Ordo-silurian assemblage in the Indochina interior: geochronological, elemental, and Sr-Nd-Pb-Hf-O isotopic constraints of early Paleozoic granitoids in South Laos

In order to verify the early Paleozoic accretionary assemblage in the Indochina interior and constrain the Prototethyan tectonic evolution in Southeast Asia, this study presents a set of new U-Pb geochronological, elemental, and Sr-Nd-Pb-Hf-O isotopic data for the fifty-two representative granitoids in South Laos. The granitoids from the Kontum terrane, Tam Ky-Phuoc Son tectonic zone, and southern Truong Son igneous zone in South Laos yield the crystallization ages of 464-485 Ma, 455-471 Ma, and 427-446 Ma, respectively, with a northerly younging trend within the Indochina interior. They are mainly monzogranite with A/CNK = 0.96-1.99 and K2O > Na2O, which are marked by enrichment in largeion lithophile elements and depletion in high field strength elements with remarkable NbTa, Sr-P, and Ti negative anomalies. Their initial Sr-87/Sr-86 ratios range from 0.70510 to 0.71559, epsilon(Nd)(t) from -9.5 to -3.0(Pb-206/Pb-204) from 18.65 to 19.72, (Pb-207/Pb-204) from 15.66 to 15.80, and (Pb-208/Pb-204), from 38.84 to 39.79. The corresponding zircon epsilon(Hf)(t) and delta O-18 values are in the range of -10.6 to +1.0 and 6.88 parts per thousand to 8.94 parts per thousand, respectively. In addition, their Sr-Nd-Pb and Hf-O isotopic compositions are generally similar with those of time-equivalent granitoids in South Tibet and SW Yunnan, China, and synchronous mafic-intermediate igneous rocks in South Laos, but distinctive from those of the supracrustal sedimentary-derived South China Paleozoic granite and Lincang-Sukhothai S-type granite. The early Paleozoic granitoids in South Laos might have originated from a mixed source of the wedge-derived juvenile crust coupled with supracrustal materials. All these data synthetically suggest the southward subduction of the Tam Ky-Phuoc Son Ocean and the northerly on-growing Ordo-Silurian accretionary orogenesis within the previously defined "single-ancient" Indochina block. The assemblage of the Indochina block might initiate at ca. 430 Ma in the Silurian and terminate in the Early-Middle Devonian.