Abstract

Geologic relations in the Jackson Mountains of northwestern Nevada have been widely cited to constrain a tectonic history for the portion of the Mesozoic arc that exists in this part of the U.S. Cordillera. New mapping, structural, and geochronologic data from the Jackson Mountains, however, require a substantial revision of earlier interpretations and have far-ranging implications for the magmatic and tectonic evolution of this part of the arc.

The most extensive rock unit in the Jackson Mountains is the Happy Creek igneous complex, which is bounded on both the east and west by older Paleozoic and Upper Triassic metasedimentary strata. The Happy Creek igneous complex has previously been regarded as a thick, Upper Triassic to upper Middle Jurassic volcanic succession composed primarily of andesitic flows and related volcaniclastic strata. New observations, however, indicate that the majority of the Happy Creek igneous complex is a monotonous expanse of massive andesite, including nonfragmental rocks intermixed with less common and irregularly distributed patches of fragmental andesite breccia. A detailed analysis of the internal features and contact relations along the margins of these massive igneous rocks indicates that they represent a mixture of hypabyssal intrusions and intrusive breccias that developed in a shallow-level subvolcanic setting. A much smaller portion of the Happy Creek igneous complex consists of stratified volcaniclastic rocks, which are interpreted to be the only surviving supracrustal elements of the Happy Creek volcanic construction, the majority of which was removed by later erosion. Hypabyssal rocks of the Happy Creek igneous complex intrude the Upper Triassic (Carnian and Norian) strata of the Boulder Creek beds, but are themselves intruded by several epizonal plutons of broadly dioritic to monzonitic composition. U-Pb zircon ages from one group of these plutons indicate ages between 196 and 190 Ma (Early Jurassic), and Happy Creek magmatism is thus restricted between latest Triassic and late Early Jurassic, and did not continue into the late Middle Jurassic. Much younger crosscutting plutons yielded U-Pb zircon ages of 170–162 Ma and thus identify a separate pulse of Middle Jurassic magmatism. Geochemical data indicate that the Happy Creek igneous complex and all the Jurassic plutons are calc-alkaline rocks formed in a volcanic arc setting.

The King Lear Formation is a younger sequence of generally coarse-grained terrigenous clastic rocks that have previously been interpreted by some workers to be gradational with the Happy Creek igneous complex. A U-Pb zircon age of 125 ± 1 Ma from an interlayered ash-flow tuff near the base of the formation and a U-Pb age of 123 ± 1 Ma from a dacitic intrusive complex that intrudes high into the section indicate that the King Lear Formation is Early Cretaceous in age and unrelated to the Happy Creek igneous complex. An angular unconformity, spanning as much as 65 m.y., separates a basement composed mostly of Happy Creek igneous complex rocks and Jurassic plutons from Lower Cretaceous strata of the King Lear Formation. Erosion associated with development of the unconformity is interpreted to have stripped most of the supracrustal portion of the Happy Creek igneous complex. This unconformity separates rocks that were previously metamorphosed and deformed (Happy Creek igneous complex and older rocks) from younger strata (King Lear Formation) that are not metamorphosed or regionally deformed. Folded and cleaved volcaniclastic rocks that were previously considered part of the King Lear Formation are here demonstrated to be part of the Happy Creek igneous complex supracrustal sequence.

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