Late paleozoic to early mesozoic paleo-tethys tectonic evolution of central NE Tibetan Plateau: Insights from the Zhiduo mafic-ultramafic complex

Abstract Recent research has identified abundant Early Permian to Late Triassic ultramafic-mafic rocks along the west Jinsha to Garze-Litang suture, central NE Tibetan Plateau. However, the petrogenesis and tectonic setting of these rocks are hotly debated. This paper presents a combination of zircon U Pb ages, major and trace element concentrations, and Sr Nd isotopic data for mafic rocks from the Zhiduo Complex in the west Jinsha suture. Three geochemical subgroups are recognized among these rocks; metagabbros showing arc-type affinity (Group 1; 286–268 Ma); isotropic gabbros with an oceanic-island basalt-like signature (Group 2; 252 ± 2 Ma); and pillow/massive basalts (Group 3; ca. 235 Ma) having back-arc basin basalt affinity. Group 1 rocks exhibit moderate-high MgO (5.98–8.07 wt%) and Al2O3 (13.75–17.40 wt%) contents and are characterized by subduction-type trace element patterns and high initial 87Sr/86Sr ratios (0.71155–0.71261) and low eNd(t) values (−7.0 to −6.0), indicating that they originated from a spinel lithospheric mantle metasomatized by slab-derived fluids. Group 2 rocks have high contents of TiO2 (2.28–4.96 wt%), total Fe2O3 (11.67–14.80 wt%), Nb (18.3–22.8 ppm), and Ta (1.52–1.81 ppm) and more radiogenic Nd isotopes [eNd(t) = +0.3 to +0.9]. These data show that they were derived from a spinel + garnet mantle source enriched by plume-type components. Group 3 rocks possess high MgO (7.29–9.29 wt%), Cr (427–488 ppm), and Ni (215–238 ppm) contents but low Nb (5.00–5.60 ppm) and Ta (0.36–0.84 ppm), coupled with low Sr (initial 87Sr/86Sr = 0.70523–0.70567) and high Nd isotopes [eNd(t) = +3.1 to +3.5]. These values suggest that they were sourced from a garnet + spinel mantle affected by slight plume contamination. Combined with previous structural and geochemical studies, the ultramafic-mafic rocks of the Zhiduo Complex and arc magmatic rocks from nearby regions indicate an integrated Early Permian to Late Triassic oceanic basin–arc–back-arc system developed in the central NE Tibetan Plateau. Thus, the west Jinsha to Garze-Litang represents a mature Paleo-Tethys Ocean rather than small back-arc basins. Southward subduction and associated back-arc spreading led to the genesis of widespread late Paleozoic to early Mesozoic magmatic rocks in the hinterland and along the northern margin of the North Qiangtang Block.