INSUGEO   12554
INSTITUTO SUPERIOR DE CORRELACION GEOLOGICA
Unidad Ejecutora - UE
congresos y reuniones científicas
Título:
Large-ion lithophile elements in K-feldspar as monitors of magmatic evolution in the Velasco Pegmatitic District, La Rioja province, Argentina
Autor/es:
SARDI, F., GROSSE, P., MURATA, M., NAVAS, A.
Lugar:
Göttingen, Alemania
Reunión:
Congreso; LAK 2009 (LateinAmerika Kolloquium); 2009
Institución organizadora:
University of Göttingen
Resumen:
The Velasco Pegmatitic District of the Pampean Pegmatitic Province (NW Argentina) is located in the central-eastern part of the Velasco range, in north-western La Rioja province. The pegmatites have three main zones: 1) marginal-external zone composed of aplite or fine-grained granite with topaz; 2) intermediate zone componed mainly of K-feldspar and accompanied by several accessory minerals such as beryl, muscovite, biotite, F-apatite, triplite and tourmaline; 3) core zone componed by quartz with subordinate beryl and/or tourmaline. The pegmatites belong to the rare element class, beryl type, and beryl-columbite-phosphate subtype. They are spatially and genetically related to Carboniferous evolved peraluminous granites. The granites are porphyritic due to the presence of abundant K-feldspar megacrysts with very common perthite textures. The large-ion lithophile elements Ba, Rb and Sr of K-feldspars from both pegmatites -essentially from intermediate zone- (K-feldpegm, n = 17) and host parental granites (K-feldgran, n = 4) were analyzed using the XRF methodology. Ba, Rb and Sr contents in K-feldpegm are variable, where as they are homogeneous in K-feldgran. Overall and average contents are very different in each rock type; the average contents of Ba and Sr in K-feldpegm (71 ppm and 14 ppm, respectively) are significantly lower than in K-feldgran (465 ppm and 96 ppm), while the average Rb content in Kfeldpegm (1214 ppm) is more than double that in K-feldgran (522 ppm). In general, the Ba, Rb and Sr contents in K-feldpegm are in agreement with other districts and fields from the Pampean Pegmatitic Province and world-wide. The K-feldpegm shows Rb enrichment and Ba and Sr depletion relative to parental K-feldgran. Since granitic pegmatites usually represent the late to post-magmatic stages of a continuous fractionation trend, large-ion elements such as Ba, Rb and Sr are excellent guides capable of explaining the magmatic evolution in the Velasco district, which is not evident using major element. So, it can affirmed that the Carboniferous magmatism developed in the center-eastern region of the Velasco range has produced several Be-mineralized and extremely fractionated pegmatites.