Autor(es):
Santos, J. 
; Moita, P. ; Marques, J.
Data: 2012
Identificador Persistente: http://hdl.handle.net/10174/7725
Origem: Repositório Científico da Universidade de Évora
Assunto(s): porphyries; calc-alkaline; ZOM; magmatism
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 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.
