Geochemistry of granitic aplite-pegmatite veins and sills and their minerals from the Sabugal area, central Portugal

Granitic beryl-columbite-phosphate subtype aplite-pegmatite veins and sills from the Sabugal area intruded a biotite > muscovite granite which is related to another two-mica granite. Variation diagrams of major and trace elements of whole rocks show fractionation trends. REE patterns and δ18O of whole rocks, BaO and P2O5 contents of K-feldspar, anorthite and P2O5 contents of plagioclase, major element and Li contents of muscovite and lithian muscovite support this series. Least squares analysis of major elements indicate that the biotite > muscovite granite and aplite-pegmatite veins and sills are derived from the earlier two-mica granite magma by fractional crystallization of quartz, plagioclase, K-feldspar, biotite and ilmenite. Modelling of trace elements shows that magmatic fl uxes and fl uids controlled the Rb, Sr and Ba contents of aplite-pegmatites, probably also lithium micas (zinnwaldite, polylithionite and rare lepidolite), cassiterite, columbite-tantalite, fl uorapatite and triplite. In aplite-pegmatites, lithian muscovite replaces primary muscovite and late lithium micas replace lithian muscovite. Complexely zoned columbite crystals are chemically characterized and attributed to disequilibrium conditions. Relations of granites and aplite-pegmatites and pegmatites from other Portuguese and Spanish areas are compared. The granitic aplite-pegmatites from Sabugal are moderately fractionated and the granitic complex type aplite-pegmatites from Gonçalo are the richest in Li and Sn, derived from a higher degree of fractional crystallization and fl uxes and fl uids control the Ba and Rb behaviours and Li, Sn, F and Ta concentrations

Micas litiníferas dos aplito-pegmatitos graníticos de Cabeço dos Poupos, Sabugal (Centro de Portugal) = Lithium micas of granitic aplite-pegmatites from Cabeço dos Poupos, Sabugal (Central Portugal)

Na região de Cabeço dos Poupos (Sabugal, centro de Portugal) afloram várias soleiras e filões aplito-pegmatíticos, com assinaturas mineralógica e geológica típicas de pegmatitos REL-Li, que se integram no subtipo berilo-columbite-fosfato. Trata-se de aplito-pegmatitos de elementos raros com um grau moderado de evolução geoquímica. A moscovite primária é facilmente reconhecida, do ponto de vista textural e geoquímico, como constituindo a mica dominante nos aplito-pegmatitos desta região. Contudo, também ocorre moscovite litinífera e zinvaldite tabular, de cristalização precoce. Em alguns destes filões, reconheceu-se a presença de micas tardias, com composições de moscovite litinífera, zinvaldite, lepidolite e polilitionite. Em geral, as micas litiníferas tardias mostram contactos bruscos com as micas mais precoces. As micas litiníferas revelaram teores de Fe+Mg, Li, Rb e F mais elevados do que a moscovite primária, sendo, por outro lado, mais empobrecidas em AlVI, AlIV+AlVI e OH

Geoquímica de soleiras e filões aplito-pegmatíticos graníticos do centro de Portugal : cristalização fraccionada, fluxos e fluídos

As soleiras e filões aplito-pegmatíticos graníticos de Arcozelo da Serra, Cabeço dos Poupos (Sabugal) e Gonçalo (Belmonte) podem ser derivados por cristalização fraccionada de quartzo, plagioclase, feldspato potássico e biotite de um magma de granito da área respectiva, o que foi testado por modelização, usando o método de regressão de mínimos quadrados para os elementos maiores e a equação de cristalização fraccionada perfeita para os elementos menores. Há aumento no grau de evolução dos aplito-pegmatitos de Arcozelo da Serra para os de Cabeço dos Poupos e destes para os de Gonçalo, pois há aumento progressivo na quantidade e número de minerais litiníferos. Os aplitopegmatitos graníticos do subtipo lepidolítico são os mais evoluídos e derivam de um maior grau de cristalização fraccionada, sendo os mais ricos em fluxos (H2O, F, P, B) e fluidos. Com o aumento do grau de evolução dos aplitopegmatitos diminui o efeito dos fluxos e fluidos nos teores de Sr e Ba, mas aumentou esse efeito no teor de Rb. A cristalização fraccionada causou aumento nos teores de Rb, Li, F, Sn e Ta nos aplito-pegmatitos graníticos, especialmente nos do subtipo lepidolítico e os fluxos e fluidos influenciaram a cristalização de lepidolite, cassiterite e óxidos de Nb-Ta

Zircon dating and inheritance of a pre-Variscan granite, SW Iberia

The Portalegre pluton is an aligned series of Pre-Variscan granitoids located along the boundary of the Central Iberian/ Ossa Morena Zones (SW Iberian Massif, Portugal). Singlezircon geochronology (U-Pb SHRIMP and Pb/Pb stepwise evaporation) yield an Upper Cambrian/Lower Ordovician age (492.7 ± 3.5 Ma) interpreted as the magmatic crystallization age. The inherited zircon cores indicate the involvement of sources with a wide range age-components: an important Late Neoproterozoic (548 ± 7 Ma and c. 611-681 Ma) population and a Mesoproterozoic (c. 0.95-1.1 Ga) and older (c. 2.6 Ga) components. Younger zircon ages of 358 ± 36 and 387 ± 7 Ma were also found and interpreted as the record of a Variscan metamorphic event. The presence of Grenvillian zirconforming events in the protholiths of Portalegre granites is significant in the regional geodynamic context of the Iberian Massif (Central Iberian affinity?). At present, the Grenvillian ages are not noticeable in the Late Neoproterozoic/Early Cambrian record of the Ossa-Morena Zone, that has been correlated with West African Craton [1]. These data suggests that the Central Iberian Zone and Ossa-Morena Zone were independent peri-gondwana terrains with diferent paleogeographic affinities before the Ordovician times. The overall chemistry for the Portalegre granites shows they are very differentiated (SiO2=74-76 wt %), peraluminous (A/CNK=1.1–1.4); have low Zr=36-125ppm, Th/Ta=2-10, ΣREE= 22-134 and 1000Ga/Al >3. Their isotopic signatures (87Sr/86Sr)493=0.7050-0.7065, εNd493(-2.88 to -0.85) and δ18O=10.5-10.8‰, are compatible with partial melting of relatively young recycled metaigneous ± enriched mantle sources. The age pattern from the inherited zircon cores in the Portalegre granites shows that the late Neoproterozoic age (Cadomian) basement was actively involved in their magma generation. The Grenvillian and Archaean zircons can be accounted for by that source component but they do not imply the presence of an older pre-Neoproterozoic basement rocks beneath SW Iberia

