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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 95, NO. Bll, PAGES 17,717-17,728, OCTOBER 10, 1990

Magmatic Inclusions in 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 occur in a 21 Ma and precursory non-welded 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, -sanidine-clinopyroxene- orthopyroxene-- latites and 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 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 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 and their 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 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 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 . A Mountain consist of dacitic to andesitic lavas, agglomerates, 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 complex [Lindsey, 1982]. X Ray Fluorescence Analyses, wt % Scattered eruptions of rhyolite and to basaltic 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 , 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 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. A. Lindsey, written communication, 1989). The tuff contains lithic inclusions Major element analyses recalculated to total 100%; LOI, weight of sedimentary rocks entrained as the pyroclastic material loss at 1000°C for 4 hours. Analyses of samples SM-31 and SM-35 from Christiansen et al. [1984]. IT, inclusion in tuff; moved through the vent. Especially in the upper 10 m of IL, inclusion in lava flow; RV, rhyolite lava vitrophyre. 17,720 CHRISTIANSEN AND VENCHIARUTTI: LIMITS ON INFERENCES FROM INCLUSIONS of talus that accumulated at the front of the moving flow into vent areas, could have been a source of lithic [Bikun, 1980]. The rhyolite lava has a maximum known inclusions entrained in the rhyolite during its eruption. thickness of 300 m [Williams, 1963]. Other volcanic rocks that could have yielded lithic The rhyolites of the Spor Mountain Formation are fragments in the rhyolites are all texturally distinctive fluorine- and rare-element rich [Christiansen et al., 1984]. rhyolitic (Joy and Dell Tuffs) or andesitic (Mt. Laird Tuff) Vitrophyres are strongly enriched in Rb (over 1000 ppm), ignimbrites. Nb, Ta, Th, U, and HREE (heavy rare elements) and The porphyritic lavas of the Drum Mountain Rhyodacite strongly depleted in Sr, Ba, Mg, and other trace elements have up to 35% phenocrysts of plagioclase, enstatite, incompatible in feldspars and mafic silicates (Table 1). , magnetite, ilmenite, and accessory zircon and Christiansen et al. [1984] concluded that the high apatite. Although the mineral assemblage in the Drum concentrations of rare elements in the rhyolite evolved by Mountain Rhyodacite is broadly similar to that in the extensive fractional crystallization. Essential minerals in ellipsoidal mafic inclusions (with the exception of sanidine vitrophyres include sanidine, quartz, sodic oligoclase, Fe- in most inclusions), the textures and mineral compositions and F-rich , with accessory fluorite, zircon, apatite, of the two are distinct [Venchiarutti, 1987]. Plagioclase xenotime, and Nb-Ta-Ti-Fe oxides. Eruption temperatures grains in the Drum Mountain Rhyodacite commonly are are estimated by two feldspar thermometry to be around simply zoned from to andesine with sieved 680 °C. Magmatic phenocrysts are accompanied by topaz interiors. In contrast, the larger plagioclase crystals in the and Fe-oxides in devitrified samples. mafic inclusions are complexly and irregularly zoned from andesine to oligoclase and skeletal with large inclusions of matrix up to 1 mm across (Figure 2). Augite and enstatite MAFIC MAGMATIC INCLUSIONS in the Drum Mountain Rhyodacite are Mg-rich compared to in the inclusions, in keeping with the more The lavas and tuffs of the Spor Mountain Formation magnesian character of the Drum Mountain Rhyodacite. contain irregularly shaped ellipsoidal inclusions of Venchiarutti [1987] calculated two temperatures porphyritic mafic material that constitute less than 0.1 % of for the Drum Mountain Rhyodacite of 880 to 925 °C (at the volume of the unit. The inclusions are vesicular and 1000 bars pressure); the mafic inclusions have pyroxene have cuspate margins. They range up to about 1 m in temperatures of 965 to 1010 °C, only slightly lower than diameter. Phenocrysts in the inclusions include irregularly Fe-Ti oxide temperatures of 1025 to 1115 °C for the same zoned plagioclase, augite, enstatite, sanidine, magnetite, samples (Table 2). ilmenite, and accessory apatite and zircon. Quartz is present as embayed, apparently resorbed, grains. The major and trace element compositions of the Drum Representative major and trace element analyses of the Mountain Rhyodacite and the mafic inclusions also differ inclusions are presented in Table 1. Full analytical data substantially. Lavas of the Drum Mountain Rhyodacite and description of methods are given by Venchiarutti range from andesite to dominant , whereas the [1987]. inclusions are latite to dominant (Figure 3). At On the basis of three lines of evidence, we conclude that similar SiO2, concentrations, the inclusions are also depleted the mafic inclusions described here formed by mingling of in Ca, Mg, Ni, and V and enriched in Fe, Na, K, and P silicic and more basic magma: (1) no lava with the compared to the Drum Mountain Rhyodacite. composition or texture of the inclusions was erupted before These differences in mineralogy, texture, and bulk the Spor Mountain Formation, (2) the compositions of the composition conclusively show that the mafic inclusions are mafic inclusions are inconsistent with liquid immiscibility, not lithic inclusions of Drum Mountain Rhyodacite, the (3) the shapes of the inclusions show that they were liquid only known candidate for a source of more mafic volcanic when entrained in the rhyolite, and (4) textures and xenoliths. mineral compositions show that mingling occurred between a hotter mafic magma and a cooler rhyolitic magma. Evidence Against Liquid Immiscibility CHRISTIANSEN AND VENCHIARUTTI: LIMITS ON INFERENCES FROM INCLUSIONS 17,721

