Mueller EarthScope Workshop Presentation

1. The Wyoming Province: A Unique Archean Craton
P. Mueller-D. Mogk-D. Henry-J. Wooden
What’s on top?
Major sub-provinces:
MMT: Montana metasedimentary
terrane
BBMZ: Beartooth-Bighorn
magmatic zone
SAT: Southern Accreted terranes
Prominent Paleoproterozoic
mobile belts:
GFTZ: Great Falls tectonic zone
THO: Trans-Hudson orogen
CB: Cheyenne Belt
FZ: Farmington zone
MMT
BBMZ
SAT
CB
FZ
THOGFTZ

2. What’s down below?
Deep Probe-SAREX Seismic Cross Section of the WP
Things to note:
1. The unusually thick, high velocity
lower crust, up to 25 km with >7.0 Vp
that underlies Archean crust, and
Proterozoic crust in the GFTZ.
2. No imbricated zones in the mantle
(fossil slabs) beneath Archean crust
3. Crustal thickness to 50 km.
4. Exposed Archean crust is mid-crust
X
Gorman et al., 2002

3. What makes the Wyoming Province unique?
1. The Mesoarchean magmatic record is characterized by distinct Pb isotopic
compositions that require involvement with an older high U/Pb crust and/or
mantle. Among all Archean cratons, only two exhibit the U-Pb systematics (crustal
Pb paradox) similar to the WP (Slave craton, northern marginal zone of the
Limpopo belt).
2. Mesoarchean rocks in the BBMZ and MMT contain xenocrysts that are up to 3.8 Ga
old, have Sm-Nd and Lu-Hf (zircon) model ages to 4.1 Ga.
3. The Wyoming craton has a keel that formed largely in the Mesoarchean based on
Sm-Nd model ages and secondary Pb isochrons of young volcanic rocks. This keel
has not interacted to any significant degree with the convecting mantle for nearly
3.0 Ga, and has apparently and almost completely resisted lithospheric subduction
during the Paleoproterozoic events that occurred along its margins.
4. 4. The WP has a distinctive, thick, high Vp lower crust up to 25 km thick (the 7x
layer). This layer extends into adjacent Proterozoic mobile belts (e.g., GFTZ).
5. In the northern BBMZ and MMT detrital zircons between 3.5 and 4.0 Ga are
present in Archean metapsammites in several localities, and in the lowest members
of the Belt-Purcell supergroup (e.g., LaHood Fm.). Lu-Hf systematics of these
zircons show a clear record of the persistence of Hadean crust into the
Mesoarchean and provide insight into the extent to which subsequent growth

4. Remnants of Mesoarchean and older
metasupracrustal rocks are exposed in the
eastern Beartooth Mountains. Extensive
deformation and metamorphism has
produced a residual association consisting
principally of metapsammites and
amphibolites.
In the Beginning: View from the Beartooth Mountains, MT
Q
A
Intercalated meta-plutonic rocks
range from 2.8 to 3.5 Ga. Much, if not
all, of the intercalation occurred during
the emplacement of the Long Lake
Magmatic Complex at 2.8 Ga,
which underlies most of the high plateau
in the photo to the right.
Q = quartzite A = amphibolite
Average Human for Scale

5. 3.5 to 4.0 Ga: The rocks may be gone, but we can still extract
useful information about them from their zircons
Samples from Archean quartzites, Eastern Beartooth Mountains
Depositional ages are < 3.1 Ga
0
10
20
30
40
50
60
2500 3000 3500 4000
Number
AGE Ma
U-Pb ages of detrital zircons in 100 Ma bins

6. Initial eHf of detrital zircons reflect the sources of the
original zircon-bearing magmas
Period from 3.4 to 4.0 Ga is broken down into arbitrary 100 Ma increments and the initial
eHf values in that increment are averaged and plotted against the midpoint of the age-
range; light blue shading reflects the range of initial eHf in each group. The occurrence
of positive eHf values in the 3.9-4.0 Ga group is based on a DM model that is a linear
function from eHf = 0 at 4.57 Ga to +16 today.
Growth trajectory of early crust with
the average Lu/Hf of lower crust

7. Ti-derived Temperatures vs. U-Pb Age for Detrital and Igneous
Zircons, Eastern Beartooth Mtns.
600
640
680
720
760
800
840
880
2700 2900 3100 3300 3500 3700 3900 4100
Temp.TiUncorrectedFiltered
Age Ma
Distribution of paired age-temperature data for zircons from the Late Archean TTG
intrusive suite (2.79-2.83 Ga) and from ancient detrital grains. The similar
distributions suggest primary igneous relationships are preserved in the older grains
and compatible with TTG genesis. Ti activity is set at 1.0 for temperature calculations.

