Petrology of the Migmatite Complex along the South Fork of the Clearwater River, Idaho


Petrology of the Migmatite Complex along the South Fork of the Clearwater River, Idaho

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A nonuraniferous migmatite complex is exposed along the South Fork of the Clearwater River in north central Idaho. The complex is situated along the northeastern border of the Atlanta lobe of the Idaho batholith in a high-grade metamorphic terrane consisting of aluminous gneiss, tonalitic migmatite, calc-silicate granofelses, quartzofeldspathic gneiss, quartzite, augen gneiss, and amphibolite. As the margin of the Atlanta lobe is approached, on the western border of the complex, the Droogs Creek granite intrudes the high-grade metasedimentary rocks in a lit-par-lit manner. Toward the east, the percentage of granitoid rocks decreases, and in situ migmatite is exposed near Dutch Oven Creek. Structures indicative of more advanced stages of migmatization increase westward through the complex. The high-grade metasedimentary rocks in the complex are steeply dipping and trend north northwest. They generally have gradational contacts, and contain abundant well-rounded zircon. Aluminous gneiss near the Crooked River contains sillimanite and garnet in addition to biotite, muscovite, feldspar, and quartz. The aluminous gneiss is on the eastern limb of an antiform cored by migmatite. The migmatite, here named the Dutch Oven Creek (DOC) migmatite, contains discontinuous pegmatite-1ike leucosomes which are enveloped by very thin biotite-rich melanosomes. Overall, the migmatite is tonalitic in composition but is granitic locally where layers contain up to 30 percent microcline. The migmatite is brecciated by quartz monzonite along its western contact and grades into calc-silicate granofelses. The calc-silicate granofelses are typically layered and contain epidote-, diopside-, and scapolite-bearing assemblages. Hornblende-rich selvages along the edges of calc-silicate xenoliths attest to local metasomatism. The granofelses grade into quartzofeldspathic gneiss and quartzite near Newsome Creek. Micaceous quartzite at Golden is folded into largescale open folds, the limbs of which contain hinges of isoclines. Augen gneiss occurs as xenoliths in the DOC migmatite, sheets in the aluminous gneiss, and as a complexly folded composite unit in sharp contact with the Golden quartzite. Biotite amphibolites are present in every unit either as blocks or sill-like lenses, and are garnet-bearing in the aluminous gneiss. Interference structures and isoclinal fold hinges on the limbs of large-scale folds suggest that the area was affected by two and possibly three progressive(?) deformational events. Tight-to-isoclinal folds in the DOC migmatite are coaxial with the larger north-south trending antiform. In the aluminous gneiss, sillimanite needles are coaxial with tight folds, whereas fibrolite is folded. Faults trend northwesterly in the eastern part of the complex and are randomly oriented in the west. Joints define at least two maxima at N83W;72S and N69W;54S. The entire complex is within the sillimanite zone of the upper amphibolite facies. The orientation of sillimanite prisms and fibrolite indicate that metamorphism and deformation were coeval. Mineral relations in the aluminous gneiss suggest that at the peak of metamorphism, temperatures between 700°-730°C and pressures exceeding 3.5 kb were attained. These conditions are within the range for partial melting to occur. Compositions of leucosomes in the DOC migmatite generally plot close to the isobaric cotectic surfaces in regions of low temperature in the Qz-Ab-Or-An-H2O system and are compatible with an origin by partial melting. Leucocratic layers that do not plot close to cotectic surfaces in areas of low temperature, may have formed by metamorphic segregation induced by(?) partial melting.


A Thesis submitted to the faculty of the Graduate School of the University of Minnesota by Diane Helen Carlson in partial fulfillment of the requirements for the degree of Master of Science, June 1981. There are 2 supplementary files also attached to this record, which contain Plates 1-2 referenced in the thesis.

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Carlson, Diane Helen. (1981). Petrology of the Migmatite Complex along the South Fork of the Clearwater River, Idaho. Retrieved from the University Digital Conservancy,

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