Precambrian crystalline rocks in the Spanish Peaks area of southwestern Montana are composed primarily of granitic and amphibolitic gneiss, quartzite, and amphibolite. Sillimanite gneiss, ultrabasic intrusions, marble, and post-Precambrian basalt are also present. Almandine-amphibolite metamorphic facies and sillimanite-almandine-orthoclase subfacies are widely distributed in the area.
Two major Precambrian orogenic events followed accumulation of a thick sedimentary sequence. An early Precambrian orogenic deformation produced isoclinal folds, folded pegmatites, and amphibolite boudins. These structural features were deformed in a second orogeny, which was characterized by large, open northeast-trending folds that developed under metamorphic conditions favorable for ductile deformation and a second generation of pegmatites. Cross folding and flexure of the second generation folds and development of kinks accompanied or followed the second orogeny. Mafic dikes and sills were emplaced following the second orogenic event and subsequently metamorphosed. They cut across the second-generation folds. Young, probably post-Precambrian quartz veins, plus felsic porphyry dikes, and basalt dikes cut all earlier structures.
Alternate correlation of thermal events whose ages have been determined radiometrically at 2.6, 1.9, and 1.6 b.y. with metamorphic and structural events are made as follows:
Amphibolitization of unfolded dikes and sills: I, 1.9 b.y; II, 1.6 b.y.
Open, subhorizontal folds: I, 2.6 b.y.; II, 1.9 b.y.
Isoclinal folds: I, age uncertain; II, 2.6 b.y.