Geochemistry of the Andesite Flank Lavas of Three Composite Cones within the Atitlfin Cauldron, Guatemala W.I, ROSE, Jr. G.T. PENFIELD 1 J.W. DREXLER P.B. LARSON i Michigan Technological University, Houghton, Michigan 49931. Oberlin College, Oberlin, Ohio 44074 Michigan Technological University, Houghton, Michigan 49931 ABSTRACT Three composite cones have grown on the southern edge of the previously existing Atit- lfin Cauldron, along the active volcanic axis of Guatemala. Lavas exposed on the flanks of these cones are generally calc-alkaline andes- ites, but their chemical compositions vary wide- ly. Atitl~n, the largest and most southerly of the three cones, has recently erupted mainly pyroclastic basaltic andesites, while the flanks of San Pedro and Tolim~n are mantled by more silicic lava flows. On Tolimfin, 74 differ- ent lava units have been mapped, forming the basis for sequential sampling. Rocks of all three cones are consistently higher in K20, Rb, Ba and REE than other Guatemalan andesites. Atitl~n's rocks and late lavas from Tolim~n have high A1.203 content, compared to similar andesites from other nearby cones. All major and trace element data on the rocks are shown to be consistent with crystal fractionation involv- ing phases observed in the rocks. If such mod- els are correct, significant differences in the relative proportions of fractionation phases are necessary to explain the varied compositions: in particular higher A120~rocks have fractionat- ed less plagioclase. We speculate that inhibi- tion of plagioclase fractionation could occur in chambers where PH20 is greater and when re- pose intervals are shorter. The distribution of volcanic vents throughout Guatemala which Present addresses: Penfield: Aeroservice Division, Western Geophysical Co., Houston, Texas, 77063. Larson: California Institute of Technology, Pa- sadena, California 91125. Bull. Volcanol., Vol. 43-1, 1980. show this postulated ~dnhibition of plagioclase fractionatlon~ is systematic with such vents lying just to the south of the main axis. The an- desites of the three cones cannot be simply re- lated to the late-Pleistocene rhyolites which are apparently associated with cauldron formation, because unlike the andesites, the rhyolites have markedly depleted heavy REE abun- dances. Recent dacitic lavas from vents south of San Pedro volcano and silicic pyroclastic rocks which mantle the slopes the San Pedro may reflect residual post-cauldron rhyolitic volcanism. INTRODUCTION The WNW-trending active volcanic axis of Central America runs parallel to the Middle American Trench which is located 100 km to the southwest. An active seis- mic belt is also parallel to the trench and the volcanic chain, and the area has been characterized as a converging plate bound- ary (MOLNAR and SYKES, 1969). Within the Guatemala Highlands, most of the youngest generation of volcanoes are com- posite cones, composed of andesitic and basaltic lavas (ROSE et al., 1977, 1978; WOODRUFF et al., 1978; EGGERS, 1971; GIERZYCKI, 1976). Three Quaternary composite cones, San Pedro, Tolim~n, and Atitl~n, are found along the south side of the Atitl~n Caul- dron in the Guatemalan Highlands. To- gether the three cones represent a volume of about 100 km 3 (Table 1) and most of
Transcript
Geochemistry of the Andesite Flank Lavas of Three Composite Cones within the Atitlfin Cauldron, Guatemala
Three composite cones have grown on the southern edge of the previously existing Atit- lfin Cauldron, along the active volcanic axis of Guatemala. Lavas exposed on the flanks of these cones are generally calc-alkaline andes- ites, but their chemical compositions vary wide- ly. Atitl~n, the largest and most southerly of the three cones, has recently erupted mainly pyroclastic basaltic andesites, while the flanks of San Pedro and Tolim~n are mantled by more silicic lava flows. On Tolimfin, 74 differ- ent lava units have been mapped, forming the basis for sequential sampling. Rocks of all three cones are consistently higher in K20, Rb, Ba and REE than other Guatemalan andesites. Atitl~n's rocks and late lavas from Tolim~n have high A1.203 content, compared to similar andesites from other nearby cones. All major and trace element data on the rocks are shown to be consistent with crystal fractionation involv- ing phases observed in the rocks. If such mod- els are correct, significant differences in the relative proportions of fractionation phases are necessary to explain the varied compositions: in particular higher A120~ rocks have fractionat- ed less plagioclase. We speculate that inhibi- tion of plagioclase fractionation could occur in chambers where PH20 is greater and when re- pose intervals are shorter. The distribution of volcanic vents throughout Guatemala which
Present addresses: Penfield: Aeroservice Division, Western Geophysical Co., Houston, Texas, 77063. Larson: California Institute of Technology, Pa- sadena, California 91125.
Bull. Volcanol., Vol. 43-1, 1980.
show this postulated ~dnhibition of plagioclase fractionatlon~ is systematic with such vents lying just to the south of the main axis. The an- desites of the three cones cannot be simply re- lated to the late-Pleistocene rhyolites which are apparently associated with cauldron formation, because unlike the andesites, the rhyolites have markedly depleted heavy REE abun- dances. Recent dacitic lavas from vents south of San Pedro volcano and silicic pyroclastic rocks which mantle the slopes the San Pedro may reflect residual post-cauldron rhyolitic volcanism.
INTRODUCTION
The WNW-trending active volcanic axis of Central America runs parallel to the Middle American Trench which is located 100 km to the southwest. An active seis- mic bel t is also parallel to the t rench and the volcanic chain, and the area has been characterized as a converging plate bound- ary (MOLNAR and SYKES, 1969). Within the Guatemala Highlands, most of the youngest generat ion of volcanoes are com- posite cones, composed of andesit ic and basaltic lavas (ROSE et al., 1977, 1978; WOODRUFF et al., 1978; EGGERS, 1971; GIERZYCKI, 1976).
Three Quaternary composite cones, San Pedro, Tolim~n, and Atitl~n, are found along the south side of the Ati t l~n Caul- dron in the Gua temalan Highlands. To- gether the three cones represent a volume of about 100 km 3 (Table 1) and mos t of
132 w.I. ROSE - T. PENFIELD - W. DREXLF-~ - P.B. LARSON
TABLE I - Comparison of physical and geologic observations of the Three Atitl~u volcanoes.
