Quartz monzonite and related rocks of the Libby Quadrangle, Montana, and the effects on them of deuteric processes

Abstract

The rocks described are, for the most part, stocks intrusive into sedimentary formations of the Belt series in northwestern Montana. The sedimentary rocks, chiefly of argillaceous and arenaceous composition, have been folded into large open anticlines and synclines which trend north-northwest. In addition the beds have been faulted along steeply-dipping, persistent faults which also trend north-northwest and show great vertical displacement. The stocks and accompanying dikes were intruded after the sedimentary rocks had been folded but before the last movement along some of the larger faults. The age of the folding, faulting, and intrusive activity is probably late Mesozoic. The principal stock, located on Dry Creek, is a quartz monzonite with a surface exposure of about 20 square miles. Three smaller stocks of quartz monzonite are present in the central and southern parts of the quadrangle. Small dikes of granite, quartz diorite, quartz latite, and lamprophyre are associated with the principal stock. Other dikes in the area, chiefly of diorite, are seemingly not closely associated with the stocks but are, nevertheless, probably to be correlated with the same period of igneous activity. The stock near the head of Bobtail Creek is porphyritic syenite and is remarkable for the large size and parallelism of its orthoclase phenocrysts. Metamorphism induced in the sedimentary rocks that adjoin the stocks is for the most part of low grade and not widespread. Minor development of typical contact-metamorphic silicates remote from any of the exposed stocks suggests the presence of other stocks not yet revealed by erosion. Deuteric effects in the stocks are widespread. Potash feldspar, sphene, magnetite, and allanite are recognized as late-stage minerals in nearly all of them. These minerals, and their relations to the other minerals of the stocks, are rather similar to those in parts of the Nelson and Idaho batholiths and other bodies of quartz monzonite and granodiorite in Idaho.