关于巴罗式变质带的最新研究及其对研究秦岭杂岩的意义

本文对苏格兰高地Grampian造山运动中产生的经典巴罗变质带进行了讨论,重点介绍了近年来关于其变质作用的特征及发生机制认识方面的重大变化。强调巴罗变质作用仅发生在整个造山运动过程中的部分时段,纪录了短暂的加热过程,时空上与区域内大规模的双峰式岩浆活动有关。经典的巴罗变质系列并非形成于地壳增厚及其热弛豫,而是代表了中地壳内大规模的接触变质作用。参考经典地区的变质特征,对发育巴罗式变质作用的秦岭杂岩进行了初步对比分析,指出其中与夕线石有关的变质P-T-t轨迹可能是等压冷却过程,而不是等温降压的顺时针演化模式。秦岭杂岩的变质作用时间仍需要进一步准确厘定。结合其它相关地质特征的分析和对比,得出如下推论:巴罗式变质带发育地区的变质作用、混合岩与花岗岩均属于同一动力系统作用的结果,花岗质岩体不是变质作用的原因;基性岩浆可能提供了巴罗式变质作用发生所需要的热能,但不是必要条件;巴罗式变质作用可以发生在正常地壳厚度情况下,不是碰撞构造的标志;伴随热异常的变形过程中产生了变质带的梯度分布(巴罗式变质带)和一些花岗岩,形成类似于底辟的冲起构造。

Recent progress on Barrovian metamorphism and its implication in studying the Qinling Complex, China

This paper has discussed the classical Barrovian metamorphism in the Grampian Orogeny, Scotland and focused on the recent progress on its driving mechanism. It is stressed that the Barrovian metamorphism occurred only during the transient, discrete period (~8Myr) of the whole orogeny evolutionary history (~27Myr). With temporal and spatial association with Grampian bimodal magmatism, the Barrovian metamorphic series is formed within the middle crust as the result of advection of heat from the lower crust and/or mantle. Thus, the conventional orogenic regional metamorphism as demonstrated by the classical Barrovian metamorphic series, was not produced through crustal thickening or thermal relaxation, instead, the typical metamorphism probably record large-scale contact metamorphism in the depth. Taking the Barrovian type as a reference, the metamorphism feature of the Qinling Complex of the Central Orogenic Belt of China is rather an analogue of the Barrovian metamorphism, during which the metamorphism P-T-t path in association with sillimanite and kyanite is possibly isobaric cooling or count clockwise, not isothermal decompression (clockwise) as generally thought. Up to now, the metamorphism time of the complex is ambiguous and more researches are needed. Through summary of some terranes with Barrovian features, we can deduce that metamorphism, migmatization and granitization involved possibly resulted from the same dynamic system and the granite is not the impetus for metamorphism. The basic magmatism is even not essential for Barrovian metamorphism which can not be taken as the symbol for collision orogeny. An alternative scenario is that the Barrovian series occurred under normal crustal thickness in which thermal gradient and some granite formed in anomalous heat flow during local deformation, such as pop-up structure.