Encarnación Roda-Robles

Chemical variations and significance of phosphates from the Fregeneda-Almendra pegmatite field, Central Iberian Zone (Spain and Portugal)

Abstract Field, textural, paragenetic, and chemical data for micas from pegmatites of the Fregeneda-Almendra pegmatitic field (Central-Iberian Zone) are used to characterize and evaluate their petrogenesis. These pegmatites show a zonal distribution from barren to evolved, with an increase in degree of evolution with increasing distance from the Mêda-Penedono-Lumbrales leucogranite. Five types of evolved pegmatites have been recognized: (1) petalite-rich, (2) spodumene-rich, (3) Li-mica + spodumenerich, (4) Li-mica-rich, and (5) cassiterite-rich pegmatites, plus six types of barren and intermediate pegmatites. Representative micas from the different pegmatite types and from the leucogranite were analyzed for major and trace elements. All micas belong to the muscovite-lepidolite series. Lithium is incorporated into Li-micas via the Li3Al-1□-2 and Si2LiAl-3 substitutions, where □ represents vacancies. The Al4Si-3□-1 and Al2□1R2+-3 substitutions, where R2+ = (Fe + Mg + Mn), account for the compositional variability of micas from the Li-mica-free pegmatites. The Li, Rb, Cs, Be, Ta, and Nb contents of micas increase in the order: leucogranites and barren pegmatites < intermediate pegmatites < spodumene-bearing and petalite-bearing dikes < Li-mica-bearing pegmatites. The Ba content decreases in the same order, and Sn and Zn are relatively abundant in the intermediate pegmatites. These variations are consistent with rare-element enrichment via fractionation processes combined with partitioning of rare elements from the pegmatite melt into the minerals and volatile phases. However, some pegmatite types occurring in this area, such as the cassiterite-rich dikes, do not seem to form part of the same evolutionary trend.

Mineralogy and geochemistry of micas from the Pinilla de Fermoselle pegmatite (Zamora, Spain)

The highly fractionated, Li-F-Be-B-P-bearing Pinilla de Fermoselle (PF) pegmatite crops out in the westernmost part of the Zamora province (Spain). This body appears as a cupola over the PF leucogranite, displaying a non-symmetrical internal zonation with a complete sequence from a barren pegmatitic facies near the granite, to a highly evolved zone in the uppermost part of the body. Representative samples of micas from the different pegmatite zones have been studied. Based on textural and chemical criteria, the micas may be grouped into two assemblages: Al-rich micas and Fe-rich micas. In general, Al-rich micas show a continuous evolution from muscovitic to lepidolitic compositions from the leucogranite to the most evolved zone. Fe-rich micas range from Fe-biotite in the leucogranite and in the least evolved pegmatite zones, to an intermediate composition between zinnwaldite and trilithionite in the most evolved pegmatitic facies. The incorporation of Li into micas appears to be controlled by the substitutions Si 2 LiAl -3 ,a nd Li3Al-1-2, AlLiR-2 ,S iLi 2R-3 ,a nd SiLiAl -1R-1, where R = (Fe 2+ + Mg + Mn). Paragenetic relationships and chemical variations in micas from different zones making up the PF pegmatite suggest that the pegmatitic system derived from a granitic melt and evolved upwards by fractionation processes. Evidence in support of this model comes from: (i) the gradual enrichment in Li, Rb, Cs and F, parallel to the decrease in Mg and Ti; (ii) the convergent evolutionary trends towards lepidolite showed by the Al- and Fe-micas; and (iii) the parallel decrease in the K/Rb ratio in micas.

Tourmaline as a petrogenetic monitor of the origin and evolution of the Berry-Havey pegmatite (Maine, U.S.A.)

