GEOLOGY AND GEOCHRONOLOGY OF THE MESOPROTEROZOIC MCDOWELL MOUNTAINS SEQUENCE IN ARIZONA

The occurrence of Mesoproterozoic sedimentary rocks older than about 1.3 Ga has been limited to a few small sequences in Arizona and New Mexico (Doe et al., 2012; Jones et al., 2011; Daniel et al., 2013). The observation that these rocks are folded and/or are cut by large thrust faults, which were then overprinted by deep-burial metamorphism, led to the identification of a new Mesoproterozoic orogeny: the Picuris orogeny. This is significant because a convincing deformation mechanism for the widespread occurrence of locally strongly deformed Mesoproterozoic plutons has been lacking, and it has prompted re-examination of rock sequences thought to be strictly of Paleoproterozoic age.

The McDowell Mountains (MDM) in south-central Arizona are composed of a recently identified ~10-km-thick section of Mesoproterozoic metamorphic rocks, mostly comprising ash-flow tuffs, quartzites, and the plutonic rocks intruding them. We will present the results of detailed geologic and structural mapping in the MDM and a geochronologic study utilizing laser ablation U-Pb dating of detrital and igneous zircon in samples collected from the MDM. The results will be used to define the relationship of the MDM sequence to the other Mesoproterozoic sequences in the region, and gain a better understanding of the timing, nature, and regional extent of the Picuris orogeny. The MDM sequence contains at least two unconformities and is cut by two large northwest-vergent thrust faults. The sequence experienced greenschist-facies metamorphism and was intruded by four separate granitic plutons, all of which are partly foliated, suggesting, intriguingly, that deformation occurred long after deposition of the supracrustal rocks. Preliminary zircon U-Pb dates on the youngest and oldest ash-flow tuffs in the sequence yield ages of 1546 ± 11 Ma and 1631.6 ± 8.2 Ma respectively. Making the MDM sequence currently the only significant igneous rocks within the North American ‘magmatic gap’ (between ca. 1.6 and 1.45 Ga) which may explain the occurrence of ~1.5 – 1.6 Ga detrital zircons in Laurentian sequences for which heretofore no local igneous source was known.