Editorial: Universidad Complutense de Madrid, Servicio de Publicaciones
Departamento: Fac. de CC. Físicas - Depto. de Física de la Tierra, Astronomía y Astrofísica I
ISBN: 978-84-669-2865-6
Notas: Tesis de la Universidad Complutense de Madrid, Facultad de Ciencias Físicas, Departamento de Física de la Tierra, Astronomía y Astrofísica I, leída el 28-06-2004
Resumen: 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).
Palabras clave: Magnetic fabrics and anisotropy; Rock and mineral magnetism; Large igneous
provinces; Magma migration and fragmentation; Europe
Resumen: The only Iberian lower Jurassic paleomagnetic pole come from the “Central Atlantic Magmatic Province”-related Messejana
Plasencia dyke, but the age and origin of its remanence have been a matter of discussion. With the aim of solving this uncertainty,
and to go further into a better understanding of its emplacement and other possible tectonic features, a
systematic paleomagnetic
investigation of 40 sites (625 specimens) distributed all along the 530 km of the Messejana Plasencia dyke has been carried out.
Rock magnetic experiments indicate PSD low Ti-titanomagnetite and magnetite as the minerals carrying the NRM. The samples
were mostly thermally demagnetized. Most sites exhibit a characteristic remanent component of normal polarity with the exception
of two sites, where samples with reversed polarities have been observed. The paleomagnetic pole derived from a total of 35 valid
sites is representative of the whole structure of the dyke, and statistically well defined, with values of PLa=70.4°N, PLo=237.6°E,
K=47.9 and A95=3.5°. Paleomagnetic data indicates that: (i) there is no evidence of a Cretaceous remagnetization in the dyke, as it
was suggested; (ii) most of the dyke had a brief emplacement time; furthermore, two dyke intrusion events separated in time from it
by at least 10,000 y have been detected; (iii) the high grouping of the VGPs directions suggests no important tectonic perturbations
of the whole structure of the dyke since its intrusion time; (iv) the pole derived from this study is a good quality lower Jurassic
paleopole for the Iberian plate; and (v) the Messejana Plasencia dyke paleopole for the Iberian plate is also in agreement with
quality-selected European and North American lower Jurassic paleopoles and the magnetic anomalies data sets that are available
for rotate them to Iberia.
