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Magmatic Inclusions in Rhyolites of the Spor Mountain Formation, Western Utah: Limitations on Compositional Inferences from Inclusions in Granitic Rocks

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Magmatic Inclusions in Rhyolites of the Spor Mountain Formation, Western Utah: Limitations on Compositional Inferences from Inclusions in Granitic Rocks JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 95, NO. Bll, PAGES 17,717-17,728, OCTOBER 10, 1990 Magmatic Inclusions in Rhyolites of the Spor Mountain Formation, Western Utah: Limitations on Compositional Inferences From Inclusions in Granitic Rocks ERIC H. CHRISTIANSEN Department of Geology, Brigham Young University, Provo, Utah DANIEL A. VENCHIARUTTI Department of Geology, University of Iowa, Iowa City Inclusions of quenched mafic magma occur in a 21 Ma rhyolite lava and precursory non-welded tuff that form the Spor Mountain Formation in west-central Utah. The mafic inclusions are not lithic inclusions; no comparable volcanic unit was present at the surface when the Spor Mountain Formation erupted. Mineral and bulk compositions preclude liquid immiscibility. The mafic inclusions show clear morphologic and textural effects of magma mingling shortly before eruption of the rhyolite. Globular inclusions from both units are vesicular, phenocryst-poor, plagioclase-sanidine-clinopyroxene- orthopyroxene-magnetite-ilmenite latites and trachytes with quench temperatures of about 1000°C. Although overlapping in SiO2, TiO2, Zr, and Hf concentrations, inclusions from the underlying tuff lack negative Eu anomalies and are enriched in P2O5, K2O, A12O3, Sr, Pb, Cr, and Ni whereas those hosted in the overlying lava have small negative Eu anomalies and two-fold enrichments in Fe2O3, MnO, HREE, Y, Ta, Th, Rb, and Cs. The most reasonable explanation for the differences between the two sets of inclusions lies in selective chemical exchange between the rhyolite lava and the mafic inclusions after eruption. Limited mechanical mixing occurred after the inclusions solidified and became chemically modified. The textures of the inclusions in the lavas and the elements selectively mobilized in the inclusions imply that vapor-phase transport occurred in this low-pressure volcanic environment. If such substantial variations in inclusion compositions can arise during what must have been a short period of time before chemical reactions were halted by rapid cooling, it seems unlikely that the compositions of mafic inclusions formed by magma mingling in slowly cooled granites preserve their original compositions, mineralogies, or information about their ultimate sources. Using the compositions of such chemically modified inclusions as end-members for mixing calculations may lead to erroneous results regarding the significance of magma mixing in plutonic rocks. INTRODUCTION us the opportunity to study the early stages of interaction between mafic magmas and their silicic hosts. The presence of inclusions composed of mafic minerals In this report we describe the evidence for mingling of in granites is often interpreted as evidence for mingling of contrasting magmas in an early Miocene rhyolite erupted in mafic magma with granite before the granite became fully the northern portion of the Basin and Range province of the consolidated [e.g., Pabst, 1928; Vernon, 1984]. Extreme western United States. Mafic (sensu lato) magmatic versions of this hypothesis hold that the chemical variability inclusions in the rhyolites preserve compositions and seen in some plutons is the result of the mixing or textures that provide evidence for selective exchange of hybridization of well-defined mafic and silicic end-members some elements before physical mixing of the two [e.g., Kistler et al., 1986]; the mafic end-member components occurred. These observations limit the represented in some cases by the compositions of mafic usefulness of compositional data obtained from mafic inclusions [e.g., Reid et al., 1983]. The preservation of inclusions in granitic rocks to define end-members for mafic magmatic inclusions in rhyolitic volcanic rocks gives magma mixing models or to infer the ultimate sources of the inclusions. Copyright 1990 by the American Geophysical Union. GEOLOGIC SETTING Paper number 89JB02957 The rhyolite lavas and tuffs of the early Miocene Spor 0148-0227/90/89JB-02957$05.00 Mountain Formation (Figure 1) are exposed in what is now 17,717 17,718 CHRISTIANSEN AND VENCHIARUTTI: LIMITS ON INFERENCES FROM INCLUSIONS Fig. 1. Generalized geologic map of the region around Spor Mountain, Utah [after Lindsey, 1979]. Sample localities are show by diamonds with sample numbers. The prefix M or SM is not shown. CHRISTIANSEN AND VENCHIARUTTI LIMITS ON INFERENCES FROM INCLUSIONS 17,719 the Great Basin, the northern part of the Basin and Range the tuff, carbonate