Atitl~n Tolim~n San Pedro
volume, kln 3 35 +i0
summit elevation, m 3535
relative age of youngest (most latest activity recent activity)
composition of surface basaltic rocks andesites
recent eruptive type pyroclastic
vents chiefly central
45 +5 25 +5
3158 3017
slightly older oldest
andesltes andesites
lava flows flows and & domes pyroclastlcs
central & central & lateral lateral
the recent lavas are andesitic (LARSON and ROSE, 1973). San Pedro and Toli- m~n are located entirely inside the Ati- tl~n Cauldron, while Atitl~n straddles the southern marginal fault of the cauldron (WILLIAMS, 1960). The cauldron is suppos- ed by WmHAMS (1960) to have originat- ed by (<collapse along ring fractures of an oval block of Tertiary and older rocks as a consequence of subterranean migra- tion of magma...>>. WILLIAMS suggested that this <<migration of magma>~ was as- sociated with the reservoirs which fed the cones of Atitlfin, Tolim~n, and San Pedro. However, recognition that the vast Pleistocene rhyolitic deposits of the Los Chocoyos Ash (HAHN et al., 1979; ROSE et al., 1979; KOCH and MCLEAN, 1975) probably had their source in the Atitl~n region suggests that the cauldron subsidence could have occurred in con- nection with eruption of the Los Choco- yes magma. HAHN et al., (1979) have pointed out that the volume of Los Cho- coyos Ash (> 300 km 3) is appropriate for the size of the AtitlKn cauldron, ac- cording to the models of SMITH (1976). The latter ideas fit bet ter with field ob- servations which are discussed below.
LOCAL GEOLOGY
The age of the Los Chocoyos Ash is > 62,000 years, based on C 14 dating (ROSE et al., 1979), and correlation of these ashes with the Y8 ash layer in 01e/018 dated cores from the Caribbean Sea (DREXLER, 1978; LEDBETTER et at., 1978) establishes an age of 85,000 ± 5,000 y. BP. Lavas of Atitl~n vol- cano clearly rest upon ash-flow deposits of the Los Chocoyos Asl~ Thus the Los Choco- yes Ash predates at least the later laves of Atitl~n volcano, and possibly the other two volcanoes as well. The three volcanoes within the Atitl~n Cauldron all seem to have grown upon topography in which the caul- dron already existed. For eYsmple, San Pedro does not have lavas which flowed to the south before the scarp caused by caul- dron subsidence developed. Instead San Pedro's lavas are constrained within the caul- dron margin by the scarp. Elevated pre- cauldron basement crops out high on the south slopes of AtitlAn Volcano, which straddles the scarp (Fig. 1). This suggests that the cone was built on the scarp. We conclude that most, perhaps all, of the erup- tive activity at the Atiti~in volcanoes occur- red after cauldron subsidence.
GEOCHEMISTRY. ' OF T H E ANDESYVE FLANK LAVAS OF T H R E E COMPOSITE CONES, ETC. 133
RELATIVE, AGES OF CONES
Among the three cones, we estimate San Pedro to have been first to decline in activity. This judgement is based mainly on geomo12ahology, and the degree of sub- mergence of the cones by Lake Atitl~n. The lake level has evidently risen as ac- tivity of Atitl~n has dammed the egress of water coastward. At present the elevation of the saddle which prevents the lake's surface drainage to the Pacific south of San Lucas Tolim~n is only about 30 m above lake level. The San Pedro cone has a very steep profile above lake level simi- lar to the upper slopes of other composite cones in the region. Its lower slopes are now submerged by waters backed up after activity of Atitl~n. The marked contrast in the profiles of Tolim~n and San Pedro is striking and supports the hypothesis that Tolim~n's activity continued long after San Pedro had ceased. The contrast in vegetative growth on the two cones (large and dense vegetation on San Pedro; scrub growth on Tolim~n) is al.~o consistent with this hypothesis.
Late Silicic Pyroclastic Ejecta [rom San Pedro
The Tertiary volcanic rocks to the west of San Pedro are draped with young de- .posits of white airfall pumice of rhyodacit: lc composition. The stratigraphy of these deposits has not been established in de- tail, but there are certainly at least two distinct deposits. Sampled on the low- er flanks of San Pedro, one of these layers overlies the A 4.0 flow (Fig. 1; 84×25), another is found on the A 1.0 sur- face (Fig. 1, 84×23). The trace chemistry of these biotite-bearing tephra, shown in Table 2, suggests correlation with the I tephra of KOCH and MCLEAN (1975) and HAHN et al., (1979). The ash which over- lies A 4.0 may correlate with a silicic member of the I sequence, since it is less Fe, Ti and Mn rich than most I tephra.
The I tephra, which also contain biotite, were thought by KOCH and MCLEAN to
have a source near Atitl~n. Deposits of I tephra are also described by HAHN (1976) near Nahual~i (northwest of Lake Atitl~n). These pumice deposits are not found on Tolim6n and Atitl~n, presumably because they are deeply buried by younger erup- tives~ Their thickness near San Pedro sug- gests that they may be associated with it, though perhaps because of steep slopes and dense vegetation, the deposit was not found on the upper slopes of San Pedro.
The source vents for these tephra may have been at Cerro Paquixt~n and Cerro Chuichumil, two lateral vents southwest of San Pedro's summit. Lavas from these vents include significant volumes of daci- tes, and trace element determinations (Table 2) suggest that these dacites could represent slightly more maflc members of a consanguinous magma. This problem will receive further atten- tion in later work.
Atitldn and Tolimdn
We have evidence of the relative ages of Tolim~n and Atitl~n. Although only Ati- tl~n has had historic activity, the flank flows and domes on Tolim~n are geomor- phologically very young. Some of these flank flows are submerged by the lake, perhaps because the lake level, controlled by activity at Atitl~n, rose after their em- placement. We believe that the active pe- riods of these two cones have overlapped to a large degree, although Atitl~n has been more active in the last several centu- ries. This is consistent with observations first made by DOLLFUS and de MONTSER- RAT (1868) that where groups of volcanoes occur within the Highlands, seaward vents seem to have the most prolific recent activity.
The structural relationship of Atitl~n and Tolim~n is emphasized by the align- ment of the summit of Atitl~n with the dual summits of Tolim~n and with some flank vents north of Tolim6n's summit. Two of these vents are marked by small domes, and the N-S orientation of this linear series of vents is transverse to the
134 W . L R O S E - T . P B N F I E L D - W. D R E X L E R - P.B. L A R S O N
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GEOCHEMISTRY OF TH]!: ANDESITE FLANK LAVAS OF THREE COMPOSITE CONES. ETC. 135
TABLE 2 - Selected major and trace element determinations of silicic airfall pumice from north- ern slopes of San Petro Volcano.
