Rock textures and mineral zoning – A clue to understanding rare-metal granite evolution: Argemela stock, Central-Eastern Portugal

0550728 - GLÚ 2022 RIV NL eng J - Článek v odborném periodiku
Breiter, Karel - Ďurišová, Jana - Korbelová, Zuzana - Lima, A. - Vašinová Galiová, M. - Hložková, M. - Dosbaba, M.
Rock textures and mineral zoning – A clue to understanding rare-metal granite evolution: Argemela stock, Central-Eastern Portugal.
Lithos. 410-411, February (2022), č. článku 106562. ISSN 0024-4937. E-ISSN 1872-6143
Grant CEP: GA ČR(CZ) GA19-05198S
Institucionální podpora: RVO:67985831
Klíčová slova: Argemela * Fractionated granite * Textures * Mineral zoning * Mica * Quartz
Obor OECD: Mineralogy
Impakt faktor: 3.5, rok: 2022
Způsob publikování: Omezený přístup
https://www.sciencedirect.com/science/article/pii/S0024493721006058

The small leucocratic strongly peraluminous P, F, Li-rich granitic system at Argemela, central-eastern Portugal, is an ideal object for the study of relations between chemical composition of granitic rocks, rock-forming minerals androck and mineral textures, and the associated Sn, Ta and W mineralization. To define the rock and mineral composition and to formulate an evolutionary model, traditional methods of bulk-rock chemical analyses, and EMPA and LA ICP-MS analyses of rock-forming minerals were combined with the study of macrotextures, TIMAautomated mineralogical mapping of typical rock samples, and CL study of internal texture of quartz and mica crystals. The Argemela stock forms a steep cylindrical body 1 km deep, with an elliptical outcrop 250 × 180 m in size, crosscutting Cambrian schists. The stock comprises two intrusions: a geochemically moderately evolved equigranular facies (0.3 wt% F, 1.2 wt% P₂O₅, 1850 ppm Li, 310 ppm Sn, 24 ppm Ta, 5 ppm W) composed of albite, quartz, muscovite and minor montebrasite followed by a rather inhomogeneous, more evolved porphyritic facies (0.25–1.2 wt% F, 1.3–1.8 wt% P₂O₅, 1600–4900 ppm Li, 600–1000 ppm Sn, 40–85 ppm Ta, 3–6 ppm W) composed of quartz, mica and subordinate K-feldspar phenocrysts embedded in an albite-quartz-mica-amblygonite matrix. The later facies is rimmed by stockscheider at the contact with slates, and with an up to 1 m thick layered zone with unidirectional solidification textures along contact with the equigranullar facies. The NE-part of the stock is crosscut by numerous thin quartz veinlets ±K-feldspar, phosphates and wolframite, while several aplitic dikes and thicker quartz+phosphate veins were found in two boreholes NW of the granite. Porphyritic facies, the most voluminous part of the system, contains strongly zoned phenocrysts of mica (phengite→lepidolite) and quartz (Tienriched to Al, Rb-enriched) indicating crystallization in two magmatic stages, while the equigranular facies and extragranitic aplitic dikes crystallized in one magmatic stage only. Hydrothermal muscovite enriched in Sn and Ta forms late overgrowths on some mica flakes in the porphyritic facies and in aplitic dikes. Based on a combination of chemical and textural observations, a four-stage evolutionary model of the Argemela stock was formulated: (i) an early magmatic stage comprises intrusion and in situ crystallization of aplitic dikes and the equigranular facies, and crystallization of quartz and mica cores in the deeper reservoir prior to intrusion of the porphyritic facies, (ii) in the late magmatic stage, crystal mush + evolved residual melt from the reservoir intruded upwards crystallizing the porphyritic facies including the stockscheider and layered zone. A small part of evolved melt formed rounded enclaves crystallizing as a phenocrysts-free ball facies. (iii) In the early (high-temperature) hydrothermal stage, magmatic fluid caused weak pervasive muscovitization of the porphyritic facies and aplitic dikes accompanied by the crystallization of disseminated cassiterite and columbite. After the transition from ductile to brittle deformation, quartz veinlets also developed, mainly in the NE part of the body. (iv) In the late, low-temperature stage, schistderived fluid enriched in Ca, Sr and Ba caused local alteration of granite, forming crandallite–goyazite–gorceixite solid solution.
Trvalý link: http://hdl.handle.net/11104/0326034