Publicado em "Acta Mineralogica-Petrographica. Abstract series 6

Dias, Patrícia Sofia Alves

Gomes, C. Leal

Universidade do Minho: RepositoriUM

GP86 – From gemstones to cell phones: The importance of pegmatites to society – poster 615Epidote populations in contaminated granitic pegmatites from Northern Portugal Dias, P.A.* & Leal Gomes, C.CIG-R – DCT, Universidade do Minho, Braga, Portugal (*patriciasdias@gmail.com) In Domo de Covas mega-anticline (Serra de Arga, N Portugal)the digestion of metacarbonate host-rocks during pegmatitic intrusion produced Ca saturation and high silica impoverishment, resulting desilicated pegmatite veins with plagioclase, scapolite, and local concentrations of phosphate and epidote.The following vein-fillings are distinguished: 1. Hybrid pegmatoids ʌ LQFOXGH SODJLRFODVH PDLQO\anorthite) r quartz r triphylite – lithiophilite. Fluorapatite occurs in border units and mark the transition to ʌ veins. Vesuvianite is discordant in exo-contact metasomatic hosts.2. Hybrid and fractionated pegmatoids (ʌoccur laterally DQG DUH WKH SURGXFWV RI IUDFWLRQDWLRQ RI ʌ SHJPDWLWHV 7KH\consist mainly of prehnite + pink clinozoisite. The clinozoisite is larger where it has crystallized first and these crystals manifest progressive zoning (Fig. 1). Textures also indicate simultaneous crystallization of zoisite and prehnite. Some latestveins only KDYHSUHKQLWHʌ-2). Oligoclase neosome, with scapolite and epidote, bearing sulphide cavities (chalcopyrite and sphalerite) fall into a more typical primary tendency, directed towards the albite pole in the haplogranite system.Fig. 1: Mineral intergrowth from veins (ʌ2) – subhedral corroded crystal of clinoizoisite (Czo) in transition to sympletitic zoisite (Zo) and prehnite (Pr) - and compositional fields of epidote from ʌ2 veins. Epidote and prehnite would come from the earlier crystallization of melts, which composition was modified by desilication. Low Fe content of zoisite (maximum XFe calculated content, 0.76) indicates low P, T conditions. The minimum temperature of crystallization is 440-500ºC (P <5Kbar), deduced from zoisite and clinozoisite compositions, in coexisting crystals, according to >1@. The lowest temperature is accompanied by a zoisite – plagioclase transition, consistent with the final reaction: 3 anorthite + 2 calciteÙ 2 zoisite + CO2. Plagioclase may result from the reaction CO2 + 2 clinozoisite Ù 3 anorthite + calcite + H2O.The contamination of Li-rich residual magma by Fe and Mn transfer, gives rise to the emergence of triphylite-lithiophilite concentrations that under normal conditions would not occur. Contamination is also expressed by high Zn concentrations in zircon (maximum content observed, 1.52%) and the deposition of sulphides in oligoclase pegmatoids. The protolith contains exhalite with high contents of Zn and Mn that were remobilized by the intrusion of pegmatite magmas.>1@ Franz, G. & Selverstone, J. (1992) Am Mineral., 77, 631-642.Compositional evolution of grossularite from leucotonalitic pegmatite Ruda nad Moravou, Czech Republic; EMPA and LA-ICP-MS studyGadas, P.* & Novák, M.Dept. of Geological Sciences, Masaryk University, Brno, Czech Republic (*pgadas@centrum.cz) The zoned leucotonalitic pegmatite Ruda nad Moravou is characterized by high Ca-contents manifested by an abundanceof plagioclase (An98-19), epidote-group minerals including allanite-dissakissite, diopside and grossularite [1]. The latter ispresent in most textural-paragenetic units from the contact inwards: G1 is almost monomineral, fine-grained, white zoned grossularite (core - Grs97-99 Adr1-3, rim - Grs89-90 Adr8-9 Sps0-1Prp0-1) from the grossularite subunit, developed along the contact with host serpentinite; G2 is microscopic, interstitial grossularite (Grs77-85 Adr11-16 Sps0-1 Prp0-1 Sch0-4) from leucocratic subunit; G3 is graphic intergrowths of pale brown grossularite + quartz (Grs73-86 Adr11-24 Sps1-2 Prp0-1 Sch0-4) from coarse-grained granitic unit; G4 is large crystals of colorless grossularite (Grs98-100 Adr0-1 Prp0-1) at the contact of the blocky unit and quartz core. Grossularite is associated with plagioclase, quartz and epidote-group minerals (G1, G2). EMPA show compositional evolution from almost pure grossularite (G1)through Fe3+-enriched (G2 and G3) to pure grossularite (G4). LA-ICP-MS study of the individual types of grossularite revealed distinct patterns of REE (Fig. 1), which exhibit similar patterns for Fe3+-poor grossularite (G1+G4) and Fe3+-enriched grossularite (G2+G3), respectively. Similar behavior was obtained also for V, Ni, Cr and Sn. Only concentrations of Ga, Zn and Hf increase during crystallization as well as Zr/Hf ratio.  Fig. 1: REE contents of the individual grossularite types compared with chondrite [2].Incorporation of most trace elements into the grossularite structure is controlled chiefly by crystal structural constraints – entering of Fe3+ into the B-site. The role of allanite-dissakissite seems low because G1, G2 show lower La but Ce and LREE are similar or higher relative to G3 and G4. Also the degree of fractionation manifested by evolution of plagioclase from anorthite to oligoclase has only minor importance on concentrations of trace elements except for Ga, Zn, and Ta. [1] Novák, M. & Gadas, P. (2010) Can. Mineral., submitted. [2] Boynton, W.V. (1984) in Henderson, P. (ed.) Rare-Earth Element Geochemistry. Elsevier, Amsterdam, 63-114. 

Epidote populations in contaminated granitic pegmatites from Northern Portugal

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Universidade do Minho: RepositoriUM