Mineralogia, petrologia e geoquímica dos granitos e seus encraves da região de Carrazeda de Ansiães
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2008
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Teixeira, Rui José dos Santos
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Na região de Carrazeda de Ansiães, sector oriental do arco magmático Monção – VilaReal – Torre de Moncorvo, afloram dez granitos (G1 a G10) sin-orogénicos que se instalaramdurante a etapa pós-colisional da orogenia Varisca, entre 330.7 Ma e 315.5 Ma. Estes granitosintruíram formações metassedimentares do Grupo do Douro, de idade Neoproterozóica –Câmbrica Inferior, ocupando o núcleo de um antiforma NW-SE, resultante das fases dedeformação D1 e D3, e originando frequentemente micaxistos e corneanas com andaluzite nas zonas de contacto.
Todos os granitos são constituídos por quartzo, microclina, plagioclase, biotite, moscovite, turmalina, zircão, silimanite, apatite, monazite, ilmenite e rútilo. De uma forma geral, possuem mais moscovite do que biotite, mas os granitos G4, G5 e G8 têm quantidades idênticas destas micas. Os granitos apresentam quase sempre encraves sobremicáceos mas, mais raramente, contêm xenólitos, “schlieren” e encraves microgranulares. Os granitos e seus encraves microgranulares são dominantemente alcalino-cálcicos e peraluminosos, com ASI variando entre 1.16 e 1.45 e corindo normativo de 2.20 a 4.77 %. Os granitos são do tipo S, apresentando perfis de terras raras enriquecidos em LREE em relação às HREE, com anomalias negativas de Eu, e valores médios de (87Sr/86Sr)i compreendidos entre 0.7097 e 0.7160 e de δ18O de 10.55 a 11.80 ‰.
Os dados estruturais, petrográficos e geocronológicos U-Pb, obtidos em zircão e monazite, permitem distinguir três grupos de granitos. Os do Grupo I (G1, G2, G3, G4, G5 e G6) são anteriores, ou contemporâneos, à idade de 320 Ma e apresentam evidências de deformação dúctil a frágil, típica dos estados tardi-magmático e sólido. Os granitos dos Grupos II (G7, G8 e G9) e III (G10) apresentam idades entre 320 e 310 Ma, tendo sido deformados pela fase D3 antes de estarem completamente consolidados, o que pode justificar a existência de uma estruturação interna magmática planar, com direcção NW-SE, concordante com as estruturas regionais. Por outro lado, o granito G10 está intimamente associado a zonas de falha com direcção NNE-SSW, tendo sido afectado por uma forte deformação frágil.
As características geoquímicas dos granitos e dos seus minerais, os perfis de terras raras dos granitos e os diferentes valores médios de (87Sr/86Sr)i e εNdt para G1 (0.7097 ± 0.0000; -6.3), G2 (0.7149 ± 0.0008; -8.2), G4 (0.7112 ± 0.0006; -8.0), G5 (0.7124 ± 0.0007; -7.5), G7 (0.7156 ± 0.0005; -8.5) e G8 (0.7155 ± 0.0007; -8.4) indicam que estes granitos G1, G2, G4, G5 e G7 correspondem a pulsações magmáticas independentes, resultantes da fusão parcial de materiais metassedimentares heterogéneas. Os granitos G7 e G8 ter-se-ão formado a partir da fusão parcial do mesmo material, sendo o granito G8 resultante do maior grau de fusão parcial.
