Publication: REE-assisted U–Pb zircon age (SHRIMP) of an anatectic granodiorite: Constraints on
the evolution of the A Silva granodiorite, Iberian allochthonous complexes
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2010
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Elsevier Science B.V., Amsterdam
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Abstract
The A Silva granodiorite is a plutonic body intruded into the metasediments of the upper unit of the Órdenes
Complex (Variscan belt, NW Spain). These metasediments represent the middle section of a magmatic arc
located in northern Gondwana. The A Silva granodiorite has been classically considered a late Variscan
granite. In this work, new field mapping, structural analysis, and SHRIMP U–Pb zircon dating indicate the
granodiorite is significantly older. However, the data indicate a concordant age range between 540 and
460 Ma, and therefore CL images are not useful toward the interpretation of the geochronological results.
This issue can be unravelled by using the hafnium and rare earth element composition of zircon in the
assessment of the age. In this way, we determined that the age distribution was the result of lead loss, rather
than a real age scatter or inheritance, and we could obtain a 206Pb/238U crystallization age of 510.28 (+1.57,
−1.44)Ma using the TuffZirc algorithm.
This age together with the well-preserved field relationships of the host rock permit us to interpret the A
Silva granodiorite as multiple sheets intruded into a sequence of metatexitic host rocks after crustal
thickening and subsequent decompression that developed coeval with partial melting during the latest
stages of a regional extensional event. Taken together with the underlying Monte Castelo gabbro (499±
2 Ma), the whole plutonic complex reaches 8 km in thickness and forms an antiformal stack structure in a
shear parallel (N–S) cross-section. This structure could be responsible for previously described, localized
granulite facies metamorphism.
The presence of a late Cambrian magmatic event has been widely reported in other areas of northern
Gondwana and it is related to the opening of the Rheic Ocean.