25 research outputs found
SHRIMP U–Pb zircon dating of anatexis in high-grade migmatite complexes of Central Spain: implications in the Hercynian evolution of Central Iberia
U–Pb SHRIMP ages obtained in zircons
from the Sotosalbos and Toledo anatectic complexes
in Central Spain give new constraints to the evolution
of the inner part of the Hercynian Iberian belt. Pre-
Hercynian ages in zircons from the Sotosalbos complex
(~464 Ma) are well preserved and reveal that an
age diversity of the Lower Paleozoic magmatism in
the area exists, as previous data on westernmost orthogneisses
yield significant older ages. Zircon ages in
the pelite-derived granites from the Toledo complex
also show an important Neoproterozoic age component
which points to a metasedimentary protolith
deposited maximally 560 Ma ago. Younger zircon
populations in both complexes at ~330 Ma in the
Sotosalbos region and ~317 Ma in the Toledo complex
indicate an important diachronism between the
anatectic processes in both areas but also that these
processes are mainly unrelated to the generation of
the later Hercynian granite batholith of Central
Spain, which could be of deeper crustal derivation. In
addition, as migmatization occurred late in the
metamorphic cycle, after peak conditions were attained,
the age of anatexis is younger than the age of
the main Hercynian metamorphic event, which still is
not well constrained
Andalusite and Na- and Li-rich cordierite in the La Costa pluton, Sierras Pampeanas, Argentina: textural and chemical evidence for a magmatic origin
The La Costa pluton in the Sierra de Velasco
(NW Argentina) consists of S-type granitoids that can be
grouped into three igneous facies: the alkali-rich Santa
Cruz facies (SCF, SiO2 *67 wt%) distinguished by the
presence of andalusite and Na- and Li-rich cordierite
(Na2O = 1.55–1.77 wt% and Li2O = 0.14–0.66 wt%), the
Anillaco facies (SiO2 *74 wt%) with a significant proportion
of Mn-rich garnet, and the Anjullo´n facies (SiO2
*75 wt%) with abundant albitic plagioclase. The petrography,
mineral chemistry and whole-rock geochemistry of
the SCF are compatible with magmatic crystallization of
Na- and Li-rich cordierite, andalusite and muscovite from
the peraluminous magma under moderate P–T conditions
(*1.9 kbar and ca. 735C). The high Li content of cordierite
in the SCF is unusual for granitic rocks of intermediate
composition
Multiple crustal sources for post-tectonic I-type granites in the Hercynian Iberian Belt
A post-tectonic plutonic array of felsic I-type
granites crops out in the western Hercynian Iberian Belt.
Isotope (Sr, N d, Pb) data favour the absence of an important
input of juvenile magmas in late- to post- tectonic
Hercynian felsic magmatism in western Iberia, but suggest
a reworking of different crustal protoliths, including
oceanic metabasic rocks accreted to mid-to-Iower crustal
levels during the early stages of the collision. I-type granites
were derived from different meta-igneous protoliths ranging
from metabasic to felsic compositions depending on their
geographical position from the external (e.g. Galicia-N
Portugal, GNP) to the innermost continental areas (Spanish
Central System and Los Pedroches Batholiths). The GNP 1-
type pIu tons related to eo-Hercynian accretional terranes
have lower initial 87Sr/86Sr ratios, lower negative εNd
values, and higher 206PbP04Pb ratios than other I-type granites of the Central Iberian zone. These more isotopically
primitive Hercynian I-type granites are important in
tracking pre-Hercynian accreted oceanic lithosphere
terranes
U–Pb isotopic ages and Hf isotope composition of zircons in Variscan gabbros from central Spain: evidence of variable crustal contamination
Ion microprobe U–Pb analyses of zircons from
three gabbroic intrusions from the Spanish Central System
(SCS) (Talavera, La Solanilla and Navahermosa) yield
Variscan ages (300 to 305 Ma) in agreement with recent
studies. Only two zircon crystals from La Solanilla massif
gave slightly discordant Paleoproterozoic ages (1,848 and
2,010 Ma). Hf isotope data show a relatively large variation
with the juvenile end-members showing ɛHfi values as high
as +3.6 to +6.9 and +1.5 to +2.9 in the Navahermosa and
Talavera gabbros, respectively. These positive ɛHfi values up
to +6.9 might represent the composition of the subcontinental
mantle which generates these SCS gabbros. This ɛHfi
range is clearly below depleted mantle values suggesting the
involvement of enriched mantle components on the origin of
these Variscan gabbros, and is consistent with previous
whole-rock studies. The presence of zircons with negative
ɛHfi values suggest variable, but significant, crustal contamination
of the gabbros, mainly by mixing with coeval granite
magmas. Inherited Paleoproterozoic zircons of La Solanilla
gabbros have similar trace element composition (e.g. Th/U
ratios), but more evolved Hf-isotope signatures than associated
Variscan zircons. Similar inherited ages have been
recorded in zircons from coeval Variscan granitoids from the
Central Iberian Zone. Granitic rocks have Nd model ages
(TDM) predominantly in the range of 1.4 to 1.6 Ga,
suggesting a juvenile addition during the Proterozoic.
However, Hf crustal model ages of xenocrystic Proterozoic
zircons in La Solanilla gabbro indicate the presence of
reworked Archean protoliths (TDM2 model ages of 3.0 to
3.2 Ga) incorporated into the hybridized mafic magma
Petrogenesis of Permian alkaline lamprophyres and diabases from the Spanish Central System and their geodynamic context within western Europe
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