Chapter 10: Mid-Miocene volcanic and plutonic rocks in the Lake Mead area of Nevada and Arizona; Production of intermediate igneous rocks in an extensional environment
Published:January 01, 1990
Eugene I. Smith, Daniel L. Feuerbach, Terry R. Naumann, James G. Mills, 1990. "Chapter 10: Mid-Miocene volcanic and plutonic rocks in the Lake Mead area of Nevada and Arizona; Production of intermediate igneous rocks in an extensional environment", The Nature and Origin of Cordilleran Magmatism, J. Lawford Anderson
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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.