Datación U-Pb de vesuvianita mediante LA-ICP-MSun ejemplo de los mármoles de Somosierra

Resumen

Vesuvianite is a common mineral in skarn ore deposits and metamorphic rocks derived of impure carbonate sediments. This mineral may contain elevated amounts of U and, consequently, be of use for age determinations by U-Pb methods. Yet, the high contents of common Pb has prevented this practice and except for the study by Romer (1992) has not been used in geo- chronology. The marbles to the N of the Somosierra mountain pass (quarries near the La Chorrera de los Litueros water falls) contain several minerals of potential use in U-Pb age studies out of which we selected Ca-rich garnet, zircon and vesuvianite for the analysis by laser ablation-ICP-MS (UP213, ICAP QC). Garnets contain U < 1 ppm and could not be dated. Idiomor- phic zircons are abundant but very small (20-50 μm) and display distinct core and rim areas. 10 results > 90 % concordant yielded 238 U/206Pb ages between 867 and 449 Ma, with a maximum of 503 ± 23 Ma, interpreted with reservations as indicative of that of the sedimentary protoliths. Vesuvianites, several mm in size, have 25 to 190 ppm U being the most interesting mine- ral for the geochronological study. In spite of the absence of matrix-matched standards for the correction of the fractionation, relatively high and variable amounts of common lead (10-30 ppm) allow to obtain an age of 274 ± 1 Ma (TW concordia plot lower intercept). From a geological point of view this age is in line with that of the youngest Variscan granites of the Guada- rrama Range and could be related to the intrusion of pegmatite dykes in the quarry area.