华北孔兹岩带古元古代高温-超高温变质演化历史的限定：原位独居石定年和相平衡模拟

The extremely thermal high-temperature-ultrahigh temperature (HT-UHT) crustal metamorphism has a geothermal gradient much higher than 20 ºC/km. Its genesis and relationship with the modern plate tectonics is a window to understand the crustal formation and evolution. It is significant for the accretion, reworking and differentiation of the crust. In the Khondalite Belt of the North China Craton, recent studies have identified several outcrops of HT-UHT metamorphic rocks, especially the sapphirine-bearing rocks and confirmed that they were regionally well-exposed. However, these rocks have different metamorphic ages, thermal conditions and P-T paths, and thus their genetic connection remains obscure. The primary cause is that their prograde metamorphic evolution, accurate age and duration are unclear or unknown. In this project, two-feldspar thermometer, single-mineral trace element thermometer combined with phase equilibria modeling will be applied to confine the peak conditions and metamorphic evolutions of these HT-UHT rocks, especially focusing on the microtextures, which possibly record the prograde metamorphic stage, such as the microdomain where spinel is replaced by sapphirine. This project will also emphasize the texturally-controlled monazite isotopic and chemical dating to confine the stages, episodes, accurate age and duration of the HT-UHT metamorphism. On the basis of the above research, we will fully compare these outcrops of HT-UHT rocks, and discuss their genetic mechanism and tectonic model as well as the Paleoproterozoic tectono-thermal evolution of the Khondalite Belt of the North China Craton, in combination with the known regional geological evolution.

壳源尺度内地温梯度远大于20 ºC/km的高温-超高温变质岩的成因和与现代板块构造的关联，关系到地壳的增生、再造和分异过程，是探索地壳形成和演化的窗口。近年来的研究发现华北孔兹岩带内的高温-超高温变质岩呈“面”状发育在变泥质岩的背景中，以假蓝宝石的出现最为典型。然而，这些岩石具有不同的变质时代、温压条件和P-T轨迹，它们间的成因联系尚不明确。根本原因是进变质演化过程、准确的变质时代和时间尺度不清楚或未知。针对这些问题，本项目拟采用二长石温度计、单矿物微量元素温度计和相平衡模拟相结合的方法限定它们的峰期条件和变质演化过程，关注可能记录进变质过程的结构，如尖晶石向假蓝宝石的转化；利用原位独居石同位素和化学定年方法来限定高温-超高温变质作用的期次、阶段性、准确的时代和时间尺度；综合对比各地高温-超高温变质岩，结合区域地质过程，阐明它们的成因机制和构造模式及华北孔兹岩带在古元古代的构造热演化历史。

超高温（UHT）变质作用代表大陆地壳最热的状态，是探索陆壳形成和分异过程的窗口，但对其成因机制和动力学过程仍有很大的争论。华北克拉通孔兹岩带为古元古代造山带，发育区域性UHT变质作用，但对其准确的变质P-T-t轨迹尚有不同的认识，尤其是进变质过程、变质时代和时间尺度。本项目针对大青山地体典型的UHT麻粒岩露头（东坡地区）开展了综合的研究，包括岩石学、矿物学、热力学模拟、微量元素地球化学和同位素地球化学等，多角度地限定了高温-UHT变质作用的变质演化和时间尺度，论证了UHT变质作用的形成机制和动力学背景。主要研究进展是（1）完成了东坡地区大比例尺填图工作，确定了UHT麻粒岩产出的状态以及与周围地质体的空间关系；（2）识别出两种不同的假蓝宝石，模拟得到围绕尖晶石生长的假蓝宝石形成于UHT峰期后近等压冷却（IBC）过程，而后成合晶状假蓝宝石形成于随后的减压-升温过程；（3）识别出进变质部分熔融过程中形成的转熔型石榴石、峰期后冷却过程中的退变质型石榴石以及随后减压升温过程中石榴石的分解作用，尤其是发现石榴石中Zr和Ti含量的变化可以指示石榴石的形成过程。确定UHT峰期之前存在一期高温-高压变质阶段和减压升温的进变质过程，最终确定了UHT麻粒岩经历了复杂的顺时针型P-T轨迹；（4）识别出UHT变质过程中多阶段的独居石和锆石生长过程，限定了进变质部分熔融作用的时代为1.94-1.90 Ga，峰期后近等压冷却过程的时代为1.90-1.86 Ga，随后减压升温阶段发生在1.86-1.82 Ga以及最后冷却到~1.80 Ga。这些研究精准识别了长期热造山过程中存在多次快速的加热作用。（5）提出UHT变质作用两次加热过程，第一次是碰撞造山的构造叠加加热，第二次是碰撞后岩石圈伸展-软流圈上涌的加热过程。该项目的完成推进了我们对UHT变质作用形成机制的认识，丰富了地球早期的动力学演化过程。