New SHRIMP ages in the Sierra de Famatina, NW Argentina: implications for the Famatinian Orogen

DAHLQUIST, J. A., PANKHURST, R. J., RAPELA, C. W., GALINDO, C., ALASINO, P., CASQUET, C., FANNING, C. M., SAAVEDRA, J., BALDO, E. AND GONZÁLEZ-CASADO, J.

Mendoza, Argentina

Simposio; Gondwana 12: Geological and Biological Heritage of Gondwana; 2005

The range of ages in the Sierra de Famatina (NW Argentina) suggested that the magmatic event in this mountain range had a prolonged development in time (from Ordovician to Devonian). However, new SHRIMP U-Pb zircon ages from different granitoids of the Ñuñorco and Cerro Toro Complex and a rhyolite located in the western flank of the Sierra de Famatina indicate that this magmatic event was essentially constrained to the Early-Mid Ordovician and was coeval with development of the Famatina retroarc basin, as indicated in recent geochronological and tectonic studies (Rapela et al. 1999; Pankhurst et al. 2000; Dahlquist and Galindo 2004). The analyzed samples are: (i) a tonalite of the Cerro Toro complex; (ii) a monzogranite of the Ñuñorco complex and (iii) a K2O-rich (K2O = 9.11%) rhyolite from outcrops in the Potrero Grande canyon. ASI values are 1.16 for the tonalite, 1.04 for the monzogranite and 1.02 for the rhyolite, i.e., they are metaluminous to slightly peraluminous igneous rocks. The tonalite has a SiO2 content = 62.73%, with Pl(48%)-Qtz(27%)-Bt(20%)±Mc±Hbl and Zrn-Aln-Ep-Opq (as accessory minerals). Its REE pattern is characterized by relatively high REEtotal (419 ppm), with [La/Yb]N = 3.0, and negative Eu anomalies (Eu/Eu* = 0.36). The monzogranite has a SiO2 content = 75.8%, with Pl(35%)-Mc(21%)-Qtz(39%)-Bt(3%) and Zrn-Opq-Aln-Ep-Ap (as accessory minerals), with a protomylonite texture. Its REE pattern is characterized by relatively high LREEtotal = 200, with [La/Yb]N = 9.6, and negative Eu anomalies (Eu/Eu* = 0.59). The rhyolite has a SiO2 content = 74.7%, with abundant Qtz and Kfs-Pl as phenocrystals and Zrn-Opq (as accessory minerals), showing distinctive spherulitic texture. Chl is a secondary mineral. Its REE pattern is characterized by relatively low REEtotal = 116, with [La/Yb]N = 2.28, and negative Eu anomalies (Eu/Eu* = 0.39). U-Pb SHRIMP zircon crystallization ages of the igneous rocks are 481 ± 4 Ma (tonalite), 463 ± 4 Ma (monzogranite), and 477 ± 4 Ma (rhyolite). Initial 87Sr/86Sr e(Sr) values calculated for the time of emplacement are, respectively, 0.7077 and -5.9 for the tonalite; 0.7066 and -3.0 for the monzogranite and 0.7000 and -2.9 for the rhyolite. Multistage TDM model ages are 1.66 Ga (tonalite), 1.43 Ga (monzogranite) and 1.44 (rhyolite). These data suggest that the parental magma was probably derived by the partial melting of older lithosphere. The anomalously low initial 87Sr/86Sr ratio for the rhyolite suggests post-crystallization modification of its Rb-Sr system. The following conclusions ca be obtained from previous studies and the new U-Pb SHRIMP data: (i) the ages from the analyzed granitoids and the ryolite from the western flank of the Sierra de Famatina are similar to those obtained in other parts of the Famatinian orogen; (ii) the available U-Pb zircon data indicate that the Famatinian magmatism was brief (~ 490-463 Ma), in remarkable contrast to the long-lived cordilleran magmatism of the Andes; (iii) the Early Ordovician development of ensialic marine basins was synchronous with  the emplacement of conspicuous crust-derived magmatism along the proto-Andean margin of Gondwana.