- Usage in publication:
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- Eldorado gneiss
- Modifications:
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- Named
- Dominant lithology:
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- Gneiss
- Diorite
- AAPG geologic province:
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- Cascades province
Summary:
Included as western unit of Skagit gneiss [!]. Occurs in area south of Cascade Pass, northern Cascade Mountains, WA. Consists of homogeneous, meta-hornblende-quartz diorite, presumably derived from basement crystalline material genetically related to LeConte belt. To west, Eldorado gneiss appears to have been tectonically mixed with Cascade River schist (new) during metamorphism. Becomes feldspathized to east where it grades into migmatitic Skagit gneiss. [Age not stated.]
Source: GNU records (USGS DDS-6; Menlo GNULEX).
- Usage in publication:
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- Eldorado Orthogneiss
- Modifications:
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- Redescribed
- AAPG geologic province:
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- Cascades province
Summary:
The gneisses that can be shown to be derived from plutonic rocks are excluded from Skagit Gneiss (restricted). These include: Eldorado Orthogneiss, Gabriel Peak Orthogneiss, Bonanza Orthogneiss.
Source: GNU records (USGS DDS-6; Menlo GNULEX).
- Usage in publication:
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- Eldorado Orthogneiss
- Modifications:
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- Age modified
- AAPG geologic province:
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- Cascades province
Summary:
Basement is presumed to be earlier Paleozoic and/or Precambrian in age. Its top is profound unconformity. Basement rocks occur as tectonic slices on western flank of range [Cascades] and as anticlinal belts within this metamorphic core. Rocks of these belts include Marblemount Meta Quartz Diorite and Eldorado Orthogneiss. Eldorado Orthogneiss was derived during Cascades metamorphism from previously non-metamorphic igneous quartz diorite similar to the parent rock of Marblemount Meta Quartz Diorite.
Source: GNU records (USGS DDS-6; Menlo GNULEX).
- Usage in publication:
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- Eldorado Orthogneiss
- Modifications:
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- Geochronologic dating
- AAPG geologic province:
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- Cascades province
Summary:
Shown on generalized geologic sketch map of central portion of Northern Cascade Mountains, WA. Samples collected at 48 deg 27.2'N and 120 deg 59.8'W gave Pb/U and PB/Pb ages of 110 +/-10 Ma and 92 +/-15 Ma on zircons. Ages of metamorphic minerals, zircons from pegmatitic material associated with migmatite, and zircons from synkinematic intrusive rocks indicate two major episodes of metamorphism in Northern Cascade terrane, one about 415 Ma and the other 90 to 60 Ma. Thus Eldorado Orthogneiss was emplaced during early stages of mid- to Late Cretaceous metamorphism in the Cascades. Unit appears to be essentially synkinematic pluton rather than premetamorphic "basement" of Marblemount type as suggested by Misch (1966).
Source: GNU records (USGS DDS-6; Menlo GNULEX).
- Usage in publication:
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- Eldorado Orthogneiss*
- Modifications:
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- Adopted
- Age modified
- AAPG geologic province:
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- Cascades province
Summary:
Eldorado Orthogneiss of Misch (1966) adopted. Principal reference area designated on Eldorado, Klawatti, and Primus Peaks and connecting ridges (sec.8 T35N R13E to sec.27 T36N R13E, Mount Baker 30'x60' quad), northern WA. Consists of metamorphosed granodiorite and tonalite. Intrusive age is (approximately 90 Ma) early Late Cretaceous; metamorphic age is Late Cretaceous to early Tertiary.
Source: GNU records (USGS DDS-6; Menlo GNULEX).
For more information, please contact Nancy Stamm, Geologic Names Committee Secretary.
Asterisk (*) indicates published by U.S. Geological Survey authors.
"No current usage" (†) implies that a name has been abandoned or has fallen into disuse. Former usage and, if known, replacement name given in parentheses ( ).
Slash (/) indicates name conflicts with nomenclatural guidelines (CSN, 1933; ACSN, 1961, 1970; NACSN, 1983, 2005, 2021). May be explained within brackets ([ ]).