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5-10-1988

Common characteristics of paired volcanoes in northern

S. P. Halsort Michigan Technological University

William I. Rose Michigan Technological University

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Recommended Citation Halsort, S. P., & Rose, W. I. (1988). Common characteristics of paired volcanoes in northern Central America. Journal of Geophysical Research, 93(B5), 4467-4476. http://dx.doi.org/10.1029/ JB093iB05p04467 Retrieved from: https://digitalcommons.mtu.edu/geo-fp/116

Follow this and additional works at: https://digitalcommons.mtu.edu/geo-fp Part of the Geology Commons, Mining Engineering Commons, and the Other Engineering Commons JOURNALOW GEOPHYSICAL RESEARCH, VOL. 93, NO. B5, PAGES4467-4476, MAY 10, 1988

COMMON CHARACTERISTICS OF PAIRED VOLCANOES IN NORTHERN CENTRAL AMERICA

Sid P. Halsor 1 and William I. Rose

Department of Geology and Geological Engineering, Michigan Technological University, Houghton

Abstract. Four pairs of active volcanoes Introduction along the northern Central American volcanic Spatial patterns of volcanoes provide a front have erupted basalt- magmas that context to study chemical evolution of volcanic show consistent intrapair behavioral and arcs. Compositional variation in rocks compositional differences. These differences are contemporaneously erupted along arcs records the found in records of volcanic activity and regional effects of changing physical parameters complete major and minor element data on over 200 (crustal thickness, segmentation of subducted samples. From northwest to southeast along the slab, spacing, and volcano size) at a volcanic front the four volcano pairs are Cerro given moment in time [Carr, 1984]. Compositional Quemado-Santa Maria, Tolim•n-Atitl•n, variation in rocks erupted across arcs may partly -Fuego, and Santa Ana-. The monitor changes in volcanic systems with time. volcano pair relations help explain compositional Some of the important across-arc variations differences, apart from those reflecting summarized by Gill [1981] are (1) increasing K20 variation in crustal thickness of about 15 km along the volcanic front, providing insight into and other incompatible element abundances away from the oceanic trench; (2) increasing silica across-arc variations and closely spaced subvolcanic plumbing systems. Intravolcano pair range of rocks containing olivine, hornblende, spacing is less than 5 km compared with an and biotite phenocrysts away from the volcanic front; and (3) in some cases a decrease in average intervolcano spacing of 25 km along the entire volcanic front. Within each volcano pair, 87Sr/86Sr and 206pb/204pb away from the plate the seaward volcano has had more frequent boundary. Increasing water and alkali content historic activity, erupting magmas that are and differing source regions and extent of generally more mafic, lower in large ion contamination in magmas away from the trench are lithophile elements and higher in Na20/K20 than often called upon to explain these across-arc variations [Sakuyama, 1977; Gill, 1981; Morrice magmas erupted from its landward counterpart. and Gill, 1986]. Each paired volcano site lies in close proximity This comparative study examines the eruptive to a rhyolitic , situated north or histories and geochemical similarities amoung northeast of the volcano pair. However, rare four pairs of active volcanoes, each pair earth element data at the Tolim•n-Atitl•n volcano situated across the northern Central American pair imply that mixing between caldera volcanic front. The geochemical data base and the mafic magma of the paired volcanoes does consists of over 200 whole rock major and trace not occur. Petrographic, isotopic, and other element analyses. (The data base used in this geochemical data from the Tolim•n-Atitl•n volcano study is a subset of a larger Central American pair suggest that separate but contemporaneous data file compiled by Carr and Rose [1987] called magma bodies beneath each volcano evolve and pass CENTAM.) Carr [1984; Carr et al., 1987] has through the crust at different rates. Atitl•n shown that crustal thinning and segmentation magraas are processed through the crust more influence some regional physical and chemical efficiently and with greater frequency than variations in Central America. A southeastward Tolim•n magmas, which undergo longer periods of 15-km thinning of the crust occurs along the stagnation interrupted by mafic injection and 250-km portion of the volcanic front that rapid eruption. This relation appears to hold at includes the pairs. Three of the volcano pairs the other paired volcano sites and is further are in , from west to east' Cerro evidence that closely spaced volcanoes, with Quemado-Santa Maria, Tolim•n-Atitl•n, similar subcrustal magma sources, evolve over Acatenango-Fuego, and the fourth pair, Santa separate magmatic plumbing systems that traverse Ana-Izalco, is in western E1 Salvador (Figure 1). the crust. The pairing pattern probably reflects Intrapair volcano spacing is less than 5 km at the regional southward migration of the volcanic all sites. Each volcano pair is also spatially front. associated with a silicic volcano center (Table 1). An aerial view of two of the Guatemalan volcano pairs is shown in Figure 2. The 1Now at Department of Earth and Environmental comparison of paired volcanoes was stimulated by Sciences, Wilkes College, Wilkes-Barre, detailed petrographic and geochemical Pennsylvania. examinations of the Tolim•n-Atitl•n , providing an extensive data base which so far exists only for that pair. Copyright 1988 by the American Geophysical Union. Age Relationships

