GEOLOGY AND LA-ICPMS U-PB GEOCHRONOLOGY OF THE WESTERN LOWER PLATE OF THE MAGDALENA-MADERA METAMORPHIC CORE COMPLEX, NORTHERN SONORA, MEXICO

The lower plate of the Magdalena-Madera metamorphic core complex (MMCC) in northern Sonora includes the Sierra La Madera in its eastern part, sierras Magdalena and La Jojoba in its northern part and sierras Las Jarillas-Potrero and Álamo Viejo (SJPAV) in its western part. The older rocks in SJPAV are part of the Jurassic continental arc of SW North America and consist of Unit 1 composed of an ~800 m thick succession of ash-fall tuffs, rhyolitic flows and subordinate fluvial strata that we dated at ca. 169 Ma (weighted mean ²⁰⁶Pb/²³⁸U ages) and that is intruded by a rhyolitic dome of 158.2±4 Ma. Two sandstone samples collected at different parts of the area from clastic Unit 2 with subordinate rhyolitic flows, yielded a detrital zircon population (n= 166) containing a combined younger age peak of 165 Ma (n=72% of the total grains), Proterozoic grains with ages ranging from 1.01 to 1.9 Ga (n=14%), scattered Neoproterozoic and Paleozoic grains (n=10%), five grains of Lower Cretaceous age, and two other grains of 85 and 73 Ma. Despite the Cretaceous grains (that probably record metamorphic overprint), we consider Unit 2 to be upper Middle Jurassic age as it is intruded by the 153.3±2.8 Ma leucocratic, two-mica La Cebolla granite. DZ (n= 98) dated from a sandstone sample of a clastic succession from the Sierra Potrero yielded a unique age peak at 119 Ma (n= 85%) which is a typical younger age for the Lower Cretaceous Morita Formation in Sonora (Peryam et al., 2012). The Jurassic and Cretaceous rocks are affected by low-grade metamorphism (Mu+Bi+Hb+Cl+Ep) with pervasive schistosity, relatively open NNW-trending folds and minor thrusting to the NE. Four sheared leucocratic, two-mica granites with garnet that intrude these rocks yielded ages of 59.3±1.5, 54.6±1.9, 51.2±1.8, and 47.62 ± 0.49 Ma. Dikes and pegmatite veins emanating from these granites are locally folded. We interpret the regional shortening to be related to widespread Late Cretaceous deformation in Sonora, whereas the ca. 59 to 47 Ma granites are similar to Paleogene anatectic granites in the nearby sierras Magdalena and Madera (Nourse et al., 2018). The post-Eocene mylonitic fabric of these granites and related folding of pegmatites, likely record late Oligocene-early Miocene exhumation by the Magdalena-Madera detachment fault.