Paper No. 5
Presentation Time: 2:05 PM
DISTINGUISHING EPISODIC ARCHEAN CRUSTAL GROWTH: THE RELATIONSHIP BETWEEN AN ARC AND ITS FOUNDATION
U-Pb ages, Lu-Hf systematics, and trace element contents of zircons from Archean metasedimentary rocks in the northern Wyoming Province (NWP) show three distinct episodes of crustal growth superimposed on a record of continuous magmatism and crustal maturation over a period of >1 Ga. This record shows variable extents of crustal recycling and juvenile (depleted mantle) input over this period. The record begins with extraction of material from a juvenile or primitive mantle source ~4.0 Ga and suggests an evolving crust with recycling dominating over juvenile additions for ~ 600 Ma. The interval from 3.2-3.4 Ga is one of prolific crustal growth that produced sufficient numbers of zircons to dominate the record in Archean metasedimentary rocks. Rocks of this age (3.2-3.4 Ga) are dominantly of the TTG suite. They exhibit geochemical traits (e.g. HFSE depletion, enriched LREE, etc.) indicative of generation by subduction. Isotopic compositions (e.g., Sm-Nd model ages 3.8-4.1 Ga) in these rocks show evidence of interaction with an even older crust, likely represented in the 3.5 to 4.0 Ga detrital zircons preserved in the intercalated, quartz-rich metasedimentary rocks. After a period of ~500 Ma, a third episode of subduction-driven magmatism and crustal growth began ~2.8 Ga, which was short lived (<100 Ma), but extensive (>400,000 sq. km). Prior to, or coeval with, this third crust-forming event, metaplutonic and supracrustal rocks were tectonically mixed with the 3.2-3.4 TTG rocks, and were metamorphosed at 6-8 Kbar and 750-800oC (kyanite is locally present); thick, stable continental crust was clearly established by this time. TTG rocks produced during this third generation of crustal growth (2.8-2.9 Ga) now comprise the dominant lithologies in the Beartooth-Bighorn magmatic zone of the northern Wyoming Province. These TTG rocks have enriched initial isotopic signatures in the U-Pb, Rb-Sr, Sm-Nd, and Lu-Hf systems and non-radiogenic trace element signatures that suggest they also were produced in an environment similar to a modern continental margin arc. The enriched initial isotopic compositions in these systems argue strongly that the late Mesoarchean (~2.8 Ga) arc was built on pre-existing, thick, continental crust.