Geochemistry and tectonic setting of the Paleogene Dillon Volcanics, southwestern Montana: Bimodal volcanism in a back-arc basin

Conference · Thu Apr 01 00:00:00 EST 1993 · Geological Society of America, Abstracts with Programs; (United States)

OSTI ID:5026318

McDowell, R J; Fritz, W J ^[1]

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1. Georgia State Univ., Atlanta, GA (United States). Dept. of Geology

The Paleogene Renova Formation of southwestern Montana consists of epiclastic, pyroclastic, and volcanic rocks deposited or erupted in the newly recognized Dillon/Renova basin. The volcanic rocks are referred to informally as the Dillon Volcanics,'' and consist of a bimodal suite of alkaline and transitional basalt and rhyolite flows, plugs, and associated pyroclastic flow, fall, and surge deposits. K-Ar dates of the Dillon Volcanics range from 53 Ma to 21 Ma. The Dillon/Renova basin was surrounded on the west, north, and east by the dominantly andesitic to dacitic Challis, Lowland Creek, and Absaroka volcanic fields. Volcanism in these areas was typical of a compressional continental arc setting. Bimodal volcanism of the Dillon/Renova basin was partly contemporaneous with volcanism in the surrounding fields, but was more typical of a continental rift zone setting and persisted after the surrounding fields had shut down. This suggests either: (1) the earliest phases of basin-and-range extension occurred in southwestern Montana and were manifested by bimodal volcanism; or (2) the Dillon/Renova basin was a back-arc basin unrelated to basin-and-range extension. Major and trace element variation diagrams indicate that the basalts and rhyolites had a common magmatic source. Chondrite normalized rae earth element patterns are very similar for both rock types, and are typical of continental rift volcanic rocks. However, the stratigraphy favors a back-arc basin. The Renova Formation exhibits a uniform eastward thinning, an eastward decrease in grain size, and gradual facies changes over a distance of 200 km. No internal unconformities have been recognized.

• OSTI ID: 5026318
• Report Number(s): CONF-9305259-; CODEN: GAAPBC
• Journal Information: Geological Society of America, Abstracts with Programs; (United States), Vol. 25:5; Conference: 89. annual meeting of the Cordilleran Section and the 46th annual meeting of the Rocky Mountain Section of the Geological Society of America (GSA), Reno, NV (United States), 19-21 May 1993; ISSN 0016-7592
• Country of Publication: United States
• Language: English

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58 GEOSCIENCES
GEOLOGIC FORMATIONS
GEOCHEMISTRY
PETROGENESIS
MONTANA
VOLCANIC ROCKS
CHEMICAL COMPOSITION
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RARE EARTHS
TECTONICS
TERTIARY PERIOD
CENOZOIC ERA
CHEMISTRY
DEVELOPED COUNTRIES
ELEMENTS
GEOLOGIC AGES
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IGNEOUS ROCKS
METALS
NORTH AMERICA
PETROLOGY
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USA
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