Subhorizontal fabric in exhumed continental lower crust and implications for lower crustal flow: Athabasca granulite terrane, western Canadian Shield

摘要

The \u3e20,000 km2 Athabasca granulite terrane is one of Earth\u27s largest exposures of continental lower crust. The terrane is underlain by heterogeneous isobarically cooled orthogneisses termed the Mary batholith. A transect across the batholith documents early, penetrative subhorizontal to gently dipping gneissic foliation (S1). Kilometer- to meter-scale domains of S1 contain lineations (L1) defined by ribbons of recrystallized K-feldspar + plagioclase + quartz + amphibole ± orthopyroxene. L1 coincides with garnet aggregates, elongate mafic enclaves, and core-and-mantle structure in feldspar porphyroclasts. Lineations are coaxial with hinges of isoclinally folded layering (F1). L1 is interpreted as a composite mineral lineation with intersection and stretching components. Kinematics are uniformly top-to-the-ESE. Thermobarometry derived from synkinematic phases is compatible with granulite-grade (∼800°C) ductile lower crustal flow during D1 at ∼0.9 GPa (∼30 km paleodepths). Results from electron probe microanalyzer (EPMA) Th-U-total Pb monazite geochronology support Neoarchean (circa 2.60–2.55 Ga) garnet growth and melt-enhanced flow. Metamorphic reactions accompanying D1 strain were synkinematic, with preferential nucleation of high-Ca garnet and amphibole in the Na-rich mantles of recrystallized plagioclase porphyroclasts. Relatively H2O-poor and/or CO2-rich conditions during D1 are required by the preservation of fine-grained microstructures. Subhorizontal tectonites in the Mary batholith may represent an important field-based analog for lower crustal flow during orogenesis or large magnitude extension. The results illustrate the evolving strength of continental lower crust. Neoarchean subhorizontal flow of weak lower crust was followed by near-isobaric cooling and strengthening. Paleoproterozoic deformation events produced steep fabrics (S2), steeply dipping shear zones, and reactivation of S1, a record of strain localization and strain hardening in an isobarically cooled anisotropic medium.