Geologic evolution in the Montana Metasedimentary Terrane from a neodymium perspective
- Univ. of Florida, Gainesville, FL (United States). Dept. of Geology
The Archean Wyoming Province, located primarily in southwestern Montana and Wyoming, can be subdivided into three distinct lithotectonic terranes defined on the basis of Late Archean assemblages. These include the Montana Metasedimentary Terrane (MMT), the Beartooth-Bighorn Magmatic Terrane (BBMT), and the Wyoming Greenstone Terrane (WGT). The MMT, located in southwestern Montana, contains a distinctive assemblage of metasedimentary and metaigneous lithologies, but is dominated by Late to Middle Archean tonalitic to granitic gneisses. The BBMT adjoins the MMT on its eastern margin and is composed primarily of Late Archean (2.75 Ga) igneous and metaigneous lithologies, although isolated Middle Archean (3.3 Ga) metasedimentary packages are present. The BBMT is exposed in Beartooth and Bighorn Mountains of Wyoming. Neodymium model ages Tchur have been determined on 55 samples and range from 2.05 to 3.78 Ga, with clusters at about 3 and 3.3 Ga. The 3.3 Ga cluster is generally corroborated by U-Pb data from single-grain, detrital zircon studies of Tobacco Root quartzites and has been documented in metasediments of the BBMT. The 2.75 Ga event which affected the BBMT has, so far, not been detected in the MMT, although 2.65 Ga lithologies have been detected in the Madison Range. It is possible that the 3 Ga ages of MMT metasediments reflect mixing of 3.3 Ga and 2.75 and/or 2.65 Ga material or, that they represent the dominant time of crustal formation in the MMT as opposed to 2.75 Ga in the BBMT. Older Tchur's (3.3 to 3.8 Ga) are found in both the Tobacco Root Mountains and the Madison Range and suggest that the MMT contains evidence of even earlier crust-forming events not documented elsewhere in the Wyoming Craton.
- OSTI ID:
- 5882071
- Report Number(s):
- CONF-921058-; CODEN: GAAPBC
- Journal Information:
- Geological Society of America, Abstracts with Programs; (United States), Vol. 24:7; Conference: 1992 annual meeting of the Geological Society of America (GSA), Cincinnati, OH (United States), 26-29 Oct 1992; ISSN 0016-7592
- Country of Publication:
- United States
- Language:
- English
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