GEOLOGY AND GEOCHEMISTRY OF THE LARAMIDE INTRUSIVES, ALONG AN EAST-WEST TRANSECT IN SOUTHERN SONORA, MEXICO

This study describes the results of a geologic, geochronologic and geochemical transect 15 km-wide and 300 km long, oriented E-W, across southern Sonora (latitude 28º 15´N), from the coast of the Gulf of California into the volcanic province of Sierra Madre Occidental (SMOc). The geology of this transect provides a regional view of the evolution and geochemistry of a continental margin active from Late Cretaceous to Late Miocene. The transect is divided into three geologic regions. The western region (Circum-Gulf belt) contains the youngest volcanic rocks, some of which are alkaline. No clastic interbeds were observed within the volcanic sections. This region also includes the oldest intrusive rocks on the transect (83 Ma). The central region (Tarahumara volcanic arc), is characterized by major outcrops of volcanic and volcaniclastic rocks of Late Cretaceous age, and batholiths that have isotopic ages from 56 to 58 Ma. This region also includes middle Tertiary clastic and volcanic rocks that are related to Basin and Range style extension, including 14 to 27 Ma, mafic rocks and 10.5 to 13 felsic rocks. The eastern region (SMOc) is characterized by broad outcrops of Tertiary volcanic rocks whose ages range from 18 to 34 Ma. The ages of Laramide intrusive rocks in this region ranges from 49 to 63 Ma. Modal compositions of the Laramide intrusives vary from tonalite to quartzmonzodiorite in the western region, and also include granite and granodiorite in the central and eastern regions. Chemically, all of the intrusives are calc-alkaline with silica contents varying from 52 to 76 in weight percent and K2O values ranging from 0.8 to 5.7%. The chondrite-normalized trace-element patterns of the Laramide plutons show the characteristic LREE enrichments that are typical of continental-margin magmatic arcs.