Evolution of the Upper Mantle beneath the Pyrenees: Evidence from Orogenic Spinel Lherzolite Massifs

Abstract

About 40 spinel lherzolite massifs occur as small isolated bodies throughout the North Pyrenean Metamorphic Zone. Peridotites are foliated and range in composition from harzburgite to lherzolite. Pyroxenites (spinel websterites to garnet pyroxenites) define a layering concordant to the foliation plane. Less commonly, the layering is cross-cut by dykes of amphibole pyroxenites and hornblendites (e.g., Lherz and Freychinède). The oldest event of the complex pre-emplacement evolution is a lowdegree partial melting (< 10% melt extracted) which was initiated in the garnet lherzolite stability field. This event may have been associated with asthenospheric upwelling of Palaeozoic age. Two magmatic episodes are superimposed on partial melting features. The first gave rise to the layered pyroxenites, interpreted as crystal segregates which separated within feeder dykes of tholeiites. These melts intruded the peridotites when the massifs were located at the top of mantle diapirs (i.e., 45–50 km). Subsequently, both peridotites and pyroxenites underwent deformation with tectonic rotation of the dykes and recrystallized within the spinel lherzolite stability field during an isobaric cooling stage from 1200 to 900 °C. Peridotites and pyroxenites contain trace amounts of Ti-pargasite which occur as interstitial grains, suggesting the arrival of volatile-rich, strongly evolved melt before the end of the isobaric cooling stage. The amphibole-rich veins correspond to crystal segregates which crystallized within feeder dykes of Mid-Cretaceous alkali basaltic magmas. The injection of alkali basalts was contemporaneous with high-stress, low-temperature plastic deformation which produced mylonitic textures in the peridotites and tectonic rotation of the amphibole-rich veins. These intra-lithospheric shear zones are related to the Mid-Cretaceous transcurrent movements of the Iberian Peninsula relative to the European Plate, which were also responsible for the tectonic introduction of the massifs into the lower crust. Several arguments suggest a slightly older age of crustal emplacement for the Western Pyrenean massifs compared with the eastern Pyrenean massifs.