东准噶尔滴水泉地区发现洋中脊型蛇绿岩
摘要
A MOR-type ophiolite unit has been recognized in the Dishuiquan region, 30-40 km to the south of the Karamaili ophiolite in east Junggar, Xinjiang, China. The Dishuiquan ophiolite contains mantle peridotite, pyroxenite, anorthosite, gabbro, basalt, and chert. LA-ICP-MS zircon U-Th-Pb dating of gabbro yielded an age of 416.8±3.2 Ma. The basalts in the Dishuiquan ophiolite are characterized by low Al<sub>2</sub>O<sub>3</sub>, TiO<sub>2</sub>, P<sub>2</sub>O<sub>5</sub>, and K<sub>2</sub>O levels; high levels of MgO and CaO; and LREE-depleted chondrite-normalized REE patterns. In N-MORB-normalized spider diagrams, the distribution patterns are very similar to those of MORB. The gabbro and pyroxenite have features of cumulate ultramafic-mafic rocks with strongly depleted trace elements. Discrimination diagrams showed that the basalts of the Dishuiquan ophiolite are of MORB and N-MORB type, and are different from the basalts from the Karamaili ophiolite to the north, which are similar to arc basalt. Therefore, the Dishuiquan ophiolite possibly formed in a MOR setting, and represents fragments of the oceanic lithosphere between the Kazakhstan-Junggar plate and the Siberian plate. Based on the regional geology, the Dishuiquan oceanic basin opened before the Ludlow and closed before the Late Devonian. The discovery of Dishuiquan MOR-type ophiolites indicates that the Karamaili ophiolite belt, the age and origin of which are disputed, may be a late tectonic mixture of MOR-type and SSZ-type ophiolites.