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.