Abstract
On the coastline of Northern Portugal, metamorphic formations and pegmatites were the subject of structural analysis with the main goal of understanding Variscan kinematics and related pegmatite intrusion. This study also aims to discriminate, select and characterize relevant aspects of the structure and the paragenesis of pegmatites, well exposed as a result of coastal erosion, justifying its inclusion in the geological heritage of the Northern coast of Portugal. The pegmatite bodies show distinctive internal and external structures that are attributable to different modes of emplacement and subsequent deformation. The pegmatitic implantation in the areas of Moledo and Afife occurs in an intragneissic and perigranitic environment, for the first area, and perigneissic and perigranitic environment, for the second. In Pedras Ruivas predominates the implantation into an exo-gneissic to exo-granitic domain. The Moledo veins show evidence of multiphase open/filling, revealing positions, shapes, attitudes, sizes and internal structures that change as a function of the host lithology and host structure, but mainly due to the dilation and the cycles number of local telescoping. The structural analysis of the pegmatite bodies allows the deduction of a local fulcrum of expansion that hypothetically overlaps a hidden stock of parental granite. In Afife and Pedras Ruivas, some pegmatitic lenses are specialized and mineralized in Li, Cs and Ta, with spodumene and tantalite ± cassiterite. Spodumene occurs as giant crystals, centimetric to pluri-decimetric in length, which mark very clearly the structures of in situ or in flow crystallization inside the pegmatites (primary structures) and also the secondary structures resulting from deformation. The geometric analysis of fabrics helps the individualization of well-defined stages of progressive evolution of the deformation of the pegmatites, allowing its correlation with major D2–D3 episodes of regional Variscan deformation.
Highlights
This seeks sought to explore the geometric analysis of intra-pegmatitic fractionation structures, in a typical dilational context, aiming to contribute to the kinematic interpretation of the internal structures of the pegmatite bodies and their vein fields in intra- and peri-granitic contexts.It tries to establish the reliability of the use of typomorphic minerals as kinematic markers, in the study of the primary internal pegmatite fabrics, either fluidal or deformational
Spodumene occurs as giant crystals, centimetric to pluri-decimetric in length, which mark very clearly the structures of in situ or in flow crystallization inside the pegmatites and the secondary structures resulting from deformation
Detailed geometric analysis of pegmatites and internal, linear and planar fabrics of spodumene; Geometric analysis of the variability of the pegmatite inner structure and the fabric of spodumene lodged therein, in distinct sectors of Northern Coast Pegmatitic Field (NCPF), which are considered to be representatives of this variability—the Moledo to Afife Sector to the North, where the pegmatites are hosted in two mica gneisses with tourmaline and garnet, and the Pedras Ruivas Sector, in the southern part, where the pegmatites were implanted in quartzites and meta-quartzphyllite (Figure 1 and Table 1); Interpretation of structures resulting from imposed and superimposed deformation, from the point of view of displacement kinematics, considering the rheological behavior of host rocks in different deformational regimes
Summary
This seeks sought to explore the geometric analysis of intra-pegmatitic fractionation structures, in a typical dilational context, aiming to contribute to the kinematic interpretation of the internal structures of the pegmatite bodies and their vein fields in intra- and peri-granitic contexts.It tries to establish the reliability of the use of typomorphic minerals as kinematic markers, in the study of the primary internal pegmatite fabrics, either fluidal or deformational. It can contribute to accessing the temporal and chronological correlation and influence of the different phases of regional deformation over different lithological supports, complementing the approaches that consider other mega-crystalline phases. This approach is similar to that proposed in [1]. Pegmatite Belt of the Central and Iberian Variscan Province. It will allow the establishment of a chronological sequence of Li, Cs and Ta (LCT) mineralizations, at least in this sector of the Variscan
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