Rapid growth of an Archean continent by arc magmatism

The voluminous Meso- to Neoarchean rocks exposed in the Beartooth Mountains of the northern Wyoming Province of western North America comprise the Long Lake Magmatic Complex (LLMC) a variably metamorphosed and deformed association of igneous and meta-igneous plutonic rocks with SiO2 ranging from at least 52 to 78 wt% Within this compositional range rock types include lineated amphibolites to hornblende-bearing gneisses of intermediate composition and multiple generations of foliated to unfoliated granitoids Emplacement ages range between approximately 2 79 and 2 83 Ga based on U-Pb zircon geochronology(SHRIMP) Field relations elemental compositions and geochronology indicate that these rocks do not represent a single fractional crystallization sequence but rather the LLMC was constructed by injection of numerous discrete magmas as sill-like bodies over an similar to 40 Ma period Although there is a continuum of compositions in the LLMC trace element abundances can be used to distinguish distinct sources and petrogenetic processes that can be broadly extrapolated to at least 3 compositional groupings (1) trondhjemitic to granitic intrusive rocks with SiO2 >70 wt% (2) variably metamorphosed granodioritic orthogneisses with SiO2 between 63 and 70 wt% and (3) amphibole-bearing mafic to intermediate gneisses with SiO2 between 52 and 63 wt% Despite the range of SiO2 contents maximum LREE abundances are similar across the compositional range and consequently exhibit a wide range of (La/Yb)(n) ratios (similar to 20-130) All LLMC rocks share a relative depletion in HFSE abundances similar to modern convergent margin magmas Initial Sr and Nd isotopic compositions across the compositional range are consistently offset from typical bulk silicate earth (BSE) values and preclude unaltered derivation from primitive or depleted mantle Common Pb isotopic data define a single array that lies above model crustal growth curves and along with the Nd and Sr data suggest relatively uniform interaction with or derivation from older lithosphere The combined isotopic and elemental data suggest the LLMC resulted from simultaneous rapid and voluminous production of diverse magmas that represent melting of isotopically similar but compositionally distinct crustal and mantle sources Dynamically Meso- to Neo-archean crustal growth in the northern Wyoming Province appears to require an environment similar to a modern ocean-continent convergent margin with a comparable rate of crustal production and diversity of magma series The resultant crust and associated mantle lithosphere (keel) appear to have suffered little-to-no modification prior to Laramide (Cretaceous) uplift and exposure (C) 2010 Elsevier B V All rights reserved