arc crustal sections: one roadmap to integrating geochemistry and geophysics george bergantz...
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![Page 1: Arc Crustal Sections: One Roadmap to Integrating Geochemistry and Geophysics George Bergantz University of Washington (how to increase the petrological](https://reader035.vdocuments.us/reader035/viewer/2022070412/56649d245503460f949fa93e/html5/thumbnails/1.jpg)
Arc Crustal Sections: One Roadmap to Integrating
Geochemistry and Geophysics
George BergantzUniversity of Washington
(how to increase the petrological signal-to-noise ratio, and avoid that groundhog day feeling…?)
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2 - 3 Science Questions: Essential Missing Piece- the Geology
Controls on architecture, a hierarchical perspective
Temporal changes: vol-comp-time “Right” physical model/template? Coupling with structure/rheology?
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Successes: Integrating Volcanic-Plutonic Observations
Magmas obviously mantle-crust hybrids aFC rulesNd, NCI < .4, oxygen, other isotopes (few
to 20% crust) Compositional continuity at large scales, Compositional
gaps within temporally related suites
Hybridization generally happens early and deep
Open system/complex crystal cargo at all scales
Further progress requires spatial-temporal controls and arc sections
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Some Arc Sections:
Kohistan (see Jagoutz poster, papers) Talkeetna-Bonanza Arc (Vancouver
Island) North Cascades Sierra Nevada/BC Fiordland Famatina (Argentina)Much agreement! Details have rarely served to address site-resolvable, specific *processes*
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Famatinian arc: exposed along 1500 km of arc length, 600 km of plutonic section
Early Ordovician (~40 my activity),
7 to ~30 km of continuous intact exposure
Voluminous hydrous mafic magmas, regionally hyper-solidus contacts
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Average magma flux rates km3/yr- low-pass filter (see Straub white paper)
Large Silicic Provinces:Altiplano-Puna: 4x10-3 – 1.2x10-2
Central San Juan: 8x10-3
Sierra Nevada: 3 - 9x10-3
North Cascades: 3x10-3
Boulder, B.C. batholith: 6x10-3 – 1x10-2
Other Arc systems (but see Jicha, Singer):
Klyuchevskoy: 3.2x10-2
Mt. Shasta: 6x10-3
Tatara-San Pedro: 6x10-5
Mt. Adams (field): 2.5x10-4
Ceboruco- Pedro: 1x10-4
Santorini: 4.6x10-4
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Given open system processes, “duration” or “residence time” can be misleading concepts.
(Grunder et al., 2008, Trans. Roy. Soc. Edin.)
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What is the significance of the 4-5 m.y. trigger?
(Grunder et al., 2008, Trans. Roy. Soc. Edin.)
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Age relations Famatinian Arc, Valle Fertil section, Argentina
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Time scales have dual nature: homogeneity at the large scale, heterogeneity at the small scale
Toba: chem oscillations in allanites > .4 M.y. before eruption; cycling of crystals through hyper-solidus domains
Fish Canyon: reverse mineral zoning, complex crystal compositions
Tuolumne Intrusive Suite: complexly zoned zircons, Spirit Mtn., Mojave system: complex rejuvenation of
intrusive sheets, zoned zircon
Complicated!…. but other than open systems processes, what general conclusions? Without in situ stratigraphic control, hard to know what it is telling us
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‘Right’ Physical Model?
Hot zone (Dufek and Bergantz, Annen et al.) : model driven, not constrained by direct geological observations. “Balloon+straw,” no assimilation or melt extraction physics
Mush column (Daly, Marsh, Bachmann et al.): Based on crustal sections and physical model for aFC and extraction
Both have features in accord with geochemical constraints
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Famatinian arc: exposed along 1500 km of arc length, 600 km of plutonic section
Early Ordovician (~40 my activity),
7 to ~30 km of continuous intact exposure
Voluminous hydrous mafic magmas, regionally hyper-solidus contacts
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Bachmann and Bergantz, 2008
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Silicic Mushes?
(Hildreth, 2004, J. Volc. Geotherm. Res., v. 136, p. 169)
Persistent hyper-solidus state
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Interplay of rheological/structural changes and other time scales?
Does it matter for compositional diversity? What controls the fundamental change
from largely horizontal regional tectonics/emplacement to vertical?
Arc sections: Linking vol-comp-structure-time
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Thanks