Paper number 7B7075. All the paired volcanoes are probably less 0148-0227/88/007B-7075505.00 than 100,000 years old. The ages of some

4467 4468 Halsor and Rose: Volcano Pairs in Central America

Fig. 1. Map of southern Guatemala and western E1 Salvador showing the locations of active paired volcanoes. Volcano pairs are numbered 1 to 4 (volcano names are listed in Table 1). Solid triangles are nonpaired active volcanoes. volcanoes have been estimated by various methods. from frequently active basaltic vents to An age of 30,000 years is estimated for Santa sporadically or rarely active, but potentially Maria based on systematic variations in the more explosive, dacitic centers. Our estimates magnetic declination of lavas [Rose et al., 1977; of the relative age and behavioral maturity of Rose, 1987]. No age information is available for landward and seaward volcanoes are given in Table 2. the Cerro Quemado dome complex, though its morphology and erosion suggest that the early Historic Activity stage of dome growth preceded the emergence of Santa Maria. Tolim•n and Atitl•n volcanoes have Changes in the focus of activity among a probable maximum age of 84,000 years based on across-arc volcano pairs are more systematic than the extrapolated position of underlying Los volcano activity patterns parallel to the Chocoyos ash [Rose et al., 1980]. Cone growth at volcanic front. Records of historic acativity these two volcanoes has largely overlapped, [Simkin et al., 1981] show that at each volcano although only Atitl•n has had historic activity. pair the seaward volcano has had more frequent Extrapolation of historic eruption rates at Fuego historic activity (Figure 3). This is consistent suggests a minimum age of 17,000 years [Chesner with observations first made by Dollfus and de and Rose, 1984]. The age of Acatenango is Montserrat [1868]. This implies migration of the unknown. Cross-cutting structural relationships volcanic front in a seaward direction. Figure 3 between Santa Ana and Coatepeque caldera suggest does not distinguish between styles of activity that Santa Ana is much older than the in which there are some important differences 10,000-year-old caldera [Carr and Pontier, 1981]. among the younger seaward volcanoes. For The first eruption of Izalco occurred in 1770 instance, the more mature seaward volcano, Santa [Cart and Pontier, 1981], making this cone Maria, underwent a major plinian eruption in clearly the youngest vent of all the paired 1902, followed in 1922 by continuous volcanoes. The behavior of these volcanoes seems dome-building eruptions which have continued to to be related to their age, with centers evolving the present. This style contrasts sharply with

TABLE 1. Principal References of Paired Volcanoes and Associated Siiicic Centers

Pair No. Landward Vent Seaward Vent Associated Silicic Center

Cerro Quemado Santa Maria volcano [Johns, 1975] [Rose et a1.,1977; [Johns, 1975] Rose, 1987]

