Geochemical and isotopic study of Mesozoic magmatism in the Sonobari Complex, western Mexico: Implications for the tectonic evolution of southwestern North America

The Sonobari Complex in northwestern Mexico preserves evidence of the consolidation of Pangaea and the Cordilleran orogenic cycle. Six Meso­ zoic magmatic pulses extending from the Early Triassic to the Paleocene are recognized in this complex. The volumetrically predominant rocks are calc­alkaline metaluminous and peraluminous granitoids. Mafic rocks are mainly tholeiitic gabbros. All studied rocks show high concentrations of large ion lithophile elements (LILE) and negative Nb, Ta, and Ti anomalies. Initial Nd and Sr isotopic ratios in granitoids (εNd(i) = –5.7 to –0.5; Sr/Sr(i) = 0.70630–0.71302) point to evolved continental sources, while radiogenic Pb isotopes (Pb/Pb(i) = 16.292–19.17; Pb/Pb(i) = 15.503–15.666; Pb/Pb(i) = 35.257–38.984) indicate a heterogeneous basement. Initial Nd and Sr isotope ratios in mafic rocks (εNd(i) = –1.9 to +5.0; Sr/Sr(i) = 0.70384–0.70626) point to mantle sources with crustal assimilation, which is also supported by the radiogenic Pb values (Pb/Pb(i) = 18.412–19.081; Pb/Pb(i) = 15.595–15.672; Pb/Pb(i) = 38.147–38.988). Geochemical and isotopic signatures suggest that magmatic rocks in the complex orig­ inated from fractional crystallization with assimilation of a heterogeneous basement isotopically similar to the Grenville orogen of Mexico. Whole­ rock compositions are compatible with volcanic­arc followed by back­arc tectonic settings, where subduction and extensional processes occurred. Therefore, the main granitic pulses in the Sonobari Complex originated af­ ter the late Paleozoic Gondwana­Laurentia collision, by subduction of oce­ anic plates or microplates along the western border of Pangea. Also, two extension­related magmatic pulses occurred after the Late Jurassic and in the Cenomanian, separated by a collisional orogenic event that is recorded by regional metamorphism. The continental arc setting of the Sonobari Complex differs from the oceanic arc context of the Guerrero­Alisitos super­ terrane, indicating that there is no genetic relation between these blocks as previously proposed; rather, a relationship with the eastern Peninsular Ranges batholith is proposed. INTRODUCTION The Sonobari Complex of northwestern Mexico is an enigmatic igneousmetamorphic block consisting of Ordovician to Late Cretaceous rocks, and whose petrogenesis and magmatic history are poorly understood. This complex is composed of metasedimentary rocks with North American and South American provenance (Vega-Granillo et al., 2008, 2013), and therefore played a role in the interaction between Laurentia and Gondwana during Pangea con solida tion. The complex includes magmatic pulses of 249–241 Ma (Early Triassic), 213–203 Ma (Late Triassic), 161–150 Ma (Late Jurassic), post-162 Ma–pre-99 Ma (latest Jurassic–Early Creteaceous), 99–97 Ma (Cenomanian), 83–80 Ma (Campanian), and 64 Ma (Paleocene) ages (Keppie et al., 2006; Vega-Granillo et al., 2012, 2013; Sarmiento-Villagrana et al., 2016), which have been described in the southern North American Cordillera, particularly in the Peninsular Ranges batho lith of Baja California. Some authors have considered the Sonobari Complex as part of the basement of the composite Guerrero terrane of western Mexico (e.g., Campa and Coney, 1983; Centeno-García et al., 2008). Others, pointing to significant differences, have regarded it as a separate terrane in its own right (Tahue terrane; Sedlock et al., 1993). In order to define the petrogenesis and tectonostratigraphic setting of the complex, we performed an extensive geochemical and Nd, Sr, and Pb isotopic study of magmatic and metaigneous rocks. Our data, coupled with previously published geochemical data, provide a framework for a discussion of the magmatic and tectonic evolution of the complex, as well as the nature of its basement, in order to establish its link with the magmatic provinces of the southern Cordilleran orogenic belt.