Crustal evolution of Paleozoic-Mesozoic granitoid in Dakrong-A Luoi area, Truong Son belt, central Vietnam: evidence from zircon U-Pb geochronology, geochemistry, and Hf isotope composition

The Dakrong-A Luoi area is located in the South Truong Son belt in the Indochina block and has been affected by multiple stages of magmatic-metamorphic events. In this paper, Paleozoic-Mesozoic granitoids from the Dakrong-A Luoi area were studied using LA ICP-MS U-Pb zircon geochronologies, Hf isotopes on zircon and whole-rock geochemistry of the Paleozoic to examine the tectonic-magmatic history of the Southern Truong Son belt. The magmatism history of the area can be divided into three phases, at ~ 452 Ma (Ordovician), ~259 Ma (Permian), and ~ 243 Ma (Triassic). The Ordovician granites were exposed in the narrow belt of NW-SE trending, while the later two granite phases were exposed in E-W trending. The Ordovician granites show consistent LA-ICP-MS zircon Ꜫ_Hf(t) values from + 6.2 to + 8.3. Their corresponding Hf model ages (T_DM2) from 0.9 Ga to 1.0 Ga suggest that the Ordovician granite was derived from the partial melting of ancient crustal rocks, which is related to the subduction of the Tam Ky-Phuoc Son (TKPS) ocean beneath the Truong Son belt. The Permian granites have Hf isotopic natures (Ꜫ_Hf(t)=-1.6 to 1.3, T_DM2 = 1.2–1.3 Ga) of partial melting from ancient mantle components with minor Paleoproterozoic ancient continental crust The Triassic granites have negative ɛ_Hf(t) values ranging from −11.9 to −7.0 and Hf model ages of 1.7–2.0 Ga suggesting that they were a product of partial melting from the Paleoproterozoic crust, corresponding to subduction/collision of the Paleo-Tethys ocean beneath the Indochina block along the Song Ma suture.

达容-阿洛伊（Dakrong-A Luoi）地区位于印支地块的南长山构造带，经历过多期岩浆-变质事件。本文采用激光剥蚀电感耦合等离子体质谱（LA-ICP-MS）锆石U-Pb年代学、锆石铪同位素分析及全岩地球化学方法，对达容-阿洛伊地区古生代-中生代花岗岩类开展研究，以揭示南长山构造带的构造-岩浆演化历史。该区域的岩浆活动可划分为三期，分别为~452 Ma（奥陶纪）、~259 Ma（二叠纪）与~243 Ma（三叠纪）。奥陶纪花岗岩呈北西-南东向狭窄带状展布，后两期花岗岩则呈东西向展布。奥陶纪花岗岩的LA-ICP-MS锆石ε_Hf(t)值稳定在+6.2至+8.3之间，对应的Hf二阶段模式年龄（T_DM2）为0.9 Ga至1.0 Ga，指示其起源于古老地壳岩石的部分熔融，该成因与南长山构造带之下的塔姆基-福松（Tam Ky-Phuoc Son，TKPS）洋俯冲作用相关。二叠纪花岗岩的铪同位素特征为ε_Hf(t)=-1.6至1.3，T_DM2=1.2–1.3 Ga，指示其源自古老地幔组分与少量古元古代古老陆壳的部分熔融。三叠纪花岗岩的ε_Hf(t)值介于-11.9至-7.0之间，Hf模式年龄为1.7–2.0 Ga，表明其起源于古元古代地壳的部分熔融，对应古特提斯洋沿松马缝合带在印支地块之下的俯冲/碰撞作用。