Tolim•n Atitl•n Atitl•n caldera [Rose et a1.,1980; [Rose et a1.,1980; [Rose et a1.,1987; Penfield et a1.,1986] Penfield et a1.,1986] Newhall, 1987]

Acatenango Fuego Baharona caldera [Chesner and [Martin and Rose, 1981; [Bornhorst and Rose, 1984] Chesner and Rose, 1984] Rose, 1982]

Santa Ana Izalco Coatepeque caldera [Carr and [Carr and [Meyer, 1964] Pontier, 1981] Pontier, 1981] Halsor and Rose: Volcano Pairs in Central America 4469

., ...

...

!

ß

Fig. 2. View looking east of two volcano pairs in Guatemala. the nearly continuous Strombolian activity at its landward counterpart (Figure 4). The bimodal Izalco, a seaward volcano in a more immature distribution of rocks erupted at Santa Maria is evolutionary stage (see Table 2). anomalous and reflects the more mature behavioral Compositional Range state of that center. Tolim•n-Atitl•n and Santa

Within each pair, the seaward volcano erupts a predominance of basalt or basaltic andesite CERRO QUEMADO which contrasts with a broader compositional SANTA MARIA range and predominance of more evolved rocks from I , ,i I I TO•.•MXN I...... ,4 ATIT L,,•N I I

ACATENANGO I,...... ,4 FUEGO I

SANTA ANA

IZALCO , , $i02

! i ! S0 60 70 ß ß ß ß ß ß A.D. 1500 1700 1900 YEAR Fig. 4. Comparison of SiO2 ranges in rocks Fig. 3. Frequency of historic activity at volcano recently erupted from paired volcanoes (dashed pairs (stippled bars represent landward volcano; line represents landward volcano; solid line, solid bars, seaward volcano). seaward volcano). TABLE 2. Descriptive Features and Relative Ages (Based on Behavioral Maturity) of Landward and Seaward Volcanoes

D•$½ription Volume.km3 RecentRock Type_

Landward volcanoes (in order of increasing behavioral maturity)

Acatenango large, rarely active 70 andesite

TolimRn large, rarely active stratovolcano 60 andesite

Santa Ana large stratovolcano, deep crater 165 andesite

Cerro Quemado dome complex on ring of older cone ? Seaward volcanoes (in order of increasing behavioral maturity)

Izalco small, continually active cone <5 basalt

Fuego large, frequently active stratovolcano 40 basalt

Atitl•n large, sporadically active stratovolcano 50 andesite

Santa Maria large strataovolcano, dome-building stage 20 dacite

! I I I •

a

K20 K20

a

a a % #a - 2

a K20 aa•a•a a K20 a n

0 I I I ,, I o 0 • 4 6

. !

Na20/K20 Na20/K20 - 4 "• o 3

-

Ooa• a a

a! a . :,

I ,

4 - _ o

. a•Oa o

n 2 -

ß•D 2 Fig. 5. Variation diagrams for volcanic rocks of paired volcanoes (open squares represent landward volcanoes; asterisks, seaward volcanoes). Halsor and Rose: Volcano Pairs in Central America 4471

60 ' showing greater intrapair differences (Figure 6). Ba and Rb vary sympathetically among seaward 50 um l.ndw.rd v01c.n0 volcanoes but not at landward vents. Cart et al. • mse.w. rdv01c. n0 [1987] report a poorly understood positive correlation of incompatible elements with volume • 40 among Central American volcanic centers. This relationship is evident at any given volcano pair (except perhaps at Cerro Quemado-Santa Ana) where the volume and LIL element contents are higher at 20 the landward volcano. However, there is no clear 10 relationship between volume and LIL elements (and behavioral maturity) in comparing only landward 0 volcanoes or only seaward volcanoes. For 1 2 • 4 instance, Izalco is clearly at an immature behavioral stage and has a very small volume, but it has Rb contents higher than two other southern 800 volcanoes (Table 2, Figure 6).

