AN EXAMPLE OF RAPID, IN-SITU CONTINENTAL GROWTH IN THE ARCHEAN, WYOMING PROVINCE, USA

An important, unresolved issue in continental evolution is establishing the extent to which new continental-scale crust is formed in situ vs. the extent to which it forms by aggregation of buoyant felsic material separated from the mantle at various locations on Earth (e.g., oceanic arcs) and later aggregated into continental-scale crust. The voluminous Meso- to Neoarchean rocks of the Beartooth Mountains of the northern Wyoming Province (Long Lake Magmatic Complex, LLMC) provide an example of rapid, in-situ continental growth. The LLMC comprises ca. 2.78 to 2.83 Ga (U-Pb zircon via SHRIMP) plutonic and metaplutonic rocks with 52 to 78% SiO2. Field relations, mineralogy, and elemental abundances suggest at least three distinct groups: 1) trondhjemitic to granitic with SiO2 >66%, low HREE contents, and negative Eu-anomaly, 2) variably metamorphosed granodiorites with SiO2 from 57 and 66% and moderately fractionated REE patterns, and 3) amphibole-bearing mafic-intermediate gneisses with SiO2 from 52 to 57% and fractionated REE patterns. Relative depletion in HFSE abundances (similar to modern convergent margin magmas) characterizes all groups. Limited variation in initial Sr and Nd isotopic compositions across a wide compositional range and similar secondary Pb isotopic systematics suggest relatively uniform interaction with, or derivation from, older lithosphere for all groups. Lack of correlation between REE patterns (e.g., (La/Yb)n from ~20-130 and variable Eu anomalies) and differentiation indices indicate that most LLMC rocks cannot be related to each other by fractional crystallization or different degrees of partial melting of a common source. Instead, these data suggest the LLMC resulted from simultaneous, rapid, and voluminous production of diverse magmas that represent melting of isotopically homogenized, but compositionally distinct, crustal and mantle sources. Dynamically, this crustal growth appears to require an environment similar to a modern ocean-continent convergent margin with a comparable rate of crustal production, diversity of magmas, and metasomatized wedge. The resultant crust and altered mantle lithosphere (keel) appears to have suffered little to no modification prior to the Cretaceous based on isotopic systematics of mantle xenoliths and crustal thermochronometry.