TTGs We Have Known and Loved: 1.5 Ga of Growth and Recycling of - PowerPoint Presentation

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.41Dept. of Earth Sciences, Montana State University, Bozeman MT, USA2Dept. of Geological Sciences, University of Florida, Gainesville FL, USA, 3U.S. Geological Survey, (retired), Marietta GA, USA4Dept. of Geology and Geophysics, Louisiana State Univ., Baton Rouge LA, USA

Ask significant questions about the Earth, its history and processesStart 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 usTake care in your work—in the field, in the lab, and particularly in interpreting data, reporting uncertainty, constraining modelsUse multiple, appropriate methods to develop integrated models Be a mentor to students and peersBe generous with your knowledge, facilities, and experienceCollaborate!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.

Major Precambrianlithotectonic 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. BBMZ : Beartooth-Bighorn Magmatic ZoneMMT: Montana Metasedimentary TerraneSAT : Southern Accreted Terranes Cheyenne Belt/Suture Colorado Province & Central Plains Orogen GFTZ “Nuclear North America” Foster et al., 2006

Great 3-D Exposures Lack of lateral continuity between rangesMetasupracrustal rocks QuartzitesPelitic schistsBanded iron formationMetabasitesUltramafites3.6-3.2 Ga QF gneiss enclaves in younger ~2.8 Ga granitoidsHellroaring Plateau, Beartooth Mountains

Beartooth Plateau Block (BPB) Older and younger relations to 2.8 magmatism Pre-existing felsic-to- mafic gneisses (3.1-3.5 Ga)Intruded by 2.8 Ga tonaliteThese share deformation and fabric elementsLate 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 3.5 Ga 2.8 Ga 2.5 Ga?

Hellroaring Plateau, Eastern Beartooth Mountains. Enclaves of Mesoarchean rocks, 3.5-3.2 Ga gneissesMetasupracrustal rocks, quartzite, BIF, pelitic schist, metabasites, UM rocks; M1 6-8 kbar 750-800ºC; Tectonic mixing of ‘old’ rocksDetrital zircons ~4.0-3.0 Ga

1. In the Beginning There Were ZirconsThe earliest record of crust formation is derived from detrital zircons in a suite of ~3.0-2.8 Ga quartzites and other metapsammitic rocks from the NWP Relative Probability for <10% Discordant 207Pb/206Pb agesMajor pulse of growth at 3.2-3.3 Ga with variable contributions from older crust; Mueller et al., 1998

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 3.2-3.3 Ga and the BBMZ (Beartooth-Bighorn magmatic zone) that formed between ~2.8-2.9 Ga). Detritus from the first arc @ 3.2-3.3 Ga dominates pre-2.8 Ga quartzites in the northern WP

Concordia plot for one of many TTG “Gray Gneisses” in the Eastern Beartooth MtnsOldest rocks in the NWP are 3.5-3.6 Ga present in eastern Beartooths, North Snowy Block, Spanish Peaks, Tobacco Root Mountains Major crust-forming event ~3.2-3.3 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

3.1-3.5 Ga (meta)igneous rocks in EB Zircons – 3.1-3.5 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

3.1-2.8 Ga Period of quiescence, deposition of platform-type sediments 2.8 Ga Second magmatic arc built on 3.2-3.5 Ga TTG gneisses: platform for building a continental arc; duration ~40 Ma

3. The Beartooth Orogenya Mesoarchean subduction-driven episode of crustal growthMost of the Archean exposures in the BBMZ are comprised of 2.8-2.9 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 MountainsThe 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 compositionsTrace element abundances of mafic members of the LLMC normalized to primitive mantle values.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) . Symbols refer to ranges of Si O 2

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

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

Image: Klein & Dutrow, 2007 a c b c b a Metasedimentary rocks (JMS): pelitic schists, quartzites , meta- turbidites and BIF Low grade: chl -and+/- staur 575-620 o C, 3.5-5.0 Kb Relict sedimentary features graded beds ( turbidite ) relict cross bedding Detrital zircons —max at 2.9-3.0 Ga not recognized in NWP; 3.2-3.33 Ga diminished; no zircons older than 3.6 Ga Granite plutons —2.8 Ga constrain age of deposition to 2.9 -2.8 Ga Goldstein et al. 2011 South Snowy Block - Yellowstone

2.79-2.81 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) 2.80 Ga Hellroaring Pluton biotite quartz monzonite, Hellroaring Pluton Aluminosity of plutons Philbrick et al. 2011 South Snowy Block - Yellowstone

Mueller et al., 1996

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

Model for Crustal Evolution in NWP Major, protracted, episodic crust-forming eventCalc-alkaline magmatism - transition to subduction style; includes juvenile additions 3.1-2.9 Absence in detrital zircon2.9-2.8 Ga Subduction involving sediment and 2nd major crust-forming event <40 Ma (BBMZ)

Phase I: Not an ArcThe Detrital Zircon Record from Archean Quartzites Average eHf values trend toward more negative values from 4.0 to 3.55 Ga, indicating a dominant component of recycled crustFrom 3.55 to 3.1 Ga average eHf values show a progressive increase, suggesting an increasing contribution from juvenile sources

Depleted Mantle 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 eclogiteV prefixes refer to volcanics (basalt and komatiite)M prefixes refer to meltsSI = sea levelIn 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.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.

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