1 2 3 4
Rb, ppm 102 - 119 87 - 94 65 - 95 62
8r ppm 71 - 107 234 - 240 260 - 415 432
Zr ppm 43 - 75 150 -171 150 - 225 209
Mnppm 435 - 511 615 - 617 700 - 870 709
Z Fe203 0.54 - 1.12 1.62 - 1.73 2,69 - 3.69 4.27
% TiO 2 0.08 - 0.15 0,21 - 0.23 0.23 - 0.30 0,51
1 Range of three determinations on pumice collected on AI.0 surface, near San Juan La Laguna (84.5 x 25 in Fig. i).
2 Range of two determinations On pumice collected on A4.0 surface, 2 km S of San Juan (84.5 x 23 in Fig. ]).
3 I tephra chemistry as reported by Hahn et al., 1978 (4 samples,
including If, 12 and 13).
4 Sample BB 3.0B dacite flow from south slcpe of Cerro PaquixtSn a small vent on the SW flank of San Pedro (85 x 18.5 in Fig. !).
All determinations by XRF using MoC as mass absorbtion correction
(Reynolds, 1963).
volcanic axis, and is siraflar to the ven t a l ignments a t Fuego Volcano 30 km east of Atitl~n (Rose et al., 1978) and at Santa Maria (ROSE et al., 1977) 30 ~km west of the lake. At Fuego, the feeder system for the volcano was postulated to be a dike
with the same t ransverse orientat ion (ROSE et al., 1978).
There is an obvious contrast in the re- cently erupted materials from Atit l~n and Tolim~n. While Atitl~n has erupted domi- nan t ly pyroclastic materials, Tol im~n 's
~-- Fm 1 - Geologic map of the Lake Atitl~n volcanoes: Designation of the lava flow units on the ¢,aes uses common letters for overlapping flows within the same sector of the cones, and numbers which reflect the stratigraphic order and age proximity of flow units containing the same letter. Overall age relationships of the flow units are given in the legend.
136 WJ. ROSE - T. pENFIELD - W. DREXLER - P,B. LARSON
most recent eruptions are viscous lava flows and domes. The more than 70 geo- morphologically young flows and domes on Tolim~n's flRn~, have been arranged in chronogical sequence by aerial photo in- terpretation and ground survey (Fig. 1). A s4m,lar approach at Atitl~n is not nearly as successful, because of the predomi- nance of easily eroded tephra there. At San Pedro, the denser vegetation cover has obscured some "detail.
ing strategy is deliberately different from that used by WOODRUFF et al. (1979), who sampled sequences of lavas and interbed- ded pyroclastic units at or near the sum- mits of cones, including Atithin. As a group, summit samples from composite cones are often more silicic than flank samples, and this is clearly true of Atitl~n Volcano as well (WOODRUFF et al., 1979; and Table 6).
SAMPLING
The flank lavas of the three cones were sampled at the localities shown in Fig. 2. We examined ]avas which span the entire age spectrum shown in Fig. 1. The sampl-
PETROGRAPHY
Modal analyses of groups of samples from the three Atitl~m volcanoes given in Table 3 show slight but significant miner- alogical differences. The more mafic flank lavas of Atitl~m are devoid of oxy-
TABLE 3 - Average modal analyses of lavas from Atitl~n volcanoes. Data were obtained from 1000 point counts of 5-10 individual samples from each group.
Tolim~n Atitl~n Flank Cerro de
Flanks Summit* Flows Oro Dome San Pedro
Phenocrysts:
Plagioclase 30 33 33 36 30
Clinopyroxene 2 6 1.5 2,5 5
Ortbopyroxene 0.5 0.5 2 4,5 ]
Olivine 1.5 1.5 1.5 0.5 1.6
Oxyhornblende 0 0 <0.5 1 2.5
Opaques 2 2 2 2 i
Groundmass (excl. vesicles) 64 57 60 53 59
*data for Atitl~n Summit from WOODRUFF et al., 1978.
GEOCHEMISTRY OF THE ANDESITE FLANK LAVAS OF THREE CO~PPOSITE CONES. ETC. 137
hornblende and the more silicic Cerro de Oro samples contain less olivine and have generally higher crystallinity.
All of the lavas are porphyritic with large (up to 4 mm) phenocrysts of strong- ly and complexly zoned plagioclase (An3s-s0) dominating. The most sodic compositions of plagioclase are found only at the rims of smaller normally-zoned phenocrysts. Larger phenocrysts have very complex oscillatory and normal zon- ing patterns and are riddled with glass
and mineral (magnetite and apatite) inclu- sions. Many of the plagioclase phenocrysts are grown together, forming a glomero- porphyritic text~ire.
The rocks contain orthopyroxene (EnTe/~s18Wo3), and augite (En4~snWo~4). The compositions of the pyroxenes very little from sample to sample; the propor- tions of the two pyroxenes change system- atically from cpx:opx = 2:1 in the mafic AtitlKn rocks to cpx:opx= 1:3 in the Cerro de Oro group. Clinopyroxene is weakly zon-
TABLE 4 - Chemical composition of pargasitic amphibole from the Atitl~n volcanoes and other nearby Guatemalan volcanoes.
Atitl~n Volcanoes I Santia~uito 2 Fue$o 3
SiO 2 43.1 (41.7 - 46.2) 42.1 42.4
AI203 11.7 (8.1 - ].5.2) 11.6 13.5
MgO 14.2 (12.1 - 15.8) 14.8 15.1
CaO 10.6 (i0.0 - ll.5) 10.6 10.4
Fe203 13.4 (11.6 - 17.6) 14.1 11.6
Ti02 2.8 (1.8 - 3.7) 2.3 2.7
Na20 nd 2.5 2.3
K20 nd 0.4 0.4
Total: 95.8 98.4 98.4
nd ffi not determined
1 Average and range of 13 microprobe analyses from three different lava samples (T-5, A3.ba, M3.0b) from andesites of San Pedro, Tolinmn and Atitl~n.
2 Average of 6 microprobe determinations of individual grains of amphibole from Santiaguito daclte (GU/S ]511A).
3 Single determination of ampbibole inclusion within plagioclase phenocryst, Fuego basalt of 1974 (Rose e~ al., 1978).
138 W.L RO~E - T. PENFIELD - W. DREXLER - P.B. LARSON
ed and microprobe traversing demon- strates it to be slightly more Fe-rich at the outer edges. Composite pyroxene grains are common.
