The Mineralogical And Geochemical Features Of Zedang Pyroxenites In Tibet

The Zedang mafic-ultramafic body crops out in the eastern Yarlung-Zangbo suture zone of Tibet.This massif consists of mantle peridotite,pyroxenite,gabbro,diabase and volcanic rocks.Two types of pyroxenites are present in Zedang area.The first pyroxenite crops out in the harzburgites as a dike body and the second one is located in the south of Zedang massif.The cpx from the pyroxenites dike has Wo=0.6-1.8,En=83.5-88.0,Fs=10.9-15.1,opx has Wo=45.3-48.2,En=43.0-47.9,Fs=5.6-9.5 and the Fo values of ol is ranging from 84.5 to 85.5.The cpx from the clinopyroxenites has Wo=45.3-48.2,En=43.0-47.9,Fs=5.6-9.5,Fo of ol changes from 84.5 to 85.5.The Cr#of sp from the clinopyroxenites varies from 29.8 to 31.2.These two pyroxenites have different Geochemical characteristics.The pyroxenites dike has higer MgO(20.08-23.23 wt%),SiO2(51.82-53.08 wt%)and ?PGE(3.42-322.06 ppb),and lower Al2O3(1.59-2.28 wt%),?REE(1.63-3.61ppm),whereas theclinopyroxeniteshasMgO(16.68-19.78wt%),SiO2(45.44-49.61wt%),?PGE(14.46-30.16 ppb),Al2O3(4.24-8.77 wt%)and?REE(5.82-7.44 ppm).Two types of pyroxenites have similar REE pattern,both are characteristic of LREE-depleted,indicating that the source of these two pyroxenites is depleted mantle.The mineralogy and geochemistry features of Zedang two types pyroxenites indicate that these pyroxenites are products of of depleted mantle-sourced magma.The body-like pyroxenite located in the harzburgites and react with the depleted harzburgites,leading the more depleted features.The two types of pyroxenite are formed in the MOR tectonic setting of the early stage of rock mass,but did not record the late subduction zone(SSZ)reconstruction process.