Origin Of The Dongqiao Peridotites And Podiform Chromite Deposit In The Central Segment Of The Bangong-Nujiang Suture Zone

The Dongqiao ophiolite is located in the central segment of Bangong-Nujiang Suture Zone and is divided into east and west massifs which were separated by 8.5 km.The ophiolite is obviously controlled by the regional tectonic setting.Due to the dismemberment and destruction of the structure,there is no complete sequence profile,mostly composed of one or several ophiolite units.On the one hand,previous studies on the tectonic setting of the Dongqiao ophiolite are mostly focused on the upper oceanic crust rocks,and so far there are no doubtless conclusions about tectonic setting.On the other hand,as the outcrop of chromitites with better mineralization in the shallow part of Dongqiao has been completely recovered,it is urgent to clarify the cause of the problem and provide a theoretical basis for subsequent prospecting exploration.We found the following studies through the petrology,mineralogy and geochemistry study of the peridotites and podiform chromitites in Dongqiao ophiolite:(1)The Dongqiao peridotites are dominated by harzburgites,and with minor proportion of dunites(~15%).The podiform chromitites mainly show banded and disseminated structures,and occur in a thick layer of far-extending dunites.(2)The clinopyroxene contents of Dongqiao peridotites are less than 3%.The mineral and the whole-rock geochemistry show that Dongqiao peridotites formed after22%~28%degree of partial melting degree of the spinel-phase mantle source,which is higher than the partial melting of abyssal peridotites(10%~22%).(3)The Cr~#values of spinels in the Dongqiao peridotites are higher than 60.The chondrite-normalized REE patterns of the Dongqiao peridotites display enrichments in LREE.In addition,the peridotites are characterized by right-leaning PGE model and relatively higher Rh/Ir than those of CI chondrite.These geochemical characteristics jointly indicate the selective enrichments of LREE and some trace elements according to intercation between the residual mantle and the fluid/melt above the subduction zone.(4)The melt-rock reaction model can be used to explain the formation process of the Dongqiao podiform chromitites:The rock combination of the upper mantle,which was strongly depleted,formed boninitic magmas after a large number of S was taken away.With the emplacement of upwelling boninitic magma,depleted harzburgites began to react with it.Inconsistent melting of pyroxene forms a silicon-rich,chromium-rich secondary melt and olivine is also formed at the same time.In the early stage of magma crystallization,IPGEs and spinel are simultaneously separated and crystallized.The incompatible elements such as Pt and Pd are difficult to combine with S and been depleted.(5)Based on the mineral composition and the whole-rock geochemistry characteristics of Dongqiao peridotites,we concluded that the peridotites and podiform chromites of the Dongqiao were formed in different tectonic settings:Dongqiao peridotites formed in the dilated environment of the mid-ocean ridge,and then were influenced by intra-ocean subduction,resulting in highly partially melted peridotites interacting with the fluid/melt above the suprasubduction zone,Dongqiao podiform chromites were formed later than peridotites in suprasubduction zone.