Geodynamic Evolution of the Proto-Tethys Ocean in the West Kunlun Orogenic Belt, NW Tibet: Implications from the Sub-Arc Crust and Lithospheric Mantle Modification

Abstract

Radiogenic isotopes serve as a crucial tool for investigating crustal evolution, playing a pivotal role in revealing magma sources and petrogenesis. However, they can be ineffective in distinguishing distinct magmatic sources with similar radiogenic isotopic compositions, a common occurrence in nature. This paper addresses this challenge in the Ordovician igneous rocks from the West Kunlun orogenic belt (WKOB), to distinguish between two potential magmatic sources (i.e., the Tarim Craton and the Tianshuihai terrane) with similar isotopic compositions, by using appropriate thermodynamic and geochemical modeling based on mineral and whole-rock geochemistry. Zircon U–Pb dating yields ages of 483 ± 3 Ma, 469 ± 2 Ma and 461 ± 2 Ma for the Pushou gabbros and the Datong monzogranites and syenites, respectively. The Pushou gabbros exhibit low SiO2 (47.4–49.1 wt.%), high MgO (5.5–6.9 wt.%), high large ion lithophile elements (LILEs, e.g., Rb, Ba, Th, and K) and low high field-strength elements (HFSEs, e.g., Nb, Ta, Zr, Hf, P, and Ti), which suggest an origin in subduction-modified mantle. They display high whole-rock (87Sr/86Sr)i ratios (0.7156 to 0.7192), negative whole-rock εNd(t) values (−7.1 to −7.8), as well as high zircon δ18O values (7.6–7.9 ‰) and enriched zircon Hf isotopic compositions (εHf(t) = −5.3 to −7.7), consistent with 1–5% subducted sediments in an enriched mantle source. Trace element models further confirm that the gabbros are most likely derived from low-degree (~15 %) partial melting of subduction-modified Tarim mantle in the spinel-garnet facies, rather than from the Tianshuihai mantle. The Datong syenite samples belong to the shoshonitic series and are characterized by medium SiO2 (59.5–61.4 wt.%), relatively low MgO (0.9–1.2 wt.%) and Mg# (37–42), enrichment in LILEs and depletion in HFSEs. They have high whole-rock (87Sr/86Sr)i ratios (0.7103 to 0.7105) and negative whole-rock εNd(t) values (−3.8 to −4.3), along with negative to slightly positive zircon εHf(t) values (–3.8 to +2.6), similar to coeval mafic rocks. Thermodynamic and geochemical modeling suggests that the Datong shoshonitic rocks likely originated via crystal fractionation of shoshonitic basaltic magmas in the SW Tarim Craton. The Datong monzogranites have high SiO2 (69.7–72.6 wt.%), low MgO (0.6–0.7 wt.%) and demonstrate a typical enrichment in alkalis, Zr, and Nb with depletion in Sr, P, and Ti, consistent with A-type granites. They are characterized by high whole-rock (87Sr/86Sr)i ratios (0.7321 to 0.7323), negative whole-rock εNd(t) (−11.3 to −11.8), negative zircon εHf(t) (−11.0 to −16.5) and high zircon δ18O (7.2–8.0 ‰), indicating derivation from the remelting of an ancient crustal source. Thermodynamic, major, and trace element modeling indicate that their parent magma may have been generated by water-deficient (~2 wt.%) partial melting of ancient crustal material beneath the SW Tarim Craton rather than that of the Tianshuihai terrane, under high-temperature (T > ~950 °C), low-pressure (P = 5–8 kbar) conditions. Based on the tectonic framework of the WKOB, we propose that the original mantle and crust beneath the southern Kunlun terrane may have been modified or partially replaced by that beneath the SW Tarim Craton during the Ordovician. Therefore, this evidence for Tarim-derived magmatism, when combined with regional sedimentary and structural records, indicates that Ordovician magmatism in the southern Kunlun terrane is most consistent with episodic northward subduction of the Proto-Tethys Ocean, commencing at ~485 Ma. Middle Ordovician slab break-off can explain the formation of the A-type granites, but re-instated northward subduction is required for the generation of late Ordovician Datong syenites.