Abstract:
Lam Sonthi area is in Lop Buri province, central Thailand. Stratigraphically, the geology of the area consists of five rocks units; Unit I: Khao Ruak siltstone which is correlated to Khao Luak Formation (Early Permian), Unit II: Khao Phaeng Ma conglomerate, Unit III: Khao Ruak volcanics, Unit IV: Lam Sonthi polymictic conglomerate, and Unit V: Kuttaphet sandstone which is correlated to Phu Kradung Formation (Early Jurassic). In addition, the volcanic rocks, which are named as Kuttaphet volcanics and Khok Khli volcanics, could be found in the top of Unit IV as the polymictic intermediate breccia. Petrographic classification identifies the volcanic rocks according to their mineral constituents that quartz is found less than 20 %, so they are named as andesite. Clasts of Kuttaphet volcanics consist of plagioclase andesite and pyroxene-plagioclase andesite while clasts of Khok Khli volcanics consist of plagioclase andesite and hornblende-plagioclase andesite. The selected large clasts of these 2 volcanics were geochemically analyzed for major oxides and some trace elements using XRF and ICP-MS, then compared to the pyroxene-plagioclase andesite which is lava from Khao Ruak volcanics. Geochemistry of all samples suggests that the volcanic rocks in this area are andesite excluding plagioclase andesite clasts of Kuttaphet volcanics are composing of dacite/rhyolite. However, all samples are identified as calc-alkaline affinity and plotting of Ti-Zr suggests that they could have been originated in the volcanic arc setting and the N-MORB normalized patterns show negative Nb and Ta which could be suggest that the volcanic rocks in the Lam Sonthi area is associated with subduction zone. Geochemical similarities indicate that the clasts of Kuttaphet volcanics (which is pyroxene-plagioclase andesite) and Khok Khli volcanics (which are plagioclase andesite and hornblende-plagioclase andesite) were derived from the similar magma source of the Khao Ruak volcanics excluding plagioclase andesite clasts of Kuttaphet volcanics.
