Geochemical Characteristics and Tectonic Setting of the Jijitaizi Ophiolite in Beishan area

The Jijitaizi ophiolite belt in the middle of the Beishan orogenic belt, between the Mingshui-Hanshan microcontinental block and the Gongpoquan unit is an important part of the Jijitaizi-Xiaohuangshan ophiolite belt, composed of ultrabasic rocks, gabbro and basalt. To reveal the tectonic setting of Jijitaizi ophiolite belt, gabbro and basalt are selected for petrological and geochemical studies. MgO of gabbro and basalt is 6.04%～6.73% and 6.21%～9.66%, Mg^# values are 32.33～37.03, 27.58～46.27, respectively, SI consolidation index is 27.66～31.55 and 24.96～42.20, Al₂O₃ is 15.82%～16.79% and 13.38%～15.38%, Na₂O higher than K₂O content (Na₂O/K₂O=9.75～17.15, 1.95～23.26), Na₂O+K₂O is 4.48%～4.90% and 3.3%～4.68%, P₂O₅ is 0.07%～0.09% and 0.16%～0.27%. Both of gabbro and basalt enrichment of LILEs and LREE, depletion of HREE and HFSEs and Eu have no obvious anomalies (δEu=0.98～1.09、0.88～1.06). (⁸⁷Sr/⁸⁶Sr)_i is 0.703700～0.704768, (¹⁴³Nd/¹⁴⁴Nd)_i is 0.512234～0.512361, ε_Nd(t) is +4.18～+6.66. These characteristics indicate Jijitaizi ophiolite belt belong to SSZ type ophiolite belt, basalt and gabbro originated from partial melting of the depleted mantle and experienced crystallization differentiation and crustal contamination. Combined with geological background, it can be concluded that the Jijitaizi ophiolite may form in a back-arc basin, resaulting from northward subduction of the Hongliuhe-Niujuanzi-Xichangjing ocean basin.