Geochronology and trace element chemistry of zircon and garnet from granulite xenoliths: Constraints on the tectonothermal evolution of the lower crust under central Spain

摘要

The nature of the lower crust under central Spain can be constrained from the study of deep-seated xenoliths entrapped within ultrabasic alkaline dykes of upper Permian age. These xenoliths are predominantly metaigneous peraluminous felsic granulites. Cathodoluminescence (CL) imaging of zircons from these rocks reveals a complex internal structure with magmatic inherited cores (sometimes showing convoluted or blurred oscillatory zoning) and metamorphic homogeneous domains. U-Pb SHRIMP dating shows that zircon core formation took place in several stages, mainly during the Upper Proterozoic (674-542 Ma). These ages are likely related to a pre-Variscan (Cadomian) tectono-magmatic episode. Most of the geochronological data correspond to metamorphic unzoned grains and rims formed during the Variscan orogeny (274-320 Ma). This time period greatly overlaps the intrusion ages of the granitic batholith in central Spain, reinforcing the hypothesis that the granulites from the lower crust might be genetically related to the extraction of the felsic magmas. Trace element concentrations in zircon and accompanying garnets, determined by Laser Ablation ICP-MS, suggest that both phases were in equilibrium in the studied granulites. HREE partition coefficients estimated in one sample are very close to 1, resembling results from previous studies in similar granulite-facies rocks. The composition of the HREE-rich magmatic zircon cores contrast markedly with metamorphic grains equilibrated with garnet (Gd_N/Yb_N>0.37). It is also possible to distinguish several types of metamorphic zircon rims on the basis of Th, U and REE composition. Temperatures estimated on these latter domains using the Ti-in zircon thermometer indicate that they formed during granulite-facies metamorphism. Zircons with high Th/U ratios (2.7-3.3) yield high-T estimates (-1000 °C) and older ages (283-320 Ma), whereas zircons with low Th/U ratios (0.04-0.38) give lower temperatures (900-950 °C) and ages (274-283 Ma). The variation in Th/U values and Ce and Eu anomalies in zircons seems to be highly influenced by element availability during metamorphic reactions and partitioning with co-existing minerals, such as monazite and feldspar. The modification of zircon geochemistry and the relationship between the temperature of crystallization and age implies a significant variation of the metamorphic conditions associated with the cooling stage at the end of the Variscan orogeny.