Thick successions of quartzose and argillaceous metasediments in the northern part of the Southern Snake Range and the central Schell Creek Mountains of eastern Nevada and the southeastern part of the Deep Creek Range of adjacent Utah are assigned to the Late Precambrian and are here named the McCoy Creek Group. This group is overlain concordantly but with sharp boundary by Lower Cambrian Prospect Mountain Quartzite (Restricted). The lower part contains minor marble and, in the Deep Creek Range, tillitic schists. The base is not exposed. Regional metamorphism ranges from very low to medium grade and is considered Mesozoic in age. It is not associated with the formation of any migmatites or gneisses in the three areas described.

In the Southern Snake Range the upper 3,600 feet of the McCoy Creek Group are exposed; six new formations are proposed most of which are also recognized in the southernmost part of the Northern Snake Range. Synkinematic regional metamorphism is of low-chlorite-zone grade and extends upward through the Lower Cambrian. Superposed dynamic metamorphism in Late Precambrian and Cambrian rocks is localized near a decollement thrust, and still later contact metamorphism occurs at post-orogenic granitic intrusions. Different combinations of the three genetically separate metamorphisms have produced various kinds of polymetamorphic rocks, including mimetically, recrystallized contact schists.

In the McCoy Creek area of the Schell Creek Mountains, 8,800 feet of the group are exposed; eight formations, designated as McCoy Creek Unit A through H, and several subdivisions are recognized. At least the upper 4,300 feet are equivalent to the formations exposed in the Southern Snake Range. The lower part includes minor marbles and local amphibolite. In the upper part of the section, as well as in overlying Prospect Mountain strata, synkinematic regional metamorphism is of the same low-chlorite-zone grade as in the equivalent rocks of the Southern Snake Range, but in the lower part the grade increases to the biotite and garnet zone. There is no orogenic unconformity or metamorphic break, and metamorphic history, analyzed in detail, is similar throughout the sequence, though on the evel of different grades. Being synkinematic and therefore orogenic, the regional metamorphism is considered to be later than the concordant Late Precambrian-Paleozoic-Early Triassic succession of the region, and to be connected with the major Mesozoic orogeny as its deep-seated expression. Though the metamorphic grade was controlled by stratigraphic depth, metamorphism was triggered by orogeny, deformation playing the role of catalyst. Superposed dynamic and contact metamorphism are absent in the McCoy Creek area but dynamic, and possibly contact metamorphism, occur in Prospect Mountain Quartzite and other Paleozoic rocks near Connors Pass farther south in the Schell Creek Range.

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In the Trout Creek area of the southeastern Deep Creek Range about 8,800 feet of metasediments are exposed which represent the lower part of the McCoy Creek Group and are believed to include strata older than any exposed in the McCoy Creek area. Seven formations, designated as Trout Creek Unit 1 through 7, and several subdivisions are recognized. The lower part includes some marble and three units of tillitic schists totalling about 1,350 feet. The erratic pebbles and boulders were derived from an earlier Precambrian terrane of gneisses, granitic rocks, migmatites, and schists with quartzite and minor marble. The erratics were dropped haphazardly into bedded, argillaceous-silty-sandy sediments. They are believed to have been rafted by icebergs, rather than to have been deposited by su aqueous slides. Metamorphism is regional-synkinematic and mostly of garnet-zone grade; it closely resembles the metamorphism in the lowest part of the McCoy Creek section. In the tillitic schists the erratics generally were fully recrystallized. Hornblendic interbeds in the lower parts of the section largely are of sedimentary derivation. Similar schists, but without tillitic members, were mapped by R. B. Nelson on the west side of the area described. These schists are overlain by the highest Precambrian strata along a shearing-off thrust. The latter strata have the same lithologic character, the same low-chlorite-zone metamorphism, and the same stratigraphic position as the equivalent upper part of the McCoy Creek Group exposed in the Southern Snake Range and the McCoy Creek area. Altho gh the later thrust faulting here has broken the continuity of sequence of the McCoy Creek Group, the medium-grade regional metamorphism of the Trout Creek area is interpreted as post-Paleozoic, in analogy with the more conclusive relationships exposed in the McCoy Creek area of the Schell Creek Mountains.

Other occurrences of Precambrian rocks in eastern Nevada are briefly outlined. The McCoy Creek Group is exposed also between the southern end of the Cherry Creek Mountains and the northern Egan Range, as well as in the Egan Range north of Ely. The stratigraphically well defined outcrops of the McCoy Creek Group are contrasted to areas of complex crystallines in which migmatites, gneisses, and related granitic rocks are associated with metaclastic rocks and some amphibolites. Such a rock association is displayed typically in the Northern Ruby-East Humboldt Range where regional metamorphism is of high and higher-medium grade and is associated with large-scale, syn- and late-kinematic migmatization. The Northern Ruby-East Humboldt Complex probably represents an earlier Precambrian baseme t, though evidence is not conclusive. In the Wood Hills, east of the East Humboldt Range, a thick Paleozoic section of marbles occurs. In the Northern Snake Range the lower plate of a decollement thrust includes limited outcrop areas of low-grade metasediments of the McCoy Creek Group and of non-metamorphic Prospect Mountain Quartzite, but the predominant rocks are medium-grade metaclastic schists, gneisses, and related granitic rocks; amphibolitic rocks also occur. The gneisses are derived from granitic intrusions; migmatization is rare and, where present, is on a small scale. The age of the clastic rocks and of their intrusion and regional metamorphism is uncertain. It is possible that Late Precambrian and Lower Cambrian rocks are involved, but the possibility that earlier Precambrian lements are present also exists.

Precambrian rocks exposed in the Grouse Creek and Raft River Ranges of northwesternmost Utah and in the Albion Mountains of adjacent Idaho comprise metaclastic schists and an amphibolitic unit, invaded by massive and gneissose granitic rocks of doubtful age. Correlation with rocks in eastern Nevada is uncertain, although two sequences can be recognized in the Raft River Range.