Cretaceous and Tertiary sedimentary, magmatic, and tectonic evolutionof north-central Sonora (Arizpe and Bacanuchi quadrangles), NorthwestMexico

Abstract

The Arizpe and Bacanuchi quadrangles provide a geologic historyrepresentative of the north-central part of Sonora, where lithologiesare dominated by late Mesozoic and Cenozoic igneous rocks. In thisstudy, new geologic mapping, (super 40) Ar/ (super 39) Ar dating, andgeochemical analyses have been combined to provide a stratigraphicframework for this area. Ten lithostratigraphic units and severaligneous and tectonic events can be recognized. The oldest outcroppingrocks are Lower Cretaceous strata of the Bisbee Group, which alongwith the Picacho Conglomerate record a Middle Cretaceous compressivetectonic event and associated sedimentation. Laramide igneous activityis widespread and represented by (1) highly altered andesitic flowsand volcaniclastic rocks (Arroyo Alcaparros andesitic rocks) of lateCampanian to Maastrichtian age, (2) less altered andesitic and daciticflows (Cerro Las Jarillas volcanic rocks) of late Paleocene age, andthe intrusive bodies of (3) Sierra El Manzanal granodiorite and (4)Rancho Vaqueria quartz monzonite. The Sierra El Manzanal granodioritewas emplaced at ca. 68 Ma on the basis of a (super 40) Ar/ (super 39)Ar biotite age (67.97 + or - 0.19 Ma) and cooled relatively rapidlyaccording to less precise (super 40) Ar/ (super 39) Ar hornblende andK-feldspar ages from the same sample (64.8 + or - 1.0 Ma and 62.8 + or- 0.3 Ma, respectively). The Cerro Las Jarillas volcanic rocks areslightly younger ( (super 40) Ar/ (super 39) Ar biotite age of 58.67 +or - 0.17 Ma). The Rancho Vaqueria quartz monzonite was emplaced atca. 57 Ma ( (super 40) Ar/ (super 39) Ar biotite age of 56.73 + or -0.14 Ma and a less precise (super 40) Ar/ (super 39) Ar hornblende ageof 55.0 + or - 0.7 Ma); a protracted cooling history of this pluton isindicated by the age spectrum of K-feldspar from the same sample. Aprobable magmatic lull and denudation seem to have occurred betweenmiddle and late Eocene time and probably until the early Oligocene.Subsequently, rhyolitic to mafic volcanism began close to lateOligocene time and lasted until the early Miocene. Felsic volcanism isrepresented by the Cerro Cebadehuachi volcanic rocks, from which(super 40) Ar/ (super 39) Ar hornblende ages of 27.25 + or - 0.09 and27.32 + or - 0.06 Ma and a biotite age of 26.97 + or - 0.06 Ma wereobtained at three different localities. The Mesa Pedregosa volcanicrocks represent the transition to younger, mafic volcanic activitythat occurred during the late Oligocene, as indicated by a sanidine(super 40) Ar/ (super 39) Ar age of 25.48 + or - 0.05 Ma. This lateOligocene and early Miocene magmatism was paired by two episodes ofextensional deformation. The first phase is characterized bynorthwest-striking normal faults and folds, which expose the deepeststructural levels of the area, and by the related basin fill, theBacanuchi Conglomerate. The second phase is represented bynorth-striking normal faults and by the syntectonic basin fill, theArizpe Conglomerate. Basaltic andesite volcanic flows at the base ofthe Arizpe Conglomerate yielded (super 40) Ar/ (super 39) Ar(whole-rock) ages of 23.52 + or - 0.17 and 21 + or - 0.20 Ma. Theextensional deformation (27 to 23 Ma) in the study area is coeval withthe development of metamorphic core complexes in neighboring areas ofSonora and with the onset of extension in southern Sonora. The maficvolcanic rocks and clastic sedimentary units associated with thisextension resemble the basin fills that in other parts of Sonora areassigned to the Baucarit Formation. Geochemical information fromsamples representing each of the igneous events displayed high-Kcalc-alkalic and mostly metaluminous compositions.

12 samples from this reference