Hongjishan Ophiolite In The Central And Western Part Of Qiangtang, Northern Tibetan Plateau: Petrologic, Geochemical, Chronological Characteristics And Its Tectonic Implications

Hongjishan ophiolite is a complete set of ophiolite rock association that was firstdefined by the authors. Few previous researches, especially the chronological andgeochemical studies, have been conducted on the Hongjishan ophiolite, which has causeda great restriction for further studying Longmucuo-Shuanghu-Lancangjiang River suturezone in the central and western Qiangtang. A series of problems, such as the petrologic,geochemical, chronological characteristics and tectonic implication of Hongjishanophiolite remain to be solved. Aiming to solve these problems, the petrologic,geochemical and chronological data for Hongjishan ophiolite were presented in this paper.The Hongjishan ophiolite stretches more than30km long and5km wide fromnorthwest to southeast. It mainly consists of metamorphic peridotite, metamorphiccumulate gabbro, metamorphic mafic dyke swarm, metamorphic basalt,radiolarian-bearing chert and plagiogranite. Therefore, every unit of ophiolite can beobserved in this region.Through the zircon U-Pb chonological study of Hongjishan ophiolite, we got twoweighted average ages, the one is274.8±2.6Ma (MSWD=0.21) obtained from21zircon grains of metamorphic cumulate gabbro, the other is271.7±2.3Ma (MSWD=0.38) from12zircon grains of plagiogranite. These two ages are the same within error,which can be interpreted as the formation age of the ophiolite, i.e., the Hongjishanophiolite formed in the Early Permian. This paper focused on the metamorphic peridotite, metamorphic cumulate gabbro,metamorphic mafic dyke swarm, metamorphic basalt, and conducted detailed geochemicalstudies for Hongjishan ophiolite. Geochemically, every unit of ophiolite has similar majorelement contents with those of typical N-MORB. Moreover, Hongjishan ophiolite isdepleted in light rare elements with low∑REE content and no obvious Eu anomalies.These features, along with primitive mantle--normalized trace element patterns, indicatethat Hongjishan ophiolite was originated from a depleted asthenosphere mantle withsimilar geochemical features of N-MORB. However, when compared to the typical maturemid-ocean ridge basalt, Hongjishan ophiolite exhibit relatively enrichment of some largeion lithophile elements (Ra、Ba、U）and depletion of Nb and Ta (especially for the Nb) inthe curves of N-MORB normalized trace element distribution pattern, which is quitedifferent from the typical mature mid-ocean ridge basalt but similar with the island arcbasalt. This kind of unique geochemical characteristics indicates the influence ofsubduction material, which suggest that Hongjishan ophiolite was formed in the marginalbasin setting, resulting from the back-arc mantle convection and back-arc oceanic crustformation in where was a spreading center near the subduction zone. In a word,Hongjishan ophiolite belongs to SSZ ophiolite.Taken all the research results above into account, we propose that the evolution ofpaleo-tethy has entered back-arc tension stage during the Early Permian time. Due toback-arc tension, Hongjishan appeared a small ocean basin, thus the Hongjishan ophioliteis a record of this tectonic movement. The research of Hongjishan ophiolite in this paperprovides important data for understanding the paleo-tethy evolution and the boundary ofEurasia and Gondwanaland.