La edad del skarn de Caladatación U-Pb mediante TIMS y LA-ICP-MS del granate cálcico

Zusammenfassung

The Cala mine depicts mineralizations of magnetite and sulfides related to the formation of a calcic skarn and to the develo- pment of an extensional shear zone (Carriedo and Tornos, 2010). We have performed a study by ID-TIMS and LA-ICP-MS methods of Ca-rich garnets from the skarn to establish their U-Pb age, and evaluate the viability of this mineral as a standard for LA dating. Anysotropic garnets studied are richer in grossular (~57 mol %) than in andradite (~41 mol %) without sig- nificant compositional zoning. Four garnet fractions analysed by ID-TIMS yield a combined 206 Pb/207 Pb age of 340 ± 2 Ma. For the LA-ICP-MS study (UP213, iCAP Qc) we performed 39 spots on different garnets. The results show low Pb (<1 ppm) and significant U contents (≈10 ppm), defining a regression line in the Tera-Wasserburg diagram with a lower intersection at 334.4 ± 0.6 Ma. Using the raw material ratio of 207Pb/ 206 Pb=0.8451 for the correction of common Pb, a concordia age of 337.8 ± 0.4 Ma is obtained. The results agree with the age of 338 ± 0.4 Ma (Zr U-Pb, TIMS; Carriedo and Tornos, 2010) pro- posed for the intrusion of the granodiorite porphyry related to the skarns formation, support the validity of Ca-rich garnets for U-Pb age studies and the potential of the garnet of Cala as standard for LA-ICP-MS dating.