Abstract The Berry-Havey pegmatite (Oxford pegmatite field, Androscoggin County, Maine, U.S.A.), enriched in Li, F, B, Be, and P, is intruded in hornblende-rich amphibolite, with minor biotite or diopside. The pegmatite has a complex internal structure, with four texturally and compositionally different zones that show an increasing degree of evolution inward: wall zone, intermediate zone, core margin, and core zone. The main minerals are quartz, feldspars, Al-micas, tourmaline, with minor Fe-micas, garnet, beryl, amblygonite-montebrasite, Fe-Mn phosphates, and apatite. Tourmaline is present in all zones of the pegmatite, showing different textures: black anhedral crystals in the wall and intermediate zones; black prisms of up to 40 cm in length in the intermediate zone; black tapered prisms, surrounded by a pseudo-graphic intergrowth of quartz or albite with black ± green/bluish tourmaline, and constituting a continuous layer under the core zone; multicolored and “watermelon” zoned crystals in the core zone; and gemmy deep green and color-zoned “watermelon” tourmaline prisms, up to 15 cm length, inside the pockets. A complete chemical evolution from Mg-rich schorl in the wall zone to elbaite with an important deprotonation in the pockets inside the core zone is observed. The most plausible exchange vectors for this chemical evolution are FeMg-1, YAlWO[YR2+W(OH)]-1 and Al[X]X(R2+Na)-1 (where R2+ = Fe2++Mg2++Mn2++Zn2+), for the tourmalines from the wall and intermediate zones. In the core margin, tourmaline composition evolves from schorl toward Li-rich species through the substitution (YAlYLiYR2+-2). Later, during the crystallization of the core zone, this exchange vector combined with the substitution ([X] YAl0.5XNa-1YLi-0.5). Finally, the gemmy tourmalines from the pockets show a deprotonation related to the exchange vector YAlWO2YLi-1W(OH)-2 and may be classified as darrellhenryite. These substitutions may reflect an increase in oxygen fugacity and a decrease in Li and F related to the crystallization of lepidolite and amblygonite-montebrasite in the core zone adjacent to or within the pockets. The crystallization of these minerals would reduce the availability of Li and F for the very latest tourmaline crystals, growing inside the pockets, where the deprotonation becomes important. Chemical and textural variation in tourmaline is consistent with a fractional crystallization process for the internal evolution of the Berry-Havey pegmatite. Crystallization of the tourmaline layer under the core zone may be related to the exsolution of the fluid phase implied in the formation of pockets.

Geology and mineralogy of Li mineralization in the Central Iberian Zone (Spain and Portugal)

Abstract Lithium mineralization is common in the Central Iberian Zone and, to a lesser extent, in the Galizia-Trás-Os-Montes Zone of Spain and Portugal, occurring along a ~500 km-long NNW-SSE striking belt. There are different styles of Li mineralization along this belt; they are mainly associated with aplite-pegmatite bodies and, to a much lesser extent, with veins of quartz and phosphate. Lithium mineralization in the Central Iberian Zone may be classified into four types: aplite-pegmatite dykes occurring in pegmatitic fields, Li mineralization associated with leucogranitic cupolas, beryl-phosphate pegmatites and quartz-montebrasite veins. The main Li minerals of these bodies include Li-mica, spodumene and/or petalite in the pegmatitic fields and leucogranitic cupolas; triphylite-lithiophilite in the beryl-phosphate pegmatites, and amblygonite-montebrasite in the quartz-montebrasite veins. The origin of these different styles of mineralization is considered to be related to differentiation of peraluminous melts, which were generated by partial melting of metasedimentary rocks during the Variscan orogeny. On the basis of paragenesis and chemical composition, the pegmatitic fields and Li mineralization associated with granitic cupolas record the highest fractionation levels, whereas the beryl-phosphate pegmatites and quartz-montebrasite veins show lower degrees of fractionation. There are a number of textural and mineralogical indicators for Li exploration in the Central Iberian Zone and in the Galizia-Trás-Os-Montes Zone, with the highest economic potential for Li being in the pegmatite fields.

