Abstract

Archean basement rocks occur in the core of a fold nappe in the northern East Humboldt Range, Nevada. The Archean rocks consist primarily of plutonic orthogneiss and metasedimentary paragneiss. Both units contain numerous amphibolitic bodies interpreted as mafic intrusions. U-Pb zircon studies indicate that the orthogneiss unit is at least 2520 ±110 Ma and therefore is an example of an Archean granitoid. These data extend the known distribution of Precambrian basement rocks into northeastern Nevada and provide new constraints on the delineation of the Archean Wyoming province.

The basement rocks are surrounded by upper amphibolite facies, metasedimentary rocks correlated with late Precambrian to Devonian(?) rocks of the eastern Great Basin miogeoclinal sequence. The contact between the Archean basement rocks and the metasedimentary mio geoclinal sequence is interpreted as a premetamorphic and prefolding, low-angle fault. The age and prefolding geometry of this low-angle fault are currently poorly constrained, but it may record important Mesozoic crustal shortening in the hinterland of the Sevier orogenic belt. The age of nappe formation is not yet fully established, but at least some isoclinal folds in the area were formed in the Tertiary and are kinematically linked to a major extensional deformation.

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