华北克拉通孔兹岩带东段超高温变质作用与多阶段部分熔融作用关系研究

Tuguiwula area, situated at the east part of Khondalite belt, North China Craton, exposed typical Mg-Al-rich pelitic ultrahigh temperature granulite, experienced intensive anatexis, and associated with synchronous Grt-bearing meta-granite, gabbronorite. This application intends to take an integrated study on the ultrahigh temperature granulite and related leucosomes and gabbronorite to explore the relationship between ultrahigh temperature metamorphism and anatexis. According to detailed regional geological survey, we will clarify the contact relationship among various rocks. Based on the HREE pattern between zircon and garnet, the genesis of garnet from various stages of leucosome will be determined, then we will clarify the metamorphic stages that melt formation. By the means of phase equilibria modeling and trace element thermobarometer, the difference and connection of the P-T paths between Grt-bearing meta-granite and ultrahigh temperature granulite will be understood. Then, SIMS isotopic geochronology will give us the specific ages of ultrahigh temperature metamorphism, crustal anatexis, mantle-derived mafic magmatism, and we will construct the temporal and spatial relationship between ultrahigh metamorphism and multiple stages of anatexis. On this basis, we will evaluate the heat contribution to ultrahigh temperature metamorphism from various type of the heat source, such as conductive heat, advective heat and radioactive heat. The geodynamic background of the ultrahigh temperature metamorphism in Tuguiwula area and the tectonic evolution of the Khondalite belt in the east part of North China Craton will be further understood.

华北克拉通孔兹岩带东段土贵乌拉地区出露典型的超高温麻粒岩。这些超高温麻粒岩普遍共生有多种类型的含石榴石花岗岩质熔体脉，并伴生有同时代的片麻状石榴石花岗岩和辉长苏长岩。该项目拟以超高温麻粒岩及各类熔体、辉长苏长岩作为研究对象，综合研究超高温变质过程与熔体的关系：通过野外地质调查，查明各地质体间的接触关系；通过锆石、石榴石微量元素配分型式判断各类熔体中石榴石成因，进而判断熔体生成时对应的变质作用阶段；基于变质作用研究，对比变质片麻状石榴石花岗岩与超高温麻粒岩变质作用P-T轨迹的区别和联系；通过同位素年代学研究，厘定超高温变质作用、地壳深熔作用、基性岩浆作用的具体时代，构建超高温变质作用过程与多阶段熔体的时空联系。在此基础上，尝试估算传导热、平流热和放射性生热等不同类型热源对该区超高温变质事件的相对贡献，探讨该区超高温变质作用的地球动力学背景，深化对华北克拉通孔兹岩带构造演化的认识。

麻粒岩相变质作用往往伴随着部分熔融。熔体的迁移、上升和侵位驱动着地壳内部的元素分异。这对大陆地壳演化和花岗岩成因研究有重要意义。在泥质岩石升温过程中，会先后经历白云母含水熔融、白云母脱水熔融和黑云母脱水熔融。由于熔融消耗的反应物不同，产生的熔体成分也不相同。我们对华北克拉通孔兹岩带东段右玉地区出露的花岗岩进行了详细的野外考察，基于岩石地球化学和相平衡模拟的研究，认为白云母含水熔融只能产生露头尺度的瓷白色花岗岩脉，具有较低的K/Na、FeO+MgO含量、Rb/Sr和Ba含量；白云母脱水熔融会形成区域上透入性的含钾长石斑晶花岗岩脉，具有较高的K/Na、Rb/Sr、Ba含量，和较低FeO+MgO含量；黑云母脱水熔融会形成富石榴子石花岗岩体，具有较高的K/Na、FeO+MgO含量、Rb/Sr和Ba含量。碰撞造山带中的花岗岩主要由含水矿物脱水熔融而来。.基于开放体系的相平衡模拟，变质泥质岩要经历至少三次熔体迁移才能达到超高温的温压条件，这一过程中会形成17–27 vol.%的S型花岗岩。熔体的迁移能够引起熔融残余体Mg和Al的升高，使得岩石在超高温条件形成假蓝宝石和石英的矿物组合。麻粒岩熔融残余体的Nb/Ta会随着温度的升高先升高再降低，而熔体的Nb/Ta会随着温度的升高持续升高。超高温条件下产生的熔体具有高于球粒陨石的Nb/Ta。变质泥质岩石的产热元素均会随着温度的升高而逐渐降低，继而导致岩石自身产热率的降低。变质泥质岩很难仅仅依靠自身的放射性产热来达到超高温的温度条件。超高温变质作用应是多种热机制共同作用的结果，这其中包括了地幔上升带来的平流热、周围地壳物质放射性生热以及韧性剪切作用带来的机械热。