%0 Journal Article
%A Silva , Pedro F.
%A Henry, Bernard
%A Marques, Fernando Ornelas
%A Font, Eric
%A Mateus, António
%A Vegas Martínez, Ramón
%A Miranda, Jorge Miguel
%A Palomino, Ricardo
%A Palencia Ortas, Alicia
%T Magma flow, exsolution processes and rock metasomatism in theGreat Messejana–Plasencia dyke (Iberian Peninsula)
%D 2008
%@ 0956-540X
%U https://hdl.handle.net/20.500.14352/50148
%X Magma flow in dykes is still not well understood; some reported magnetic fabrics are contradictoryand the potential effects of exsolution and metasomatism processes on the magneticproperties are issues open to debate. Therefore, a long dyke made of segments with differentthickness, which record distinct degrees of metasomatism, the Messejana–Plasencia dyke(MPD), was studied. Oriented dolerite samples were collected along several cross-sectionsand characterized by means of microscopy and magnetic analyses. The results obtained showthat the effects of metasomatism on rock mineralogy are important, and that the metasomaticprocesses can greatly influence anisotropy degree and mean susceptibility only when rocksare strongly affected by metasomatism. Petrography, scanning electron microscopy (SEM)and bulk magnetic analyses show a high-temperature oxidation-exsolution event, experiencedby the very early Ti-spinels, during the early stages of magma cooling, which was mostlyobserved in central domains of the thick dyke segments. Exsolution reduced the grain sizeof the magnetic carrier (multidomain to single domain transformation), thus producing compositefabrics involving inverse fabrics. These are likely responsible for a significant numberof the ‘abnormal’ fabrics, which make the interpretation of magma flow much more complex.By choosing to use only the ‘normal’ fabric for magma flow determination, we havereduced by 50 per cent the number of relevant sites. In these sites, the imbrication angle of themagnetic foliation relative to dyke wall strongly suggests flow with end-members indicatingvertical-dominated flow (seven sites) and horizontal-dominated flow (three sites).
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