Late Triassic post-collisional high-K two-mica granites in Peninsular Thailand, SE Asia: Petrogenesis and Sn mineralization potential

Triassic magmatism in Thailand provides the critical geological record for understanding the evolution of the Paleo-Tethys Ocean. This paper presents zircon U-Pb dating, zircon trace element compositions, elemental geochemistry and Sr-Nd-Hf isotopes for the newly identified Late Triassic granite intrusions in the Upper Peninsula of Thailand where voluminous Late Cretaceous-Paleogene granite intrusions occur. Zircon U-Pb dating results indicate that the two-mica granites were emplaced at ca. 217–216 Ma. The unaltered two-mica granite samples are high silicic (SiO2 = 74.03–75.06 wt%) and display strongly peraluminous characteristics, with A/CNK ratios of 1.21–1.33 and CIPW normative corundum contents of 2.86–3.88 wt%. They have fractionated REE patterns ((La/Yb)N = 5.64–7.34) and notable negative Eu anomalies (Eu/Eu* = 0.35–0.49), with enrichment of Rb, Th, U, Ta and Pb and depletion of Ba, Nb, Sr and Ti. The two-mica granites can be assigned as the highly fractionated S-type granites. They have negative whole-rock εNd(t) (−13.2 to −12.3) and zircon εHf(t) (−11.5 to −7.4) values with corresponding two-stage Nd and Hf model ages ranging from 1.99 to 2.06 Ga and from 1.72 to 1.98 Ga, respectively, indicating a Paleoproterozoic crustal source. The combined geochemical and isotopic data reveal that these rocks were derived by fluid-absent muscovite melting of metapelite at the middle-upper crustal depths and modified by fractional crystallization. In combination of the results in this study with regional geological data, it can be concluded that the two-mica granites were more likely formed during the post-collisional stage of the Sibumasu-Indochina collision, induced by slab break-off of the Paleo-Tethys Ocean. Based on the integrated consideration of a set of factors, including temporal and spatial proximity, source lithology and source enrichment, redox state, and fractional crystallization coupled with melt-fluid interaction, it is suggested that the ca. 216 Ma two-mica granites exhibit high Sn mineralization potential and could be helpful for exploring the primary Sn deposit of Late Triassic age nearby.