D. Orejana

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

Basic and intermediate meta-igneous xenoliths are very scarce within the granulite population transported by the Permian alkaline lamprophyric dyke swarmof the Spanish Central System(SCS). These xenoliths aremetaluminous pyroxene-bearing charnockites (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 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.

Composition and evolution of the lithospheric mantle in central Spain: inferences from peridotite xenoliths from the Cenozoic Calatrava volcanic field

Abstract Spinel lherzolite xenoliths from the Cenozoic Calatrava volcanic field provide a sampling of the lithospheric mantle of central Spain. The xenoliths are estimated to originate from depths of 35–50 km. Trace element content of clinopyroxene and Cr-number in spinel indicate low degrees of partial melting (≤ 5%) of the xenoliths. Although a major element whole-rock model suggests wider degrees of melting, the Calatrava peridotite chemistry indicates a moderately fertile mantle beneath central Spain. Calatrava peridotite xenoliths bear evidence for interaction with two different metasomatic agents. The enrichment in LREE(light rare earth element), Th, U and Pb, and the negative anomalies in Nb–Ta in clinopyroxene and amphibole from xenoliths of El Aprisco, indicate that the metasomatic agent was probably a subduction-related melt, whereas the enrichment in MREE in clinopyroxene from xenoliths of the Cerro Pelado centre suggests an alkaline melt similar to the host undersaturated magmas. These metasomatic agents are also consistent with the chemistry of interstitial glasses found in xenoliths of the two volcanic centres. Differences in metasomatism but also in mantle composition is supported by Sr–Nd whole-rock data which show a more radiogenic nature for Sr isotopes of samples from the El Aprisco centre (87Sr/86Sr ratios of 0.7035–0.7044 instead of 0.7032–0.7037 for samples from Cerro Pelado). The timing of the subduction-related metasomatic stage is unconstrained, although the Calatrava intraplate volcanism intrudes an old Variscan lithospheric section reworked during the converging plate system affecting SE Iberia in the Tertiary. The presence of wehrlite types within the Calatrava peridotite xenoliths is here interpreted as a reaction of host lherzolites with silica-undersaturated silicate melts that could be related to the Calatrava alkaline magmatism. The Sr–Nd isotopic composition of Calatrava peridotites plot within the European athenospheric reservoir(EAR) mantle, these values represent more enriched signatures than those found in the other Spanish Cenozoic alkaline province of Olot.

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 Permian alkaline lamprophyres from the Spanish Central System (SCS) are highly porphyritic rocks which carry a heterogeneous population of clinopyroxene and kaersutite zoned phernocrysts. Clinopyroxene phenocrysts may show 1) normal zoning (Cpx-I), 2) reverse zoning with Fe-rich green cores (Cpx-II), and 3) reverse zoning with colourless Al-poor, silica-rich cores (Cpx-III). Kaersutite phenocrysts also show a slight reverse zoning. Major and trace element composition of Cpx-I suggests that their compositional variation is related to a crystal fractionation process from melts similar to the host lamprophyres. The Cpx-II cores represent crystallization from highly evolved melts (low Mg-Cr contents and incompatible element enrichment), genetically related with the SCS alkaline magmatism, and the growth or surrounding Mg-rich inner rims points to a magma mixing process. The major and trace element composition of Cpx-III cores supports derivation from a magma which has fractionated plagioclase. This characteristic, together with their similarities when compared to clinopyroxenes from charnockite xenoliths, suggests that they might be xenocrysts from deep calc-alkaline cumulates. The composition of melts in equilibrium with clinopyroxene and amphibole phenocrysts supports a model in which Cpx-II and Cpx-III cores would have been incorporated into a more primitive lamprophyric magma stagnated at lower crustal levels. The low pressure composition of all phenocryst outer rims indicates that they crystallised directly from the host alkaline magma at their subvolcanic emplacement levels.

Heterogeneous metasomatism in cumulate xenoliths from the Spanish Central System: implications for percolative fractional crystallization of lamprophyric melts

Abstract The alkaline lamprophyres and diabases from the Spanish Central System carry a heterogeneous suite of xenoliths including a group of highly altered ultramafic pyroxenites that contain Cr–Mg-rich high-T hydrous minerals (Ti-phlogopite and pargasitic to kaersutitic amphibole), indicative of modal metasomatism. The trace element mineral compositions of these xenoliths show three patterns: type A xenoliths, with light rare earth element enriched clinopyroxenes with high field strength element (HFSE) negative anomalies; type B xenoliths, with clinopyroxenes and amphiboles with high incompatible trace element contents (large ion lithophile elements (LILE), HFSE and REE); type C xenoliths, with relatively REE- and HFSE-poor clinopyroxenes and amphiboles. These metasomatic signatures suggest the involvement of three different metasomatic agents: carbonate, silicate and hydrous fluids or melts, respectively. These agents could have been derived from the progressive differentiation of a CO2–H2O-rich highly alkaline magma, genetically related to the Late Permian alkaline magmatism. Because of the original sub-alkaline nature of the pyroxenite xenoliths, they might have been formed originally as pyroxene-rich cumulates associated with underplated Hercynian calc-alkaline basic magmas. Metasomatism as a result of the infiltration of alkaline magmas within these cumulates might explain the relatively high radiogenic Nd composition of the altered ultramafic xenoliths.