| The Luobusa chromite deposit, which is the largest podiform chromite deposit with maximum study in China, occurred in the mantle peridotite in the east part of Yarlung Zangbo ophiolite belt in south Tibet. In this paper, the tectonic setting, genesis and evolution of the deposit is discussed throuth the study of petrology, geochemical contrast of chrome spinel, and chronology, on the basis of previous studies.The main results are as follows:Isotopic study of the Luobusha peridotites reveals that the rocks are characterized by high initial 87Sr/86Sr, low initial 143Nd/144Nd ratios, this does not represent the isotopic composition of its source, because they may be altered by the crustal material or seawater; Its isotopic composition of Pb is similar to the Indian Ocean, The samples are distributed between the DMM and EM II, which reflects the addition of crustal material.The highΔ7/4Pb andΔ8/4Pb values indicates the enrichment of the radiogenic Pb, combined with the highΔSr value, exhibit a source with Dupal anomaly.The chromium spinels are classified more detailed through different structural types of ores, and then we compare them with the accessory chromium spinel in the mantle peridotite. The result indicates that there are significant differences whether in the composition or the evolutionary trends between them. The composition of the accessory chromium spinel vary more clearly, which showing a clear evolutionary trand from harzurgite to dunite, that is the chromium spinel become more enriched in Cr and Mg with the increasing degree of partial melting of the wall rocks, and, that mears they have close genetic relationship with the mantle peridotite; The composition of the chromium spinels in different types of ores are very stable, show high Cr# values. Combined with other research, we conclude that the chromium spinels in the ores may come from the deep mantle, they may be the carrier of the UHP minerals from the deep mantle to the surface, and so, their origin has nothing to do with the mantle peridotites.The contents of trace elements and REE in the different types of chromium spinels are tested though LA-ICP-MS. The general trend of the REE patterns is the left side is slightly low, show strong negative Ce anomaly and defferent levels of Tm-Er peak. The primitive mantal normalized trace elements distribution patterns seem flat overall, show Nb, Ta, Pr, Ti, Tm peaks and Ba, Ce, Sr, Y low. The negative Ce anomaly indicates that chromium spinels both in rocks and ores were influenced by the crustal material in their evolution because the primitive mantal does not have Ce anomaly.Zircon and apatite samples were selected from the Luobusha conglomerate for fission track dating. A two- humped age of 36 and 21 Ma were obtained from the zir -com samples.Combining of the grology feature, we infer that the 36Ma represent the diagenetic age of conglomerate, and the 21Ma is on behalf a large scale thrusting event, which is also a time limit for the tectonic evolution of late mineralization. The central age from apatite is 6.6~7.5Ma, which should reflect the uplift and erosion of Luobusa chromite deposit.At last, the metallogenic evolution of the Luobusha chromite deposit is discussed, we divide the whole process into three phases: a) in the mid-ocean ridge, the mantle plume brings the material rise to the shallow of mantle quickly, the initial enrichment of chromite occurred in the mantle shear flow; b) Above the subduction zone (SSZ) the main mineralization stage, the fluid released from the subducted slab of oceanic crust dehydration, metasomatized the depleted mantle peridotite, which promote the occurrence of partial melting and the formation of chromite deposit;c)Large scale thusting stage, the chromite deposit was thrusted over Luobusa Group with the ophiolite units,and they were reversed during this stage. |
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