新疆东准噶尔北部地区中泥盆世双峰式火山岩组合

THE MIDDLE DEVONIAN BIMODAL ASSOCIATION OF VOL.CANIC ROCKS IN THE NORTHERN AREA OF EAST JUNGGAR,XINJIANG

新疆北部额尔齐斯河和乌伦古河之间的东准噶尔北部地区的中泥盆世火山岩,可以分成基性火山岩和酸性火山岩两类。中性火山岩的缺乏使之不能成为钙碱性火山岩系列,并明显表现出火山岩成分间断,说明这是一个典型的双峰式火山岩组合。双峰式火山岩组合的形成和大陆裂谷作用或大陆拉伸作用有密切关系。在地壳减薄较强烈的地方,地幔部分熔融产生的岩浆直接上升到地表喷发,形成基性火山岩,但岩浆在上升过程中经历了橄榄石和(或)部分辉石的分离作用;在地壳减薄较小的地方,上升的幔源岩浆在下地壳中聚集,导致下地壳部分熔融,所产生的岩浆在地表喷发,形成酸性火山岩。

The northern area of East Junggar, located between the Ertix and Ulungur rivers of northern Xinjiang, is one of the regions in China where Late Palaeozoic volcanic rocks are well developed. Middle Devonian volcanic rocks in the area are widespread and more than 5000m thick; they were formed in marine, terrestrial, and paralic environments. Studies on the petrography, petrochemistry, and geochemistry of rare earth elements and trace elements of the volcanic rocks indicate that Middle Devonian volcanic rocks in the area may be divided into two groups: basic and acid. Basic volcanic rocks mainly occur in the lower Middle Devonian Beitashan Formation (D_2b), consisting of basalt, basaltic andesite, trachybasalt, and basaltic trachyandesite, and acid volcanic rocks appear in the upper Middle Devonian Yundukela Formation (D_2y), dominated by rhyolite, with minor dacite and trachydacite. The striking compositional gaps between basic and acid groups resulted from the absence of intermediate rocks. These features are different from those of calc-alkaline volcanic rocks. Therefore, the Middle Devonian volcanics in the area are a typical bimodal association made up of basic and acid rocks. At present, it is accepted that the formation of a bimodal association of volcanic rocks has a close relation to continental rifting or continental extension. In such a Middle Devonian tectonic setting, magmas were first produced by partial melting of the mantle. Where crustal thinning was greater, the magmas ascended and erupted on the surface directly, forming basic volcanic rocks, but olivine and/or some pyroxene fractionation occurred in the magmas during their ascent through the crust; and where crustal thinning was less, ascending mantlederived magmas accumulated within the lower crust, resulting in partial melting of the lower crust. Acid volcanic rocks were formed after the eruption on the surface of the magmas derived from the lower crust. Thus, the two groups of volcanic rocks were derived from two differing sources of magmas, respectively. The establishment of the Middle Devonian bimodal association of volcanic rocks in the northern area of East Junggar implies that the area was in an extensional (rather than a compressional) environment during the Middle Devonian.