600 T o 1 im•n-At it 1 •n

400 Petrographic and geochemical differences observed at Atitl•n and Tolim•n volcanoes (Figure 7) offer insight into the causes of intrapair 200 compositional and behavioral variation. Like other landward volcanoes, Tolim•n erupts more silicic products than its seaward neighbor (Figure 4); however, at a given SiO 2 value, 2 • 4 Tolim•n rocks possess more magnesium than Atitl•n VOLCANO PAIR rocks (Figure 8). This is contrary to compositional variation produced by simple liquid line of descent of common parental magma. The Fig. 6. Comparison of LIL element abundances presence of forsteritic olivine showing among volcano pairs. disequilibrium texture (Figure 9) reflects the high-magnesium bulk composition and nonequilibrated nature of Tolim•n silicic Ana-Izalco pairs show similar compositional andesires. ranges even though their southern volcanoes Tolim•n rocks have much more conspicuous differ in behavioral maturity. Although Izalco evidence of disequilibrium than do Atitl•n is the youngest seaward volcano, Fuego has specimens. Tolim•n samples show stronger erupted the least evolved basaltic compositions. bimodality in plagioclase composition, more prevalent reverse zoning in pyroxene phenocrysts, Variation Diagrams and a persistent nonreaction relationship between orthopyroxene and olivine (Figure 10) . Some of Other consistent intrapair geochemical these disequilibrium features are observed at differences can be seen in elemental variation other landward volcanoes, such as Cerro Quemado diagrams. Seaward volcanoes show higher Na20/K20 [Johns, 1975] and Santa Ana [Cart and Pontier, ratios and flatter slopes on K20 versus MgO plots 1981]. Several Tolim•n also contain (Figure 5). Sodium enrichment with increasing embayed or reaction-rimmed quartz xenocrysts and crustal thickness is part of a regional large, silicic glass inclusions in some sodic plagioclase phenocrysts. northwesterly trend present along the Central American volcanic front [Carr et al., 1979; Carr, The above-mentioned disequilibrium features 1984]. Rose [1987] has shown that in andesires are evidence for magma mixing and have been cited from several volcanoes in the vicinity of Santa with growing frequency at other andesite localities [Anderson, 1976; Eichelberger, 1978; Maria, where crustal thickness approaches a Luhr and Carmichael, 1980; Gerlach and Grove, regional maximum (~45 km), Na enrichment is a 1982; Sakuyama, 1981, 1984; Koyaguchi, 1986]. At very erratic localized phenomenon. Regionally, Tolim•n, the common coexistence of orthopyroxene, K20 correlates positively with the volumes of quartz, hornblende, and silicic glasses included volcanoes [Carr, 1984]. This is consistent among in sodic plagioclase points to a silica-rich volcano pairs where greater volumes and potash end-member (dacite-rhyolite) mixing with a contents are associated with landward volcanoes. basaltic end-member containing forsteritic olivine and clinopyroxene. Because of the close Large Ion Lithophile Element Abundances spatial and temporal relationahip between Tolim•n volcano and the Atitl•n caldera, strontium Potash enrichments at landward volcanoes are isotope and rare earth element (REE) analyses mirrored by elevated large ion lithophile (LIL) were made on andesires from the volcano and element abundances. Rubidium and barium from the caldera to test whether abundances normalized to 55% SiO 2 for each andesites are basalt-rhyolite mixing products. volcano are higher at landward volcanoes, with Rb Although andesite and rhyolite have nearly 4472 Halsor and Rose: Volcano Pairs in Central America

Fig. 7. View looking southwest at Tolim•n-Atitl•n volcano pair. Tolim•n is the foreground volcano.

