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Eocene potassic magmatism in the Highwood Mountains, Montana: Petrology, geochemistry, and tectonic implications
- Source :
- Journal of Geophysical Research: Solid Earth. 96:13237-13260
- Publication Year :
- 1991
- Publisher :
- American Geophysical Union (AGU), 1991.
-
Abstract
- Potassic volcanics and intrusives of Eocene age (52±1 Ma) in the Highwood Mountains of north-central Montana provide evidence for interaction of asthenospheric magmas with Archean mantle lithosphere of the Wyoming Craton. Diverse rock types were produced by shallow level degassing, fractional crystallization and magma mixing within two separate magma series whose parental liquids were latite and olivine minette. Halogen systematics of apatite microphenocrysts and other evidence have established that shallow degassing of the phlogopite-diopside-phyric minettes to yield heteromorphic leucite-salite-phyric mafic phonolites and shonkinites was an important process. Geochemical and mineralogical data suggest that fractional crystallization of olivine, clinopyroxene, mica and leucite, accompanied by widespread magma mixing, produced a spectrum of more evolved magmas such as leucite phonolites, malignites, alkali syenites and trachytes along with mica clinopyroxenite cumulates. Sr, Nd and Pb isotopic compositions and trace element data for the primitive olivine minette magmas are explicable by a multistage model involving three source components. One component is ancient Ba-LREE-enriched, U-Th-HFSE-depleted subcontinental lithospheric mantle, which has been recognized in other alkalic rocks of the region, including those from the Crazy Mountains and Smoky Butte. Glimmerite-veined harzburgite and phlogopite dunite xenoliths (one with -eNd of −33 at 52 Ma) found in the most primitive Highwood olivine minette are probably samples of this material. The other two components are asthenospheric mantle with isotopic composition near that of bulk earth (and dominant in Montana alnoitic diatremes) and a young subduction-related component (probably Eocene, but possibly as old as late Cretaceous), which is required to explain the Rb/Sr-87Sr/86Sr systematics of the Highwood rocks. A consistent model for the petrogenesis of the Highwood parental mafic magmas involves partial melting of asthenospheric mantle wedge triggered by infiltration of melts released from the metasomatized carapace above a low-angle subducted slab of Farallon Plate lithosphere, followed by assimilative interaction of these melts with ancient, phlogopite-rich, metasome veins upon ascent through the Wyoming Craton mantle keel.
- Subjects :
- Atmospheric Science
Mantle wedge
Geochemistry
Soil Science
Aquatic Science
engineering.material
Oceanography
Geochemistry and Petrology
Earth and Planetary Sciences (miscellaneous)
Xenolith
Petrology
Earth-Surface Processes
Water Science and Technology
geography
Olivine
geography.geographical_feature_category
Fractional crystallization (geology)
Ecology
Paleontology
Forestry
Craton
Geophysics
Space and Planetary Science
Latite
engineering
Igneous differentiation
Mafic
Geology
Subjects
Details
- ISSN :
- 01480227
- Volume :
- 96
- Database :
- OpenAIRE
- Journal :
- Journal of Geophysical Research: Solid Earth
- Accession number :
- edsair.doi...........710f7816a860870fa68273d65361edba