Origin of and : Evidence of mixing at Glass in California and at other circum-Pacific volcanoes

JOHN C. EICHELBERGER* Department of , Stanford University, Stanford, California 94305

ABSTRACT subtracting appropriate proportions of appropriate phases from a hypothetical parent . Likewise, by choosing an appropriate The intimate association of , andesite, dacite, and hypothetical source and adjusting conditions, nearly any ob- within a volcanic center suggests that these rocks are genetically served composition could be produced by . The test related. Individual flows that show a gradation in composition of such models is whether they agree with the phase assemblages in may preserve maximum evidence of the magmatic processes pro- . It is often impossible to make this evaluation for intrusive ducing this association. One such flow of rhyolite to dacite compo- rocks because it is difficult to look back through the sition, Glass Mountain in northern California, was formed by con- process to the magmatic stage. However, fresh volcanic rocks rep- tamination of rhyolite as it intruded the basaltic flows of resent quenched magma samples in which the liquid remains as a the Medicine Lake Highland shield . Although dacite flows metastable glass or finely crystalline groundmass, and the and domes commonly show less variation in composition than the phases remain as . Although it cannot be assumed that Glass Mountain flow, many show similar evidence of contamina- volcanic rocks are representative of all igneous rocks, they do allow tion by basalt by the presence of abundant basaltic inclusions and interpretation of magmatic processes with a minimum of assump- phenocrysts and clots from those inclusions. Similarly, tions. many andesite flows contain rhyolitic inclusions, rhyolitic bands, The association of rocks of different composition at a volcano and phenocrysts appropriate to rhyolite. These observations indi- suggests a genetic relationship. If magmas evolve from one another, cate that andesite and dacite are hybrid rocks that are formed when it is reasonable to expect that some volcanic products will show rising primary basalt and rhyolite magmas either become contami- gradations in composition, thereby preserving evidence of funda- nated with the glassy debris of the volcanic pile or mix with each mental petrogenetic processes. Zonation has been studied in ash other directly. Linear variation in bulk composition, phenocryst sheets, but these are formed when magma is disaggregated at the assemblages of intermediate rock, and frequency distribution of vent and reassembled elsewhere, producing serious obstacles to lava compositions in the southern , Chilean , interpretation. The ideal object of study is a compositionally zoned , and Tongan support this hypothesis. lava flow in which the original zonation of the magma body can be It appears that partial melting usually produces magma of rhyolitic inferred from the mode of flow emplacement. Further requirements and basaltic compositions and that any subsequent fractional crys- are lack of chemical alteration and complete exposure of the flow. tallization is of limited importance. Key words: igneous , These requirements are fulfilled by the Glass Mountain lava flow, contamination of magma, mixing of magma, basalt, andesite, da- Medicine Lake Highland, California. cite, rhyolite. METHOD INTRODUCTION A volcanic unit represents a rearrangement of material that was The basalt-andesite-dacite-rhyolite series of volcanic and chemi- previously beneath the 's surface. A key problem is to dis- cally equivalent plutonic rocks is a particularly perplexing problem cover the mode of flow emplacement in order to infer the orienta- in . Lava ranging in composition from basalt to tion of compositional gradients within the magma prior to erup- rhyolite can be erupted from the same volcano or found within the tion. This was accomplished at Glass Mountain by mapping flow same plutonic complex, and rocks of differing composition com- fronts, lava streams, levees, and shear zones and by observing pres- monly occur in apparently random sequences. Models that have sure ridge patterns on aerial photographs. All mappable features of been proposed to account for the series include crystal-liquid frac- the flow were recorded. These were phenocryst content, inclusion tionation of parental basalt, andesite, or both; production of the content, glossiness of the glass, and amount of surface . entire suite by partial melting; and magma contamination or mix- Lava streams were then sampled at regular intervals from distal end ing. Experimental studies have shown how these processes could to vent to obtain a suite of samples representing the eruptive se- operate. Much attention has been focused on the content quence. of coarse-grained plutonic rocks and on the bulk composition of Phases within selected samples were analyzed on an ARL-EMX volcanic and plutonic rocks, but these data do not necessarily yield electron microprobe, using an accelerating potential of 15 kv and information on conditions and processes prior to final crystalliza- sample current of 0.03 ¡xa. For glasses, a 30-/u.m beam diameter tion or recystallization. The common igneous rocks contain six to was necessary to avoid loss of . Crystalline phases were eight principal components and could have contained at least this analyzed with a 15-/xm beam. For concentration profiles, a 5-/xm number of different crystalline phases during magmatic evolution. beam was used. Complete analysis of the glasses was done with a Thus it is possible to account for nearly any bulk composition by microlite-free standard of nearly identical composition. An 55 was used as the standard for plagioclase pheno- * Present address: Geosciences Group, Los Alamos Scientific Laboratory, University crysts, and clinopyroxene was used for the clinopyroxene, or- of California, Los Alamos, New Mexico 87544 thopyroxene, and phenocrysts. Correction was made for

Geological Society of America Bulletin, v. 86, p. 1381-1391, 7 figs., October 1975, Doc. no. 51007.


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background and drift. Primary standards were checked with sec- lobes. Craters are limited to the part of the flow within 1 km of the ondary standards and found to deviate relative to known values by vent, and therefore they may represent a change in physical proper- 2 percent or less. ties of the last magma extruded or more explosive release due to less stirring by flow movement than earlier erupted material. A SETTING, HISTORY, AND DESCRIPTION less common surface feature is upwellings 2 to 5 m high and as OF GLASS MOUNTAIN much as 10 m long, which resemble an open book. They consist of flakes of lava folded back away from and on either side of a linear Glass Mountain is located in the Medicine Lake Highland (Fig. vent. 1), one of the most active centers of in the Cascade In contrast to the flow surface, the surface of the domes consists Range (Powers, 1932; Anderson, 1941). The dominant and earliest entirely of polished spines with very little debris, and the lava is structure of Quaternary volcanism was a broad of uniformly gray and finely vesicular. From each dome there is a basalt and lava flows and 50 km in diameter gradation from domelike features into the flowlike features of the and 1 km high; but even early in its evolution, rhyolite and dacite last stream that issued from that vent. The transition occurs over a flows were present. Prior to the end of glaciation, the summit of the distance of about 0.5 km. The difference, in surface morphology is shield volcano collapsed by about 150 m to form an ellipsoidal clearly a result of lack of horizontal movement in the domes, and 8 km by 6 km. Then viscous andesite (olivine free, unlike the variety of vesicularity of the flow-surface lava probably results most shield lava) erupted along the caldera rim to form a rampart from the variety of conditions for vesiculation provided by churn- of small steep cones. After glaciation occurred, numerous rhyolite ing of the flow. flows, dacite flows, and the Glass Mountain rhyolite and dacite flow erupted at high elevations inside and outside the caldera. Development of the Flow Fresh cones and associated floods of basalt and basaltic andesite, one of which is as young as Glass Mountain, the A reconstruction of the probable sequence of flow emplacement flanks of the Medicine Lake Highland. Nowhere is the shield consistent with the lava stream pattern and the overlapping of deeply eroded. lobes is presented in Figure 2. Contrary to suggestions of Anderson The most stark and imposing feature of the Medicine Lake High- (1933) and Chesterman (1955), the entire mass of fresh lava at land is Glass Mountain, located on the east rim of the caldera. The Glass Mountain was extruded during a single uninterrupted pulse mountain is a single flow with a volume of 1 km3, composed of of activity. Although the uppermost northeast lobe from the north rhyolite and dacite, nearly free from vegetation or alteration, and vent overlies the main lobe from the middle vent, lava from both free from the effects of and erosion. A charred but vents coalesces to the west. From some vantage points, the upper standing cedar engulfed by blocks at the flow's edge yielded C14 lobe appears to be a separate flow surrounded by a margin of talus, ages of 100 to 400 yr (Friedman, 1968). Such an age is consistent but the west slope is a series of pressure ridges connecting the north with Modoc Indian legends and early reports of seismic and vent and the upper lobe with the rest of the flow. activity (Finch, 1928). Fumarole activity persists today Anderson (1933) suggested that of lava at Glass 0.5 km to the west at the Hot Spot, where temperatures exceed Mountain destroyed a very large pumice cone and that a remnant 80°C at 0.5-m depth in an area of about 1 a. of it is engulfed in the flow on the side of Glass Mountain. Anderson (1933) described the general features of Glass Moun- The postulated remnant is a crescent-shaped area of ridges topo- tain, and Chesterman (1955) added some details. During its erup- graphically identical to pressure ridges that are parallel to it to the tion, four lobes of lava poured eastward down the flank of the north and south, and it differs from the surrounding flow only in shield volcano, and a broad front of lava advanced westward on the higher degree of brecciation and fumarolic alteration. Clasts in flatter ground toward the caldera floor. The thick slab of lava thus the are not pumice bombs like those that compose the small generated is surmounted by a flat-topped summit dome, the last cones around the domes north of the flow but rather are fragments viscous extrusion of the north vent. There are two other vents for of lava with all degrees of vesicularity, like the material that occurs the flow, each plugged with a small dome. These lie along a fissure intact in the surrounding flow. Like the surrounding flow, the from which one dome south of the flow and nine domes north of it crescent-shaped area is intruded by spines and pocked with explo- were also extruded. sion pits. Whatever the cause of brecciation, this area is clearly part Like other viscous flows, the Glass Mountain flow is a rippled of the flow, and the concordance of pressure ridges in the vicinity jumble of lava blocks dropping abruptly at the sides in a steep wall indicates that movement of the flow was not disrupted by any of talus 30 to 100 m high. Where the flow overlies a convex bed, obstacle. open transverse cracks have developed similar to the crevasses of valley glaciers. The transverse cracks and solid protrusions through Lithologic Variation of the Flow the marginal talus reveal that the interior of the flow is dense massive obsidian with subhorizontal flow banding. Only the top 2 Lithologic variation within Glass Mountain lava is quantitatively to 4 m is broken and deformed. The 0.2- to 2-m blocks that com- large (Table 1) and visually striking. Viewed from the summit, the pose the surface range from highly pumiceous to nonvesicular lava. cream-colored rhyolite pumice, blotched with black obsidian Surface morphology is also heterogeneous; dominant features are spines, merges downstream with the charcoal-gray scoriaceous da- 5-m-high pressure ridges, shear zones, 3- to 5-m polished spines, cite of the eastern lobes. The mappable units are described below. and 15-m-deep craters. The primary features resulting Rhyolite. The rhyolite is black to gray, and some of it is flow from flowage are the pressure ridges, which are convex down- bands 1 mm to 1 cm wide. Phenocrysts and inclusions are scarce or stream and normal to flow direction at the center of each lava absent. Except on the rhyolite domes, pumice is abundant and of stream, and shear zones, along which pressure ridges are truncated. very low . The rhyolite obsidian is very glossy except where These were later modified by upward intrusion of spines and exca- frosted or, rarely, crystallized to . vation of craters. Spines tipped with shear-zone breccia rise well Rhyolite with Inclusions. This unit is similar to the rhyolite unit above the lava stream—levee interface, showing that much vertical but contains abundant inclusions of basalt. Dark, dull, readjustment occurred after horizontal flow had ceased. Because porphyritic bands surround some of the inclusions. few craters show distortion from a circular shape, they are appar- Dacite. The dacite is dull charcoal-gray, porphyritic lava that ently also a late feature. Spines can be found truncated by craters forms coarse surface . Porphyritic basalt inclusions of all sizes and intrusive into them. The craters clearly result from degassing are abundant. There are scattered glossy streaks of phenocryst-free of the lava, because they occur on the upper lobe of overlapping glass.

