摘要

The similar to 2.5 Ga Huronian Supergroup volcanic rocks in northern Ontario represent one of the better preserved Paleoproterozoic continental flood basalt sequences on Earth. The youngest age of Huronian Supergroup volcanism is refined to 2452.5+/-6.2 Ma based on new U-Pb zircon analyses of the Copper Cliff rhyolite. Recent mapping and geochemical analyses of Thessalon Formation volcanic rocks define a well-developed chemostratigraphy; a lower series consisting of four geochemically distinct mafic lava sequences and an upper basalt-andesite volcanic series. These two mafic volcanic series are separated by felsic volcanic rocks (rhyolite and dacite). The mafic lavas are low-Ti tholeiites, dominated by basalt and andesite, and are characterized by enriched light rare earth element profiles and high Th contents (up to 12 ppm). Most basalt-andesite sequences show some internal geochemical variation reflecting a combination of fractional crystallization and, in some cases, limited crustal contamination but each sequence represents a discrete magma batch derived by different degrees of partial melting in the subcontinental lithospheric mantle. All magma batches display prominent negative Nb-Ta and Ti anomalies on primitive mantle normalized multi-element diagrams, interpreted to reflect melting of a heterogeneous and previously metasomatized garnet lherzolite that acquired some chemical characteristics of a source typical of subduction zone magmatism. At least three distinct mantle source compositions are required to explain the range in the observed lava compositions. The most unusual of these reservoirs has a number of geochemical features akin to upper continental crust and is characterized by high Th/Yb (similar to 10) and low Zr/Nb (similar to 11). This reservoir is interpreted to be peridotite modified through metasomatism and entrapment of arc magmas during northward Neoarchean subduction along the southern Superior craton margin.

  • 出版日期2013-8