SLOW DIFFERENTIATION OF CONTINENTAL CRUST IN AN ARCHEAN GRANULITE TERRANE (Invited Presentation)

Petrologic studies of Archean granulite terranes provide important insights into the tectonic styles and mechanisms for crustal heating, differentiation, and stabilization in the Archean. We present a focused petrochronologic study on a single outcrop in the Pikwitonei Granulite Domain (PGD), a large (>15,000 km2) Archean granulite terrane in the northwestern Superior Province. Field observations suggest peritectic garnet growth during partial melting of a migmatitic pelite, melt drainage from the pelite, and amalgamation of melt in a leucogranitic pluton. Such relationships provide an opportunity to quantify the timescales over which the crust was melt-bearing, as well as the timescales of melt segregation and amalgamation during high temperature Archean metamorphism.

Monazite included in peritectic garnet in the residual pelite has high-Y cores and low-Y rims, dated to ~2690 and 2670 Ma respectively. The latter date places a minimum constraint on the timing of peritectic garnet growth and partial melting. Monazite on the border of melanosome and leucosome has been dated at ~2670 Ma and has distinctly lower Y, HREE, and U. This is interpreted to record monazite growth after an episode of melt removal. In the leucogranite pluton, zircon grains exhibit core and rim textures typical of growth during melt crystallization and yield U-Pb dates spanning 2712 to 2640 Ma that do not overlap within their uncertainties. Lu-Hf isotopic analyses of the same zircon domains show decreasing 176Lu/177Hf and Th/U through time. We interpret these data to record zircon saturation of distinct melt batches that were produced in the presence of progressively increasing abundances of peritectic garnet in the residuum, and amalgamated over a ~60 Ma duration. This duration of crustal melt amalgamation is substantially longer than durations of leucogranite magmatism and pluton construction in modern collisional settings (e.g. ~1-10 Ma in the Himalaya), where crustal melting is intimately related to tectonic exhumation. In contrast, the PGD cooled slowly at mid-crustal depths for several hundred Ma, and was melt-bearing for at least 60 Ma. Our results suggest: 1) tectonic processes in the PGD may have been fundamentally different from those in modern orogens, and 2) tectonic transport of melt-bearing crust promotes rapid crustal differentiation.