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).
University of Minnesota Ph.D. dissertation. January 2009. Major: Geology. Advisors: Donna L. Whitney, Christian Teyssier. 1 computer file (PDF); xviii, 379 pages. Ill. (some col.) + 1 computer file (XLS); 1 table titled Table 4.1: Ion microprobe analytical U-Pb zircon isotopic data from depth profiling
Gordon, Stacia Michelle.
Timescales of migmatization, metamorphism, and deformation in a collapsed Orogenic Plateau..
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