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Date of Award
Master of Science (MS)
Brown, Edwin H.
Talbot, James L.
Suczek, Christopher A., 1942-2014
The Tonga Formation of the North Cascades, Washington, is spatially situated between rocks of the crystalline core and those of the Northwest Cascades System. Its affinity to one or the other, or both, of these flanking units has been uncertain but is of importance to orogenic models for the region.
The Tonga Formation consists of weakly-deformed bedded feldspathic graywackes and shale metamorphosed at chlorite to staurolite grade. Comparison of protolith lithologic assemblage and Rb/Sr isotopic signature in addition to plutonic, metamorphic, and deformational histories suggests that the Tonga Formation is correlative with the Chiwaukum Schist and is not equivalent to the Easton Metamorphic Suite as previously suggested.
A 87Sr/86Sr to 87Rb/86Sr plot of 18 samples from the Tonga Formation is indistinguishable from Chiwaukum Schist data. Both data sets contrast with the Darrington Phyllite of the Easton Metamorphic Suite, which has a steeper slope on the isotope plot, has a lower initial 87Sr/86Sr ratio, and ranges to much higher 87Rb/86Sr values. The similarity of isotopic characteristics of the Tonga and Chiwaukum strongly supports a common history for the two units.
Metamorphic grade reflected in distribution of metamorphic minerals and thermobarometry increases continuously northward from chlorite zone (kilobars) to staurolite zone (~7 kilobars) in the Tonga Formation and then to kyanite zone (~7 kilobars) in Chiwaukum Schist.
Deformation is expressed by the formation of folds of compositional layers which formed in the southern part of the study area before intrusion of the Beckler Peak Stock (~90 Ma); in the north the dominant fabric formed after crystallization of a 96 Ma (U/Pb zircon, N. Walker) porphyritic dike and continued to develop after intrusion of the 90 Ma Excelsior Mountain Orthogneiss.
High pressure metamorphic recrystallization occurred after deformation, and after shallow intrusion of the 90 Ma Excelsior Mountain Orthogneiss (Aluminum-in-hornblende pressure ~3 kilobars).
The Tonga Formation provides a continuous transect into the high-grade Chiwaukum Schist in the core of the Late Cretaceous orogen. Findings do not support the concept that the Darrington Phyllite and the Chiwaukum Schist are correlative or that the Cascades crystalline core evolved via a Barrovian overprint from the Easton blueschist terrane. Results of this study suggest a relatively simple two-stage metamorphic history represented by ~90 Ma shallow contact metamorphism overprinted by high-pressure metamorphism increasing sharply from southwest to northeast and reaching peak conditions after 90 Ma and prior to ~80 Ma.
Tonga Formation, Darrington, Phyllite, Chiwaukum Schist
Western Washington University
Subject – LCSH
Geology, Structural--Washington (State)--Cascade Range; Metamorphic rocks--Washington (State)--Cascade Range
Cascade Range; Washington (State)
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Duggan, Kathleen (Kathleen Maura), "Regional Significance of the Tonga Formation, North Cascades, Washington" (1992). WWU Graduate School Collection. 834.