Emplacement of the Twin Sisters dunite, Washington

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ID: 295106
1963
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Abstract
The large, elliptical, 36-sq-mi Twin Sisters dunite and 2 smaller dunite bodies are located along a NW.-trending, nearly vertical fault and are intrusive into several thrust plates and locally into the unconformably overlying Swauk Formation \[Cretaceous-Paleocene\]. The time of emplacement was post-Paleocene, and definitely later than the main Cretaceous orogeny. The Twin Sisters mass is composed of a virtually unaltered, coarse-grained enstatite-bearing dunite with accessory amounts of chromite and chromium diopside. Every section displays abundant evidence of cataclasis, including granulation and bending of mineral grains, and translation bands in olivine. Completely identical textures have recrystallized. These recrystallized cataclastic features now consist of fine-grained, unstrained olivine mosaic zones rounding and embaying large, strained porphyroclasts. Locally thin mosaic zones cut single, large crystals. The translation bands have recrystallized into bandlike forms with irregular, sutured boundaries. This recrystallization is thought to have taken place at moderate temperatures. Serpentinites are marginal to and gradational with the large Twin Sisters mass and 1 exposed smaller body. The width of this narrow marginal zone is essentially the same for both the large and the small, and is thus independent of the size of the mass. In detail, there is an outward transition from unaltered dunite to partially serpentinized dunite to massive serpentinite and finally to schistose serpentinite at the actual contact. Serpentine is very rare in the core area. It is concluded that the Twin Sisters dunite mass moved and deformed while still hot enough to recrystallize (possibly during the intrusive rise), later by cataclasis alone. Near the last stages of emplacement serpentinizing fluids were added from the country rock to the marginal portions. This reduced friction at the contact, and further intrusion was accomplished by marginal faulting.
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Authors Donal M. Ragan
Journal american journal of science
Year 1963
DOI
10.2475/ajs.261.6.549
URL
https://doi.org/10.2475/ajs.261.6.549
Keywords Keywords not found

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