Structural development of high-temperature mylonites in theArchean Wyoming province, northwestern Madison Range,Montana

摘要

The Crooked Creek mylonite, in the northwestern Madison Range, southwestern Montana, is definedby several curved lenses of high non-coaxial strain exposed over a 7-km-wide, northeast-trending strip. Thecountry rocks, part of the Archean Wyoming province, are dominantly trondhjemitic to granitic orthogneisswith subordinate amphibolite, quartzite, aluminous gneiss, and sills of metabasite (mafic granulite). Datapresented here support an interpretation that the mylonite formed during a period of rapid, heterogeneousstrain at near-peak metamorphic conditions during an early deformational event (D_1) caused by northwest-southeast-directed transpression. The mylonite has a well-developed L-S tectonite fabric and a fine-grained,recrystallized (granoblastic) texture. The strong linear fabric, interpreted as the stretching direction, is definedby elongate compositional "fish," fold axes, aligned elongate minerals, and mullion axes. The margins of themylonitic zones are concordant with and grade into regions of unmylonitized gneiss. A second deformationalevent (D_2) has folded the mylonite surface to produce meter- kilometer-scale, tight-to-isoclinal, gentlyplunging folds in both the mylonite and country rock, and represents a northwest–southeast shortening event.Planar or linear fabrics associated with D_2are remarkably absent. A third regional deformational event (D3)produced open, kilometer-scale folds generally with gently north-plunging fold axes.Thermobarometric measurements presented here indicate that metamorphic conditions during Di werethe same in both the mylonite and the country gneiss, reaching upper amphibolite- to lower granulite-faciesconditions: 700 ± 50° C and 8.5 ± 0.5 kb. Previous geochronological studies of mylonitic and cross-cuttingrocks in the Jerome Rock Lake area, east of the Crooked Creek mylonite, bracket the timing of this high-grade metamorphism and mylonitization between 2.78 and 2.56 Ga, nearly a billion years before the 1.78-GaBig Sky orogeny, which overprinted the basement rocks exposed in adjacent ranges of the Wyoming province.