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…?)
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?
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
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*
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
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
Given open system processes, “duration” or “residence time” can be misleading concepts.
(Grunder et al., 2008, Trans. Roy. Soc. Edin.)
What is the significance of the 4-5 m.y. trigger?
(Grunder et al., 2008, Trans. Roy. Soc. Edin.)
Age relations Famatinian Arc, Valle Fertil section, Argentina
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
‘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
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
Bachmann and Bergantz, 2008
Silicic Mushes?
(Hildreth, 2004, J. Volc. Geotherm. Res., v. 136, p. 169)
Persistent hyper-solidus state
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
Thanks