Transition from mildly-tholeiitic to calc-alkaline suite: the case of...

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Ž .Journal of Volcanology and Geothermal Research 86 1998 117–136

Transition from mildly-tholeiitic to calc-alkaline suite: the case ofChichontepec volcanic centre, El Salvador, Central America

Silvio G. Rotolo a,), Francesca Castorina b

a ( )Dipartimento di Chimica e Fisica della Terra CFTA , Via Archirafi 36, 90123 Palermo, Italyb Dipartimento di Scienze della Terra, UniÕersita La Sapienza, Piazzale A. Moro 5, 00185 Rome, Italy`

Accepted 30 May 1998

Abstract

The Chichontepec volcano is a Plio-Pleistocene composite volcano that erupted lavas ranging from high-alumina basaltsto dacites. It experienced a caldera-forming paroxysmal eruption during the early Pleistocene. Pre-caldera lavas are mildlytholeiitic and they evolved mainly by low pressure crystal fractionation, notwithstanding the fact that most mafic lavasŽ .low-MgO high-alumina basalts retain traces of polybaric evolution. Conversely, post-caldera lavas, which are mainlypyroxene andesites, are clearly calc-alkaline, having evolved by open-system crystal fractionation. Sr–Nd isotopic data andtrace elements characteristics indicate that the same mantle source was involved in the petrogenesis of these series.Modelling the AFC process showed that it did not play any role in the petrogenesis of these rocks; a crystal fractionationmodel is considered to be more relevant. A slight variation in the fractionating assemblage could have caused the transitionfrom an early mildly tholeiitic trend to a late calc-alkaline one. Mineralogical evidence, mass-balance calculations andelemental chemistry support this hypothesis, assuming that the greater amount of pyroxene on the liquidus is at the expenseof plagioclase; this would have prevented the trend in iron enrichment. q 1998 Elsevier Science B.V. All rights reserved.

Keywords: Central America; El Salvador; andesites; high-alumina basalts; Al-spinel; radiogenic isotopes

1. Introduction

Tholeiitic and calc-alkaline suites do not normallycoexist in volcanic arcs. Usually, immature islandarcs are tholeiitic in character, while active volcanicarcs, which have developed on the continental litho-sphere, are mostly calc-alkaline. In some regions,however, there is a close spatial and chronologicalassociation between tholeiitic and calc-alkaline seriesŽ .Baker et al., 1994 . Tholeiitic lavas, such as thoseemitted by the Masaya and Granada volcanoes in

) Corresponding author. E-mail: [email protected]

ŽNicaragua and Boqueron in El Salvador Ui, 1972;.Fairbrothers et al., 1978 , are rare among lavas in

Central America, and, in particular, along the Sal-vadorian volcanic front. These lavas are mostly an-desites and seldom high-alumina basalts.

This study is concerned with the magmatic evolu-Ž .tion of the Chichontepec volcano El Salvador ,

which evolved from an early, mildly tholeiiticrtran-sitional stage to a clearly calc-alkaline stage, througha paroxysmal plinian phase. We have focused on themineralogy, geochemical and petrological aspects ofpre- and post-caldera lavas. We here propose a sim-ple crystal fractionation model to explain magma

0377-0273r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.Ž .PII: S0377-0273 98 00076-6

( )S.G. Rotolo, F. CastorinarJournal of Volcanology and Geothermal Research 86 1998 117–136118

evolution at the Chichontepec volcano. Furthermore,we suspect that an increase in pyroxene in the frac-tionation assemblage was responsible for a transitionfrom a tholeiitic to a calc-alkaline trend.

2. Geology and the eruptive history of the Chi-chontepec volcano

Ž .Chichontepec or ‘Volcan de S. Vicente’ is an`andesitic, composite volcano, the second most volu-

Ž 3.minous in El Salvador 130 km , after the SantaŽ 3 .Ana volcano 265 km ; Carr et al., 1981 . It is

located approximately 50 km east of the capital city,San Salvador. The remnants of an older volcaniccenter lie immediately West of the main volcanic

Ž .edifice Fig. 1 . They consist of a series of pro-Žnounced hills arranged in a semicircle La Carbonera

.hills .Chichontepec is a paired volcano whose eastern

Ž .crater elevation: 2180 m appears to be morphologi-Žcally younger while the western crater elevation:

Fig. 1. Generalized geological map of the Chichontepec volcanic centre. Same horizontal scale in the cross section; elevation in meters.

( )S.G. Rotolo, F. CastorinarJournal of Volcanology and Geothermal Research 86 1998 117–136 119

.2105 m seems older. Several parasitic vents, whichappear to be the most recent in the whole complex,are located on the lower flanks of the volcano,mostly on the northeastern side. As a consequence,this area recorded a progressive eastward migrationof the focus of volcanism during its lifetime.

The whole edifice rises inside a late-PlioceneŽ .extensional structure 20 to 30 km wide , the so-

called Central Graben, that runs along the length ofEl Salvador and is roughly parallel to the Pacificcoast. The graben cuts through a Tertiary volcanicbasement, which locally consists of basalt and an-desite lavas, tuffs and agglomerates. Tertiary vol-canic rocks lie in turn on a thick Mesozoic sedimen-tary cover and on a Paleozoic metamorphic basementŽWilliams and Meyer-Abich, 1955; Pichler and Weyl,

.1973; Wiesemann, 1975; Carr, 1976 . The northernmargin of the Central Graben is marked by a seriesof E–W-trending faults with displacements of up to300 m. The southern margin is much less evident,having been partially buried by recent Chichontepeclavas. It is represented by a series of minor faultswhich extend westward through La Carbonera hillstowards lake Ilopango.

