Interlayered granitoid gneiss and amphibolite of the Twilight Gneiss: An Early Proterozoic intrusive complex in the western Needle Mountains, southwestern Colorado
- Univ. of Kansas, Lawrence, KS (United States). Dept. of Geology
- Geological Survey, Flagstaff, AZ (United States)
Early Proterozoic basement (ca. 1.75 Ga) in the western Needle Mountains is comprised of intimately interlayered trondhjemitic to tonalitic gneiss and amphibolite assigned to the Twilight Gneiss (TG) of Barker (1969), and metavolcanic rocks of the Irving Formation. Previous workers have interpreted the interlayered assemblage in the TG as either granitoid intrusives with screens of mafic volcanic rocks from the adjacent Irving Formation or dacitic and basaltic flows and tuffs. Medium- to coarse-grained and locally porphyritic granitoid gneiss layers in the TG are meters to hundreds of meters thick. They contain numerous sheets of texturally and compositionally homogeneous fine- to coarse-grained amphibolite that extend hundreds of meters along strike and are as much as 4 m thick. Compositional layering in these rocks typically is subparallel to S[sub 1] and S[sub 2] axial parallel foliations produced during polyphase isoclinal to tight folding. Despite this complicated tectonic overprint the amphibolites locally cut layering in the granitoid gneisses. The authors interpret the trondhjemitic to tonalitic gneisses in the TG as metamorphosed and multiply deformed intrusives whose crosscutting relationship with older volcanogenic rocks of the Irving Formation is locally preserved. Amphibolite layers are interpreted as swarms of gabbroic dikes or sills that intruded the granitoid complex prior to isoclinal to tight folding and amphibolite facies metamorphism. These dike swarms thus reflect extension and mafic magmatism that preceded or was contemporaneous with the main pulse of pre-1.7 Ga compressional deformation. Intrusion of undeformed granitic dikes in the southern margin of the Twilight Gneiss is attributed to local melting and limited remobilization of trondhjemitic gneisses during a thermal event that post-dated basement deformation.
- OSTI ID:
- 6138542
- Report Number(s):
- CONF-9303212-; CODEN: GAAPBC
- Journal Information:
- Geological Society of America, Abstracts with Programs; (United States), Vol. 25:1; Conference: 27. annual Geological Society of America (GSA) South-Central Section meeting, Fort Worth, TX (United States), 15-16 Mar 1993; ISSN 0016-7592
- Country of Publication:
- United States
- Language:
- English
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