汉诺坝新生代玄武岩中下地壳麻粒岩包体的变形显微构造特征及其地质意义

Deformation microstructures of lower crustal granulite xenoliths from Hannuoba Cenozoic basalts,China and their geologic implications.

新生代玄武岩中的下地壳包体,由于从下地壳被快速携带至地表,因此保留了下地壳的直接信息。华北北部汉诺坝新生代玄武岩中除了含有丰富的幔源包体之外,还含有许多下地壳麻粒岩包体。本文的主要目的是通过对该区下地壳麻粒岩包体的变形显微构造和位错亚构造特征的详细研究,探讨下地壳的变形特征和变形机制。光学显微镜下观测表明,下地壳麻粒岩包体的低温(<800℃)样品中确实发育显微破裂,但变形双晶、变形条带、扭折带也同样发育,动态重结晶作用也开始出现。随着温度、压力的升高,变形双晶、变形条带、变形纹、扭折带和重结晶新晶粒等塑性变形特征占主导地位,而显微破裂则主要表现为由塑性失配引起的显微破裂以及流体包裹体面。而明显不同于Ivrea带地体麻粒岩,在这些包体中未发现与韧性剪切有关的变形显微构造特征。透射电镜观测表明,包体中的斜长石和辉石颗粒普遍发育自由位错、位错列、亚晶界、新晶界、变形双晶、包裹体列和出溶片晶等位错亚构造。上述观测结果表明,下地壳变形作用以塑性变形为主而不是准脆性变形,其变形机制主要为位错的滑移和攀移机制,其中包括机械双晶作用和动态重结晶作用。

The lower crustal xenoliths brought up rapidly by Cenozoic basalts onto the earth surface may provide direct information about the deformation features and mechanism of the lower crust.The Hannuoba Cenozoic basalts in Hebei Province,North China, contain not only abundant mantle xenoliths,but also lower crustal xenoliths of granulite compositions.The main purpose of this study is to get an insight into the deformation process and mechanism of the lower crust through the microstructural observation of lower crustal xenoliths collected from this region.Optical microscopic observation reveals that mierocracking do occur in low-temperature(<800℃) xenolith samples,accompanied by the existence of deformation bands,kink bands,deformation twinning and the initiation of dynamically reerystallized new grains.With increasing temperature and pressure,deformation microstructures representative of intracrystalline plasticity,such as undulatory extinction,deformation twins,intraerystalline deformation bands,kink bands, deformation lamellae and new grains become predominant,while the development of microcraeks are mostly caused by plastic mismatch.However,microstruetures representative of ductile shear zone in Ivrea granulite terrain are not found in these lower crustal xenoliths.Transmission Electronic Microscopic(TEM)observation reveals that free dislocations,dislocation array,sub-grain boundary,sub-grains,new grain boundary,deformation twins,and fluid inclusion array are widely developed in plagioclase and pyroxene grains of the xenoliths.All these results indicate that the deformation process in the lower crust of the studied area is dominated by plastic deformation rather than semi-brittle regime.The predominant deformation mechanism is intracrystalline plasticity represented by dislocation glide and dislocation climb including mechanical twining and dynamic reerystallization by grain boundary migration and subgrain rotation.