The youngest Chichontepec lava flows are cov-Ž .ered by 3000-year-old dacitic ashes ‘Tierra blanca’ ,

which have been emitted from the nearby IlopangoŽ .volcano about 20 km northwestwards ; there is no

evidence of younger eruptions. Its current state isone of solfataric activity, characterized by low-tem-perature fumaroles and hot springs, which are lo-

Žcated on the northern side of the volcano Aiuppa et.al., 1997 .

2.1. The pre-caldera stage. The Plio-Pleistocene LaCarbonera Õolcano

The earliest edifice is called La Carbonera. Thebeginning of the activity of La Carbonera volcano ispoorly defined. Limited whole-rock KrAr dates on

Ž .two samples samples 98 and 14 yielded ages of2.2"0.4 and 1.2"0.2 Ma, respectively. During thistime span, mildly tholeiitic to transitional lavas rang-

Žing from low-MgO high-alumina basalts MgO-5. Žwt.%; Al O )19 wt.% to dacites SiO s63 to 762 3 2

.wt.% were emitted from the La Carbonera edifice.The lavas are porphyritic, containing phenocrysts of

mainly plagioclase, olivine, clinopyroxene, and mag-netite, which are set in a black microcrystallinematrix.

The silicic dome of Nahuistepe is located 8 kmwestwards of the Chichontepec summit cones. It iscomposed of rhyolites containing phenocrysts of pla-gioclase, orthopyroxene and hornblende, in a white,glassy, variably devitrified, groundmass. Althoughthe eruption of this dome falls within the time spanof pre-caldera lavas, as based on a single KrAr dateon plagioclase of 1.7"0.3 Ma, there is no fieldevidence for a comagmatic association between rhy-olites and La Carbonera lavas.

La Carbonera volcano experienced a pyroclasticeruption that led to the collapse of the edifice, therebygiving rise to a caldera, which is regarded to as the

Ž .La Carbonera caldera CEL, 1992 . Caldera rims arepreserved only in the western side of the edificealong the La Carbonera hills. They form a gently

Žcurved structure of cinder cones and lava flows Fig..1 .

2.2. The caldera stage

Intercalated between La Carbonera lavas and theoverlying post-caldera andesites, there is a pyroclas-tic sequence whose thickness is approximately 25 m.Pyroclastic rocks crop out mostly northwest from theChichontepec volcano, but they have also been drilled

Ž .under the post-caldera lavas Fig. 1 in the SV-1Ž .deep geothermal well CEL, 1992 on the northern

slopes of Chichontepec. A few pyroclastic sectionshave been found in the southwestern sector and haveall been heavily reworked by post-depositional pro-cesses.

Ž .One ignimbrite 5 to 10 m thick directly overliesLa Carbonera lavas, marking the beginning of thepyroclastic sequence. This deposit is composed of abrownish yellow, poorly welded, unsorted mix of ash

Ž .and pumice clasts f30–40 vol.% and less abun-Ž .dant lithic lapilli f10–20 vol.% . The ignimbrite is

separated from overlying pyroclastic rocks by aŽ .brown-coloured paleosol 1 to 2 m thick , which in

Žturn is overlain by a dacitic pumice-fall deposit 0.6.to 1.3 m thick . The latter is composed of highly

vesiculated pumice lapilli with minor basaltic–ande-sitic lithic fragments. This sequence continues with

Žalternating coarse pumice-falls and ash deposits 2 to


.3 m thick . Higher up the sequence continues withŽ .surge deposits 0.2 to 0.8 m thick made up of

well-developed plane-parallel beds, containing ash-sized particles and accretionary lapilli. The top surgelayer is cut by an erosional surface which, in turn, is

Ž .overlain by a sequence 2 to 3 m thick of alternatingcoarse-grained layers containing variable amounts ofpumice and lithic fragments and ash layers. Thewhole pyroclastic sequence terminates with a 2-m-thick paleosol.

The explosive phases, as recorded by the depositsdescribed above, led to the collapse of the La Car-bonera volcano and to the formation of the LaCarbonera caldera. The pattern of isopachs of daciticpumice deposit locates the vent position inside theannular structure outlined by La Carbonera hills.

2.3. The post-caldera stage. The Pleistocene Chi-chontepec Õolcano

The renewed volcanic activity after the pyroclas-Ž 3.tic stage formed a considerable volume 130 km of

Ž .thick ;20 m and lobate lava flows, which built upthe Chichontepec edifice inside the La Carboneracaldera. Lavas on the northern and southern flanks ofthe volcano were probably emitted from a centralvent, while those of the eastern flank were emittedfrom a parasitic vent, which is located to the east ofthe summit craters.

ŽLavas are mostly two-pyroxene andesites SiO s2.57 to 63 wt.% , ranging from basaltic andesites

Ž .SiO s52 to 57 wt.% to dacites, while those of the2

parasitic cones are mostly dacites. All post-calderalavas are highly porphyritic, containing phenocrystsof plagioclase, orthopyroxene, clinopyroxene, mag-netite, very rare ilmenite, and "hornblende, whichare all set in a grey, glassyrmicrocrystalline matrix.Abundant crystal clots and enclaves in pre-calderalavas distinguish them from post-caldera ones.