Olivine (Fovs-90) is subordinate in both size and abundance. It commonly has cor- roded margins.
Oxyhornblende is present in all sam- ples except those from Atitl~n. Table 4 shows results of electron microprobe de- terminations of these minerals. The com- positions reported are similar to those of pargasitic amphibole in andesites given by ALLEN et al. (1975, p. 1080) or to basaltic hornblendes given in DEER et al. (1965, p. 317). The amphibole phenocrysts invar- iably have reaction rims composed of pla- gioclase, clinopyroxene and opaque miner- als. This pargasitic amphibole is chemi- caUy nearly identical to that found in Fuego basalt and in Santiaguite dacite (Table 4). The preservation of amphibole in Guatemalan lavas is apparently in- fluenced by the magma composition. In si- licic lavas, like the Santiaguite dacites (62- 65% SiO2) it exhibits thin reaction rims. In the andesitic lavas of the Atitl~n volca- noes it is more strongly resorbed. In Fue- go's basalts it can be found preserved only as inclusions in other minerals (especially plagioclase) which have evidently shielded it from the melt. We conclude that, with ascent and volatile release, the pargasitic amphibole, which is a common phenocryst in a wide range of magmatic compositions, resorbed much more readily, often com- pletely in hasalts, but tends progressive- ly to be preserved in the more silicic rocks. This could occur because the reac- tion rates in cooler viscous sflicic melts are much slower, or perhaps because the amphibole is relatively more stable in the sflicic magmas.
An opaque mineral, chiefly euhedral magnetite, makes up about 2% of all lavas examined. Apatite is also in all lavas, oc- curring as ti~y crystals in the groundmass and as inclusions in the phenocrysts, espe- dally plagioclase. Tiny, barely recogniz- able crystals of cristobalite, and rarely tridymite, line vesicle walls in some sam- ples. The groundmass of most lava sam- ples is hyalocrystalline.
Clots of mafic minerals (pyroxenes,
magnetite and olivine together with pla- gioclase) are found in the lavas, and they are especially characteristic in the Cerro de Oro samples. The Cerro de Oro rocks also contain larger xenolithic inclusions, some of with are described in Table 5. Judging from textural criteria, these xenoliths appear to represent cog- hate dike rocks, rather t.ha~ crystal cu- mulates.
Taken together, the rocks of the Atitlgm volcanoes are rather typical calc-alkaline basaltic andesites and andesites. The pe- trography of lavas of Atitlfin volcano itself are very similar to Santa Maria (ROSE et al., 1977), and contain much more pyrox- ene and less olivine than the nearby high-A1 basalts of Fuego Volcano (ROSE et al., 1978).
ANALYTICAL METHODS
Chemical analyses for major elements were determined by atomic absorbtion spectroscopy using LiBO2]HNOs fusion-so- lution techniques (MEDLIN et al., 1969). Trace elements were determined by X- ray fluorescence (Rb, Sr, Zr) using Mo- Compton as a mass absorbtion estimate (REYNOLDS, 1963) and by graphite-furnace atomic absorption (V, Ni, Ba) after HF]HNOs solution. Data for rare earth elements and for Co, Hf, Sc and Cr were obtained by instrumental neutron activa- tion analyses at the Phoenox Memorial Laboratory, Ann Arbor. International geo- chemical reference standards W-l, BCR-1, GH, GSP-1, G-2 and AGV-1 were used as standards in the determinations (FLANA- GAN, 1973).
CHEMICAL RESULTS
Chemical analyses of selected samples from the Atitl~n volcanoes are given in Table 6.
Nearly all of the individual composi- tions are andesitic, showing the clear dom- inance of andesites in the exposed lavas of the three cones. The Atitl~n lava are
GEOCHEMISTRY OF THE ANDESITE FLANK LAVAS OF THREE COMPOSITE CONES. ETC. 139
TABLE 5 - Chemical analyses of mafic xenoli ths in lavas of d o m e s on nor th slope of Tol im~n Volcano.
Wt. % T-8a MI. 0a Q0.9a
SiO 2 49.3 47.7 52.4
AI203 18.5 20.7 19.3
Fe203 8.9 9.1 8.4
MgO 5.9 4,8 3.9
CaO 8.9 i0.0 7.7
Na20 3.9 3.7 3.4
K20 I. 1 0.5 i.i
TiO 2 0.9 1.4 1.0
N20 0.9 0.5 1.8
MnO nd 0. ] 0.2
Total: 98.3 98.5 99.2
ppm
Rb 15 8 40
Sr 700 660 610
Zr nd 120 140
*Total Fe as Fe203
T-8a - hypabyssal xenolith containing plagioclase, hornblende, magnetite and minor olivine, Cerro de Oro.
Mi.0a - fine grained hypabyssal xenolith from north slope of Cerro de Oro.
QO.9a - dense, lamprophyric xenollth with hornblende phenocrysts from
dome south of Cerro de Oro at 96.3W, 20.2N.
nd = not determined
140 W.L ROSE - T, PENFIELD - W. DREXLER - P.B. LARSON
generally the most mafic, although basalt- ic andesites with SiO~ = 52% are found on both Tolirn~ and San Pedro. The rocks of the Cerro de Oro dome, one of the youngest features on the flanks of To- lim~n, are generally the most silicie of the sample groups, although individual sam- ples from both Tolim~n and San Pedro have SiO2 percentages higher than the Cerro de Oro average (58.4%). Although we looked for a relationship between the silica content and the relative volume of units on Tolim~n, present data do not demonstrate one.
Figure 3 shows that the rocks from the Atitl~n volcanoes have generally slightly potash contents than other Guatemalan volcanic rocks of similar silica content. This difference signals similarly higher Rb, Ba and REE contents for the suite. Figure 4 shows an attempt to examine the regional chemical differences along the
Guatemalan volcanic chain. To construct the figure, representative basaltic compo- sitions were selected from each volcanic center of the Guatemalan Highlands. Re- sults show apparently K-rich basalts in the AtitlKn region. On an AFM plot (Fig. 5) the AtitIAn volcanoes are also slightly displaced from the main trend of Guate- malan volcanics.
