Study On The Genesis Of Ophiolites And Podiform Chromite Deposits Of The Western Zhungeer Area, Xinjiang

Study on the Genesis of Ophiolites and PodiformChromite Deposits of the Western Zhungeer Area, XinjingHao Ziguo(Chinese Acaemy of Geological Sciences)The author first diyides the ophiolites of the Western Zhungeer area into two types in this paper. The one is of the assemblage of metemorphic peridotite+ troctolite + gabbro( abbreviated to PTG lineage), the other is of the assemblage of metamorphic periaotite＋pyroxenite＋gabbro(abbreviated to PPG lineage), The former is represented by the Dalabute and Hebukesaier ophiolite belts; the latter is represented by the Tangbale and Mayileshan ophiolite belts. The metamorphic peridotite can be divided into two types of high AL(AL'＞1) and low AL(AL'＜1) on the basis of the aluminium saturation coefficient(AL' =AL/(2Ca＋Na＋K)). The PTG lineage corres- ponds to the high AL type, but the PPG lineage corresponds to the low AL type. The PTG lineage follows an AL-rich evolution trend, and the PPG lineage follows a Ca-rich evolution trend. Therefore, the two types of the ophiolite are very different in rock associa-tion, mineralogy, rock chemistry, RE,geochemistry, ore deposit type and the time of formation and napping.The author first points out that there are two t chromite deposit in the two ophiolite types of the Western Zhungeer area. The one is refrcatory type chromite deposit which occurs in PTG lineage ophiolite, the other is metallurgical type chromite deposit which occurs in PTG lineage optic!ire. The former is examplified by the Saertuohai, Jingyu and Hongguleleng blocks, the latter is exemplified by the Tangbale and Saleinuohai blocks. The chrome spinels(Cr-sp) of ore-forming and accessory mineral in the chromite deposit of refractory type are high AL and low Cr, and in the chromite deposit of metallurgical type are high Cr and low AL. The composition range of Cr-sp in the two types of chromite deposit are distictly different. The compositions of accessory Cr-sp exhibit a same evolution trend of Cr-rich and Al-poor fromlherzolite to dunite in the two types of chromite deposit. The compositions of ore-forming Cr-sp corresponds to the composition range of accessory Cr-sp of the dunite in the metallurgical type's chromite deposit, and these of dunite + harzburgite in the refractory type,s chromitedeposit. They are all residues of the primary pyrolite at itshigh degree's partial melting. So, they show a consistency in thediagenetic and metallogenetic process.The major mechanism of formation of the ophiolites and the podi-form chromite diposits in the Western Zhungeer area includes partialmelting of the primary pyrolite and differentiation of the melt-outmaterials( basaltic magmas) after they intruded. The differencesbetween the two ophiolite types are related to the degree of partialmelting of the pyrolite and depth of formation of magma chambers aswell as oxidation state of the mammas. Two type's chromite deposits in the ophiolites are also controlled by the composition of primary rock and late metasomatism in the mantle, in addition to the above mentioned factors. The degree of partial melting and the depth of formation of chromite deposit of metallurgical type are higher than that of chromite deposit of refractory type.Analysis of geotectonic conditions for the Western Zhungeer area shows that the ophiolites and chromite deposits formed in back-are spreading environment. The time of formation of the oceanic crust is about from the late Cambrian epoch(508+20 M.Y.) to the Middle Devo - ninon epoch. The sedimentory stage of the basin is from the Ordovic-ian period to the Middle Carboniferous epoch。The napping time of the crust is different. It is The Early Ordovician epoch in the Tangbale, and the Late Devonian epoch in the Mayileshan, as well as the Middle Carboniferous epoch in the Dalabute and Hebukesaier。Obxionsly, the forming and nappling time of the ophiolites get gra- dually younger from South to Morth in the Western Znungeerarea. The undevelopment of the sheeted sill complexs( included cumulate comp-lexes) in the Western Zhungeer ophiolites is concerned with the slow spreading of the back-arc basin.