内蒙石炭纪锡林浩特辉长岩成因—对地幔源区性质及深部流体循环的初步制约

Although the petrogenesis of the Carboniferous-aged gabbros is crucial for an understanding the evolution of the Xing’an-Mongolia orogenic belt, there is neither consensus on their origin nor on the tectonic setting of their emplacement. The principle problems appear to be the lack of detailed information on their mantle-source characteristics, partial-melting conditions as well as an understanding of magma evolution. Integrated whole-rock geochemical and isotopic analyses, in situ mineral chemical and isotopic compositions, and petrographic examination of selected rock units can address those problems. Previous studies have shown that Late Carboniferous amphibole-bearing gabbroic rocks crop out over a wide area near Xilinhot, Inner Mongolia, China, and can provide key information for understanding the characteristics of the mantle source. Therefore, this study will focus on those Late Carboniferous, in order to determine the thermal and chemical state of their mantle source, and to provide preliminary constraints on deep-Earth geodynamics of the Xing’an-Mongolia orogenic belt during the Late Paleozoic. This study will analyse clinopyroxene and whole-rock trace-element compositions, and mineral modes in order to constrain the trace-element compositions of the parental magmas. The proposed study will further determine pressure and temperature conditions, and water contents of the parental magmas through the major-element contents of plagioclase, clinopyroxene, orthopyroxene and amphibole, and thereby reveal the role of water in the generation and subsequent evolution of parental magmas. Combined whole-rock Sr-Nd-Hf isotopic compositions, major-element contents and oxygen isotopic composition of olivine will be used to determine the evolution of recycled crustal components in the mantle source. These results will provide crucial constraints on the understanding of the source of water and on associated cycling process beneath the Xing’an-Mongolia orogenic belt. Together with available data, the outcomes of this study will provide new perspectives on the mantle source and geodynamic processes related to these Carboniferous mafic igneous rocks. This study will help to establish a dynamic link between deep-Earth fluid cycling and Late Paleozoic magmatism in Xing’an-Mongolia orogenic belt, and provide an independent constraint on the tectonic evolution of Xing’an-Mongolia orogenic belt.

石炭纪基性岩的成因是理解兴蒙造山带演化的关键之一，但是目前对于其岩石成因及动力学背景仍存在较大争议，缺乏对其地幔源区的系统研究是造成争议的主要原因。前期工作表明内蒙锡林浩特地区出露晚石炭世含角闪石辉长岩，是示踪地幔源区性质的重要窗口。因此，本项目拟以晚石炭世锡林浩特辉长岩为研究对象，通过全岩微量元素和矿物比例以及单斜辉石原位微量元素分析，反演母岩浆的微量元素组成；根据斜长石、单斜辉石、斜方辉石和角闪石的主量元素含量，确定母岩浆的温压条件和水含量，分析水在岩浆形成和演化过程中的作用；通过橄榄石主量元素和氧同位素以及全岩Sr-Nd-Hf同位素分析，确定地幔源区中再循环地壳组分的演化。这些信息是揭示地幔源区水来源及其循环过程的关键。结合已有的成果，本研究将初步建立深部水循环与兴蒙造山带晚古生代岩浆活动的成因联系，探讨形成锡林浩特辉长岩的动力学过程，为解决兴蒙造山带演化的关键科学问题提供重要制约。

内蒙中部晚石炭世岩浆活动的成因与动力学背景对于揭示古亚洲洋俯冲以及兴蒙造山带演化的具有重要意义。本课题以锡林浩特和西乌旗地区晚石炭世辉长-辉长闪长岩为主要研究对象，运用SIMS锆石U-Pb年代学、锆石原位O-Hf同位素组成以及单矿物成分分析方法，结合全岩主微量和Sr-Nd-Hf同位素组成，厘定了母岩浆的水含量以及部分熔融的温度和压力条件，探讨了地幔源区的性质和再循环组分的贡献，初步建立了深部流体循环与兴蒙造山带演化之间的联系。本研究获得了锡林浩特-西乌旗地区辉长岩体的侵位时间为311~317 Ma，表明其为晚石炭世双峰式岩浆活动的产物。单斜辉石-角闪石-斜长石温压计算结果表明岩浆部分熔融压力为5~6 kbar，温度为1000~1100 °C。角闪石和斜长石水含量计算显示母岩浆的水含量>2 wt.%，指示其为富水岩浆。全岩样品具有较低的87Sr/86Sr初始比值（0.7034~0.7041）但相对高的εNd(t)（+5.58~+6.88）和εHf(t)值（+12.07~+13.44），并富集Rb、Sr、U和Pb等流体活动性元素，但亏损Nb和Ta等非流体活动性元素，暗示其起源于遭受含水流体交代富集的地幔源区。与橄榄岩来源的熔体相比，辉长岩样品具有相对高的SiO2（51.7~53.2 wt.%）、Cr（138.4~757.2 ppm）和Ni（50.4~141.1 ppm）含量以及Zn/Fe比值（10.8~11.5），但相对较低的Al2O3（13.1~16.8 wt.%）含量，指示母岩浆起源于由橄榄岩夹辉石岩脉组成的地幔源区。部分锆石δ18O值（高达6.97‰）明显高于地幔的值，也反映了源区存在再循环地壳组分的加入。壳源富硅熔体的加入并与橄榄岩相互反应很可能是地幔源区产生辉石岩脉的关键。模拟计算表明大约2%沉积物熔体和0.5%沉积物流体加入到地幔源区，随后5~10%比例的部分熔融可形成这套辉长岩体的母岩浆。综合分析显示早石炭世之前古亚洲洋已闭合，而晚石炭世兴蒙造山带处于陆内伸展构造背景。因此，本研究认为锡林浩特及其邻区晚石炭世岩浆活动是遭受地幔转换带滞留俯冲板块析出富水流体交代的地幔源区部分熔融的产物，从而建立了深部流体循环过程与造山后伸展背景下岩浆活动的成因联系。