Petrogenesis of anorthosite in the Laguoco ophiolite western part of the Bangong-Nujiang suture zone and its constraint to the evolution of the Meso-Tethys Ocean

The Laguoco ophiolite is distributed in the Gaize area, north Tibetan plateau, and is one of the most complete and well-preserved ophiolites in the Shiquanhe-Yongzhu belt. The petrological, zircon U-Pb geochronology and geochemistry of anorthosite intruding gabbros are studied to manifest the genesis and tectonic affinities of the Laguoco ophiolite. The anorthosite was emplaced within gabbros mainly as irregular dikes or lenses, with zircon U-Pb ages of 162±1 Ma, slightly later than mafic rocks of the ophiolite. All zircons yield remarkable positive ε_Hf(t) values (+15.8~+19.7), indicating that the magma was derived from a long-term depleted mantle source. All anorthosites are characterized by low SiO₂, TiO₂ and high CaO contents, MORB-like REE patterns with low trace element concentrations, enrichment of Rb, Ba, Th and Sr, and depletion of Nb, Ta and Zr. Moreover, these samples show high Nb/La but low Th/Nb and Ba/Nb ratios, which suggests their magma was affected by enriched mantle with variable inputting of subduction materials. The comprehensive analysis suggests that the Laguoco anorthosite was derived from high degree partial melting of depleted mantle source with re-enrichment of plume. The primary magma deriving from (enriched) mantle wedge rapidly ascended into oceanic crust in company with the crystallization differentiation of Mg-Fe oxides. Then, residual Ca, Al-bearing magma injected into the gabbros and formed anorthosite with the decrease of pressure. Combining with regional geology, it is suggested that the Laguoco ophiolite was formed in a subduction-related setting during the Jurassic.