Genesis Of Hi-Al Chromitite Of The Sartohay Ophiolite,Xinjiang

More recently, diamonds and a range of minerals from deep mantle have been increasingly discovered from chromitites in Luobusa, Tibet, and Polar Urals, Russia, which is challenging the shallow genesis of chromitite formation. Why do these minerals preserve in chromitite? What is the mechanism of formation? Are these minerals ubiquitous in ophiolitic chromitite and mantle peridotite of different type and different tectonic environment all over the world? What is the scientific implication? The new discovery makes the mechanism of ophiolitic chromitites formation, especially, the origin of these deeply-derived minerals and deep mantle interaction to be a extremely challengeable and totally new frontier scientific problem for plate tectonic.The Sartohay chromitite and the hosted Darbute ophiolite in Xinjiang are chosen as the study area of this thesis. Sartohay chromitite is a type of high-Al chromitite occurring in the late Paleozoic ophiolite, which is different from the diamonds-bearing Luobusa high-Cr chromitite in Tibet and Polar Urals high-Cr chromitite in Russia, they occur in the early Paleozoic and Mesozoic ophiolite, respectively. The type of chromitite and the hosted orogenic belt of Sartohay chromitite differ from the chromitites in Tibet and Russia, consequently, it is pretty scientificly significant to search for whether diamonds and other minerals from deep mantle preserve in Sartohay chromitite and their genesis.The exact field geological investigation for Darbute ophiolite and Sartohay chromitite, comprehensive studies indoor and mineral separation have been carried out, including major, trace elements and PGEs analyses of ophiolite and chromitite, mineralogy of peridotite, in situ LA-ICP-MS trace elements analyses of minerals, microprobe analyses of inclusions of chromitite, heavy mineral separation of chromitie and analyses of the separated minerals’ composition, texture and inclusions, and LA-ICP-MS U-Pb age dating and Hf analysis of zircons from mafic rocks of ophiolite. Finally, the main progresses of the thesis are as follows:(1) The Darbute ophiolite is suggested to form in back arc spreading ridge and influenced by subducted fluid/melt. The mantle peridotites consist mainly of harzburgite and minor dunite and Iherzolite. The Mg#of spinels in harzburgite range from54to62, and Cr#range from39-48, and the Fo of olivines in harzburgite range from90-93. The contents of I REE of harzburgite and dunite are lower than Cl chondrite, and display the enriched LREE and the U-type REE pattern. Σ REE contents of clinopyroxene in harzburgite are higher than SSZ perodotites and lower then abyssal peridotites, and the HFSE Ti vs Yb and Ti vs Dy plots indicate that the clinopyroxene is probably residue of hydrous melting of peroditite. Oxygen fugacities recorded by olivine-spinel of harzburgite equilibria are+0.96log units below and-0.14log units above the FMQ buffer, overlapping the range from MORB harzburgite to SSZ harzburgite, or from back arc basin peridotite to island arc peridotite. The major elements of clinopyroxene, orthopyroxene, olivine and spinel in peridotites also indicate the characteristics of both MOR and SSZ. The Cr#vs Mg#plot of spinel and the Cr#vs Fo plot of spinel and associated olivine show that the harzburgite is residue from about20%-25%partial melting of primitive peridotite. The mafic rocks in ophiolite display the both N-MORB and E-MORB geochemical affinity, are depleted in HFSE Nb, Ta, Zr, Hf, Ti and enriched in LILE U, K, Pb, Sr, which states clearly the added subducted component. Furthermore, the εHf of zircons in mafic rocks also demonstrate the involvement of crustally-derived materials.(2) It is suggested that the Darbute oceanic basin spread until at least early Carboniferous in terms of the U-Pb age dating of zircons of mafic rocks. The U-Pb dating of Muhatayi basalt yielded the age of (393.9±3.0) Ma, which is the age of the Darbute ophiolite formation. The U-Pb dating of Sartohay gabbro vein yielded the age of (359.4±1.0) Ma, The U-Pb dating of Darbute gabbro vein yielded the age of (342.0±2.7)) Ma, The U-Pb dating of Akebasitao gabbro vein yielded the age of (357.6±2.4)Ma, The U-Pb dating of Akebasitao pyroxenite vein yielded the age of (350.6±4.0) Ma. These veins originated from the partial melting of asthenosphere mantle and then intruded in the mantle peridotite during the extension of ocean, they reflect that the seafloor spreading at early Carboniferous.(3) More than20mineral species, such as ultra-high pressure, highly reduced and crust-derived minerals, were firstly discovered from847kg sartohay high-Al chromitite, including:native elements:Cr, diamond, Si, Fe; carbides:SiC; metallic alloys:FeCr, FeNiCr; Oxides:wilstite, rutile, ilmenite, hematite, chromite, magnetite, quartz; sulfides:Galena, sphalerite, millerite, heazlewoodite, arsenopyrite; silicates:forsterite, enstatite, diopside, serpentine, zircon, feldspar. These minerals can totally compare with the minerals from high-Cr chromitite from Luobusa,Tibet and Polar Ural, Russia, which would indicate that Sartohay high-Al chromitite may also have the metallogenic stage in deep mantle like the high-Cr chromitites.(4) A large number of anhydrous and hydrous in situ inclusions were discovered in the chromites from Sartohay chromite, including diopside, enstatite, forsterite, Na-phlogopite(aspidolite), Na-Cr pargasite, albite and so on. The rutiles separated from chromitites came from metamafic rocks, and the zircons from mineral separation contain crustally hydrous inclusions. All evidence imply that the metallogenic stage in shallow hydrous upper mantle. The contents of A12O3and TiO2and value of FeO/MgO of parental magma got by calculation are similar to MORB, which means the chromitite formation was related closely to the hydrous MORB-like melt.(5) The model that the metallogenic stages both in deep mantle and in shallow upper mantle for the genesis of Sartohay high-AI chromitite has been proposed. The strongly reduced fluid enriched in Si, Cr, C, Fe, Ni produced by the partial melting of subducted slab in mantle transition zone, meanwhile, the diamond, moissanite, native Si, Fe, Cr, wustite and FeNi alloy. These minerals then moves upward beneath oceanic spreading centers and are encapsulated in chromite near the top of the transition zone (<14GPa). At this stage, the chromite preliminarily enriches and upwells into the upper oceanic lithosphere mantle with the UHP and highly reduced minerals. The upward-migrating magmas result from slab breakoff pass through the slab and assimilate the subducted materials containing crustal rutiles and zircons. The magmas then go into the magma chamber beneath the back arc spreading ridge and lead to the melt/rock interaction happen on a large scale, and the former preliminarily enriched chromitite is redistributed, which facilitates Sartohay chromitite crystallization. Neither PGMs or many BMSs have been found as inclusions in chromite in that the parental MORB-like magma carried very little PGEs and was extremely S-poor. The BMSs such as millerite, heazlewoodite, and PGMs began to crystallize in the crack or interstitial matrix between chromite grains at the last stage of magmatic evolution.