Paper No. 19-2
Presentation Time: 8:25 AM
DIORITES AND TONALITES AND GRANITES! OH MY! 1.5 GA OF GROWTH AND RECYCLING OF ARCHEAN CONTINENTAL CRUST IN THE NORTHERN WYOMING PROVINCE (NWP)
The NWP has evolved through episodic pulses of magmatism over a 1.5 Ga period. The oldest record of magmagenesis is in detrital zircons with ages of 4.0-3.6 Ga preserved in high-grade metasedimentary rocks across the province. Initial εHf values for these zircons systematically decrease with time suggesting extensive recycling of older crust. The oldest rocks exposed in the NWP are 3.5-3.6 Ga TTG, that are likely derived from garnet-bearing mafic lower crust. A major crust-forming event occurred across the NWP over the protracted interval 3.4-3.2 Ga, producing rocks of the TTG suite, monzodiorites and granites. Detrital zircons from these rocks are dominant in younger metasupracrustal rocks. Geochemical signatures of these rocks include LIL enrichment, negative Eu anomalies and HFSE depletion, indicating derivation in a subduction setting involving older crust (4.0-3.5 Ga inherited zircons) as well as juvenile material (based on εHf values). After a 400 Ma hiatus, a second major subduction event began at ~2.8-2.9 Ga and produced the voluminous (>400,000 sq. km) magmatic rocks that comprise the Beartooth-Bighorn Magmatic Province. This suite of rocks includes diorites, tonalites, granodiorites and granites, with trace element signatures and enriched initial isotopic values (U-Pb, Rb-Sr, Sm-Nd, and Lu-Hf) that suggest they were also produced in an environment similar to a modern continental arc. This range of compositions was generated over a restricted time interval (<50 Ma), suggesting coeval melting of multiple mantle and crustal sources. Intracrustal decompression melting produced local leucogranites in shear zones during Late Archean (2.55 Ga) tectonic adjustments. Magmatic evolution of the NWP 1) began 4.0-3.5 Ga by melting of primitive mantle and derivative mafic crust in a plume setting (i.e. stagnant lid model), 2) produced thick sialic crust through protracted, episodic magmatism from depleted and enriched sources in the interval 3.4-3.2 Ga by 'sag' or subduction processes, 3) developed a modern-style continental arc on older, thick and stable crust in the Mesoarchean (2.8 Ga); and 4) generated ~2.55 Ga partial melts along shear zones during a Neoarchean period of tectonic juxtaposition or rifting associated with a “break-out” of Wyoming from a larger craton that may have included the Slave craton.