Fig. 2. Photomicrographs of magmatic inclusions from the rhyolitic lavas and tuffs of the Spor Mountain Formation, (a) Large phenocrysts of plagioclase with abundant inclusions of fine-grained matrix dominate samples collected from the tuff. Small phenocrysts of enstatite, augite, magnetite, ilmenite, and apatite are set in matrix of acicular pyroxenes and feldspars. (6 mm across) (b) Detail of matrix included in plagioclase phenocryst. Acicular pyroxene and plagioclase crystallized from walls of melt trapped in plagioclase phenocryst. (3 mm across) (c) Ladder-like crystals of pyroxene(?) dominate matrix of a tuff-hosted inclusion. Such extreme crystal morphologies are evidence for undercooling during crystallization [cf., Bacon 1986]. (3 mm across) (d) Detail of matrix of tuff- hosted inclusion shows elongate bow-tie crystal of apatite, with augite and magnetite, in matrix of acicular pyroxene and oxides. (3 mm across) (e) Margin of inclusion from lava illustrates the contrast between the mafic inclusions and the rhyolite lava. Note the coarse rim(R ) of biotite, oxides, and feldspar, interpreted to form by reaction of the mafic inclusion (MI) with vapor released from the cooling rhyolite lava (L). Groundmass of the mafic inclusion consists of same phases surrounding a large plagioclase phenocryst. Note concentration of biotite grains around altered oxide grain. (6 mm across) (/) Detail of groundmass of lava-hosted inclusion. Texture is dramatically different from tuff- hosted inclusions and groundmass is dominated by secondary biotite which has replaced pyroxenes, feldspars, and oxides. (3 mm across.) 17,722 CHRISTIANSEN AND VENCHIARUTTI: LIMITS ON INFERENCES FROM INCLUSIONS

TABLE 2. Mineralogical Contrasts Between the Drum Mountain Rhyodacite and the Mafic Inclusions and Rhyolites of the Spor Mountain Formation

Mineral Drum Mountain Spor Mountain Rhyodacite Formation Mafic Inclusions Rhyolite

Feldspar Plagioclase Ab35An61Or4- Ab^An^Orio- Abg j AngOrj j Ab50An45Or5

Sanidine rare Ab36An3Or61 Ab45An1Or54 64 68 Two-feldspar SiO2 (wt%) Temperature, °C 935 - 1020 690 Fig. 3. IUGS chemical classification of volcanic rocks Pyroxene [LeBas et al., 1986] contrasting the compositions of the Augite En43Fs15Wo42 En35Fs29Wo36 Drum Mountain Rhyodacite and the mafic inclusions found

Hypersthene En^Fs^Wog En58Fs38Wo4 in the rhyolites of the Spor Mountain Formation.