3. Petrography

3.1. Pre-caldera laÕas

Pre-caldera lavas are mostly composed of por-Ž . Žphyritic 30–33% phenocrysts , pilotaxitic lavas Ta-

.bles 1 and 2 with the most representative samplesof this group comprising low-MgO high-alumina

basalts. The composition of olivine phenocrysts is tobe found in the Fo range with a CaO content of74 – 80

0.10–0.25 wt.%. They frequently show resorptionŽ .embayments or thin, pigeonite rims. Al-rich spinelŽ .Al O s50 to 55 wt.% phenocrysts are typical of2 3

pre-caldera series with maximum grain sizes of up to1 mm. The crystals are rounded to lobate and theyare not found as inclusions in other phases. Backscattered electron examination has not shown zoningor exsolutions, except in one case of a corona ofmagnetite micrograins around pleonaste. AnorthiteŽ .An is abundant among phenocrysts, together89 – 93

Ž .with a less calcic plagioclase An . Both are82 – 88

free of inclusions, dusty zones or resorption surfaces.Ž .Plagioclase microphenocrysts An have a mean55 – 62

) Ž .FeO content 1.1 wt.% , which is distinctly higherthan post-caldera plagioclase microphenocrystsŽ ) . Ž .mean FeO s0.6 wt.% Table 1 . The characteris-

Ž .tic features of pre-caldera plagioclases are: 1 TheŽvery slight zoning in anorthite crystals DAn core–

. Ž .rim is commonly 2–3 mol% . 2 The normal zoningŽpatterns of other plagioclase crystals i.e., cores

An , rim s An , microphenocrysts s82 – 88 77 – 84. Ž . Ž .An ; Fig. 2, Table 1 . 3 The absence of dusty55 – 62

Ž .zones. Augite phenocrysts Wo En Fs33 – 36 40 23 – 25

have a low Ca-Tschermak component, ranging from0.4 to 1.0 mol%. Pigeonite is present in the ground-mass and its presence is typical for pre-caldera lavasŽ .Fig. 3 , while it is almost absent in other groupsŽ . Žexcept in the post-caldera lava, sample 117 Table.1 . Orthopyroxene is rare, being present only in the

Ž .most evolved samples Wo En Fs , and3 59 – 68 37 – 28

amphibole is absent.Lavas from the Nahuistepe dome are represented

by white coloured, glassy and variably devitrified,rhyolites with plagioclase occurring as euhedral phe-

Ž . Ž .nocrysts An and microphenocrysts An . The40 20Ž .latter is the most important phase 13 vol.% , while

Ž . Ž .orthopyroxene 1 vol.% , hornblende 2 vol.% andŽ .biotite 1 vol.% are rarer.

3.2. Post-caldera laÕas

The post-caldera lavas are characterized by thepresence of abundant glomerocrysts, microgranularenclaves and coarse-grained gabbroranorthositicxenoliths. The phenocryst content is higher thanpre-caldera lavas, in the 37–42% range. Olivine is


Table 1Main petrographic characteristics of pre-caldera and post-caldera lavas

Pre-caldera lavas Post-caldera lavas

Ž .Zoning in plagioclase Normal: high-alumina basalts Either reversed 0.52–0.71Ž . Ž .phenocrysts mean X core–rim s0.93–0.82; other rockss0.88–0.77 or normal 0.52–0.47An

X of plagioclase Lower than phenocrysts Frequently higher thanAnŽ . Ž . Ž .microphenocrysts range 0.62 to 0.55 phenocrysts 0.57 to 0.75

Ž .Mean FeO) wt.% in 1.1 0.6plagioclase microphenocrysts

Ž . Ž .Zoning in orthopyroxene Normal 0.68–0.59 Either reversed 0.62–0.70Ž . Ž .phenocrysts mean X core–rim or normal 0.63–0.59En

X of orthopyroxene Lower than phenocrysts Frequently higher thanEnŽ . Ž . Ž .microphenocrysts range 0.60 to 0.53 phenocrysts 0.67 to 0.56

Low-Ca pyroxene in groundmass Pigeonite Orthopyroxene

Ž .Olivine Often present 55% of lavas Quite rarely presentŽ .18% of lavas

Al-rich spinel Present Absent

Hornblende Absent Present from SiO )63%2

Ž .Crystal clots, cognate inclusions, Crystal clots only, Abundant 15% of lavasŽ .xenoliths but very rare -2% of lavas

Table 2Ž .Modal analyses of the most representative lavas 800 to 1700 points counted for each sample

Sample: Pre-caldera Post-caldera Domerock-type: 14 73 98 1 36 117 161 85

high- dacite high- andesite dacite basaltic hybrid rhyolitealumina alumina andesite lava

Ž .basalt basalt mingledaPlg ph 40.1 7.8 31.3 24.0 28.3 30.8 19.1 12.5

bPlg mph 33.5 17.9 35.1 14.3 13.9 2.8 4.5 3.0Ol ph 4.0 4.0 trOl mph tr 2.0Cpx ph 1.0 0.5 1.2 3.0 4.2 3.8Cpx mph 12.4 2.9 14.5 0.2 0.1 0.9Opx ph 6.3 3.0 3.4 1.8Opx mph 1.6 0.3 0.5 1.3 0.7Hbl 0.8 0.5 2.0Bt 1.0Qz 2.0Oxides ph 0.8 0.1 1.0 2.1 2.0 0.3 0.7Oxides mph 8.2 2.8 13.0 2.1 2.6 1.0 13.3 0.3Crystals percent 100.0 32.0 100.0 50.7 54.0 44.8 45.5 22.2

cGdm percent 68.0 49.3 46.0 55.2 54.5 77.8

a Ž . b Ž . c Ž .Phenocrysts )0.5 mm ; microphenocrysts )0.05 mm ; groundmass -0.05 mm .Bt: biotite; Cpx: clinopyroxene; Hbl: hornblende; Ol: olivine; Opx: orthopyroxene; Plg: plagioclase; Qz: quartz.


