Mineralogical study of the hornblende gabbro in Zetang ophiolite, southern Tibet, and its genetic implications

Abstract: Zetang ophiolite is mainly composed of peridotite, gabbro and basalt. Hornblende gabbro, which was intruded into the peridotite, comprises amphibole, prehnite+pumpellyite and minor Fe-Ti oxides. The amphibole in hornblende gabbro is brown euhedral with homogeneous interference color and has higher Al2O3 (7.0%-11.0%), TiO2 (1.0%-2.5%), as well as Na2O (1.0%-1.9%) values than the amphibole of metasomatic genesis, implying that the amphibole is of primary magmatic origin. The Fe-Ti oxides consist of magnetite (TiO2=1.2%-13.2%; Fe2O3=40.6%-61.4%; FeO=31.1%-43.6%) and ilmenite (TiO2=36.2%-50.8%; Fe2O3=2.5%-24.6%; FeO=33.0%-43.7%). The ilmenites always exhibit trellis and sandwich textures or are closely spaced (composite type) in the magnetite host. The oxygen fugacities (expressed as -logfo2) of the Fe-Ti oxides calculated by ilmenite-magnetite geothermobarometry are 12.4-22.8 (approximately NNO+1), indicating that the presence of high water activities during crystallization. The positive εHf(t) values (+11.0 to +19.8) of the sample suggest that it came from depleted mantle (DM). The hornblende gabbro is characterized by high Al2O3 (20.4%-22.4%) and TiO2 (1.0%-1.5%), low MgO (4.1%-5.5%) values, similar to features of high-alumina basalt (HAB). The hornblende gabbro might have been formed by evolved mantle-derived hydrous magma rich in volatile, Ti and Al2O3. However, the sample shows depleted light rare earth element (LREE) patterns, which is different from features of HAB in the subduction zone. The rocks were probably formed by partial melting of mantle wedge induced by dehydration of serpentinized subducted oceanic lithosphere.