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Petrographic and petrofabric study of the metamorphic rocks north of Carson City, Ormsby County, Nevada
AuthorZones, Christe P
Geological Sciences and Engineering
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The metamorphic rocks of the report area are predominantly pyroclastic rocks in the southern part of the area and altered shales and limestones interbedded with pyroclastic rocks in the northern part. They were intruded by granitic rocks during the Nevadan orogeny. The age of the metamorphic rocks can not be determined but they may be Triassic. The rocks were subjected to low rank regional metamorphism, followed by medium to high rank contact metamorphism. During the dynamic metamorphism the rocks we re folded about a northwest axis and a fabric was imprinted on them. Recrystallization of the mineral components during contact metamorphism took place along directions of easy growth, governed by the anisotropism of the earlier metamorphic fabric. The rocks show a strong lineation, which parallels the northwest-trending fold axis, and a we11 developed foliation. The fabric was probably imposed on the rocks predominantly by slip along the foliation plane but there is evidence in the pattern of preferred orientation of quartz that rotation of slip planes or mineral grains about the a and b fabric axes was also operative. Following the metamorphisra, the area may have been folded or warped about a northeast trending axis and cut by step faults whose strikes paralleled the trend of the fold axis. As an alternative possibility, differential tilting of adjacent fault blocks may have simulated folding.
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