2. The Plume to Plate Transition: Hadean and Archean Crustal Evolution in the Northern Wyoming Province, U.S.A.

The 2.8–4.0 Ga record of crustal evolution preserved in the northern Wyoming Province of western North America provides insight into the role of plume- and plate-regimes in the generation of Hadean and Archean continental crust, and the associated elemental depletion of the primitive mantle. The most complete record is exposed in the Beartooth Mountains (Montana-Wyoming), which lie within the Beartooth-Bighorn magmatic zone (BBMZ) sub-province of the Wyoming Province. The BBMZ (> 100,000 km²) is characterized by a single, voluminous suite of Mesoarchean (~ 2.8–2.9 Ga) TTG (tonalite-trondhjemite-granodiorite) plutonic and metaplutonic rocks. In the Beartooth Mountains these Mesoarchean rocks are exposed along an ~ 100 km E-W cross-section, along which they intrude greenschist grade turbidites in the west (South Snowy block) and high grade, older gneisses in the east (Beartooth Plateau block). The most complete assemblage of pre-2.8 Ga crust is preserved as enclaves within the plutonic Mesoarchean rocks of the Beartooth Plateau block. These older gneisses consist of 3.1–3.5 Ga, tectonically interleaved meta-plutonic (principally TTG and associated migmatites) and metasupracrustal lithologies (e.g., quartzites, schists, banded iron formation, and a range of paragneisses).

The arc-like elemental abundances and enriched Pb and Nd isotopic systematics of the Mesoarchean magmatic suite and the 3.1–3.5 Ga older enclaves in conjunction with Lu-Hf data from 3.3 to 4.0 Ga detrital zircons suggest a model of crustal evolution that began with a Hadean, mafic proto-continent that likely developed over a zone of mantle upwelling. Lu-Hf systematics of the 3.6 to 4.0 Ga zircons suggest substantial recycling within the proto-continent in this interval, and that this recycling involved a low Lu/Hf (~ 0.1) system. A ubiquitous component of 3.2–3.4 Ga detrital zircons with more juvenile Hf isotopic compositions occurs throughout the northern Wyoming Province and suggests a major period of crustal growth and generation of TTG-suite rocks from more depleted sources. Following a period of relative quiescence (2.8–3.1 Ga) in the BBMZ, late Mesoarchean arc magmatism (TTG, adakites, etc.) largely reconstituted the older crust during a relatively brief period between 2.79 and 2.83 Ga; it has remained essentially undisturbed since that time.

Placing this history in a global context suggests that Hadean-Eoarchean crust formed in diachronous and spatially diverse environments that were both plume-like (e.g., Pilbara, northern Wyoming Province) and subduction-like (e.g., West Greenland). The relative importance of plume-type crustal growth declined and subduction-type growth increased through time as a consequence of a progressive decline in terrestrial heat production and mantle potential temperature, with a concomitant increase in hydrous mantle melting in subduction zones.