Individual K20-SiO2 plots for Central American volcano groups may be con- structed from data sets of 5-55 samples. We did this for 13 volcanos. Slopes com- puted by least squares linear fit through these data sets are shown in Fig. 3. Systematically lower slopes were found at volcanoes south of the principal WNW volcanic axis and higher slopes at vol- canoes to the north. Within the AtitlKn volcanoes, AtitIAn itself, the most south- erly of the three peaks, has the lowest K20,SiO2 slope. Rare-earth elemental
TABLE 6 - Chemical data on selected samples of lavas from the AtitIAn volcanoes. Samples nos. keyed to map units of figure.
14.8 Ii, 7 13,6 28. i 19.7 20 . 9 4.07 3.20 3.27 0.93 1.03 1.09 0.33 0.26 0,32
*Total Fe as Fe0 **Data on Ati~l~ summit samples from WOODKUl~ et al., 1979
ud = not de~ermtued
GEOCHEMISTRY OF THE ANDESITE FLANK LAVAS OF THREE COMPOSITE CONES; ETC. 141
LAVA SAMPLING SITES
• Alitlbn Flanks Atlll~n Sumrnd Se~ Pedro
• TohmSn ~Sor~ple numbers as shown]
A21m =~Alb
A'~ 011] "- . , - N (
- ~30 -I
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LI 5 • IIL40 L193' flIT-2 '~
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ATCI-8
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A-~-&
91"30!w 9r~20'w
FIc, 2 - Sketch map showing sites of sampling for this study.
data for the Atitl~n sample groups, nor malized to chondritic abundances, are shown in Fig. 6. The plot clearly shows that the andesites of the Atitl~n volcanoes are higher in rare earths (particularly the light rare earth elements) than basalts an- desires from Fuego (east of Atitl~n) and Santa Mari~ (west). Data for the rhyolitic Los Chocoyos Ash, believed to have come from the Lake Atitl~n cauldron, produce a very steeply sloping data field with markedly depleted heavy REE on the same plot.
Fig. 7 summarized the major element changes observed in stratigraphy se- quence in Tol im~l 's flank lavas. The most significant trends with time are a de- crease in MgO and an increase in A1203.
INTERPRETATIONS
Previous petrologic investigations of Guatemalan volcanic suites have relied heavily on crystal fractionation hypoth- eses (ROSE, 1972; ROSE et al., 1977, 1979;
WOODRUFF et al., 1979) to explain the chemical diversity of sequential lavas from individual vents. The case of crystal fractionation is particularly compelling for the 1974 Fuego eruption, which apparent- ly produced an inverted sequence of differ- entiated Hi-A] basalts (ROSE et al., 1978). For composite cones, generally increas- ingly silicic lavas seem to be produced as the cone grows, and it has been suggested that this tendency is due to greater inhibi- t-ion of eruption and therefore progressive- ly longer fractionation periods as the stature of the cone increases (Rose et al., 1977).
Chemical data for the Atitl~n volcanoes generally are consistent with crystal frac- tionation hypotheses involving mineral phases directly observed in the rocks. Pla- gioclase fractionation is suggested by cor- relations such as Rb/Sr vs. SiO2 and CaO vs. K20 (Fig. 8). Generally low concentra- tions of Cr and Ni (compared to MORB) suggest that early fractionation of clino- pyroxene and olivine could have occurred (see also ROSE et al., 1977). Vanadium
142 W.L ROSE - T. PENFIELD - W. DREXLER - P.B. LARSON
concentrations decrease with SiO2 con- tents (Fig. 8) and suggest that magnetite may also be fractionating. The occurrence of magnetite phenocrysts and especially the existence of magnetite inclusions with plagioclase phenocrysts shows that mag- netite is in equilibrium with the AtitlAn parental magmas, just as it was in the case of Fuego's 1974 magma (Rose et aL, 1978).
In order to examine rigorously the com- patibility of chemical data with a crystal
fractionation hypothesis, least squares (WRIGHT and DOHERTY, 1970) and Ray- leigh (SHAW, 1970) modeling of fractiona- Lion was performed. Results, s, lmmarized in Table 7, show that crystal fractionation involving phases observed in the rocks can account for the derivation of all of the AtitiAn magmas from a common high AI- basalt parent similar to that observed at Fuego (ROSE et al., 1978) and postulated at Santa Maria (ROSE et al., 1977). Two types of fractionation models are shown,
2 o ,v"
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FiG. 3 - K20-SiO2 data summary for the Guatemalan Highlands. Upper left shows all data with Atit1~n volcanoes data indicated. Lower left shows best linear fit for individual volcano subsets, ke- yed to locat/on map at lower right Upper right shows general location map.
GEOCHEMISTRy OF THE ANDE,WfTE FLANK LAVAS OF THREE cOMPOSITE CONES. ETC. 143
t " • , ppm 20 ..am'" "": Ce
116 1 ;'" " j ,o ~
.a ° •
~ e e l o 6
" Rb/Sr -.o5 .-'" • i= a° =6
- - e " •
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_ O: • ".
f l.2 U.~.. " " ' - .
o" " . . • ." •
1.o :." K2 0 : " . . • I0:- .
1Okra
it I I , WNW ESE
FI(~. 4 - Compositio[~al variation of basaltic lavas from the cones of the Guatema|an High- lands. Abscissa is distance along the volcano axis shown in Fig. 3 (lower right). Lavas of 49-52% SiO2 only are used in this summary.
one of which uses olivine, clinopyroxene, plagioclase and magnetite as fractionating phases, and one which uses pargasitic am- phibole (Table 4) as well.
CHOICE OF PARENT
Whenever a ~parent>> magma is postu- lated which cannot be found at the sur- face the postulation must be subject to critical evaluation. In this case the choice is suggested by: (1) the existence of similar
(to the parent) basaltic magmas at several nearby volcanoes such as Fuego (ROSE et al., 1978) and Pacaya (EGGERS, !97I); (2) the occurrence of mafic hypabyssal xeno- liths of basaltic composition in the Atitl~n lavas (Table 5) and (3) evidence that com- posite cones such as nearby Santa Mari~ have produced lavas which tend to be se- quentially more silicic as the cone grows (ROSE et al., 1977). This suggests that more mafic lavas may be found in the in- teriors of large composite cones. If, as sug- gested by ROSE et al., (1977), this evolu- tion is tied to longer and longer repose pe- riods as the magmas become generally more silicic, then the relative inactivity of the Atitl~n volcanos in the historic period, compared to the frequent eruptions of the basaltic vents of Fuego and Pacaya, is con- sistent.
