阿拉善北部地区石炭纪火山岩岩石成因及构造意义

Petrogenesis and Tectonic Significance of Carboniferous Volcanic Rocks in Northern Alxa and Its Neighboring Areas,Inner Mongolia,China

阿拉善北部地区石炭纪火山岩分布广泛,目前对其成因和构造环境研究还很薄弱.通过对该区石炭纪火山岩岩石学和地球化学特征的分析,探讨其岩石成因和形成时的构造背景,为判定石炭纪盆地性质与古构造环境提供岩石地球化学约束.研究区内石炭纪火山岩主要为中-酸性火山岩,少量基性火山岩.玄武岩、玄武安山岩的大多数样品显示亚碱性系列特征,Mg#介于0.29~0.69之间,高场强元素Nb、Ta、Ti明显亏损,岩石轻度富集轻稀土元素(LREE),(La/Yb)N=2.19~10.10,Eu异常不明显(δEu=0.81~1.08),稀土配分曲线右倾较缓,εNd(t)值较高(+1.10^+6.35).总体上既显示板内构造环境特征,又携带俯冲带地球化学印记.综合区域地质特征及前人研究结果,认为阿拉善北部及其邻区石炭纪火山岩形成于板内裂谷环境,且可能与地幔柱事件有关,岩浆在上升过程中受到地壳物质不同程度的混染.

Carboniferous volcanic rocks are widely distributed in northern Alashan League and its neighboring areas,and their genesis and tectonic setting have always been an issue of controversy.Based on analysis of petrology and petrogeochemistry characteristics of Carboniferous volcanic rocks in the study area,this paper attempts to clarify the palaeo-tectonic setting and provides independent evidence for understanding the properties of Carboniferous basin.The volcanic rocks are dominated by intermediate-acidic volcanic rock.The majority of basalts,basaltic-andesite and andesite fall into sub-alkaline series.Mg#number of this suit ranges from 0.29to 0.69.These volcanic rocks samples exhibit strong depletion in the high field strength elements Nb,Ta and Ti,and minor enrichment in the LREE.In terms of REE,（La/Yb） N number ranges from 2.19to 10.10,showing less obvious Eu depletion（δEu=0.81to 1.08）,and slightly right-inclined REE distribution patterns,with higherεNd（t）value（＋1.10to＋6.35）.The samples show not only the marks of those within plate setting as a whole,but also those of subduction zone.Combining with the regional geological characteristic,we conclude that the Carboniferous magmatic activity in the study area generated in a within-plate setting,with variable degrees of contamination of crust during magma ascending,and probably related to mantle plume event.