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

摘要

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 Sr-87/Sr-86 ratios (0.71155-0.71261) and low epsilon(Nd) (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 wet), 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 [epsilon(Nd)(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 (036-0.84 ppm), coupled with low Sr (initial Sr-87/Sr-86 0.70523-0.70567) and high Nd isotopes [epsilon(Nd)(gt) = +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. (C) 2020 Elsevier B.V. All rights reserved.