Granulite-facies gneisses and meta-igneous xenoliths from the Campo de Calatrava volcanic field (Spain): Implications for the tectonics of the Variscan lower crust

Abstract

The microstructural, petrofabric and geochronological study of deep-seated xenoliths from the La Encomienda volcano (Neogene Campo de Calatrava Volcanic Field) has helped to characterize the nature and dynamics of processes operative in the lower crust and subcontinental mantle domains of the Central Iberian Zone in the Iberian Massif (Spain). We present the finding of three new xenolith types: noritic andesine meta-anorthosites, aluminous granulites and felsic granulites. Plagioclase (andesine), hypersthene, ilmenite, apatite and zircon make up the mineral assemblage in equilibrium in the anorthosite xenoliths, whereas K-feldspar (sanidine), antiperthitic plagioclase (oligoclase-andesine), quartz, sillimanite, garnet, rutile and biotite conform the mineral assemblage in the aluminous granulites. Felsic granulites differ from the aluminous type in the smaller amount of garnet, the occurrence of perthitic microcline and the lack of sillimanite and biotite. The presence of significant fabrics in the constituent minerals indicates that they underwent high-temperature deformations representative of low crustal domains.U-Th-Pb analyses on anorthositic zircons provide a Variscan Late Carboniferous Concordia age of 309 ± 3 Ma, interpreted as the age of the igneous protolith of the granulitic anorthosite. Estimations for zircons in the felsic granulites yield three age groups at 288 ± 2 Ma, 398 ± 2 Ma and 490 ± 2 Ma. The first one would represent a late- to post-Variscan (Cimmerian) regional metamorphic event under granulite-facies conditions. The last two might be related to the age of an igneous component of the protolith at ca. 490 Ma and possibly that of a previous tectonothermal event at 400 Ma. 206Pb/238U results on the aluminous and felsic granulitic rutiles give ages of 5 ± 1 Ma that may correspond to resetting during Neogene volcanic events.A tentative scenario would contemplate the existence of Upper Carboniferous (late-Variscan) lithospheric shear zones along which mantle-derived components would have raised until the mantle-crust interface. The associated thermal budget would have acted as an important heat source responsible for heat conduction into the lower crust, along with eventual crustal melting. Removal of the mafic portions would have led to formation of low-density plagioclase-rich accumulations propagating upwards until its eventual incorporation in low crustal domains along with the formation of deep-seated granulites. The occurrence of these lithologies and the large volumes of hybrid monzogranites to granodiorites in the Central Iberian Zone might indicate, congruently, that, more than expected, mantle-derived magmas would have also played an important role during crustal growth processes related to the final stages of the Variscan orogeny in this part of the Iberian Massif.