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

摘要

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 high-strain zones. New field, 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 PT "relative" thermobarometry confirms 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.