EVIDENCE FOR POLYPHASE EXHUMATION OF THE HARCUVAR METAMORPHIC CORE COMPLEX, WESTERN ARIZONA

Cordilleran metamorphic core complexes are generally understood to be zones of large-scale Tertiary crustal extension; however, recent work indicates some core complexes may also have experienced Late Cretaceous exhumation. If correct, this would significantly change our understanding of core complexes, including their rate, magnitude, timing, and driving force. New results from the Harcuvar core complex, western Arizona, suggest the area underwent considerable metamorphism and exhumation during the Late Cretaceous and support a polyphase exhumation history.

The footwall of the core complex comprises Proterozoic crystalline gneisses and Late Cretaceous leucogranites with minor pelitic schists. These schists, including Har77 and Har92, document important aspects of the tectonic history of the core complex. Both Har77, a staurolite-kyanite-garnet-quartz-muscovite schist, and Har92, a K-feldspar-sillimanite-garnet-biotite-quartz-muscovite schist, contain largely homogenized almandine-rich garnets. Remaining zoning suggests growth during decreasing T and P and is interpreted as garnet growth during exhumation. Preliminary thermobarometry yields ~10 kb pressures for Har77 cores and ~4 kb at ~715 °C for rims, allowing for retrograde staurolite growth. Har92 records P up to ~6 kb, while coarse, striated white mica (possibly after fibrolite) and K-feldspar augen (some pseudomorphed by quartz-white mica symplectite) may suggest retrogression through the second sillimanite isograd.

Monazite from these schists all yield Late Cretaceous U-Pb ages (mean age ~72 Ma by LA-ICP-MS), whether located in the matrix or included in garnet or kyanite. Older (~76–70 Ma), low Y and HREE monazite cores are interpreted to record garnet growth during uplift, constraining significant decompression of ~4–6 kb in the Late Cretaceous. Younger (~70–63 Ma), higher Y and HREE rims suggest growth during garnet breakdown. These new results indicate significant burial and exhumation during the Late Cretaceous–early Tertiary, supporting a model of Late Cretaceous crustal thickening and partial melting that drove gravitational collapse and Laramide exhumation of the footwall. Subsequent Miocene extension was superimposed on this earlier event, highlighting the polyphase nature of exhumation in the core complex.