USING ZIRCON AND TITANITE GEOCHRONOLOGY TO EVALUATE PROTOLITH STRATIGRAPHY AND THE TIMING OF METAMORPHISM AND MIGMATIZATION IN MONARCH CANYON

In the deepest structural levels of the Funeral Mountains metamorphic core complex, the contact separating middle amphibolite facies metasediments (above) from migmatitic paragneisses (below) has been interpreted as: (1) an unconformity between metasediments of the Crystal Spring Fm (CSF) and Paleoproterozoic basement (2) the Monarch Spring fault separating non-migmatitic CSF from migmatitic CSF, and (3) an in-situ anatectic front within an attenuated but continuous stratigraphic package. Detrital zircon (DZ) U-Pb geochronology through the metamorphosed section has allowed us to reconstruct the protolith stratigraphy and to test these alternatives. Two DZ age spectra patterns are noted: DZ samples that exhibit modes at 1.0-1.3, 1.3-1.5, 1.6-1.8 and 2.4-2.6 Ga, interpreted as Neoproterozoic Horse Thief Springs Fm (HTS); and DZ samples that exhibit a unimodal peak between 1.6-1.8 Ga, interpreted as Mesoproterozoic CSF. The DZ data suggest that Monarch Canyon preserves a nearly continuous stratigraphic sequence from the Neoproterozoic Johnnie Fm to the Mesoproterozoic CSF, and places the (ca. 300m.y.) unconformity, separating Neoproterozoic HTS from Mesoproterozoic CSF, within the migmatite complex; thus, unmelted middle amphibolite facies metasediments grade to migmatites at an in-situ anatectic front within the HTS. U-Pb ages of zircon rims in unmelted rocks, interpreted as metamorphic overgrowths, range from 70.5 ±2.0 to 105.1 ±1.8 Ma, and in migmatites, from 68.4 ±2.5 to 104.7 ±3.7 Ma. Tera-Wasserberg lower intercept U-Pb ages of metamorphic titanite in middle amphibolite facies metasediments are 77.7 ±3.1 Ma and 80.4 ±1.8 Ma. This U-Pb geochronology reveals that partial melting of the HTS and CSF occurred in the Late Cretaceous, coeval with metamorphism of the middle amphibolite facies metasediments. These coeval metamorphic packages preserve orthogonal shear sense: top-NW in the mylonitic stack above the proposed anatectic front and top-SW below. We hypothesize that rocks in Monarch Canyon preserve top-driven Late Cretaceous top-NW extensional shear and middle amphibolite facies metamorphism in the structurally shallower rocks, and bottom-driven top-SW shear and migmatization in deeper rocks during Laramide flat slab subduction and associated fluid infiltration.