identical isotopic signatures and these data lavas. This hypothesis is consistent with the produce inconclusive results, REE patterns show observed petrographic relations- forsteritic that andesites cannot be simple two-component olivines, calcium-enriched plagioclase rims, mixtures of high-alumina basalt and Atitl•n reversely zoned pyroxenes from a basaltic rhyolite (Figure 11). end-member, and orthopyroxene, quartz, and Tolim•n andesites probably preserve their hornblende from a silicic end-member. Because high degree of disequilibriumby rapid eruption basaltic andesites of Atitl•n contain following internal mixing in a graded magma equilibrated olivines, augite phenocrysts with chamber [McBirney, 1980]. Mixing in the chamber normal and reverse zoning, mildly bimodal might be induced by an injection of hot mafic plagioclase rim compositions, and lack magma which subsequently causes the chamber to erupt poorly mixed, nonequilibrated intermediate

• Atitl•n 70 20 I;;'7ATolim•n

-

_ oø o c• 50 iO-

- _ 0 , , , , ø .L ß •'5 ' ' •'o 50 5'5' ' '6'0' ' ' . 64 Fo in o]ivine Si02 Fig.. 9a. Frequency distribution of forsterite Fig. 8. Variation in Mg# (100 [Mg/Mg+Fe2+]) of component (Fo = atomic % Mg) in Tollm•n and Tolim•n and Atitl•n rocks. Atitl•n olivines. Halsor and Rose: Volcano Pairs in Central America 4473

90 rapid recharge, causing frequent eruptions of : "Tolim•n completely mixed magma. Frequent eruptions * Atitl•n o KD=.3 impede the development of a silicic cap. The magma chamber beneath Tolim•n endures longer ,__--._c 85 + • static evolution with infrequent mafic injections .c_80 " quickly followed by eruptions. o .• Magma Reservoir Piracy

E 75 • Cart [1984] and a series of other workers [Chesner and Rose, 1984; Grant et al., 1984; 70 ...... Rose, 1987; Halsor et al., 1985] have shown that 40 5'0 6'0 70 magma and crustal densities together with whole rock Mg• volcanic edifice heights imply that Central American volcanoes have been supplid by magma Fig. 9b. Most forsteritic olivine compositions accumulated at the base of the crust. The sizes versus whole rock Mg#. The equilibrium curve of magma reservoirs are proportional to edifice assumes KD = 0.3, where KD = volumes. Rose [1987] has estimated the (FeO/MgO)ol / (FeO/MgO)liq- deep-level magma body beneath Santa Maria to have had a size of at least 30-40 km3. Because Santa Maria has one of the smaller edifice volumes, its orthopyroxene, quartz, and hornblende, they may magma reservoir volume can be viewed as generally represent more thorough mixtures of basalt and small when compared with the size of reservoirs intermediate andesitic magmas (Figure 12). underlying the other paired volcanoes. The The behavioral patterns at AtitlAn and envisioned dimensions of deep-level magma TolimAn volcanoes are linked to the differences reservoirs, then, is greater than intrapair in the nature of their open-system magma bodies. volcano spacing (less than 5 km) . This means The magma chamber beneath Atitl•n may undergo

Tolim•n I normal •2 30- •II type EZ• reverse

20-

o ß ! 3o 45 Anmole 60 % 75 90 Tol•man. ,, Atitl•n Fig. lob. Atitl•n • ty.. EZ• type 6- Tolim•n Ol•v•ne Opx Hnblnd 4- Quartz

Atitl•n 2- OllVlne Opx , , Hnb' -d absent qua-•z absent

o 3'o 60 75 90 45 Anmole

Fig. 10a. Fig. 10c.

