U-PB GEOCHRONOLOGY AND GEOCHEMISTRY OF PALECENE TO EOCENE LEUCOCRATIC PLUTONS OF THE SIERRAS MAGDALENA AND LA JOJOBA, NORTHERN SONORA, MEXICO

The basement of north-central Sonora in the Guacomea, La Jojoba and Magdalena mountains consists of Jurassic volcanic, metasedimentary and plutonic rocks and strata of the Late Cretaceous Cocospera formation that were intruded by granites of Late Cretaceous to Eocene ages. In this report we present new U-Pb ages obtained from these units that help to constrain the geologic events of the lower plate of the Magdalena-Madera metamorphic core complex in this region. Two samples of different metavolcanic and quartz rich-sandstone outcrops of sierra La Jojoba that we dated by detrital zircons (n = 188) yielded only Jurassic zircons and similar, unique age peaks at 172 Ma. These strata are intruded by a ca. 171 Ma foliated granite dated by Nourse et al. (2018). The Late Cretaceous Cocospera formation, that is also metamorphosed, yielded Proterozoic (n = 25%), Paleozoic (n = 4%) and Late Cretaceous detrital zircons (n = 40%). These rocks are intruded by several leucocratic, two-mica granites with garnet that include El Blanco (58.78 ± 0.41 Ma), La Mariana (48.8 ± 1.4 Ma), Lluvia de Oro (42.62 ± 0.52 Ma), El Pozole (41.79 ± 0.80 Ma) and Terrenate (45.84) ± 0.52 Ma) granites. Three U-Pb ages from the Sierra Magdalena granites indicate they are 45.53 ± 0.63 and 37.69 ± 0.38 Ma, including the Cañada Honda granite of Nourse et al. (2018) that is 41.2 ± 1.7 Ma. An large augen gneiss xenolith in the southern part of the sierra Magdalena that yielded a 1.07 Ga U-Pb age, represents part of the missing Proterozoic basement in this region. Geochemically, most of these granitoids classify as granites and granodiorites that have SiO2 contents ranging from 67.6 to 77.3 wt. %. They are mostly peraluminous, medium- to high-K calcalkaline rocks. Multielemental diagrams of the analyzed samples suggest a clear affinity for a magmatic arc tectonic setting, however some granites have irregular REE patterns denominated in the literature as "lanthanide tetrad effect", marked by an increase in the enrichment of heavy REE, and a clear negative Eu anomaly, a characteristic found in highly evolved granites with strong hydrothermal interaction between residual fused and hydrothermal fluids (eg, Masuda et al., 1987, Masuda and Akagi, 1989).