Geochemistry of REE for the Carboniferous Huaco granitic complex, sierra de Velasco, Argentina

SARDI, FERNANDO G.; MURATA, M. ; GROSSE, P.; FERNANDEZ LOZANO, R.

Florianopolis

Simposio; The 8th Hutton Symposium on Granites and Related Rocks; 2015

Sociedad Brasilera de Geologia

The REE geochemistry of facies of the Huaco granitic complex, Velasco range (Sierras Pampeanas) of Northwestern Argentina is presented. The Huaco granitic complex has a more or less circular shape and occupies an area of around 620 km2. It has a Lower Carboniferous age and geochemical and geologic features of A-type granites. We can distinguish several petrographic facies within the Huaco complex. Their lithological compositions vary between granodiorite and syeno-monzogranite, but they have notable differences in occurrence, texture and accessory minerals. The identified facies are:BG ?Border Granite- is the border facies of the complex that grades transitionally into the main facies, RPG. It has a slightly porphyritic texture and biotite > muscovite. RPG ?Regional Porphyritic Granite- constitutes the bulk of the Huaco granite. The texture is predominantly porphyritic with ~30-35% megacrysts of perthitic microcline, and the biotite/muscovite ratio is ~2.APG ?Adjacent Porphyritic Granite- is a facies that surrounds Be-pegmatites, which are abundant in the complex. The contact with the RPG is transitional, but with the MAP facies it is sharp. The APG has a porphyritic texture, but the microcline megacrysts are smaller and slightly less abundant than in the RPG. It contains slightly more muscovite and less biotite than the RPG.MAP ?Marginal Aplite of Pegmatite- constitutes the granitic border of the Be-pegmatites. The texture is equigranular and the biotite/muscovite ratio is ~0.3.LGE ?LeucoGranite Enclaves- are small, up to 2 m in diameter, leucogranitic enclaves hosted within the RPG in sharp contact. They have equigranular textures, and biotite and muscovite are found in variable amounts; some contain garnet.EqG ?Equigranular Granite-, are granites bodies hosted in the RPG with dimensions from < 100 m in diameter to over 1 km2. The texture is equigranular and muscovite > biotite; tourmaline is often present.Geochemically, the facies are calc-alkaline and peraluminous (ASI > 1); they show enrichments in LIL and HFS elements.The ƩREE is higher in the RPG (avg. 236 ppm), followed by the BG (205 ppm), the APG (175 ppm), the EqG (86 ppm), the LGE (68 ppm) and the MAP (13 ppm). LREE > HREE, except for one garnet-bearing sample belonging to the LGE facies.Chondrite-normalized REE patterns for the BG, RPG, APG and EqGfacies are similar, with negative slopes attributed to LREE fractionation by accessory minerals, essentially monazite; the average value of Lan/Ybn is ~15 for the BG facies and ~9 for the other three. The MAP facies shows a flatter pattern (Lan/Ybn ~ 2), while the LGE patterns are very irregular, with variable Lan/Ybn (0.4 ? 8.4). The negative Eu-anomaly is notable in all facies (Eu/Eu* < 1), except for garnet-bearing sample of the LGE facies.In light of the REE geochemistry, the BG, RPG, APG and EqGfacies can be formed by fractional crystallization from a common Huaco magma source. Although the granites of the MAP and LGE facies are spatially related to the main phase of Huaco granitic complex (RPG), they seem to have had an independent magmatic evolution.