Magmatism of the Snake River Plain – Yellowstone region: Implications for continental lithosphere...
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Transcript of Magmatism of the Snake River Plain – Yellowstone region: Implications for continental lithosphere...
Magmatism of the Snake River Plain – Yellowstone region:
Implications for continental lithosphere evolution above a mantle plume
Bill Leemannow at
National Science Foundation
Theme of this presentation• Earthscope and related geophysical investigations
will provide a snapshot of crust-lithosphere structure
• This will be particularly useful in evaluating near real-time geological processes
• A focus on the active Yellowstone-Snake River Plain magma system would provide an unprecedented opportunity to understand large-scale magmato-tectonic processes and their interactions with and effects on existing lithosphere.
Key topics to be addressed
• Nature of the underlying lithosphere - isotope constraints
• Space-time migration of bimodal volcanism - the ‘hot spot track’
• Volumes, rates, and sources of magmatism - geodynamic implications
• Specific problems and the role of Earthscope
Architecture of the lithosphere - N. Rocky Mtns.Setting for mid-Miocene magmatic flareup
WISZ
Setting for mid-Miocene magmatic flareup
WISZ
artifactrealistic
On-craton
Off-craton
Isotopes signify distinct mantle sources across prominent tectonic boundaries
41.0
42.0
43.0
44.0
N L
at.
(°)
112.0114.0116.0118.0120.0
W. Long. (°)
<0.708<0.707<0.706
<0.7055<0.705<0.704
NV
OR
UT
ID
Sr isotopic compositions of Cenozoic basalts
< 0.706> 0.706
< 0.706
Circles ≥ 0.706Diamonds < 0.706
N. Lat. (°)
Map view
Cratonedge
E-W Crossection
off-cratonon-craton
y = 0.1597x + 12.733
R2 = 0.9682
15.2
15.3
15.4
15.5
15.6
15.7
15.8
16.0 16.5 17.0 17.5 18.0 18.5 19.0
206Pb/204Pb
207P
b/20
4Pb
SRP-YNP basaltsIsochron age = ca. 2.5 Ga
YNP
ESRP
WSRPCSRP
Pb-Pb systematics imply Archean age for SRP basalt sourceswith increasingly radiogenic Pb to the west
15.3
15.4
15.5
15.6
15.7
15.8
15.9
16.0 17.0 18.0 19.0 20.0
206Pb/204Pb
207Pb/204Pb
SRPYNPEOR
YellowstoneESRP
WSRPy = 0.181x + 12.348 r2 = 0.95 ca. 2.67 Ga
NHRL (oceanic array)
Rhyolites
off-craton
Pb in SRP rhyolites becomes progressively more radiogenic to west, and also is consistent with an Archean source; compositions dramatically change near the
inferred craton edge.
Zircon Geochronology of Lower Crustal XenolithsVervoort, Wolf & Leeman (unpub.)
Leeman et al., 1985
Proterozoic sediments
Archean Crust
2.6 2.8-3.2
2.9-3.2 Ga
SRP 2600
2500
2400
2300
2200
2100
0.28
0.32
0.36
0.40
0.44
0.48
0.52
6.5 7.5 8.5 9.5 10.5 11.5 12.5
207Pb/235U
206 Pb/238U
Intercepts at 27 ± 100 & 2582.4 ± 8.7 [±11] Ma
MSWD = 0.28
SM
These data enlarge the known extent of the Archean Wyoming province
Post-mid Miocene magmatic progressions
dashed lines mark isotope discontinuites
Following CRB ‘event’, magmatism expanded NE-ward with time into the SRP with a minor bifurcation into SE Oregon.
Early silicic magmatism requires precursor basaltic intrusions.
CRB flood lavas
Migration of SRPmagmatism
(Armstrong, Leeman &Malde, 1975)
MREC
EMBH Tiv
Apparent propagation rate
Extension?
WSRP
Yellow boxes = anomalies
Problems in estimating volcanic propagation rates
• Locations of vents/sources
• Correlations of distal units to source
• Causes for silicic magmatism
• Tectonic displacement (extension)
Space-time distribution of Yellowstone hotspot track silicic
volcanic rocks(Perkins & Nash, 2002)
**
In detail, not a simple age progression!
Main trendAnomalousBasalt
*
1
2
3
Ignimbrite flare-up between 11.7-10.0 Ma coincided with widespread outbreaks of distinct rhyolites
These occurrences signify that: (1) Large pockets of compositionally diverse silicic magmas existed coevally within wide expanses of the crust, and(2) Mafic magmatism must have been similarly widespread
0
2
4
6
8
10
12
14
16
Age(m.y.)
0 1 2 3 4 5 6
FeO*MBH
RTF
BJ Rhy
Tephras
McD
Tyd
Tmr/yt
YP
JM
OF
CPT avgs
Yellowstone
W.-Central SRP
MREC
McDermitt
Juniper Mtn.
