1. TTGs We Have Known and Loved: 1
TTGs We Have Known and Loved: 1.5 Ga of Growth and Recycling of Archean Continental Crust in the Northern Wyoming Craton (NWC)
 Mogk, D. W.1, Mueller, P.A.2, Wooden, J.L.3 , and Henry, D.J.4 1Dept. of Earth Sciences, Montana State University, Bozeman MT, USA 2Dept. of Geological Sciences, University of Florida, Gainesville FL, USA, 3U.S. Geological Survey, (retired), Marietta GA, USA 4Dept. of Geology and Geophysics, Louisiana State Univ., Baton Rouge LA, USA

2. What this talk is really about: (Lessons learned from Paul Mueller)
Ask significant questions about the Earth, its history and processes
Start with Nature—the answers are always in the rocks
Do your homework—there’s a lot of information and wisdom from those who worked before us
Take care in your work—in the field, in the lab, and particularly in interpreting data, reporting uncertainty, constraining models
Use multiple, appropriate methods to develop integrated models
Be a mentor to students and peers
Be generous with your knowledge, facilities, and experience
Collaborate!
What this talk is really about:
(Lessons learned from Paul Mueller)
This talk, and others following, are the embodiment of Paul Mueller’s contributions to our Science, and our Scientists.

3. “Nuclear North America”
Major Precambrian
lithotectonic provinces of Laurentia immediately east of the rifted Neoproterozoic margin. The Wyoming Province is completely surrounded by Proterozoic orogenic belts, but does not host any Proterozoic magmatic belts.
MMT: Montana Metasedimentary Terrane
GFTZ
BBMZ: Beartooth-Bighorn Magmatic Zone
SAT: Southern Accreted Terranes
Cheyenne Belt/Suture
Colorado Province & Central Plains Orogen
Foster et al., 2006

4. Hellroaring Plateau, Beartooth Mountains
Great 3-D Exposures
Lack of lateral continuity between ranges
Metasupracrustal rocks
Quartzites
Pelitic schists
Banded iron formation
Metabasites
Ultramafites
Ga QF gneiss enclaves in younger ~2.8 Ga granitoids

5. Beartooth Plateau Block (BPB)
3.5 Ga
2.5 Ga?
2.8 Ga
Older and younger relations to 2.8 magmatism
Pre-existing felsic-to- mafic gneisses ( Ga)
Intruded by 2.8 Ga tonalite
These share deformation and fabric elements
Late mafic dikes (multiple generations, 2.6 – 0.75 Ga)
Must take these outcrops apart rock by rock to get full story!!!
Photo credit: Darrell Henry

6. Hellroaring Plateau, Eastern Beartooth Mountains.
Enclaves of Mesoarchean rocks, Ga gneisses
Metasupracrustal rocks, quartzite, BIF, pelitic schist, metabasites, UM rocks; M1 6-8 kbar ºC;
Tectonic mixing of ‘old’ rocks
Detrital zircons ~ Ga

7. 1. In the Beginning There Were Zircons
The earliest record of crust formation is derived from detrital zircons in a suite of ~ Ga quartzites and other metapsammitic rocks from the NWP
1. Note distribution of Ga detrital zircons. 2. Maxima at Ga. 3. None younger than 3.0 Ga except for Jardine
Relative Probability for <10% Discordant 207Pb/206Pb ages
Major pulse of growth at Ga with variable contributions from older crust; Mueller et al., 1998

8. 2. Mesoarchean: The Time of the First Arcs
There are two major crustal age-provinces in the northern Wyoming Province, the MMT (Montana metasedimentary terrane) that formed largely at Ga and the BBMZ (Beartooth-Bighorn magmatic zone) that formed between ~ Ga).
Detritus from the first Ga dominates pre-2.8 Ga quartzites in the northern WP

9. Oldest rocks in the NWP are 3. 5-3
Oldest rocks in the NWP are Ga present in eastern Beartooths, North Snowy Block, Spanish Peaks, Tobacco Root Mountains
Major crust-forming event ~ Ga; TTG suite in EBT, Madison Range, Tobacco Root Mtns
Recorded in detrital zircons from quartzites across the NWP
A protracted period of magmatism, numerous small events created a huge volume of continental crust
Concordia plot for one of many TTG “Gray Gneisses” in the Eastern Beartooth Mtns

10. 3.1-3.5 Ga (meta)igneous rocks in EB
Zircons – Ga
Period of protracted magmatism; many small events that in aggregate produce a large amount of crust; “yo-yo” tectonics oscillating, numerous closely spaced small arcs?
TAS whole rock classification = basalt to rhyolite (equivalent)
Dominantly TTG Suite
Similar Age, Composition in Madison, Tobacco Root

