HAPPY-CREEK IGNEOUS COMPLEX AND TECTONIC EVOLUTION OF THE EARLY MESOZOIC ARC IN THE JACKSON MOUNTAINS, NORTHWEST NEVADA

Geologic relations in the Jackson Mountains of northwestern Nevada hav e been widely cited to constrain a tectonic history for the portion of the Mesozoic are that exists in this part of the U. S. Cordillera. Ne w mapping, structural, and geochronologic data from the Jackson Mounta ins, however, require a substantial revision of earlier interpretation s and have far-ranging implications for the magmatic and tectonic evol ution of this part of the arc. The most extensive rock unit in the Jac kson Mountains is the Happy Creek igneous complex, which is bounded on both the east and west by older Paleozoic and Upper Triassic metasedi mentary strata. The Happy Creek complex has previously been regarded a s a thick, Upper Triassic to upper Middle Jurassic volcanic succession composed primarily of andesitic flows and related volcaniclastic stra ta. New observations, however, indicate that the majority of the Happy Creek complex is a monotonous expanse of massive andesite, including nonfragmental rocks intermixed with less common and irregularly distri buted patches of fragmental andesite breccia. A detailed analysis of t he internal features and contact relations along the margins of these massive igneous rocks indicates that they represent a mixture of hypab yssal intrusions and intrusive breccias that developed in a shallow-le vel subvolcanic setting. A much smaller portion of the Happy Creek com plex consists of stratified volcaniclastic rocks, which are interprete d to be the only surviving supracrustal elements of the Happy Creek vo lcanic construction, the majority of which was removed by later erosio n. Hypabyssal rocks of the Happy Creek complex intrude the Upper Trias sic (Carnian and Norian) strata of the Boulder Creek beds, but are the mselves intruded by several epizonal plutons of broadly dioritic to mo nzonitic composition. U-Pb zircon ages from one group of these plutons indicate ages between 196 and 190 Ma (Early Jurassic), and Happy Cree k magmatism is thus restricted between latest Triassic and late Early Jurassic, and did not continue into the late Middle Jurassic. Much you nger crosscutting plutons yielded U-Pb zircon ages of 170-162 Ma and t hus identify a separate pulse of Middle Jurassic magmatism. Geochemica l data indicate that the Happy Creek complex and all the Jurassic plut ons are calc-alkaline rocks formed in a volcanic are setting. The King Lear Formation is a younger sequence of generally coarse-grained terr igenous elastic rocks that have previously been interpreted by some wo rkers to be gradational with the Happy Creek complex. A U-Pb zircon ag e of 125 +/- 1 Ma from an interlayered ashflow tuff near the base of t he formation and a U-Pb age of 123 +/- 1 Ma from a dacitic intrusive c omplex that intrudes high into the section indicate that the King Lear Formation is Early Cretaceous in age and unrelated to the Happy Creek complex. An angular unconformity, spanning as much as 65 m.y., separa tes a base ment composed mostly of Happy Creek complex rocks and Juras sic plutons from Lower Cretaceous strata of the King Lear Formation. E rosion associated with development of the unconformity is interpreted to have stripped most of the supracrustal portion of the Happy Creek c omplex. This unconformity separates rocks that were previously metamor phosed and deformed (Happy Creek complex and older rocks) from younger strata (King Lear Formation) that are not metamorphosed or regionally deformed. Folded and cleaved volcaniclastic rocks that were previousl y considered part of the King Lear Formation are here demonstrated to be part of the Happy Creek complex supracrustal sequence.