei-Phetchabun-Nakhon Nayok Volcanic Belt. The purpose of this study is to clarify the geochemistry and REEs characteristic of Khao Yai Volcanics that is useful for identifying the paleo-environment or tectonic setting eruption in this area. The least-altered, mafic volcanic rocks from the Permo-Triassic Khao Yai volcanics, are seriate-textured to porphyritic, with variable amounts of phenocrysts/microphenocrysts. The mineral compositions include plagioclase, clinopyroxene, orthopyroxene, olivine, minor Fe-Ti oxide mineral, amphibole, apatite, biotite/phlogopite, monazite/zircon, glassy, and quartz. The geochemical characteristic of the Khao Yai Volcanics informed that the rocks had the same parental magma with different degrees of crystal fractionation. Most of the mafic volcanic rocks are subalkalic andesite on the basis of their Zr/TiO2 and Nb/Y ratios and calc-alkalic on diagrams Ti-Zr, Ti-Zr-Y, Hf-Th-Ta and Y-La-Nb. The least-altered mafic volcanic rocks have (La/Sm)cn and (Sm/Yb)cn ranging from 2.41 to 2.71 and 1.86 to 2.22, respectively.  The studied calc-alkalic andesite are analogous to the Quaternary calc-alkalic dacite and andesite from Maca Volcano, Patagonian Andes and the Middle Eocene andesite from Shimokoh cauldron, SW Japan in terms of chondrite-normalized REEs and N-MORB normalized patterns. Accordingly, the studied Khao Yai volcanics have been developed in an active continental margin, formed by eastward underthrusting Paleo-Tethys, the leading edge of Shan-Thai, beneath Indochina in the Late Permian to Early Triassic. HIGHLIGHTS New tectonic model of the Permo-Triassic Khao Yai mafic volcanic rocks in Nakhon Nayok Province, Thailand The geochemistry and rare earth elements (REEs) analysis by XRF and ICP-MS The REEs analog to modern volcanic rock The classification of pre-Jurassic volcanic belt in Thailand GRAPHICAL ABSTRACT