贺兰山高压与超高温两类麻粒岩的变质PTt轨迹及构造意义

The Helanshan may be the only place in the Khondalite belt where both high-pressure (HP) and ultrahigh-temperature (UHT) pelitic granulites, beside of the medium pressure (MP) pelitic granulites and mafic granulites, have been discovered. Thus, it is a natural laboratory to study the thermodynamic conditions and tectonic setting required to form both the HP and UHT pelitic granulites. In this study, we intend to conduct the combined study of petrology, whole-rock element and isotope geochemistry, and geochronology on these different types of granulites, particularly the HP and UHT pelitic granulites. The P–T–t paths will be constructed based on the important replacement and reaction textures, the T–X、P–X and P–T pseudosections calculated by Thermocalc software, and zircon and monazite datings. Based on these results, we will determined the relationship between the P–T paths and origins of the HP and UHT pelitic granulites, and the thermodynamic conditions for their formations. In addition, we will determine the tectonic setting of the granulite-facies metamorphisms in the Helanshan and their tectonic implications for the orogenic evolution of Khondalite belt. And then the plate tectonic and the geodynamic process of continent-collisional orogeny during the Paleoproterozoic era will be discussed.

贺兰山是孔兹岩带同时发育高压(HP)和超高温(UHT)泥质麻粒岩的地区，此外还发育中压(MP)泥质麻粒岩及基性麻粒岩，是研究HP和UHT两类泥质麻粒岩变质作用热力学条件和构造背景的理想场所。本项目拟以HP和UHT泥质麻粒岩为重点研究对象，通过对HP和UHT泥质麻粒岩、MP泥质麻粒岩及基性麻粒岩细致的岩相学观察，锆石和独居石年代学研究，结合NCKFMASHTO(+ZrO2)体系下运用Thermocalc软件进行T–X、P–X和P–T视剖面图模拟，确定各类麻粒岩的PT轨迹与时代，构建它们的PTt轨迹；对基性麻粒岩进行地球化学研究确定它们的原岩成因。在此基础上，确定HP和UHT泥质麻粒岩的PT轨迹关系和成因关系，阐明在同一地区发生HP和UHT变质作用的热力学条件和构造背景；揭示贺兰山麻粒岩相变质作用的地质背景及对孔兹岩带造山演化的构造意义。进而探讨古元古代板块构造和碰撞造山的地球动力学过程。

本项目对贺兰山麻粒岩相变泥质岩进行了深入的岩相学观察、变质相平衡模拟、SHRIMP和LA–ICP–MS锆石/独居石U-Pb年代学、TIMA矿物成分分析以及石榴子石微量元素分析研究。含尖晶石石榴矽线石片麻岩峰期矿物组合稳定的温压条件为960–1030°C和6.3–7.3kbar，与三元长石温度计得到的结果吻合。因此，含尖晶石石榴矽线石片麻岩经历了超高温变质作用，其顺时针的PT演化轨迹以温度峰期前的降压和之后的近等压降温为特征。高压泥质麻粒岩样品在峰期变质M2-1阶段位于蓝晶石稳定域，对应的温压条件为794–815°C/9.8–10.6kbar；而M2-2阶段位于矽线石稳定域，对应的温压条件为804–823°C/8.9–10.0kbar。中压泥质麻粒岩记录了从∼10.1kbar/∼830°C到∼5.8kbar/∼800°C的近等温降压过程，之后经历近等压降温过程。SHRIMP独居石U–Pb测年揭示中压麻粒岩的变质时代为1944.4±4.2Ma，这与锆石U–Pb年龄1959±18Ma在误差范围内一致。SHRIMP独居石U–Pb测年揭示超高温变质作用的时代为1930.8±2.6和1933.6±3.1Ma，指示贺兰山超高温变质作用与集宁地区超高温变质作用为同时代的，因此认为整个孔兹岩带都经历了大约1.93Ga的超高温变质作用。LA-ICP-MS独居石U–Pb年代学研究显示中压/高压泥质麻粒岩样品的变质年龄为1940–1920Ma。LA-ICP-MS独居石U–Pb分析显示含尖晶石超高温麻粒岩样品记录了两组变质年龄，分别为1940–1920Ma和~1870Ma，前一组年龄与SHRIMP独居石U-Pb年龄结果相似，而1870Ma年龄可能代表降温至固相线的时代。中压和高压麻粒相变质作用与阴山和鄂尔多斯地块在大约1.95 Ga碰撞有关，而超高温麻粒岩相变质作用与后碰撞伸展有关，在大约1.95Ga到1.93Ga之间贺兰山经历了由碰撞到伸展的转变过程。石榴子石记录了明显的生长环带，核部微量元素记录的温压条件为4.0–4.5kbar/518–534°C，指示古元古代碰撞造山沉积物埋深的地温梯度明显高于现今碰撞造山带。贺兰山闪长岩来源于地幔岩浆作用，结晶年龄为1938–1931Ma，该期地幔岩浆作用可能为超高温变质作用提供了热源。