Two-pyroxene Temperature, °C 870-925 965 - 1010 same compositions as those in the rhyolite, as well as fine- Biotite grained fragments of rhyolite matrix and oxidized remnants Fe/(Fe+Mg) 0.96-0.99 of the mafic inclusions. These lenses appear to represent Fe-Ti oxide the material sheared away from the solid margins of some temperature, °C not analyzed 1025-1115 inclusions that intersected shear planes during viscous flow of the lava. Collectively the rounded pillow-like shapes, vesicle Data compiled from Venchiarutti [1987], Bikun [1980], and Christiansen etal. [1984]. patterns, re-entrants of vesicular glass and lava in the mafic Dashes, not present. inclusions in the tuff and in the lava suggest that they were still liquid when entrained in the rhyolite. However, substantial cooling of these inclusions of mafic magma to Morphologic Evidence for Magma Mixing below their solidus temperature occurred before the final emplacement of the rhyolite lava, as evidenced by the Mafic inclusions are found as moderately vesiculated brittle textures found in the brecciated mafic lenses [cf. porphyritic globules in the beryllium tuff member of the Sparks and Marshall, 1986]. Some mafic inclusions also Spor Mountain Formation. Contacts between mafic have angular shapes defined by planar surfaces that truncate inclusions and pumiceous glass (from the lower part of the some plagioclase phenocrysts and vesicles. Many mafic tuff) are smoothly lobate or crenulate. Tuffaceous material inclusions in the lavas not associated with lenses of is infolded into the mafic inclusions along their margins. microbreccia were probably carried along passively, The surfaces of inclusions extracted from weakly between shear planes, in the moving lava. This consolidated tuff are corrugated and ropy. These inclusions combination of evidence for mixing of two liquids and later consist of up to about 20% vesicles up to 3 mm across. brittle deformation of solid mafic inclusions is found in Vesicles occur as trains parallel to the margins of the other examples of magma mixing in viscous lavas [Bacon, inclusions, suggesting that they formed when the inclusions 1986]. took their final shapes. Mafic inclusions in the lava are similar to those in the underlying tuff in phenocryst Petrographic Evidence for Magma Mixing assemblage and texture, as well as general composition. Inclusions in the lava rarely exceed about 20 cm across and Microscopic textures and mineral compositions provide are usually less vesicular than those found in the tuff. evidence consistent with formation by rapid cooling of a Moreover, fine-grained lenses of mafic material stretched mafic magma against a cooler rhyolite magma of the Spor along the flow foliation of the lava emanate from many Mountain Formation. For example, flow-aligned crystals inclusion margins. These brownish lenses are themselves and trachytic fabrics, such as those seen in many lavas flow foliated and are commonly 1 to 2 cm thick and range including those in the Drum Mountain Rhyodacite, are to about 1 m in length. Minerals in these lenses include absent in the mafic inclusions. Fine-grained margins brecciated fragments of those found in the mafic inclusions several millimeters thick occur around many inclusions in as well as those found in the rhyolite; fragments include the tuff suggesting that quenching occurred more rapidly quartz, sanidine, sodic plagioclase, and biotite with the along their margins. Light haloes, 1 to 2 cm wide, CHRISTIANSEN AND VENCHIARUTTI: LiMrrs ON INFERENCES FROM INCLUSIONS 17,723

