Adakites in the Truong Son and Loei fold belts, Thailand and Laos: Genesis and implications for geodynamics and metallogeny

LA-ICP-MS U-Pb zircon dating reveals that the Phu Kham adakites in the Truong Son Belt were emplaced during the Late Carboniferous (ca. 306 to 304 Ma), whereas the Puthep 1 (PUT 1) adakites in the Loei Fold Belt were formed during the Middle Triassic (ca. 244 to 241 Ma). These rock formation ages are largely coeval to Re-Os molybdenite ages (Phu Kham: ca. 304 Ma; PUT 1: ca. 246 Ma), suggesting close temporal links between adakite formation and copper-gold mineralization. New geochemical results show that the Phu Kham and PUT 1 adakites are characterized by low HREE and Y contents, but elevated LREE, Sr, Sr/Y, and La/Yb values. In addition, the two adakites have relatively high ¹⁴³Nd/¹⁴⁴Nd ratios (0.512648 to 0.512719), Mg# (Phu Kham: 37-68; PUT 1: 40-65), and Cr and Ni contents. These results are best interpreted as representing slab melts that have interacted with supra-subduction zone mantle wedge during ascent. Tectonically, the Phu Kham and PUT 1 adakites were most likely formed during the initiation of subduction of the Ailaoshan-Song Ma and the Main Paleo-Tethys ocean plates respectively. Close temporal relations between the adakite formation and the copper-gold mineralization suggest strong genetic links between the two in the mainland SE Asia region.