Magma flow, exsolution processes and rock metasomatism in the Great Messejana–Plasencia dyke (Iberian Peninsula)



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Silva , Pedro F. and Henry, Bernard and Marques, Fernando Ornelas and Font, Eric and Mateus, António and Vegas, Ramón and Miranda, Jorge Miguel and Palomino, Ricardo and Palencia Ortas, Alicia (2008) Magma flow, exsolution processes and rock metasomatism in the Great Messejana–Plasencia dyke (Iberian Peninsula). Geophysical Journal International, 175 . pp. 806-824. ISSN 0956-540X

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Magma flow in dykes is still not well understood; some reported magnetic fabrics are contradictory
and the potential effects of exsolution and metasomatism processes on the magnetic
properties are issues open to debate. Therefore, a long dyke made of segments with different
thickness, which record distinct degrees of metasomatism, the Messejana–Plasencia dyke
(MPD), was studied. Oriented dolerite samples were collected along several cross-sections
and characterized by means of microscopy and magnetic analyses. The results obtained show
that the effects of metasomatism on rock mineralogy are important, and that the metasomatic
processes can greatly influence anisotropy degree and mean susceptibility only when rocks
are strongly affected by metasomatism. Petrography, scanning electron microscopy (SEM)
and bulk magnetic analyses show a high-temperature oxidation-exsolution event, experienced
by the very early Ti-spinels, during the early stages of magma cooling, which was mostly
observed in central domains of the thick dyke segments. Exsolution reduced the grain size
of the magnetic carrier (multidomain to single domain transformation), thus producing composite
fabrics involving inverse fabrics. These are likely responsible for a significant number
of 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 have
reduced by 50 per cent the number of relevant sites. In these sites, the imbrication angle of the
magnetic foliation relative to dyke wall strongly suggests flow with end-members indicating
vertical-dominated flow (seven sites) and horizontal-dominated flow (three sites).

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Uncontrolled Keywords:Magnetic fabrics and anisotropy; Rock and mineral magnetism; Large igneous provinces; Magma migration and fragmentation; Europe
Subjects:Sciences > Geology > Geodynamics
ID Code:16781
Deposited On:19 Oct 2012 09:45
Last Modified:07 Feb 2014 09:35

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