Alguns dos granitos e seus minerais mostram evolução por fraccionação, distinguindo-se três séries de diferenciação magmática: a) G2 e G3; b) G5 e G6; c) G8, G9 e G10. Os perfis de terras raras subparalelos, com decréscimo de terras raras e ligeiro aumento da anomalia negativa de Eu dentro dos granitos de cada série, bem como as isócronas Rb-Sr estabelecidas pelos granitos: a) G2 e G3 e b) G8, G9 e G10, também apoiam a existência do mecanismo de cristalização fraccionada para cada série de duração de 0.2 Ma para G2 e G3 e 0.6 Ma para G8, G9 e G10. A modelização de elementos maiores e menores indica que: a) as amostras mais ricas em SiO2 do granito G2 e o granito G3 derivaram do magma granítico G2 por cristalização fraccionada de quartzo, feldspato potássico, plagioclase e biotite; b) o granito G6 resultou do magma granítico G5 por cristalização fraccionada de feldspato potássico, plagioclase e biotite; c) as amostras mais ricas em SiO2 do granito G8 e os granitos G9 e G10 derivaram do magma granítico G8 por cristalização fraccionada controlada pela separação de quartzo, feldspato potássico, plagioclase, biotite e ilmenite. Os valores de (87Sr/86Sr)i não apresentam uma variação significativa de G2 para G3, nem de G8 para G9 e G10, indicando que a cristalização fraccionada foi o principal mecanismo responsável pela sua génese, o que é apoiado pela existência de valores relativamente semelhantes, mas irregulares de εNdT e δ18O nesses granitos de cada série. Contudo, na série de diferenciação G5 e G6, terá ocorrido assimilação contemporânea de material metassedimentar, como indicado pelos dados isotópicos de (87Sr/86Sr)i, εNdt e δ18O e pela errócrona Rb-Sr. Este processo AFC durou 1.6 Ma.
No granito G9 há encraves de monzogranito que pertencem à série de diferenciação G8 a G10, que corresponderão a fragmentos iniciais de margens arrefecidas, mobilizadas durante a ascensão do magma. Por outro lado, as características geoquímicas e isotópicas do encrave granodiorítico que ocorre no granito G1 e dos encraves tonalíticos que ocorrem nos granitos G5 e G8 indicam uma origem distinta dos respectivos granitos hospedeiros, envolvendo provavelmente processos de mistura entre magmas crustais e magmas do manto superior ou da crusta inferior. O encrave de monzogranito existente no granito G1 resultou da anatexia de materiais metassedimentares, mas apresenta características geoquímicas e isotópicas distintas do seu granito hospedeiro, o que é atribuído a heterogeneidade dos materiais metassedimentares. Os granitos G1, G2, G3, G6, G7 e G10 são estaníferos, com teores médios de Sn superiores a 18 ppm e o granito G5 com 16 ppm Sn, estão associados a mineralizações de estanho e volfrâmio que foram exploradas no passado. Porém, os principais filões de quartzo com cassiterite e volframite cortam os G3 e G10, ambos com teores médios de Sn de 31 ppm. A cristalização fraccionada terá sido o mecanismo responsável pelo aumento dos teores de Sn nos granitos das séries de diferenciação e respectivas micas. Embora a biotite seja geralmente mais rica em Sn do que a moscovite primária coexistente, a moscovite é geralmente responsável pela maior retenção de Sn nestes granitos.
Ten syntectonic granites (G1 to G10) occur in the Carrazeda de Ansiães area and were emplaced between 330.7 to 315.5 Ma during the post-collisional Variscan orogeny. They intruded metasedimentary materials of Douro Group in the core of the NW-SE antiform, resulting from D1 and D3 deformation phases, and caused micaschist and hornefelses of contact metamorphism. All the granites contain quartz, microcline, plagioclase, biotite, muscovite, tourmaline, zircon, sillimanite, apatite, monazite, ilmenite and rutile. In general, they have more muscovite than biotite, but the granites G4, G5 and G8 present similar amounts of both micas. The granites contain surmicaceous enclaves and more rarely xenoliths, schlieren and microgranular enclaves. Granites and their microgranular enclaves are mainly alkaline-calcic and peraluminous with ASI ranging between 1.16 and 1.45 and normative corundum of 2.20 – 4.77 %. They are S-type