Relative contribution of Precambrian metamorphic rocks and Cretaceous-Tertiary igneous rocks to Oligocene and Holocene fluvial sands and the unroofing of a magmatic arc

Conference · Fri Mar 01 00:00:00 EST 1991 · AAPG Bulletin (American Association of Petroleum Geologists); (United States)

OSTI ID:7259877

Molinaroli, E; Basu, A ^[1]

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1. Indiana Univ., Bloomington (United States)

Oligocene and Holocene fluvial sands were deposited in small extensional basins in a magmatic arc in southwestern Montana under relatively humid and semi-arid conditions, respectively. The source rocks are roof-pendants and thrust-slices of Precambrian metamorphic rocks (PCM) and Cretaceous-Tertiary igneous rocks (KTI) that make up the arc. The authors have surveyed 143,607 heavy mineral grains (HMGs) in polished thin sections of 55 samples collected from adjacent but discrete geomorphologic units. In the Holocene sands, of 5440 HMGs 519 are garnets and of 97,667 HMGs 395 are zircons. In the Oligocene sandstones, of 6397 HMGs 998 are garnets, and of 45,940 HMGs 331 are zircons. Garnets are absent in the igneous rocks and zircons are extremely rare in the metamorphic rocks. Garnets ar estimated to be about 100 times as abundant in the metamorphic rocks as the zircons are in the igneous rocks. Mass balance calculations show that the proportion of PCM/(PCM+KTI) ranges from 0 to 21% in Oligocene sandstones, and from 3 to 76% in Holocene sands in different local units. However, the overall PCM/(PCM+KTI) proportions in the Holocene and the Oligocene sands in southwestern Montana are 19% and 18%, respectively. This suggests that the roof pendants, thrust slices, and magmatic arc rocks have been unroofed in constant proportions since the Oligocene although locally the proportions have been different.

• OSTI ID: 7259877
• Report Number(s): CONF-910403-; CODEN: AABUD
• Journal Information: AAPG Bulletin (American Association of Petroleum Geologists); (United States), Vol. 75:3; Conference: Annual meeting of the American Association of Petroleum Geologists (AAPG), Dallas, TX (United States), 7-10 Apr 1991; ISSN 0149-1423
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
ALLUVIAL DEPOSITS
MINERALOGY
ORIGIN
MONTANA
COMPARATIVE EVALUATIONS
CRETACEOUS PERIOD
GARNETS
IGNEOUS ROCKS
METAMORPHIC ROCKS
PETROGRAPHY
PRECAMBRIAN ERA
QUATERNARY PERIOD
SANDSTONES
TERTIARY PERIOD
ZIRCON
CENOZOIC ERA
DEVELOPED COUNTRIES
EVALUATION
FEDERAL REGION VIII
GEOLOGIC AGES
GEOLOGIC DEPOSITS
MESOZOIC ERA
MINERALS
NORTH AMERICA
OXYGEN COMPOUNDS
ROCKS
SEDIMENTARY ROCKS
SILICATE MINERALS
SILICATES
SILICON COMPOUNDS
USA
580000* - Geosciences