New petrographic, geochemical and geochronological data for the Reguengos de Monsaraz pluton (Ossa Morena Zone, SW Iberian Massif, Portugal)

A Antunes,M H Mendes,J F Santos,S Ribeiro,M R Azevedo

doi:10.3989/egeol.40162.123

Abstract

The Reguengos de Monsaraz pluton is a late to post-tectonic Variscan intrusion occurring in the Ossa Morena Zone (Iberian Variscan Chain). The dominant lithological types are tonalites and granodiorites, but the internal area of the massif is composed of gabbro-dioritic rocks. Field evidence shows that the intrusion is heterogeneous at mesoscopic scale suggesting that the emplacement of mafic and felsic magmas was contemporaneous. Petrographic and geochemical studies reveal that the different lithologic types define a continuous sequence with compositions varying from metaluminous to slightly peraluminous and a typical calc-alkaline signature. In Harker variation diagrams, it is possible to observe systematic rectilinear correlations pointing to the involvement of magma mingling/mixing processes in the petrogenesis of this sequence. Rb-Sr isotopic data, using a mineral-mineral pair from a granodiorite sample, yielded an age of 298 Ma, interpreted as a cooling age after igneous crystallization.

Highlights

The Reguengos de Monsaraz pluton is located in the Portuguese part of the Ossa Morena Zone (OMZ), one of the major geotectonic units of the Iberian Massif (Fig. 1)
The major elements were determined using Inductively Coupled Plasma Atomic Emission (ICP-AES) whilst the trace elements were analysed by Plasma Mass Spectrometry (ICP-MS), after alkaline melting with lithium metaborate/tetraborate followed by nitric acid dissolution
An origin involving fractional crystallization of mantle derived magmas, at deep crustal levels, followed by mixing/mingling during ascent and emplacement is proposed to account for the observed geochemical variation

Summary

Introduction

The Reguengos de Monsaraz pluton is located in the Portuguese part of the Ossa Morena Zone (OMZ), one of the major geotectonic units of the Iberian Massif (Fig. 1). The tonalites and the granodiorites are the dominant lithological types and contain abundant mafic microgranular enclaves (Carvalhosa, 1967 and Carvalhosa & Zbyzewski, 1991) These authors mentioned the occurrence of subordinate amounts of gabbro-dioritic rocks occupying the central part of the massif along an area with a maximum length of 3 km. A conspicuous mesoscopic feature observed during the field survey is the heterogeneous appearance of all the different lithological types This is mainly due to the widespread occurrence of mafic microgranular enclaves within the tonalites and the granodiorites (Fig. 2), first mentioned by Carvalhosa & Zbyzewski (1991). In terms of mineralogical composition, the sequence displays a gradual increase of microcline and quartz proportions and decrease of mafic mineral contents (specially, amphibole), from the gabbro-diorites to the tonalites and the granodiorites These variations are accompanied by a consistent increase of the albite component in the plagioclase. Single, zoned grains of allanite with rounded shapes have been sporadically found in the different members of the Reguengos de Monsaraz intrusion

Analytical methods

Conclusions