Data of Figs. 3 and 5 demostrate that the most mafic lavas of the Atitl~n volca- noes seem to be higher in K, Rb/Sr and REE than their counter-parts at other Guatemalan vents. This suggests that Atithin <<parent>> magmas could differ from other nearby parents in the same respect. We evaluated fractionation mod- els using parent compositions obtained at Fuego Volcano (ROSE et al., 1978) and those inferred from data on basaltic andes- ires at Santa Maria (ROSE et a1.1977). Since both of these .pa ren t s . may be low in K, Ba, Rb and REE (Fig. 4) we also considered parents somewhat higher in these elements.
In order to get consistent models it must be postulated that the parent mag- mas are enriched in K, Rb, Ba and REE by 50-80% compared to Santa Mari~ or Fuego basalts. Since these elements be- have chiefly as residual elements in fraction- ation, the effect of using <<unenriched. parents is simply to produce consistent negative residuals for these elements. Be- cause Figs. 3 and 5 show the enrichment of Atitl~n rocks in these elements, we be- lieve such an adjustment in the parental magma composition is justified.
ASSUMPTIONS USED IN MODELING
Crystal/liqued partition coefficients for trace elements are compiled in Table 8.
144 W . L R O S E - T . P E N ' F I E L D - W , D R E X L E R - P . B . L A R S O N
- +~ + +^ ÷+.~ x
÷ ÷
y+ + + Guatemalan Quaternary Volcanics \ A Atitl~n Summit • Atifl6n Flanks • San Pedro Flanks \ • Tolim(~n Flanks Mg \0\
0 V
FIG. 5 - AFM plot for Guatemalan Quaternary rocks, showing the Atitl~n volcano subsets studied in this paper.
Stoichiometric albite, anorthite, forsterite and fayalite and microprobe compositions 100 for clinopyroxene, orthopyroxene, amphi- bole and 6 tanomagnet i te were used in the 50 m i ~ n g calculations.
REE/Ch RESULTS OF MODELING
Tables 7 and 9 show the results of calculations modeling crystal fractionatioz~ In general the modeling shows that chem- ical data are consistent with fractiona- tion hypotheses. In detail two further conclusions can be advanced. (1) The dis- finction between models which include fractionating amphibole and those which do not is small, so much so that no real preference is indicated by the data. (2) The proportions of phases which fraction- ate from the parent very considerably from one group of rocks to another. The principal variation in the models without
10
0s
I J L I l i 1 Le Ce SmEu Tb Yb Lu
FIG. 6 - Chondrite-normMized (Leedy L-5) rare earth element plot for selected subsets of Guatema|an volcanic rocks.
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146 w.L ROSE - T. PENFIELD - w. DREXLER - P:B. LARSON
TABLE 8 - Crystal-liquid partition coefficients used in the Rayleigh calculations (plag -- Plagio- clase, cpx = Clinopyroxene, opx = Orthopyroxene, ol = Olivine, mag = Magnetite and ap ffi Apa- rite).
plag cpx opx ol mag ap
Rb .05 .003 .022 0 0 0
Sr 1.83 .]2 .017 .014 0 2.0
Ba .3 .022 .013 0 0 .0]
La . Ol .1 .02 . Ol .002 30
Ce .2 .175 .05 .007 0 30
Sm .15 .6 .075 .01 0 50
Eu .34 .65 .06 .01 0 20
Lu .2 .8 .75 .02 0 25
Ni .26 1.8 3.8 8.7 6.6 0
Co .i 1,5 1.4 3.8 8.0 0
V 0 2.31 .5 .09 24.0 0
Note--Data from ALLEGRE and others (1977), AI~TH (1976). LEEMAN and VITALIANO (1976), EWART and others (1973) and ZIELINSKI and FREY (1970).
amphibole is in the proportion of plagio- clase. In the models with amphibole, the amphibole proportions vary markedly, and display an inverse relationship to pla- gioclase.
The variability of the proportion of pla- gioclase in the fractionation models can be directly related to geologic groups of sam- ples. Frank lavas of Atitl~n volcano are markedly higher in in Al20 s (Fig. 4) and require fractionation models with signifi- cantly lower relative proportions of pla- gioclase (.50) (Table 7) than the lavas of summit (.64). Late lavas of Tolim~n's flanks have higher Alg.0s (Fig. 7) and re- quire fractionation models with proportion-
ally less plagioclase (.55) than earlier la- vas on the same flanks (.66) (Table 9).
The modeling of rare earth elements produces results consistent with the frac- #donation model of Table 7. Figure 9 shows results for the Atitl~n summit grouping. Uncertainties in two main imputs to the modeling cause a range of calculat- ed results for each case. One principal uncertainty is the <<apatite effect>> as ex- plained by WOODRUFF et al., (1979). Tiny apatite inclusions occur in plagioclase and other phenocrysts in the AtitI~n lavas. Fractionation of apatite, even in tiny amounts, has a significant effect on the rare earth elements (see Table 8). Thus
GEOCHEMISTRY OF THE ANDEBITE FLANK LAVAS OF THREE COMPOSITE CONES, ETC. 147
some means of quantifying the amount of apatite fractionation taking place is neces- sary. Because P20.~ remains almost con- stant at 0.2% throughout the sequence, a fractionation model would necessitate about 1% of the fractionating material to be apatite (or 0.3% apatite for 30% total fractionation). Because apatite inclusions have a very inhomogeneous distribution in phenocrysts, petrographic tests so far can- not confirm whether this is a reasonable proportion, but we believe it is appro~mate- ly so. A second uncertainty in the rare
earth modeling comes in the choice of Eu partition coefficients for plagioclase, which vary from 0.3 in basalts to about 2 in andesites. Since plagioclase is the major phase fractionating, it makes a considerable difference what values one uses.
Both of these uncertainties are incor- porated in Fig. 9. In general the ob- served pattern falls within the range of possible solutions~
TABLE 9 - Crystal fractionation models of single samples representative of early (A1203-poor) and late (AI.20rrich) Tolim~n flank lavas from a common high A1-basalt parent.
Parent Early (Li.0b) Late (L4.3)
0BS. CALC 0BS. CALC
Si02 50.9 58.6 58.6 54.6 54.9
A]203 19.3 17.9 17.9 2] .5 21.~
Fe0(tot) 9.3 6.1 6.2 6.7 6.,~
Mg0 6.3 4.0 4.0 3.9 3.9
Ca0 9.4 7.2 7.3 7,1 7.]