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Glass Mountain Flow and Domes



Mountain sample locations and I I BANDED LAVA () lithologic units, with outline map of California showing lo- ^H DACITE

cation of flow. Heavy lines in- X/\ BRECCIA ZONE dicate lobe margins. Light lines indicate sharp boundaries be- tween lithologic units; these usually occur at stream-levee interfaces. Note that lithologic boundaries in lava stream of large northeast lobe are grada- tional. Breaks in stream of rhyolite with inclusions in nar- row eastern lobe result from upwelling of banded lava.

Banded Lava. Banded lava is gradational between rhyolite with inclusions and dacite in appearance of the nonvesicular lava and in I KILOMETER degree of development of the pumiceous surface phase. Although the rhyolite with inclusions contains occasional dacitelike streaks and the dacite contains occasional rhyolitelike streaks, this lava consists of rhyolitic and dacitic streaks in subequal proportions, as ,+ ( bands 1 mm to 1 cm thick. Phenocrysts are generally confined to the dull dacitelike bands, which swell and pinch around inclusions. a Breccia Zone. Breccia zones are areas of intense brecciation and fumarolic alteration, consisting of obsidian fragments in a of red . Locally there is unaltered breccia of obsidian in a densely welded matrix of ground glass.

Distribution of Units

The distribution of units can be understood in terms of the mode of emplacement of the flow. Domes must have been the final mate- rial erupted, because they plug the vents. The streams extending from the domes consist of lava that becomes progressively older (by a few days) with distance from the vent. Lava at the distal end of the streams is the "oldest" intact lava. At a given distance from the vent, levees contain lava erupted earlier than the enclosed stream. Crosses denote flow vents. Heavy lines indicate lobe margins. Light lines This accounts for the pattern of lava types shown in Figure 1. show lava stream boundaries. Dacite forms the levees and the distal end of the large northeast lobe from the middle vent, and the lava stream of this lobe grades upstream through banded lava to rhyolite with inclusions. The long may have contributed dacite and banded lava to the northeast lobe eastern lobe from the middle and south vents has levees of banded at the beginning. lava surrounding a stream of rhyolite with inclusions. The levees of Just as stream velocity decreases outward from the stream the southeast lobe from the south vent are composed of banded center, causing the lava erupted later to occur at the center of the lava and rhyolite with inclusions, whereas the enclosed stream is lobes, stream velocity also decreases downward from the surface. inclusion-free rhyolite. Applying the reconstruction of flow de- Thus, lava erupted earlier is encountered downward through the velopment shown in Figure 2, these observations suggest that the stream. The same early dacitic lava that occurs as levees around middle vent first discharged dacite, then banded lava, and finally rhyolitic streams is also found as upwellings and spines intruded rhyolite with inclusions. The south vent discharged banded lava, upward into the streams. This juxtaposition of contrasting lava then rhyolite with inclusions, and finally rhyolite. The north vent types resulting from differences in flow velocity and late-stage ver- discharged copious rhyolite toward the end of the eruption, but tical readjustment gave previous workers the impression that two

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separate and internally homogeneous magmas were erupted simul- large shield volcano. However, the upward gradation from rhyolite taneously and mixed during extrusion. Actually, the change in lava to dacite is surprising because of the higher density of the dacite, was a gradual one, and the progression was the same at the middle and it is the opposite chemical gradient from what would be ex- and south vents. pected if the body were compositionally zoned because of crystal We can draw the following conclusions, for the reasons stated, settling or because of a temperature gradient. about the magma body that gave rise to Glass Mountain: (1) The lava of Glass Mountain was derived from a single magma body or Groundmass of Lava well-connected series of bodies, since the extrusive activity at all flow vents was simultaneous and continuous. (2) The large volume Table 2 shows the groundmass composition of Glass Mountain of banded material implies that a mixing process was in operation lava. For samples from the longest and best developed lava stream, prior to extrusion. (3) Because the eruption tapped the magma groundmass composition is plotted against distance from the vent body at a series of points along a fissure, a crude cross section of the (Fig. 4). Along with the gradation in bulk composition from rhyo- magma body can be inferred (Fig. 3): a pocket of dacitic magma lite to dacite (Table 1), there is a corresponding gradation in existed under the south, middle, and possibly north vents, and the groundmass from rhyolite to rhyodacite. Because the rhyolite near dacite magma graded downward through increasingly rhyolitic the vent is phenocryst free, its bulk composition is the groundmass heterogeneous magma to rhyolite magma with inclusions and composition, but this is not the case for the porphyritic banded lava finally to pure rhyolite magma. and dacite. Consequently, the overall variation in bulk composition The upward increase in basalt inclusion content within the is considerably greater than the variation in groundmass compo- magma body is not surprising, since the magma was intruding a sition. Except for minute but abundant rod-shaped microlites, the RHYOLITE VENT I | RHYOLITE MAGMA rhyolite groundmass is a nearly homogeneous glass. Segregation of ¡¡§°°| RHYOLITE MAGMA WITH INCLUSIONS RHYOLITE AND DACITE VENT the microlites into bands gives rise to the flow banding of the I | RHYODACITE (BANDED) MAGMA rhyolite. However, any difference in composition between [\ \ ''J LAVA OF SHIELD VOLCANO DACITE MAGMA microlite-rich and microlite-poor bands is below the level of detec- tion by the electron microprobe. In contrast, the dacite groundmass VENTS FOR NORTH DOMES VENTS FOR FLOW VENT FOR I 1 I 1 SOUTH DOME is markedly heterogeneous, and in the banded lava the boundary between heterogeneous dacitelike bands and homogeneous rhyolitelike bands is abrupt. Electron-microprobe data and micro- scopic examination indicate that the groundmass of the dacite and of the dacitelike bands in the banded lava is a fine mixture of rhyolitic glass and microlites of intermediate plagioclase and au- gite. The groundmass of the basalt inclusions is even more heterogeneous. It is a very finely felted mass of plagioclase and microlites and brown glass and therefore is not suitable for analysis.