Fig. 10. Petrographic differences observed in Tolim•n and Atitl•n rocks. (a) Frequency distribution of compositions of two types of plagioclase phenocrysts- type 1 generally have clear interiors, normal > reverse zoning, and type 2 possess dusty or cellular morphology, reverse > normal zoning. (b) Diagram showing the relative proportions of normal and reverse zoning in augite phenocrysts. (c) Phenocryst mineralogy versus SiO2 (plagioclase and cpx are omitted but occur throughout the compositional range at each volcano). 4474 Halsor and Rose- Volcano Pairs in Central America

•00 volcano tends to defuse magmatic activity slowly from the landward volcano. Furthermore, we • I I I I I I I i I I I 0 LCI rhyoliteI I I _ believe the time at which the seaward volcano ß Tol andesite emerges is critical to the evolving activity at ß At andesite the landward volcano. Rose [1987] has outlined a A F basalt three-phase history of activity at Santa Maria: a long period of growth of the composite cone, a dramatic plinian eruption, and a period of dome extrusion. It is possible that the large-scale plinian eruption requires an evolutionary period •0 __

_ lasting centuries and regular supply of magma _ from depth. We suggest that other landward

_

_ volcanoes have not had large ptinian eruptions _ because the "defusing" effect, or shift in focus of activity to the seaward volcano, inhibits supply of magma from depth. Santa Maria may not have evolved to a plinian eruption stage if a new vent had emerged to the south of it. The defusing effect of the "magma reservoir piracy" ! I I I I I I I I I I I I I I concept suggests that it is unlikely the landward La Ce Sm Eu Lu volcanoes Tolim•n, Acatenango, and Santa Ana will produce plinian eruptions. It also suggests that Fig. 11. Average chondrite normalized REE nonpaired volcanoes in the same area, like Agua, abundances in Tolim•n (Tol, n=10) and Atitl•n (At, are potential sites for major explosive activity. n=6) andesires. The stippled field is bracketed by Fuego high alumina basalt (F basalt, n=31) and Los Chocoyos rhyolite (LC rhyolite, n=15). Conclusions

The aim of this study is to call attention to volcano pairing. Several aspects of pairing of that parental magmas rising beneath volcano pairs volcanoes in Central America appear to be present could originate from the same magma reservoir, in parts of other volcanic arcs' Colima and and implies that their magmas develop Nevado de Colima volcanoes in the Mexican compositional differences as they traverse the volcanic belt, San Pedro and San Pablo volcanoes crust along different routes. in the central Andes (J. Luhr, personal We propose that the reason the southern communication, 1987), and across-chain volcanoes volcanoes are younger and have experienced more of the Mariana arc (P. Fryer, personal frequent historic activity is due to southward communication, 1987). We encourage consideration migration of magma reservoirs near the base of of this concept in other areas. the crust. Carr et at. [1982] have shown that Conclusions from this study are as follows. offsets and changes in strike of volcanic 1.) Across-arc paired volcanoes occur in lineaments along the Central American volcanic several places in northern Central America. front are due to transverse breaks that segment 2.) Intrapair volcanoes overlie separate the plate margin (Figure 13). This concept plumbing systems fed by common parental magmas. suggests that melting occurs at a common depth on 3.) Landward volcanoes erupt poorly mixed each segment. Segments with steeper dipping underthrust slabs have volcanoes that lie nearer andesite, possibly triggered by influx of mafic basalt. to the trench. If the dip continues to increase, the magma reservoir at the base of the crust will 4.) Seaward volcanoes erupt more frequently and tend to produce well-mixed basaltic andesite. move oceanward (southwest) as the focus of 5.) Southward migration of the volcanic melting moves "updip" near the slab. Steepening front is a probable cause of pairing and of the slab and southward migration of the preserves the spacing of centers. reservoirs might be caused by a decrease in the 6.) Magma reservoir piracy occurs when a convergence rate between the Cocos and the seaward volcano emerges and slowly defuses overriding Caribbean plates. Admittedly, this is magmatic activity from the landward volcano. only one speculative explanation, and various geometric mechanisms exist to shift relative plate positions. Tolim•n The shift in focus of activity from the landward to seaward volcano in each pair is not ¸ abrupt but gradual, such that both volcanoes may Basalt Dacite experience activity in the same interval of time. Izalco, the youngest southern volcano, has dominated the site of activity since its emergence; still, Santa Ana has had several historic eruptions. The same observations seem SiO 2 to apply for Fuego and Acatenango. At a more mature volcano pair, Totim•n and Atitl•n, active 5'0 •0 7'0 periods have largely overlapped, although Atitl•n has been more active in the last several Fig. 12. Compositional range relationship between centuries [Rose et at., 1980]. These intrapair end-members and mixed Tolim•n and Atitl•n eruptive relationships suggest that the seaward andesites. Halsor and Rose- Volcano Pairs in Central America 4475

