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Monazite is a common accessory phase in felsic granulite ribbon mylonites exposed in the Upper Deck domain of the Athabasca granulite terrane, western Canadian Shield. Field relationships, bulk rock geochemistry and phase equilibria modelling in the Na 2 O-CaO-K 2 O-FeO-MgO-Al 2 O 3 -SiO 2 -H 2 O-TiO 2 -Fe 2 O 3 system are consistent with the garnet-rich rocks representing the residual products of ultrahigh temperature melting of biotite-bearing paragneisses driven by intraplating of mafic magma in continental lower crust. The c. 2.64-2.61 Ga Y-rich resorbed monazite cores included in garnet are interpreted as relicts of detrital grains deposited on the Earth's surface after c. 2.61 Ga. Yttrium-poor monazite domains in garnet are depleted in Sm and Gd and linked to fluid-absent melting of biotite + plagioclase + quartz AE sillimanite during a prograde loading path from ≤0.8 to ≥1.4 GPa. The c. 2.61-2.55 Ga Y-depleted, Th-rich monazite domains crystallized in the presence of garnet + ternary feldspar AE orthopyroxene + peraluminous melt. The c. 2.58-2.52 Ga monazite rims depleted in Th + Ca and enriched in Eu are linked to localized melt extraction synchronous with growth of high-pressure (HP) grossular-rich garnet at the expense of plagioclase during crustal thickening, culminating at >950°C. Re-heating and dextral transpressive lower crustal reactivation at c. 1.9 Ga resulted in syn-kinematic growth of (La + Ce)-enriched monazite and a second generation of garnet, concurrent with recrystallization of feldspar and orthopyroxene at 1.0-1.2 GPa and 600-700°C. Monazite grains in this study are marked by positive Eu-anomalies relative to chondrite. A direct link is implied between Y, Sm, Eu and Gd in monazite and two major phases in continental lower crust: garnet and plagioclase. Positive Eu-anomalies in lower crustal monazite associated with modally abundant garnet appear to be directly related to Eu-enrichment and depletions of Y, Sm and Gd that are consequences of garnet growth and plagioclase breakdown during HP melting of peraluminous bulk compositions.

The >20,000 km2 Athabasca granulite terrane is one of Earth's 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.

The Paleoproterozoic orogen of the southwestern United States is characterized by a segmented, block-type architecture consisting of tens of kilometer-scale blocks of relatively homogeneous deformation and metamorphism bounded by subvertical highstrain zones. New fi eld, microstructural, and petrologic observations combined with previously published structural and geochronological data are most consistent with a tectonometamorphic history characterized by a clockwise, looping pressure-temperature (P-T) path involving: (1) initial deposition of volcanogenic and turbiditic supracrustal rocks at ca. 1.75-1.74 Ga, (2) passage from <12 km (below pressures equivalent to the aluminosilicate triple point) to ~25 km depths (~0.7 GPa) between ca. 1.70 and 1.69 Ga, (3) decompression back to ~12 km depths (0.3-0.4 GPa) by ca. 1.68 Ga, and (4) a protracted period of near-isobaric cooling (ca. 200-250 Ma). The general geometry of this looping P-T path is similar for rocks across the entire traverse; however, significant differences in peak temperatures are recorded (~500 to >750 °C). Notable variations along the transect are also primarily thermal in nature and include differences in the temperature of the prograde history (i.e., early andalusite versus kyanite), equilibrium pressures recorded at peak temperatures, and intensity of late-stage thermal spikes due to local dike emplacement. High-precision ΔPT "relative" thermobarometry confi rms lateral temperature variations on the order of 100-250 °C with little to no variation in pressure. The Upper Granite Gorge thus represents a subhorizontal section of lowermost middle continental crust (~0.7 GPa). Results imply that the entire ~70-km-long transect decompressed from ~0.7 to ~0.3-0.4 GPa levels as one large coherent block in the Paleoproterozoic.The transect represents a 100% exposed fi eld laboratory for understanding the heterogeneity and rheologic behavior of lowermost middle continental crust during orogenesis. Hot blocks achieved partial melting conditions during penetrative subvertical fabric development. Although these blocks were weak, large-scale horizontal channel fl ow was apparently inhibited by colder, stronger blocks that reinforced and helped preserve the block-type architecture. Development of dramatic lateral thermal gradients and discontinuities without breaks in crustal level is attributed to: (1) spatially heterogeneous advective heat fl ow delivered by dense granitic pegmatite dike complexes and (2) local transcurrent displacements along block-bounding high-strain zones over an ~15-20 m.y. time interval. Exhumation of the transect from 25 to 12 km depths is interpreted to refl ect erosion synchronous with penetrative development of steeply dipping NE-striking foliations and steeply plunging stretching lineations, consistent with an orogen-scale strain fi eld involving NW-SE subhorizontal shortening and subvertical extension during crustal thickening.

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