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The Elk River-Clarkia area lies from 10 to 35 miles northwest of the Idaho batholith in a zone of moderately to highly metamorphosed sedimentary rocks of the Precambrian Belt series. The Prichard formation, which is the oldest unit in the Belt series, covers the northeastern part of the area and consists mainly of garnet-mica schist with a minor amount of quartzite. It is separated by a fault zone from the younger formations. The massive Revett quartzite is fragmentarily exposed along and near this fault zone and is overlain by a fairly thin layer of schist of the St. Regis formation. The youngest formation, the Wallace, covers the western and southern part of the area. It consists of aluminous schist and interbedded calcareous gneiss and quartzite. The Burke formation, which should occur between the Prichard and Revett formations, could not be identified in this area. Two large and a number of small bodies of igneous rocks occur in the metasedimentary series. One of the large bodies is quartz diorite and the other is quartz monzonite. The composition of the small bodies ranges from gabbro to quartz diorite, quartz monzonite, and granite. The grade of metamorphism of the Belt series increases gradually from northwest to southeast parallel to the major structural trend and suggests that the metamorphism is related to the intrusion of the Idaho batholith, the nearest outcrops of which are exposed about 10 miles southeast of the area under discussion. The critical mineral assemblage in the schists in the northern part is almandine--muscovitebiotite-quartz; in the calcareous layers of the gneisses the common assemblage is hornblende-biotite-epidote mineralsalbite--microcline--quartz with or without scapolite and calcite. These mineral assemblages are typical of the biotitealmandite subfacies of the epidote amphibolite facies. Toward the south, kyanite appears with staurolite in the aluminumrich layers of the garnet-mica schist and diopside becomes stable in the calcareous layers of the gneissic units. Oligoclase instead of albite occurs with the epidote minerals. Microcline abounds in many thin biotite-rich layers interbedded with diopside and biotite gneisses. These mineral associations suggest metamorphic temperatures of the staurolite-kyanite subfacies of the epidote amphibolite facies. The occurrence of sillimanite instead of staurolite and kyanite in the schist in the southern part of the area suggests the sillimanite-muscovite subfacies of the amphibolite facies. Grossularite occurs with calcite in a few calcareous layers, but the assemblage diopside--biotite--plagioclase (An30 ) is most common. Metamorphic temperatures are estimated to rise from 380°C in the northern border to 570°C in the southern border. Contrary to the normal distribution of the potassium feldspar in the regionally metamorphosed area, microcline is rare or lacking in the gneisses of the southernmost pa.rt-that is, in the zone of highest grade of metamorphism. This lack of microcline can be either depositional or due to a later partial removal of potassium from a contact zone of the Idaho batholith about 20 miles wide. In the light of the regional geology the latter possibility seems more likely and would suggest that potassium was mobilized in a wider contact aureole than the other elements (calcium, aluminum, iron, and magnesium). It was removed from the inner contact aureole, which is the zone of metasomatic enrichment of basic components (Hietanen, 1962), and probably also from the southern part of the area under discussion, which part belongs to an outer contact aureole of the batholith. No other traces of regional metasomatism could be found in this outer zone.

Metamorphism of the Belt series in the Elk River-Clarkia area, Idaho