VOLCANIC

FRO

MONOGENETIC so

lOO

15o

KM

Fig. 13. Block diagram illustrating principal features of a transverse break in Central America [Carr et al., 1982]. Note that the position of volcanic front segments is controlled by the intersection of lithosphere and asthenosphere with the underthrust slab. One possible way to develop volcano pairing in a trenchward direction is to increase the subduction angle of the underthrust slab.

Acknowledqments. Financial support for this J. Volcanol. Geotherm. Res., 11, 277-292, work came from the National Science Foundation 1981. through grant EAR 82-05606, the Smithsonian Carr, M. J., and W. I. Rose, CENTAM--A data Institution, and Michigan Technological base of Central American volcanic rocks, J. University. Michael Carr, Alexander McBirney, Volcanol. Geotherm. Res., 33, 239-240, and an anonymous reviewer helped us improve the 1987. text. Carr, M. J., W. I. Rose, and D. G. Mayfield, Potassium content of lavas and depth to References the seismic zone in Central America, J. Volcanol. Geotherm. Res., f, 387-401, 1979. Anderson, A. T., Magma mixing: Petrological Carr, M. J., W. I. Rose, and R. E. Stoiber, process and volcanological tool, J. Central America in Andesites, edited by R. Volcanol. Geotherm. RES., l, 3-33, 1976. S. Thorpe, pp. 149-166, Wiley, New York, Bornhorst, T. J., and W. I. Rose, Quaternary 1982. Barahona caldera complex, Guatemala Chesner, C. A., and W. I. Rose, Geochemistry abstract), Eos Trans. AGU, 63, 1155, and evolution of the Fuego complex, 1982. Guatemala, J., Volcanol. Geotherm. Reu., Carr, M. J., Symmetrical and segmented 21, 25-44, 1984. variation of physical and geochemical Dollfus, A., and E. de Montserrat, Voyage characteristics of the Central American geologique dans les republiques de volcanic front, J. Volcanol. Geotherm. Guatemala et E1 Salvator, Paris. Res., 20, 231-252, 1984. Imperiale, 539 pp., 1868. Carr, M. J., M.D. Feigenson, and J. A. Eichelberger, J. C., Andesitic volcanism and Walker, Models of Central American crustal evolution, Nature, 275, 21-27, volcanism inferred from systematic 1978. regional variations in geology, Gerlach, D.C., and T. L. Grove, Petrology of geochemistry and volcanology (abstract) in Medicine Lake Highland volcanics: Hawaii S_vmposium on How Volca•o• Work, Characterization of end members of magma p. 29, Hilo, Hawaii, Jan. 19-25, 1987. mixing, Contrib. Mineral. Petrol., 80, Carr, M. J., and N. K. Pontier, Evolution of 147-159, 1982. a young parasitic cone towards a mature Gill, J., Oroqenic Andesites and Plate central vent- Izalco and Santa Ana Tectonics, 390 pp., Springer, New York, volcanoes in E1 Salvador, Central America, 1981. 4476 Halsor and Rose: Volcano Pairs in Central America

