Petrological Characteristics Of The Dalabute Peridotite In The Western Junggar,Xingjiang,and Its Tectonic Significance

As one of the most typical and largest ophiolite belts in Xinjiang,the Dalabute ophiolite is considered to be the remnant of the paleo-asian ocean.The basite of the upper units in ophiolite are used to explore a series of key scientific problems such as its formation times and tectonic background,but the study of mantle peridotite in ophiolite is relatively weak and its genesis is controversial.In this study,some fresher mantle peridotite was found in this ophiolite belt,which can provide a lot of useful information for the restoration of mantle properties and tectonic environment in this area and help to solve the major basic geological problems such as tectonic evolution history in Western Junggar area of Xinjiang.In this thesis,the mantle peridotite of Sartohy and Suluchok from Dalabute ophiolite is selected as the research objects in the West Junggar.A detail field geological survey is conducted,using petrology,petrogeochemical and minerageochemical evidence of the mantle peridotite to explore its petrogenesis and tectonic setting.The mantle peridotite is mainly harzburgite from Sartohy ophiolite,band-shaped and lenticular dunite from Suluchok ophiolite.The olivine Fo 90.92?91.40 in harzburgite,olivine Fo 89.63?95.91 in band-shaped dunite,olivine Fo 90.34?92.11 in lenticular dunite,all of which are forsterite.The clinopyo-xene Mg^#93.05?96.85,Wo 42.89?50.84,En 46.77?53.15,Fs 1.65?3.97,belongs to diopside.The orthopyroxene Mg^#89.81?91.69,En 80.09?89.76,Fs 7.68?9.98,Wo2.10?12.23,belongs to enstatite.The spinel Cr^#value is 48.64?50.40,Al₂O₃content ranges from 28.12%to 28.86%,which is a high alumina spinel from Sartohy harzburgite.The spinel Cr^#value of lenticular dunite ranges from 92.17 to 97.29,and the spinel Cr^#value in band-shaped dunite varies between 89.90 and 94.73 from Suluchok ophiolite,all of which are high chromium type spinel.The trace elements of the clinopyroxene obtained from the In-situ LA-ICP-MS Analysis states that the clinopyroxene are depleted in light rare earth elements?LREE?and enriched heavy rare earth elements?HREE?,the content and distribution pattern of HREE is similar to Izu-Bonin-Mariana fore-arc harzburgite.The trace element characteristic shows that the large ion lithophile elements Rb,U,Pb and Sr are enriched significantly,the high field strength elements Zr,Nb are negatively anomalous,which is similar to the feature of clinopyroxene from Mariana fore-arc harzburgite.The trace elements of the spinel obtained from the In-situ LA-ICP-MS Analysis states that the spinel has affinity with boninite from dunite,while spinel of harzburgite is related to mid-ocean-rige basalt?MORB?.The lenticular dunite has Mg O contents that range from 43.71%to 45.11%,Al₂O₃contents range from 0.43%to 0.72%,Ca O contents range from 0.07%to 0.17%.The band-shaped dunite Mg O contents range from 43.05%to 44.39%,Al₂O₃content ranges from 0.89%to1.25%,Ca O contents ranges from 0.07%to 0.6%.The harzburgite Mg O contents range from 43.71%to 45.11%,Al₂O₃content ranges from0.43%to 0.72%,Ca O contents ranges from 0.07%to 0.17%.The harzburgite Mg O contents ranges from 43.05%to 44.39%,Al₂O₃contents range from 0.79%to 1.05%,Ca O contents range from 0.88%to 1.37%.Compared with the depleted mantle,the Dalabute mantle peridotite has higher Mg O and lower Al₂O₃,Ca O.The total rare earth element contents of the harzburgite,lenticular dunite and band-shaped dunite are are 0.25ppm?0.47ppm,0.24ppm?0.68ppm and 1.25ppm?2.40ppm.The chondrite normalized REE distribution patterns of the Dalabute mantle peridotite are a flat pattern,which is obviously different from the typical depleted mantle displaying strongly depleted LREE,suggesting that it experienced the reaction of fluid with the rocks.Trace elements are obviously enriched in compatible elements Cr?Co?Ni,characterized by residual mantle.Mineral geochemical and whole rock geochemical characteristics indicate that the Dalabute mantle peridotite has the characteristics of abyssal mantle peridotite and fore-arc mantle peridotite.According to those diagrm Ti-Dy and Y_N-Yb _Nin clinopyroxene,Cr^#in spinel-Al₂O₃in clinopyroxene,Fo in olivine-Cr^#in spinel,Cr^#-Mg^#in spinel,the degree of partial melting from Dalabut mantle peridotite have been estimated,the results show that the partial melting degree of the Saltohy harzburgite is22%?28%,and the partial melting degree of the Suluchok dunite is more than 40%.Based on the petrological,mineral geochemical and whole rock geochemical characteristics of the Dalabute mantle peridotite,this article considers that the harzburgite from Saltohy is formed by the depleted mantle rock with reaction about MORB melt and the melt/fluid from subduction zone.The dunite from Suluchok is formed by melting/fluid reaction of depleted mantle rock with boninite,island arc basalt and the melt/fluid from subduction zone.The mantle peridotite from Dalabute ophilite is formed in the initial stage of fore-arc subduction,and the Saltohy harzburgite and Suluchok dunite is the product of different periods of this stage.Combining with the formation age and regional geological background,it is considered that the Dalabute mantle peridotite is a remnant of the paleozoic oceanic lithosphere of western Junggar formed in the fore-arc environment,which indicates that the subduction of the Western Junggar oceanic basin has begun in the Middle Devonian.