Does AMS constrain granite emplacement fabrics? Evidence of decoupling between AMS and K-feldspar fabrics during post-emplacement crystallization of interstitial melt.

HR: 14:00h
AN: GP73A-01 INVITED [Abstracts]
TI: Does AMS constrain granite emplacement fabrics? Evidence of decoupling between AMS and K-feldspar fabrics during post-emplacement crystallization of interstitial melt.
AU: * Kratinova, Z
EM: kratinova@ig.cas.cz
AF: Geophysical Institute, Czech Academy of Sciences, Bocni II/1401, Praha, 14131, Czech Republic
AU: * Kratinova, Z
EM: kratinova@ig.cas.cz
AF: Centro de Geofisica da Universidade de Lisboa (IDL), Campo Grande, Edifício C8, Lisboa, 1749 - 016, Portugal
AU: Jezek, J
EM: jezek@natur.cuni.cz
AF: Institute of Applied Mathematics and Computer Sciences, Charles University, Faculty of Science, Prague, Albertov 6, Praha, 12843, Czech Republic
AU: Schulmann, K
EM: schulman@illite.u-strasbg.fr
AF: Centre de Géochimie de la Surface, EOST, UMR 7516 Université Louis Pasteur, Rue Blessig 1, Strasbourg-cedex, 67084, France
AU: Hrouda, F
EM: fhrouda@agico.cz
AF: Institute of Petrology and Structural Geology, Charles University, Prague, Albertov 6, Praha, 12843, Czech Republic
AU: Hrouda, F
EM: fhrouda@agico.cz
AF: AGICO Inc., Jecna 29a, Brno, 621 00, Czech Republic
AU: Lexa, O
EM: lexa@natur.cuni.cz
AF: Institute of Petrology and Structural Geology, Charles University, Prague, Albertov 6, Praha, 12843, Czech Republic
AU: Shail, R
EM: R.K.Shail@ex.ac.uk
AF: Camborne School of Mines, School of Geography, Archaeology and Earth Resources, University of Exeter, Cornwall Campus,Cornwall, Penryn, TR10 9EZ, United Kingdom
AB: A multistage model for the origin of magmatic fabrics in porphyritic granites is proposed. It is based on a comparative study of the anisotropy of magnetic susceptibility (AMS), preferred orientation of large K-feldspar phenocrysts and quantitative microstructure analysis in the late Variscan Land's End Granite (Cornwall, UK). There is a decoupling between the AMS method and the feldspar phenocryst fabric indicating that each fabric reflects different parts of the magma cooling history and related stress field. The AMS method has a high sensitivity to post-emplacement regional tectonic overprinting while the feldspar phenocryst fabric records magma emplacement and has a low sensitivity to post-emplacement deformation. The post-emplacement crystallization of interstitial melt and re-orientation of groundmass micas significantly modifies the AMS fabric and reduces its 'memory' of emplacement-related processes. It may explain the commonly observed low intensity of granite AMS fabrics. Fabric features, intensity and symmetries are evaluated with respect to different scales of observation and sample location. The general agreement between AMS and K-feldspar fabrics at the pluton scale contrasts with important discrepancies in fabric intensity and mutual orientations at outcrop scale. The AMS fabric is predominantly homogeneous and stable at pluton scale while the K-feldspar shows a complex fabric pattern, characterized by meter-scale variations in orientation, symmetry and intensity. We suggest that the AMS fabric is controlled by late regional deformation, overprinting the partially crystallized and weakly anisotropic matrix. In contrast, the K-feldspar fabric reflects heterogeneous flow of the phenocryst-rich magma, mainly related to magma chamber construction and pluton emplacement. The model of fabric development associated with a pure-shear overprint on a variable intensive vertical fabric was confirmed by numerical modeling.
DE: 1518 Magnetic fabrics and anisotropy
DE: 8030 Microstructures
DE: 8035 Pluton emplacement
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2009 Joint Assembly