Helena Martins

Hercynian Acid Magmatism and Related Mineralizations in Northern Portugal

Abstract Cabeceiras de Basto (CB), Vieira do Minho (VM) and Vila Pouca de Aguiar (VPA) granite plutons are located in the so-called Central Iberian Zone, northern Portugal. U-Pb zircon and monazite geochronology yielded the minimum emplacement age of 311 Ma for CB and the crystallization ages of 311 Ma for VM and 299 Ma for VPA, constraining the time interval of the third Hercynian deformation phase, relative to which these massifs can be grouped as follows: syn- to late-tectonic (CB and VM) and post-tectonic (VPA) granites. CB is a composite peraluminous two-mica granite pluton, occupying the core of a N 130° E antiform. Geochemical and isotopic data suggest a heterogeneous crustal source of mid-Proterozoic age (1.2 Ga). The granites are specialized in Sn, Li and, to a lesser extent, W mineralizations. Fractional crystallization is the main concentrator mechanism for these elements. Late-magmatic hydrothermal alteration processes (feldspar albitization and plagioclase and biotite muscovitization) enhanced the concentration promoted by the magmatic differentiation. Both the VM and VPA plutons are controlled by deep regional fault zones and consist of composite biotite granites, generated in the lower crust and installed in higher structural crustal levels, resulting in thermal metamorphism (P:2 kb; T: 500–600°C) capable of generating crustal convecting fluids. They are spatially related to important Sn, W and Au mineralizations. Although they are not highly specialized, they have contributed significantly as heat sources for the ore concentrations.

Assessment of Concrete Aggregate for ASR Potential by Petrography. The Work Developed by RILEM TC-ACS (2007–2013)

Alkali-silica reaction is a mechanism of deterioration of concrete structures which is associated with the composition of the rocks used as aggregates. The assessment of aggregates to evaluate their potential reactivity to alkalis is made both by visual methods and laboratory expansion tests. There are a number of national standards regarding the petrographic examination of new aggregates but the identification of the features that are related with the reactivity is not always easy. Supplementary to the recommendation of the petrographic method, RILEM has developed in recent years a worldwide petrographic atlas aiming at providing information regarding the features typical of each rock type that might be involved in alkali-silica reactions. In this work, a presentation is made on the results of the petrographic study of about 70 samples received from 25 countries in all continents. The samples include rock fragments, sand and gravel, concrete from structures and from prims subjected to expansion tests.

Building up of a nested granite intrusion: magnetic fabric, gravity modelling and fluid inclusion planes studies in Santa Eulália Plutonic Complex (Ossa Morena Zone, Portugal)

The Santa Eulalia Plutonic Complex (SEPC), located in the Ossa Morena Zone (south Portugal), is composed of a medium- to coarse-grained pink granite (G0-type) and a central grey medium-grained biotite granite (G1-type). Available Rb–Sr data indicates an age of 290 Ma. An emplacement model for the SEPC is proposed, taking into account magnetic fabric, 2D gravity modelling and fluid inclusion planes studies. The G0 and G1 types demonstrate different magnetic behaviour: G0 is considered a magnetite-type granite and G1 is an ilmenite-type granite. The formation of G0 required oxidized conditions related to the interaction of mafic rocks with a felsic magma. The 2D gravity modelling and subvertical magnetic lineations show that the feeder zone of the SEPC is located in the eastern part of the pluton, confirming the role of the Assumar and Messejana Variscan faults in the process of ascent and emplacement. The magma emplacement was controlled by ENE–WSW planar anisotropies related to the final brittle stages of the Variscan Orogeny. The emplacement of the two granites was almost synchronous as shown by their gradational contacts in the field. The magnetic fabric however suggests emplacement of the G0-type first, closely followed by emplacement of the G1-type, pushing the G0 laterally which becomes more anisotropic towards the margin. The G1-type became flattened, acquiring a dome-like structure. The SEPC is a nested pluton with G0-type granite assuming a tabular flat shape and G1-type forming a rooted dome-like structure. After emplacement, SEPC recorded increments of the late Variscan stress field documented by fluid inclusion planes in quartz.