Research On The Drilling Cores(DSD-1) Of Dongbo Ophioliticmassif In The Western Segement Of Yarlung Zangbo Suturezone

Dongbo peridotite massif is located in the western segments of the Yarlung-Zangbo suture with an area of 400 km2. In order to find out the origin of this massif and to evaluate the chromitite prospect, a borehole(DSD- 1) has been drilled in the massif carried out by Sino-Probe program. The upper part with a thickness of 23 m of the drill hole is composed of Quaternary sediments. The rest part of this drill hole is dominated by peridotite. According to the rock type identified under the microscope, the drilled cores can be divided into two main rock units. These two rock units can be further divided into 44 sub-rock units. From 23.1 m to 340.7 m of the drilling core, it mainly consists of strongly serpentinized Cpx-bearing harzburgites with Opx contents ranging from 15% to 20 % and Cpx contents less than 5%. Spinels from these harzburgites have Cr# varying between 11.1 and 23.0 consistent with those of the abyssal peridotites. From 310.17 to 1002.06 m of this borehole, the drilling core mainly consists of intermediately serpentinized harzburgites with some dunites or mafic rock veins. The Opx contents of the harzburgites from 310.17 to 1002.06 m ranges from 10% to 15 % with no or very little Cpx. Spinel grains have red brown color with Cr# of 40.5 ~ 62.8. In the Mg# vs. Cr# diagram, these spinels fall in the SSZ peridotites fields. According to the occurrence and mineral assemblage characteristics, the dunites from this borehole can be divided into two types. Type-Ⅰdunites occur as lenses with a varying thickness from several centimeters to tens of centimeters. There is no Opx and Cpx mineral in type-Ⅰdunites and the Cr# values of type-Ⅰdunites vary between 63.6 ~ 67.3 indicating that type-Ⅰdunites may represent the residues after high degree of partial melting of the harzburgites. Type-Ⅱ dunites are layered dunites with a thickness of 14 m. Minor Cpx, Opx and amphiboles can be observed in the type-Ⅱdunites. The occurrence of amphiboles in type-Ⅱ dunites may suggest the modification by the fluids/melts from the subduction zone. Cpx and Opx from olivine orthopyroxenites and orthopyroxenites have similar Al2O3 and CaO contents. Amphiboles from these olivine orthopyroxenites and orthopyroxenites have consistent Na2 O and TiO2 contents. The similar mineral chemistry of these rocks may indicate similar material source and origin. Diabase from DSD-1 borehole with a thickness of 18 m has chondrite-normalized REE patterns similar to those of the N-MORB. However, the deletion of Nb and Ti suggest suprasubduction zone affinities. Based on the integration of petrological, mineralogical and geochemical date of different rocks from the DSD-1 borehole, we proposed that Dongbo peridotite massif was originated in the mid-ocean ridge setting and later modified by the fluids/melts from the intra-oceanic subduction zone.