Grant, N. K., W. I. Rose, and L. A. Fultz, Rose, W. I., Santa Maria, Guatemala: Bimodal Correlated Sr isotope and geochemical soda-rich calcalkalic stratovolcano, J. variations in basalts and basaltic Volcanol. Geotherm. Res., 33, 109-129, andesites from Guatemala in Andean 1987. Ma_c•natism, edited by R. S. Harmon and B. Rose, W. I., N. K. Grant, G. A. Hahn, I. M. A. Barreiro, pp. 139-149, Birkhauser Lange, J. L. Powell, J. Easter, and J. M. Boston, Cambridge, Mass., 1984. Degraff, The evolution of Santa Maria• Halsor, S. P., W. I. Rose, T. J. Bornhorst, volcano, Guatemala, J. Geol., 85, 63-87, and G. T. Penfield, Strato-geochemical 1977. relationships at Lake Atitl•n, Guatemala- Rose, W. I., C. G. Newhall, T. J. Bornhorst, Multiple magmatic processes beneath three and S. Self, Quaternary silicic andesitic volcanoes, Eos Trans. AGU, 66, pyroclastic deposits of Atitl•n caldera, 1146, 1985. Guatemala, J. Volcanol. Geotherm. Res., Johns, G. W., Geology of the Cerro Quemado 33, 57-80, 1987. Volcanic Dome Complex, M.S. thesis, 124 Rose, W. I. Jr., G. T. Penfield, J. W. pp., Michigan Technological University, Drexler, and P. B. Larson, Geochemistry of Houghton, 1975. the andesite flank lavas of three Koyaguchi, T., Textural and compositional composite cones within the Atitl•n evidence for magma mixing and its cauldron, Guatemala, Bull. Volcanol., mechanism, Abu volcano group, southwestern 43-1, 131-153, 1980. Japan, Contrib. Mineral. Petrol., 93, Sakuyama, M., Lateral variations of H20 33-45, 1986. contents in Quaternary magma of Luhr, J. F. and I. S. E. Carmichael, The northeastern Japan, Bull. Volcanol. Soc. Colima volcanic complex, Mexico, I., Jpn., 22• 263-271, 1977. Post-caldera andesites from volcan Colima, Sakuyama, M., Petrological study of the Myoko Contrib. Mineral. Petrol., 71, 343-372, and Kurohime volcanoes, Japan- 1980. Crystallization sequence and evidence for Martin, D. P., and W. I. Rose, Behavior magma mixing, J. Petrol., 22, 553-583, patterns of Fuego volcano, Guatemala, J. 1981. Volcanol. Geotherm. Res., 10, 67-81, 1981. Sakuyama, M., Magma mixing and magma plumbing McBirney, A. R., Mixing and unmixing of systems in island arcs, Bull. Volcanol., magmas, J. Volcanol. Geotherm. Res., !, 47-4, 685-702, 1984. 357-371, 1980. Simkin, T., L. Siebert, L. McClelland, D. Meyer, J., Stratigraphie des BimKiese und Bim Bridge, C. Newhall, and J. H. Latter, ashen des Coatepeque Vulkans in Westlichen Volcanoes of the World, 232 pp., E1 Salvador (Mittel Amerika), Neues Jahrb. Smithsonian Institution, Washington, D.C., Geol. Paleontol., 119, 215-246, 1964. 1981. Morrice, M. G. and J. B. Gill, Spatial patterns in the mineralogy of island arc magma series: Sangihe arc, Indonesia, J. Volcanol. Geotherm. Res., 29, 311-353, S. P. Halsor, Department of Earth and 1986. Environmental Sciences, Wilkes College, Newhall, C. G., Geology of the Lake Atitl•n Wilkes-Barre, PA 18766. area, Guatemala, J. Volcanol. Geotherm. W. I. Rose, Michigan Technological University, Res., 33, 23-55, 1987. Houghton, MI 49931. Penfield, G. T., W. I. Rose, and S. P. Halsor, Geologic map of the Lake Atitl•n (Received May 14, 1987; volcanoes, Geol. Soc. Am. Map Chart Ser., revised September 21, 1987; MC-55, 1986. accepted October 20, 1987.)