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Journal of Petrology | Volume 29 | Number 3 | Pages 527-557 | 1988
© Oxford University Press 1988

research-article

Open System Magmatic Evolution of the Taos Plateau Volcanic Field, Northern New Mexico: 3. Petrology and Geochemistry of Andesite and Dacite

NANCY J. McMILLAN* and MICHAEL A. DUNGAN

Department of Geological Sciences Southern Methodist University, Dallas, TX 75275

Received September 30, 1987; ABSTRACT

Calc-alkaline olivine andesite and two-pyroxene dacite of the Taos Plateau volcanic field evolved in an open magmatic system. mg-numbers of spatially and temporally associated Servilleta Basalt (54–61) and ohvine andesite (49–59) are comparable and preclude fractional crystallization of ferromagnesian minerals as the major differentiation process. If Servilleta olivine tholeiite is assumed to be the parental magma type, enrichments of highly incompatible trace elements (up to 17 ×) oVer concentrations in the basalts require that andesitic and dacitic magmas contain a substantial proportion of assimilated crust. Isotopic compositions of andesite and dacite, which have slightly higher 87Sr/86Sr ratios than the basalts but lower 143Nd/144Nd, 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios, are consistent with contamination of parental basalt by old, low Rb/Sr, low U/Pb, and low Th/Pb continental crust. Concentrations of highly incompatible trace elements in andesite and dacite lavas are decoupled from major element compositions; the highest concentrat ions of these elements occur in andesitic, rather than dacitic compositions, and andesite lavas are more variable in trace element contents. Assimilation of heterogeneous crust concurrent with fractional crystallization of varying mineral assemblages could cause this decoupled behavior. High mg-numbers in andesite and dacite, skeletal olivine phenocrysts, and reversely zoned pyroxene phenocrysts are manifestations of mafic replenishment and magma mixing in the Taos Plateau magmatic system.

Taos Plateau volcanoes are monolithologic and are distributed in a semi-concentric zoned pattern that is a reflection of the complex subvolcanic magmatic system. A central focus of basalt shields developed above the main basaltic conduit system; these magmas contain 10–35% admixed andesitic and dacitic magma. Basalt shields are surrounded by a partial ring of olivine andesite shield volcanoes, where replenishment of basaltic magma provided the heat necessary for prolonged assimilation of crust, resulting in intermediate-composition lavas. Dacite shields are located around the periphery of the more mafic volcanoes and reflect a decrease in mafic input on the fringes of the magmatic system.


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