Xenoliths in basaltic rocks of the Roca Negra volcano: inferences on the composition of the upper mantle and lower crust beneath the Catalan Volcanic Zone (NE Spain)
摘要
Xenoliths hosted in alkaline basaltic lavas provide constraints on the composition and evolution of the upper mantle and lower crust, as well as on deep igneous processes beneath volcanic zones. This study investigates xenoliths occurring in pyroclasts and lavas of the Roca Negra volcano (Neogene–Quaternary Catalan Volcanic Zone). Petrographic analysis and major- and trace-element mineral chemistry were used to characterise xenolith lithologies, to estimate their pressure-temperature equilibrium conditions and constrain the nature of the upper mantle and lower crust. The xenolith assemblage includes ultramafic and mafic rocks corresponding to classical Type II (aluminous-augite) xenoliths in basaltic lavas, together with leucocratic igneous to meta-igneous rocks. Ultramafic and mafic xenoliths are cumulates dominated by clinopyroxene and/or amphibole and formed from the crystallisation and mixing of alkaline and tholeiitic melts more evolved than the host basalts. Most leucocratic xenoliths (gabbronorites, gabbros and anorthosites), with cumulate and granoblastic textures, derive from more saturated tholeiitic or high-Al basalts emplaced earlier within the lower-middle crust. Thermobarometric estimates for the ultramafic and mafic xenoliths (1140–949 °C; 1.0–0.6 GPa) indicate equilibrium conditions spanning the upper mantle and lower crust, whereas estimates for leucocratic xenoliths (1159–933 °C; 0.7–0.4 GPa) correspond to the lower to middle crust. These results indicate that the crust-mantle transition beneath the Catalan Volcanic Zone, at approximately 29 km depth, is composite, and consists of ultramafic-mafic cumulates intruded into mantle spinel peridotites and older layered igneous rocks of the lower to middle crust prior to the eruption of the host alkaline lavas.