Geochemical and Geochronological Constraints on the Petrogenesis and Tectonic Evolution of Li‐Pegmatites and Granites in Phang Nga and Phuket, Southern Thailand

ABSTRACT Lepidolite‐bearing pegmatite, recently recognized as a major lithium (Li) resource, is exposed in Phang Nga, southern Thailand. This study investigates the geological relationships between Li‐pegmatites and granitic rocks and their formation processes within the tectonic evolution of the study area, using petrological, geochemical, and geochronological data. The granitic rocks in the study area are classified into three types: I‐type biotite granite, both I‐ and S‐type biotite–muscovite granite, and S‐type tourmaline granite. The Li‐pegmatite shows S‐type characteristics with peraluminous features. The primitive mantle‐normalized spider diagram of Li‐pegmatite and tourmaline granite reveals similar patterns, with enrichment in large‐ion lithophile elements (e.g., Cs, Rb, Li, and U) and depletion in Ba, Nb, and Sr, suggesting that the Li‐pegmatite evolved from a highly fractionated tourmaline granitic melt. Zircon U–Pb dating of the granites indicates three main magmatic events: Late Triassic biotite granite and tourmaline granite (216–214 Ma), Late Cretaceous biotite–muscovite granite and tourmaline granite (83–81 Ma), and Paleocene biotite granite (60–58 Ma). The cassiterite U–Pb ages of Li‐pegmatites are 81 and 69 Ma, which are coeval with the Late Cretaceous tourmaline granite. Newly found Late Triassic granites in the Western Granitoid Belt suggest that the Triassic granitic magmatism of the Central Granitoid Belt may extend further into southern Thailand than previously known. The formation of the Late Triassic biotite granite and tourmaline granite is related to the collision events between the Sibumasu and Indochina Terranes following the closure of the Paleo‐Tethys Ocean. Subsequently, during the Late Cretaceous, biotite–muscovite granite, tourmaline granite, and associated Li‐pegmatites formed due to the West Burma–Sibumasu collision and the subduction of the Neo‐Tethys oceanic plate beneath the western Sibumasu Terrane. The Paleocene biotite granite likely resulted from asthenospheric upwelling caused by the rollback of the Neo‐Tethys oceanic plate.