surround the mafic inclusions in the rhyolite lava. These enrichment of Na and K toward the rims. The large range haloes consist of coarser-grained devitrified rhyolite in plagioclase compositions is consistent with the textural enriched in topaz and oxides compared to rhyolite outside evidence for rapid crystal growth during cooling and of the haloes. We suggest that hot mafic inclusions differentiation of the residual melt. The crystallization of promoted devitrification and vapor release from the rhyolite anhydrous solids quickly led to volatile exsolution to produce these distinctive envelopes. In addition, [Eichelberger, 1980] and the development of vesicle trains abundant skeletal plagioclase with feathery rims occurs in around megacrysts, xenocrysts of rhyolite, and inside re- the mafic inclusions (Figure 2) and may be evidence for entrants. rapid growth from an undercooled feldspar-saturated melt. Mineral thermometry also suggests that significant COMPOSITIONAL CONTRASTS AMONG undercooling would be expected as the mafic magma MAGMATIC INCLUSIONS rapidly cooled against the rhyolite. The crystal-growth The chemical compositions of the mafic inclusions in the experiments of Lofgren [1980] produced similar textures for lava and tuff members of the Spor Mountain Formation are plagioclase grown from melts undercooled by 100 to 150 significantly and systematically different. Two element °C. Groundmasses of the inclusions are dominated by variation diagrams, like those shown in Figure 4, illustrate acicular pyroxenes and dendritic or plumose the fact that although the bulk compositions of the (Figure 2); such textures indicate even greater degrees of inclusions from the two rock types overlap for some undercooling as the groundmasses formed. According to elements (including Si and Zr as well as Ti, K, Ga, and heat transport equations presented by Sparks et al. [1977], Hf), concentrations of other elements are decidedly different even a mafic inclusion as large as 20 cm across would cool for inclusions taken from the tuff compared to those taken to the temperature of the rhyolite (well below the expected from the later erupted lava. For example, the inclusions solidus of the more mafic magma) in less than an hour. from the lava are enriched in Fe, Mn, HREE, Y, Nb, Ta, Plagioclase crystals are commonly rimmed by sanidine in Th, Rb, Cs, and Sc and depleted in Al, Na, P, Eu, Ba, Sr, inclusions found in the tuff and lava. Even in those Pb, and Ni compared to the inclusions in the tuff (Figures plagioclase grains not rimmed by sanidine there is an 4 and 5).

Fig. 4. These variation diagrams show that, for some elements, the compositions of inclusions found in the lavas differ from those found in the tuffs of the Spor Mountain Formation. The selective nature of these enrichments and depletions is inconsistent with simple mixing between a mafic magma and the rhyolite. 17,724 CHRISTIANSEN AND VENCHIARUTTI: LIMITS ON INFERENCES FROM INCLUSIONS