Phenocrysts of Lava Figure 3. Cross section along Glass Mountain fissure, just prior to erup- tion, showing possible configuration of magma body. Although the rhyolite is almost phenocryst free, with only scarce plagioclase, the banded lava and dacite contain phenocrysts of plagioclase, clinopyroxene, orthopyroxene, and olivine in increas- TABLE 1. WHOLE-ROCK COMPOSITION OF LAVA AND AN INCLUSION FROM GLASS MOUNTAIN FLOW ing abundance toward the dacitic distal ends. The basalt inclusions Rhyolite Dacite Inclusion are markedly porphyritic, with abundant plagioclase, clinopyrox- CS) (1) (1) (1) ene, and olivine. There are also minor amounts of inclusions simi-

Si02 73.0 67.2 65.7 54.7 lar to the basalt in phenocryst content but with less groundmass, TiOi 0.3 0.3 0.8 0.9 perhaps representing more highly crystallized parts of a basaltic flow AI2O3 14.0 16.2 16.3 18.5 FeO 1.2 3.3 4.7 7.0 or or a basalt cumulate. MgO 0.3 1.3 1.8 5.8 CaO 1.4 3.4 3.7 8.7 Electron-microprobe analysis of the phenocrysts confirms what NaîO 4.0 4.0 4.0 3.0 is apparent from microscopic examination of the lava (Fig. 5): most k2o 4.2 3.4 2.8 1.0 of the phenocrysts of the banded lava and dacite are identical to the Note: Numbers in parentheses are analyses in average. Data from Anderson (1941). phenocrysts of the basalt inclusions. Composition of the pheno-


Banded lava Rhyolite with inclusions Rhyolite

224+ D 230 231 264 308 246 248 311 274 261 284 239d 239g 299 268 272

Si02 67.0 67.8 68.8 70.2 70.6 70.8 71.0 71.8 72.4 73.1 73.2 73.5 74.2 74.5 73.7 74.3 74.7 ai2o3 15.1 15.2 14.6 14.4 14.8 13.6 15.2 13.9 14.1 13.8 13.9 13.8 13.4 13.7 13.9 13.5 13,4 FeO 3.8 3.6 3.6 2.8 1.9 2.4 2.4 2.0 1.7 1.6 1.7 1.6 1.5 1.3 1.4 1.4 1.4 CaO 3.1 2.9 2.7 2.2 2.0 2.1 2.1 2.0 1.8 1.4 1.6 1.1 1.2 1.2 1.4 1.2 1.2 Na20 4.6 4.5 4.1 4.3 4.7 4.1 4.4 4.2 4.2 4.1 4.3 4.1 4.1 4.2 4.2 4.1 4.2 k2o 3.2 3.3 3.7 3.7 3.8 4.0 3.8 4.0 4.1 4.3 4.1 4.4 4.4 4.3 4.2 4.3 4.2 •Dotal 96.8 97.3 97.5 97.6 97.8 97.0 98.9 97.9 98.3 98.3 98.8 98.5 98.8 99.2 98.8 98.8 99.1

Note: MgO and Ti02 were rot determined because of low concentration (Doth were 0.3% in 239 and 264. •Determined by electron microprobe. tSanple mmbers correspond to nunbered locations in Figure 1; 239d and 239g are frcm same location but represent dull (finely crystallized) and glassy obsidian, respectively.

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crysts, as determined by electron-microprobe analysis, is presented Clearly, the mechanical aspect of this event was critically impor- in Figure 6 and Table 3. The most significant observations are the tant, and the weakness of the glass in the basalt, heated to perhaps

following: (1) Except for rare plagioclase (An35) phenocrysts in the 800° to 900°C, must have been a significant factor. But the degree of rhyolite, phenocrysts occur only in association with inclusions. chemical interaction can be evaluated on the basis of appearance, Phenocrysts in the banded lava are largely confined to the dark disappearance, or change in composition of phases. When a crystal- bands that swirl around and trail inclusions. (2) Most of the line phase comes into contact with a liquid with which it is not in plagioclase and all of the olivine and clinopyroxene phenocrysts are equilibrium, it will tend to react with that liquid and change to a virtually identical in composition, size, and shape to phenocrysts composition appropriate to the new system or be resorbed and within associated basalt inclusions. However, phenocrysts of replaced by a new appropriate phase. This simply fulfills the olivine and clinopyroxene tend to be more rounded than those in equilibrium condition that the chemical potential of each compo- the inclusions. These three phenocryst phases are of a composition nent must be the same in all coexisting phases. Energy for this appropriate for basalt. (3) Unlike phases mentioned in (2), some reaction is supplied by lowering the temperature of the system and phenocrysts are associated with inclusions but not also contained by crystallization. The reaction rate obviously depends on the within them. These are orthopyroxene and a plagioclase that is crystal and liquid phases involved. A phase such as plagioclase that distinctly more sodic than most of the plagioclase phenocrysts yet commonly exhibits strong zoning may not react completely with a more calcic than the rare plagioclase of the rhyolite (Fig. 6). Both liquid already saturated with more sodic plagioclase. Thus, these phases are usually euhedral and sometimes exhibit weak re- most of the plagioclase phenocrysts show little evidence of reaction verse zoning. The latter was not found in any of the other pheno- by either change of shape or of composition compared with the crysts. In addition, the orthopyroxene phenocrysts are always sur- plagioclase still within the basalt inclusions. Olivine and rounded by a -enriched boundary layer (Fig. 7). clinopyroxene responded somewhat more to their new environ- ment by slight resorption, as suggested by their rounded outline. INTERPRETATION OF FIELD AND But there are plagioclase and pyroxene phenocrysts of distinctly ANALYTICAL RESULTS different composition (Fig. 6) and shape that are absent in the inclusions. Their association with the inclusions, euhedral form, It is highly unlikely that systems with bulk compositions as dif- and weak reverse zonation suggest that they grew during contami- ferent as basalt and dacite (or even rhyodacite) would crystallize the same calcic plagioclase, magnesian olivine, and clinopyroxene (Bowen, 1928). Indeed, the appearance of the basalt inclusions (Fig. 5) suggests that they were disintegrating, strewing out bands of debris as they rotated in the moving rhyolite magma. Fragments of the basalt became the phenocryst clots of the dacite, and indi- vidual phenocrysts of the basalt became the phenocrysts of the dacite. The cause of disaggregation was apparently destruction of the glassy basalt matrix that contributed material to the ground- mass of the dacite. The progressive breakdown of inclusions, as they were engulfed by the rising rhyolite magma body within the shield volcano, produced the gradation from dacite at the top, downward through prominently banded lava where the process was beginning, into as yet uncontaminated rhyolite.

O <


Figure 5. Photomicrographs of representative Glass Mountain samples. A, Rhyolite (sample 239g), showing rod-shaped microlites and absence of 5 6 DISTAL END phenocrysts. B, Rhyolite with inclusions (sample 286), showing porphyritic Distance kilometers basalt inclusions and dark, dacitelike bands around them. C, Banded lava Figure 4. Plot of composition of groundmass versus distance from vent (sample 311), showing inclusions and continued development of dark for lava stream extending from middle vent to distal end of northeast lobe. bands. D, Dacite (sample D), with glomeroporphyritic texture and a few In order of increasing distance from vent, samples are 239, 311, 308, 230, rhyolitelike bands. Note progressive increase in phenocryst content and 231, and D. darkening of groundmass in this sequence. Bar = 2 mm.

