Genesis of Dingqing mantle peridotite in the eastern segment of the Bangong-Nujiang suture zone: Evidence from mineralogy and geochemistry of mantle peridotite from borehole ZK02

Dingqing ophiolite is located in the eastern segment of the Bangong-Nujiang suture zone, which is the largest exposed ophiolite in the suture zone. A 165. 19m-deep borehole has been drilled in the eastern of the Dingqing ophiolite to help understanding the genesis of the mantle peridotite. The cores are consisted of about 0. 5m thick Quaternary sediments in the upper part and the rest mantle peridotite which can be divided into 17 lithologic unit layers according to the microscopic identification. The mantle peridotites are mostly harzburgites mixed with a small amount of dunites and chromite-containing dunite. The Fo values of olivine (88. 79 similar to 93 73) and the Cr-# of chromium spinet (44. 33 similar to 81. 66) suggest that the Dingqing mantle peridotite may have experienced about 20% similar to 40% partial melting. All the petrochemical features, including rich MgO (45. 98% similar to 49. 45%) and poor aluminum (Al2O3 = 0. 19% -1. 37%) and calcium (CaO = 0. 28% similar to 0. 70%), are attribute to the highly -melted residual mantle. Judging from the LREE enrichment that the mantle peridotite is obviously different from the Alpine ophiolite since it may experience strong partial melting and the subsequent fluid metasomatism during the subduction. The Fe0/Mg0 value corresponding to the Al2O3 content of the parent melt is calculated using the chromium spinet composition in the mantle peridotite, demonstrating that they mainly belong to the boninite comparing to the primitive magma in various tectonic environments. The FMQ values of oxygen fugacities of dunite and harzburgite are - 3. 05 similar to 0. 71 and similar to 3. 89 - + 1. 47 respectively, indicating they have experienced the subduction process. Basing on the data of drilling core, it is proposed that the eastern of the Dingqing ophiolite may be formed in the forearc environment in the supra subduction zone.