Geology of the Humboldt igneous complex, Nevada, and tectonic implications for the Jurassic magmatism in the Cordilleran Orogen

The Jurassic Humboldt igneous complex in west-central Nevada consists of a comagmatic suite of intrusive and extrusive rocks and is tectonically intercalated with Triassic to Lower Jurassic shelf sequence and basinal successions. Its plutonic rocks include, from bottom to top, olivine-gabbro, melatroctolite, hornblende gabbro, microgabbro, and diorite transitional upward into quartz diorite, tonalite-granodiorite, and monzonite. Contacts between these plutonic subunits are commonly gradational, but mutual intrusive relations, characterized by the existence of brecciated and altered zones and xenoliths, are also common. Mafic to felsic plutonic rocks are cut by generally N-S to NW-SE striking dikes that form local dike swarms with one- and two-sided chilled margins. Dikes are composed of fine- to medium-grained rocks ranging in composition from basalts to andesites and feed into and/or are overlain by extrusive rocks consisting of lava flows intercalated with volcanic tuff and breccia. Lava flows at stratigraphically lower levels are more mafic and locally display pillow shapes reminiscent of submarine lava flows, whereas lava flows at higher levels are more felsic and are commonly interleaved with a fine-grained tuffaceous material. Volcanic rocks range in composition from basalts, basaltic andesites, andesites, to latites and dacites and mineralogically and texturally are similar to the dikes. The major element compositions of the analyzed rocks suggest relatively evolved basaltic magmas, whereas strongly incompatible trace element ratios (e.g., Ce/Ta) have high values typical of subduction related magmas. Lavas, dikes, and gabbros commonly display similar rare earth element (REE) patterns, although more felsic rocks are light rare earth (LREE) enriched, suggesting a cogenetic suite of rocks. These REE patterns are characteristic of basaltic andesites from volcanic arcs and suggest, coupled with field relations, that the rocks of the Humboldt complex might have evolved from subduction originated magmas in a volcanoplutonic arc setting. The tectonic nature of the contact between the Humboldt complex and the underlying Triassic-Jurassic sedimentary strata indicates that it was displaced eastward from its original arc environment following its igneous evolution. Both the Humboldt complex and the sedimentary strata are intruded at all structural levels by numerous northeast-striking dikes and dike swarms, which strongly altered and metasomatized their country rocks. These dike rocks are Miocene in age and have geochemical characteristics distinctly different from those of the Humboldt rocks. Based on regional correlations and paleogeographic reconstructions, we interpret the Humboldt igneous complex as the northern continuation of a Jurassic continental margin arc that extended from the Sonora Desert region in the south to northwestern Nevada and northern California in the north. This continental margin arc was a site of regional subsidence and crustal extension that accompanied magmatic activity and penecontemporaneous deposition of both arc and craton-derived detritus in submarine to subaerial conditions during much of the Jurassic. We emphasize that the coeval ensimatic arc terrane west and outboard of this extending continental margin arc might have represented a fringing oceanic realm that was subsequently collapsed into the outer continental margin of North America in Middle to Late Jurassic time.