Geochemistry and geochronology of mafic rocks from the Spanish Central
System: Constraints on the mantle evolution beneath central Spain
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Abstract
The Spanish Central System (SCS) contains several suites of Palaeozoic mafic igneous intrusions with contrasting
geochemical affinity: Ordovician tholeiitic metabasites, Variscan calc-alkaline gabbros (Gb1) and microdiorites
(Gb2), shoshonitic monzogabbros (Gb3) and alkaline diabases and lamprophyres (Gb4). Not all of these rocks are
accurately dated, and several aspects of their genesis are still poorly understood. We present new whole-rock
geochemical data (major and trace elements, and Sr–Nd isotopes), U–Pb and Lu–Hf isotopic ratios on
magmatic zircons and 40Ar/39Ar amphibole geochronology results in order to establish a precise chronology for
the successive events of magmatism in the SCS, and discuss the nature of their mantle sources. Accurate ages have
been determined for the Variscan gabbros (305–294 Ma), the microdiorites (299 Ma) and the accompanying felsic
porphyries (292 Ma), the shoshonitic monzogabbros (285 Ma), and the alkaline diabases (274 Ma) and monzosyenites
(271–264 Ma). According to this information, the Variscan mafic magmatism would be mainly
concentrated in the range of 305–294 Ma, with a final manifestation represented by the minor shoshonitic dykes.
The alkaline magmatism proved to be slightly older than previously thought and yielded at least two distinct
pulses: diabases and lamprophyres–monzosyenites. Zircon Hf isotopes evidence the involvement of depleted and
slightly enriched mantle sources. The bulk of the εHf values are in the broad range of 8 to þ11, indicative of
melting both depleted and enriched mantle regions. The high within-sample Hf isotope variation (up to ~11
epsilon units) shown by samples from the Variscan series (gabbros, microdiorites and monzogabbros) could be
explained mainly by hybridisation of magmas derived from heterogeneous lithospheric mantle sources. Pressure
estimates indicate that the Variscan mafic magmas were extracted from the lithosphere. The Nd–Hf isotopic
composition of these suites of rocks suggests the recycling of pelitic sediments during the Cadomian orogeny.
Deeper (asthenospheric) mantle levels were involved in the generation of the alkaline suite, whose anomalous
negative εHf values (moderately decoupled with respect to radiogenic Nd) could be associated with subducted
oceanic components raised by mantle upwelling associated with lithosphere thinning and extension during the
Permian.
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