High initial Sr isotopic ratios and evidence for magma mixing in the Pioneer batholith of southwest Montana
The northeast part of the composite Pioneer batholith of southwest Montana consists of Late Cretaceous calc-alkaline plutons that vary in composition as a function of age, changing from minor hornblende-biotite quartz diorite at about 76 m.y., to a small pluton of hornblende-biotite tonalite at about 74 m.y., to abundant biotite-hornblende granodiorite and biotite granite at about 72 m.y., to some biotite granodiorite and 2-mica granite at 67 to 65 m.y. Initial /sup 87/Sr//sup 86/Sr ratios (SIRs) for the plutons range from 0.7112 to 0.7160. Magmas having a SIR near 0.7112 appear in most episodes of emplacement, whereas those having higher values are restricted to individual episodes. Two granodiorite plutons of 72 m.y. age show a mutual gradational contact, across which the SIR varies over a distance of 7 km from 0.711 to 0.714. The variation is ascribed to mixing between contemporaneous but isotopically distinct granodiorite magmas. The high SIRs of the batholith are interpreted as reflecting partial melting of portions of Precambrian lower crustal lithosphere to produce mafic to intermediate magma. The source may be similar to parts of Proterozoic gneisses now exposed at the surface. Mafic to intermediate magma may have fractionally crystallized at depth and released residual magma episodically to the upper crust over approximately 11 m.y. The high SIR of the Pioneer batholith is broadly similar to that of large parts of the Idaho batholith to the west, but is distinct from the SIR of the Boulder batholith to the northeast. A significant crustal discontinuity may separate the Pioneer-Idaho region from the Boulder region.
- Research Organization:
- Geological Survey, Reston, VA
- OSTI ID:
- 6740027
- Journal Information:
- J. Geol.; (United States), Vol. 94:3
- Country of Publication:
- United States
- Language:
- English
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GEOLOGIC HISTORY
MONTANA
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ISOTOPE RATIO
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STRONTIUM 87
ALKALINE EARTH ISOTOPES
BETA DECAY RADIOISOTOPES
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ELECTRON CAPTURE RADIOISOTOPES
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USA
580202* - Geophysics- Volcanology- (1980-1989)
580400 - Geochemistry- (-1989)