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

Abstract

The Sonobari Complex in northwestern Mexico preserves evidence of the consolidation of Pangaea and the Cordilleran orogenic cycle. Six Mesozoic magmatic pulses extending from the Early Triassic to the Paleocene are recognized in this complex. The volumetrically predominant rocks are calc-alkaline 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; 87Sr/86Sr(i) = 0.70630-0.71302) point to evolved continental sources, while radiogenic Pb isotopes (206Pb/204Pb(i) = 16.292-19.17; 207Pb/204Pb(i) = 15.503-15.666; 208Pb/204Pb(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; 87Sr/86Sr(i) = 0.70384-0.70626) point to mantle sources with crustal assimilation, which is also supported by the radiogenic Pb values (206Pb/204Pb(i) = 18.412-19.081; 207Pb/204Pb(i) = 15.595-15.672; 208Pb/204Pb(i) = 38.147-38.988). Geochemical and isotopic signatures suggest that magmatic rocks in the complex originated from fractional crystallization with assimilation of a heterogeneous basement isotopically similar to the Grenville orogen of Mexico. Wholerock compositions are compatible with volcanic-arc followed by back-arc tectonic settings, where subduction and extensional processes occurred. Therefore, the main granitic pulses in the Sonobari Complex originated after the late Paleozoic Gondwana-Laurentia collision, by subduction of oceanic plates or microplates along the western border of Pangea. Also, two extension-related 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 Guerrero-Alisitos superterrane, indicating that there is no genetic relation between these blocks as previously proposed; rather, a relationship with the eastern Peninsular Ranges batholith is proposed.