The Effect of Melt Infiltration on Metagranitic Rocks: the Snieznik Dome, Bohemian Massif

Abstract

Highly deformed banded phengite-biotite metagranite from the Snieznik dome in the Bohemian Massif has been modified locally to have stromatic, schlieren or nebulitic textures typical of migmatites. This occurred mostly along subvertical deformation zones at eclogitefacies conditions, at a scale of several centimetres to several metres, mostly parallel to the foliation. The transition from banded to migmatite types of orthogneiss is marked by an increase in the amount of phases interstitial along grain boundaries in the dynamically recrystallized monomineralic feldspar and quartz aggregates, and by increasing consumption of recrystallized K-feldspar grains by fine-grained plagioclase and quartz, as well as myrmekite (intergrowth of Pl_Qz). The new minerals are in textural equilibrium with phengite. The myrmekite, quartz and feldspars can be coarse-grained (grain size 0.5 cm).These features are considered to be the result of grain-scale melt infiltration that caused dissolution-reprecipitation along grain boundaries in the presence of phengite. The infiltration was pervasive at the grain scale, but localized at hand-specimen to outcrop scales.All the rock types have the same mineral assemblage of Grt_Ph_Bt_Ttn_Kfs_Pl_Qz6Rt6Ilm; they have similar garnet, phengite and biotite compositions, and based on mineral equilibria modelling we infer equilibration at a pressure of 1.5-1.7 GPa and a temperature of 690-740

Syn-deformational melt percolation through a high-pressure orthogneiss and the exhumation of a subducted continental wedge (Orlica-Śnieżnik Dome, NE Bohemian Massif)

“…In order to put valorisation to the field observations, we make here some basic interpretation of the observed textural transitions (for similar interpretation see Hasalová et al 2008a,b,c;Schulmann et al 2008;Chopin et al 2012b;Štípská et al 2019). From the gradual transitions of the rock types in the field, it is concluded that the banded type II, schlieren and nebulitic orthogneiss types resulted from progressive disappearance of the originally monomineral augen/banded texture typical of the augen/banded type I orthogneiss.…”

“…From the observed textural gradients occurring parallel to the S2 foliation and ranging from the banded type II to schlieren and nebulitic orthogneiss types, it is concluded that these transformations occurred mainly during the D2 deformation. As the schlieren and nebulitic textures are commonly assigned to migmatites (Mehnert, 1971), it is supposed that the observed textural transitions are a result of transformation under the presence of melt (for similar interpretation see also Hasalová et al 2008a,b,c;Schulmann et al 2008;Chopin et al 2012b;Závada et al 2018;Štípská et al 2019).…”

“…4a-d). The nucleation of interstitial phases is typically explained by crystallization from melt (for identical interpretation of similar rock sequence see also Hasalová et al 2008a,b,c;Sc ulmann et al 2008;C opin et al 2012b;Štípská et al 2019).…”

“…Lexa et al 2006;Hasalová et al 2008a). Likely mechanism for crystallization of albite and other phases from melt, while the melt content of the rock increases, is also a process of dissolution-precipitation of these phases promoted by the presence of melt and possibly also by crystallization from melt (Sawyer, 2001;Hasalová et al 2008b;Holness and Sawyer, 2008;Závada et al 2018Závada et al , Štípská et al 2019. The observed mineral assemblage q-pl-ksp-g-bi-wm-ilm with supposed former melt in all rock types together with rim compositions of garnet (alm 65-70 ; grs 5-10 ; sps 20-25 ; Fig.…”

“…Petrological studies characterizing role of melt for vertical transfer of rocks through crust are scarce, but the existing studies indicate that chemistry of crystallized interstitial melt and compositional variations of minerals can record exhumation of rocks of the order of ~10-20 kilometers (Hasalová et al 2008b;Štípská et al 2019;Nahodilová et al 2020). In addition, these studies indicate that dynamically moving melt plays a major role in exhuming large portions of continental crust.…”