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