新疆北部卡拉麦里地区石炭纪火山岩成因及其构造意义

Genesis and tectonic significance of Carboniferous volcanic rocks in Karamali area,northern Xinjiang

新疆北部卡拉麦里地区石炭系广泛发育安山岩、凝灰岩等火山岩,关于其形成的构造背景仍存在争议。选取卡拉麦里山2条典型的石炭系剖面,针对下石炭统姜巴斯套组、上石炭统巴山组和弧形梁组的安山岩、凝灰岩等岩石样品开展了镜下观察和主微量元素等测试分析。结果表明,样品具有较低-中等的MgO含量(0.5%~2.35%)和低-中等的Mg#(15.7%~42.5%),说明岩石样品在形成过程中发生了结晶分异作用。岩石样品的微量元素分布曲线与N-MORB、E-MORB和OIB存在明显差异,在Ta/Yb-Th/Yb图和La-Y-Nb图中岩石样品几乎全投在钙碱性系列区域中,在Nb/Yb-La/Yb图中全部落入俯冲组分范围并靠近地幔区。在Nb/Y-Rb/Y和Ba/La-Ba/Nb图中,表明样品受到过流体富集和熔体富集的影响。样品的Nb/U和Ce/Pb值分别为1.6~11.1和0.47~12.2,很接近大陆地壳的范围,指示物质来源可能有大陆地壳物质的混染。Th-Hf/3-Nb/16图投点指示所有样品均属于岛弧玄武岩范畴。综合分析表明,卡拉麦里地区从早石炭世到晚石炭世均处于活动大陆边缘的大陆岛弧环境,且这种环境至少从早石炭世一直延续到了晚石炭世。

Volcanic rocks,such as andesite and tuff,are widely developed in the Carboniferous system in Karamali area,northern Xinjiang.The tectonic background of their formation is still controversial.In this paper,two typical Carboniferous profiles from Karamali Mountain were selected.Andesite and tuff samples from the lower Carboniferous Jiangbasitao Formation,the upper Carboniferous Bashan Formation,and Huxingliang Formation were observed under a microscope,and their main trace elements were analyzed.The results show that the contents of MgO(0.5%-2.35%)and Mg#(15.7%-42.5%)are low to medium in samples,indicating that the crystallization differentiation of rock samples occurs during the formation process.The distribution curve of trace elements in the samples is obviously different from that of N-MORB,E-MORB,and OIB.The samples are almost all in the calcalkaline series in Ta/Yb-Th/Yb and La-Y-Nb diagrams,while the samples fall subduction components and approach the mantle series in Nb/Yb-La/Yb diagrams.The Nb/Y-Rb/Y and Ba/La-Ba/Nb diagrams also show that the samples are affected by both fluid enrichment and melt enrichment.The Nb/U and Ce/Pb ratios of the samples are 1.6-11.1 and 0.47-12.2,respectively,which are close to the range of continental crust and indicate that the source of the material may be mixed with continental crust materials.According to the Th-Hf/3-Nb/16 diagram,all the samples are located in the island arc basalt region.The comprehensive analysis shows that Karamali area was in the continental island arc environment of active continental margin from the early Carboniferous to the late Carboniferous,and this environment lasted at least from the early Carboniferous to the late Carboniferous.