The ca. 350 Ma Beja Igneous Complex: A record of transcurrent slab break-off in the Southern Iberia Variscan Belt?

We report the results of an isotopic study of the large gabbro-dioritic Beja Igneous Complex (BIC) in the between the highly contrasting Ossa-Morena (OM) and South Portuguese (SP) Zones of the Southern Variscan orogen. This boundary is interpreted as a major suture zone materialized by discontinuous, strips of mafic/ultramafic rocks (the so-called Beja-Acebuches ophiolite complex, BAOC), and by melange of Middle to Late Devonian age in the Pulo do Lobo accretionary prism (PLAP). The Beja gabbro was either as part of the ophiolite-like units, or as a broadly arc-related massif reflecting the northward oceanic lithosphere. U-Pb zircon (ID-TIMS) dating of two diorites and a granodiorite points to placement ages of 350 +/- 2 Ma (Serpa), 352 +/- 2 Ma (Torrao), and 353 +/- 4 Ma (Sao Pedro), respectively, whereas dyke yields a slightly younger age of 345 +/- 2 Ma. These results show that published Ar/Ar dates do not igneous crystallization ages, but merely reflect regional cooling below ca. 500 degrees C. at least 10 Ma after intrusive event, probably as a result of uplift of the OMZ side of the suture zone relative to the subsiding SPZ Sr-87/Sr-86(350) and epsilon Nd-350 display a large range of values (from 0.7041 to 0.7093 and from +4.0 to -6.1 respectively) which documents a rather complex petrogenetic history with an important role played by crustal processes. The more primitive Sr and Nd isotope signatures are measured in the mafic cumulates, while Sr and unradiogenic Nd isotope compositions occur in the more evolved rock-types. The broad trend of epsilon Nd-350 with decreasing Sm/Nd and increasing SiO2 concentration is reminiscent of crustal assimilation with fractional assimilation (AFC). epsilon Nd values of flasergabbros and associated cumulates ascribed to the like unit in the Guadiana valley are close to zero or even slightly negative, irrespective of the age chosen correction for in situ radioactive decay of Sm-147 in the range 350-500 Ma, assumed to encompass geological age. This observation conflicts with a truly oceanic derivation of these rocks, which could represent early stage, deformed and metamorphosed equivalents of the Beja gabbro. Most metabasalts of area have positive, albeit variable epsilon Nd, consistent with mildly depleted mantle sources and/or significant assimilation. Only a few samples reach highly positive values (epsilon Nd350-500 Ca. +8) similar to those typical MORBs, as do the Acebuches amphibolites near Aracena (epsilon Nd350-500 ca. +9). These data suggest that the only disrupted but also composite, since the gabbroic rocks are not cogenetic with the basalts. Based on Devonian age of the collision between the OMZ and SPZ, the emplacement of the Beja gabbro cannot be an active subduction of oceanic lithosphere. Instead, the generation in the mantle, and emplacement overlying continental crust of copious volumes of mafic magmas with concomitant LP-HT tentatively ascribed to a process of transcurrent slab break-off that followed oblique subduction and (C) 2008 Elsevier B.V. All rights reserved.