The relevance of vein texture in understanding the past hydraulic behaviour of a crystalline rock mass: reconstruction of the palaeohydrology of the Mecsekalja Zone, south Hungary

Abstract

This study reconstructs the palaeohydrogeologic evolution of the shallow-to-moderate Mesozoic subsidence history for the Mecsekalja Zone (MZ), a narrow metamorphic belt in the eastern Mecsek Mountains, Hungary. Brittle deformation of the MZ produced a vein system with a cement history consisting of five sequential carbonate generations and one quartz phase. Vein textures suggest different fluid-flow mechanisms for the parent fluids of subsequent cement generations. Combined microthermometric and stable-isotope measurements permit reconstruction of the character of subsequent fluid generations with different flow types, as defined by vein textures, yielding new information regarding the hydraulic behaviour of a metamorphic crystalline complex. Textural observations and geochemical data suggest that fracture-controlled flow pathways and externally derived fluids were typical of some flow events, while percolation through the rock matrix and the relationship to the Cretaceous volcanism and dyke emplacement were typical of others. The difference in the mode of calcite deposition from pervasive fluids (i.e. pervasive carbonatisation along grain boundaries versus deposition in antitaxial veins) between two calcite generations related to the volcanism inspired a stress-dependent model of antitaxial vein growth. Textural and isotope variations in a vein generation produced by the same parent fluid indicate rock-dependent hydraulic behaviour for different rock types, distinct action of the contemporaneous fracture systems and different extents of fluid-rock interaction. Cathodoluminescence microscopy and fluid-inclusion microthermometry shed light on the possible role of hydraulic fracturing in the formation of massive calcite. The time of formation was estimated from the isotope composition of the oldest calcite generation and its presumptive relationship with the sedimentary sequences to the north, whereas microthermometry permitted conciliation of the reconstructed flow sequence with the Mesozoic subsidence history of the Mó rá gy Block (including the MZ).

A BAF–2 fúrás töréses szerkezeti elemeinek mikroszerkezeti és cementáció vizsgálata (Bodai Agyagkő Formáció)

“…Mindemellett azonban nem zárhatóak ki a triász-jura passzív kontinensperemi helyzet kapcsán fennálló folyamatos süllyedés és tektonikai tagolódás, valamint a Tiszai-főegység európai kontinensről történő leválásának és a mecseki terület azt követő gyors süllyedésének hatásai sem, amelyet a Mecsekalja-övben kimutatott, feltehetően triász és jura korú, tektonikus erek igazolhatnak (DABI et al 2011(DABI et al , 2017. A krétában végbement deformáció során létre -jöttek a Mecsek szerkezetének jellemző, ÉK-DNy-i tengelyű gyűrődései (NÉMEDI-VARGA 1983), továbbá a redő -tengellyel megegyező csapású balos eltolódások (MAROS et al 2004), mindemellett a BAF-ban rétegpárhuzamos ráto -lódások keletkeztek (MAROS et al 2000).…”

Post-metamorphic palaeofluid evolution of the Mecsekalja Zone metamorphic complex

“…4.4), the most depleted compositions of which are concurrent with the stable isotope compositions of dyke ocelli from the Eastern Mecsek volcanic dykes (i.e., primary carbonate in the magma), published by Demény and Harangi (1996). The trend points towards the compositions of the first vein-filling calcite phase (Cal EB 1, Dabi et al, 2011, where EB stands for elongated blocky texture), which is the most widespread occurring carbonate phase in the northern part Demény and Harangi (1996) and vein-filling calcite generations in the study area, published by Dabi et al (2011), are also displayed.…”

“…The same fluid produced a vein generation with syntaxial vein geometries, the stable isotope composition of which suggest different flow paths and degrees of fluid-rock interactions from those of the crack-seal veins (Fig. 3.1A, Dabi et al, 2011). …”

“…The gneissic rocks of the MZ are cross cut by a generation of antitaxial crack-seal calcite 69 veins, the parent fluid of which is related to the Early Cretaceous dyke emplacement (Dabi et al, 2011). The same fluid produced a vein generation with syntaxial vein geometries, the stable isotope composition of which suggest different flow paths and degrees of fluid-rock interactions from those of the crack-seal veins (Fig.…”

“…Earlier studies on rocks of the Mórágy Granite to the south have described pervasive carbonatisation of the granitoid rocks and proposed its "direct relationship with the process of the Cretaceous volcanism, which triggered the explosion-like escape of the volatile components and induced an intensive carbonatisation" (Balla et al, 2009) cloud-like clusters, the discussed fluid was interpreted to be primary, i.e., it was interpreted to be the parent fluid of the abovementioned calcite generation and also, the microcrack calcite was assumed to be identical to the vein calcite (Dabi et al, 2011). As Cal SF calcites have distinct stable isotope compositions (Fig.…”