Formation Process Of The Podiform Chromitites

Podiform chromitite are output in mantle peridotite as the main source of chromium that is a shortage and most important strategic resource in china. Chromitite and mantle peridotite researching is a must way to deal with the chromitite resource bottleneck necessary and looking for more deposits. Previous studies showed chromitite formed shallow mantle environment in mid-ocean ridge or subduction zones(eg. Roberts,1988; Schiano,1998). However, with more and more diamond and other UHP minerals found in the chromitite and mantle peridotite, people have begun to question the shallow portion of the chromitite genesis theory(eg. Bai et al.,2001; Yang et al.,2007). This thesis are based on Luobusa mantle peridotite and chromitite that are eastern ophiolite of Yarlung Zangbo, and western suture Purang and Dongbo mantle peridotite and chromitite. Basing on detailed field survey and taking a large number of mantle peridotite and chromitite samples, then carried out systematic mineralogy, geochemical composition and Re-Os isotope study for study on chromitite and mantle peridotite formation process, various proceedings and recognitions have been achieved:(1) Identified two types chromitite, massive chromitite have harzburgite as envelope rock, and disseminated chromitite have dunite as envelope rock. Spinel composition, oxygen fugacity, mineral inclusions and exsolution, UHP minerals and other features of massive chromitite are instructed its formed in deep mantle. And disseminated chromitite spinel mineral composition and oxygen fugacity characteristics, as well as disseminated chromitite also show systematic changes in olivine and spinel composition from the dunite envelope to the disseminated chromitite, indicating melt-rock reaction.(2) High-aluminum Cr#=12-60 and high - chromium Cr#=60-80 two different types chromitite found in Purang mantle peridotite that is extremely rare in the world. Field output feature, mineral composition, geochemistry and PGM and other characteristics, which are show Purang mantle peridotite and chromitite had the characteristic of abyssal peridotite and fore-arc peridotite, indicating that it throw a multi-stage evolution and partial melting process.(3) There are high-aluminum and high - chromium chromitite found in Dongbo mantle peridotite. Individual ore bodies are 1-3 m long,0.5-2 m wide and strick NW, parallel to the main structure of the peridotite. Lherzolite and harzburgite envelope of the high aluminum chromitite are alpine peridotite LREE depleted characteristics, and harzburgite and dunite are more as "U" or "V" typed REE patterns, indicating different degree modified by later fluid. There are many difference between the two type chromitite in mineral composition, geochemistry and isotopic feature, show that they may formed in different tectonic settings or magmatic processes. And massive chromitite are from deep mantle and disseminated chromitite probably from shallow environments.(4)Proposed chromitite formation of a multi-stage new model, included four stages. a) Earlier slab of the continental crust and oceanic crust dehydrated and decomposed subducted into the mantle transition zone (410-660km), and thermal and fluid generated of the transition zone led to mantle melting and release and aggregated Cr. b) Basing on mantle plume, chromitite magma migrated and consolidation in the top of transition zone with capture diamond, stishovite and other minerals in strong reductive fluid. Chromitite enter into the high pressure phase of the "plasticity - semiplastic mantle peridotite". c) As depth change to shollow and material upward, earlier phase UHP minerals change into low - pressure phase minerals, stishovite into coesite, and early high pressure chromitite exsolution into coesite and clinopyroxene. d) Hydrous melt reaction with harzburgite formed disseminated chromitite and dunite envelope without UHP minerals when emplacement and subduction zone.(5) Seven small, lenticular bodies of chromitite ore have been found in the Purang, western of Yarlung Zangbo suture. Comparative study with Luobusa and Purang, Dongbo mantle peridotite, Yarlung Zangbo suture. With diamond and other UHP minerals found in the Purang and Dongbo mantle peridotite, PGM and chromitite Re - Os isotopic results, indicating there are many similarities between several western large massif and Luobusa, which had undergone tectonic and magmatic evolution, as well as the formation of chromitite. There is a big space exploration chromitite for the several large massif in western Yarlung Zangbo suture.