granites, showing REE patterns enriched in LREE relatively to HREE, having negative Eu anomalies and mean values of (87Sr/86Sr)i of 0.7097 – 0.7160 and δ18O of 10.55 – 11.80 ‰. Structural, petrographic and U-Pb geochronological data obtained on zircon and monazite enabled to distinguish three granite groups. Group I consists of G1, G2, G3, G4, G5 and G6, which are ≤ 320 Ma old, show evidences of a ductile and fragile deformation, characteristic of late magmatic and solid states. Granites of Group II (G7, G8 and G9) and Group III (G10) are 320 – 310 Ma old, were deformed by D3 before they were solidified, explaining the NW-SE concordant planar magmatic structure. Granite G10 is associated with the NNE-SSW fault zones and was deformed by a strong fragile deformation. The geochemical characteristics of granites and their minerals, their REE patterns and different mean values of (87Sr/86Sr)i and εNdt for G1 (0.7097 ± 0.0000; -6.3), G2 (0.7149 ± 0.0008; -8.2), G4 (0.7112 ± 0.0006; -8.0), G5 (0.7124 ± 0.0007; -7.5), G7 (0.7156 ± 0.0005; - 8.5) and G8 (0.7155 ± 0.0007; -8.4) show that these granites G1, G2, G4, G5 and G7 correspond to distinct pulses of granite magmas, resulting from partial melting of heterogeneous metassedimentary materials. The granites G7 and G8 result from partial melting of the same metassedimentary material, but granite G8 is derived from a higher degree of partial melting than G7. Some granites and their minerals show fractionation trends and three series are distinguished: a) G2 and G3; b) G5 and G6; c) G8, G9 and G10. The subparallel REE patterns showing decrease in all REE and a slight increase in the negative Eu anomaly within each series and Rb-Sr isochrons for a) G2 and G3 and b) G8, G9 and G10 suggest a fractional crystallization for each series, which took 0.2 Ma for G2 and G3 and 0.6 Ma for G8, G9 and G10. Modelling of major and trace elements show that the richest samples in SiO2 of granite G2 and also granite G3 are derived from granite magma G2 by fractional crystallization of quartz, potash feldspar, plagioclase and biotite; b) granite G6 is derived form granite G5 by fractional crystallization of potash feldspar, plagioclase and biotite; c) the richest samples in SiO2 of granite G8 and granite G9 and G10 are derived from granite magma G8 by fractional crystallization of quartz, potash feldspar, plagioclase, biotite and ilmenite. (87Sr/86Sr)i values do not show any significant variation neither from G2 to G3 nor from G8 to G9 and G10, showing that fractional crystallization was the main mechanism, which is supported by the relatively similar, although irregular, εNdt and δ18O data within each series. However, in the G5 and G6 series, there was also a contemporaneous assimilation of metassedimentary material, as shown by (87Sr/86Sr)i, εNdt data and Rb-Sr errorchron. The AFC mechanism took 1.6 Ma. Granite G9 has monzogranite enclaves which belong to series G8 – G10 and will correspond to fragments of early cold margins, removed during the magma ascendancy. On the hand, the geochemical and isotopic characteristics of the granodiorite enclave in granite G1 and tonalitic enclaves in granites G5 and G8 suggest that these enclaves are not derived from the host granite magma and probably result from mixing of a crustal magma and an upper mantle magma or lower crustal magma. The monzogranite enclave in granite G1 is derived by anatexis of metassedimentary materials, but has distinct geochemical and isotopic characteristics from those of host granite, which is attributed to heterogeneity of metassedimentary materials. The tin-bearing granites G1, G2, G3, G6, G7 and G10 have ≥ 18 ppm Sn and the granite G5 shows 16 ppm Sn an are associated with Sn and W mineralizations. The main quartz veins containing cassiterite and wolframite cut granites G3 and G10, which contain 31 ppm Sn. Fractional crystallization was responsible for the increase in Sn content in granites from the series and their micas. Although biotite is richer in Sn than coexisting primary muscovite, muscovite retains a higher percentage of total granite tin content.
