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IRTG-StRATEGY

Causes and Consequences of Diversity in the Central Volcanic Zone in NW

Integrating petrology, geology, geophysics & modelling

Robert Trumbull GFZ - Potsdam IRTG-StRATEGY

SFB-267 DFG Collaborative Research Centre „Deformation Processes in the “ 1993 - 2005

Geophysics, FUB, GFZ: C. Haberland, F. Schilling, H. Brasse, B. Schurr, X. Yuan

Geodynamic modelling, GFZ: S. Sobolev, A. Babeyko, I. Koulakov

Volcanology & petrology, TUB, GFZ: J. Lindsay, A. Schmitt, W. Schnurr, A. Risse, W. Siebel

Geology, GFZ, UP, FUB, TUB: O. Oncken, M. Strecker, F. Lucassen, U. Riller, E. Scheuber

Collaborations: M. Gardeweg, J. Clavero: Chile P. Caffe, B. Coira, J. Viramonte: Argentina S. de Silva, S. Kay: USA IRTG-StRATEGY

Central Andes Superlatives

• second-highest continental on earth • “type locality“ for crustal delamination at Cordilleran margins • among Earth's largest provinces • largest crustal magma body ever detected geophysically

Does this all fit together (how) ?

IRTG-StRATEGY

Central Volcanic Zone magma diversity

Stratovolcanoes (-): Oligo- to present-day Arc and along transverse zones

Ignimbrites (dacite-): Oligo-Miocene to Patchy distribution, arc and back-arc

Mafic monogenetic centers: Pliocene and younger Mainly southern Puna back-arc IRTG-StRATEGY

70°W 70°W 68°W 66°W 10°N NVZ 0°

10°S CVZ 20°S 20°S Uyuni H 30°S Altiplano-Puna SVZ B O L I V I A 40°S I Pastos Grandes Volcanic Complex 50°S Guacha 22°S 70°W Case study Panizos Calama La Pacana Atacama Antofagasta Co. Tuzgle Antofagasta 24°S

Aguas Arizaro Calientes Antofalla Hombre Muerto Neogene La Isla 26°S Pedernales Co. Galán arc volcanoes

Maricunga Co. Blanco back-arc volcanoes Wheelwright G 28°S 00 100100 200200 kmkm A 70°W 68°W 66°W Case study S. Puna IRTG-StRATEGY

Altiplano-Puna ignimbrite “flare-up” 9-4 Ma

APVC ignimbrites: ca. 20 to 40 km3 /Ma /km

All Neogene volcanoes together: ca. 3 km3 /Ma /km IRTG-StRATEGY

Ignimbrite magma volumes:

APVC total erupted volume > 15,000 km3 ignimbrites

Mega-eruptions with single flow units of 100‘s to >1000 km3 Pinatubo, 5 km3 Mt. St. Helens, 1km3 IRTG-StRATEGY

Case Study: La Pacana

Bolivia

Purico

a

n i t

n

e

g r

A

30 km

(Gardeweg & Ramierz, 1987; Lindsay et al., 2001) IRTG-StRATEGY

600 Atana pumice Atana & Tononao Units:

50% Zoned magma chamber ?

) Pl+Qtz+Bt

m Pl+Hbl+Bt+Cpx+Mag

p +Hbl+Sa

p 100

(

r 10%

S

0 Toconao pumice

10 100 200 300 Rb (ppm) 100 ATANA dacite 1000 Toconao pumice Atana pumice 4.1 Ma 140 500

)

m

p TOCONAO rhyolite

p

(

a 4.6 Ma

B 220m AtanaAtana glass melt 100 inclusions

60 40 100 200 300 Zr (ppm) Lindsay et al. (2001) J. Petrology IRTG-StRATEGY

P - T conditions of magma storage

different geothermometers

Pressure:

150 - 200 ± 50 MPa

H2O-CO2 solubility in melt inclusions Al-in

Lindsay et al. (2001) J. Petrology IRTG-StRATEGY IRTG-StRATEGY

Lindsay et al. (2001) J. Petrology IRTG-StRATEGY

Magma diversity: Plateau vs. Arc ignimbrites

Creataceous mantle rocks (alkali basalts, xenoliths)

Stratovolcanoes

Small-volume Large-volume „arcigs. „plateauigs.

(Lindsay et al., 2001, J.Petrol. / Siebel et al., 2001, Chem. Geol.) IRTG-StRATEGY

Crustal melts in the ignimbrite source IRTG-StRATEGY

Crustal recycling: Pliocene U-Pb zircon age, Neoproterozoic TDM age

Inherited zircons in Neogene Igs.

(Schmitt et al. 2002) ( Lucassen et al., 2001 Tectonophysics) IRTG-StRATEGY

Crustal melt productivity and source depth: melt experiments of natural gneiss at 5 to 10 kbar

low-P

1 Bt + 1 Qz + 0.5 Pl = 0.6 Opx + 1.8 melt

high-P 1 Bt + 0.8 Qz + 0.14 Sill = 0.7 Gt + 1.7 melt

(Patino-Douce & Beard, 1996, J. Petrol.)

