Cretaceous and Tertiary sedimentary, magmatic, and tectonic evolution of north-central Sonora (Arizpe and Bacanuchi Quadrangles), northwest Mexico

The Arizpe and Bacanuchi Quadrangles provide a geologic history representat ive of the north-central part of Sonora, where lithologies are dominated by late Mesozoic and Cenozoic igneous rocks. In this study, new geologic mapp ing, Ar-40/Ar-39 dating, and geochemical analyses have been combined to pro vide a stratigraphic framework for this area. Ten lithostratigraphic units and several igneous and tectonic events can be recognized. The oldest outcr opping rocks are Lower Cretaceous strata of the Bisbee Group, which along w ith the Picacho conglomerate record a middle Cretaceous compressive tectoni c event and associated sedimentation. Laramide igneous activity is widespre ad and represented by (1) highly altered andesitic flows and volcaniclastic rocks (Arroyo Alcaparros andesitic rocks) of late Campanian to Maastrichti an age, (2) less altered andesitic and dacitic flows (Cerro Las Jarillas vo lcanic rocks) of late Paleocene age, and the intrusive bodies of (3) Sierra El Manzanal granodiorite and (4) Rancho Vaqueria quartz monzonite, The Sie rra El Manzanal granodiorite was emplaced at ca, 68 Ma on the basis of a Ar -40/Ar-39 biotite age (67.97 +/- 0.19 Ma) and cooled relatively rapidly acc ording to less precise Ar-40/Ar-39 hornblende and K-feldspar ages from the same sample (64.8 +/- 1.0 Ma and 62.8 +/- 0.3 Ma, respectively). The Cerro Las Jarillas volcanic rocks are slightly younger (Ar-40/Ar-39 biotite age o f 58.67 +/- 0.17 Ma). The Rancho Vaqueria quartz monzonite was emplaced at ca. 57 Ma (Ar-40/Ar-39 biotite age of 56.73 +/- 0.14 Ma and a Less precise Ar-40/Ar-39 hornblende age of 55.0 1 +/- Ma); a protracted cooling history of this pluton is indicated by the age spectrum of K-feldspar from the same sample. A probable magmatic lull and denudation seem to have occurred betw een middle and late Eocene time and probably until the early Oligocene, Sub sequently, rhyolitic to mafic volcanism began close to late Oligocene time and lasted until the early Miocene. Felsic volcanism is represented by the Cerro Cebadehuachi volcanic rocks, from which Ar-40/Ar-39 hornblende ages o f 27.25 +/- 0.09 and 27.32 +/- 0.06 Ma and a biotite age of 26.97 +/- 0.06 Ma were obtained at three different localities. The Mesa Pedregosa volcanic rocks represent the transition to younger, mafic volcanic activity that oc curred during the Late Oligocene, as indicated by a sanidine Ar-40/Ar-39 ag e of 25.48 +/- 0.05 Ma, This tate Oligocene and early Miocene magmatism was paired by two episodes of extensional deformation. The first phase is char acterized by northwest-striking normal faults and folds, which expose the d eepest structural levels of the area, and by the related basin fill, the Ba canuchi conglomerate. The second phase is represented by north-striking nor mal faults and hy the syntectonic basin fill, the Arizpe conglomerate. Basa ltic andesite volcanic flows at the base of the Arizpe conglomerate yielded Ar-40/Ar-39 (whole-rock) ages of 23.52 +/- 0.17 and 21 +/- 0.20 Ma. The ex tensional deformation (27 to 23 Ma) in the study area is coeval with the de velopment of metamorphic core complexes in neighboring areas of Sonora and with the onset of extension in southern Sonora, The mafic volcanic rocks an d elastic sedimentary units associated with this extension resemble the bas in fills that in other parts of Sonora are assigned to the Baucarit Formati on. Geochemical information from samples representing each of the igneous event s displayed high-K calc-alkalic and mostly metaluminous compositions, The o lder units including the Arroyo Alcaparros andesitic rocks, the Cerro I,as Jarillas volcanic rocks, the Sierra El Manzanal granodiorite, and the Ranch o Vaqueria quartz monzonite are characterized by steep chondrite-normalized REE (rare earth element) slopes and generally well-developed negative Eu a nomalies, suggesting garnet and plagioclase removal in the source. The youn ger igneous events including the Cerro Cebadehuachi and Mesa Pedregosa volc anic rocks, and the basaltic flows associated with the Arizpe conglomerate, showed basin-shaped REE slopes with no Eu anomalies, suggesting clinopyrox ene or amphibole fractionation.