35 Rheological Information from Structures and Microstructures

Tuesday, 19 March 2013
8:45 AM - 12:00 PM
Jefferson Room

USING DETAILED AMS AND MEGACRYST FABRIC ANALYSES TO UNDERSTAND MAGMA FLOW WITHIN THE SAN MARTINO MULTILAYER LACCOLITH SYSTEM, ELBA ISLAND, ITALY

Paper number: 35-8
Program: Theme Sessions
Time: 10:40 AM – 10:55 AM

Authors

Abstract

Magmatic fabric may preserve the final increments of strain as magma moves and solidifies, and can help explain the filling of felsic magma chambers. Analysis of such fabrics can be done using shape-preferred orientations of crystals in the field, in thin section, and by using anisotrophy of magnetic susceptibility (AMS) analysis. Stresses generated by non-coaxial, differential magma flow can be simplified as divergent, convergent, and shear, each generating distinctly different stress fields in relation to particle paths. Divergence generally produces flattening perpendicular to flow lines and lineations in the direction of stretching. Convergence causes stretching parallel to particle paths and within the plane of flattening. Shear typically generates imbricated foliations with stretching slightly oblique to the primary particle paths. To move from fabric analysis to understanding magma flow requires either an additional line of evidence to inform that relationship, or a process of forward modeling from the known geology to determine if a resulting flow model is “geo-logical” and internally consistent. Based on examination of the literature in which fabric analysis leads to a flow model, this latter approach is generally taken, even if not acknowledged.

A study of the late Miocene, multilayer San Martino laccolith complex and its feeder dikes, comprised of sanidine megacrysts accompanied by biotite phenocrysts, reveals details about feeding and growth mechanisms of shallow-level, felsic intrusions. Fabric data comes from field measurements of K-feldspar attitudes (50 sites/2500 measurements) and from AMS determinations (150 sites/1500 cores). Strong foliations show good correlation between AMS and K-feldspar attitudes, strengthening the validity of their use as markers for final magma movement. Detailed mapping and reconstruction of original emplacement geometries suggest a central dike fed the laccolith complex. Fabric analyses support this model, with magma initially confined in steeply-dipping dikes to produce vertical imbricated foliations. Divergent lateral spreading of magma followed, as propagating and inflating pulses arranged suspended crystals perpendicular to magma displacement directions within the filling laccolith layers.