Fig. 2. Feldspar ternary diagram. Each point represents one analysis. Sample numbers are reported on the left of each triangle.

quite rare and, when present, it is almost completelyresorbed. Plagioclase is present in two types thatcould be distinguished by their size and the pres-

Ž .encerabsence of inclusions: 1 Medium-sized phe-Ž . Žnocrysts 0.5–1.0 mm either normally zoned cores:

. ŽAn , rims: An or reversely cores: An ,54 – 63 47 – 54 62 – 51.rims: An . In the most evolved samples they72 – 69

Ž .host tiny apatite needles. 2 Complexly-zonedŽ . Ž .megacrysts up to 4 mm with calcic cores An ,65 – 70

often consisting of many annealed crystals. Near-rimzones are often sieved, either by glass, which isrhyolitic in composition, or by fine-grained matrix

material. Plagioclase microphenocrysts are in theAn range. It is important to note that microphe-57 – 75

nocrysts are often enriched in the An component, asŽ .compared to phenocrysts Fig. 2, Table 1 .

Orthopyroxene phenocrysts have a composition inthe Wo En Fs range. They often show2 – 3 62 – 73 35 – 24

Ž .reverse zoning i.e., Mg-richer rims and are fre-quently jacketed by clinopyroxene. Groundmass pi-geonite is almost completely absent while orthopy-roxene is present. Clinopyroxene is an augiteŽ .Wo En Fs showing either normal or reverse42 42 16

Ž .zoning predominant . The Ca-Tschermak compo-


Ž .Fig. 3. Pyroxene compositions for pre-caldera 43 analyses andŽ .post-caldera lavas 74 analyses .

Žnent is higher than in pre-caldera lavas mean value.s2.6 mol% . The presence of brown hornblende,

rimmed by fine-grained magnetite and glassy mate-rial, is typical of post-caldera evolved lavas. Horn-

Ž .blende is a Mg-hastingsite X s0.66–0.72 with aMgŽ .low K O content 0.3–0.4% and a NarK ratio of2

approximately 5.6–9.0.Enclaves are relatively common in post-caldera

Ž .lavas about 15% of lava samples contain enclaves ,Ž .while they are almost absent i.e., -2% in pre-

caldera lavas. Their length ranges from 0.5 to 1 cmacross. We have divided the enclaves into two groups:

Ž .1 Fine-grained mafic enclaves occurring asholocrystalline to hypocrystalline fine-grained en-

Ž .claves 0.5 to 1.5 mm . They show intersertalrhy-alophitic textures, with subhedral orthopyroxene andeuhedralrsubhedral plagioclase laths. When glass is

Ž .present 5 to 10 vol.% , it forms variably devitrifiedpale brown pools enclosing plagioclase. The enclavetextures suggest that they are tightly packed crystal-rich mushes, that is, depositional cumulates frag-

Ž .ments, as defined by Irvine 1982 . Mineral composi-tions of enclaves are comparable to those of hosting

Žlavas e.g., pyroxene in enclaves s Wo 2 – 3

En Fs ; pyroxene in hostsWo En61 – 64 35 – 37 2 – 3 59 – 62

Fs ; plagioclase in enclavesAn , plagio-36 – 38 65 – 75.clase in hostsAn . Thus, they are probably50 – 58

cognate.Ž . Ž2 Coarse-grained anorthositic enclaves 2 sam-.ples . They consist of hypidiomorphic granular

anorthosites, containing 90 vol.% of coarse-grainedŽ . Ž .-7 mm subhedral plagioclase An , unzoned ,90 – 93

Ž . Žinterstial olivine Fo and Al–Ti-spinel Al O s78 2 3.11 to 12 wt.% . They are characterized by a greater

compositional difference compared to the host lavas,Žplagioclase in enclavesAn , plagioclase in90 – 93

.hostsAn , while olivine and spinel are absent57 – 62

in the lavas. The lack of any evidence of equilib-rium, of post-caldera lavas with an anorthitic plagio-clase strongly suggest that these enclaves are cumu-lates, which are unrelated to hosting lavas. Bothgroups of enclaves show a very small degree ofresorption or reaction with the liquid and absence of

Table 3Thermobarometric results

a Ž . Ž . Ž . Ž . Ž .Sample Group Rock-type T 8C T 8C T 8C P kbar P kbar1 2 3 1 2

1 pre-caldera and 966 980 1005 1.5 1.436 pre-caldera and 931 980 993 1.6 2.0

117 pre-caldera ba 993 – – 3.0 2.1122 pre-caldera and 974 930 961 3.0 1.5124 pre-caldera and 953 – – 2.8 2.014 post-caldera hab 1000 1025 1075 5.4 2.3

163 post-caldera and – 920 909 4.5 –85 Rhyolite dome rhy – 800 798 1.8 0.7

aand: andesite; ba: basaltic andesite; hab: high-alumina basalt; rhy: rhyolite.Ž .T : temperature calculated using cpx–opx pairs Wells, 1977 ; T and T temperature calculated using mt–ilm pairs according to1 2 3

Ž . Ž .Buddington and Lindsley 1964 and Ghiorso and Sack 1991 , respectively.Ž .Pressure estimates based on plg–liquid equilibria Kudo and Weill, 1970; see text for details . P : phenocrysts—whole rock; P :1 2

microphenocrysts—groundmass.


