Mineral Compositions of Syn-collisional Granitoids and their Implications for the Formation of Juvenile Continental Crust and Adakitic Magmatism
Mineral Compositions of Syn-collisional Granitoids and their Implications for the Formation of Juvenile Continental Crust and Adakitic Magmatism
复制标题
同碰撞花岗岩的矿物成分及其对新生陆壳和埃达克岩浆作用形成的意义
DOI：
10.1093/petrology/egaa038
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发表时间：
2020-10
影响因子：
3.9
通讯作者：
Dong Wang
中科院分区：
文献类型：
--
作者：
Yuanyuan Xiao;Shuo Chen;Yaoling Niu;Xiaohong Wang;Qiqi Xue;Guodong Wang;Yaijie Gao;Hongmei Gong;Juanjuan Kong;Fengli Shao;Pu Sun;Meng Duan;Di Hong;Dong Wang
大陆碰撞带被认为是大陆地壳净生长的主要地点。因此，研究具有镁铁质岩浆包体的同碰撞花岗岩类是验证这一假说的必要条件。青藏高原北方北祁连造山带的宝积山和曲母山同碰撞岩体中存在着与寄主花岗岩类密切接触的丰富的镁铁质岩体，其矿物组成相似，但镁铁质岩体中镁铁质矿物的模态丰度较高。QMS寄主花岗岩具有较高的Sr/Y和La/Yb比值，显示出与BJS花岗岩不同的埃达克质成分。基于岩石成分和锆石U-Pb年龄测定，最近对这两个岩体的研究认为，MMEs代表与其寄主花岗岩类相同的岩浆系统中早期结晶的堆积岩，其母岩熔体最好理解为俯冲的上洋壳与角闪岩相下的一些陆源沉积物碰撞后部分熔融产生的安山质岩浆。
在这里，我们专注于微量元素地球化学的组成矿物相的MMEs和他们的主机花岗岩的QMS和BJS岩体。我们发现，不同的矿物相优先宿主不同的微量元素，例如，大多数稀土元素（REE和Y）存在于钛铁矿（仅在QMS岩体中发现）、角闪石、磷灰石、绿帘石和锆石（大多数为重稀土元素）中，高场强元素（HFSE）存在于黑云母、钛铁矿、角闪石和锆石中。根据矿物化学数据，我们证明这两个岩体的MMEs具有相似的堆积成因，结晶于花岗岩类岩浆活动早期的原始安山质熔体。这些同碰撞花岗岩类的原始安山质熔体很可能是由洋壳部分熔融产生的，支持大陆地壳生长的假设，认为同碰撞花岗岩类代表幼年大陆地壳。
不同的矿物成分表明，这两个岩体具有不同的母岩浆成分，更高的TiO 2含量，更高的Sr/Y和La/Yb比在QMS母岩浆，一个签名最好理解为从源继承。较高的TiO 2含量的QMS岩体的母岩浆导致常见的存在下，在QMS岩体（不存在的BJS岩体），结晶的微量元素（REE，Y，Nb，Ta和其他）系统学的残余熔体埃达克质签名。因此，具有高TiO 2含量和高Sr/Y和La/Yb比值的母岩浆，以及它们对钛铁矿的进一步分馏，是埃达克质成分发展的重要因素，如QMS寄主花岗岩类所代表的。这一模式为埃达克质岩石的成因研究提供了新的视角。本研究进一步表明，在一般情况下，矿物化学为揭示花岗岩类岩石的成因提供了必要的信息。
Continental collision zones have been proposed as primary sites of net continental crustal growth. Therefore, studies on syn-collisional granitoids with mafic magmatic enclaves (MMEs) are essential for testing this hypothesis. The Baojishan (BJS) and Qumushan (QMS) syn-collisional plutons in the North Qilian Orogen (NQO) on the northern margin of the Tibetan Plateau have abundant MMEs in sharp contact with host granitoids, sharing similar constituent minerals but with higher modal abundances of mafic minerals in MMEs. The QMS host granitoids have high Sr/Y and La/Yb ratios showing adakitic compositions, different from the BJS granitoids. Based on bulk-rock compositions and zircon U-Pb age dating, recent studies on these two plutons proposed that MMEs represent cumulates crystallized early from the same magmatic system as their host granitoids, and their parental melts are best understood as andesitic magmas produced by partial melting of the underthrusting upper ocean crust upon collision with some terrigenous sediments under amphibolite facies.
Here, we focus on trace element geochemistry of the constituent mineral phases of both MMEs and their host granitoids of the QMS and BJS plutons. We show that different mineral phases preferentially host different trace elements, e.g., most rare earth elements (REEs and Y) reside in titanite (only found in the QMS pluton), amphibole, apatite, epidote and zircon (mostly heavy-REEs), and high field strength elements (HFSEs) reside in biotite, titanite, amphibole and zircon. Based on the mineral chemical data, we testify that for these two plutons, MMEs are of similar cumulate origin, crystallized from primitive andesitic melts in the early stage of granitoid magmatism. The primitive andesitic melts for these syn-collisional granitoids are most likely produced by partial melting of the oceanic crust, supporting the hypothesis of continental crustal growth considering the syn-collisional granitoids represent juvenile continental crust.
As evidenced by distinct mineral compositions, the two plutons have different parental magma compositions, e.g., higher TiO2 content, higher Sr/Y and La/Yb ratios in the QMS parental magmas, a signature best understood as being inherited from the source. The higher TiO2 content of the parental magma for the QMS pluton leads to the common presence of titanite in the QMS pluton (absent in the BJS pluton), crystallization of which in turn controls the trace element (REE, Y, Nb, Ta and others) systematics in the residual melts towards an adakitic signature. Therefore, parental magmas with high TiO2 content and high Sr/Y and La/Yb ratios, as well as their further fractionation of titanite, are important factors in the development of adakitic compositions, as represented by the QMS host granitoids. This model offers a new perspective on the petrogenesis of adakitic rocks. The present study further demonstrates that in general, mineral chemistry holds essential information for revealing the petrogenesis of granitoid rocks.
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