海洋地质与环境重点实验室知识库

 地幔交代作用会使岩石圈地幔的矿物成分和地球化学组成改变。与大洋岩石圈地幔相比，大陆岩石圈地幔，特别是克拉通岩石圈地幔受到的地幔交代作用更加复杂。地幔交代作用能够改变克拉通地幔矿物组成和化学组成，进而使得克拉通岩石圈地幔容易被破坏。所以，研究交代富集岩石圈地幔的成因对理解“克拉通破坏”具有重要意义。起源于交代岩石圈地幔部分熔融的火山岩和地幔捕虏体是研究克拉通岩石圈地幔富集演化过程的极佳样品。华北克拉通是现存最古老的克拉通之一，其上广泛分布有中生代和新生代的火山岩，这些火山岩中还携带有大量地幔捕虏体，这为研究大陆环境下岩石圈地幔的交代作用提供了绝佳样品。本文选择了华北克拉通中生代早白垩世基性火山岩以及中国东部新生代玄武岩中携带的地幔辉石岩捕虏体作为研究对象，以此来探讨富集岩石圈地幔的成因。

 本文研究的华北克拉通中生代早白垩世基性火山岩来自位于华北克拉通西部的乌拉特中旗和黑石头沟两个地区，本文对这些玄武岩样品进行了全岩³⁹Ar/⁴⁰Ar年龄、全岩主、微量元素及Sr-Nd-Hf同位素的分析测试，以此来研究华北克拉通西部陆块交代富集岩石圈地幔的形成机制。这些玄武岩的形成年龄约为116 Ma，具有较高的不相容元素丰度（[La/Sm]_N=2.80-4.56）和富集的Sr-Nd-Hf同位素组成（⁸⁷Sr/⁸⁶Sr_i = 0.7062-0.7075，ɛ_Nd(t) = -6.0 - -13.0，ɛ_Hf(t)= -8.3 - -17.4）。所有这些特征都与华北克拉通东部陆块＞110 Ma玄武岩的地球化学特征相似。ɛ_Nd(t)与ɛ_Hf(t)之间、ɛ_Nd(t)和ɛ_Hf(t)与母/子体元素比值（分别为Sm/Nd和Lu/Hf）之间以及ɛ_Nd(t)和ɛ_Hf(t)与[La/Sm]_N之间均具有良好的相关关系，表明玄武岩的源区是经历了熔体交代的古老岩石圈地幔。微量元素配分模式图上，这些玄武岩表现出了明显的Nb-Ta（与K相比）-Ti负异常和Ba-Pb正异常，而且Nd和Hf同位素也显示出与SiO₂、Ti/Sm和Ba/Ta良好的相关性，说明其源区存在俯冲沉积物的贡献。此外，玄武岩的同位素组成与微量元素组成远比俯冲沉积物富集，这表明：俯冲进入地幔的沉积物是以交代熔体的方式贡献到玄武岩源区中的。因此，本文认为华北克拉通西部陆块之下交代岩石圈地幔的形成过程就是地幔橄榄岩与陆源沉积物部分熔融熔体之间的反应过程，该过程会在原有岩石圈地幔中形成含角闪石/金云母的交代脉体。相较于地幔橄榄岩，这些交代脉体的熔点更低，易融熔形成富碱的熔体。我们认为华北克拉通西部陆块北缘早白垩世的岩浆活动是由软流圈地幔上涌加热交代岩石圈地幔引起的，同时，本文认为该热扰动的成因与蒙古鄂霍次克洋闭合后俯冲板片的断离有关。

 本文研究的辉石岩捕虏体采自中国东部达里诺尔地区鸽子山附近，其围岩是新生代时期的玄武岩。本文对这些辉石岩捕虏体进行了岩石学和矿物学研究，并对其中的金云母、单斜辉石、斜方辉石和粒间熔体进行了原位微区主量、微量元素的分析测试。数据结果表明，单斜辉石和斜方辉石都具有较高的Al₂O₃含量和较低的Mg^#、Cr₂O₃，此外，斜方辉石还具有较高的MnO含量。结合这些辉石岩所具有的堆晶结构以及单斜辉石、斜方辉石的主量元素特征，可以认为研究区辉石岩代表了交代熔体在地幔条件下冷却堆晶形成的脉体。简单的计算表明，单斜辉石以及粒间熔体之间并没有达到地球化学的平衡，说明粒间熔体是在辉石岩形成以后的不同期次的地幔交代作用的产物。金云母高TiO₂和低Mg^#的特点表明其形成自碱性玄武质熔体的结晶，本文认为这些基性熔体来自于地幔的部分熔融。粒间熔体含有较高的MgO和FeO以及较低的SiO₂，指示其形成于地幔超基性岩的部分熔融，K-Pb正异常及Nb-Ta的负异常说明其源区可能也存在少量俯冲沉积物的贡献。

 综上所述，中生代时期华北克拉通富集岩石圈地幔的起源与地质历史时期经历的多次俯冲事件息息相关：板片俯冲过程中携带大量陆源沉积物进入地幔，这些沉积物熔融形成的富Si熔体交代上覆难熔的岩石圈地幔，改变其矿物组成和地球化学特征，形成“富集岩石圈地幔”。新生代时期，中国东部之下的岩石圈地幔继续被交代，交代介质主要是来自于软流圈地幔的熔体，但也有少量沉积物的贡献。地幔交代作用的持续发生最终会导致原本亏损的岩石圈地幔向富集的岩石圈地幔逐渐转变，从而使其变的易熔。因此，我们认为地幔交代作用的发生对中国东部岩石圈岩石圈的减薄与破坏有着促进作用。