Fig. 4. T y f calculated by magnetite–ilmenite pairs. FilledO 2

circles: pre-caldera lavas; empty circles: post-caldera; filledŽ . Žsquares: rhyolitic dome; stars represent solutions of Eq. 1 see

.text .

quench textures. These observations support a slowcooling in a nearly thermal equilibrium betweenenclaves and the host liquid.

4. Thermodynamic constraints

4.1. Geothermobarometry

Among the various geothermometers based on theshape of the orthopyroxene–clinopyroxene miscibil-ity gap, we have chosen the calibration of WellsŽ . Ž1977 . Although associated errors are large "608C

.according to the author , this method is consideredparticularly accurate in the temperature range of

Ž .andesitic magmas Wallace and Carmichael, 1994 .ŽWe avoided pairs with disequilibrium textures such

.as Mg-rich rims, overgrowths, etc. , preferring, in-stead, to choose contiguous pairs, as close as possi-ble to the X opxrcpx s1, as a constraint for equilib-Mg

Žrium. We obtained pre-eruptive temperatures using.the rims of phenocrysts from 15 selected pairs that

cluster around 9508C for andesites and up to 10008CŽ .for pre-caldera high-alumina basalt Table 3 .

The Fe–Ti oxide geothermometer was applied tocoexisting mt–ilm pairs, preliminarily checked for

Table 4Ž . Ž .Mineral phases used in thermodynamic calculations of f according to Eq. 1 see textO 2

Sample: Pre-caldera Post-caldera

14r358 Mt 14r359 Ol 14r360 Opx 122r309 Mt 122r316 Ol 122r315 Opx

SiO 0.09 37.22 51.48 0.06 37.93 52.722

TiO 3.77 0.02 0.30 10.15 0.01 0.222

Al O 3.16 0.21 0.63 1.75 0.06 2.042 3

Cr O 0.02 0.00 0.00 0.00 0.04 0.002 3

FeO 85.67 30.73 20.63 81.08 26.10 16.69MnO 0.38 0.62 1.13 0.79 0.69 0.63MgO 1.13 29.45 21.18 1.50 36.33 24.78CaO 0.13 0.25 3.00 0.02 0.16 2.01Na O 0.00 0.00 0.16 0.00 0.00 0.062

Total 94.40 98.50 98.58 95.41 101.32 99.15X 0.891 0.713Mt

aa 0.891 0.713Mt

X 0.596 0.689Enba 0.578 0.656En

X 0.326 0.260Fsca 0.316 0.248Fs

X 0.624 0.706Foda 0.389 0.498Fo

a Ž .Activity of magnetite in spinel: a sX Xsmole fraction; asactivity .Mt Fe O ,Ti – Mt3 4b Ž 2 .0.5Activity of enstatite in orthopyroxene: a s X X X .En Mg,M2 Mg,M1 Si,TETc Ž 2 .0.5Activity of ferrosilite in orthopyroxene: a s X X X .Fs Fe,M2 Fe,M1 Si,TETdActivity of forsterite in olivine: a sX .Fo Mg SiO ,Ol2 4Ž . Ž . Ž . Ž . Ž . Ž .En enstatite; Fs ferrosilite; Fo forsterite; Mt magnetite; Ol olivine; Opx orthopyroxene.


Fig. 5. Plagioclase–liquid equilibria. K Ca – Na were calculatedDŽ .between groundmass i.e., chilled liquid and microphenocrysts.

Filled rectangles: pre-caldera; empty rectangles: post-caldera;banded rectangle: rhyolitic dome. Horizontal bars represent themean An value of each sample. Numbers inside circles representthe K Ca – Na values. Sample numbers are reported close to eachD

vertical rectangle.

equilibrium according to the method of Bacon andŽ .Hirschmann 1988 . Due to the extreme scarcity of

the rhombohedral phase, only eight pairs were used.Temperatures were estimated using two methods:empirical calibration of Buddington and LindsleyŽ .1964 and thermodynamic solution model of Ghiorso

Ž .and Sack 1991 . Agreement between these twomethods was nearly always within the error experi-

Ž .enced by the authors "308C . Estimated tempera-tures were as high as 1025 to 10758C for high-alumina basalt, 920 to 9808C for andesites, and 780

Ž .to 8008C for rhyolite Table 3 .The equilibrium between Fe–Ti oxides also al-

Ž .lowed the estimation of oxygen fugacity Fig. 4Ž .with log f ranging from y13.2 at 8008C to y10.6O 2

Ž .at 10008C , i.e., 1–2 log units above the Ni–NiOŽbuffer with the exception of sample 163 which

.straddles the NNO line . We also performed inde-pendent thermodynamic calculations based on equi-

Ž .librium Luhr and Carmichael, 1980 :

6Mg SiO q6FeSiO qO s2Fe O q12MgSiO2 4 3 2 3 4 3

1Ž .Ž .Thermodynamic data DG8, DG8 and ln K weref r

calculated using the SUPCRT92 computer program

Ž .Johnson et al., 1992 at the temperature fixed by thegeothermometers. We ignored the P effect becauseof the small DV of solids participating in the reac-tion. Activities of various components in the mineral

Ž .phases used in the calculations Table 4 were takenŽ .from the activity models of: a Kerrick and Darken

Ž . Ž . Ž .1975 for olivine and b Carmichael et al. 1977 ,for enstatite, ferrosilite and magnetite.

