Multiple magmatic fabrics in plutons: an overlooked tool for exploring interactions between magmatic processes and regional deformation?

Abstract

This paper elaborates on the concept of multiple magmatic fabrics in plutons. After a general overview of various types of multiple fabrics that may develop in magmatic rocks, two case examples of porphyritic granite and melasyenite plutons in the Bohemian Massif are examined. In the Jizera granite, complex variations in K-feldspar phenocryst shape-fabric revealed by image analysis of a 200 m long section of an underground tunnel are in contrast with homogeneously oriented magnetic (AMS) fabric carried by coaxial contributions of biotite, magnetite and maghemite. In the Knížecí Stolec melasyenite pluton, emplacement-related margin-parallel feldspar foliation was overprinted by flat-lying foliation; the latter is interpreted to record regional tectonic strain. At the two stations examined in detail, the crystallographic-preferred orientation of biotite and amphibole in the inter-phenocryst matrix (measured using electron back-scatter diffraction – EBSD) differed from both feldspar fabric and also from the AMS principal directions. Multiple magmatic fabrics in these two plutons are interpreted in terms of fabric superposition, where late weak strain is superposed onto a high-strength phenocryst framework, but is accommodated preferentially by small mineral grains (biotite, magnetite) in the melt-bearing matrix. This mechanism may explain the discrepancy between mesoscopic feldspar fabric and AMS. We conclude that multiple magmatic fabrics in plutons may thus result from accumulated strain caused by different processes during final crystallization and, as such, may serve as a sensitive indicator of the evolving interactions between magmatic and tectonic processes in the Earth's crust.

Noncoaxial K‐feldspar and AMS subfabrics in the Land's End granite, Cornwall: Evidence of magmatic fabric decoupling during late deformation and matrix crystallization

“…Other detailed field‐based structural studies have indicated that plutonic fabrics result from internal magmatic processes such as magma pulses and surges driven by contrasts in gravitational potential within magma chambers [ Miller and Paterson , 1999, 2001]. These studies have highlighted the complexity of magma chamber construction, whereby material transfer processes may result in the development of multiple superposed magmatic fabrics [e.g., Žák et al , 2005, 2008]. It has also been recently demonstrated that, in some cases, late and often weak fabrics crosscut boundaries between magmatic units and result from late deformation overprints that are not causally related to magma emplacement processes [ Žák et al , 2007, 2008].…”

“…These studies have highlighted the complexity of magma chamber construction, whereby material transfer processes may result in the development of multiple superposed magmatic fabrics [e.g., Žák et al , 2005, 2008]. It has also been recently demonstrated that, in some cases, late and often weak fabrics crosscut boundaries between magmatic units and result from late deformation overprints that are not causally related to magma emplacement processes [ Žák et al , 2007, 2008].…”

Microstructure Features in Paleo and Neoproterozoic Granitic Rocks, Southeastern Region of Brazil

“…Feldspar features may represent key ways to better understand the magmatic and post-magmatic features [18][19][20][21][22], among which may be highlighted the different zoning patterns, exsolution, corrosion features, and those regarding deformation processes. Even a normal zoning, with grains showing Ca-rich cores and Na-rich rims, can show a more complex magmatic process when the Na enrichment from the core is not progressive, and there is an abrupt transition to a Na-rich boundary (Figure 2A).…”

Fabric transpositions in granite plutons — An insight from non-scaled analogue modelling

“…Bouchez et al 1990;Benn et al 2001;Kratinová et al 2007;ák et al 2008). These fabrics, usually interpreted in terms of internal magmatic processes, illustrate the complexity of flow in large magma chambers and represent processes, which cannot be easily constrained by field observations.…”