Late Cretaceous to early Eocene magmatic evolution of the Laramide arc in the Nacozari quadrangle, northeastern Sonora, Mexico and its regional implications

Abstract

Late Cretaceous to early Eocene volcanic and plutonic rocks that crop out in the Nacozari quadrangle compose the Laramide magmatic arc in northeastern Sonora, northwestern Mexico. The purpose of this work is to characterize the arc rocks based on new cartography, stratigraphy, U-Pb and K-Ar geochronology, and major, trace and isotope geochemical data and interpret them in the regional context of evolution of the arc. The study area lies within the Proterozoic Mazatzal crustal block which is limited to the south by the Caborca block. The volcanic rocks regionally correlate with the Tarahumara Formation and consist of volcanic flows, breccias, tuffs and volcaniclastic sandstones with an estimated thickness of 4km. They range in composition from andesite to rhyolite and have isotopic ages from 76.8±0.4 to 57.1±0.4Ma. The granitic plutons and rhyolitic domes that intrude them yielded ages from 59.3±0.8 to 53.3±1.6Ma. Initial 87Sr/86Sr and epsilon Nd values for these rocks vary from 0.70553 to 0.70841 and −3.7 to −8.5, respectively, and have 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb isotope ratios that range from 18.05 to 19.05, 15.52 to 15.63, and 38.25 to 39.07, respectively. Regional integration of our U-Pb ages suggests that the volcanic activity lasted between ca. 81 and 57Ma, with two main peaks of activity at 74.5 and 61Ma. Similarly, regional magmatic activity indicated by dated plutons initiated at ca.91 and ended by 50Ma, after main peaks of activity at 71 and 60–55Ma. The plutonism between 91 and 80Ma was subordinate and regionally restricted to coastal and central Sonora and might represent an older event unrelated to the Laramide arc. Two K-Ar cooling ages of 55.4±0.7 and 48.5±0.7Ma obtained from La Púrica granite dated at ca. 59Ma (U-Pb) in the study area, correlate regionally with a possible event of rapid uplift between 68 and 50Ma. The geochemical composition of the studied rocks indicate high-K, calc-alkaline, metaluminous to weakly peraluminous compositions, typical of subduction arc-related magmas which characterize the Laramide magmatic belt in northwestern Mexico. The Nd, Sr and Pb isotope compositions of these and other samples documented for northeastern Sonora, closely correlate with isotope signatures of the Mazatzal crustal block in southeastern Arizona and differ from those in the Caborca block, which display affinity with the Mojave-Yavapai province. The data also allow to best constrain the location of the boundary between the Mazatzal and Caborca crustal blocks in east-central Sonora.