Ln f was then calculated according to:O 2

2 12 6 6ln f s a a r a a ln KŽ . Ž . Ž . Ž .O mt en fo fs2

ŽEstimates of log f with this method stars inO 2

.Fig. 4 are in broad agreement with those calculatedby the Fe–Ti oxide method.

Ž .The Kudo and Weill 1970 plagioclase-liquidempirical geothermometer was used for an indirect

Ž .estimation of P see Luhr and Carmichael, 1980 ,H O2

solving the quadratic equations given by the authors,and then fitting pressure for an independent tempera-ture estimate. Errors associated with this methoddepend on the accuracy of temperature estimates:"508C shift pressure of "0.7 kbar. Whole-rock andgroundmass compositions were assumed to be repre-sentative of the liquid as regards phenocrysts andmicrophenocrysts, respectively. The estimated PH 02

Ž .was unexpectedly high 5–5.5 kbar for high-Anphenocrysts of pre-caldera lavas. Microphenocrysts

Ž . Žgave consistently lower pressures 2–2.5 kbar Ta-.ble 3 .

Fig. 6. SiO vs. FeO)rMgO. The dividing line between tholeiitic2Ž .and calc-alkaline trends was taken from Miyashiro 1974 .


Fig. 7. Harker diagrams for major and minor elements. Symbols as in Fig. 6. Major elements: the liquid line of descent starts from sample98 and at each step 5% of solid was removed from each produced liquid. Mineral proportions of the fractionating assemblage were chosen

Ž . Ž .bearing in mind phenocryst abundance counted and phase relations from Grove and Baker 1984 : plgs0.75, olivs0.15, spinels0.10,for SiO2)50.5%; plgs0.67, olivs0.10, cpxs0.10, mts0.13, for SiO2)54%; plgs0.67, cpxs0.10, opxs0.10, mts0.13, forSiO2)58%. Equilibrium mineral compositions were calculated at each step according to the following crystal–liquid exchange partition

Fe-Mg Ca – Na Ž Fe – Mg x tal liq x tal liq Ca – Nacoefficients: olivine K s 0.31–0.32; plagioclase K s 2.5–1.1 where K s X ) X rX ) X ; K sD D D Fe Mg Mg Fe Dx tal liq x tal liq .X ) X rX ) X ; Al-spinel, magnetite and clinopyroxene compositions were kept constant. Compositions of the solids, which wereCa Na Na Ca

removed at each step, were calculated multiplying the wt.% oxide in each phase )wt. proportion of the phase )wt. proportion of the solidŽ . n0.05 . The resulting residual liquids were then normalized to 100%. The remaining liquid fraction after each step was: 0.95 , n being theprogressive number of the step. Minor elements: stepwise FC lines were drawn using the same starting point and mineral assemblages ofmajor element model.


As a further barometric constraint, we used theŽ .simple P–T–H O grid proposed by Brophy 1986 ,2

which is based on the sequence of appearance of theliquidus minerals. The petrographically-inferred se-quence of the crystallization of pre-caldera high-alumina basalts, i.e., plagioclase, and later co-pre-cipitation of olivine plus clinopyroxene, sets a rough

Župper pressure limit under slightly hydrous condi-.tions of 6 kbar.

4.2. Plagioclase–liquid equilibria and water contentestimates

In order to evaluate if the minerals had once beenin equilibrium with the inferred liquid, we consid-ered groundmass–microphenocrysts pairs, choosing

Žthe means of groundmass compositions at least four.analyses as representative of the quenched liquid.

Ž .Groundmass SiO wt.% ranges from 51.6 to 62.52

for pre-caldera lavas, from 67.7 to 71.9 for post-caldera lavas, and from 76.8 to 77.4 for Nahuisteperhyolites.

Plagioclase–liquid equilibria: the experimentalŽ .data of Sisson and Grove 1993a , for high-alumina

basalts and basaltic andesites, demonstrated thatCa – NaŽ x tal liq x tal liq .K sX PX rX PX for plagioclase–D Ca Na Na Ca

melt equilibria is strongly dependent on water con-tent and very little on pressure. They stated that theK Ca – Na changes from nearly 1 under anhydrousD

conditions to 5.5 at H O saturation. Microphe-2

nocrysts of the post-caldera group show K valuesD

that range from 2.8 to 6, considerably higher thanŽ . Ž .post-caldera microphenocrysts 1 to 2 Fig. 5 .

The high H 0 content of post-caldera lavas, as2Ca – Na Žinferred from the high K Sisson and Grove,D

.1993a , is in accordance with the presence of horn-Žblende in post-caldera andesites. Hornblende which

.never occurs in pre-caldera rocks gives a roughindication of a high water content because its stabi-lization in an andesitic melt at 950–10008C and 3 to4 kbar requires a melt water content of 4–6 wt.%ŽEggler and Burnham, 1973; Baker and Eggler, 1983;

.Rutherford and Devine, 1988 .

5. Geochemical data

We selected the simple FeO)rMgO vs. SiO2Ž .diagram introduced by Miyashiro 1974 , which we

consider offers the best distinction between the pre-caldera and the post-caldera series. The pre-calderaseries indicates a transitional to mildly tholeiiticcharacter, while the post-caldera series are typical

Ž .calc-alkaline Fig. 6 . The tholeiitic affinity of pre-caldera lavas is also supported by the presence of

Ž .pigeonite microphenocrysts Kuno, 1959 and by theFeO) enrichment in plagioclase microphenocrystsŽ .Aramaki and Katsura, 1973 .

