Amphibolite to granulite facies metamorphism and deformation in the Jerome Rock Lakes area, Spanish Peaks, Montana

Abstract

Structural analysis, bedrock mapping, and quantitative thermobarometry reveal a protracted metamorphic history for the multiple lithologies present in the Jerome Rock Lakes area of the Spanish Peaks, located in the northern Madison Range, southwest Montana. Volumetrically-dominant quartzofeldspathic gneisses are primarily of the tonalite-trondhjemite-granodiorite (TTG) suite and record multiple episodes of high-temperature deformation. Coarse-grained hornblende in association with K feldspar-bearing leucosome indicates locally vapor-present melting. Clinopyroxene-bearing metabasic enclaves in the quartzofeldspathic gneisses record the subsolidus hydration of clinopyroxene porphyroclasts to hornblende and subsequent dehydration melting of hornblende in the transitional granulite facies to produce garnet coronae and clinopyroxene neoblasts under vapor-absent conditions. Variably migmatized garnet-sillimanite schists record amphibolite facies metamorphism at ca. 5.5 kbar and 640 °C, with extensive melt production via melting of biotite. Inclusions of staurolite in kyanite-bearing metaquartzite are inferred to indicate the presence of modern thicknesses of continental crust and a geothermal gradient similar to Phanerozoic values. Infrequent layers containing tschermakitic amphibole layers may represent the residuum of vapor-present melting of biotite and indicate local variation in the activity of water. Migmatized garnet amphibolites record peak upper-amphibolite to transitional granulite facies metamorphism at ca. 10 kbar and 800 °C. A clockwise P-T path of Alpine character is proposed for the lithologies of the Jerome Rock Lakes area, with peak pressures in the kyanite field attained before peak temperatures, followed by rapid, steeply adiabatic to isothermal decompression. Early isoclinal folds developed during prograde metamorphism and were rotated into the regional foliation. Progressive deformation resulted in the formation of later isoclinal folds coplanar with foliation. Peak temperatures were attained after cessation of deformation, indicated by undeformed leucosomes. A second episode of deformation produced outcrop-scale parallel folds trending north-south. The Precambrian tectonic history of the northern Madison Range is inferred to have terminated in a continental collision with northwest-southeast directed shortening that produced steeply-dipping, NNE-trending regional foliation. This style is generally consistent with that observed elsewhere in the Spanish Peaks and in the Tobacco Root Range, generated by the ca. 1.7 Ga collision of western North America with the Medicine Hat craton.