11. 3.1-2.8 Ga Period of quiescence, deposition of platform-type sediments
2.8 Ga Second magmatic arc built on Ga TTG gneisses: platform for building a continental arc; duration ~40 Ma

12. a Mesoarchean subduction-driven episode of crustal growth
3. The Beartooth Orogeny
a Mesoarchean subduction-driven episode of crustal growth
Most of the Archean exposures in the BBMZ are comprised of Ga TTG-suite granitoids, but granites are also present.
Evidence for a subduction-driven magmatic system is found in elemental and isotopic abundances of Mesoarchean crust in the Beartooth and Bighorn Mountains
The key to the subduction interpretation is the crustal production rate
Major element chemistry reflects calc-alkaline evolution of a low K/Na suite (TTG) and adakite-like compositions
Symbols refer to ranges of SiO2
Normalized abundance patterns show relative enrichment and depletion in the same elements evident in modern arc magmas (e.g., enriched in Pb, depleted in HFSE) .
Trace element abundances of mafic members of the LLMC normalized to primitive mantle values.

13. Intimate interlayering of dioritic to granitic rocks.
Magmatic Field Relations
Intimate interlayering of dioritic to granitic rocks.
Emplacement interpreted as mesozonal sheeted dike complex.
Local mingling of magmas.
No evident liquid line of descent, all magmatic rocks overlap in space and time

14. A Mesoarchean Magmatic Arc at 2.8-2.9 Ga
built on an ancient continental margin
Mueller et al., 2010
Greenschist to granulite facies rocks of the Beartooth arc are exposed across the range

16. South Snowy Block - Yellowstone
Metasedimentary rocks (JMS): pelitic schists, quartzites, meta-turbidites and BIF
Low grade: chl-and+/-staur oC, Kb
Relict sedimentary features
graded beds (turbidite)
relict cross bedding a c b
Goldstein et al. 2011
Detrital zircons —max at Ga not recognized in NWP; Ga diminished; no zircons older than 3.6 Ga Granite plutons—2.8 Ga constrain age of deposition to Ga
Image: Klein & Dutrow, 2007

17. South Snowy Block - Yellowstone
2.80 Ga Hellroaring Pluton
biotite quartz monzonite, Hellroaring Pluton
Aluminosity of plutons
Ga magmatic rocks
Undeformed bulbous, epizonal plutons
Peraluminous, primary muscovite (~3.8 Kb)
Jardine metasediments are allochthonous (detrital zircons), accreted prior to 2.8 Ga (magmatic zircons)
Philbrick et al. 2011

18. Mueller et al., 1996

19. Model for Crustal Evolution in NWP
Differentiation of proto-continent
Continued input of magma from additional mantle up-welling
”Stagnant Lid” model?
Mantle up-welling dominant
Anhydrous melting of mantle
Proto-continent with minor or no keel

20. Model for Crustal Evolution in NWP
Major, protracted, episodic crust-forming event
Calc-alkaline magmatism - transition to subduction style; includes juvenile additions
Absence in detrital zircon
Ga Subduction involving sediment and 2nd major crust-forming event <40 Ma (BBMZ)

21. The Detrital Zircon Record from Archean Quartzites
Phase I: Not an Arc
The Detrital Zircon Record from Archean Quartzites
Pattern seen in Minnesota River Valley, Yilgarn; but not everywhere, not Isua
Average eHf values trend toward more negative values from 4.0 to 3.55 Ga, indicating a dominant component of recycled crust
From 3.55 to 3.1 Ga average eHf values show a progressive increase, suggesting an increasing contribution from juvenile sources

22. Schematic of the initial stage of mantle upwelling associated with plateau development adapted from Bedard (2006).
T prefixes refer to generations of TTG
E prefixes refer to generations of eclogite
V prefixes refer to volcanics (basalt and komatiite)
M prefixes refer to melts
SI = sea level
Eclogite forms at the base of the crust and then delaminates and melts, these melts may interact with melts produced higher in the column. More importantly, however, the eclogite that separates from the new crust will reduce the Lu/Hf ratio of the bulk crust.
In the earliest stages of crust formation (e.g., >4.0 Ga), the rising mantle diapir may be composed of primordial or partially depleted mantle. In either case, this mantle will likely have higher incompatible element contents than modern depleted mantle (e.g., 2x Zr). In particular, this anhydrous melting will yield Lu/Hf ratios higher than estimates of average crust.
Depleted
Mantle

23. The real reason Paul keeps coming back to Montana!