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nation, in response to resorption of the -rich olivine and rejecting calcium from a nutrient liquid richer in calcium. How- of calcium-rich and magnesium-rich clinopyroxene. ever, both the uncontaminated rhyolite and the rhyolitic glass Thus, there was partial reaction of crystalline phases from the within the groundmass of the contaminated lava contain slightly inclusions with rhyolite liquid, but the glass was probably the most less calcium than the orthopyroxene. Apparently there was limited abundant and chemically reactive phase in the inclusions. It is more direct assimilation of components from the glass of the basalt by reactive because it is metastable and lacks the rigid bond arrange- the rhyolite liquid, producing the calcium-enriched liquid in which ments of crystalline phases. It is clear that the groundmass of the the orthopyroxene phenocrysts grew. Subsequent growth of the inclusions disintegrated, because it is largely absent from small plagioclase and microlites then lowered calcium concentra- clots of phenocrysts and completely absent from around individual tion in the liquid to its precontamination level. inclusion-derived phenocrysts. Other than phenocryst content, the only difference between contaminated and uncontaminated lava is SUMMARY OF RESULTS the abundant microlites and brownish color of the groundmass of the contaminated lava. The glass within the two lava types is essen- Partial assimilation and partial crystallization of the groundmass tially the same. These microlites and brown color are apparently of basalt inclusions engulfed by rhyolite magma caused mechanical the result of disintegration of the groundmass of the inclusions. The weakening of the inclusions. Debris was strewn outward in bands dacite groundmass assemblage of plagioclase and augite microlites from inclusions rotating in the velocity gradients of the rising plus rhyolite glass could result if the glass of the basalt was dis- magma. Phenocrysts of the inclusions became phenocrysts of the solved by the rhyolite liquid, with subsequent crystallization during contaminated magma, while the groundmass of the inclusions con- extrusion yielding the microlites and a new rhyolite liquid that tributed to the "groundmass" of the contaminated magma. Re- quenched to glass. Another possibility is that heat from the rhyolite sorption of the inherited olivine and clinopyroxene phenocrysts magma triggered growth of plagioclase and pyroxene microlites in and partial assimilation of the glass from the inclusions caused the glass of the basalt inclusions and that internal stresses thus coprecipitation of reverse-zoned orthopyroxene and plagioclase introduced aided in the mechanical disaggregation of inclusions. phenocrysts and later growth of plagioclase and augite microlites. Both these chemical and mechanical mechanisms probably played a Contamination was greatest at the top of the magma body and role. The first possibility requires greater heat from the magma decreased downward, resulting in a magma zoned gradationally because it involves less crystallization during contamination. The downward from porphyritic dacite to nonporphyritic rhyolite. Ex- near absence of phenocrysts in the rhyolite, however, implies that trusion of this body produced a flow with distal ends of dacite the magma was at or above its liquidus. If such is the case, it might grading toward rhyolite at the vents. well have partially dissolved the glass of the basalt. On the other hand, heating of the glass of the basalt must have caused growth of IMPLICATIONS microlites even before contact with the rhyolite magma. The question therefore becomes, Was there any substantial Glass Mountain represents a case in which contamination might change in composition of the liquid phase as a result of magma- be expected to be minimal, since the magma involved was rhyolite inclusion interaction? The weak reverse zoning of the intermediate and the contaminant was pre-existent basaltic lava. The con- plagioclase and orthopyroxene implies that the initially rhyolitic taminating material consisted of crystalline phases which, in liquid became somewhat enriched in calcium and magnesium dur- Bowen's (1928) terminology, came from earlier in the reaction ing contamination. Further supporting evidence is the calcium- series than the sodic plagioclase with which the rhyolite was satu- enriched boundary layer surrounding the orthopyroxene (Fig. 7). rated. It also contained a groundmass that is clearly more mafic Although a calcium peak in a single electron-microprobe traverse than the rhyolite and therefore could not have been entirely liquid of the phenocryst-groundmass interface could result from the coin- at the temperature of the rhyolite. Nevertheless, contamination cidental juxtaposition of a plagioclase microlite, the consistent pres- amounted to as much as 25 percent basalt, by weight, of the hybrid ence of the peak suggests that this is a boundary layer formed at an dacite. active interface. Such a layer could result if the growing crystal was Because there are many areas where rhyolite and dacite have passed through mafic volcanic piles prior to eruption, it is reasona-

0.5 CaSiOs ble to expect that the kind of contamination that occurred at Glass phenocrysts in rhyodacite and dacite core phenocrysts in basalt Orthopyroxene inclusions Enes Mg

phenocrysts in rhyolite boundory (scarce) loyer • Fe microlite\ \f ****' MgSi03 Y\ ...Ca Figure 7. Electron- J OLIVINE PLAGIOCLASE microprobe traverses of v 0.12 0.10 0.08 0.06 0.04 0.02 0 orthopyroxene (sample ¿ core rim rim 248) and intermediate An 46 plagioclase (sample 311) c An5l An s 2 phenocrysts. o No l ì i v : nn Ca 0.2 0.4 0.6 Figure 6. Composition of crystalline phases or Glass Mountain lava and Plagioclase inclusions. All compositions are in mole fraction. N is number of . |\jk For pyroxene, A is rhyolite with inclusions (sample 286), B is banded lava V (sample 311), C is banded lava (sample 248), and D is dacite (sample D). 0.2 0.1 0 0.1 0.2 Thus, sequence A, B, C, D represents increasingly dacitic lava. Distance from core in mm

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Mountain has occurred elsewhere. Furthermore the reverse type of Mixing of basalt and rhyolite to produce intermediate lava is a contamination, basalt magma intruding rhyolitic and dacitic rocks, very old idea (Bunsen, 1851), but it has been given less and less should proceed even more easily because the contaminant is rich in attention for several reasons. Bowen (1928) tied the idea of direct components with which the magma is not saturated. Indeed, there mixing of basalt and rhyolite magmas to liquid immiscibility or is much evidence that both types of contamination are common. To "splitting" as the source of these primary melts. He then showed the extent that intermediate lava can be shown to form by mixing that immiscibility is unlikely for these . Basalt and rhyolite of more extreme compositions, the possible importance of frac- need not originate by liquid immiscibility, however. They could tional crystallization or primary liquids of intermediate composi- form from a single source material by fractional melting (Yoder, tion is diminished, and the significance of primary magmas of ex- 1973). Bowen (1928) tied rock-magma mixing to assimilation of treme compositions is enhanced. Thus, the logical consequence of crystalline rocks and showed that significant assimilation of such demonstrated volcanic contamination is a mixing model in which rocks is unlikely because of the high energy requirement. More intermediate volcanic rocks are ultimately the product of two par- recently, data have shown that old radiogenic crustal rocks ent magmas — rhyolite and basalt. I have made a general statement do not play a significant role as contaminants (Dickinson, 1970). of such a model (Eichelberger, 1974). The evidence on which the The latter two arguments do not rule out rock-magma mixing, model is based is that (1) banded and variegated lava of intermediate because they do not apply to the most abundant and reactive ma- composition is abundant; (2) variation in composition within indi- terial at any volcanic center — the flows and ash sheets themselves. vidual volcanic centers of the basalt-andesite-dacite-rhyolite as- Fenner (1926, 1938) proposed volcanic contamination as the sociation is essentially linear, and basalt and rhyolite sometimes origin of certain at Katmai and Yellowstone. Production of occur without intermediate ; (3) most dacite and composi- andesite by contamination of basalt with rhyolitic material was tionally equivalent contain abundant fine-grained shown at Paricutin by Wilcox (1954) and at by Finch mafic inclusions of basaltic composition; and (4) intermediate lava and Anderson (1930). Most petrologists, however, have accepted commonly contains phenocrysts in disequilibrium with each other the idea that mixing has not stood the test of time as an important and the groundmass and inappropriate for the bulk composition petrogenetic process. The rest of this paper will present the evi- but identical to phases in associated basalt and rhyolite. dence for mixing at several volcanic areas, selected for discussion to illustrate the kinds of reactions involved and the influence of lava distribution on the resultant lava assemblage. For each ex- TABLE 3. COMPOSITION OF PHENOCRYSTS IN LAVA AND INCLUSIONS FROM GLASS MOUNTAIN FLOW ample, it will be shown that andesite and dacite reflect their hybrid origin by inheriting the dominant phenocrysts of the parent rhyo-

Sample Plagioclase Olivine Clinopyroxene Orthopyroxene lite and basalt. Table 4 gives a comparison of intermediate com- positions predicted by mixing with average compositions actually Lava Incl. Lava Incl. Lava Incl. observed at each locality.

