高温-超高温变质作用成因研究——来自华北克拉通西部孔兹岩带和南非Kaapvaal克拉通西南部Namaqua活动带与Bushveld变质杂岩体的启示

超高温变质作用是在变质地质学领域,继超高压变质作用研究高峰之后的又一重要前缘课题,对于认识地壳构造-热演化具有重要意义。本文总结了华北克拉通西部孔兹岩带和南非Kaapvaal克拉通西南部Namaqua活动带与Bushveld变质杂岩体的高温-超高温麻粒岩的化学成分、矿物组合、变质演化特征,及其相应的变质事件与构造属性。我国的超高温变质作用带,包括华北克拉通西部的孔兹岩带——从内蒙西段的大青山到东段的集宁-凉城地区的超高温变质岩,皆为Al-Mg质和Al饱和体系的超高温变质岩石,常见假蓝宝石+石英、尖晶石+石英的典型超高温变质组合,以及含假蓝宝石±尖晶石、但缺少石英的非典型超高温变质组合。南非Namaqua活动带与Bushveld变质杂岩体分别发现有独特的Fe-Al饱和的铁尖晶石+石英+大隅石、刚玉+高温石英等超高温矿物组合,罕见的高温硼硅酸盐和硅硼铝镁石等超高温矿物组合;以及Ca-Mg质饱和的钙镁橄榄石+镁硅钙石镁黄长石+镁橄榄石等超高温矿物组合的麻粒岩。研究的核心问题是矿物和岩石在高温-超高温条件下的特殊行为方式,不同构造环境和岩石化学成分下的变质反应及其热动力学过程。由此提出超高温变质作用成因研究中的科学问题:包括不同类型和地质属性的高温-超高温麻粒岩的成因特征;麻粒岩的形成条件演化过程和构造背景;高温-超高温变质过程中部分熔融和重新水化过程中流体的作用以及岩体形变过程中的部分熔融;变质反应以及变质作用P-T-t轨迹、元素地球化学和熔体作用行为;岩石保留的可能的变质事件和年代学记录,定量评价高温-超高温过程中变质演化的时间跨度和演化速率。

Probe into the genesis of high temperature-ultrahigh temperature metamorphism: The enlightenment from the Western Khondalite Belt of the North China Craton and the Namaqua mobile belt and the Bushveld metamorphic complex of South Africa

In recent years there have been increased efforts to understand mineral equlibria at very high temperatures, to quantify pressure and temperature in such rocks and to model geological environments that can create such temperatures. The high temperature (HT)-ultrahigh temperature (UHT) metamorphism is another important frontier subject in the field of metamorphic geology, following the unprecedented upsurge in the study of ultrahigh pressure metamorphism. This paper summarizes the chemical composition, mineral assemblage, metamorphic evolution characteristics, and the tectonic background of the HT-UHT granulites from the Khondalite Belt in the west of the North China Craton (NCC) and the Bushveld metamorphic complex in the Namaqua active belt in the southwest of the Kaapvaal Craton in South Africa. The HT-UHT metamorphic rocks of the Western Khondalite Belt of the NCC in Inner Mangolia, including UHT metamorphic rocks from the western side Daqingshan to the eastern side Jining-Liangcheng, are all Al-Mg and Al saturated HT-UHT granulites with common occurred mineral assemblages of sapphirine+quartz, spinel+quartz, and quartz absent sapphirine±spinel. On the other hand, rare occurred HT-UHT mineral assemblages of hercynite+quartz+osumilite, corundum+quartz granulites and borosilicates-bearing mineral assemblage such as kornerupine+sapphirine in the Namaqualand-Natal Mobile Belt adjacent to Kaapvaal Craton, which are Fe-Al saturated HT-UHT granulites. There are also Ca-Mg saturated UHT granulites, such as that with mineral assemblage of monticellite+merwinite+akermanite+forsterite in the Bushveld metamorphic Complex of South Africa. The core research question is in what particular way rocks and minerals behave at high to ultrahigh temperatures, with emphasis on mineral reactions in the broader sense, from seemingly closed-system phase equlibria to metasomatic reactions across bulk-compositional discontinuities. The scientific issues on the genesis study of hyperthermal metamorphism, therefore, are proposed in this paper:they include genetic features and geological characteristics of different HT-UHT rock types; the forming condition, tectonic setting and evolution processes of the HT-UHT granulites; the role of fluids in terms of triggering melting or rehydration, and the controls of fluids on mineral equilibria; gain of insight into the effects of partial melting, including the leucosome-melanosome separation process; the effect of melt being presented during deformation; application of appropriate high-T thermometers and examine the consistency between different thermobarometric methods; and to characterize the various types of high-T reaction textures observed and consider implications for reaction kinetics.