Ductile shearing and migmatization at mid‐crustal levels in an Archaean high‐grade gneiss belt, northern Gallatin Range, Montana, USA

Abstract

A high‐grade Archaean gneiss terrane in the northern Gallatin Range, south‐western Montana, USA, contains a trondhjemite–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°C, pressures in excess of 8 kbar, 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.