73 83 79 44,18 1*3,16 3,41 In most of the following discussion, the phenocryst assemblage, 81 <13,17 3,32 73 75 inclusions, and bulk composition of lava are treated as evidence of 73 77 75 ^3,16 3,29 74 42,18 53 76 111, 111 5,18 mixing in general. Whether intermediate igneous rocks form by direct magma-magma mixing or magma- mixing, the 74 78 73 76 starting materials are the same and the results are nearly the same. 72 76 Curtis (1968) felt that Fenner's (1926) interpretation of the Katmai 46 75 73 dacite presented an energy problem; Curtis proposed that the mafic 70 material was molten when mixed with rhyolite. Such mixing might 248 3,32-21 still produce mafic inclusions in the resulting dacite, because the 3,38 311 82 83 78 83 1(3,18 "»3,15 81 79 78 82 43,17 43, lit 3,37 basaltic magma would be chilled in the cooler rhyolite. 75 77 78 82 Iti, 14 lt3, lit 3,27 74 77 77 82 42,16 The Glass Mountain study has shown that production of dacite 74 76 76 82 Itl.lli by volcanic contamination is largely a mechanical process, and 71 74 69 82 40.15 70 74 82 39,20 therefore "superheat" is not necessary. The zonation of the flow is 69 73 81 39.16 consistent with contamination, but difficult to explain by 68 73 80 38,19 64 72 75 38,13 magma-magma mixing. Furthermore, the nearly universal occur- 54 54 rence of basaltic inclusions in dacite is evidence against direct mix- 50 ing because of the coincidence required to bring two magmas into 48 76 contact. The coincidence is particularly severe where eruptions are 73 73 infrequent and of relatively small volume, but where large magma 69 69 bodies are involved, magma mixing is more probable and may be 78 8o 79 83 43,16 44.15 3,41 the dominant process. 77 78 77 82 42,15 43.14 3,41 76 78 76 81 42.16 3,38 75 77 80 40.15 Medicine Lake Highland 74 76 80 40,14 73 75 80 38.13 72 74 80 36.14 70 73 79 35.15 The Medicine Lake Highland is a broad shield volcano com- 69 72 79 posed largely of basalt and basaltic andesite flows and tuff sheets. 49 72 79 63 Rhyolite, dacite, and siliceous andesite are confined to the central 38 part of the shield, and floods of basalt have erupted from its lower 37 37 flanks (Anderson, 1941). Extreme basalt and rhyolite compositions 36 are well represented, but there is a gap in the 28 reported analyses 33 32 between 60.0 percent and 65.7 percent Si02. 32 The presence of calcic plagioclase, magnesian olivine, and as- Note : Plagioclase composition is expressed as mole percent (Na, Ca determined), olivine composition as mole percent (Mg, Fe determined), and sociated basalt inclusions reveals the hybrid origin of the dacite, clinopyroxene and orthopyroxene composition as mole percent wollastonite, ferrosllite, which shows the range of homogeneity to be expected in a mixing respectively (Ca, Fe, Mg determined). Samples 259 and 286 are similar to samples 261 and 284, respectively. process. The banded rhyodacite of Glass Mountain, the rhyodacite northeast of Mount Hoffman, and the Hoffman dacite are charged

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with inclusions trailing swirls of debris, and they represent the pronounced mafic- banding. Even much of the intermediate beginning of contamination. The more uniform dacite of the distal lava that is not strongly banded has a markedly heterogeneous end of Glass Mountain represents the next step. The final groundmass of brown and clear glass. homogenization is illustrated by the Medicine Lake dacite, in The 1851 andesite and 1915 dacite are of particular interest. The which most of the inclusions have disintegrated, but a glomeropor- 1851 andesite is a flow erupted from the base of Cinder Cone, and

phyritic texture is retained. it is commonly called basalt despite its 55 to 57 percent Si02 Much of the basaltic andesite contains phenocrysts of embayed content. It is similar to adjacent central lava in form and

with rims, together with An65_85 plagioclase composition, although it probably has a lower quartz phenocryst and Fo78_88 olivine similar to the porphyritic basalt. The matrix content than most. Phenocrysts are embayed quartz with an augite plagioclase is labradorite. The most likely source of the andesine is corona, olivine, pyroxene (mostly Mg-rich orthopyroxene), and the porphyritic rhyolite of the Medicine Lake Highland, since an- corroded, cloudy plagioclase. The lava also contains inclusions desine is the dominant crystal phase of these rocks. This interpreta- with about 50 percent clear glass plus corroded phenocrysts of tion is consistent with the bulk composition of the basaltic andesite quartz, andesine to , and . In 1915, after a period (Table 4). Calcic plagioclase and forsteritic olivine were inherited of phreatic , a dacite flow was extruded from the summit from the parental basalt, and labradorite rims grew on the andesine of , followed immediately by a very violent eruption of after the rhyolite glass had dissolved in the basalt magma. banded pumice. This pumice is remarkably similar in appearance Some of the basaltic andesite lacks andesine phenocrysts but is to pumice from Glass Mountain's banded part, which was also otherwise similar to the andesine-bearing basaltic andesite. Since extruded from vents that had just erupted dacite. The plagioclase much of the Medicine Lake Highland rhyolite lacks phenocrysts phenocrysts have sodic andesine cores and sodic labradorite rims, but is similar in bulk composition to andesine-bearing rhyolite, it is and the groundmass is labradorite. Phenocrysts of quartz likely that such rhyolite served as the contaminant for the an- and forsteritic olivine rimmed with hypersthene are also present. desine-free basaltic andesite. All the abundant basaltic inclusions of Lassen area dacite contain Siliceous andesite of the caldera rim presents a problem in view brown glass, but the crystal content varies. Usually there are of the anomalous composition (high Na, low Ca) of the two phenocrysts of bytownite or labradorite, , and occa- analyzed samples (Table 4) and apparent lack of inherited pheno- sionally augite. Where phenocrysts are absent, there is only a felted crysts. It may represent an extreme contamination of basalt that mass of plagioclase and hypersthene in brown glass. Some of the traveled through the thickest and most intensely fractured and al- inclusions of contain quartz and olivine phenocrysts, tered part of the pile, or it may have a different origin. both with augite rims. The reported analyses show that the inclu- If mixing is accomplished primarily by assimilation in magma of sions are equivalent in composition to basaltic andesite and ande- previously erupted material, the Daly Gap of this suite is explained site of the area. as the compositional region beyond the limits of contamination of Because the rhyolite of the Lassen area is so persistently por- rhyolite with basalt or basalt with rhyolite. Furthermore, the dis- phyritic and much of the intermediate lava is so poorly stirred, the tribution of andesite is controlled by the localization of rhyolite at resultant banding and abundance of phenocrysts inherited from the center of the Medicine Lake Highland. Basalt magma is most rhyolite give clear evidence of mixing. Andesite inherited quartz, likely to reach the surface without contamination at the fringes of plagioclase, sanidine, and inclusions from the rhyolite contami- the volcanic pile. nant, while retaining olivine from the basalt parent magma. In some cases, augite grew on the quartz and olivine in response to con- Lassen tamination. Progressive contamination of the rhyolite magma caused calcic rims to grow on its plagioclase phenocrysts, while The volcanic highland around Lassen Peak, located 120 km hypersthene reaction rims formed on olivine inherited from the south of Medicine Lake, is a region of great diversity in lava com- basalt contaminant. Along with the sodic andesine cores of the position and volcanic structures. Volcanism in the Lassen area has plagioclase phenocrysts, the contaminated rhyolite (dacite) re- produced marginal flows of fluid basalt like the flows that flood the tained its original quartz and sanidine. As at the Medicine Lake Hat Creek graben 20 km north of Lassen Peak, and the interior of Highland, the areal distribution of lava types apparently resulted the highland is characterized by intimately associated andesite from localized generation of rhyolite magma within a broader field cones and dacite domes on a platform of andesite and concealed of basalt activity. rhyolite. (1932) provided a thorough general description On the basis of phenocryst and inclusion content, Finch and of the region. Detailed discussions of the dacite and its inclusions Anderson (1930) suggested that the 1851 Cinder Cone andesite were given by Williams (1931), of Cinder Cone and its flow by was formed by contamination of basalt with rhyolite. Peterman Finch and Anderson (1930), of the Hat Creek basalt flow by An- and others (1970) later noted that the Sr87/Sr86 values of lava in the derson (1940), and of the 1915 lava of Lassen Peak by Macdonald area are consistent with this interpretation. But despite the abun- and Katsura (1965). dance of basaltic inclusions in the dacite and the prominent band- Except in the basalt, phenocrysts in the Lassen area lava are ing of both andesite and dacite, only the 1851 andesite flow and often larger and more numerous than at Medicine Lake. Much of 1915 dacite pumice have been recognized as hybrid rocks. Dacite the basalt lacks phenocrysts, but the porphyritic basalt usually con- has presented a problem because it is not as obvious that the mafic tains bytownite or labradorite, augite, and forsteritic olivine. The inclusions within it are fragments of basaltic lava as it is for the andesite usually has phenocrysts of labradorite or andesine, augite, Glass Mountain inclusions. Williams (1931) argued that the mafic hypersthene, forsteritic olivine, quartz with augite rims, and occa- inclusions were early precipitates torn from the wall of the magma sionally sanidine. Dacite is almost always charged with basaltic chamber during eruption. Yet these inclusions have a high glass inclusions, which make up as much as 50 percent of the rock. content and are similar in composition to associated mafic lava. Typical phenocrysts are andesine or oligoclase (often zoned in re- The Chaos Crags inclusions are particularly significant because verse), quartz, hornblende, , pyroxene, and occasionally they contain the same phenocrysts, even to the reaction rims, as the sanidine and forsteritic olivine. The rhyolite is also strongly por- 1851 andesite. An early precipitate could not be forsteritic olivine phyritic, with andesine or oligoclase, quartz, and sanidine. A strik- and quartz plus brown glass. These inclusions serve to illustrate the ing feature of much of the andesite, particularly in the central multistage nature of mixing, because in this case the contaminant plateau of Lassen Peak and on West Prospect Peak, and of the was already slightly contaminated itself. The difference between dacite, as on Raker Peak and in the 1915 pumice, is the very these inclusions and nearby mafic lava is the somewhat coarser