 Mantle metasomatism would change the mineral composition and geochemical characters of the lithospheric mantle. Compared with the oceanic lithospheric mantle, the continental lithospheric mantle, especially craton lithospheric mantle, has more complex metasomatism history. The minerals or geochemical composition of the craton lithospheric mantle would change through the process of mantle metasomatism, which may lead to the destruction of the original stability of the lithospheric mantle. Therefore, the study of the origin of the enriched lithospheric mantle is of great significance for understanding the destruction of the craton lithosphere. Volcanic rocks deriving from partial melting of metasomatic lithosphere mantle and mantle xenolites are excellent samples for understanding the enrichment and evolution of craton lithosphere mantle. The North China Craton is one of the oldest cratons in the world, which is widely distributed with Mesozoic and Cenozoic volcanic rocks. These volcanic rocks also carry a large number of mantle xenoliths, which provide excellent samples for the study of lithospheric mantle metasomatism in the continental environment. we selected the early Mesozoic basalts in the North China Craton and mantle pyroxenite xenoliths carried by Cenozoic basalts in eastern China as research objects to explore the origin of the enriched lithospheric mantle.

 In this paper, we present new bulk-rock ³⁹Ar/⁴⁰Ar age, major and trace elements and Sr-Nd-Hf isotopic data on the early Cretaceous intra-plate alkali basalts from the Western North China Craton (WNCC) to study the origin of the metasomatized cratonic lithosphere mantle. The age of these basalts is ~116 Ma. These basalts have elevated incompatible element abundance with high [La/Sm]_N (2.80-4.56) and enriched Sr-Nd-Hf isotopic compositions (⁸⁷Sr/⁸⁶Sr_i = 0.7062-0.7075, ε_Nd(t) = -6.0 to -13.0 and ε_Hf(t) = -8.3 to -17.4), being similar to the contemporary analogues from the East North China Craton. The WNCC basalts also show good correlations between ɛ_Nd(t) and ɛ_Hf(t), and both ɛ_Nd(t) and ɛ_Hf(t) correlate with their parent/daughter ratios (i.e., Sm/Nd and Lu/Hf, respectively) and [La/Sm]_N. All these geochemical observations are consistent with the interpretation that these basalts originated from partial melting of the ancient lithospheric mantle that experienced melt metasomatism. The obvious negative Nb-Ta (compared with K)-Ti and positive Ba-Pb anomalies observed in these basalts suggest the contribution of sediments in the mantle source. Furthermore, the isotopic composition and trace elements of basalts are much more enriched than that of sediments, which constrain that the the subducted terrigenous sediment contributed in the mantle in the form of metasomatic melt. Collectively, we suggest that the formation of the metasomatized mantle source beneath the WNCC is the process of metasomatic reaction between mantle peridotite and terrigenous sediment-derived silica melts to generate metasomatic veins containing amphibole/phlogopite. The melting point of the metasomatic veins is lower than peridotite, so the metasomatic veins would be easy to melt and could form alkali melts. Partial melting of the metasomatic lithospheric mantle in the WNCC was considered to be induced by thermal perturbation that was ultimately related to the breakoff of the subducted oceanic slab following the closure of the Mongolia-Okhotsk ocean.

 This paper also reports the petrology and mineralogy data of the newly discovered phlogopite-bearing pyroxenite xenolites contained in Cenozoic basalts near Gezishan Mountain in Darinoor area of eastern China, and the in-situ in suit major and trace elements compositions of phlogopites, clinopyroxenes, orthopyroxenes and intergranular melts (melts trapped along grain boundaries) in these pyroxenites. The geochemical data suggest that both clinopyroxenes and orthopyroxenes have higher abundance of Al₂O₃ and lower Mg^# and Cr₂O₃. In addition, the orthopyroxenes also have higher abundance of MnO. These characters suggest that the xenolites in this study represent the metasomatic veins formed by segregation from metasomatic melts under upper mantle conditions. The simple calculation shows that there is no geochemical equilibrium among clinopyroxenes and intergranular melts, so we suggest that intergranular melts are more likely the products in metasomatism event by silicate melt after the formation of pyroxenites. The phlogopites with high TiO₂ and low Mg^# indicate that they were crystallizations of alkaline basalt melt. We thought that these basalt melts come from partial melting of mantle. The intergranular melt has higher abundance of MgO and FeO and lower abundance of SiO₂, indicating that the melts originated from partial melting of mantle ultrabasic rocks. The positive anomalies of K-Pb and the negative anomalies of Nb-Ta suggest that the melts source was mantle with contributions of subducted sediments.

 In conclusion, we propose that the origin of the NCC metasomatized lithospheric mantle was related to the subduction of terrigenous sediments into the mantle during multiple plate subduction underneath the NCC in geological history, which continuously supplied materials to form silica melt to metasomatize the overlying refractory lithospheric mantle. During the Cenozoic, the lithospheric mantle below eastern China was metasomatized continually by the melts from asthenosphere with contributions of subducted sediments. The continuous mantle metasomatism would eventually transform the lithospheric mantle from depletion to enrichment, finally make the lithosphere mantle fusible. Therefore, we propose that the mantle metasomatism is a prerequisite for lithosphere thinning.