Cadomian magmatism and metamorphism at the Ossa Morena/Central Iberian zone boundary, Iberian Massif, Central Portugal: Geochemistry and P–T constraints of the Sardoal Complex

ABSTRACT: A well preserved Cadomian basement is exposed in the Iberian Massif, Central Portugal, at the Ossa Morena/Central Iberian zone boundary, which allows the determination of reliable geochemical data. A sequence of Cadomian and Variscan magmatic and tectonometamorphic events has been already described for this area and are documented in other areas of the Avalonian-Cadomian orogen. However, the geochemical information concerning the Cadomian basement for this area is still limited. We present whole rock geochemical and oxygen isotopic information to characterize the igneous protoliths of the Sardoal Complex, located within the Tomar-Badajoz-Cordoba Shear Zone, and identify their tectonic setting. We use detailed petrography, mineral chemistry and P-T data to characterize the final Cadomian tectonometamorphic event. The Sardoal Complex contains orthogneiss and amphibolite units. The protoliths of the orthogneiss are calc-alkaline magmas of acid composition and peraluminous character that were generated in an active continental margin in three different stages (ca. 692 Ma, ca. 569 Ma and ca. 548 Ma). The most significant processes in their petrogenesis are the partial melting of old metasedimentary and meta-igneous crust at different crustal levels and the crystal fractionation of plagioclase, alkali feldspars, apatite, zircon and Fe-Ti oxides. The protoliths of the amphibolite, older than ca. 540 Ma, are tholeiitic and calc-alkaline magmas of basic composition that display N-,T- and E-MORB affinities. They were generated in an active continental margin. Crustal contamination and fractional crystallization of hornblende and diopside were involved in their petrogenesis. However, the fractional crystallization was not significant. The magmatic activity recorded in the Sardoal Complex indicates the existence of a long-lived continental arc (ca. 692-540 Ma) with coeval felsic and mafic magmatism. The final stage of the Cadomian metamorphism is usually represented in other areas of the Cadomian basement as a LP-HT metamorphic event. However, the P-T data obtained by thermodynamic modelling indicates medium pressure/high temperature conditions at ca. 540 Ma. These data suggest that the Sardoal Complex represents a deeper level of the exhumed Cadomian basement where the final stage of the Cadomian metamorphism was recorded.info:eu-repo/semantics/publishedVersio

Geochemistry and metamorphism of the Mouriscas Complex, Ossa-Morena/Central Iberian zone boundary, Iberian Massif, Central Portugal: Implications for the Cadomian and Variscan orogenies

ABSTRACT: The Mouriscas Complex is a deformed and metamorphosed predominantly mafic igneous complex of Ediacaran and Ordovician age and crops out at the Ossa-Morena/Central Iberian zone boundary in the Iberian Massif, Central Portugal. It comprises amphibolite with Neoproterozoic protoliths (ca. 544 Ma), protomylonitic felsic dykes derived from younger trondhjemitic protoliths (ca. 483 Ma) and garnet amphibolite derived of even younger dioritic protoliths (ca. 477 Ma). The protoliths of the Neoproterozoic amphibolites are calc-alkaline magmas of basic to intermediate compositions with intraplate and active continental margin affinities and are considered to represent the final phase of the Cadomian arc magmatism. They are interpreted to have originated as coarse-grained intrusions, likely gabbro or diorite and generated from the, partial melting of meta-igneous lower crust and mantle. Their emplacement occurred near the Cadomian metamorphic event dated at ca. 540 Ma (P = 7-8 kbar and T = 640-660 degrees C) which is interpreted to represent a continental collision. During the Late Cambrian-Early Ordovician an extensional episode occurred in the central-southern Iberian Massif and was also observed in other areas of the Variscan Orogen. It led to mantle upwelling and to the development of an aborted intracratonic rift located at the Ossa-Morena/Central Iberian zone boundary and to the opening of the Rheic Ocean to the south of the area studied in present coordinates (i.e., between the Ossa-Morena and South Portuguese Zones). This event has been dated at ca. 477 Ma and was responsible for the melting of deep ancient mafic crust and mantle with formation of bimodal magmatism in an intra-plate setting, as indicated by the protoliths of the protomylonitic felsic dykes with trondhjemitic composition and of the garnet amphibolite. Subsequent Variscan metamorphism took place under amphibolite facies conditions (P = 4-5.5 kbar; T = 600-625 degrees C) at lower P-T conditions than the Cadomian metamorphic event. It was followed by greenschist retrogression as suggested by the appearance of actinolite rims and formation of chlorite and epidote.info:eu-repo/semantics/publishedVersio