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Zircon dating and inheritance of a pre-Variscan granite, SW Iberia
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Abstract(s)
The Portalegre pluton is an aligned series of Pre-Variscan
granitoids located along the boundary of the Central Iberian/
Ossa Morena Zones (SW Iberian Massif, Portugal). Singlezircon
geochronology (U-Pb SHRIMP and Pb/Pb stepwise
evaporation) yield an Upper Cambrian/Lower Ordovician age
(492.7 ± 3.5 Ma) interpreted as the magmatic crystallization
age. The inherited zircon cores indicate the involvement of
sources with a wide range age-components: an important Late
Neoproterozoic (548 ± 7 Ma and c. 611-681 Ma) population
and a Mesoproterozoic (c. 0.95-1.1 Ga) and older (c. 2.6 Ga)
components. Younger zircon ages of 358 ± 36 and 387 ± 7 Ma
were also found and interpreted as the record of a Variscan
metamorphic event. The presence of Grenvillian zirconforming
events in the protholiths of Portalegre granites is
significant in the regional geodynamic context of the Iberian
Massif (Central Iberian affinity?). At present, the Grenvillian
ages are not noticeable in the Late Neoproterozoic/Early
Cambrian record of the Ossa-Morena Zone, that has been
correlated with West African Craton [1]. These data suggests
that the Central Iberian Zone and Ossa-Morena Zone were
independent peri-gondwana terrains with diferent
paleogeographic affinities before the Ordovician times. The
overall chemistry for the Portalegre granites shows they are
very differentiated (SiO2=74-76 wt %), peraluminous
(A/CNK=1.1–1.4); have low Zr=36-125ppm, Th/Ta=2-10,
ΣREE= 22-134 and 1000Ga/Al >3. Their isotopic signatures
(87Sr/86Sr)493=0.7050-0.7065, εNd493(-2.88 to -0.85) and
δ18O=10.5-10.8‰, are compatible with partial melting of
relatively young recycled metaigneous ± enriched mantle
sources. The age pattern from the inherited zircon cores in the
Portalegre granites shows that the late Neoproterozoic age
(Cadomian) basement was actively involved in their magma
generation. The Grenvillian and Archaean zircons can be
accounted for by that source component but they do not imply
the presence of an older pre-Neoproterozoic basement rocks
beneath SW Iberia.
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Keywords
Citation
Solá, Ana Rita; Ribeiro, Maria Luísa; Neiva, Ana Margarida R. Zircon dating and inheritance of a pre-Variscan granite, SW Iberia. In: Goldschmidt Conference Abstracts, 2008.