Microstructural changes during melt-assisted modification of quartzofeldspatic rocks. An example from the Eger Complex, North-Western Bohemian Massif

Abstract

The Eger Complex is situated in the Saxothuringian domain at the western margin of Bohemian Massif (Czech Republic). Migmatitic orthogneisses associated with granofelses and high-pressure felsic granulites make up the majority of the complex, which is interpreted as an upper crystalline nappe exhumed from underneath the fore-arc Teplá-Barrandian domain during the Variscan orogeny. Studies conducted in the Eger Complex suggested rapid exhumation and cooling after a static heating event from temperatures of ~760° estimated for the granofelses to ~850°C estimated for the granulites at isobaric conditions of ~16 kbar. The heating event led to substantial modification of microstructure in the granitoid rocks of the Eger Complex and the processes responsible for these changes are the focus of investigation in this work. There is a progressive change from a banded orthogneiss consisting of monomineralic layers of recrystallized K-feldspar, plagioclase and quartz to a macroscopically equigranular microstructure observed in the granulite. Such microstructural change is studied in four samples representing the two end-member microstructures and two intermediate microstructural stages represented by a migmatitic orthogneiss and a granoblastic granofels. Microstructural changes leading to the granulitization have been quantified and described through manual digitization and subsequent statistical analysis of rock microstructures coupled with analysis of crystallographic preferred orientations and both macroscopic and microscopic observations. Grain size analysis of K-feldspar, plagioclase and quartz suggests that the largest change in microstructure occurs at the beginning of anatexis when the strength of the aggregate distribution and the crystallographic preferred orientations are significantly reduced. This change is attributed to melt crystallization. Statistical evaluation of the transition from the migmatitic orthogneiss towards the granofels suggests considerable ripening of the microstructure. In the granulite, the temperature-increase from upper amphibolite to granulite facies resulted in increased melting and subsequent crystallization leading to an almost complete homogenization of the microstructure. Previously estimated melt proportions of ~0-8.5 % are considered insufficient to completely rework the originally strongly anisotropic fabric and a model of cyclic melt infiltration is proposed as the most likely mechanism for the destruction of the original rock fabric.