Evolution of a metamorphic gradient in the footwall of the northern Snake Range detachment, Nevada

Abstract

The northern Snake Range metamorphic core complex exposes an extensive section of the footwall below a low-angle detachment, and is therefore an ideal location to study the metamorphic evolution of core complex mylonites. Metamorphic grade in the footwall of the northern Snake Range décollement (NSRD) has been observed to increase with structural depth from the chlorite zone to the garnet-staurolite zone. In a transect through footwall schists at Hendry's Creek, textural and mineral composition analyses reveal systematic changes with depth in addition to the metamorphic gradient. Fe/Mg ratios in muscovite are consistent with the metamorphic gradient and increase with depth, whereas Fe/Mg ratios in garnet and chlorite porphyroblasts show the opposite trend. An inflection in chlorite composition occurs in garnet schist ~100 m below the detachment, where Ca zonation patterns in garnet are indicative of more extensive retrogression than in structurally lower samples. Schist at ~100 m below the detachment also contains abundant fluid inclusion planes and shows textural evidence of staurolite breakdown, indicating the presence of fluids at structurally high levels in the footwall. Results suggest that all structural levels sampled experienced amphibolite facies metamorphism, and the structurally highest levels of the footwall were later retrogressed during synextensional, greenschist facies metamorphism localized along the NSRD.