DETRITAL ZIRCON DATA FROM THE MAZATZAL GROUP OF CENTRAL ARIZONA: INSIGHTS FROM NEWLY IDENTIFIED DISCONFORMITIES

Proterozoic exposures in Arizona include a series of deformed metasedimentary sequences, some bounded by unconformities, that evolved as Laurentia grew southward from 1.8 to 1.4 Ga. Detrital zircon geochronology provides a critical test of correlations of these discontinuous and highly segmented sequences both locally and throughout southwestern Laurentia. Prior work identified a ca. 170 m.y. discontinuity within the Hess Canyon Group in the Salt River Canyon, central Arizona. The Yankee Joe and Blackjack Formations were previously thought to be conformable with the ca. 1657 Ma White Ledges Formation. However, a new maximum depositional age of ca. 1488 Ma indicates that the upper Hess Canyon Group was deposited after 1488 Ma and the whole section was deformed and metamorphosed ca. 1488–1436 Ma. We have recently identified another discontinuity within the Mazatzal Group of central Arizona. Detrital zircon ages from Deadman Quartzite indicate a dominant population at ca. 1745 Ma and a younger population at 1701 Ma. The younger population is consistent with the age of underlying rhyolite (1702 Ma) and, together with older detrital zircon populations, reflect a mixture of local sources and recycling of older material from the surrounding region. The overlying Maverick Shale has a dominant population at ca. 1703 Ma and a younger population with an average age of ca. 1631 Ma, indicating it is at least 70 Ma younger than the Deadman Quartzite. A disconformity of 70 Ma within the Maverick Shale implies that the overlying Mazatzal Peak Quartzite and Hopi Springs Shale are also younger than ca. 1631 Ma and correlative with parts of the Hess Canyon Group 50 km to the southeast. It also suggests that fold-and-thrust style deformation of the upper Mazatzal Group is younger than ca. 1631 Ma. However, published studies suggest that strong fold and thrust fabrics in the Slate Creek and Four Peaks shear zones are cross-cut by ca. 1630-1650 Ma granite intrusions. Resolving the extent and significance of 1.6 vs. 1.4 Ga deformation in southwestern Laurentia remains an important challenge, and increasing recognition of unconformities within Proterozoic metasedimentary successions is providing new key constraints.