Fig. 5. Ratios of average composition of inclusions found in rhyolite lava (n= 10) to those found in tuff (n=8) of the Spor Mountain Formation. Fig. 6. Chondrite-normalized rare earth element patterns for mafic inclusions and rhyolite lavas of the Spor In other studies of mafic magmatic inclusions in silicic Mountain Formation. Inclusions collected from the rhyolite rocks, compositional differences among inclusions have lava are enriched in all REE (except Eu) compared to those been attributed to at least four different processes: (1) found in the tuff; HREE are enriched more than LREE. mixing of different proportions of rhyolite and mafic magma which created a compositional spectrum in the mafic inclusions, (2) incorporation of distinct and independent mafic magmas in the same silicic magma, (3) zonation in 1984]. We cannot entirely preclude a role for magma the magma chamber from which the mafic inclusions were mixing in producing the latitic to trachytic bulk derived, and (4) selective transport of several elements compositions of the inclusions; we suggest only that the between the mafic and silicic components. differences between tuff-hosted and lava-hosted inclusions A possible explanation for the chemical variation between are not the result of hybridization with rhyolite. the two types of inclusions relies on variable proportions of It is conceivable that the tuff-hosted and lava-hosted hybridization between rhyolite and mafic magma [e.g., inclusions could represent mingling of unrelated mafic Bacon and Metz, 1984], Taken at face value, many of the magmas in the chamber from which the rhyolites erupted. elemental enrichment patterns are broadly consistent with We cannot accept this hypothesis because phenocrysts this mechanism and suggest that inclusions hosted in the (plagioclase, clinopyroxene, and orthopyroxene) in both lava resulted from increased hybridization of mafic magma types of inclusions have the same compositional ranges, with incompatible element rich rhyolite. HREE, Rb, Cs, derived temperatures, and textures. Moreover, the and other incompatible elements enriched in the rhyolite, compositional similarities and trace element patterns of the are also enriched in inclusions in the lava; Sr, Ni, P, and two types of mafic inclusions suggest a strong relationship other compatible elements depleted in the rhyolites are also between them. depleted in the lava-hosted inclusions as compared to those Marshall and Sparks [1984] and Mattson et al. [1986] erupted with the tuff. However, for some of the describe compositionally heterogeneous groups of mafic to incompatible elements (e.g., Rb and REE), the magmatic inclusions in silicic concentrations in the lava-hosted mafic inclusions exceed intrusions. They attributed variations in inclusion those found in rhyolite vitrophyres (Figure 6 and Table 1). compositions to pre-mingling zonation, caused by fractional These vitrophyres are probably end-member compositions crystallization, in a mafic magma body. The compositional for any mixing process. Moreover, for other elements the contrasts between tuff-hosted and rhyolite-hosted inclusions enrichment patterns are the reverse of those expected for a from Spor Mountain cannot be explained by fractional rhyolite-mafic magma mixing relationship. For example, crystallization of the mafic magma. This is shown by the Fe and Mn are enriched in the Rb-rich inclusions taken lack of coherence between the enrichment and depletion from the lava which is itself poor in Fe and Mn but rich in patterns of certain elements. For example, assuming the Rb. Such relationships cannot be the result of any simple differences between the inclusions in the lava and those in mixing process wherein two compositions are physically the tuff are the result of fractional crystallization and that mixed. In such a case, the final composition of the hybrid Rb behaved as a completely incompatible element, about is a simple function of the proportions of the two end- 70 % of the inclusion-forming magma (as represented by the members in the mixture and lies on a linear mixing line average composition of inclusions in the tuff) must have between the two end-members [e.g., Bacon and Metz, crystallized to produce the higher Rb concentrations in the CHRISTIANSEN AND VENCHIARUTTI: LIMITS ON INFERENCES FROM INCLUSIONS 17,725 lava-hosted inclusions. However, fractionation of 70% explained by vapor exchange between the rhyolite lava and plagioclase, Fe-rich pyroxenes, and Fe-Ti oxides in the the mafic inclusions and the formation of a new stable proportions observed in the inclusions would simultaneously mineral assemblage. With such a model, element cause substantial depletions of Fe, Mn, and Ca as well as concentrations are not the result of simple physical mixing enrichment of SiO2. In contrast to this prediction, Fe, Mn, or hybridization of two components but they instead depend and Ca are enriched in the lava-hosted inclusions (Figure 5) on the partitioning of elements into the water- and fluorine- and the range of SiO2 concentrations is the same in both rich vapor, its subsequent reaction with the inclusions to types of inclusions. We are not suggesting that the mafic form new mineral assemblages, and the affinity of component that mingled with the rhyolite was entirely transported elements for the newly formed minerals. For homogeneous; in fact, the covariation of SiO2 and Zr example, it is difficult to explain the low P2O5 and Sr (Figure 4) suggests that the mafic magmas may have been concentrations of the inclusions in the lavas by zoned and the rhyolite lava and tuff sampled the same unidirectional transport from rhyolite to mafic inclusions. zoned sequence. Phosphorous concentrations should be very low in vapors We suggest that the compositional differences between the [London et al.9 1988] released from the cooling P2O5-poor two groups of inclusions can be explained by selective lava, but the five-fold decrease in P2O5 in the inclusions in exchange of elements after rhyolite and mafic magma the lava compared to those in the tuff implies that mingled. Rhyolitic volcanic rocks, lavas and tuffs, phosphorous was also transported out of the mafic typically release water and fluorine rich (in the case of these inclusions as apatite was destroyed. rhyolites) vapors that eventually crystallize or react with In the present case, the elemental differences between the pre-existing minerals to form crystal lined vesicles, two types of inclusions cannot be explained by continued lithophysae, crystal coated fractures, and sprays of crystals thermal diffusion between mafic liquid and the cooler disseminated in the groundmass. In rhyolite lavas, quartz, rhyolite. In thermal diffusion experiments, Fe and Mn topaz, fluorite, feldspar, Fe-oxides, Mn-oxides, Mn-rich concentrate at the cold ends of diffusion couples and behave garnet, beryl, and cassiterite may crystallize from vapor. antithetically to the alkalies [Lesher, 1986]. In contrast, These vapor-phase minerals demonstrate that many elements lava-hosted mafic inclusions are enriched in Fe, Mn, Rb, are transported in vapors exsolved from rhyolite as it cools and Cs, suggesting that other processes, probably acting on and crystallizes. Bacon [1986] also noted that vapor-phase already solid mafic inclusions, produced the observed crystallization occurs in voids within mafic inclusions. differences. Likewise, chemical diffusion between co- These empirical observations are corroborated by existing silicic and mafic melts produces progressive experiments using vitrophyres from Spor Mountain enrichments of P, Fe, and REE in the mafic melt until [Webster et al., 1989] and the similar Macusani glasses equilibrium is reached [Watson, 1976; 1982]. These are [London et aL, 1988] that yield partition coefficients near not the differences observed between the two inclusion one for the exchange of elements between rhyolite melt and types. Apparently, rapid thermal equilibration and vapor. Only slight incongruent partitioning is suggested by consequent solidification of the mafic inclusions prevented these experiments; London et al. [1988] noted nearly substantial exchange of elements driven by thermal or complete extraction of Fe from macusanite melt in chemical diffusion across melt interfaces. equilibrium with vapor but attributed this to uptake in the Figure 7 summarizes our notion of how magma mixing platinum capsules used to contain the experimental charges. occurred in the rhyolites of the Spor Mountain Formation. We suggest that element exchange between the rhyolite and (1) Mafic magma with a temperature of about 1000 °C was mafic inclusions was accomplished by vapor-phase transport entrained into cooler, about 680 °C, rhyolite magma and of some elements and the reaction of this vapor with the disrupted into small ellipsoidal bodies. Because of their mafic inclusions. This process selectively enriched or small size, these mafic magmatic inclusions cooled rapidly depleted several elements. The lava-hosted inclusions to the temperature of the rhyolite. Narrow quench rims appear to have been affected the strongest by this process* formed on the mafic inclusions. Crystallization occurred The textures of the mafic inclusions in the Spor Mountain with several hundred degrees of undercooling to form lavas are consistent with the transport of some elements in distinctive phenocryst and then groundmass morphologies vapors that reacted with the mafic inclusions after eruption. in the mafic inclusions. Vesiculation occurred when the The groundmasses of mafic inclusions in the lavas are mafic inclusions became fluid saturated as a result of the altered to secondary assemblages that contain variable crystallization of anhydrous minerals. Because of their amounts of biotite (low TiO2; less than 1.9%), fluorite, small size and the large temperature contrast, most hematite, and Mn oxides in contrast to the glass, pyroxene, inclusions became solid and mechanically brittle in less than and magnetite found in inclusions analyzed in the tuffs one hour. (2) Eruption of the tuff shortly after (Figure 2). Inclusions in the lava are also surrounded by entrainment [Venchiarutti, 1987] allowed some of these light-colored haloes enriched in acicular sprays and inclusions to be preserved more-or-less intact. (3) irregular clots of groundmass topaz interpreted to be the Inclusions erupted in the lava experienced pronounced result of vapor phase crystallization, described above. The chemical modification that selectively mobilized some transport of Fe, Mn, Rb, and other elements is most likely elements and altered the compositions and groundmass 17,726 CHRISTIANSEN AND VENCHIARUTTP. LIMITS ON INFERENCES FROM INCLUSIONS

