- Usage in publication:
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- Dillon granite gneiss
- Modifications:
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- Named
- Dominant lithology:
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- Gneiss
- AAPG geologic province:
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- Northern Rocky Mountain region
Summary:
Name applied to a thick sheet-like mass of gray to red-brown granite gneiss that is widespread in the Ruby Mountains, Beaverhead and Madison Cos, MT in Northern Rocky Mountain region. No type locality designated. Is generally resistant to weathering, forming rounded to angular outcrops between the Cherry Creek group and the pre-Cherry Creek group. Intrudes Cherry Creek group. Ranges from nearly massive with a foliation that is inconspicuous megascopically to strongly layered, even lineated, gneissic. Forms a pluton and numerous aplite and pegmatite bodies. Contains quartz, microcline, plagioclase, and minor biotite. Has abundant quartz veins and pods. Geologic map. Of pre-Beltian, Precambrian age.
Source: GNU records (USGS DDS-6; Denver GNULEX).
- Usage in publication:
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- Dillon Granite Gneiss
- Modifications:
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- Age modified
- Overview
- AAPG geologic province:
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- Northern Rocky Mountain region
Summary:
Has multiple origins. Some is metamorphosed sedimentary rock; some is of igneous origin; some is remobilized pre-existing basement rock. Authors see little justification for separating Dillon from other similar quartzo-feldspathic gneisses of the region, unless the term Dillon is restricted to the single body at the crest of Ruby Range, in Madison Co, MT in Northern Rocky Mountain region. Considered to be of Archean age. Previously assigned to Precambrian.
Source: GNU records (USGS DDS-6; Denver GNULEX).
- Usage in publication:
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- Dillon Granite Gneiss
- Modifications:
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- Not used
Summary:
First paper (abstract) to publish name did not include a complete definition. Name has been applied to granite gneiss between the pre-Cherry Creek and the Cherry Creek Group, and to only the basement quartzofeldspathic gneiss. Redefining the Dillon at this time would probably not eliminate the confusion that has surrounded the name since its introduction. Rocks previously assigned to the Dillon are designated: 1) quartzofeldspathic gneiss, the name applied to the rocks that underlie the Christensen Ranch Metasedimentary Suite (new); or 2) granite gneiss, the name applied to smaller bodies that intruded or developed within metasedimentary strata in this report on the Ruby Range, Madison Co, MT in Northern Rocky Mountain region.
Source: GNU records (USGS DDS-6; Denver 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 ([ ]).