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 GFTZ THO BBMZ SAT FZ CB

Deep Probe-SAREX Seismic Cross Section of the WP 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

What makes the Wyoming Province unique? 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). 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. 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. 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). 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 events were juvenile.

In the Beginning: View from the Beartooth Mountains, MT 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. Q A Average Human for Scale Q = quartzite A = amphibolite 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.

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 U-Pb ages of detrital zircons in 100 Ma bins

Growth trajectory of early crust with the average Lu/Hf of lower crust Initial eHf of detrital zircons reflect the sources of the original zircon-bearing magmas Growth trajectory of early crust with the average Lu/Hf of lower crust 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.

Ti-derived Temperatures vs Ti-derived Temperatures vs. U-Pb Age for Detrital and Igneous Zircons, Eastern Beartooth Mtns. 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.

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

Initial eHf of zircons in Mesoarchean magmas suggest Paleoarchean and Hadean crust influenced their petrogenesis Initial eHf data (laser ablation) for120 Mesoarchean (2.79-2.83 Ga) zircons from the Beartooth Mountains Grain number -2.8 +/- 1.6 EHf(t) BTR-34 xenocrysts BTR-16 Credit to Jennifer Staffenburg

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

Beartooth Mesoarchean common Pb Recycling of ancient crust in the Mesoarchean magmas of the BBMZ is evident in the crustal Pb paradox 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. 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.

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.

Old, High U/Pb heritage, Thick crust, Robust keel Where Do We Stand? Old, High U/Pb heritage, Thick crust, Robust keel Wyoming Province Mesoarchean, including the Stillwater 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).

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 GFTZ THO BBMZ SAT FZ CB