magma (Figure 8). In this case, only limited mechanical mixing occurred after chemical modification. On the other hand, if similar processes of selective chemical modification followed by substantial mechanical mixing occur in plutonic environments and produce hybrid rocks, compositional inferences and petrogenetic models based on the elemental, isotopic, or mineralogical composition of the mafic inclusions in such granitoids are suspect (Figure 8). Our observations suggest several limitations for inclusions found in plutonic rocks.

Fig. 7. Schematic model of magma mixing process at Spor Mountain, Utah. Small pillows of mafic magma became entrained in the rhyolite and rapidly cooled to the temperature of their silicic host. Crystallization and vesiculation resulted and were probably complete by the time of eruption. Chemical modification of inclusions in the lavas occurred after solidification of the mafic inclusions as elements were selectively mobilized by vapor- phase processes continuing during and after eruption. Chemically modified inclusions were then mechanically disaggregated and mixed with the rhyolite during flow. Less Mobile Component Fig. 8. Contrasted models for the physical and chemical mixing of magmas in plutonic environments shown on a textures of the lava-hosted mafic inclusions. Vapor-phase schematic variation diagram. Simple mechanical mixing of exchange was the most likely mechanism of transport. (4) composition A (a mafic magma) and B (a silicic magma) Eruption of these now solid inclusions in lava flows would produce the linear trend shown. Studies of the resulted in very limited mechanical mixing of the two magmatic inclusions in the Spor Mountain Formation components where the inclusions were cut by shear planes suggest that selective chemical modification of a mafic in the flowing lavas. One component was brittle magma can occur before it becomes physically mixed with (chemically modified mafic inclusions) and the other still the silicic magma. viscous (largely unmodified rhyolite lava) during mixing.

