Geochronology and petrogenesis of late triassic-early jurassic LCT pegmatites from the Yamon-Kazat area, southern Myanmar: Implications for magmatic evolution

Abstract

The Mesozoic granitoids in the Southeast Asia Tin Belt, stretching over 2800 km from Myanmar to Indonesia, represent a significant geological and economic resource due to their association with diverse mineral resources and complex tectonic history. Despite the extensive documentation of magmatic phases within this belt, there have been no records of Late Triassic to Early Jurassic magmatism in southern Myanmar until recent investigations. A systematic geological study in the Yamon-Kazat (YK) area of the Myeik Sn-W district in southern Myanmar addresses this gap by focusing on the age, petrogenesis, and tectonic context of Jurassic magmatism. This study integrates field work, petrographic identification, bulk geochemistry data, in situ zircon U-Pb-Lu-Hf isotopic analyses, and zircon trace element chemistry. Field studies in the YK area identified granitic pegmatites occurring as small-scale intrusions, veins, and dyke swarms. These pegmatites display mineralogical assemblages dominated by quartz, alkali-feldspar, plagioclase, lepidolite, and muscovite with minor alteration minerals (sericite, chlorite, and iron oxides). Geochemically, the parental magma of YK pegmatites is derived from S-type granitic sources and exhibits a moderately peraluminous affinity. They are enriched in Li (178–>10,000 ppm), Sn (93–>10,000 ppm), Rb (325–>10,000 ppm), Cs (21–1800 ppm), Ta (20–173 ppm), Nb (42–167 ppm), and Be (23–407 ppm). They show a negative Eu anomaly (average Eu/Eu* = ∼0.69) and moderate enrichment of light rare-earth elements, revealing geochemical signatures similar to Lithium–Cesium–Tantalum (LCT) pegmatite. The zircon chemistry displays high contents of U, Th, Pb, Y, and REEs, suggesting generation through extensive fractional crystallization from a residual parental granitic source. Geochronologically, three types of zircons have been identified in the YK samples. The first group consists of xenocrystic zircons with ²⁰⁶Pb/²³⁸U ages ranging from 3619.4 ± 61.97 Ma to 339.6 ± 7.8 Ma, characterized by anhedral-subhedral crystal morphologies. These zircons exhibit oscillatory zoning in their inherited cores and are surrounded by overgrowth rims. The second group comprises magmatic zircons, which are observed as euhedral prismatic to subhedral circular grains with distinct oscillatory zoning, commonly surrounded by bright, thin recrystallized rims. These grains yielded concordia ages ranging from 201.6 ± 1.5 Ma to 199.1 ± 1.4 Ma, indicating the emplacement of YK pegmatites in the Late Triassic to Early Jurassic. The third group consists of younger secondary zircons, dated from overgrowth rims, with ²⁰⁶Pb/²³⁸U ages ranging from 180.6 ± 3.91 Ma to 46 ± 1.09 Ma. These grains contain elevated concentrations of Nb, Ta, Ti, and P (avg. 286.2, 103.5, 105.4, and 1415 ppm, respectively) compared to the inherited and magmatic zircons. The ¹⁷⁶Hf/¹⁷⁷Hf ratios (0.282184–0.282329), negative εHf(t) values (−16.6 to −12.9), and Hf model ages (1.33–1.56 Ga) of magmatic zircons indicate a continental crustal-derived source. Tectonically, the YK pegmatites formed during the late to post-collisional phase (∼200 Ma) following the Sibumasu-Indochina collision and the closure of the Paleo-Tethys Ocean (Early Triassic). This event facilitated the partial melting of older crustal components, resulting in pegmatite emplacement in a transitional tectonic setting. Subsequently, the YK pegmatites underwent three distinct magmatic-hydrothermal events during the Middle Jurassic, Middle-Late Cretaceous, and Early Eocene periods (∼171 Ma, 105 Ma, and 50 Ma, respectively). These findings provide new insights into magmatic evolution in the Myeik Sn-W district and serve as a benchmark for future Sn-Li production in the region.