Rare-element class, beryl type, beryl-columbite-phosphate subtype pegmatites of the Velasco district from the Pampeana Pegmatite Province, Argentina, contain varieties of green, yellow, and aquamarine beryl. The major and trace element chemistry and dimensions of the unit cell parameters of beryl from ten pegmatites from the Velasco district were used to identify differences among beryl types, determine chromophore elements, and determine the relative degree of fractionation of the pegmatites. Concentrations of Rb, Li, Cs, and Na in all beryls analyzed are among the lowest measured in pegmatitic beryl of similar colors worldwide. Within and among individual pegmatites, an increase in Li and Cs contents and decrease in Na/Li ratios in the order green beryl → yellow beryl → aquamarine suggest that green beryl formed in the early stages of crystallization of the pegmatites while yellow beryl and aquamarine formed from more evolved fluids in the late stages of crystallization. The chemical variations from green beryl to yellow beryl to aquamarine reflect crystallization during melt fractionation and/or crystallization of Fe-Mn phosphate minerals. Beryl from the El Bolsoncito and El Principio pegmatites, with the highest Li and Cs and some of the lowest Fe + Mg contents in the studied beryl, reflects the highest degree of evolution among the Velasco pegmatites. Within individual pegmatites Cr contents in beryl decrease in the order green beryl → yellow beryl, and green beryl → aquamarine, consistent with Cr being a chromophore element for green beryl. Niobium contents increase from green to yellow beryl, and from green beryl to aquamarine, indicating that Nb tends to concentrate in late forms of beryl. The absence of a systematic change in the size of unit cell parameters a and c with varying compositions and the small size of the c parameter reflect the extremely limited incorporation of Li, Na, and Cs in the mineral. A compilation of beryl compositions worldwide shows the highest degree of fractionation for the Tanco, Koktokay No. 3, Minas Gerais, Kaatiala, Namivo, Bikita, Czech Republic, and Greer Lake pegmatites.

The Carboniferous post-orogenic Huaco and Sanagasta granites are located in the central sector of the Velasco Range, northwestern Argentina, and host rare-element (Be) pegmatites. They are highly fractionated peraluminous granites with porphyritic textures having variable sizes and abundances of K-feldspar megacrysts. Two different types of parental granites are recognized: a Regional Porphyritic Granite (RPG), distant from the Be-pegmatites, and an Adjacent Porphyritic Granite (APG) surrounding the Be-pegmatites. At the pegmatite margins, aplites (MA), or more rarely equigranular leucogranites, are in contact with the APG. The granite-to-pegmatite transition is characterized by a decreasing biotite/muscovite ratio and TiO 2 -Fe 2 O 3 t -MgO contents, which is consistent with fractional crystallization of biotite, and also by decreasing megacryst and grain size. A continuous magmatic crystallization trend from the RPG to the APG is observed, whereas the MA constitutes the earliest stage of pegmatite formation, characterized by depletion in ferromagnesian elements, calcium and some trace elements (Ba, Sr, Y, Nb, Ce, Zr, Th). However, the granite-pegmatite evolution occurred by differentiation from a common magma source.

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ABSTRACT. The A-type Huaco granite pluton of the Velasco range (Sierras Pampeanas of northwest Argentina) is formed by three coeval granitic facies and contains subordinate coeval-to-late facies, as well as enclaves, dikes and stocks that show different temporal relations, textures and compositions. The dominant facies (Regional Porphyritic Granite; RPG) is a porphyritic two-mica monzo-to syenogranite, with abundant microcline megacrysts up to 12 cm in size. It was emplaced in a dominant extensional setting and has a mainly crustal source but with participation of a mantle-derived component. The RPG transitions towards two coeval and co-genetic granite facies, at its margins (Border Granite; BG) and around Be-pegmatites (Adjacent Porphyritic Granite; APG). These two facies have a finer-grained texture and smaller and less abundant megacrysts. They are also monzo-to syenogranites, but a slight decrease in the biotite/muscovite ratio is observed from the BG to the RPG to the APG. Trace element modeling suggests that the RPG, BG and APG differentiated from the same magma source by fractional crystallization. Temporally older mafic (ME) and felsic (FE) enclaves are common in the pluton. The ME can be considered partially assimilated remnants of a mafic component in the genesis of the RPG, whereas the FE seem to be remnants of premature aplites. Other subordinate rocks intrude the RPG and are, hence, temporally younger: felsic dikes (FD), dioritic dikes (DD) and equiganular granites (EqG) are clearly posterior, whereas coeval-to-late Be-pegmatites (BeP) and orbicular granites (OG) formed during the final stages of crystallization of the pluton. The BeP, OG and FD indicate the presence of abundant water and volatiles. The EqG form small stocks that intrude the RPG and were possibly originated from purely crustal sources. The DD probably correspond to a younger unrelated episode of mafic magmatism. Keywords: Granitic facies, REE and LIL composition,Fractional crystallization model, Huaco granite, Velasco range, Sierras Pampeanas.RESUMEN. Estructura interna y geoquímica del plutón granítico carbonífero de Huaco, Sierra de Velasco, NW de Argentina. El Plutón Huaco, ubicado en la sierra de Velasco (en Sierras Pampeanas del noroeste de Argentina), es de afinidad granítica A y está conformado por tres facies graníticas y contiene varias rocas ígneas subordinadas, las que muestran diferentes relaciones temporales, texturas y composiciones. La facies granítica dominante (Granito Porfírico Regional; RPG) es un monzo-sienogranito de dos micas, de textura porfírica con abundantes megacristales de microclino que pueden alcanzar hasta 12 cm de longitud. Ha sido emplazado en un marco tectónico dominantemente extensional y habría tenido una fuente de origen cortical pero también con participación de componentes mantélicos. En los bordes del Plutón Huaco y envolviendo a pegmatitas de berilo contenidas en el mismo (BeP), se reconocen las facies cogenéticas BG y APG respectivamente, las cuales tienen contacto transicional con el RPG. E...