Autor(es):
Santos, J. F. 
; Moita, P. ; Marques, J.
Data: 2012
Identificador Persistente: http://hdl.handle.net/10773/10660
Origem: RIA - Repositório Institucional da Universidade de Aveiro
Assunto(s): Calc-alkaline magmatism; SW border of the Ossa-Morena Zone; Porphyries Gabbro-diorites; Sr and Nd isotope composition; Magma mixing; Hydrothermal alteration
Descrição
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 calc-alkaline 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.
