Sr and Nd isotope composition of the Alcáçovas calc-alkaline rocks (Ossa- Morena Zone, Portugal)

Abstract

The Alcáçovas area is located in the SW sector of the Ossa- Morena Zone (OMZ), close to a major fault that separates this geotectonic unit from the South Portuguese Zone (SPZ). Along this boundary, in the OMZ, testimonies of low-K tholeiitic and calcalkaline magmatism are common and have been interpreted as being related to the operation of a subduction zone between OMZ and SPZ during the Variscan cycle [1]. Two main igneous lithologies, both displaying calc-alkaline compositions, can be found in the studied area: gabbro-diorites and dacitic-rhyolitic porphyries [2,3]. Outcrop conditions have not yet allowed to establish unequivocally the sequence of magma emplacement. In previous geochronological studies on the porphyries, whole-rock Rb-Sr dates and K-Ar ages cluster around 320 Ma [4,5,6]. According to field observations, sometimes felsic dykes cut mafic rocks, but there are also gradual transitions from gabbroic to tonalitic compositions, within bodies mapped as gabbro-diorite, revealing that different melts coexisted. In this study, rock samples of both gabbro-dioritic bodies and porphyries were analysed for Rb-Sr and Sm-Nd isotopes. Considering the whole set of samples, no isochron was obtained, showing that they can not be simply related by crystal fractionation processes. Rb-Sr data of porphyries from a single quarry (at Lameira, 7 km to the SW of Alcáçovas) give 323±16 Ma (MSWD=1.9; initial 87Sr/86Sr=0.7097±0.0018). Taking into account that the rocks of the Lameira outcrop show strong hydrothermal alteration, this date must be viewed as a consequence of a very efficient redistribution of mobile elements during aqueous fluid circulation and, as such, it places a minimum limit to the actual magmatic age The plot of compositions of the gabbro-dioritic bodies, including their transitions to tonalites and the associated felsic dykes, in the εNd-87Sr/86Sr diagram, define an almost perfect hyperbole (from εNd323 = +3.9 and 87Sr/86Sr323 = 0.7058 to εNd323 = -3.8 and 87Sr/86Sr323 = 0.7085), as expected in a mixture between mantle-derived melts and crustal materials. In the same diagram, samples from the Lameira quarry show an almost constant εNd323, between -2.4 and -2.9, and 87Sr/86Sr323 varying from 0.7092 to 0.7106. Therefore, the Lameira porphyries could represent a member of the same mixture, with the Sr signature modified by hydrothermal fluids with a stronger crustal component. Funding: FCT through projects Petrochron (PTDC/CTEGIX/ 112561/2009) and Geobiotec (PEst-C/CTE/UI4035/2011). [1] Santos et al. (1990) Comun. Serv. Geol. Portugal 76, 29-48. [2] Gonçalves et al. (1992) Not. Expl. Carta Geol. Torrão, 86 pp. [3] Caldeira et al (2007) Comun. Geol. INETI 94, 05-28. [4] Andrade (1974) Mem. Not. Univ. Coimbra 78, 29-36. [5] Coelho et al. (1986) Ciências da Terra UNL 8, 65-72. [6] Priem et al. (1986) Comun. Serv. Geol. Portugal 72, 03-07.