Na20 3.1 3.5 3.5 3.7 3.6
K20 0,95 1.6 1.7 1.5 ].A
T~02 i.i 0.78 0.79 0.8 0.8
PLAG 42 28
OLIV 9 6
CPX 7.7 13:
MAG + ILM 5.0 4.0
ZR 2 0.09 0.]8
Rb 21 45 56 39 42
$r 650 498 46~ 510 592
~i 40 7 8 7 12
Co 30 25 21 27 22
Ba 350 560 785 440 630
V 275 155 70 145 105
148 W . I . R O S E - T . P E N F I E L D - W. D R E X L E R - P,B, L A R S O N
K20-Si02 PLOTS AND CRYSTAL FRACTIONATION
The slope produced by crystal fraction- ation or any variation diagram (such as K20-SiO2) is of course dependent on the proportions of phases fractionatod, the compositions of the phases, and the com- position of the parent. Figure 10 gives an example. A high Al-basalt can produce a whole spectrum of slopes on a K20-SIO2 plot, depending on the proportions of the specified phases (in this case, magnetite, olivine, clinopyroxene, plagioclase and am- phibole) which are fractionating. Phase proportions which are relatively low in plagioclase and pyroxene and high in oli- vine, magnetite and oxyhornblende have relatively gentle slopes. And the converse is true as well. Figure 10 shows the results of Table 7 plotted on a K20-SiO2 plot. The highest calculated proportions of pla- gioclase fractionating occur in the Cerro de Oro rocks, and these are the steepest slopes on the plot. Atiti~n with the lowest calculated proportions of fractionating pla- gioclase, has the flattest range of slopes. Although differences in details of parent composition and phase compositions will alter the figure, the principles illustrated
would still apply, Do differences in the proportions of phases fractionating cause the K20-SiO2 slope differences enumerat- ed in Figs. 3 and 10. The hypothesis is still quite speculative, and its survival de- pends upon at least (I) better data docu- menting elemental variations among lavas of the same apparent parentage, (2) detailed modeling of all the elements for frac- tionation of varying proportions, and (3) a good explanation for why these variations in fractionation could occur.
A possible insight comes from the region- al distribution of K20-Si02 slopes dis- cussed above (Fig. 3). Why should lava se- ries with gentle slopes lie to the seaward of the volcanic axis? In the next section we of- fer a possible, speculative explAnAtion.
CAUSES OF DIFFERENT CRYSTAL FRACTIONATION PROPORTIONS
T he work of YODER and TmL~,y (1962) showed that the sequence of crystalliza- tion of minerals in a particular high-A1 ba- salt is dependent on the H20 content~ At low PH20 plagioclase is on the liquidus (therefore an <<early>~ crysta]llzlug phase), bu t at higher PH20 the early phases are
300 I- ppm Zr
I 0 0
" pp~ s, ~soo Io
5 0 0 8
4.00 6
?OF .= ppm Rb 4
OLD ~ YOUNG
.° f 18
16 --16,~
A z%.J x _152
/ \ x - - ~ /~ FezOs (t°tal) x . _
'¢--x /~- A / \ ~ / ~
-'V q2 K2o
OLD ~YOUNG
FIG, 7 - Stratigraphic chemical changes in flank lavas of Tolim~n volcano, Absissa in relative age, with no attempt at quantitative evaluation. Age range extends from oldest recognizable units in Fig. 1 to the youngest.
GEOCHEMISTRY OF TI:IE ANDESITE FLANK LAVAS OF THREE COMPOSITE CONES. ETC, 149
more likely to be olivine and magnetite, and plagioclase moves very far from the liquidus. Thus the relative proportions of plagioclase and olivine in equilibrium with a high-A1 melt should be sensitive to the H20 content. High H20 content should in- hibit the crystallization of plagioclase and therefore its fractionation (see also FAIR- BROTHERS et al., 1978). At the same time it may aid in the crystallization of magnetite (OsBORN, 1959) and amphibole.
We know that some basaltic magmas from Guatemala have high H20 content. The basalts of the 1974 Fuego eruption were shown to contain 3% H20 based on the composition of included glass in phe- nocrysts (ROSE et aL, 1978). Another im- portant vm~iable which could affect the proportions of fractionating phases is sim- ply the duration of the fractionation proc- ess. Since the density contrast of plagio- clase with basaltic and andesitic melts is very small, much smaller than for the ma- fic phases, it will be very much more diffi- cult for plagioclase crystal settling to occur in short periods. The combination of short fractionation time and high H~O content in a shallow magma chamber will produce conditions relatively unfavorable for both
E ¢3L
25 t~
] l
I I
I ]
OJfi
O.IC
0.05
700 • 200
SOOt• • ,~ •
ooF.,o. , , . o.,-. .ool 4 0 0 L - ~ I I I ]
54 56 58 60 62 % S i O 2
I I I • 1
I
•A
plagioclase crystallization and fractiona- tion, yet, because H20 decreases the vis- cosity of magma the fractionation of mafic minerals may still proceed.
Atitlfin may be an example of such pla- gioclase-poor fractionation, as suggested by our models. Its pyroclastic eruptive style is suggestive of higher volatile con- tent, and its greater frequency of historic activity is suggestive of shorter repose pe- riods. Atitl~n located along the orographic barrier where it intercepts more of mon- soonal rains than the other vents. I t may also be that Atitl~n's magmas interact with a thicker section of hydrated crustal rocks, since there may be more sedimen- tary material towards the coast, and more crystalline basement inland. Either could explain why Atitl~n shallow magma bod- ies may contain more water and erupt more energetically. Such explanations ex- plain why other volcanoes seaward of the main axis also show relatively more fre-
z~l I I t~
• &• I •
• | •
I 1 l 5 4 56 58 60
% SiO 2
t 6O o. 40
2O
E
h _ o ~
Atit~on • ,~ Toliman • San Pedro •
I,; . , . , 1 _ . 0 ° 8 • ~ . • •
I 1 i l 1 I 1.0 IA L8 22
% K20 8o [ I I ~ I | J • !
• g~% cA"
4 0 0 500 600 700 p p m Sr
FIG. 8 - Variation diagrams for lava samples from the Lake Atitl~n volcanoes.
