GEOCHEMISTRY AND ALTERATION OF LAMPANG - TAK VOLCANIC ROCKS, THAILAND

Abstract

Volcanic rocks in southern Lampang and northern Tak are a part of the pre-Cretaceous Chiang Khong-Lampang-Tak volcanic belt situated in the northern region of Thailand. These studied rocks are composed of rhyolite and basalt with porphyritic texture, associated with pyroclastic rocks. Rhyolites consist of quartz and alkali-feldspar phenocrysts, while basaltic microphenocrysts are plagioclase and unidentified mafic minerals (pyroxene?) highly replaced by chlorite. Petrographically analysis describes alkali-feldspar altered to clay minerals, plagioclase was replaced by sericite, and volcanic glass devitrified to cryptocrystalline quartz. The Zr/TiO2 and Nb/Y ratio suggest that these volcanic rocks are rhyodacite and alkali basalt. In addition, REE Patterns classified these volcanic rocks were generated from two magma series; 1) alkali basalts were from the mildly calc-alkaline magma, and 2) rhyodacites were exploded from the alkaline magma. A petrographical study suggests that mineral compositions were vastly altered to phyllosilicates agreeable with geochemical data that describe these volcanic rocks grain Si4+ and lose alkaline cations. The rhyodacites have a high sericitization index (SI) and Ishikawa alteration index (AI) related to sericitization, illitization, silicification, and chloritization processes, which are caused by mass transfer in the hydrothermal system and maybe affected by the metamorphic process. Meanwhile, Triassic(?) basalts with moderately high CCPI present chlorite, pyrite, and carbonates key minerals, resulting from weathering and alteration processes. These alteration processes may affect the potential of new georesources for the ceramic industry similar to central Lampang, especially clay minerals.