Harker-type diagrams show quite different trendsŽ .for the two series Fig. 7 . Pre-caldera lavas depict a

liquid line of descent, which is driven by the earlyfractionation of spinel plus olivine; Ti–mt appears asa liquidus phase at a silica value of 55% and itsonset causes an abrupt drop in FeO, TiO , MnO and2

V. P O has an initial positive correlation with silica,2 5

up to the inflection point at 57% SiO , which marks2

the onset of apatite on the liquidus. Post-calderalavas show a flatter and more scattered correlation ofthese elements when plotted against silica.

The pre-caldera series comprises rather evolvedtholeiites with K O contents in the range of2

Fig. 8. N-MORB normalized spiderdiagram for pre- and post-caldera lavas. Normalization values from Sun and Mc DonoughŽ .1989 .

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Table 5Representative whole-rock analyses

Pre-caldera Dome Post-caldera

Sample 14 73 79 98 163 85 1 36 43 117 122 124 131 133 161 162aRock-type hab dac and hab ba rhy and dac and ba and and and and hyb hab-enc

SiO 50.81 66.86 58.71 50.49 54.99 73.48 61.29 64.09 58.58 56.11 59.00 60.55 62.49 62.70 67.07 50.232

TiO 0.86 0.65 0.70 0.81 1.01 0.23 0.53 0.52 0.65 0.80 0.59 0.60 0.52 0.52 0.55 1.182

Al O 22.14 15.51 18.19 21.83 18.62 14.41 17.09 16.63 18.69 18.95 18.28 17.99 17.84 17.80 15.38 20.082 3

Fe O 3.83 3.75 2.63 4.38 3.62 0.80 2.94 3.91 1.69 1.35 1.64 1.92 3.31 2.73 4.20 5.672 3

FeO 4.50 1.02 4.40 4.22 5.57 1.37 2.72 1.53 4.96 6.22 4.97 4.27 2.38 2.93 0.32 4.23FeO) 7.95 4.39 6.77 8.16 8.83 2.10 5.37 5.05 6.48 7.43 6.45 6.00 5.36 5.38 4.10 9.36MnO 0.15 0.11 0.16 0.15 0.18 0.02 0.13 0.11 0.15 0.16 0.15 0.15 0.13 0.14 0.11 0.18MgO 3.62 0.81 2.82 4.44 2.83 0.25 3.53 1.98 3.49 3.51 3.23 3.39 2.88 2.92 1.46 4.14CaO 9.41 3.30 6.21 9.26 7.05 2.03 5.64 4.63 6.95 7.38 6.65 5.21 5.28 5.17 3.90 9.82Na O 3.07 4.84 4.24 3.02 3.98 4.22 3.63 3.47 3.51 3.50 3.49 3.63 3.58 3.57 4.14 3.102

K O 0.51 1.94 1.16 0.50 0.89 3.14 1.29 1.79 1.19 0.98 1.13 1.29 1.36 1.37 1.97 0.462

P O 0.16 0.25 0.29 0.17 0.30 0.02 0.16 0.19 0.13 0.16 0.15 0.17 0.21 0.16 0.13 0.252 5

LOI 0.94 0.95 0.50 0.73 0.96 0.03 1.04 1.16 0.66 0.87 0.72 0.83 0.92 0.99 0.70 0.56

Mga 44.8 24.7 42.6 49.2 36.3 17.6 54.0 41.1 48.3 45.7 47.2 50.2 48.9 48.5 38.8 44.1V 296 31 108 277 225 19 131 124 171 198 157 139 105 116 93 294Cr 15 3 8 15 11 2 7 9 10 11 8 5 7 7 5 6Co 28 11 22 31 31 0 19 16 23 26 24 20 19 18 13 35Ni 7 3 6 10 5 2 4 4 6 4 6 3 4 4 2 26Rb 10 40 23 10 15 65 27 38 27 23 29 18 25 25 13 15Sr 539 362 489 544 512 160 368 383 415 541 382 356 369 396 548 461

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Y 16 37 23 18 18 15 18 30 20 20 19 18 18 19 21 31Zr 37 186 115 39 61 94 105 129 105 76 100 86 107 108 81 87Nb 2 3 3 3 5 – 3 – 3 2 5 3 4 3 – –Ba 359 1260 594 435 531 1182 628 892 598 588 564 819 669 663 981 475La 5 20.7 10 7.2 9 16.9 11.0 19.1 10 8 10.0 4 11 10 15 11Ce 15 42.2 29 13.7 32 29.9 25.9 30.2 23 18 22.8 24 25 24 27 29Nd 31.2 12.2 13.0 16.1 27.5 – – 15.0 – – – – –Sm – 7.8 – 3.1 – 2.4 3.9 6.9 – – 3.6 – – – – –Eu 2.0 1.2 0.7 1.0 1.8 – 1.0 – – – – –Gd – 6.5 – 3.1 – 2.0 3.3 5.5 – – 3.2 – – – – –Tb – 1.1 0.5 0.3 0.6 0.9 – 0.6 – – – – –Dy – 7.0 – 3.5 – 2.1 3.8 5.6 – – 3.4 – – – – –Er – 4.4 – 1.9 – 1.3 2.3 3.4 – – 2.2 – – – – –Yb – 4.5 – 2.1 – 1.6 2.6 3.7 – – 2.4 – – – – –Lu – 0.6 – 0.3 – 0.2 0.4 0.6 – – 0.3 – – – – –

