Regional Significance of the Tonga Formation, North Cascades, Washington

Author

Kathleen (Kathleen Maura) Duggan, Western Washington University

Date Permissions Signed

6-14-2018

Date of Award

Spring 1992

Document Type

Masters Thesis

Degree Name

Master of Science (MS)

Department

Geology

First Advisor

Brown, Edwin H.

Second Advisor

Talbot, James L.

Third Advisor

Suczek, Christopher A., 1942-2014

Abstract

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.

Type

Text

Keywords

Tonga Formation, Darrington, Phyllite, Chiwaukum Schist

Publisher

Western Washington University

OCLC Number

1083265793

Subject – LCSH

Geology, Structural--Washington (State)--Cascade Range; Metamorphic rocks--Washington (State)--Cascade Range

Geographic Coverage

Cascade Range; Washington (State)

Format

application/pdf

Genre/Form

masters theses

Language

English

Rights

Copying of this document in whole or in part is allowable only for scholarly purposes. It is understood, however, that any copying or publication of this thesis for commercial purposes, or for financial gain, shall not be allowed without the author's written permission.

Recommended Citation

Duggan, Kathleen (Kathleen Maura), "Regional Significance of the Tonga Formation, North Cascades, Washington" (1992). WWU Graduate School Collection. 834.
https://cedar.wwu.edu/wwuet/834