Montserrat Liesa

Partial melting of subducted continental lower crust in the Pyrenees

A magnetotelluric profile through the Central Pyrenees indicates the présence of very high conductive zones at lower crustal and upper mantle depths. High conductivity at upper mantle depths is interpreted as partial melts within subducted lower crust, whereas high conductivity at lower crustal depths is interpreted as rising magmas derived from the melting crustal slab. Such melts suggest that the continental lower crust, together with its lithospheric mantle may have been subducted into the mantle during the Pyrenean continental collision. Thus, magma generation can be related to thermal reequilibration of a subducted lower crust, a scenario that may serve as a model for understanding the late evolution of other collisional orogens.

Integrated two-dimensional lithospheric conductivity modelling in the pyrenees using field-scale and laboratory measurements

Abstract Recent magnetotelluric (MT) studies have shown that the lower crust in the Pyrenees contains a high conductivity zone consistent with a subducting continental slab, whose conductivity is 0.33 S/m. Partial melting has been interpreted to be the most plausible explanation for this high conductivity. Here we report a two-dimensional conductivity model of the lithosphere by integrating field-scale and laboratory determinations of the conductivity of continental crustal and mantle rocks. The laboratory data provide empirical formulas which allow us to determine the fluid saturated rock and melt conductivity when temperature, pressure and lithology are known. Consequently, we have also calculated the density, lithostatic pressure, and several alternative temperature profiles for use in the model from gravity, seismic and thermal field data. These can be used with a prescribed melt fraction to predict the electrical conductivity at depth, which can be compared with the MT conductivity data. Alternatively, the laboratory data can be combined with the MT conductivity data to predict the melt fraction at depth. The primary outputs of the modelling are conductivity and melt fraction prediction profiles for six mixing models; (i) Waff’s model/Hashin–Shtrikman (HS) upper bound, (ii) HS lower bound, (iii) parallel layers, (iv) perpendicular layers, (v) random melt areas, and (vi) a modified Archie’s law that takes account of the presence of two conducting phases. The modelling results indicate that a good match to the MT data can be obtained along the whole profile by the influence of pressure, temperature and the fluid phase with the only exception being the subducted slab, where a minimum of 4.7% melt fraction is necessary to explain the data.

U-Pb zircon age of Ordovician magmatism in the Albera Massif(Eastern Pyrenees)

New geochronological data from the Albera Massif confirm the presence of an Early – Mid Ordovician igneous event (472 - 465Ma) recorded in the pre-Variscan rocks of the Pyrenees. This event resulted in the emplacement of a large granitic body in the lower part of the pre-Upper Ordovician metasedimentary succession and in the intrusion of a series of metric sized dykes in the middle and upper parts of it. The two types of igneous rocks were gneissified during subsequent Variscan deformation. The geochronological data confirm the occurrence of the gneiss as having derived from an Ordovician intrusive sheet, as in other Pyrenean massifs. The dykes are considered to be the subvolcanic equivalent of the intrusive sheet. The data also provide insight into the age of the metasedimentary series of the massif and enable us to correlate the dated rocks with other gneissic and subvolcanic bodies of the Variscan massifs of the Pyrenees and Iberia.

Late Variscan metamorphic and magmatic evolution in the eastern Pyrenees revealed by U–Pb age zircon dating

Variscan migmatites cropping out in the eastern Pyrenees were dated together with Late Variscan plutonic rocks. Upper Proterozoic–Lower Cambrian series were migmatized during a thermal episode that occurred in the interval 320–315 Ma coeval with the main Variscan deformation event (D1). The calc-alkaline Sant Llorenç–La Jonquera pluton and the gabbro–diorite Ceret stock were emplaced during a later thermal episode synchronous with the D2 deformation event. A tonalite located at the base of La Jonquera suite intruded into the upper crustal levels between 314 and 311 Ma. The gabbro–diorite stock was emplaced in the middle levels of the series in two magmatic pulses at 312 and 307 Ma. The thermal evolution recorded in the eastern Pyrenees can be correlated with that of neighbouring areas of NE Iberia (Pyrenees–Catalan Coastal Ranges) and SE France (Montagne Noire). The correlation suggests a NW–SE-trending zonation where the northeasternmost areas (Montagne Noire and eastern Pyrenees) would occupy relatively more internal zones of the orogen than the southwesternmost ones. Supplementary material: Methods and U–Th–Pb isotopic and REE geochemical data for zircon are available at www.geolsoc.org.uk/SUP18703.

Petrogenesis of Ordovician Magmatism in the Pyrenees (Albera and Canigó Massifs) Determined on the Basis of Zircon Minor and Trace Element Composition

Zircon composition (U, Th, rare earth elements, and Hf) was tested as a tracer of petrogenetic processes in a set of metaigneous rocks from two pre-Ordovician massifs in the Pyrenees, Canigó and Albera. Two groups were differentiated after analyzing a number of elements in zircon: (1) Casemí gneiss and Marialles amphibolite and (2) subvolcanic metaporphyries and Cadí and Sureda orthogneisses. Casemí gneiss and Marialles amphibolite from the Canigó massif have high Th, Th/U, and Ce/Sm and low Yb/Gd and U/Ce that define linear trends in most of the plots used. The anomalous trend of the data in the Th/U-versus-Hf plot suggests mantle involvement in the origin of these rocks and the participation of fractional crystallization during their evolution. Zircon of the metaporphyries and the Cadí and Sureda orthogneisses exhibit similar characteristics despite a difference in age. Zircon has low Th, Th/U, Ce/Sm, and and high Yb/Gd and U/Ce, suggesting that this mineral grew in a melt formed by anatexis of a continental crust, with stable plagioclase. These petrogenetic data are consistent with the previously studied whole-rock geochemistry and Sr-Nd isotopes and confirm the use of zircon as a marker of petrogenetic processes in preference to a lithological tracer.

On the Structure and Metamorphism of the Roc de Frausa Massif (Eastern Pyrenees)

AbstractThe Roc de Frausa Massif, located at the Eastern Pyrenees, is formed by a stratoid Pre-Hercynian deformed granite (orthogneiss) interbedded with metasedimentary series. Hercynian granitoids (St. Llorenc — La Jonquera pluton) surround the southern and eastern part of the massif and Hercynian basic igneous rocks (Ceret stock) occupy the central part of it. The Pre-Hercynian granite and the sedimentary series were involved, during the Hercynian orogeny, in complex polyphasic tectonics and metamorphism. As a result, an ubiquitous penetrative foliation was developed during the earlier stages. This foliation was subsequently folded into a complex antiformal structural formed by a double dome : Roc de Frausa dome and Mas Blanc dome. Main lithological boundaries (gneiss — metasediments and metasediments — granitoids) are broadly parallel to the regional foliation, and they all display the dome geometry. Interference fold pattern between two late phases, an ealier one with NE-SW trending folds and a younge...