阿拉善晚古生代陆壳增生研究:以宗乃山和雅布赖山岩体为例

Crustal growth refers to the process that mantle material is added into crust. However, the geochemical disequilibrium between granite and mantle suggests that the former can not be formed by directly partial melting of mantle rocks.Thus, it is important to investigate the origin and formation mechanism of granites for constraining continental growth. In the Alxa Block, there are abundant Late Paleozoic granites as a magmatic response to tectonic evolution of the Paleo-Asian Ocean. Some of them exhibit positive εNd-εHf, while others have negative values. In general, the former is considered as products of continental growth, while the latter is interpreted to be produced by remelting of ancient crustal rocks. Evidently, the origin of granites has been ignored and continental growth would be not well understood. So for example, it is possible that partial melts of metaigneous rocks have positive εNd-εHf values and those granites with negative εNd-εHf values are likely derived from enriched lithospheric mantle. All in all, continental growth in the Alxa Block need to be further investigated. In this work, we plan on carrying out mineralogy, petrology and geochemistry on the granites and associated mafic enclaves from the Zongnaishan and Yabulaishan complexes. Our main objectives are to further constrain the Late Paleozoic continental growth in the Alxa Block.

地壳增生是指地幔物质加入到地壳的过程，而花岗岩与地幔间存在的化学不平衡性导致了前者很难从后者直接部分熔融产生。因此，从花岗岩的角度研究陆壳增生首先要查明其岩石学成因和形成机制。在阿拉善地块广泛产出与古亚洲洋演化有关的晚古生代花岗岩，其中部分岩体具有正的εNd-εHf同位素值，而其他岩体则相反。前者通常被看作陆壳增生的证据，而后者则被认为是老地壳重融的产物。然而，这种认识忽略了花岗岩成因的研究，可能会导致对陆壳增生的理解出现偏差。例如,正变质岩（如斜长角闪岩）的部分熔融可能形成具有正εNd-εHf的花岗岩，而具有负εNd-εHf的花岗岩也可能来自于富集岩石圈地幔。因此，阿拉善地区的陆壳增生过程需要进一步调查。本项目，我们拟选取晚古生代的宗乃山和雅布赖山杂岩体中的花岗岩及镁铁质包体进行矿物学，岩石学，岩石地球化学的对比研究，以期能为该地区晚古生代陆壳增生过程提供进一步制约。

项目背景：具有亏损同位素组成的花岗岩的产出是陆壳增生过程的识别标志之一，而具有富集同位素组成的花岗岩则被看作是陆壳再造的结果。这些花岗岩对陆壳增生是否有贡献还缺乏深入探讨。主要研究内容：本项目结合前人研究成果，选取了晚古生代的宗乃山和雅布赖山杂岩体中的花岗岩及相关镁铁质岩石在矿物学，岩石学，岩石地球化学和地质年代学等方面开展了对比研究，以期能为上述科学问题提供制约。重要结果和关键数据：1.地质年代学的结果表明雅布赖岩基中的花岗岩和辉长岩侵入于二叠纪（~ 272 Ma）；2.古亚洲洋的闭合不早于中二叠世(~ 262 Ma)；3.雅布赖辉长岩起源于浅部受俯冲物质改造的不均匀的岩石圈地幔源区，而雅布赖花岗岩尽管具有富集同位素组成但也是来源于底侵的新生玄武质地壳；4.雅布赖岩基和宗乃山-沙拉扎山岩基具有明显不同的同位素组成和岩石学成因，但是都代表了陆壳发生了明显增生；5.结合前人和新的数据，综合分析结果显示阿拉善地块在二叠纪发生了明显的陆壳增生。科学意义：具有富集同位素组分的花岗岩可能也记录了新地壳的形成，如果忽视地幔物质对于这些花岗岩形成的贡献，有可能造成对陆壳增生速率的低估。