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texture of the groundmass of the inclusions. In this feature, the Considering all the lava types of the Clear Lake area (Brice, Lassen inclusions are typical of inclusions in dacite domes and 1953), sanidine phenocrysts reach a maximum concentration of differ from those of the Glass Mountain flow. This may be because 10 percent by volume in the rhyolite and 5 percent in the dacite. magma that produces domes cools more slowly than the hot, Quartz averages 6 percent in the rhyolite and 2 percent in the quickly quenched magma of Glass Mountain, and so the inclusions dacite. Olivine averages 15 percent in the "olivine basalt" but com- have a better opportunity for recrystallization. poses as much as 2 percent by volume of the dacite. Brice (1953) recognized that the large amount of rhyolite and Clear Lake dacite relative to andesite and "basalt" at Clear Lake is incompati- ble with fractional crystallization, and that only mixing of rhyolite Pleistocene to Holocene volcanism of the Clear Lake region (An- and basalt could account for the phase assemblages of the dacite derson, 1936; Brice, 1953; Bowman and others, 1973), 140 km and andesite. Indeed, mixing not only explains the bulk composi- north of San Francisco, first produced mafic flows that are referred tion of the intermediate lava types (Table 4) but also, allowing for to as quartz olivine basalt, although they differ a little in silica some resorption, explains the phenocryst abundances. The pres- content from later andesite. They interfinger with and postdate the ence of abundant inclusions, of phenocrysts more "acid" in charac- sedimentary Cache Formation, and contain abundant sedimentary ter than the groundmass, and of variations within flows are all in- and metamorphic fragments showing varying degrees of assimila- dicative of mixing. Although the uncontaminated rhyolite end tion. The dominant phenocryst is forsteritic olivine, accompanied member is well represented, all the mafic lava for which analyses by quartz, labradorite, and augite. are reported show petrographic evidence of contamination. The The main, later phase of activity produced andesite, dacite, and parent mafic magma is therefore more basaltic, most likely a high- rhyolite. Individual flows show considerable variation in composi- alumina basalt. The petrographic evidence suggests that basalt was tion and contain abundant . Most of the andesite and the first magma to be generated and was contaminated by older dacite contains phenocrysts of both An5S-70 plagioclase and An25-35 crustal rocks. Once the plumbing system became established and plagioclase. The calcic variety predominates in the andesite, rhyolite appeared, however, rhyolite replaced the country rocks as whereas sodic plagioclase predominates in the dacite. In both cases, the dominant contaminant. The later intermediate lava inherited the sodic plagioclase phenocrysts are more sodic than the ground- calcic plagioclase and forsteritic olivine from the basalt parent mass plagioclase. Other phenocrysts include quartz with augite along with sodic plagioclase, quartz, and sanidine from the rhyo- rims, Fog4 olivine, hypersthene, and augite. In addition, most of the lite. For testing mixing against observed compositions, the two dacite contains Or75 sanidine phenocrysts, although sanidine is ab- analyses (Anderson, 1936) of early "quartz olivine basalt" were sent from the groundmass, and fine-grained inclusions consisting of omitted. labradorite and mafic . Nearby, at Borax Lake, there is a complex of rhyolite overlying



Rhyolite Dacite Andésite Basaltic Basalt Rhyolite Dacite Andesite Basalt Rhyolite Decite Andesit 1»:1 1:1 Olivine Platy 1:3 andesite 1» :1 2:3 1:1 (8) W (1) (2) (3) (5) CO (6) (l"t) (2) (3) (5) (2)

S102 73-2 68.3 67.2 61.0 58.Ii 6o.o 51».9 51».9 1(8.8 71.0 66.3 66.1 56.8 57.7 It7-1» 7"».5 65.9 66.0 57.2 Ti 02 0.3 0.1» O.U 0.6 0.9 1.3 0.8 0.9 1.0 0.2 0.1» 0.1» 0.7 0.6 1.1 0.2 0.7 0.6 1.1 A1203 13-9 15.0 16.2 16.7 17.0 17.0 18.1 17.9 19-5 15.2 15.8 16.1» 17.1 17.0 18.3 13.0 15.1 15.5 17.1 FeO 2.2 3-5 3-6 5-5 7.6 7.2 7.1 7-2 8.7 2.2 3-6 3-5 6.5 6.0 9.1» 1-3 3-9 3-5 6.4 MnO O.O6 o.ofi 0.07 0.13 0.13 0.17 MgO ò'.h Ü8 i'.i» 3*8 3-1» 2.2 5-5 5.3 7.2 1.2 2.8 2.1 5-9 5-1 9-1 o!i» 2*6 2*8 Ü8 CaO 1.5 3.1» 3-5 6.3 6.6 1».8 8.7 8.3 11.1 2.1 3-7 i».i» 6.9 7.5 10.1 1.0 l»-3 3.9 7.6 Na20 3-9 3-7 3-9 3-3 3.1» 5.0 3.1 3-3 2.8 3-8 3-5 3-9 2.8 3-1» 2-2 i» .0 3-6 3-5 3-1 K20 l».2 3-l> 3-3 2-3 1-5 1.7 1.3 1.2 0.1» 3-2 2.7 2.2 1.6 1.6 0.6 l»-5 3.0 2.8 1-5


Rhyolite Andesite Basaltic Rhyolite Dacite Andésite Basalt Dacite Andesite Basaltic 2;3 andesite 7:3 3:2 andesite (29) ' (3T) (20) (26) (8) (3) (13) (11) (2) (8)

Si02 73-9 61.2 60.I 52-7 71».2 67-3 66 2 58.8 58.8 51.1 65.1 60.5 60.3 53-7 Ti 02 0.2 0.7 O.9 1.1 O.3 0.5 0 1» 1.1 0.6 0.6 0.7 0.6 AI203 13.2 16.2 I6.9 18.2 13-3 11».6 15 3 16 !i 17-0 17.5 llt.l 15.1 l'i.6 16.6 FeO 1.2 5.6 5.1» 8.5 1.7 l».l 3 9 7.0 6.8 9.6 7.8 8.7 9-9 10.1 MnO O.O6 0.08 0.1 0.1 0.05 0.1 0 1 0.2 0.2 0.2 0.2 0.2 MgO 0.1» 3-3 2.8 5-2 0.3 2.2 2 2 ¿'5 5.0 6.6 1-5 2-9 2.6 1».9 CaO 1.2 5.3 5.5 8.1 1.6 M 1» 1» 7.6 7.3 10.6 5-9 7-9 7.1» 11.0 Na20 3-7 3-9 3.7 l».l !» .2 3.7 3 5 3-2 2.9 2.7 3.0 2-5 2.7 1.8 K20 1» .6 2.3 2.8 0.8 3-2 2.1» 2 3 1.1» 1.2 0.5 1.1 0.9 0.8 0.5

Ba 630 538 778* 1»78* 280 210 205 105 Co 1».9 22 18 33 11» 21 28 31 Cr 1.7 1+7 58 69 1» 19 6 1»1 Cu 18 ¿5 37 ¿3 6 25 39 31» 27 70 35 135 Ha 16 15 16 15 Li 35 16 19 Î Ni Fb 1» .0 3 3 2.9 2 Rb 108 ¿3 3¿ 11.0 16 12 11 6 Se 1».7 27 26 38 Sr 118 k¿2 651 692 125 261» 233 333 300 270 300 225 V 36 109 126 157 8.5 158 178 232 93 I80 185 310 Zr i»38 288 208 188 160 121» 103 106 47 39 39 26

Note: Columns headed with rock names show average composition of that lava type for the volcanic region in which it occurs. Numbers in parentheses are the number of analyses in each average. Columns headed with ratios show the composition of a mixture of the average rhyolite and basalt, or average extreme compositions where rhyolite and (or) basalt are absent. Ratios are parts rhyolite to parts basalt, by weight. Major are in weight percent and minor elements are in ppm. References for analyses are as follows: Medicine Lake Highland: Anderson (1941); Lassen: Williams (1932), Anderson (1940); Clear Lake: Anderson (1936); Chilean Andes: Siegers and others (1969), Pichler and Zeil (1972); Taupo volcanic zone: Ewart and others (1968), Lewis (1968b), Cole (1973); Tongan Islands: Ewart and Bryan (1973). * These columns represent averages of 11 and 5 analyses, respectively.

