Abstract. The steeply tilted Mount Perkins block, northwestern Arizona, exposes a cross section of a magmatic system that evolved through the onset of regional extension. New 4øAr/39Ar ages of variably tilted (0-90 ø) volcanic strata bracket extension between 15.7 and 11.3 Ma. Preextensional intrusive activity included emplacement of a composite Miocene laccolith and stock, trachydacite dome complex, and east striking rhyolite dikes. Related volcanic activity produced an -18-16 Ma stratovolcano, cored by trachydacite domes and flanked by trachydacite-trachyandesite flows, and -16 Ma rhyolite flows. Similar compositions indicate a genetic link between the stratovolcano and granodioritic phase of the laccolith. Magmatic activity synchronous with early regional extension (15.7-14.5 Ma) generated a thick, felsic volcanic sequence, a swarm of northerly striking subvertical rhyolite dikes, and rhyolite domes. Field relations and compositions indicate that the dike swarm and felsic volcanic sequence are cogenetic. Modes of magma emplacement changed during the onset of extension from subhorizontal sheets, east striking dikes, and stocks to northerly striking, subvertical dike swarms, as the regional stress field shifted from nearly isotropic to decidedly anisotropic with an east-west trending, horizontal least principal stress. Preextensional trachydacitic and preextensional to synextensional rhyolitic magmas were part of an evolving system, which involved the ponding of mantle-derived basaltic magmas and ensuing crustal melting and assimilation at progressively shallower levels. Major extension halted this system by generating abundant pathways to the surface (fractures), which flushed out preexisting crustal melts and hybrid magmas. Remaining silicic melts were quenched by rapid, upper crustal cooling induced by tectonic denudation. These processes facilitated eruption of mafic magmas. Accordingly, silicic magmatism at Mount Perkins ended abruptly during peak extension -14.5 Ma and gave way to mafic magmatism, which continued until extension ceased.

Middle Tertiary volcanic rocks of the Lake Mead field are calc-alkalic to alkalic-calcic and vary continuously in composition from basalt to rhyolite. These volcanic rocks formed during Basin-and-Range extension and are spatially and genetically associated with diorite-to-granite intrusions of the Wilson Ridge pluton. Locally, igneous rocks were subjected to potassium metasomatism.Field relations and petrography provide evidence of disequilibrium mineral assemblages and liquid-liquid mixing of basalt and granite magmas to form the intermediate rock types of the Lake Mead volcanic field. Evidence of mixing includes incompatible phase assemblages of euhedral olivine, embayed quartz, and sodic plagioclase within andesite flows. Plagioclase occurs in rounded and partially resorbed clusters of equant crystals and commonly displays oscillatory zoning and outer glass-charged zones (fretted texture). Quartz phenocrysts are commonly surrounded by rims of prismatic augite and glass. Fine-grained spheroidal to ellipsoidal inclusions of basalt are common in dacite flows and dikes. Thus, various mixing ratios of olivine basalt and granite end members may be responsible for the textural variations observed in volcanic rocks of the Lake Mead field.The evolution of the igneous rocks of the Lake Mead field was evaluated by petrogenetic models involving both crystal fractionation and magma mixing. These processes may have operated together to produce the compositional range in volcanic and plutonic rocks of the Lake Mead area. Open-system models provide estimates of the relative importance of the two processes and suggest that mixing was more important in the derivation of andesite and diorite (mass mixed component/mass crystallizing phase R = 0.8 to 2.2) than dacite, quartz monzonite, or granite (R = 0.1 to 0.65).The calc-alkaline-alkalic-calcic nature of the igneous rocks of the Lake Mead field, and possibly other similar rock suites that formed in the Great Basin during regional extension, may result from magma mixing. Basalts and rhyolites may mechanically mix in various proportions to produce intermediate rock types. The classic bimodal assemblages may only occur where mixing is incomplete or in structural situations where different magma types cannot mix. Smith, E. I., Feuerbach, D. L., Naumann, T. R., and Mills, J. G., 1990, Mid-Miocene volcanic and plutonic rocks in the Lake Mead area of Nevada and Arizona; Production of intermediate igneous rocks in an extensional environment, in Anderson, J. L., ed., The nature and origin of Cordilleran magmatism: Boulder, Colorado,

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