Open AccessOn the reliability of AMS in ilmenite‐type granites: an insight from the Marimanha pluton, central Pyrenees
Author(s) -
OlivaUrcia B.,
Casas A. M.,
Ramón M. J.,
Leiss B.,
Mariani E.,
RománBerdiel T.
Publication year - 2012
Publication title -
geophysical journal international
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.302
H-Index - 168
eISSN - 1365-246X
pISSN - 0956-540X
DOI - 10.1111/j.1365-246x.2011.05355.x
Subject(s) - pluton , geology , electron backscatter diffraction , biotite , ilmenite , mineral , quartz , texture (cosmology) , anisotropy , mineralogy , orientation (vector space) , paramagnetism , geochemistry , materials science , composite material , geometry , condensed matter physics , metallurgy , microstructure , optics , seismology , mathematics , paleontology , image (mathematics) , physics , artificial intelligence , computer science , tectonics
SUMMARY The anisotropy of magnetic susceptibility (AMS) at room temperature has been used for decades to obtain the petrofabric orientation in granites as a kinematic marker to establish models explaining the emplacement of plutons. To assess the significance of AMS in terms of mineral orientation, we have performed a multidisciplinary study at five sites of an ilmenite‐type pluton (Marimanha, central Pyrenees) with significant facies changes. To test the reliability of AMS measurements at room temperature, the following methods were applied: low temperature AMS; image analyses and X‐ray texture goniometry (XTG) of biotites; and electron backscatter diffraction (EBSD) to obtain c ‐axes directions of quartz. The total (para‐, ferro‐ and dia‐)magnetic fabric analysed by AMS is compared with the paramagnetic fabric (low‐T AMS), mica orientation (with image analyses and XTG) and the diamagnetic fabric (EBSD). Results indicate that weakly oriented paramagnetic minerals can give well‐defined magnetic fabrics (AMS at room and low temperatures). Furthermore, the AMS ellipsoid is the result of composite biotite fabrics resulting from both orientation and spatial distribution of crystals, as demonstrated by 2‐D mathematical models presented in this study. AMS is the most effective technique for quickly measuring composite fabrics. In addition, the advantage of using AMS analyses is twofold: (1) it is a fast way of analysing standard samples that can give clues for subsequent image/mineral orientation analysis and (2) it is a volume‐related method that gives a picture of the rock fabric as a whole.