石榴石微量元素地球化学及其在沉积物源分析中的应用

石榴石是沉积物中常见的重矿物,其可来源于多种岩石,而且不同类型母岩中石榴石具有多样的地球化学组成,因此碎屑石榴石的地球化学分析在沉积物源研究中应用广泛。通过电子探针分析可以容易地获得单颗粒碎屑石榴石的主量元素地球化学组成,可借此探讨其母岩类型,但也存在一定的局限性,比如中酸性火成岩和部分变沉积岩来源的石榴石通常都具有高Fe、Mn的特征,不易于区分。本文系统地收集了不同岩石类型的石榴石微量元素数据,尝试利用微量元素地球化学的差异性对碎屑石榴石物源分析进行补充。最终得出以下结论:(1)石榴石的稀土元素(REE)组成与钇(Y)元素指标可区分中酸性火成岩和变沉积岩来源的碎屑石榴石;(2)基性岩(橄榄岩、辉石岩)及所对应的变基性岩石(榴辉岩)中石榴石的微量元素地球化学组成相近,但部分橄榄岩来源的石榴石在镨/钬(Pr_N/Ho_N)值和重稀土总量(ΣHREE含量)上与辉石岩和榴辉岩的有显著差别,这一特点可运用于以基性岩母岩为主的碎屑沉积物源研究中;(3)夕卡岩中的石榴石在主量元素地球化学组成上表现为高度一致的高Ca特征,而稀土元素组成具有两种典型的分配模式,岩浆型(指示富铁、氧化环境)与热液型(指示富铝、还原环境)。综上所述,石榴石微量元素地球化学可以有效地运用于沉积物源分析研究中,是其主量元素物源分析方法的重要补充。

Garnet trace elemental geochemistry and its application in sedimentary provenance analysis

Garnet is a common heavy mineral in sediments. It can come from a variety of rocks and shows a variety of geochemistry compositions in different parent-rocks. Therefore, geochemical analysis of detrital garnet is widely used in sedimentary provenance research. The elemental geochemistry of single detrital mineral is easily obtained by electron microprobe and applied to the provenance analysis. However, there are certain limitations. For instance, the major elemental compositions of garnet in both intermediate-acid igneous rocks and metasedimentary rocks are usually characterized by high Fe and Mn. Here, we systematically investigated trace elemental geochemical data of garnet from different types of rocks and attempted establishing some useful trace elemental proxies for detrital garnet provenance analysis. The main conclusions are: (1) Rare earth elements and yttrium can be employed to distinguish detrital garnets from intermediate-acid igneous and metasedimentary rocks; (2) Garnets from basic rocks (peridotite, pyroxenite) and the corresponding meta-basic rocks (eclogite) had fairly consistent major and trace elemental geochemistry, but some garnets from peridotite rocks had unique praseodymium/holmium (Pr_N/Ho_N) values and total heavy rare earth elements (ΣHREE), which can be applied to sedimentary provenance analysis with basic rock-dominanted backgrounds; (3) Garnet in skarn was characterized by overwhelmingly high Ca, but rare earth element compositions were diverse, which can be interpreted as magma type (indicating rich iron and oxidation environment) or contact metasomatic type (reflecting rich aluminum and reducing environment). Collectively, the trace elemental geochemistry of garnet can be effectively used in sedimentary provenance analysis, which is an important supplement to the major element-based provenance analysis.