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Recognition of MOR- and SSZ-type ophiolites in the Bangong Lake ophiolite mélange, western Tibet: evidence from two kinds of mantle peridotites

SHI Ren-deng~1,2, YANG Jing-sui~1, XU Zhi-qin~1 and QI Xue-xiang~1(1. Institute of Geology, CAGS, Beijing 100037, China; 2. State Key Laboratory of Mineral Deposit Research, Nanjing University, Nanjing 210093, China)  
Two kinds of mantle peridotites are recognized in Rutog at the westernmost part of the Bangong Lake - Nujiang ophiolite mélange in Tibet. One is a brecciform harzburgite, composed of about 80% olivine with Fo 90.76~91.84, 91.09 on average, 15% orthopyroxene with Mg~# 90.97~91.41, 91.16 on average, 2% clinopyroxene with Mg~# 93.24~94.60, 93.96 on average, and 3% magnetite and light brown chromium spinel with Cr~# 0.20~0.25, 0.23 on average (lower than 0.60), and characterized by low MgO (41.41%~42.02%), high Al_2O_3 (1.63%~1.94%), CaO (1.34%~1.60 %) and Ti (133.04~134.52 μg/g), and depletion of REE with ΣREE being 0.17~0.22 times that of chondrite. Major and trace element characteristics indicate that the brecciform harzburgite is restite resulting from 10%~15% partial melting of primitive mantle. The other type is massive harzburgite without clinopyroxene, composed of about 85% olivine, 13% orthopyroxene, and 2% magnetite and red-brown chromium spinel with Cr~# 0.69~0.74, 0.71 on average (higher than 0.60). In contrast to the brecciform harzburgite, the massive harzburgite is characterized by high MgO (42.96%~44.69%), low Al_2O_3 (0.23%~0.61%), CaO (0.08%~0.11%) and Ti (68.55~68.82μg/g) ,and considerable depletion of REE, withΣREE being 0 .03 ~0 .05 ti mes that of chondrite . Major and trace element charac-teristics suggest that the massive harzburgiteis restite formed by 30 %~40 %partial melting of the pri mitive mantle .It is preli minar-ily heldthat the brecciformharzburgiteis restite of ancient oceaniclithosphere emplacement inthe orogeny zone ,andthat the massiveharzburgiteis restite formed by another partial melting of residual peridotites during the subduction of ancient oceanic lithosphere .They are typical mantle peridotites of MOR- and SSZ-type ophiolite ,respectively ,and consistent withthe two types of lavasin Ban-gong Lake ophiolite mélange ,i .e .,P-MORB and Boninite .
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