Abstract
Quantifying magma ascent rates and crustal contamination timescales in a continental magmatic system; a case study of the Block Mountain lava flow, SW Montana
Abstracts with programs - Geological Society of America, Vol.47(5), p.96
Geological Society of America, North-Central Section, 49th annual meeting
05/2015
Abstract
The ascent of mantle-derived magmas through the continental crust and subsequent magma-crust interactions are commonly preserved in continental magmatic rocks. Here we use in situ crystal chemistry, thermobarometry, and geospeedometry to reconstruct the pressure-temperature-time pathway associated with an Eocene extensional magmatic event in SW Montana. The approximately 45 Ma Block Mountain basaltic-andesite ( approximately 56-58 wt.% SiO (sub 2) ) contains three texturally and compositionally distinct types of clinopyroxene that equilibrated in distinct P-T environments. Large (100-500 mu m), subhedral clinopyroxene phenocrysts (En (sub 48) Wo (sub 41) Fs (sub 11) ) characterized by relatively high Mg# ( approximately 82), Al (sub 2) O (sub 3) ( approximately 5.7 wt.%) and Cr (sub 2) O (sub 3) ( approximately 0.29 wt.%), are in chemical equilibrium with the bulk-rock basaltic-andesite, and equilibrated at high temperatures and pressures ( approximately 1210-1220 degrees C and 610-940 MPa) consistent with crystallization within the upper mantle. Smaller (<100 mu m), euderal to subhedral, clinopyroxene microphenocrysts (En (sub 46) Wo (sub 37) Fs (sub 17) ) with notably lower Mg# ( approximately 73), Al (sub 2) O (sub 3) ( approximately 2.5 wt.%), and Cr (sub 2) O (sub 3) ( approximately 0.20 wt.%) make up approximately 40% of the basaltic-andesite groundmass (GM), and appear to have crystallized over a larger, lower range of temperatures and pressures (1166-1180 degrees C and 380-860 MPa), presumably during ascent through the lower crust ( approximately 31-14 km). An additional population of clinopyroxene microphenocrysts (<100 mu m; En (sub 44) Wo (sub 40) Fs (sub 15) ) is found exclusively in the diffusion coronas (clinopyroxene-glass reaction rims) surrounding resorbed quartz crystals. These microphenocrysts have Mg#s ( approximately 74) similar to microphenocrysts in the GM but have significantly lower Al (sub 2) O (sub 3) ( approximately 0.45 wt.%) and Cr (sub 2) O (sub 3) (b.d.l.) concentrations, and crystallized at significantly lower temperatures and pressures (1006-1101 degrees C and 140-300 MPa). Corona textures and glass compositions ( approximately 73 wt.% SiO (sub 2) ) indicate that the microphenocrysts formed during the peritectic reaction of quartz and the ascending mafic magma in the upper crust (5-11 km). We utilize crystal growth and diffusion-based element modeling in the groundmass and corona glass to determine (1) the rate at which the mafic magma ascended from the upper mantle to the upper crust (groundmass clinopyroxene), and (2) the timescale over which ascending mafic magma interacted with the upper crust prior to eruption.
Details
- Title: Subtitle
- Quantifying magma ascent rates and crustal contamination timescales in a continental magmatic system; a case study of the Block Mountain lava flow, SW Montana
- Creators
- Dale H Burns - University of Iowa, Department of Earth and Environmental Sciences Iowa City, IA USA United StatesDavid W Peate
- Resource Type
- Abstract
- Publication Details
- Abstracts with programs - Geological Society of America, Vol.47(5), p.96
- Conference
- Geological Society of America, North-Central Section, 49th annual meeting
- Publisher
- Geological Society of America (GSA)
- ISSN
- 0016-7592
- Alternative title
- Geological Society of America, North-Central Section, 49th annual meeting
- Language
- English
- Date published
- 05/2015
- Academic Unit
- Earth and Environmental Sciences; Honors Program
- Record Identifier
- 9984240785302771
Metrics
5 Record Views