Petrography and petrochemistry of cenozoic volcanic rocks in southern margin of khorat plateau

Abstract

Cenozoic volcanic rocks in Thailand distribute in the Inthanon zone, Sukhothai fold belt, and Khorat Plateau. The important Cenozoic basaltic rocks are along the southern margin of the Khorat Plateau and divide into 12 areas that trend in an east-west direction from Nakhon Ratchasima province to Ubon Ratchathani province. The study of the variation and relationship of these basaltic rocks is impressive and brings an understanding of their tectonic evolution in Khorat Plateau. In this study, the occurrences of basaltic rocks had studied in Nakhon Ratchasima, Phu Phra Angkhan, Khao Prai Bat, Khao Phanom Rung, Khao Kradong, Khao Phanom Sawai, Phu Ngoen, Phu Khmint, Phu Kom, Phu Fai, and Nam Yun area. Fifty-three at least altered basaltic samples were collected from each volcanic mass to study rock textures, mineral assemblages, whole-rock chemical composition, trace elements, and rare earth element compositions. This study consists of a petrographic method, X-ray fluorescence spectrometer (XRF) analysis, and multiple collector inductively coupled plasma spectrometry (MC-ICP-MS) analysis. The Cenozoic basaltic rocks in Khorat Plateau occurred as a subaerial lava flow. The occurrence of basalt is a lava mound that is found mainly on the western and eastern margins. In comparison, the central basalt appears like a lava cone. Porphyritic-microporphyritic texture is the typical texture of these basalts. Phenocrysts mainly consist of olivine and minor clinopyroxene. Plagioclases phenocryst is observed in some areas. Groundmass is fine-grained, consisting of lath-plagioclase, clinopyroxene, magnetite, and glass. In some areas, such as Nakhon Ratchasima, Phu Fai, Nam Yuen, Etc., nepheline was observed in the groundmass by intergranular between the plagioclase lath. In addition, diabasic textures were found only in the Phu Fai area, and seriate texture is observed in basalt around Nam Yuen and Phu Fai. The disequilibrium feature of phenocrysts and Mg-numbers between 0.40 and 0.61 suggest that these basalts have been solidified from evolved magma mainly by fractional crystallization of olivine. The clinopyroxene or plagioclase are partially crystallized in some regions. Enriched in light-rare earth and depleted in heavy-rare earth, these characteristics are similar to those of OIB-like basaltic magma. The study of petrography and geochemistry of Cenozoic basalts in the Khorat Plateau can be divided these basalts into 5 groups with distribution in 3 areas: the western margin in Nakhon Ratchasima area (Group A); The central consists of Phu Phra Ang Khan- Khao Prai Bat - Khao Phanom Rung - Khao Kradong - Khao Phanom Sawai (Group B) and the eastern margin can be divided into 3 groups, namely Phu Kom - Nam Yun (Group C-1), Phu Ngoen - Phu Khmint (Group C-2) and Phu Fai (Group C-3). Group A consists of basalt and trachybasalt (46.5-48.9wt% SiO2); Group B consists of trachybasalt and basaltic-trachyandesite that have high silica content (49.3-52.9wt% SiO2), low MgO and more medium-rare earth (MREE: Sm, Eu, Gd) than other groups; Group C-1 consists of basalt and trachybasalt (46.9-51.2wt% SiO2); Group C-2 consists of basanite and trachybasalt that low silica content (42.5-47.8wt% SiO2) indicates a low degree of partial melting; Group C-3 consists of basaltic trachyandesite (49.8-51.1wt% SiO2). The relationship of Zr-Ti-Y-Nb supports that the eruptions of Cenozoic basaltic rocks in the southern margin of the Khorat Plateau are within-plate tectonic. Basaltic magma is possibly derived from two different magmatic sources. OIB-like basaltic magma, which exhibited enriched LREE and depleted HREE has been observed in western and eastern marginal basalts (Group A, Group C-1, Group C-2, and Group C-3). After that, the enriched MREE magmatic source was melted into the first magmatic group. They exhibited similar characteristics to the basalt group found at the margins but with more enriched in MREE. These characteristics were found in basalt rocks in the central area (Group B).