IMPLICATIONS FOR COMPOSITIONAL STUDIES 1. Even if mafic inclusions were formed by magma OF INCLUSIONS IN GRANITES mixing, the compositions of the inclusions may not accurately reflect the isotopic or elemental compositions of From the observations summarized above it is obvious their sources or the processes by which they evolved. that even in volcanic environments where mingled magmas 2. Common practice in studies of plutonic rocks has been are erupted and cooled rapidly, significant changes in the to define magma mixing end-members as points lying near composition and mineralogy of the mafic end-member can the ends of more or less linear trends on two-oxide occur before cooling inhibits continued exchange of variation diagrams. If modification of the mafic magma chemical components between the two divergent occurred before physical mixing, the original composition compositions. In the case of the distinctive F- and of the mafic inclusions may not lie on mixing trends at all. incompatible-element rich Spor Mountain rhyolite, these 3. The observation that most mafic inclusions in granitic changes in bulk composition of mafic inclusions can be rocks consist mostly of hydrous minerals (hornblende readily detected. Moreover, the compositional changes that and/or biotite) whose compositions are similar to those in occurred in the mafic inclusions were not the result of the host granite [Vernon, 1984; Furman and Spera, 1985; simple mixing processes. Instead, components were Reid and Hamilton, 1987; Dodge and Kistler, 1988; selectively exchanged between the mafic and the Bedard, 1988] is itself illuminating. Apparently, components and the resultant compositions do not lie on compositional contrasts between inclusions and silicic linear mixing arrays between the voluminous felsic magma magmas were obliterated as the mafic materials equilibrated and the smaller amount of injected or entrained mafic with their lower temperature and compositionally distinctive CHRISTIANSEN AND VENCHIARUTTI: LIMITS ON INFERENCES FROM INCLUSIONS 17,727

silicic hosts. Moreover, few basic magmas have crystallization of the mafic globules; mafic inclusions compositions that could crystallize to form assemblages of would be left with a cumulate signature and hybrids would biotite and hornblende without substantial prior be substantially different from simple mixtures of the modification as modeled in the experiments of Johnston and originally introduced mafic magma and the silicic magma. Wyllie [1988]. In the case of the Spor Mountain Formation, hydrous mineral phases developed in mafic Acknowledgments. We thank M. D. Druecker, D. Tingey, and D. V. inclusions as a result of sub-solidus interaction with the Walsh for technical assistance in obtaining electron microprobe and x-ray fluorescence analyses. Discussions with M. Sheridan, D. Burt, M. rhyolite host. Such processes of hydration and Reagan, T. Foster, and G. McCormick clarified our thinking regarding recrystallization are also likely for magmatic inclusions in magma mixing. The comments of D. A. Lindsey and an anonymous slowly cooled plutonic rocks, perhaps accounting for some reviewer were also helpful. Permission of Brush-Wellman Inc. to study of the "metamorphic" textures described in these inclusions. and sample outcrops on their property is appreciated. This work was In addition, Debon [1980] and Letterier and Debon [1978] supported in part by funds from the University of Iowa and NSF grant EAR-8618323. concluded that metasomatic reactions were important in determining the compositions of inclusions in granites. REFERENCES Due to the prolonged cooling history of plutonic rocks, selective modification of mafic magmatic inclusions, as Bacon, C. R., Magmatic inclusions in silicic and opposed to simple mixing, is probably more common than intermediate volcanic rocks, J. Geophys. Res., 91, 6091- in volcanic environments. Mafic inclusions in granitic 6112, 1986. plutons may provide evidence of mingling of disparate Bacon, G. R., and J. 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