lithic inclusions were altered to colorful province. The Cenozoic volcanic history of the Great Basin nodules of silica minerals, fluorite, clay, manganese oxides, was reviewed by Best et ah [1989] and of the area around and bertrandite during mineralization of the tuff. Spor Mountain by Lindsey [1982] and Shawe [1972]. The rhyolite lavas, mapped as the porphyritic rhyolite Cenozoic magmatism in the northern Great Basin began member of the Spor Mountain Formation [Lindsey, 1979], about 42 Ma with the emplacement of a calc-alkaline overlie the beryllium tuff member in most places but locally sequence of intermediate-composition lavas, ash flows, and the tuff is absent and the lava rests directly on Paleozoic small intrusions. The oldest volcanic rocks near Spor sedimentary rocks or the Drum Mountain Rhyodacite. A Mountain consist of dacitic to andesitic lavas, agglomerates, breccia zone, containing blocks of vitrophyre, is present at and ash flow tuffs. The Drum Mountain Rhyodacite, a the base of the rhyolite lava in several open-pit beryllium prominent member of this association, erupted to form a mines. This breccia is interpreted as an over-ridden apron stratovolcano with peripheral lava flows. Lindsey [1982] reports a fission track age on zircon of 42 Ma for this unit. At 38 Ma (Joy Tuff) and again at 32 Ma (Dell Tuff) TABLE 1. Representative Chemical Analyses of Inclusions rhyolitic ash flows were erupted. Most of the tuffs are and Rhyolites, Spor Mountain Formation, West Central Utah found only as relatively thin remnants of intracaldera deposits. After an 11 m. y. lull in magmatic activity in the Sample M 39 M 43 M 32 SM 8101 SM 31 SM35 region, the 21 Ma Spor Mountain Formation erupted on the western margin of the Caldera complex [Lindsey, 1982]. X Ray Fluorescence Analyses, wt % Scattered eruptions of rhyolite and basalt to basaltic andesite Rock type IL IL IT IT RV RV lavas occurred in this part of the eastern Great Basin after SiO2 61.4 61.1 60.1 58.9 75.0 75.5 about 10 Ma, including the eruption of the Topaz Mountain TiO2 0.97 0.90 0.85 1.23 0.06 0.06 Rhyolite in the adjacent Thomas Range and Keg Mountains. A12O3 16.5 16.5 17.8 17.0 13.5 13,4 Fe2O3 7.13 8.06 5.28 6.97 1.51 1.46 High-angle block faulting typical of Basin and Range MnO 0.14 0.17 0.08 0.11 0.07 0.06 extension formed between 21 and 7 Ma. MgO 1.12 0.56 2.35 1.48 0.05 0.08 The rhyolites of the Spor Mountain Formation take their CaO 3.51 4.11 3.98 5.19 0.62 1.30 Na2O 3.76 3.44 3.16 3.95 4.11 4.42 name from Spor Mountain which is a block of tilted and K2O 5.30 5.15 5.83 4.45 5.10 3.73 intricately faulted lower and middle Paleozoic sedimentary P2O5 0.18 0.07 0.43 0.65 0.00 0.00 rocks composed chiefly of carbonates (Figure 1). LOI 0.90 0.19 1.88 2.39 2.96 3.55 Numerous, relatively small plugs, dikes, and breccia pipes of Spor Mountain Formation rhyolite intruded the X Ray Fluorescence Analyses, ppm Rb 1306 1505 520 225 970 1060 sedimentary sequence. Post eruption basin-and-range Sr 440 400 640 590 5 5 faulting has complicated the structure making it difficult to Y 200 260 90 42 92 135 estimate the number of vents involved. Lindsey [1979] Zr 510 470 510 535 99 120 identified at least 11 vents, including breccia pipes. Nb 50 140 48 58 90 120 Eruptions of the Spor Mountain Formation commenced V 36 32 25 35 5 5 Cu 1 5 2 2 6 6 with emplacement of a series of ignimbrites, pyroclastic fall Zn 320 260 1200 82 60 60 deposits, and pyroclastic surge deposits, and ended with Ga 21 28 21 16 50 50 extrusion of rhyolite lavas over the tuff [Bikun, 1980]. The Ba 1512 877 1239 1210 25 25 tephra deposits and local accumulations of tuffaceous Pb 26 30 64 - 30 30 sandstone and conglomerate have been mapped as the Instrumental Neutron Activation Analyses, ppm beryllium tuff member of the Spor Mountain Formation Sc 15 10 9 11 2.6 2.7 [Lindsey, 1979]. The pyroclastic rocks overlie Paleozoic Cs 72 87 36 67 55 58 sedimentary rocks, the Drum Mountain Rhyodacite, and the La 126 74 94 92 60 59 fluvial sediments of the Spor Mountain Formation Ce 200 144 131 167 144 137 mentioned above. The upper part of the tuff was Nd 97 78 84 78 52 51 Sm 20 19 13 14 18 hydrothermally altered and mineralized (Be-U-F-Li-Mn) by 13 Eu 3.3 2.2 3.5 3.3 0.2 0.1 fluids trapped beneath the impermeable lava cap [Lindsey, Tb 3.9 3.7 1.6 1.5 3.3 3.4 1977; Burt and Sheridan, 1981]. Alteration mineral Yb 13.9 18.3 7.2 3.3 15.6 15.7 assemblages in the tuff include smectite resulting from Lu 2.0 2.5 1.1 0.5 2.5 2.6 incomplete argillization of glass, local sericite, and Hf 11 12 10 10.6 6.2 7.1 secondary potassium feldspar [Lindsey, 1977]. The tephra Ta 5.9 11 5.9 3.7 26 25 Th 40 53 18 21 64 69 reaches a thickness of almost 100 m; a central welded zone U 20 12 21 5.6 36 38 is developed in thick sections [Williams, 1963] and basal zones at a few localities (D.
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