Archean to paleoproterozoic crustal evolution in the Phan Si Pan zone, Northwest Vietnam: evidence from the U-Pb geochronology and Sr-Nd-Hf isotopic geochemistry

We studied zircon U-Pb geochronology, Sr-Nd-Hf isotopes, and whole-rock geochemistry of Mesoarchean-Paleoproterozoic igneous rocks in the Phan Si Pan zone, NW Vietnam. The Mesoarchean-Paleoproterozoic magmatism in the area can be divided into three phases as represented by ca. 2.9 Ga Mesoarchean tonalite-trondhjemite-granodiorites (TTGs), ca. 2.3 Ga Paleoproterozoic mafic dykes, and ca. 1.8 Ga Paleoproterozoic granites. The ca. 2.9 Ga Mesoarchean TTGs serve as part of the oldest basement rocks in SE Asia, which have zircon ε_Hf(t) and whole-rock ε_Nd(t) values ranging from −2.8 to +2.1. Their ancient zircon Hf and whole-rock Nd isotopic model ages (T_DM2) of 3.4 Ga to 3.2 Ga suggest an origin from partial melting of ancient crustal rocks. The ca. 2.3 Ga mafic dykes were affected by post-tectonic thermal events at ca. 1.9–1.8 Ga. Their zircon ε_Hf(t) (+1.4 to +5.9) and whole-rock ε_Nd(t) (−4.1 to −1.6) and ⁸⁷Sr/⁸⁶Sr ratio (0.703459 to 0.719079) isotopic characteristics show that they originated from partial melting of mantle components with a minor input of Archaean continental crust. The Paleoproterozoic granites in the Phan Si Pan zone crystallized at 1803–1864 Ma, and show negative ε_Hf(t) values of −17.9 to −14.8 and zircon Hf model ages (T_DM2) ranging from 3.48 to 3.66 Ga, suggesting a derivation by partial melting of Paleoarchean continental crust during Paleoproterozoic time. These isotopic results shed light on the Precambrian tectonic evolution of NW Vietnam and its relationship with continental blocks in Yangtze and India.

本研究针对越南西北部番西邦带的中太古代-古元古代火成岩开展了锆石U-Pb年代学、Sr-Nd-Hf同位素及全岩地球化学分析。该区域内的中太古代-古元古代岩浆作用可划分为三个阶段，分别以约2.9 Ga的中太古代英云闪长岩-奥长花岗岩-花岗闪长岩（tonalite-trondhjemite-granodiorites，TTGs）、约2.3 Ga的古元古代镁铁质岩脉，以及约1.8 Ga的古元古代花岗岩为代表。约2.9 Ga的中太古代TTGs是东南亚最古老的基底岩石之一，其锆石ε_Hf(t)与全岩ε_Nd(t)值范围为-2.8至+2.1。该类岩石的锆石Hf及全岩Nd同位素模式年龄（T_DM2）为3.4 Ga至3.2 Ga，指示其起源于古老地壳岩石的部分熔融作用。约2.3 Ga的镁铁质岩脉受到了约1.9~1.8 Ga的构造后热事件改造。其锆石ε_Hf(t)（+1.4至+5.9）、全岩ε_Nd(t)（-4.1至-1.6）以及⁸⁷Sr/⁸⁶Sr比值（0.703459至0.719079）的同位素特征表明，它们源自地幔组分的部分熔融，且伴随有少量太古代大陆地壳的物质加入。番西邦带内的古元古代花岗岩结晶年龄为1803~1864 Ma，其锆石ε_Hf(t)值为-17.9至-14.8，锆石Hf同位素模式年龄（T_DM2）介于3.48 Ga至3.66 Ga之间，指示其起源于古元古代时期古太古代大陆地壳的部分熔融。上述同位素研究结果为越南西北部前寒武纪构造演化及其与扬子、印度大陆块体的亲缘关系提供了关键约束。