1 5 0 W , L R O S E - T . p E N F I E L D - W. D R E X L E R - P.B. L A R S O N
quent and energetic activity, less relative plagioclase fractionation and lower slopes on K20-SiO2 plots (Fig. 3) than their land- ward neighboring volcanoes. But in the absence of data which prove meteoric wa- ter entrance into these magma chambers this conclusion is premature.
The change in the proportions of phases fractionating at Tolim~n (Table 9), must also be accomodated in such an hy- pothesis. Could the later Tolim~n mag- mas have crystallized at higher PH20? Again circumstantial support for increased meteoric water availability exists, because Lake Atitl~n water level must have in- creased greatly in the latter stages of activity.
DEEP SOURCES OF ATITLAN LAVAS
Eclogite is an unlikely source material for Central American basaltic andesites because (1) to derive the magmas in a one-stage process Cr, Ni and Co abun- dances demand a high degree of partial melting (40-60% yet K, Rb and REE abun- dances require a much lower degree (5-10%); GILL, 1974). (2) The lack of depletion of heavy REE in the lavas (Fig. 8) shows that it is unlikely that a rock having gar- net as a residual phase could be the par- ent, even if the rock were later fractionat- ed (a two-stage derivation) (see also LEE- MAN, 1976).
5 O
4 0
3 O
.= 2 0
I I I I I
~ ~ S e n g e of calculated results
~: " ..~ .Observed
I I ; 1 Lo Ce S m Eu Lu
FIG. 9 - Modeling of rare earth element changes for Atitl~n summit lavas after model described in Table 6. Shaded area represents calculated area of uncertainty discussed in text.
Because of the low (relative to ul t ram,- fic concentrations) concentrations of Cr and Ni in Central American lavas, ultra- mafic mantle material is a possible source only if two-stage generation is accepted, in which fractionation of olivine and pyrox- ene (reducing Ni and Cr) occurs after partial melting and before the magma reaches near surface reservoirs. The ultra- marie source material does not have gar- net as a residual phase, because the lavas do not show depletion of heavy REE.
The origin of Atitl~n andesites from a lesser degree of partial melting of the same material as more mafic Guatemalan magmas is unlikely. Although the REE plots shown in Fig. 8 bear a close resem- blance to plots presented by IJ~.MAN (1976, p. 1583, fig. 1) showing the effect of var- ying degrees of partial melting of a spinel lherzolite on REE patterns, such a model predicts that the andesites generated would have relatively depleted heavy REE and low Na20/K20 because the SiO2 increase with lesser degree of partial melt- ing depends on the existence of residual garnet and pyroxene (RINGWOOD, 1974). The Atitl~n lavas fail to show these char- acteristics and the explanation that varia- tions in basaltic compositions along the Guatemalan chain (Fig. 4) are due to changes of the degree of partial melting of a homogeneous mantle is therefore un- likely to be correct. An inhomogeneous mantle source richer in K, Rb, Ba and REE under Atitl~n is an explanation that is compatible with the data.
An alternate hypothesis is suggested, however, by geologic features at the sur- face. The Atitl~n volcanoes are located at the site of the Atitl~n cauldron, the presumed source of the largest Quater- nary rhyolite eruption in the region. I t is possible that basaltic magmas near Ati- tl~n became contaminated with residual rhyolite from the Los Chocoyos magma. At present the spatial correlation is the only evidence for this idea. The occur- rence of late silicic pyroclastics and flows possibly associated with vents south of San Pedro along the cauldron margin sug- gests that residual rhyolitic liquid associat- ed with the cauldron still is found near the surface.
GEO~STRY OF THE ANDESITE FLANK LAVAS OF TIIREE COMPOSITE CONES. ETC, 151
~ . 5 I I I I l i I I ]
2.0
0
1.5
1.0
50
Tolim6n and Cerro de Oro
':~ =~ii,i:iiiiii?"
::~,,!!iii!!ii i i ~ San Pedro . i i i i i i i i i !~,;
.... ::.::::.
tl6n
m
I I, I 1 I I 1 I 1
55 60 % Si 0 z
FIG. 10 - Effect of fractionation of 5% of individual phenocrysts from a high-A1 basalt parent, shown on a K20-SiO,~ plot. As vector quantities (c = clinopyroxene, p = plagioclase, ol = olivine, ox = oxyhornblende, m = magnetite). Shaded area show the range of sloped observed in Table 6, for each of the Atitl~n volcanoes.
CONCLUSIONS
T h r e e closely-spaced composite volca- noes total ing about 100 km 3 in volume have grown on the south side of the Lake Ati t l~n Cauldron since t he grea t erupt ion ( > k m 3) and cauldron subsidence of about 85,000 years B.P. T h e style of the mos t r ecen t activity a t the cones differs mark- edly. Atitl~n, larges t and mos t souther ly of the three, has produced ma in ly pyro-
clastic erupt ions of basal t ic andes i te while the other cones have produced a much higher propor t ion of more silicic rocks e rup ted as lava flows. Exposed lavas of the cones are p r edominan t l y calc-alkaline andesi tes , bu t t hey vary widely in chem- istry. As a group t h e y have higher K20, Rb, Ba and rare ear th e l e m e n t s t han o ther G u a t e m a l a n andesi tes . Model ing shows tha t der ivat ion of the Lake Ati t l~n andes i tes from a single high-Al ba sa l t par -
152 W . I . R O S E - T . P E N F I E L D . W . D R E X L E R - P . B . L A R S O N
ent, by fractionation of observed phe- nocrysts, approximately in observed pro- portions, is consistent with major and mi- nor element chemistry. If such models are correct, it is probable that these andesites evolve separately in small magma cham- bers where repose intervals and H20 con- tents may vary markedly in both time and space and cause differing amounts and proportions of phases to fractionate.
Development of the basalt-andesite-da- cite suite of lavas at the three cones is a separate process in time from the genera- tion and eruption of rhyolitic magma which proceded it. though it is possible that minor contamination of the more ma- fic lavas by residual rhyolite did occur.
ACKNOWLEDGEMENTS
Funding for the work came from the National Science Foundation, through Grants GA38435 and EAR74-19025.
Logistic support of Dartmouth College and the Instituto Geogr~fico Nacional of Guatemala is gratefully acknowledged.
Norman K. Grant gave constant encour- agement and frequent constxuctive criti- cism. Laurel G. Woodruff helped with cal- culation and criticism. Michael J. Carr helped with some of the sampling and was an interested critic. Christopher Newhall reviewed our first draft.
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