Do basaltic intrusions trigger felsic super-eruptions? Matthew Ferguson1 (
[email protected]) 1School
of Physical Sciences, University of Tasmania, Hobart, TAS, 7001, Australia
Problem: Unravelling genesis and evolution of ancient volcano-plutonic complexes is complicated by alteration, erosion, deformation, age uncertainties, and by difficulties in constraining formation settings Objective: Compare ancient and recent felsic magmatism for insights into development of ancient rocks
Ancient
Recent
Gawler Range Volcanics (GRV) and Hiltaba Suite (HS), South Australia
1590 Ma
Taupo – New Zealand San Juan, Cascades, Yellowstone (SRP) – North America
< 30 Ma
After Agangi et al., 2011
Wilson, 2008
• Huckleberry Ridge Tuff – 2.1 Ma, 2500 km3, largest Yellowstone eruption, others in hotspot trail
• Dacite and rhyolite lavas • Volume up to ca 3500km3
• Taupo Oruanui eruption – 26.5 ka, most recent super eruption worldwide, approx. 1200 km3
• Erupted at 850–1000°C
Petrography: Textures show that minerals have complex histories; they do not tell us about protoliths 500µm
Opx
Mineral aggregates – complex history
Zircon Apatite
•
Titanomagnetite, pyroxenes, plagioclase, apatite and zircon
Gawler Range Volcanics •
Titanomagnetite
Spongy and anhedral crystal textures – mineral destabilisation Gawler Range Volcanics
Subhedral-anhedral habits, simple to complex growth relationships
Eucarro Rhyolite (GRVPW20)
Snake River Plain Bachmann et al., 2002
Eucarro Rhyolite (GRVPW20)
Plagioclase
Ellis and Wolff, 2012
Ellsia and Wolff, 2012
Snake River Plain
Snake River Plain
Requires heat and/or volatile addition
1mm
Ellis and Wolff, 2012
Rejuvenation established in recent examples
Geochemistry: Crystal-liquid separation, young crust or juvenile input involved at source
2.5 1.5 0.5
Pondanna Dacite (int. Rb/Sr, high Mg)
Eucarro Rhyolite (int. Rb/Sr) Basal Eucarro Rhyolite/Paney Rhyolite (high Si-Rb/Sr) Roxby Downs Granite (RD2488)
K2O (wt. %)
Eucarro Rhyolite (low Si-Rb/Sr, high Ti)
70 75 SiO2 (wt. %)
2.5 1.5
80 6.5
6.0
6.0
5.0 4.5 4.0
Bonnichsen et al., 2008 Central Snake River Plain
65
6.5
5.5
Cpx
12
70 75 SiO2 (wt. %)
80
10 8 6 4
70 75 SiO2 (wt. %)
10 8 6 Compilation of cSRP pyroxene analyses
4
GRV pyroxenes
GRV and HS
2
2 0
5 10 CaO (wt. %)
15
0
20
5 10 CaO (wt. %)
15
Eucarro Rhyolite aggregates Eucarro Rhyolite Pondanna Dacite Eucarro Rhyolite enclave Moonaree Dacite aggregates Moonaree Dacite
5.0 4.5
80
65
70 75 SiO2 (wt. %)
80
•
Coexisting, high-Fe pyroxenes
•
Whole rock and mineral compositions controlled by crystal-liquid separation
Established mafic input Mafic lavas (thin crust)
Felsic units (thick crust) Nash et al., 2006
Snake River Plain
20
20
5.5
4.0 65
Not xenocrysts
12
3.5
0.5 65
Pondanna Dacite (high Si)
4.5
14
7.5 1590 Ma 5.0 Evolved baseline composition 2.5 CHUR 0.0 -2.5 -5.0 Mafic and felsic -7.5 rocks with similar ɛNdt -10.0 -12.5 -15.0 1500 1550 1600 1650 1700 Age (Ma)
10
Zircon ɛHft
3.5
SRP
14
Opx MgO (wt. %)
4.5
FeO(T) (wt. %)
Pondanna Dacite (low Rb/Sr, high Ca)
GRV and HS
K2O (wt. %)
Basal Moonaree Dacite (high Si, low Mg)
FeO(T) (wt. %)
Moonaree Dacite (red facies, low Si)
5.5
MgO (wt. %)
5.5
Moonaree Dacite (Qtz-bearing, high K-Rb/Sr)
Same crystallising assemblage
Whole-rock ɛNdt
Ferropotassic, A-type rocks
Suspected mafic input
0
CHUR
-10 -20
Requires juvenile input or very young crust
-30
All data
Zircon Hf
Eucarro Rhyolite Moonaree Dacite
-40 1500 1550 1600 1650 1700 Age (Ma)
Drew et al., 2013
Conclusion: Mafic magmatism is involved in the development of large volume felsic magmatism Mafic magmas at different scales – prime candidates for suppliers of heat, volatiles and metals Mafic microgranular clot
Lower GRV Upper GRV Yardea enclave Eucarro enclave Yardea enclave margin Eucarro enclave margin
8
MgO (wt. %)
Groundmass
10
6 4
Acknowledgements We are grateful to Karsten Goemann and Sandrin Feig for analytical assistance. I would like to thank Alex Cherry and Nathan Chapman for constructive criticism which improved this poster.
2.5 2
1.5 1 Wilson and Charlier, 2016
2
0.5
0
0 45
1mm
3
TiO2 (wt. %)
Quenched blobs of mafic magma in Upper GRV
Interpreted vertical structure of active, large volume silicic systems
50
55
60 65 70 SiO2 (wt. %)
75
80
45
50
55
60 65 70 SiO2 (wt. %)
75
80
Mafic rocks in the Lower GRV, and compositions of mafic blobs in Upper GRV
Significant mafic influence in lower crust
References Agangi et al. 2011 PhD Thesis. Bachmann et al. 2002 J. Petrol. 43, 1469-1503.
Bonnichsen et al. 2008 Bull. Volcanol. 70, 315-342. Drew et al. 2013 Earth Planet. Sci. Lett. 381, 63-77.
Ellis and Wolff 2012 J. Volcanol. Geotherm. Res. 211, 1-11. Wilson 2008 Elements 4, 29-34. Nash et al. 2006 Earth Planet. Sci. Lett. 247, 143-156. Wilson and Charlier 2016 Elements 12, 103-108.
