JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 94, NO. B5, PAGES 6041-6058, MAY 10, 1989 Evolution of a Chemically Zoned Magma Body' Black Mountain Volcanic Center, Southwestern Nevada THOMAS A. VOGEL Department of Geological Sciences, Michigan State University, East Lansing DONALD C. NOBLE Department of Geological Sciences, University of Nevada at Reno LELAND W. YOUNKER Earth ScienceDepartment, Lawrence Livermore National Laboratory, Livermore, California Rocks of the Black Mountain volcanic center consist of four ash flow sheets and units of lava that underlie,interfinger with, and overlie the sheets. Rocksfrom the center represent three magma types. Magma type c was present throughthe history of the center, whereastypes a and b were available after the eruption of the Rocket Wash Member, during the eruptionsof the Pahute Mesa and Trail Ridge members. The magma types are defined by trace elementratios;for example, magmatypes a, b, and c have La/Th values of 1.0-3.5, >7.5, and 3.5-7.5. Silica contents in the magma typesa, b, and c range from 71.5 to 74.1, from 65.8 to 69.2, and from 55.6 to 73.8 wt %, respectively. The stratigraphic distribution of chemically distinct pumice fragmentswithin the ash flow sheetsis used to show that magmatype a was locatedin the uppermost part of the chamberand was underlain successively by magma types b and c. Because pumice fragments that belong to all three magma types occur in individualcooling units, a zoned magma body musthave existedduringthis period. Magma mixing is indicatedby the disequilibrium phenocrysts which are commonin pumice fragments from all magma types; however, this mixing did not destroythe originalzoningof the upper part of the magmabody. Most of the chemicalvariation of magma type c is consistent with fractionationof feldspar, olivine, andpyroxene,but abundant disequilibrium, maficphenocrysts indicatethat magma replenishment and mixing were common. Magma type b had much higher La/Th and light rare earth element (LREE)/heavy rare earth elementvaluesand must have originated independently from magmatype c. Most likely the two typeswere derivedfrom differentsource material. The low La/Th valuesof magma type a can be explainedby separation of a phenocrystassemblage containingboth a LREE-bearing phaseand zircon from either magma types b or c, or possibly by the partial meltingof sourcematerial containing these phases. INTRODUCTION Zoned magmabodieshave been inferred for many alkaline [cf. Wolff and Storey, 1984] and silicic [cf. Smith, 1979; Hildreth, 1981; Mahood, 1981; Ferriz and Mahood, 1987] volcanic centers. Most of these studies have evaluated compositionalzoning by studying the compositional varia- tion in a single pyroclastic unit or a series of pyroclastic units. This study evaluatescompositional zoning in a magma body by studying a sequenceof ash flow sheets and inter- fingering lava flows from the Black Mountain volcanic center. This center is part of the middle to late Miocene volcanic field of southwestern Nevada. It is one of the youngest, best preserved and exposed, and petrologically most interesting caldera complexes in the western United States. The Black Mountain volcanic center representsthe systematic evacu- ation of a compositionally layered magmabody. The center consists of a number of zoned ash flow sheetsthat, in large part, were erupted over a very short period oftime [Noble et al., 1984] and are associated with a number of units of lava of widely varying composition. These eruptive products appear to provide a nearly complete sampling of the magma compositions during the evolution of the magma body and Copyright 1989by the American Geophysical Union. Paper number 88JB03529. 0148-0227/89/88JB-03529505.00 have a large range of chemical compositions that fall into three discrete compositionalgroups. In this study, more emphasis is placed on the ash flow sheets becauseeach sheet representsan instantaneous sam- pling of the upper part of the magma body, where most of the chemicallayering occurs [Smith, 1979; Baker and McBirney, 1985]. Because of the short time periodsbetween eruption of some of the ash flow sheets at the Black Mountain volcanic center [Noble et al., 1984], we infer that the magma body was nearly continuously erupted from the highly evolved top, down to the less evolved dominant volume. The ash flow sheets sampledthe upper portion of the Black Mountain magma body, where a distinct chemical layering developed during the middle period of the evolution of the magma system. In contrast, the lavas apparently sampled the lower portions of the Black Mountain magma body (see below). Thus a study of the lava samplesand pumice samplesfrom the ash flow sheets allows us to evaluate the evolution of a large portion of the Black Mountain magma body, much of which was erupted over a short time. BLACK MOUNTAIN VOLCANIC CENTER The Black Mountain volcanic center was mappedby R. L. Christiansen and D.C. Noble, largely during 1963 [Chris- tiansen and Noble, 1968; Noble and Christiansen, 1968]. Summariesof the geology of the center have been given by Noble et al. [1964], Christiansen and Noble [1968], Noble 6041