Multistage boron metasomatism in the Alamo Complex (Central Iberian Zone, Spain): Evidence from field relations, petrography, and 40Ar/39Ar tourmaline dating

Abstract The Alamo Complex consists of structural-metamorphic domes surrounded by low-grade metasedimentary rocks of Upper Proterozoic to Lower Cambrian age that form part of the Schist Graywacke Complex (Central Iberian Zone, Spain). Tourmaline is ubiquitous throughout the domes, in which it occurs in tourmalinites, psammo-pelitic schists, quartzites, gneisses, migmatites, leucogranites, aplo-pegmatites, and quartz veins. Overall, tourmaline compositions can be described within the four component system schorl-dravite-foitite-magnesiofoitite, with K2O < 0.23%, low Ca contents, Mg/(Mg+Fe) = 0.25-0.71 and X/(X+Na) = 0.18-0.43, where X = vacancies in the X site. Field relations and petrographic observations, combined with tourmaline 40Ar/39Ar data, provide evidence of intense boron metasomatism affecting this region. Tourmaline 40Ar/39Ar step-heating spectra for Cambrian-Ordovician orthogneisses are complex, yielding a pseudo-plateau age of ~370 Ma that is interpreted to reflect Variscan rejuvenation of older tourmaline. Tourmaline 40Ar/39Ar data for mylonitized and folded tourmalinites yield disturbed spectra with pseudo-plateau ages of ~355-342 Ma that are unsuitable for precise age determination. These ages, however, are consistent with published ages (340-350 Ma) for the second Variscan deformation (D2). Tourmaline from fine-layered tourmalinite and metasedimentary rocks yield well-defined plateau ages of 317 and 315 Ma, respectively, recording an additional metasomatic event concomitant with anatexis and evolution of B-bearing granites, pegmatites, and hydrothermal fluids. The different tourmaline-forming stages reflect significant boron cycling within the continental crust of the Central Iberian Zone, driven by deformation, metamorphism, and magmatism during the Variscan orogeny. Boron-rich aqueous fluids related to Cambro-Ordovician magmatism are considered to be the primary source of boron.

Insights into petrogenesis of the Jálama pluton (Central Iberian Zone, western Spain)

ABSTRACT The Jálama pluton (JP) is a Variscan peraluminous granitoid that intruded into low-grade metasediments from the Central Iberian Zone (CIZ). It comprises a sillimanite-bearing two-mica monzogranite in the inner zone, followed by a tourmaline-bearing two-mica monzogranite, and a marginal tourmaline-muscovite leucogranite in the northern half of the pluton. Microgranitoid enclaves and metasedimentary xenoliths occur locally in monzogranites. The change in rock type from the central monzogranite to the marginal leucogranite corresponds to decreasing TiO2, MgO, FeO, CaO, Sr, Ba, Zr, and ΣREE, and increasing SiO2, Na2O, P2O5, Rb, Li, Cs, Ta, Sn, and W. Fe/(Fe+Mg) ratios in biotite, muscovite and tourmaline increase with increasing Fe/(Fe+Mg) in bulk rock, suggesting an important control of the bulk-rock composition on mineral chemistry. The high peraluminosity, the low CaO and high P contents, as well as the similarity of ε(Nd)300 values in both the granites and metasediments of the southern CIZ constitute strong evidences for a crustal origin of the granite suite, probably by melting of these metasedimentary rocks. Field and petrographic observations, together with mineralogical and geochemical data, suggest that assimilation and mingling/mixing acted in concert with fractional crystallization during the formation of the JP. These processes may also have been important in the evolution of other granitoids from this region.

Mica and feldspar as indicators of the evolution of a highly evolved granite-pegmatite system in the Tres Arroyos area (Central Iberian Zone, Spain)

