Timescales of migmatization, metamorphism, and deformation in a collapsed Orogenic Plateau.

Abstract

Migmatites play an important role in the evolution of mountain systems by inducing rheological contrasts and focusing strain. In modern orogenic plateaux, investigations have suggested that a layer of partially molten crust is located in the mid- to lower crust. To understand the role of partially molten crust in orogenic systems, it is important to determine how much of the crust was partially molten for how long, and to link the conditions, timing, and consequences of partial melting to tectonic processes at different crustal levels during construction and collapse of orogens. The Skagit Gneiss, in the highgrade core of the North Cascade continental magmatic arc of Washington, USA and BC, and the Valhalla complex, in the Shuswap metamorphic core complex of southeastern British Columbia, Canada, both contain abundant migmatites and represent the western and eastern margins, respectively, of a proposed orogenic plateau that was once present in western North America during the Late Mesozoic-Early Cenozoic. In the Valhalla complex, samples of migmatite were collected from the dome core to the bounding detachment fault for conventional, in situ, and depth profiling ion microprobe analyses. The conventional and in situ analyses of stromatic migmatites and leucosomes crystallized in boudin necks yield concordant U-Pb zircon ages that cluster near 60 Ma, interpreted as the timing of melt crystallization. Monazite Th-Pb ages range from 57-49 Ma. Patchy zoning and the range of dates suggest that the monazite was recrystallized under fluid-mediated conditions. To better understand the late history recorded in the monazite, depth profiling U-Pb ages were obtained from the outermost rims of zircons and yield a consistent age of 51 Ma. Oxygen isotopic measurements of the unpolished crystal faces systemically yield heavier δ18O (up to 9.0 ‰) relative to

interior compositions (down to 5.5 ‰). Furthermore, Ti concentrations of unpolished crystal faces and grain interiors yield temperatures of ~650 ºC. The depth profiling zircon results and the conventional Th-Pb monazite results indicate that deformation- and fluidmediated recrystallization of zircon and monazite occurred at high-T conditions as late as 51 Ma. The Ar cooling ages overlap with the youngest monazite and zircon results and cluster from 51 to 49 Ma. The geochronometric, geochemical and trace element results from the Valhalla complex, combined with field, structural, and petrologic data from this and previous studies of the Omineca domes, show that a large region of orogenic crust in this part of the Cordillera was partially molten in the early Tertiary. Rapid cooling is associated with extension and exhumation of migmatites in the domes. In the Skagit Gneiss, monazite and zircon were dated using the U-Pb TIMS method from migmatites in 3 localities. Zircons from the mesosome of the westernmost locality commonly yield Cretaceous dates, with younger dates clustering at 61 Ma. Leucosomes yield zircon with concordant dates that range from 68 to 47 Ma, interpreted as representing the timing of melt crystallization. In comparison, monazite reveal bimodal results, with one group clustering near 48 Ma and a second set of older dates from 69 to 65 Ma. The latter monazite dates are consistently older than the zircons from the same leucosome, consistent with the possibility that the older monazites record the timing of prograde to possibly peak metamorphism. The Eocene zircon and monazite dates are at the young end of the age spectrum for the North Cascades arc system and overlap with the timing of transtensional basin formation, suggesting that partial melting was an active process during at least the initial stages of extension and exhumation of the high-grade rocks. In addition, in the Skagit Gneiss, a detailed study of part of the eastern

bounding strike-slip fault zone suggests that a dynamic system was present between the high-grade Skagit rocks, the fault, and the adjacent basin. A step-over zone in the strikeslip fault may have developed during transpression and allowed part of the basin to be incorporated into the high-grade core and undergo metamorphism and deformation with the Skagit Gneiss. Although ~ 300 km separate the North Cascades from the Shuswap metamorphic core complex today, the two regions share many similarities: 1) both areas expose deformed high-grade gneiss that underwent isothermal decompression; 2) both areas contain abundant deformed migmatites that crystallized at similar times; and 3) Ar cooling ages from the two regions are similar. In both the Skagit Gneiss and the Valhalla complex, the partially molten crust played a significant role in the decompression and exhumation of the terranes. The similarities in P-T-t-d between the two regions strongly indicate that the North Cascades and the Omineca belt were dynamically linked and that the two areas represent the collapsed margins of an orogenic plateau. The migmatites in both areas are evidence of the layer of partially molten crust that once flowed beneath the proposed plateau. The data from the two areas suggest that partial melting must play a major role in the tectonic evolution of orogenic systems that contain abundant melt (e.g., Himalaya-Tibet; Andes-Altiplano-Puna; Cordillera).