Petrochemistry of Volcanic and Plutonic Rocks in Loei Province, Loei- Petchabun Fold Belt, Thailand Kamonporn Kromkhun 1  , Graham Baines 2 , Peangta Satarugsa 1 and John Foden 2 1 Geotechnology, Department of Technology, Khon Kaen University, 40002, Thailand 2 Geology and Geophysics, School of Earth and Environmental Sciences, University of Adelaide, Adelaide, SA 5005, Australia Abstract. The petrography and geochemistry of Permo – Triassic volcanic and plutonic rocks of the Loei- Phetchabun Fold Belt located in Amphoe Wang Sa Phung and Maung areas, Loei Province were studied. These intermediate igneous rocks have calc-alkaline affinities and are characterized by moderately fractionated LREEs and relatively flat HREEs with negative Eu anomalies. Both volcanic and plutonic samples display positive Rb, Th, Ce, Sm and Y anomalies and negative Nb and Ti anomalies. The geochemical data indicate that fractional crystallization of plagioclase, hornblende, biotite, titanite and apatite and/or magma mixing were important processes during the generation of these igneous rocks. Therefore, the data indicate magmatism at an east-dipping continent–ocean subduction zone where a former ocean between the Indochina and Sibumasu blocks was subducted beneath the Indochina block. Available geochronology data indicate that subduction was active from at least 244-230 Ma. The subduction-derived melts were probably contaminated by the overlying continental crust. Keywords: Loei-Phetchabun Fold Belt, Petrochemistry, Calc-alkaline, Thailand 1. Introduction The Loei-Petchabun Fold Belt also known as the Petchaboon Volcanic Belt [1] or the Central Thailand Volcanic Belt [2] is located on the western edge of the Korat Plateau, Northeast Thailand. The fold-belt consists of diverse volcanic and plutonic rocks that may have been generated and deformed during ocean closure and collision between two micro-continents, Sibumasu and Indochina, during the Permian and Triassic [3]. This collision resulted in crustal shortening and thickening, the amalgamated continental block then drifted northward before colliding with the rest of Asia in the late Triassic to Jurassic. The Loei-Petchabun fold belt hosts several important mineral deposits (e.g. the Chatree epithermal Au- Ag deposit, Thung Kam Cu-Au skarn deposit and Phu Lon Cu porphyry deposit). Epithermal, skarn and porphyry deposits are often closely related to igneous activity, this igneous activity is commonly a consequence of plate tectonics. In Thailand, previous plate tectonic studies have mainly focussed on sedimentary basin analysis and palaeo-biogeographic studies but there is limited data on the petrochemistry of volcanic and plutonic rocks and how they relate to the geodynamic setting. This paper presents new geochemical data from volcanic and plutonic rocks in the Western Loei area (Amphoe Wang Sa Phung and Maung, Loei Province) which we use to infer the magma source and tectonic setting. 2. Geology of the Study Area The Loei-Phetchabun Fold Belt consists of sequences of Palaeozoic strata of which the oldest sequences are Silurian-Devonian meta-volcanic and meta-volcaniclastic rocks. These volcanic rocks are overlain by Carboniferous clastic and carbonate rocks which are intruded by Carboniferous granites. N-S trending  Corresponding author. Tel.: +6643362125; fax: +6643362126. E-mail address: kamokr@kku.ac.th 2013 2nd International Conference on Geological and Environmental Sciences IPCBEE vol.52 (2013) © (2013) IACSIT Press, Singapore DOI: 10.7763/IPCBEE. 2013. V52. 11 55