DUCTILE SHEARING AND MIGMATIZATION AT MIDCRUSTAL LEVELS IN AN ARCHEAN HIGH-GRADE GNEISS BELT, NORTHERN GALLATIN RANGE, MONTANA, USA

A high-grade Archaean gneiss terrane in the northern Gallatin Range, south-western Montana, USA, contains a trondhiemite-tonalite gneiss (TTG) sequence that was migmatized during pervasive ductile shearing. Metamorphism of these rocks is in the upper amphibolite to granulite facies at temperatures of 680-735-degrees-C, pressures in excess of 8kbar, and a 'clockwise' P-T-t path is inferred. Ductile shearing occurred in metre-scale anastomosing bands of high strain throughout the area. The TTGs have been extensively migmatized via vapour-present melt reactions involving the incongruent melting of biotite-bearing TTG to produce hornblende and granitic melt. The granitic melt is produced in narrow envelopes adjacent to ductile shear zones in response to infiltration of water-rich solutions. Melt migration occurred on a local scale, and extraction of melt from the system left behind a plagioclase-hornblende residuum with minor interstitial microcline. Ductile shearing and migmatization in the TTG operated in a positive feedback mechanism; the entire volume of gneiss was chemically and mechanically reworked through the cyclical infiltration of aqueous solutions, vapour-present melting and melt-enhanced deformation. The proposed melt reaction may be an important crustal differentiation process considering that (1) many collisional orogens do not attain temperatures high enough to permit vapour-absent melting, (2) pervasive networks of ductile shear zones at mid-crustal levels may serve as channels for fluid ingress and melt extraction, and (3) the large volumes of TTGs in Archaean and Phanerozoic orogens may constitute a significant source reservoir for certain types of high-level granites.