Geochemistry, U–Pb dating, and Tectonic settings of the Silurian-Devonian Pyroclastic and Associated Rocks, Uthai Thani and Kanchanaburi Provinces

Abstract

The pyroclastic and associated rocks were mapped as a Silurian-Devonian unit in the lower part of the study area (Kanchanaburi area) and are mapped as a Mesozoic unit in the upper part of the study area (Uthai Thani area). These rocks were poorly investigated and their geotectonic setting is uncertain. Their outcrops are generally massive and elongate trending NW-SE direction which unconformably overlay meta-sedimentary rocks. These rocks are metamorphosed in a low-grade metamorphism. This study researches both meta-igneous and associated meta-sedimentary rocks. Based on geochemical and geochronological data, the meta-igneous rocks can be classified into three groups, which are typical calc-alkaline series and have continental arc-affinity: Group I Kanchanaburi Calc-Alkaline Rock I, Group II Kanchanaburi Calc-Alkaline Rock II, and Group III Uthai-Thani Calc-Alkaline Rock. Group I consist of meta-crystal and meta-lithic tuffs and can be chemically classified as dacitic-rhyolitic rocks. U-Pb zircon dating indicates that this group was emplaced during ca. 482-480 Ma. Group II consist of meta-lithic tuffs and can be chemically classified as trachyandesitic rocks. U-Pb zircon dating indicates that this group was emplaced during ca. 498-494 Ma. Both groups are analogous to the Quaternary rhyolite from the Andean Northern volcanic zone, Ecuador, related to active continental margin. Group III consist of meta-crystal and meta-vitric tuffs and can be chemically classified as dacitic-rhyolitic rocks. U-Pb zircon dating indicates that this group was emplaced during ca. 301 Ma. This group is analogous to the Pliocene rhyolitic ignimbrite from the Andean Southern volcanic zone, Argentina, related to active continental margin. The meta-sedimentary rocks consist of quartzites and meta-sandstones and can be chemically classified as arkose, subarkose, sublitharenite, and quartz-arenite. Their tectonic settings are the passive continental margin Their provenances are the quartzose sediments of mature continent, which is possibly the Sibumasu Terrane. The meta-sedimentary rocks are analogous to the Post-Archean Average Australian Sedimentary rock (PAAS) and the Present Upper Continental Crust (UCC), which have been formed as a passive margin. The results of this study suggest that the emplacement of the Kanchanaburi Calc-Alkaline Rock I and II (Pak Phraek Tuff) in the Sibumasu Terrane is related to the Proto-Tethys ocean closure during Late Cambrian to Early Ordovician (ca.500-480 Ma). The emplacement of the Uthai-Thani Calc-Alkaline Rock in the Sukhothai Arc is related to the Palaeo-Tethys ocean closure during Late Carboniferous-Early Permian (ca. 300 Ma). Furthermore, the meta-sedimentary rocks (Bo Phloi and Ban Rai formations) have taken place in the Palaeo-Tethys ocean extension during Middle Devonian.