8. The Mesoarchean of the BBMZ and MMT shows evidence of
crustal recycling and arc-like trace element abundances
LILE enriched
HFSE depleted
Sr/Y > 40 suggest
thicker crust where
garnet is stable.

9. -16
-14
-12
-10
-8
-6
-4
-2
0
2
4
0 20 40 60 80 100 120 140
-2.8 +/- 1.6
EHf(t)
Grain number
BTR-16
xenocrysts
BTR-34
Initial eHf data (laser ablation) for120 Mesoarchean
(2.79-2.83 Ga) zircons from the Beartooth Mountains
Credit to Jennifer Staffenburg
Initial eHf of zircons in Mesoarchean magmas suggest
Paleoarchean and Hadean crust influenced their petrogenesis

10. The Mesoarchean: Sm-Nd model ages suggest significant
recycling of Paleoarchean and older crust in the BBMZ

11. Recycling of ancient crust in the Mesoarchean magmas of
the BBMZ is evident in the crustal Pb paradox
Beartooth Mesoarchean
common Pb
(Mueller et al. papers)
Beartooth and Bighorn
Mesoarchean common Pb ( adding
data from Frost et al. papers)
The mean 238U/204Pb value for these rocks is <6.0
The mean 238U/204Pb for island arcs is ~6
The mean 238U/204Pb for MORB is >10
So, the Pb stored in BBMZ Mesoarchean crust is part of the early enriched reservoir
needed to solve the first (mantle) Pb paradox and persisted through subduction.
The mantle or first Pb paradox is that MORB’s Pb
compositions are “future” Pb and cannot be
generated in the age of the earth by decay.

12. What are the implications of these observations for the
Wyoming Province, and Archean crustal growth in general?
Our proposal is that the craton began to form in a plume-like setting at >4.0 Ga. By
3.4 Ga plate tectonic-systems began to impact the craton:
1. The oldest zircons (4.0 Ga) have initial Hf isotopic compositions that fall in the narrow gap
between primitive mantle and depleted mantle model values.
2. This early crust was involved in magma genesis into the Mesoarchean based on Sm-Nd and
Lu-Hf (zircon) model ages >4.0 Ga. This seems to best fit a model of a stagnant lid regime
similar to that on Mars (e.g., Tharsis), Venus, and other planetary bodies where large volcanic
edifices form in a mono-plate tectonic system with anhydrous melting of peridotite.
3. Beginning with a major pulse of crust production at 3.2 Ga and then at 2.8 Ga, we envision a
modern analog of a subduction system in which hydrous melting of mantle and “oceanic’’
lithosphere yield the HFSE depleted TTGs. This plate tectonic-like system can also help explain
the quiet periods we see after the major growth pulse at 3.2-3.3 Ga.
4. We are led to this plume to plate transition model to explain the enriched Pb isotopic
evolution that appears in younger melts for 100’s of millions of years.
5. To appreciate this deduction, it is important to realize that anhydrous melting of peridotite
yields an increase in U/Pb in the melt. Hydrous melting yields lower U/Pb in the melt by ~50%.
So an early anhydrous melting regime, as in a plume, is important to creating the early high
U/Pb regime (aka early enriched reservoir).
6. Plate tectonics is preferred over sagduction because of the volume of crust we know was
produced at 3.2 and 2.8 Ga. This is only possible if new source material was continually
supplied to produce melts that grew the craton.

13. Where Do We Stand?
Old, High U/Pb heritage, Thick crust, Robust keel
Fields and trends for common Pb isotopes in select cratons made the WP
unique In 1988 and remains so today (from Mueller and Wooden, 1988).
Wyoming Province
Mesoarchean, including
the Stillwater

16. The Wyoming Province: A Unique Archean Craton
P. Mueller-D. Mogk-D. Henry-J. Wooden
Major sub-provinces:
MMT: Montana metasedimentary
terrane
BBMZ: Beartooth-Bighorn
magmatic zone
SAT: Southern Accreted terranes
Prominent Paleoproterozoic
mobile belts:
GFTZ: Great Falls tectonic zone
THO: Trans-Hudson orogen
CB: Cheyenne Belt
FZ: Farmington zone
MMT
BBMZ
SAT
CB
FZ
THOGFTZ