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dacite that may be a near replica of Glass Mountain. The dacite is augite, olivine with augite reaction rims, hypersthene with augite glomeroporphyritic with clots and inclusions containing An50-65 rims, plagioclase with oligoclase cores surrounded by a cloudy zone plagioclase, hypersthene, and forsteritic olivine. The inclusions and then bytownite rims, and embayed quartz with augite rims. have a groundmass of hornblende and andesine. Bowman and The hypersthene is identical in composition to the hypersthene of others (1973) suggested that the complex is a single flow, and they adjacent (Cole, 1970b). The groundmass feldspar is lab- have shown that the lava varies continuously in a remarkably linear radorite. Cole proposed that the 1886 basalt, which is 51.2 percent fashion in major and composition. This could not be Si02 and contains few phenocrysts, followed the path of the 1020 a result of crystal-liquid fractionation, because the crystal fraction andesite through the rhyolite pile, as the basalt contains xenoliths consists of multiple crystal phases. Linearity requires of the frac- of andesite but lacks rhyolite xenoliths. tionation process that all crystalline phases appear at the same time Thus, the first injection of basalt into the rhyolite pile produced and behave identically. This is made all the more impossible be- andesite, with quartz, oligoclase, and hypersthene inherited from cause one of the phenocryst phases is forsteritic olivine, which the rhyolite contaminant and olivine retained from the basalt par- could not form in the dacite. ent. The second injection of basalt magma followed that of the first so closely that significant contamination did not occur, and lava Chilean Andes close to the primary basalt composition erupted. Similar evidence of mixing is found at Ngauruhoe Volcano at the The voluminous late Cenozoic rocks of the Chilean Andes (Pich- southwest end of the Taupo belt, where the greatest concentration ler and Zeil, 1972) have been divided into the Rhyolite and Ande- of andesite occurs. Steiner (1958) described a 1954 andesite flow site Formations, which are of comparable size. The Rhyolite For- (56 percent Si02> that contains abundant siliceous xenoliths, many mation is chiefly composed of rhyolite sheets, is 1 km or of which are glassy and have relict quartz, oligoclase, and pyroxene more thick, and forms a high plateau. The Andesite Formation and are similar in bulk composition to nearby rhyolite. forms the great Andean stratovolcanoes that lie within, partially The Tauhara Volcano, roughly midway between Tarawera and postdate, and hence pierce the ignimbrite plateau. Although de- Ngauruhoe, consists of five dacite domes described by Lewis velopment of the began first, it continued during (1968a, 1968b). Some of the domes show an upward gradation growth of the andesite volcanoes. Together, the two formations from banded rhyodacite to dacite (the same sequence as Glass form a continuous spectrum from andesite to rhyolite, but basalt is Mountain), all are charged with "basic cognate xenoliths," and the scarce. In the rhyolite, phenocrysts of quartz, sanidine, and oligo- lava is glomeroporphyritic. Most plagioclase phenocrysts are about clase predominate. In the andesite, quartz phenocrysts occur in An40, but there is more calcic euhedral plagioclase identical to the rocks with as little as 55 percent Si02. These are accompanied by phenocrysts of the inclusions. Quartz is present in all the lava, but phenocrysts of plagioclase with embayed An 25-35 cores and lab- it is embayed and rimmed with clinopyroxene. Hypersthene is vari- radorite rims, large An65_80 plagioclase, abundant euhedral An45_62 able in composition and sometimes zoned in reverse. The En 52 cores plagioclase, hypersthene, and forsteritic olivine with augite rims. of these phenocrysts are more rich than the groundmass The groundmass plagioclase of the andesite is An^-^. hypersthene but identical to hypersthene in associated rhyolite This phenocryst assemblage indicates that the Chilean andesite lavas. Other phenocrysts are augite, hornblende, and Fo83 olivine was formed by contamination of basalt magma as it passed rimmed with hypersthene. The ubiquitous inclusions are porphyrit- through the rhyolite ignimbrite sheets or residual rhyolite magma ic, with a groundmass of labradorite, hornblende, and pyroxene bodies beneath them. Because rhyolite activity was so voluminous and phenocrysts of calcic plagioclase, clinopyroxene, and or- and largely predated basalt activity, little basalt reached the surface thopyroxene. In bulk composition they are nearly identical to without contamination. An andesite-rhyolite association is the re- Taupo basaltic . The similarity of Tauhara lava to Glass sult. Andesite inherited quartz phenocrysts and oligoclase cores of Mountain lava is remarkable, the only difference being that the parent rhyolite magma contained phenocrysts prior to contamina- plagioclase phenocrysts from the rhyolite contaminant. An65_8o plagioclase phenocrysts and olivine cores in the augite phenocrysts tion by basalt and that domes were formed rather than a flow. were retained from the parent basalt magma. Contamination re- Although mixing is consistent with bulk compositions of Taupo

sulted in growth of new euhedral An45_62 plagioclase, as well as zone lava (Table 4), Ewart and Stipp (1968) have argued against it labradorite rims on the inherited oligoclase and augite rims on the on the basis of isotope abundances. The Sr87/Sr86 values inherited olivine. for the andesite are intermediate to values for rhyolite and basalt, but the average ratio for the andesite is 0.7056, whereas mixing an Taupo Volcanic Zone average basalt and rhyolite gives 0.7049. But the small number (4) of basalt samples analyzed may not be isotopically representative Like the Chilean Andes, the Taupo volcanic zone of New Zea- of the parent basalt magma, and involvement in the contamination land contains voluminous rhyolite ignimbrite sheets. But here process of minor amounts of radiogenic sediment, older concealed andesite and basalt are less important, with a total estimated vol- ignimbrites, or altered contaminants would tend to close the small ume of only 400 km3, compared to 16,000 km3 of rhyolitic lava gap of 0.0007. (Healy, 1964). The zone of rhyolitic volcanism is elongate, with mafic lava scattered throughout but concentrated at the ends. Tongan Islands Taupo basalt (Cole, 1973) is of the high-alumina type common around the Pacific. The usual phenocrysts are olivine, augite, and Lava of the active western chain of the Tongan Islands (Bryan labradorite or bytownite. Andesine or oligoclase is the most abun- and others, 1972; Ewart and Bryan, 1973) ranges from basaltic dant phenocryst phase in the rhyolite and is often accompanied by andesite to dacite, with andesite subordinate in volume. In bulk orthopyroxene, , , quartz, and hornblende, but composition, the suite is somewhat unusual — high in Fe and Ca rarely sanidine (Ewart and Stipp, 1968). and low in K. This makes interpretation of the phenocryst assem- The Tarawera volcanic complex (Cole, 1970a), near the center blage difficult, although the limited range of plagioclase composi- of the Taupo zone, consists primarily of rhyolite domes and pumice tion in the suite, An80-88, indicates that plagioclase may be inher- deposits, but blocks of andesite were ejected in 1020 A.D., and ited from a basaltic parent. One lava flow that is clearly a hybrid is basalt erupted in 1886 (Cole, 1970c). The 1020 andesite has the 1967-1968 Metis Shoal dacite. This flow is glomeroporphyrit- been called a "pyroxene basalt," although it is 58.5 percent Si02 ic, with phenocrysts of augite, En63 and En79 orthopyroxene, and similar to other lava of the same silica content. Phenocrysts are bytownite, titanomagnetite, and Fo93 olivine with orthopyroxene

