Resumen: 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.
Resumen: 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 â—¦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.
Palabras clave: Granulite xenoliths, Pyroxene mineral chemistry, Hercynian granites, Igneous petrology, Lower crust, Central Spain
Resumen: Basic to ultrabasic alkaline lamprophyres and
diabases intruded within the Spanish Central System (SCS)
during Upper Permian. Their high LREE, LILE and HFSE
contents, together with positive Nb–Ta anomalies, link
their origin with the infiltration of sublithospheric K-rich
fluids. These alkaline dykes may be classified
in two distinct
groups according to the Sr–Nd isotope ratios: (1) a
depleted PREMA-like asthenospheric component, and (2) a
BSE-like lithospheric component. A slight enrichment in
radiogenic 207Pb and 208Pb allows the contribution of a
recycled crustal or lithospheric component in the mantle
sources. The intrusion of this alkaline magmatism is likely
to have occurred due to adiabatic decompression and
mantle upwelling in the context of the widespread rifting
developed from Carboniferous to Permian in western
Europe. The clear differences in the geochemical affinity of
Lower Permian basic magmas from north-western and
south-western Europe might be interpreted in terms of a
more extensive separation of both regions during that
period, until they were assembled during Upper Permian.
Palabras clave: Lamprophyres; Alkaline magmatism; Asthenospheric mantle; Permian rifting; Western Europe
Resumen: There is a significant enrichment in some trace elements in the major residual minerals of peraluminous granulite xenoliths from
the lower crust. Those trace elements are released from the breakdown of accessory phases at high-T granulite-facies conditions
(N850 °C). Around 10–35% of Zr is hosted in granulite rutile and garnet, whereas, the entire LREE–Eu
budget is controlled by
feldspar. The Zr- and REE-compatible behaviour of the major granulite phases, combined with the scarcity of accessory phases,
which are mostly included in major granulite minerals, leads to a disequilibrium in accessory dissolution in the peraluminous
partial melts. Thus the melt extracts less Zr and LREE and, consequently, generates the false impression of having lower-T when
applying current accessory phase dissolution models.
Resumen: The alkaline lamprophyres and diabases of the Spanish Central System carry a heterogeneous suite of xenoliths which
includes scarce pyroxenitic and hornblenditic types that can be divided in two groups: (a) pyroxenite xenoliths, including spinel
clinopyroxenites and spinel websterites with granoblastic textures, and (b) hornblende-bearing clinopyroxenites and
hornblendites
(here after called hornblenditic xenoliths) characterised by the presence of Ti-rich kaersutitic amphibole and magmatic
textures. Both groups of xenoliths can be assigned to the Al-augite series of Wilshire and Shervais (1975) [Wilshire, H.G.,
Shervais, J.W., 1975. Al-augite and Cr-diopside ultramafic xenoliths in basaltic rocks from western United States. Phys. Chem.
Earth 9, 257–272] with Al-rich and Cr-poor mafic phases. Clinopyroxenes show a very similar trace element composition in all
of the ultramafic xenoliths, characterised by convex-upward chondrite-normalised REE patterns and low contents of incompatible
elements such as Rb, Ba, Th and Nb. Kaersutite in the amphibole-bearing xenoliths shows a similar convex-upward
REE pattern as clinopyroxene. Whole-rock and mineral geochemistry support an origin as cumulates from alkaline to
subalkaline melts for most of the pyroxenites and hornblendites that have been studied. The Sr–Nd isotope ratios of pyroxenite
xenoliths display two extreme compositional poles: one clinopyroxenite plots in the OIB field towards depleted values
(87Sr/86Sr=0.7028 and qNd=6.2), whereas the other pyroxenites plot in enriched lithospheric fields (0.705 to 0.706 and
2.8 to 3.4, respectively), which implies that different magmas have been involved in their genesis. The hornblenditic
xenolith suite has a very homogeneous isotopic composition, close to the isotopically depleted values of high qNd and low
87Sr/86Sr ratios of one of the pyroxenite xenoliths. Some of these ultramafic xenoliths fall within the isotopic compositional
range of their host alkaline dykes, which also define a bipolar compositional field, suggesting that most of them are cogenetic
with the lamprophyres. P–T estimates yield temperatures in the range of 970–1080 8C and pressures mainly from 0.9 to 1.2 GPa
for pyroxenites, whilst hornblenditic xenoliths give lower (and probably underestimated) pressures (0.7–0.9 GPa). This pressure
range is in agreement with pyroxenites being formed by an underplating event at the upper mantle–lower crust boundary,
whereas pressure estimates for hornblenditic xenoliths suggest equilibration within the lower crust.
Palabras clave: Ultramafic xenoliths; Pyroxenites; Hornblendites; Spanish Central System; Lamprophyres
