Metaluminous pyroxene-bearing granulite xenoliths from the lower continental crust in central Spain: their role in the genesis of Hercynian I-type granites

摘要

Basic and intermediate meta-igneous xenoliths are very scarce within the granulite population transported by the Permian alkaline lamprophyric dyke swarm of the Spanish Central System (SCS). These xenoliths are metaluminous pyroxene-bearing charnockites (sensu lato). They show LREE-poor plagioclase and orthopyroxene-clinopyroxene. Crystallization conditions were estimated at about 850 to 1000 degrees C and 9 to 11 kbar, a slightly higher range than that estimated for the associated peraluminous granulites, but indicating derivation from the lowermost crust. Whole-rock geochemistry suggests that the charnockite samples are not a cogenetic suite. The more basic varieties have affinities with cumulates from previous calc-alkaline underplated protoliths, whereas intermediate charnockites have a restitic origin. The similarity in Sr-Nd-Pb isotopic signatures between these restitic charnockites and some SCS I-type granites suggests a genetic relationship. This study, including Pb isotopic data from the whole granulite xenolith suite, reinforces the lower-crustal derivation of the SCS Hercynian granitic batholith.