Ten syntectonic granites (G1 to G10) occur in the Carrazeda de Ansiães area and were emplaced between 330.7 to 315.5 Ma during the post-collisional Variscan orogeny. They intruded metasedimentary materials of Douro Group in the core of the NW-SE antiform, resulting from D1 and D3 deformation phases, and caused micaschist and hornefelses of contact metamorphism. All the granites contain quartz, microcline, plagioclase, biotite, muscovite, tourmaline, zircon, sillimanite, apatite, monazite, ilmenite and rutile. In general, they have more muscovite than biotite, but the granites G4, G5 and G8 present similar amounts of both micas. The granites contain surmicaceous enclaves and more rarely xenoliths, schlieren and microgranular enclaves. Granites and their microgranular enclaves are mainly alkaline-calcic and peraluminous with ASI ranging between 1.16 and 1.45 and normative corundum of 2.20 – 4.77 %. They are S-type granites, showing REE patterns enriched in LREE relatively to HREE, having negative Eu anomalies and mean values of (87Sr/86Sr)i of 0.7097 – 0.7160 and δ18O of 10.55 – 11.80 ‰. Structural, petrographic and U-Pb geochronological data obtained on zircon and monazite enabled to distinguish three granite groups. Group I consists of G1, G2, G3, G4, G5 and G6, which are ≤ 320 Ma old, show evidences of a ductile and fragile deformation, characteristic of late magmatic and solid states. Granites of Group II (G7, G8 and G9) and Group III (G10) are 320 – 310 Ma old, were deformed by D3 before they were solidified, explaining the NW-SE concordant planar magmatic structure. Granite G10 is associated with the NNE-SSW fault zones and was deformed by a strong fragile deformation. The geochemical characteristics of granites and their minerals, their REE patterns and different mean values of (87Sr/86Sr)i and εNdt for G1 (0.7097 ± 0.0000; -6.3), G2 (0.7149 ± 0.0008; -8.2), G4 (0.7112 ± 0.0006; -8.0), G5 (0.7124 ± 0.0007; -7.5), G7 (0.7156 ± 0.0005; - 8.5) and G8 (0.7155 ± 0.0007; -8.4) show that these granites G1, G2, G4, G5 and G7 correspond to distinct pulses of granite magmas, resulting from partial melting of heterogeneous metassedimentary materials. The granites G7 and G8 result from partial melting of the same metassedimentary material, but granite G8 is derived from a higher degree of partial melting than G7. Some granites and their minerals show fractionation trends and three series are distinguished: a) G2 and G3; b) G5 and G6; c) G8, G9 and G10. The subparallel REE patterns showing decrease in all REE and a slight increase in the negative Eu anomaly within each series and Rb-Sr isochrons for a) G2 and G3 and b) G8, G9 and G10 suggest a fractional crystallization for each series, which took 0.2 Ma for G2 and G3 and 0.6 Ma for G8, G9 and G10. Modelling of major and trace elements show that the richest samples in SiO2 of granite G2 and also granite G3 are derived from granite magma G2 by fractional crystallization of quartz, potash feldspar, plagioclase and biotite; b) granite G6 is derived form granite G5 by fractional crystallization of potash feldspar, plagioclase and biotite; c) the richest samples in SiO2 of granite G8 and granite G9 and G10 are derived from granite magma G8 by fractional crystallization of quartz, potash feldspar, plagioclase, biotite and ilmenite. (87Sr/86Sr)i values do not show any significant variation neither from G2 to G3 nor from G8 to G9 and G10, showing that fractional crystallization was the main mechanism, which is supported by the relatively similar, although irregular, εNdt and δ18O data within each series. However, in the G5 and G6 series, there was also a contemporaneous assimilation of metassedimentary material, as shown by (87Sr/86Sr)i, εNdt data and Rb-Sr errorchron. The AFC mechanism took 1.6 Ma. Granite G9 has monzogranite enclaves which belong to series G8 – G10 and will correspond to fragments of early cold margins, removed during the magma ascendancy. On the hand, the geochemical and isotopic characteristics of the granodiorite enclave in granite G1 and tonalitic enclaves in granites G5 and G8 suggest that these enclaves are not derived from the host granite magma and probably result from mixing of a crustal magma and an upper mantle magma or lower crustal magma. The monzogranite enclave in granite G1 is derived by anatexis of metassedimentary materials, but has distinct geochemical and isotopic characteristics from those of host granite, which is attributed to heterogeneity of metassedimentary materials. The tin-bearing granites G1, G2, G3, G6, G7 and G10 have ≥ 18 ppm Sn and the granite G5 shows 16 ppm Sn an are associated with Sn and W mineralizations. The main quartz veins containing cassiterite and wolframite cut granites G3 and G10, which contain 31 ppm Sn. Fractional crystallization was responsible for the increase in Sn content in granites from the series and their micas. Although biotite is richer in Sn than coexisting primary muscovite, muscovite retains a higher percentage of total granite tin content.
Descrição
Tese de Doutoramento em Geologia
Palavras-chave
Geologia , Petrogenese , Granitos