East-west variations in age, chemical and isotopic composition of the Laramide batholith in southern Sonora, Mexico

Abstract

We examined an east-west transect from the Gulf of California to the volcanic province of the Sierra Madre Occidental (28°30’N). The transect is divided into three geologic regions: 1) The coastal region (COR) is characterized by scattered exposures of Laramide plutons lying beneath the modern and late Tertiary sediments. No outcrops of the volcanic component (Tarahumara Formation) were found there; 2) The central region (CER) is dominated by widespread outcrops of both the Laramide batholith (LB) and coeval volcanic and volcaniclastic rocks. Exposures of intrusive contacts confi rm the close relationship of the plutonic and volcanic rocks there; 3) The eastern segment (SMOc) with exposures of the Laramide arc (LA) that are mostly concealed by the Tertiary Sierra Madre Occidental volcanic rocks. The plutonic rocks in the COR are granodiorite and tonalite and, further inland, they change to granodiorite and granite. Extensive hydrothermal alteration largely obscures textures in the volcanic rocks, but fresh samples range from andesite, dacite and less rhyolite. Chemically, the plutons are medium-K to high-K calc-alkaline, LREE-enriched and with relatively minor europium anomalies. K-Ar and U/Pb dating shows that plutons are older (79–83 Ma) to the west and younger to the east, where age ranges between 55 and 65 Ma throughout the remainder of the transect. The coeval volcanic facies yielded zircon U/Pb ages of 60 Ma, and 70 to 90 Ma within the CER. A younger pulse at ~50 Ma is present in the west of the SMOc region. Radiogenic Sr and Nd isotope values range from initial 87Sr/86Sr 0.70547 to 0.70715 and εNd varies from -3.3 to -6.3. All of these results are consistent with development of a subduction-related magmatic arc that was emplaced within mature continental crust. The present-day width of the arc is >300 km, which suggests development of a low-angle subduction confi guration that produced a signifi cant volume of magmatism. Although the age decreases with the distance to the plate margin, the trend is not regular, and the activity between 55 and 65 Ma is at least 200 km in width. The results also support a confi guration in which the Laramide arc of the mainland of southern Sonora is a continuation of the Peninsular Ranges batholith of Baja California. However, the LA shows signifi cant contrasts with the batholith of southern Sinaloa, which is narrower and has a larger age range. The southern Sinaloa batholith also has compositional and isotopic characteristics consistent with emplacement within a younger and less mature crustal domain (Guerrero terrane).