Mesoproterozoic rocks of the McDowell Mountains, Arizona—Journey into the magmatic gap

ABSTRACT The McDowell Mountains of central Arizona contain one of the best preserved and thickest sections of pre–Apache Group Mesoproterozoic rocks in the state. The oldest formation, ash-flow tuff, has an age of ca. 1650 Ma and is overlain by a quartzite-shale-quartzite triplet. These two units are interpreted to correlate with the Red Rock Group and Mazatzal Group, respectively. Significantly, these formations are overlain by ~6 km of ash-flow tuffs and minor psammite and metabasalt. Preliminary U-Pb analysis of igneous zircons in the youngest ash-flow tuff (the Taliesin tuff) provides an age of 1546 ± 11 Ma. The oldest granite pluton, the Antiguo granite, intrudes the Taliesin tuff and has a U-Pb age of 1525.6 ± 9.5 Ma. If these preliminary results are accurate, they record volcanism, burial and metamorphism, and plutonism that occurred within the Proterozoic ‘magmatic gap’ (between ca. 1.6 and 1.45 Ga) and may be the only voluminous igneous rocks within this age range yet recognized in North America. Additionally, the stratigraphic section was deformed into km-scale folds during an episode of contraction that formed two northwest-vergent thrust faults in the northern part of the mountain range. Both thrusts were subsequently intruded by mafic hypabyssal sills and then buried to greenschist-facies depths and intruded by the coarse-grained Carefree granite at 1425 Ma.