Gregory Dumond

Granite genesis and mafic-felsic magma interaction in the lower crust

Field observations from an exposed section of deep continental crust, the Athabasca granulite terrane (AGT), Saskatchewan, Canada, provide a view of granite genesis and a mechanism for deep-seated contamination of felsic and mafic magmas. The 1.9 Ga Chipman mafic dike swarm was emplaced into the Chipman Tonalite (ca. 3.3 Ga) and the megacrystic Fehr granite (ca. 2.6 Ga) at a crustal depth of ∼40 km. The Fehr granite shows evidence for extensive partial melting and generation of granitic leucosome. Mafic dikes and granitic leucosome display magma mingling and mixing textures similar to those widely described from shallow crustal exposures. The AGT provides a view of a dynamic, heterogeneous, and locally fertile deep crust. Mantle-derived mafic magma promotes extensive partial melting of fertile granitoids, which in turn filter and entrap later mafic dikes and sills. The result is almost inevitable mingling and hybridization (i.e., contamination) of mafic and felsic end members. This interaction of magmas in the deep crustal environment may account for the isotopic and compositional signatures of igneous rocks at shallower crustal levels that typically record contamination of crustal melts by mantle material and vice versa.

Crustal segmentation, composite looping pressure-temperature paths, and magma-enhanced metamorphic field gradients: Upper Granite Gorge, Grand Canyon, USA

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 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.

Transpressive uplift and exhumation of continental lower crust revealed by synkinematic monazite reactions

Exposures of continental lower crust provide fundamental constraints on the thermal-mechanical behavior of continental lithosphere during orogeny. The applicability of fi results, however, requires knowledge of whether these data pertain to deformation during lowercrustal residence or during uplift and exhumation of deep crust. Dating synkinematic monazite-producing reactions provides one way to evaluate deformation styles in the deep crust. We report on the implications of monazite reaction dating for the timing of fabric formation and movement along three crustal-scale shear zones in northern Saskatchewan, western Canadian Shield. The structures accommodated dextral transpressive strain during oblique- and thrust-sense displacement that was coeval with uplift and exhumation of >20,000 km 2 of continental lower crust (>1.0 GPa) to middle-crustal levels (<0.5 GPa). In situ Th-U‐total Pb monazite data reveal that monazite rims in all three shear zones grew synkinematically at 1849 ± 6 Ma (2σ, mean square of weighted deviates = 0.8). The style of deformation involved localized strain concurrent with segmentation and translation of rheologically strong blocks of deep crust along mutually interacting shear zones during transpression.