THE ARCHEAN PLUME-TO-TRENCH TRANSITION AND CRUSTAL GROWTH

The Meso- to Neoarchean rocks of the Long Lake Magmatic Complex (LLMC; Beartooth Mtns., Wyoming Province) comprise igneous and meta-igneous plutonic rocks with SiO₂ from at least 52 to 78 wt %. Emplacement ages range from ~2.79 to 2.83 Ga, based on U-Pb zircon geochronology. 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 of: 1) trondhjemitic to granitic rocks with SiO₂ >70 wt %, 2) granodioritic rocks with SiO₂ from 63 to 70 wt %, and 3) mafic to intermediate rocks with SiO₂ from 52 to 63 wt %. Maximum LREE abundances are similar across the compositional range with variable (La/Yb)_n ratios (~20-130) and all rocks share a relative depletion in HFSE abundances similar to modern convergent margin magmas. Initial Sr and Nd isotopic compositions are consistently offset from typical bulk silicate earth values and preclude unaltered derivation from primitive or depleted mantle (DM). Common Pb isotopic data define a single array (~2.8 Ga) that lies above model crustal growth curves and exhibit a “Pb-paradox” similar to that of the depleted mantle (radiogenic Pb paired with low U/Pb). This relationship requires an inversion of U-Pb systematics similar to that required for the modern DM on a global scale, i.e., early evolution in a high U/Pb environment, followed by formation a new reservoir that derives its Pb isotopic composition from the older reservoir, but it formed by processes that reduce the U/Pb ratio. This inversion can be viewed as a natural outgrowth of a global transition from plume-based crustal growth (anhydrous mantle melting) to subduction-related crustal growth (hydrous mantle-slab melting). The combined isotopic and elemental data suggest the LLMC resulted from such a scenario with simultaneous, rapid, and voluminous production of diverse magmas in a tectonic setting akin to a modern ocean-continent convergent margin with transfer of ancient enriched isotopic signatures to the ~2.8 Ga magmas via incorporation of sediment into the subduction zone. The resultant crust and associated mantle lithosphere (keel) appear to have suffered little-to-no modification prior to Laramide uplift and exposure.