1.China University of Geosciences（Beijing）;2.Chinese Academy of Geological Sciences;3.Jiangsu Donghai Continental Deep Hole Crustal Activity National Observation and Research Station
变质变形岩石中副矿物的变形及元素分布特征能记录变质变形岩石的变质变形信息，特别是结合原位定年，可以更为精确地揭示岩石的变质变形历史。磷灰石是变质岩中常见的副矿物，伴随着磷灰石原位定年技术的快速发展，使其成为岩浆岩和矿床学研究当中重要的定年矿物，在不同变质-变形过程中，磷灰石的变形机制与行为及其对元素扩散过程的制约效应等一系列重要问题还未得到解决。本研究选取喜马拉雅造山带中构造变形和变质作用最强的东构造结内的岩石开展研究。通过SEM和CL观测，显示麻粒岩中的磷灰石并未显示明显的成分结构，而糜棱岩中的磷灰石则呈现明显的明暗变化，也即成分变化。通过对薄片中磷灰石的展布特征统计发现两类变质岩中磷灰石的长轴方向近平行于面理的方向。EBSD组构分析显示麻粒岩中的磷灰石呈现出明显的粒内形变，而糜棱岩中的磷灰石几乎未发生粒内形变。EPMA对磷灰石成分面扫的结果显示麻粒岩中磷灰石的主量元素分布相对均匀，而糜棱岩中磷灰石的Si元素分布呈现明显的环带或者不均匀，这和CL图像的结果相一致。综合以上的磷灰石变形和元素分布特征可初步得出：(1) 尽管麻粒岩中的磷灰石定向分布，参与了岩石的变形，但可能由于其变质温度较高，促使了元素的快速扩散，使元素分布趋于均一，但是后期的流体作用在沿着低角度颗粒边界或者近垂直于最大拉伸应力方向的裂隙发生了成分交代；(2) 糜棱岩中磷灰石平行于糜棱面理排列，但其几乎未发生粒内形变，可能是由于形变温度较低 (< 450o)，或者是由于糜棱岩的应变主要集中在石英和云母域，而相对强硬的磷灰石并未参与变形，其CL图像和部分主量元素分布图显示的成分变化可能是原生的，这预示着较低变质变形温度可能并未导致磷灰石中元素的快速扩散；(3) 初步的年龄结果显示麻粒岩中磷灰石记录了多期年龄，指示继承的磷灰石发生元素再平衡和晚期流体对磷灰石同位素体系的改造，而长英质糜棱岩中磷灰石则是在糜棱岩化过程中流体不断活动致使磷灰石不断结晶的结果。这些重要的认识为变质变形岩石中磷灰石年龄的解释提供了重要的信息。
The metamorphism and deformation of rocks can be well recorded by the structures and element distribution of accessory minerals. Especially, the metamorphic/deformation history can be revealed and analysized by in-situ chronologic dating. Apatite is a common accessory mineral in metamorphic rocks. With the rapid development of in-situ dating technology of apatite, it has become an important dating mineral in magmatic rocks and mineral deposit studies. In different metamorphic-deformation processes, a series of important problems about the deformation mechanism and behavior of apatite and its restriction effect on the element diffusion process have not been solved. In this study, rocks in the Eastern Himalayan syntaxis, which has the strongest tectonic deformation and metamorphism, are selected for research. SEM and CL observations show that the apatite in granulite does not show obvious compositional structure, while the apatite in mylonite shows obvious changes in light and dark, that is, composition changes. By statistical analysis of apatite distribution characteristics in thin sections, we found that the long axis direction of apatite in the two metamorphic rocks is approximately parallel to the main foliation. EBSD fabric analysis shows that the apatite in granulite shows obvious intra-granular deformation, while the apatite in mylonite shows almost no intra-granular deformation. The results of EPMA scanning of apatite composition show that the distribution of major elements of apatite in granulite is relatively uniform, while the distribution of Si elements of apatite in mylonite is obviously zonal or uneven, which is consistent with the results of CL scanning. Based on the above apatite deformation and element distribution characteristics, it can be preliminarily concluded that: (1) Although apatite in granulite is directionally distributed parallel to main foliation and occurred intra-granular deformation, it may be due to relative higher metamorphic temperature, which promotes the rapid diffusion of elements and makes the redistribution of elements tend to be uniform. Moreover, the late fluid action occurs element metasomatism along the low-angle grain boundary or near the fracture which are always perpendicular to the maximum tensile stress. (2) The apatite in mylonite is arranged parallel to the mylonite foliation, but almost no intra-granular deformation occurs, possibly due to the low deformation temperature (< 450o), or because the strain of mylonite is mainly concentrated in the quartz and mica domains, while the relatively tough apatite does not participate in deformation. The composition changes shown in the CL image and some major elements distribution maps may be indicating that the low metamorphic/deformation temperature have not led to the rapid diffusion of elements in apatite. (3) The preliminary age results show that the apatite in the granulite records multiple ages, indicating that the inherited apatite occurs in the element redistribution during metamorphism and late stage metasomatize of the apatite which has influence mineral isotope system, while the apatite in the felsic mylonite is the result of continuous fluid activity during the mylonitization process resulting in continuous crystallization of apatite. New results provide important information for the interpretation of apatite age in metamorphic and deformation rocks.