The aim of this study is to establish the petrogenetic links between the different granitic and aplopegmatitic units occurring in the Tres Arroyos granite-aplopegmatite system (Central Iberian Zone, Spain), from the textural and chemical variations in micas and feldspars. We aim to understand the differentiation mechanisms that allowed the extreme fractionation levels observed in the Li–F-richest dykes occurring in this aplopegmatite field. Major and trace elements in micas and feldspar from the different facies were analyzed by electron microprobe and LA-ICP-MS. Mica compositions define three different trends: muscovite-zinnwaldite-polylithionite, muscovite-trilithionite-polylithionite and biotite-zinnwaldite. The substitution mechanisms depend on the type of trend and on the stage of evolution. The K/Rb ratio and the Ba contents decrease in micas and K-feldspar with fractionation, whereas Li, Rb and Cs values increase. Lithium, Rb, Cs, Ba, Nb, and Ta show a clear tendency to get into the structure of the Fe-rich mica, whereas Be partitions preferably into the Al-micas. When Fe-micas are absent, Al-rich micas become the major sink for those trace elements. Phosphorus and Pb preferably get into the feldspars. In general, a linear and continuous variation of the K/Rb ratio with incompatible elements such as Cs, Rb or Li, and compatible such as Ba, both in micas and K-feldspar, supports a petrogenetic link between the Alburquerque batholith and the aplopegmatites via fractional crystallization.ResumenEl propósito de este trabajo es establecer las relaciones petrogenéticas entre las distintas facies graníticas y aplopegmatíticas del sistema de Tres Arroyos (Zona Centro Ibérica, España) mediante las variaciones texturales y químicas en micas y feldespatos. También se intenta entender los diferentes mecanismos responsables del fraccionamiento extremo que se observa en los diques más ricos en Li-F. Se han analizado los elementos mayores y traza de las micas y de los feldespatos de las distintas facies por microsonda electrónica y ablación láser. Las micas pertenecen a tres tendencias composicionales: moscovita-zinnwaldita-polilitionita, moscovita-trilitionita-polilitionita y biotita-zinnwaldita. Los mecanismos de substitución varían según el tipo de mica y el grado de fraccionamiento. La razón K/Rb y los contenidos de Ba en micas y feldespato-K descienden con el fraccionamiento, mientras que los valores de Li, Rb y Cs aumentan. El Li, Rb, Ba, Nb y Ta se incorporan preferentemente en la estructura de las micas de Fe, mientras que el Be tiende a entrar en las de Al. En ausencia de micas de Fe, los elementos traza se incorporan principalmente en las micas de Al. El P y el Pb son los únicos elementos que se acumulan en los feldespatos. En general, la continua tendencia linear que presentan las razones de K/Rb versus elementos incompatibles (Cs, Rb o Li), o elementos compatibles (Ba), tanto en micas como feldespato-K apoya la teoría de que las aplopegmatitas están genéticamente relacionadas con el granito de Alburquerque mediante un mecanismo de cristalización fraccionada.

Granitic pegmatites: the State of the Art. Preface.

The PEG2007 symposium was held on May 6–12, 2007 in Porto, Portugal. The conference was organised by the Department of Geology of Porto University, the Department of Mineralogy and Petrology of the University of the Basque Country, Bilbao, Spain, the Laboratory of Mineralogy of the University of Liège, Belgium, and the Laboratory of Mineralogy of Paul Sabatier University, Toulouse, France. The aim of this symposium was two-fold. On the one hand, specialists from all over the world had the opportunity to meet, discuss and learn about current research ideas and findings on granitic pegmatites. On the other hand, the tradition of gathering members from the scientific community every two years to advance our understanding on pegmatites was maintained, following on the “LERM” conference in the Czech Republic in 2003 and the “Crystallization processes in granitic pegmatites” conference on Elba Island (Italy) in 2005. Research on granitic pegmatites will continue to expand and, in 2009, the PEG2009 meeting will take place in Recife (Brazil). Information on the PEG2009 meeting is already available on the webpage at http://www.ufpe.br/geologia/peg2009brazil/. The PEG2007 conference was attended by 74 participants, representing 16 countries. There were 24 oral presentations and 19 poster presentations, abstracts of which are available at http://www.fc.up.pt/peg2007/. A selection of six papers presented at PEG2007 constitutes this thematic issue of the European Journal of Mineralogy. The topics of the papers presented in this issue encompass several aspects of modern pegmatite research, from regional (Neiva et al.) to detailed mineralogical studies (Novák et al., Uher et al.) through crystal-growth (Sánchez-Muñoz et al.) and trace-element (Müller et al.) aspects, as well as a review on the latest discoveries and theories on pegmatites by Simmons & Webber. We would like to thank Chief Editor Fernando Nieto and special issues coordinator Walter Maresch who helped us with the manuscripts. We are also in debt to the referees who helped us in the reviewing process of the manuscripts. A thank you also to Tania Martins and Romeu Vieira for helping in the PEG2007 organization. Although 2007 was a good year for “pegmatologists”, thanks to this meeting, it was also a sad year, because of the loss of two well-known researchers who contributed to the development of studies on granitic pegmatites. We take this opportunity to dedicate these contributions to the memory of François Fontan and Bernard Charoy.