Origin Of The Kangjinla Podiform Chromite Deposit And Mantle Peridotite, South Tibet

The Yarlung Zangbo ophiolites in southern Tibet are remnants of the Neo-Tethys oceanic lithosphere and are considered as one of the suture zone representing the breakup and reconnection of Gondwana during Mesozoic. Recently, numerous unusual mantle minerals with the possible deep source (>300km) have been recovered from podiform chromities in the Luobusa ophiolite within the eastern Yarlung Zangbo suture zone, Tibet. This new finding has attracted high attention in geosciences field and arose many significant issues: does these deep mantle minerals also occur in the similar chromitite deposite in the Luobusa? Can we discover the similar mantle minerals from the mantle rocks associated with the chromitite? What is the origin of the chromitite and mantle rocks containing these minerals like diamond, together with the origin of the ophiolite? This study wants to perform researches on the Kangjinla chromitite and its host rock-mantle peridotite, to determine the existence of special mantle minerals, to discuss the origin of the chromtite and mantle peridotite together with the genetic relation between them. Basing on detailed field survey and laboratory researches on lithological features, mineralogical composition, geochemical and zircon SHRIMP U-Pb data, together with the mineralogical study of chromitite and mantle peridotite heavy mineral bulk sample, various proceedings and recognitions have been achieved:(1) The mantle peridotite is the remnants of partial melted MOR mantle altered by SSZ fluids. The evidences include: the mantle peridotite mainly consists of dunite, harzburgite and lherzolite; the highly depleted composition; significant variation of chrome spinel suggesting the huge difference in melting degree, which might indicate the production of various melting events; the harzburgite and lherzolite with low Cr#30-40 are abyssal peridotite while the harzburgite and dunite with high Cr#40-77 are island-arc peridotite, the latter is formed by the melting of the former during intra-ocean subduction; the geochemical characteristics can also separate the depleted and enriched LREE, the former is attribute to the MOR mantle peridotite and the latter is to the SSZ fluids action.(2) Special mantle minerals have been discovered from manual heavy minerals of chromitite and mantle peridotite. For a 1116Kg chromitite and a 384Kg mantle peridotite sample, by heavy mineral separation and picking up under a binocular microscope following productions are obtained: (a) the UHP minerals including moissanite and over 1000 diamond grains are discovered firstly from the Kangjinla chromitite orebody Cr-11, which is far more abundant than the Luobusa district; (b) a series of unusual mantle minerals like moissanite and a great deal of diamond grains are discovered firstly from the host rocks of chromitite orebody Cr-11, providing the foundation of discussing the chromitite origin and its relation with the host rocks; (c) besides the UHP minerals like moissanite and diamond, various special mantle minerals are revealed from the chromitite orebody Cr-11 and its host rocks including some native elements, alloys, oxides, sulphides, silicates, carbonates, and tungstates. The unusal minerals assembledge discovered from the Kangjinla district is similar to that from the Luobusa district. Especially, that the same mineral assembledge was found firstly from mantle peridotite, the host rocks of chromitite, which provided new key evidence for discussing the origin of the chromitite, the hosted mantle rocks and the relation between them.(3) The deep origin of Kangjinla podiform chromitite is suggested by: that for the Kangjinla chromitite and the mantle peridotite rocks, the compositions of the chrome spinel, olivine and clinopyroxene are rather different, which indicates that the chrome spinel in the chromitite is neither the simple concentration of that in the host rocks nor with certain genic relation with mantle peridotite. Though the chromitite might be exotic for host rocks, both of them were carried from deep mantle to the shallow place by mantle plum. According to the geochemical data, the formation of mantle peridotite might be: during the spreading of ocean floor, the MOR mantle underwent variable degrees of partial melting and formed the depleted mantle peridotite with podiform chromitite; during the spreading of ocean basin, intra-ocean subduction happened, REE and LILE rich fluids containing water derived from the subduction slab migrated upwards and metasomatism happened to peridotite relic.(4) The ancient crustal zircons discovered from the chromitite are key evidence of mantle inhomogenity and recycled crust materials. Zircon grains are separated from the mantle peridotite, the host rocks of the chromitite orebody Cr-11, and the SHRIMP U-Pb age reveals sophisticated background and multiple sources of zircons. The weighted mean value of four measuring points (130.0±2.8Ma) might be the crystallization age of dunite. Many crustal zircons have the age (up to 2770Ma) that is much earlier than the formation of ophiolite considered by previous researchers, indicated the abundant subduction crust components occur in the zircons separated, which was preserved in the mantle, and outcropped with the mantle cycle process. Considering the coexistence of unusual mantle minerals and possible cyclic crustal materials, the origin of the Luobusa chromitite might bear certain relationship with mantle plume.