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

Abstract

Basic and intermediate meta-igneous xenoliths are very scarce within the granulite population transported by the Per mian alkaline lamprophyric dyke swarm of the Spanish Central System (SCS). These xenoliths are metalurninous pyroxene-bearing chamockites (sensu lato). They show LREE-poor plagioclase and orthopyroxene-clinopyroxene. Crystallization conditions were estimated at about 850 to 1000 °C and 9 to 11 kbar, a slightly higher range than that estimated for the associated peralurninous granulites, but indicating derivation from the lowerrnost crust.\¥hole-rock geochemistry suggests that the chamockite 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 chamockites 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.

Geochemistry of mafic phenocrysts from alkaline lamprophyres of the Spanish Central System: implications on crystal fractionation, magma mixing and xenoliths entrapment within deep magma chambers

“…The San Bartolome de Pinares outcrop, where Cpx III phenocrysts may be found, also include plagioclase bearing xenoliths of charnockite affinity (Villaseca et al, 2007). There is an evident resemblance between the ma jor and trace element composition of clinopyroxenes from these xenoliths and Cpx-III colourless cores, including the deep Eu and Sr anomalies ( Fig.…”

“…These enclaves are rounded in shape, equigranular and small in size (usu ally < 1 cm). They have been recently interpreted as calc alkaline metabasic cumulates granulitized at lower crustal levels (Villaseca et al, 2007).…”

“…Cpx-III does not derive from basic ultrabasic magmas, which is a requirement of Nimis & Ulmer geobarometer. Nevertheless, the pressure range es timated for the lower crustal charnockitic xenoliths from the Spanish Central System (0.9-1.0 GPa; Villaseca et al, 2007) may indicate the equilibrium conditions for these Cpx-III cores, as they are fragments from these xenoliths. Thus, the San Bartolome de Pinares dyke carries xenocrys tic Cpx-III and Cpx-II green core phenocrysts from lower crustal levels.…”

“…Part of the charnockites from Villaseca et al (2007) have been interpreted as lower crustal fragments formed as frac tionates from calc-alkaline basic magmas. Thus, it is pos sible that the charnockite enclaves presented here (and also the accompanying Cpx-III xenocrysts) correspond with cu mulates associated with highly evolved subalkaline mag mas.…”

Mineral chemistry of late Variscan gabbros from central Spain: constraints on crystallisation processes and nature of the parental magmas

“…Several models have been proposed to explain the origin of this huge batholith: a) hybridization between crustal melts and mantle-derived magmas (Pinarelli and Rottura, 1995;Moreno-Ventas et al, 1995); b) variable degrees of crustal assimilation by mantle derived magmas (Ugidos and Recio, 1993;Castro et al, 1999); and c) partial melting of essentially crustal sources, either from lower crustal derivation (Villaseca et al, 1998(Villaseca et al, , 1999 or from midcrustal levels (Bea et al, 1999(Bea et al, , 2003. Clear geochemical similarities between SCS granites and lower crustal xenoliths carried by the SCS Upper Permian alkaline dykes, suggest that the lower crust is the most likely crustal source for the formation of the SCS batholith (Villaseca et al, 1999(Villaseca et al, , 2007. This is reinforced by coincidence in the U-Pb zircon age ranges recorded in the SCS granites and the lower crustal xenoliths (Fernández-Suárez et al, 2006;Orejana et al, 2011).…”