Point 1: without external fluid, maximum 20% melt production for average gneiss with 10% biotite

Point 2: melting at >10 kbar should produce garnet effect on REE patterns IRTG-StRATEGY

APVC ignimbrites lack garnet signature: source depth < 30 km IRTG-StRATEGY

Large-volume plateau ignimbrites - main points

Eruptions: Huge - 1000 km3 single flow units • Homogeneous, -rich dacite, viscous, water-poor (2-3 wt.%)

Magma storage: Shallow • Pre-eruptive magma temperature 750-850°C • Magma chamber depths ca. 5-8 km

Magma source: Crustal • Sr-Nd-Pb isotopes overlap with basement • Not pure crustal melts: 20-30% mafic magma (arc?) mixed in • Not lower (no garnet signature, moderate T, dehydration melting IRTG-StRATEGY

Geophysical evidence for partial melts

Gravity and Seismic velocity Schmitz et al., 1996

Electrical conductivity Schilling et al. 1997 Brasse et al. 2002

Seismic attenuation tomography Haberland & Rietbrock, 2001 Schurr et al., 2002

Seismic receiver functions Chmielowski, 1999 Yuan et al., 2000 Zandt et al., 2003

High surface heat-flow density Springer, 1999

IRTG-StRATEGY

• Areal extent and depth of the partial-melt zone

• Proportion of melt present within it IRTG-StRATEGY

Area of geophysical anomaly coincides with caldera complexes

Caldera complexes

InferredAltiplano-Puna magma Magma Body “sill”Zandt at 20 et km al. (2003) IRTG-StRATEGY

Depth to top of the anomaly is ca. 20 km, thickness is poorly defined

Electrical resistivity modified from Brasse et al. (2002)

0 km 20 40 60 S-wave velocity -depth profiles from Vs 3 4 5 3 4 5 3 4 5 3 4 5 3 4 5 receiver functions km/s Yuan et al. (2000) IRTG-StRATEGY

Melt proportion in the zone around 20% if connected

Electrical conductivity at 20 km depth

(Schilling & Partsch, 2001, PEPI) IRTG-StRATEGY

The A-P Magma Body is a “live” regional migmatite IRTG-StRATEGY

Thermo-mechanical modelling:

Can we explain formation of the APMB in terms of crustal heat budget during deformation?

Model temperature development at 20-25 km depth with time for different heat sources:

Internal crustal (radiogenic, frictional heat)

Active arc: random, pluton intrusions at 1200°C distributed in 5 km-thick layer for a total flux of 30km3/km/My

Hot Mantle: heat flow 60mW/m2 at Moho IRTG-StRATEGY

Initial conditions: 35 km crust, 25°C/km geotherm

Required: > 800°C at 20 km within 10-15 Ma after onset of shortening

Model lithosphere section, 200 km wide (APVC dimension):

35 km crust (initially) rheology 95 km mantle with rheology

constant shortening by 100% for 30 My

(Babeyko et al. 2002, EPSL) IRTG-StRATEGY

Result: average temperatures at 20-25 km depth during 30 my

Required: > 800°C at 20 km within 10-15 Ma after onset of shortening

(Babeyko et al. 2002, EPSL)

Hot mantle, convection

Hot mantle, conduction

Internal heat plus arc intrusions

Internal heat sources IRTG-StRATEGY

Hot mantle, convection Conclusions from modelling:

mid-crustal melt zone can be established if:

Mantle heat input is high

Heat is transported by crustal convection

Convection (ductile flow) is possible for crust with quartz rheology and active shortening IRTG-StRATEGY

Evidence for hot upper mantle: seismic tomography

Tuzgle

(Koulakov et al., 2005) IRTG-StRATEGY

Temperature distribution in the mantle wedge from S-wave tomography inversion

Delamination ?

(Koulakov et al., 2005) IRTG-StRATEGY

Thermo-mechanical model of the Andean Orogeny (temperature distribution)

delamination

Sobolev et al. (2006) IRTG-StRATEGY

Conclusions

1. Information from APVC ignimbrites and various geophysical anomalies in the plateau region are entirely consistent and probably expressions of the same process.

2. The geophysical anomalies detect a „live migmatite zone in the mid-crust with an overall, average proportion of melt ca. 20%.

3. Crustal melting on a large scale is not an arc process per se: Arc are unimportant as a heat source; melting is not happening in the south; and „arc ignimbrites in the CVZ are quite different from the large, „plateau ignimbrites.

4. Starting and maintaining the mid-crustal melt zone requires high mantle heat and intracrustal convection enhanced by deformation.

5. All available evidence supports the idea of lithospheric delamination under the Puna plateau with upwelling of hot asthenosphere in the gap. IRTG-StRATEGY

Causes and Consequences of Magma Diversity in the Central Volcanic Zone in NW Argentina

Integrating petrology, geology, geophysics & modelling

Thank you for listening! Geodynamics Geophysics

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