BJ/TF/MBH
Tmr
Tmc
Figure X14. Temporal variation in chemistry of West-Central SRP rhyolites (14-3 Ma). Included are data for Bruneau-Jarbidge (CPT and BJ), Mt. Bennett Hills (MBH), and Rogerson/Twin Falls (RTF) areas (our averages), Owyhee front (OF), Magic Reservoir Center (Tmr/yt, Tyd), and regional ashes (Tephras). Comparative data are shown for the younger Yellowstone (YP) and older Juniper Mtn. (JM) and McDermitt (McD) eruptive centers. Regression lines through data from most eruptive centers have negative slopes consistent with magmas becoming more evolved with time. BJ/RTF/MBH data differ dramatically in showing increasing ‘maficity’ with time.
OF
0.5115
0.5120
0.5125
0.5130
143Nd/144Nd
05101520
Age
MREC Rhy
SRP-OR Rhy
MREC
AVT
YP
W ofcraton
SRP basalts
(ESRP)
IB
From Leeman, Oldow, and Hart (1992) and unpublished data
(0-15 Ma)
Archean xenoliths < 0.5115
Caldera-Forming Stage Rifting Stage
IgnimbriteFlare Up
Eruption Rate (km3/Ma)
Cumulative Volume(as percent of total)
500
100
12 9 6
0
Age (M.y.)
80
60
40
20
0
400
300
200
100
∑Volume = ca. 10000 km3
Comparison of the three ash-flow tuffs of the Yellowstone Group and resulting calderas
Ash-flow
Tuff
Age
(Ma)
Volume
(km3)
Area
(km2)
Dimen-sions (km)
Caldera
name
Lava Creek Tuff
0.640 1000 7500 85 x 45 Yellowstone
Mesa Falls Tuff
1.3 280 2700 16 x 16 Henry’s Fork
Huckleberry Ridge Tuff
2.1 2450 15500 ~85 x 50 Big Bend Ridge, etc.
(segments)
Total duration: >2.1 Ma Total AFT eruptive volume > 3700 km3
(Total volume of rhyolitic magma is considerably greater)
How much basalt are we talking about?
1. Yellowstone analog - rhyolites produced by crustal melting due to intrusion of basalts; assuming I:E = ~2 (this could be >10), volume production is constrained by thermal balances:rhyolite volume = ~10000 km3 (produced over 2 Ma)partial melt zone = 100000 km3 (for 10% melting) thickness of pmz = ~6-13 km (for radii of 70 to 50 km)
2. Heat budget requires crystallization of ~2g of basalt for each 1g of rhyolite produced, or about 20000 km3 over 2 Ma - a supply rate of ~0.01 km3/yr (~1/10 the rate for Kilauea): equivalent total thickness of basalt intruded = ~1.3-2.5 km (for radii of 70 to 50 km), or about 1 km/Ma
3. For a lithosphere block (width = 100 km, thickness = 100 km)migrating over plume heat source at 2-4 cm/yr (20-40 km/Ma), the required volume of basalt amounts to 5% partial melting of SCLM (assuming greater lithosphere volume or faster migration decreases % pm).
Implications and questions 1. Large volume (~10000 km3/Ma) injection of basalt into crust,
with near constant crustal thickness along the SRP, implies accommodation by lithosphere stretching (parallel to SRP axis):
extension = V/(tL• width) = ~1 km/Ma strain rate for SRP = (1 km/Ma • 15 Ma)/500 km = ~3%
2. The inferred magnitude of extension (~1 cm/yr) is similar to the difference between plate velocity estimated from time-distance relations for silicic eruptive centers (~3.5-4 cm/yr) vs. estimates based on other methods (e.g., NUVEL-1 model, 2.2±0.8 cm/yr).
3. Ongoing B&R style extension may account for extended magmatism distal from the plume center.
4. More work is needed to reconcile the inferred basalt production with apparent thermal inertial of either SCLM or a plume deflected by a thick lithosphere. E.g., just how thick is the mechanical boundary layer wherein reside the old isotopic components that contribute to Y-SRP magmatism?
Model for SRP crustal evolution - assuming an averaged crustal extension rate ( ~5%/Ma) and original crustal thickness of 40 km. Original Moho and midcrust (Conrad discontinuity) shallow with time according to lines ‘M’ and ‘C’. To maintain near-constant crustal thickness (based on available seismic refraction data) requires addition of under- or intra-plated basalt over depths equivalent to those between curves ‘M’ and ‘Moho’ (though not restricted to the geometry shown). Final mass distribution is such that ~3/4 of the present-day WSRP crust has a lower crustal average P-wave velocity (~6.7 km/sec).
YP WCSRP (Distance ->)
0
10
20
30
40
50
0 5 10 15 20Time (Ma)
Thi
ckne
ss (k
m)
total crustupper crustOrig Moho
Upper crust
Lower crust‘M’
Lithospheric mantle
Vol. new crust
New ‘Moho’
shallow basaltic
intrusions
‘C’ rhyolite
basalt
What is the source of Y-SRP basalts? • Upwelling plume material
a. If t > ~100 km, a plume is unlikely to melt unless Tp >1500°C
b. Plume could contribute heat to SCLM and volatiles (e.g., He)
c. If melting occurs, expect OIB- or MORB-like magmas
• Lower SCLM (isotopic compositions depend on age of SCLM)
a. If strongly refractory (e.g., residual peridotite), perhaps no melt
b. Low % melts of hydrated lithosphere (--> lamproite melts?)
c. Larger % melts of mafic/pyroxenitic veins (--> basaltic melts?)
