A peri-Gondwanan arc in NW Iberia. II: Assessment of the intra-arc tectonothermal evolution through U–Pb SHRIMP dating of mafic dykes

Abstract

The arc-derived upper units of the Órdenes Complex, NW Iberia, are emplaced above the Variscan suture and contain a low-grade metasedimentary uppermost section, with a maximum depositional age of 510–530 Ma, intruded by a number of mafic dykes. Three deformational events affect the metasedimentary section. The youngest deformation event (D3) is of undifferentiated Variscan age and consists of metre- to decametre-scale, close upright folds with axes plunging gently towards N20°E. The most important D2 structure is a regional S2 foliation axial planar of minor foldswith dextral asymmetry. The presence of a stretching lineation parallel to the D2 fold axes is related to a top-to-the-north sense of shearing in a context of regional extension. The oldestmeterscale D1 folds are developed in suitable greywacke–pelite alternations and consist of tight folds with chevron and similarmorphologies, axes plunging gently toward N20°E, and a continuous S1 axial planar foliation. The essential characteristic of the D1 deformation event is depicted by a set of west-vergent folds with reverse limbs less than 2 kmin wavelength, that are affected in their lower part by the generalised presence of the regional S2 foliation. The age of D2 and D1 structures is notwell constrained. The diabase dykes intruding the low-grade turbidites cut the D1 folds and their field relationships suggest that they were emplaced at the end of the D2 shearing event and prior to the upright D3 Variscan folds. Zircon grains obtained from one of the diabase dykes were analysed for U–Pb at the SHRIMP-RG facility at Stanford University. An age of c. 510 Ma, based on the analysis of 31 individual zircon grains, is interpreted to date the crystallization of the Ares dyke. The tectonic and magmatic evolution of the top turbiditic series of the Órdenes Complex is tentatively related to the dynamics of a peri- Gondwanan arc developed during active subduction beneath Gondwana and suggests: (1) accretion beneath the arc during west-vergent (present coordinates) nappe development (D1); (2) extension of the arc during top-tothe- north shearing (D2); and (3) final intrusion of the diabasic dykes into an intra-arc turbiditic series. This evolution spans the end of volcanic arc activity and the onset of the opening of the Rheic Ocean.