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rims. The more magnesian orthopyroxene is the same composition plex: Jour. Geology and , v. 13, p. 903-924. as orthopyroxene in the basaltic andesite. Evidence that this dacite 1970c, Petrology of the basic rocks of the Tarawera volcanic complex: has been contaminated with basaltic material implies the existence New Zealand Jour. Geology and Geophysics, v. 13, p. 925-936. of magma that is more rhyolitic in composition. 1973, High alumina of Taupo volcanic zone, New Zealand: Lithos, v. 6, p. 53-64. More general evidence for the hybrid origin of intermediate lava Curtis, G. H., 1968, The stratigraphy of the from the 1912 eruption of the suite is the remarkably linear variation in bulk composition of and , , in Coats, R. R., Hay, R. L., (Table 4). The mafic parent magma may well be a true basalt that and Anderson, C. A., eds., Studies in (Williams volume): erupts as uncontaminated lava only at the submerged fringes of the Geol. Soc. America Mem. 116, p. 153-210. volcanic pile, like the fringing basalt of Lassen and Medicine Dickinson, W. R., 1970; Relations of andesites, , and derivative Lake. sandstones to arc-trench : Rev. Geophysics and Space Physics, v. 8, p. 813-864. CONCLUSIONS Eichelberger, J. C., 1974, Magma contamination within the volcanic pile: Origin of andesite and dacite: Geology, v. 2, p. 29-33. Ewart, A., and Bryan, W. B., 1973, The petrology and of the The abundance of rhyolite relative to other lava types at many Tongan Islands, in Coleman, P. J., ed., The western Pacific: volcanic centers has always presented a serious obstacle to accep- arcs, marginal , geochemistry: Perth, Univ. Western Australia tance of the hypothesis that it is the end product of fractional Press, p. 503-522. crystallization. This paper has shown that in may areas, dacite, the Ewart, A., and Stipp, J. J., 1968, Petrogenesis of the volcanic rocks of the logical predecessor of rhyolite in fractional crystallization, is actu- central North Island, New Zealand, as indicated by a study of Sr87/Sr88 ally derived from rhyolite rather than the reverse. Rhyolite must ratios, and Sr, Rb, K, U, and Th abundances: Geochim. et Cos- represent a primary melt, and dacite often does not. mochim. Acta, v. 32, p. 699-736. Andesite presents a more difficult problem. Although many Ewart, A., Taylor, S. R., and Capp, A. C., 1968, Trace and minor element andesites are clearly hybrid rocks, others lack evidence of mixing geochemistry of the rhyolitic volcanic rocks, central North Island, New Zealand: Contr. and Petrology, v. 18, p. 76-104. (Milhollen, 1975). Some of the latter may be well-stirred hybrids, Fenner, C. N., 1926, The Katmai magmatic province: Jour. Geology, v. 34, and some may represent primary melts or be derived from limited p. 673-772. fractional crystallization of basalt magma. Nevertheless, the wide- 1938, Contact relations between rhyolite and basalt on Gardiner spread evidence of mixing in intermediate lava of the circum-Pacific River, Yellowstone Park: Geol. Soc. America Bull., v. 49, p. volcanoes discussed here suggests that primary andesite magma is 1441-1484. not as important as has been thought. Finch, R. H., 1928, Lassen report no. 14: Volcano Letter 161, p. 1. Finch, R. H., and Anderson, C. A., 1930, The quartz basalt eruptions of ACKNOWLEDGMENTS Cinder Cone, Lassen Volcanic National Park, California: California Univ. Pubs. Geol. Sci., v. 19, p. 245-273. Friedman, I., 1968, Hydration rind dates rhyolite flows: Science, v. 159, p. I thank J. G. Blencoe, W. R. Dickinson, K. L. Williams, and 878-880. especially W. C. Luth for help and suggestions. My field work on Healy, J., 1964, Volcanic mechanisms in the Taupo volcanic zone, New Glass Mountain would not have been possible without the assis- Zealand: New Zealand Jour. Geology and Geophysics, v. 7, p. 6—23. tance of David McConaughy and my wife, Alice. National Science Lewis, J. F., 1968a, Tauhara Volcano, Taupo zone, Pt. I — Geology and Foundation Grant GA-1684 to R. H. Jahns, W. C. Luth, and O. F. structure: New Zealand Jour. Geology and Geophysics, v. 11, p. Tuttle and a Geological Society of America Penrose Grant to me 212-224. defrayed the expenses of field and analytical work. Final prepara- 1968b, Tauhara Volcano, Taupo zone, Pt. II — Mineralogy and pe- tion of this work was performed at Los Alamos Scientific Labora- trology: New Zealand Jour. Geology and Geophysics, v. 11, p. tory under the auspices of the U.S. Energy Research and Develop- 651-684. Macdonald, G. A., and Katsura, T., 1965, Eruption of Lassen Peak, Cas- ment Administration. D. B. Slemmons provided helpful suggestions cade Range, California, in 1915: Example of mixed magmas: Geol. regarding the manuscript. Soc. America Bull., v. 76, p. 475-482. REFERENCES CITED Milhollen, G. L., 1975, Magma contamination within the volcanic pile: Origin of andesite and dacite: Comment: Geology, v. 3, p. 164—168. 87 86 Anderson, C. A., 1933, Volcanic history of Glass Mountain, northern Peterman, Z., Carmichael, I., and Smith, A., 1970, Sr /Sr ratios of California: Am. Jour. Sci., v. 226, p. 485-506. Quaternary lavas of the Cascade Range, northern California: Geol. 1936, Volcanic history of the Clear Lake area, California: Geol. Soc. Soc. America Bull., v. 81, p. 311-317. America Bull., v. 47, p. 629-664. Pichler, H., and Zeil, W., 1972, The Cenozoic rhyolite-andesite association 1940, Hat Creek lava flow: Am. Jour. Sci., v. 238, p. 477-492. of the Chilean Andes: Bull. Volcanol., v. 35, p. 424-452. 1941, Volcanoes of the Medicine Lake Highland, California: Califor- Powers, H. A., 1932, The lavas of the Modoc Lava-Bed quadrangle, nia Univ. Pubs. Geol. Sci., v. 25, p. 347-422. California: Am. Mineralogist, v. 17, p. 253-294. Bowen, N. L., 1928, The evolution of the igneous rocks: New York, Doyer Siegers, A., Pichler, H., and Zeil, W., 1969, Trace element abundances in Pubs., Inc., 322 p. the Andesite Formation of northern : Geochim. et Cosmochim. Bowman, H. R., Asaro, F., and Perlman, I., 1973, On the uniformity of Acta, v. 33, p. 882-887. composition in and evidence for magmatic mixing: Jour. Steiner, A., 1958, Petrogenetic implications of the 1954 Ngauruhoe lava Petrology, v. 81, p. 312-327. and its xenoliths: New Zealand Jour. Geology and Geophysics, v. 1, p. Brice, J. C., 1953, Geology of the Lower Lake quadrangle, California: 325-363. California Div. Mines Bull. 166, p. 34-49. Wilcox, R., 1954, Petrology of the Paricutin volcano, Mexico: U.S. Geol. Bryan, W. B., Stice, G. D., and Ewart, A., 1972, Geology, petrography, and Survey Bull., v. 965-C, p. 281-353. geochemistry of the volcanic islands of Tonga: Jour. Geophys. Re- Williams, H., 1931, The dacites of Lassen Peak and vicinity, California, and search, v. 77, p. 1566-1585. their basic inclusions: Am. Jour. Sci., v. 222, p. 385-403. Bunsen, R., 1851, Uber die Prozesse der vulkanischen Gesteinbildungen 1932, Geology of Lassen Volcanic National Park: California Univ. Pubs. Geol. Sci., v. 21, p. 195-385. Islands: Annals Physics and Chemistry, v. 83, p. 197-272. Yoder, H. S., Jr., 1973, Contemporaneous basaltic and rhyolitic magmas: Chesterman, C. W., 1955, Age of the obsidian flow at Glass Mountain, Am. Mineralogist, v. 58, p. 153-171. Siskiyou County, California: Am. Jour. Sci., v. 253, p. 418-424. Cole, J. W., 1970a, Structure and eruptive history of the Tarawera volcanic complex: New Zealand Jour. Geology and Geophysics, v. 13, p. MANUSCRIPT RECEIVED BY THE SOCIETY APRIL 8, 1974 879-902. REVISED MANUSCRIPT RECEIVED MARCH 27, 1975 1970b, Petrography of the rhyolite lavas of Tarawera volcanic com- MANUSCRIPT ACCEPTED APRIL 24, 1975 Printed in U.S.A.

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