Evidence for Late Cretaceous–early Tertiary lower plate mylonitization and extension in the Harcuvar metamorphic core complex, Arizona: Evidence from U-Pb geochronology

Metamorphic core complexes in the North American Cordillera are interpreted as sites of large–magnitude crustal extension, yet many aspects of their development remain controversial. Most core complexes in the central and southern Basin and Range are thought to have been exhumed by a single fault that evolved from ductile to brittle during the Miocene. However, new field relationships and U-Pb zircon geochronology from the lower plate of the Harcuvar core complex in western Arizona raise questions about the age and tectonic significance of mylonites in the evolution of this core complex.

Variably mylonitic plutonic rocks dominate the lower plate. Plutonic units yield three major age populations. A foliated biotite granite yielded a U–Pb zircon age of 152 ± 3 Ma, and a gneiss with an E–W lineation yielded a similar 152 ± 4 Ma age, with some 179 ± 9 Ma cores. Leucogranite plutons and sills dominate much of the footwall and yield 71–72 Ma ages, with some inherited ~152 and ~180 Ma cores. A migmatitic biotite gneiss yielded younger U–Pb zircon ages of ~55 Ma and ~61 Ma with some ca. 1.4–1.5 Ga cores. Pegmatite dikes that cross–cut top-NE-directed mylonitic shear zones at high angles and are only weakly deformed also yield 55–64 Ma ages.

These results indicate that a phase of Middle to Late Jurassic magmatism was followed by a major magmatic event at ~72 Ma that emplaced large volumes of leucogranite. Minor melting (mainly pegmatites) continued until ca. 55 Ma. The pegmatites are likely late to post–tectonic and suggest that much of the lower plate mylonitization, including shearing related to top–NE extension, occurred during the Late Cretaceous to early Tertiary (Laramide) rather than the Miocene. Laramide extension may have been driven by Late Cretaceous crustal thickening and partial melting that weakened the crust and caused gravitational collapse. Thus, Miocene extension mainly occurred by brittle fault slip rather than widespread ductile shearing. These results raise questions about the time scales of core complex development and the role of tectonic inheritance in Miocene extension. U–Pb zircon and monazite geochronology currently in progress will further refine the timing of plutonic and metamorphic events and their role in the development of this core complex.