Abstract The Truong Son and Kon Tum terranes in northeastern Indochina are thought to, respectively, form part of the Indian and Australian margins of Gondwana and separated one another by the Tam Ky–Phuoc Son Ocean during the early Paleozoic. In this paper, we present results of a comprehensive geochronological and geochemical study on a trondhjemite–tonalite suite identified in the Tam Ky–Phuoc Son Suture Zone. LA–ICP–MS zircon U–Pb dating yielded weighted mean 206Pb/238U ages of 518.5 ± 7.1 Ma and 502.1 ± 6.0 Ma for the trondhjemite and tonalite, respectively. These are the oldest magmatic zircons recorded in this area so far. Both rocks consist primarily of plagioclase (oligoclase), quartz, with minor, variable amounts of hornblende and biotite. They have high SiO2, Na2O and Y, but low Al2O3, K2O, Sr and Rb contents, and are characterized by depletion in LREEs with flat HREE patterns. These features are similar to those of typical oceanic plagiogranites. Our new evidences from field geology, geochemistry, and zircon Hf isotopic data, along with previously published data from the Tam Ky–Phuoc Son Suture Zone, suggest that the plagiogranites were originated from extensive fractionation of hydrous basaltic magmas. Furthermore, they were generated in an intra-oceanic arc system above a northward subduction zone within the Tam Ky–Phuoc Son Ocean. The final closure of this ocean took place before the late Silurian, marked by collision of the Truong Son Terrane with the Kon Tum Terrane along the Tam Ky–Phuoc Son Suture Zone to form the Indochina Block. After elimination of the southeastward offset along the Red River shear zone and the clockwise rotation of Indochina relative to South China since the Tertiary, the eastern part of the Tam Ky–Phuoc Son Suture aligns well with the Kuungan Suture on Hainan Island, together they have likely recorded the final assembly of Gondwana in this region.

中文翻译：

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越南中部 Tam Ky-Phuoc Son 缝合带的寒武纪洋内弧长长闪长岩和闪长岩：对印度支那地块早期古生代组合的影响

摘要 印度支那东北部的 Truong Son 和 Kon Tum 地体被认为分别构成冈瓦纳大陆印度和澳大利亚边缘的一部分，并在早古生代被 Tam Ky-Phuoc Son 洋隔开。在本文中，我们展示了对在 Tam Ky-Phuoc Son 缝合带中确定的长闪长岩-辉长岩组的综合地质年代学和地球化学研究结果。LA-ICP-MS 锆石 U-Pb 测年的加权平均 206Pb/238U 年龄分别为 518.5 ± 7.1 Ma 和 502.1 ± 6.0 Ma 的长闪长岩和方长岩。这些是迄今为止在该地区记录的最古老的岩浆锆石。这两种岩石主要由斜长石（寡长石）、石英组成，还有少量的、数量不等的角闪石和黑云母。它们具有高 SiO2、Na2O 和 Y，但 Al2O3、K2O、Sr 和 Rb 含量低，并且以具有平坦 HREE 模式的 LREE 耗尽为特征。这些特征与典型的海洋斜长花岗岩相似。我们来自野外地质学、地球化学和锆石 Hf 同位素数据的新证据，以及先前发表的来自 Tam Ky-Phuoc Son 缝合带的数据，表明斜花岗岩起源于含水玄武质岩浆的广泛分馏。此外，它们是在 Tam Ky-Phuoc Son 海洋内向北俯冲带上方的洋内弧系统中生成的。这片海洋的最终闭合发生在志留纪晚期之前，其标志是长山地体与 Kon Tum 地体沿 Tam Ky-Phuoc Son 缝合带碰撞形成印度支那地块。