Large-ion lithophile elements in K-feldspar as monitors of magmatic evolution in the Velasco Pegmatitic District, La Rioja province, Argentina

SARDI, F. G.; GROSSE, P.; MURATA, M.; NAVAS, S.

Gottingen

Congreso; XXI Colloquium on Latin American Earth Sciences; 2009

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 composed mainly of K-feldspar and accompanied by several accessory minerals such as beryl, muscovite, biotite, F-apatite, triplite and tourmaline; 3) quartz core, sometimes with 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 evolved Carboniferous granites of S-type affinity. 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 (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 K-feldpegm (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 show 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 elements. The Carboniferous magmatism developed in the center-eastern region of the Velasco range has produced a great number of Be-pegmatites, some of them extremely fractionated.