摘要
The Zedong ophiolites in the eastern Yarlung-Zangbo suture zone of Tibet represent a mantle slice of more than 45 km2. This massif consists mainly of mantle peridotites, with lesser gabbros, diabases and volcanic rocks. The mantle peridotites are mostly harzburgite, lherzolite; a few dike-like bodies of dunite are also present. Mineral structures show that the peridotites experienced plastic deformation and partial melting. Olivine （Fos9.7-91.2）, orthopyroxene （Enss-92）, clinopyroxene （En4-49Wo47-slFS2-4） and spinel [Mg^#=lOOxMg/（Mg＋Fe）]=49.1-70.7; Cr^#=（100xCr/（Cr＋Al）=18.8-76.5] are the major minerals. The degree of partial melting of mantle peridotites is 10%-40%, indicating that the Zedong mantle peridotites may experience a multi-stage process. The peridotites are characterized by depleted major element compositions and low REE content （0.08-0.62 ppm）. Their ＂spoon-shaped＂ primitivemantle normalized REE patterns with （La/Sm）N being 0.50-6.00 indicate that the Zedong ultramafic rocks belong to depleted residual mantle rocks. The PGE content of Zedong peridotites （18.19-50.74 ppb） is similar with primary mantle with Pd/Ir being 0.54-0.60 and Pt/Pd being 1.09-1.66. The Zedong peridotites have variable, unradiogenic Os isotopic compositions with 187Os/18Os=0.1228 to 0.1282. A corollary to this interpretation is that the convecting upper mantle is heterogeneous in Os isotopes. All data of the Zedong peridotites suggest that they formed originally at a mid-ocean ridge （MOR） and were later modified in supra-subduction zone （SSZ） environment.
The Zedong ophiolites in the eastern Yarlung-Zangbo suture zone of Tibet represent a mantle slice of more than 45 km2. This massif consists mainly of mantle peridotites, with lesser gabbros, diabases and volcanic rocks. The mantle peridotites are mostly harzburgite, lherzolite; a few dike-like bodies of dunite are also present. Mineral structures show that the peridotites experienced plastic deformation and partial melting. Olivine （Fos9.7-91.2）, orthopyroxene （Enss-92）, clinopyroxene （En4-49Wo47-slFS2-4） and spinel [Mg^#=lOOxMg/（Mg＋Fe）]=49.1-70.7; Cr^#=（100xCr/（Cr＋Al）=18.8-76.5] are the major minerals. The degree of partial melting of mantle peridotites is 10%-40%, indicating that the Zedong mantle peridotites may experience a multi-stage process. The peridotites are characterized by depleted major element compositions and low REE content （0.08-0.62 ppm）. Their ＂spoon-shaped＂ primitivemantle normalized REE patterns with （La/Sm）N being 0.50-6.00 indicate that the Zedong ultramafic rocks belong to depleted residual mantle rocks. The PGE content of Zedong peridotites （18.19-50.74 ppb） is similar with primary mantle with Pd/Ir being 0.54-0.60 and Pt/Pd being 1.09-1.66. The Zedong peridotites have variable, unradiogenic Os isotopic compositions with 187Os/18Os=0.1228 to 0.1282. A corollary to this interpretation is that the convecting upper mantle is heterogeneous in Os isotopes. All data of the Zedong peridotites suggest that they formed originally at a mid-ocean ridge （MOR） and were later modified in supra-subduction zone （SSZ） environment.
基金
supported by the Project IGCP–649
the China Geological Survey(No.DD20160023–01)
