Formation of A-type granites in the Lower Yangtze River Belt: A perspective from apatite geochemistry
Formation of A-type granites in the Lower Yangtze River Belt: A perspective from apatite geochemistry
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长江下游带A型花岗岩的形成：磷灰石地球化学视角
DOI：
10.1016/j.lithos.2018.02.005
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发表时间：
2018-04
期刊：
影响因子：
3.5
通讯作者：
Sun Wei-Dong
中科院分区：
文献类型：
--
作者：
Jiang Xiao-Yan;Li He;Ding Xing;Wu Kai;Guo Jia;Liu Ji-Qiang;Sun Wei-Dong
磷灰石普遍存在于A型花岗岩中，可用于阐明硅酸盐熔体的挥发分含量，反映其来源特征。 A型花岗岩已被认为是一组独特的花岗岩。 A1 和 A2 型子群是在不同的扩展设置下产生的。然而，A1 型和 A2 型花岗岩独特的地球化学特征背后的机制细节仍然不清楚。白垩纪A1型和A2型花岗岩带产于中国东部长江下游带。本文对长江下游不同地点的当代A1型和A2型花岗岩中磷灰石的主量和微量元素组成进行了研究，以破译其不同的成因过程。 A1 型和 A2 型花岗岩的磷灰石显示出相似的主量元素和微量元素，但 F 和 Cl 浓度不同。与西部A2型花岗岩相比，长江下游带东部A1型花岗岩的磷灰石F含量较低，Cl含量较高。而且，从东到西，A1型花岗岩磷灰石中F浓度逐渐增加，而Cl浓度逐渐下降。在俯冲板块中，F在角闪岩阶段被角闪石、绿泥石、蛇纹石和云母矿物保留，最后在榴辉岩阶段被多硅白云母和硬长石矿物保留，而Cl则受角闪石、绿泥石和蛇纹石控制。因此，A1 亚组磷灰石中高且变化的 Cl 浓度可能归因于俯冲洋壳部分熔融过程中角闪石、绿泥石和/或蛇纹石的分解，在较浅的深度释放出大量 Cl。相比之下，F在俯冲洋壳中被输送到更深处，并通过白硅白云母和硬钠长石的分解而释放出来，对A2型花岗岩的形成做出了重要贡献。 A1型和A2型花岗岩样品中的磷灰石随着距俯冲带位置距离的增加，LREE/HREE、LREE/MREE和MREE/HREE比值出现规律性变化，这可能是由于俯冲过程中蛇纹石和白白云岩在不同阶段不同步脱水的结果。我们认为，长江下游带的A1型和A2型花岗岩分别来自角闪石、绿泥石和/或蛇纹石分解流体交代的来源，其中Cl较高，F较低，而F多白云母和/或硬铝石岩的F相对较高，Cl较低。
Apatite is ubiquitous in A-type granites, and can be used to elucidate the volatile contents of the silicate melt, which reflect its source characteristics. A-type granites have been recognized as a distinct group of granites. A1- and A2-type subgroups are produced under different extensional settings. However, the details of the mechanisms behind the distinctive geochemical characteristics of A1- and A2-type granites remain obscure. Belts of Cretaceous A1- and A2-type granites occur along the Lower Yangtze River Belt in eastern China. Here we investigated the major and trace element compositions of apatites from contemporary A1- and A2-type granites at different localities along the Lower Yangtze River Belt, in order to decipher their discrepant source processes. Apatites from A1- and A2-type granites show similar major and trace elements, but differ in their F and Cl concentrations. Apatites from A1-type granites in the eastern part of the Lower Yangtze River Belt have much lower F and higher Cl concentrations compared to A2-type granites in the western part. Moreover, from the east to the west, the F concentrations of apatites from A1-type granites increase, while the Cl concentrations decline. In a subducted plate, F is retained by amphibole, chlorite, serpentine and mica minerals through the amphibolite stage, and finally by phengite and lawsonite during the eclogite stage, whereas, Cl is controlled by amphibole, chlorite and serpentine. The high and varied Cl concentrations in A1subgroup apatites, therefore, may be attributed to the breakdown of amphibole, chlorite and/or serpentine decomposition during partial melting of subducted oceanic crust releasing a large amount of Cl at shallower depth. In contrast, F is transported to deeper depths in the subducted oceanic crust, and released through breakdown of phengite and lawsonite, making an important contribution to the formation of A2-type granites. Apatites from A1- and A2-type granite samples show regular changes in LREE/HREE, LREE/MREE and MREE/HREE ratios with increasing distance from the location of the subduction zone, probably as the result of nonsynchronous dehydration of the serpentine and phengite at different stages during subduction. We propose that A1- and A2-type granites on the Lower Yangtze River Belt were derived from sources metasomatised by fluids originating from the breakdown of amphibole, chlorite and/or serpentine with higher Cl, lower F, and from phengite and/or lawsonite with relatively higher F but lower Cl, respectively.
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