• Combination models?
a. Plume melts modified systematically during ascent & storage by SCLM-derived melts
b. Hybrid source consisting of plume mantle & thermally eroded SCLM material
Arguments for a lithospheric mantle source
• Pb isotope array and Archean isochron age• Enriched Sr isotope ratios with low Rb/Sr• All radiogenic isotopes consistent with ingrowth
within an isolated Archean source• Similarities to OIB-MORB wrt K-Zr, Ba-Th, B-
Nb, etc. trace element systematics (precludes crustal contamination)
• HREE profiles are flat, and inconsistent with melting of deep mantle (garnet-bearing)
It appears that if an asthenospheric mantle plume is involved, it cannot contribute significant amounts of melt. However, elevated 3He/4He could signify outgassing of volatiles from a deep mantle domain.
101.1.1
1
10
100
1000
MORB avgsOIB avgsSRP
SKIP-D
SKIP-ASKIP-B
SKIP-C
(Rb/Hf)/PM
Th/PM
PM
E-MORB sourceN-MORB
source
OIB avgs.
N-MORB avgs.
SRP
FC
melting
Rb-depleted sources
Rb-depletion in SRP source coupled with elevated 87Sr/86Srimplies old source (consistent with Pb-Pb model age ca. 2.5 Ga)
10000100010010100
1000
10000
100000
1000000SROT
SKIP-B
SKIP-CSKIP-DLoihi
KoolauRhyolitesCrustOIBLamproitesMORB
SKIP-A
Zr
K
UC
LC
YP Rhyolites
Lamproites
50
K/Zr = 20
Intraplate basalts
FC
SRP basalts and OIB are identical for K-Zr systematics
RbTh Ba K Nb Ta La Ce Sr P Nd ZrSmHf Ti Y.1
1
10
100
1000
6YC-142L74-26N-MORBMinetteKimberlite70-15BR
ock/PM
SROT
HAOTs
super-enriched SCMLmelts
Relatively flat HREE profiles in SRP basalts suggest shallow (ca. <70 km) spinel-lherzolite sources lacking garnet
30201000
15
3He/4He (R/Ra)
Arcs
MORB
Continental basalts
YNP springs
SRP basalts (Reid)
SRP basalts (C&L)
Imnaha basalt
Siletzia basalts
Kerguelen (xenoliths)
Loihi
Hawaii
Iceland
Helium Isotope Summary He isotope data for SRP basalts (olivines) show greater 3He enrichment than in MORB, and over-lapping ranges for many inferred hot spot suites.
Schematic lithospheric structure, NW USA
200
150
100
50
Asthenosphere
WISZ YP CAS
Lithosphere
Crust
Accreted
SCLM
NE SW
NW USA Mantle Structure Plume (?)
TBL
SZ
Subducting
Slab
Precambrian craton Accreted terranes
SRP
magma
transport &
storage
pmz
T (°C)
P (GPa)
ca. 1400°C adiabat
0 3 6
100 2000 Z (km)
1200
1600
1000
Thick lithosphere retards melting of upwelling mantle;
Melting requires either higher Tp or lithospheric thinning
Mantle melting considerations
mantle
solidus
lithosphere lid
Decompression melting scenario
Yellowstonevelocity profiles
Schutt
Controls on eruptions & ‘out of sequence’ events?
1. Oceanic hot spot volcanism displays a simple time-volume relation, SRP volcanism does not. This could be explained by different lithosphere structures.
2. Assuming existence of a sufficient magma supply, and ascent by bouyant forces, to get eruptions through continental crust requires a minimum depth (~50 km) to magma reservoir.
3. Shallower reservoirs (e.g., near Moho) cannot support eruption of basalt through normal continental crust, but can support intrusion at shallower levels (est. intrusion of basalt is equivalent to ~1 km thickness/Ma).
4. Magmatic processes gradually increase crustal density thus increasing likelihood of basalt eruptions from increasingly shallower reservoirs. Petrologic constraints suggest that typical SROTs are fed from mid-crust reservoirs (≤ 25 km)
Suggested research goals• High-resolution reflection/refraction seismology - determine geometry
of intrusive structures, mass distribution within crust• Anisotropy and 3-D structure - constraints on deformation style and
magnitude along and adjacent to SRP track• Nature of inferred lithosphere boundaries - isotope contrasts• Attenuation - melt distributions with depth within the crust• Definition of base of lithosphere as a physical/chemical/thermal entity• Modelling deformation of weakened crust (due to magma injection) -
contributions to regional tectonics• Petrology-geochemistry - understanding processes of continental
evolution• Development and extrapolation of understanding of large igneous
systems
Image of compressional-wave velocity structure at 100 km depth(Dueker et al., 2001).
SRP