Origin of andesite and dacite: Evidence of mixing at Glass Mountain in California and at other circum-Pacific volcanoes JOHN C. EICHELBERGER* Department of Geology, Stanford University, Stanford, California 94305 ABSTRACT subtracting appropriate proportions of appropriate phases from a hypothetical parent liquid. Likewise, by choosing an appropriate The intimate association of basalt, andesite, dacite, and rhyolite hypothetical source rock and adjusting conditions, nearly any ob- within a volcanic center suggests that these rocks are genetically served composition could be produced by partial melting. The test related. Individual lava 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- magmas. 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 crystallization sition, Glass Mountain in northern California, was formed by con- process to the magmatic stage. However, fresh volcanic rocks rep- tamination of rhyolite magma as it intruded the basaltic flows of resent quenched magma samples in which the liquid remains as a the Medicine Lake Highland shield volcano. Although dacite flows metastable glass or finely crystalline groundmass, and the crystal and domes commonly show less variation in composition than the phases remain as phenocrysts. 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 phenocryst 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 Cascade Range, Chilean Andes, interpretation. The ideal object of study is a compositionally zoned Taupo volcanic zone, and Tongan Islands 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 petrology, 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 Earth'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 igneous petrology. 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 pumice. 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 mineral 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 sodium. 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 obsidian standard of nearly identical composition. An 55 plagioclase 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 olivine phenocrysts. Correction was made for Geological Society of America Bulletin, v. 86, p. 1381-1391, 7 figs., October 1975, Doc. no. 51007. 1381 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/10/1381/3418239/i0016-7606-86-10-1381.pdf by guest on 29 September 2021 1382 J. C. EICHELBERGER 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 gas 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 volcanism 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 shield volcano of uniformly gray and finely vesicular. From each dome there is a basalt and basaltic andesite lava flows and tuff 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 caldera 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 cinder cones and associated floods of basalt and basaltic andesite, one of which is as young as Glass Mountain, mantle 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 weathering 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.