Amdo Neoproterozoic orthogneisses: a review of geochemical and isotopic characteristics, petrogenesis, and reconstruction of Rodinia

Tectonic configuration of the Rodinia Supercontinent and palaeogeographic framework of terranes incorporated into the Tibetan Plateau remain enigmatic due to the paucity of Neoproterozoic crystalline outcrops. The Amdo Terrane, emplaced in the central Tibetan Plateau, plays a key role in regulating the Precambrian geodynamics of the Himalayan–Tibetan Orogen. Recent investigations regarding Amdo Neoproterozoic orthogneisses have elucidated their petrogenesis and tectonic implications; however, their wide range of crystallization ages (ca. 910–767 Ma) and differentiated geochemical characters (e.g. varied SiO₂ contents of 45.83–78.92 wt. %, etc.) leave the Precambrian origin and tectono-thermal evolution of the Amdo Terrane controversial. In this contribution, we present comprehensive geochemical and geochronological datasets concerning reported Amdo Neoproterozoic orthogneisses, accompanied by comparative analyses and interpretive discussions. These Amdo orthogneisses are predominantly granitic gneisses; the majority of these granitoids (Gg-1, ca. 893–767 Ma) exhibit high-K calc–alkaline compositions and heterogenous zircon εHf_(t) values (−9.3 to + 7.0), indicating derivation from multiple magma sources. Their negative Nb, Ta, and Ti anomalies, coupled with volcanic-arc (VAG) or syn-collision granites (syn-COLG) affinities suggest Gg-1 belong to an Andean-type arc setting. The other group of granitic gneisses (Gg-2, ca. 820, 802, and 801 Ma) display geochemical signatures typical of A2-type granites, with elevated concentrations of high-field strength elements (HFSEs, Zr+Nb+Ce+Y > 330 ppm) and high Y/Nb, Rb/Nb, Ce/Nb, Sc/Nb, and Yb/Ta ratios. Integrated with the coeval Amdo arc-related granitoids and back-arc igneous rocks (initiated at ca. 839 Ma) in the Present-day North Lhasa Terrane (PNL), we perceive that Amdo Gg-2 gneisses may represent products of a back-arc extensional environment driven by upward counterflow of asthenosphere. Moreover, subordinate intermediate orthogneisses may be granitic variants after moderate mantle contributions; likewise, they could also be classified into arc (Gi-1, ca. 807 and 767 Ma) and back-arc (Gi-2, ca. 842 and 824 Ma) groups based on their distinct geochemical features and tectonic conditions. Additionally, the rarely reported basaltic gneisses (gneissic amphibolites, ca. 863 Ma) are overprint by E-MORB traits (relatively high Nb/Yb, Zr/Y ratios and slightly right-inclined rare earth element patterns), possibly defining evolved remnants of the reduced Mozambique oceanic crust. We tentatively interpret the Amdo Terrane as the western segment of Original North Lhasa Block (ONL, comprising the PNL and Amdo Terrane). After rift-related magmatism (separation from the Rodinia) triggered by a mantle plume (ca. 925–900 Ma), the ONL initiated westward drifting and underwent protracted oceanic subduction by Mozambique; thus, generating extensive volcanic-arc granitoids in the western domain of ONL, especially of the Amdo part (ca. 893–857 Ma). Subsequent rollback of the deeply subducted oceanic slab (possibly started at ca. 850 Ma) induced asthenospheric upwelling and back-arc extension, giving birth to the high-temperature A2-type granitoids (ca. 842–801 Ma). Furthermore, the westward transporting history of Amdo Terrane could be further extended considering the younger Andean-type magmatic events (ca. 838–767 Ma). In addition, the E-MORB-like amphibolite gneisses (ca. 863 Ma) entrained within the Amdo Terrane might fingerprint allochthonous intra-oceanic fragments after within-plate enrichment which were scraped onto the western active continental margin of ONL due to the subduction of the Mozambique Ocean.

由于新元古代结晶露头的匮乏，罗迪尼亚超大陆（Rodinia Supercontinent）的构造格架以及纳入青藏高原的地体的古地理格架至今仍存在诸多未解之谜。安多地体（Amdo Terrane）位于青藏高原中部，对于约束喜马拉雅-青藏高原造山带的前寒武纪地球动力学具有关键意义。近期针对安多新元古代正片麻岩的研究阐明了其岩石成因与构造指示意义，但其广泛的结晶年龄范围（约910~767 Ma）以及分异的地球化学特征（如二氧化硅（SiO₂）含量变化为45.83~78.92 wt.%等）使得安多地体的前寒武纪起源与构造热演化仍存在争议。本研究针对已报道的安多新元古代正片麻岩，构建了综合的地球化学与年代学数据集，并辅以对比分析与解释性讨论。这些安多正片麻岩以花岗质片麻岩为主；其中绝大多数花岗岩类（Gg-1，约893~767 Ma）表现出高钾钙碱性的成分特征，且锆石εHf(t)值（-9.3至+7.0）具有非均质性，指示其起源于多套岩浆源区。它们具有负的Nb、Ta、Ti异常，结合火山弧（VAG）或同碰撞花岗岩（syn-COLG）的亲缘性，表明Gg-1形成于安第斯型弧环境。另一组花岗质片麻岩（Gg-2，约820、802和801 Ma）具有典型的A2型花岗岩地球化学特征：高场强元素（HFSEs，Zr+Nb+Ce+Y > 330 ppm）浓度升高，且Y/Nb、Rb/Nb、Ce/Nb、Sc/Nb以及Yb/Ta比值较高。结合现今北拉萨地体（PNL）中同期的安多弧相关花岗岩类与弧后火成岩（起始于约839 Ma），我们认为安多Gg-2片麻岩可能代表了由软流圈向上逆流驱动的弧后伸展环境的产物。此外，少量中性正片麻岩可能是经历了中等程度幔源贡献的花岗岩变种；同样，根据其独特的地球化学特征与构造环境，可将其划分为弧类（Gi-1，约807和767 Ma）与弧后类（Gi-2，约842和824 Ma）两组。另外，鲜有报道的玄武质片麻岩（片麻状角闪岩，约863 Ma）叠加了富集型洋中脊玄武岩（E-MORB）的特征（相对高的Nb/Yb、Zr/Y比值以及略呈右倾的稀土元素配分模式），可能代表了演化后的亏损莫桑比克洋壳残余。我们初步将安多地体解译为原始北拉萨地块（ONL，包含现今北拉萨地体与安多地体）的西部段。在由地幔柱（约925~900 Ma）触发的裂谷相关岩浆作用（与罗迪尼亚超大陆分离）之后，原始北拉萨地块开始向西漂移，并经历了由莫桑比克洋引发的长期洋壳俯冲，从而在原始北拉萨地块的西部区域，尤其是安多地区（约893~857 Ma）形成了广泛的火山弧花岗岩类。随后，深俯冲洋壳的板片回转（可能始于约850 Ma）引发了软流圈上涌与弧后伸展，形成了高温A2型花岗岩类（约842~801 Ma）。此外，考虑到更年轻的安第斯型岩浆事件（约838~767 Ma），安多地体的西向运移历史可进一步延伸。再者，赋存于安多地体中的E-MORB型角闪片麻岩（约863 Ma）可能记录了板内富集后的异地体洋内碎片，这些碎片因莫桑比克洋的俯冲而被刮擦至原始北拉萨地块的西部活动大陆边缘。