Stratigraphy and volcanic facies In Khao Noi Area, Tha Takiap district, Chachoengsao province

Abstract

The Khao Noi volcanics is in Tha Takiap district, Chachoengsao province, eastern Thailand. This volcanics is a part of the Lampang volcanic belt, a potential host rocks for mineralizations, especially gold and antimony epithermal style deposits. Based on field investigation, drill core logging, and petrographic study, the Khao Noi host volcanic sequence has a thickness of at least 150 meters. The sequence can be divided into three units, namely 1) Sedimentary unit (Unit 1), 2) Mafic-intermediate volcanic unit (Unit 2), and 3) Felsic volcanic unit (Unit 3). Unit 1 forms as a basement of the Khao Noi volcanic sequence and is characterized by laminated limestone, laminated sandstone, limestone breccia, and laminated mudstone. Unit 2 consists of plagioclase-phyric andesite, monomictic andesitic breccia, and mafic volcanic facies. This unit is mainly characterized by lava and hyaloclastite facies. Unit 3 consists of lithic-rich pumice breccia, crystal-rich pumice breccia, and quartz-phyric rhyolite facies. Both lithic-rich pumice breccia and crystal-rich pumice breccia facies were interpreted to form from the explosive eruption and deposited in the submarine environment below the wave-base.

The volcanic rocks (lava) from the mafic-intermediate volcanic unit (Unit 2) were selected for geochemistry study. Based on trace elements and REEs, the mafic-intermediate volcanic rocks range in composition from andesite to trachyte-andesite with calc-alkaline magma affinity. These volcanic rocks can be separated into two suites; Suite I shows a higher concentration of Ni, Ti, and V, and Suite II shows a higher concentration of Y and Zr. Both Suites I and II have negative Eu and Sr anomalies. Besides, they are characterized by enrichment of LILE and LREE with depleted Nb indicating the crust-derived magmatic source. A ratio of Ta/Yb vs. Th/Yb indicates that both suites originate in a continental arc or alkaline oceanic arc setting. According to the modern analogs of subduction-related settings, Suite I resembles a high-K calc-alkaline rock from the Riggit-Beser complex, Pleistocene age, East Java Indonesia. While Suite II similar to the high-K calc-alkaline rocks from the Aeolian islands of Pleistocene age from the southern part of the Tyrrhenian Sea, Italy. Furthermore, both Suites I and II are similar to the Lampang area's volcanic rocks, which occur in subduction-related settings during the Middle Triassic age. Thus, the Khao Noi volcanic rocks might be formed in an arc tectonic setting as a part of the Lampang volcanic belt.