GREENSTONE BELTS AND EVOLVED CONTINENTAL CRUST: ARE LESSER ENDOWED GREENSTONE BELTS A PRODUCT OF INHERITED LITHOSPHERIC ARCHITECTURE?

Understanding the lithospheric architecture coeval with greenstone belt formation is critical for determining factors that controlled their precious and base metal endowment. To identify these factors, integrated mapping, geophysical imaging, and a regional comparison of U-Pb-TE zircon analyses by LA-ICP-MS from major igneous suites were conducted in a lesser endowed region of the Archean western Superior Province, Canada. Investigations focused on where ~3000-2720 Ma gneissic-plutonic rocks of the Marmion terrane are juxtaposed with ~2750-2720 Ma bimodal volcanic rocks and ~2705-2695 Ma syn-deformational volcano-sedimentary basins of the western Wabigoon subprovince (WWS). Mapping along this boundary reveals that the contact is structural as it forms a wide (>1km) shear zone defined by intense fabric development and abundant dextral north-side-down kinematic indicators. Field relationships indicate that this phase of deformation occurred relatively late as shear is recorded in syn-deformational (~2700 Ma) intrusive rocks, possibly forming coeval with gravitational collapse. Zircon grains from TTG gneisses of the Marmion terrane indicate major episodes of crustal growth are recorded at ~3000 Ma, 2900-2800 Ma, 2750-2720 Ma, and 2700-2665 Ma. Older, Mesoarchean ages are interpreted as initial crustal growth and reworking events, while the Neoarchean ages are interpreted as magmatism associated with back-arc to arc formation coeval with regional amalgamations. Mesoarchean inheritance and/or evolved trace element signatures are recorded by magmatic suites throughout the WWS, suggesting that the older crust underlies the region. Newly obtained seismic sections display strong, moderate- to shallow-dipping reflectors that extend from surficial exposures of the Marmion terrane to middle crustal depths below the WWS along its southern extent. Thus, the integrated, multi-disciplinary results indicate that the crustal architecture of the WWS reflects a peri-continental setting. The presence of a dry, infertile and multiply reworked Mesoarchean continental crust at depth likely limited fluid flux and base and precious metal availability during the Neoarchean evolution of the WWS.