87 86Srr Sr 0.70385 0.70379 – 0.70382 0.70378 0.70393 0.70378 0.70374 0.70379 0.70383 0.70385 – – 0.70385 0.70385 0.70377143 144Ndr Nd – 0.51300 – 0.51302 – 0.51297 – – 0.51304 – – – – 0.51302 0.51302 0.51299

X Y X Y X Y X Y X Y X Y X Y X Y X Y X Y X Y X Y X Y X Y X Y X YLatitude, 36 00 36 50 35 58 36 50 31 50 33 52 37 49 37 55 34 40 36 59 35 10 35 50 33 51 34 20 33 50 33 50138N

X Y X Y X Y X Y X Y X Y X Y X Y X Y X Y X Y X Y X Y X Y X Y X YLongitude, 52 41 54 13 53 33 56 05 50 52 57 20 50 10 49 23 50 55 48 43 47 55 46 54 48 18 48 20 50 55 50 55888E

a Ž .and: andesite; ba: basaltic andesite; dac: dacite; hab: high-alumina basalt; hab-encl: hab enclave which is hosted in a post-caldera lava; hyb: hybrid mingled lava; rhy: rhyolite.Dashes in columnssnot analyzed.Mga: at.% with all iron as Fe.87Srr86 Sr: errors"0.00002; 143Ndr144 Nd: errors"0.00001.


Fig. 9. REE spectra for selected lavas. Symbols as in Fig. 6.Sample 98 is a high-alumina basalt. For the other rock-types, see

Ž .Table 5. Normalization values from Anders and Ebihara 1982 .

medium-K rock series. When compared with N-typeMORB, we find that most of the rocks are enriched

Ž .in K and related elements Fig. 8 . Mga ranges from24.7 to 49.2, and TirV ratios are in the range ofthose of the N-MORB. Typical characteristics, espe-cially of pre-caldera lavas, are the very low Cr and

Ž .Ni contents -20 ppm and -13 ppm, respectively ,Ž .even in the most mafic samples Table 5 . Incompat-

ible trace elements, such as Zr and Y, show variableconcentration, while the ZrrY and YrNb ratios arein the E-MORB range. Some samples show a well

Ž .developed negative Ce anomaly Fig. 9 which, to-gether with high BarLa ratios, are typical of arc

Žmagmas White and Patchett, 1984; Carr et al.,.1990 . Eu exhibits a positive anomaly in high-

Ž .alumina basalts sample 98 and a negative anomalyŽ .in more evolved rocks. Spiderdiagrams Fig. 8 of

trace-element concentrations, which have been nor-Ž .malized to N-MORB Sun and Mc Donough, 1989

show a general slight depletion of HFSE, with BarNband BarLa values always being high. The represen-

Fig. 10. 87Srr86Sr vs. 143Ndr144 Nd. Symbols as in Fig. 5. The thick line represents the mantle array. Pre- and post-caldera lavas are plottedin the inset. See text for discussion.


Ž .tative high-alumina basalt sample 98 shows well-developed positive Sr positive anomalies. As regards

Ž .some evolved samples samples 73, 85 , negativespikes of Ti and P probably reflect the removal ofmagnetite and apatite. A pronounced trough is pre-sent at Nb, which is a common feature of arc mag-mas. Ti and P depletions are missing in high-aluminabasalts.

The post-caldera series comprises rocks with Mga

Žranging from 41.1 to 47.5, low TiO content 0.5–2.0.8 and TirV ratios, which are comparable to those

of pre-caldera rocks, as well as the abundance of LILelements. The chondrite-normalized REE pattern

Ž .show an enrichment in LREE with LarYb f2.8N

and HREE patterns, which are flat or slightly frac-tionated. The spider diagram is similar to those ofthe pre-caldera suite. Sr displays a bulk distributioncoefficient in both groups, which is always )1 andwhich produces the observed negative correlation

Ž .with silica Fig. 7 .The isotopic composition of selected whole-rock

samples are reported in Table 5 and plotted on theŽ .Sr–Nd isotope correlation diagram Fig. 10 . Mea-

sured strontium isotopic ratios range from 0.70378 to0.70393, while neodimium isotopic ratios range from

Ž0.513040 to 0.512971 mean values: 0.70381"

0.00004 1s , and 0.51301"0.00002 1s , respec-.tively , for a silica content that ranges from 51 to 74

wt.%. The homogeneity of 87Srr86Sr and 143Ndr144

Nd ratios does not allow a distinction between pre-and post-caldera lavas.

6. Interpretation of data and discussion

Although coexisting tholeiitic and calc-alkalinesuites are not very frequent in volcanic arcs, it iswell known that a tholeiitic parent could generate acalc-alkaline derivative magma, due either to lowpressure or to source-related processes. The formerinclude changes in the fractionating mineral assem-blage, such as a response to variations in inten-siverextensive variables, e.g., pressure, water con-

Žtent or oxygen fugacity Fairbrothers et al., 1978;.Grove and Baker, 1984 , while different degrees of

partial melting or the degree of source-hydrationŽ .accounts for the latter Baker et al., 1994 .

6.1. Geochemical and isotopic constraints

Post-caldera chemical trends are much more scat-tered for most elements, if compared to pre-calderatrends. They cluster at a silica value of between60–65% and this behaviour is thought to be due to aheterogeneous distribution of mafic phases andrormicroenclaves.

In the most mafic samples, low Cr and Ni con-Žtents contrast with high V abundance 250 to 350

.ppm ; this, in combination with low Mga, reflectsŽextensive olivine plus pyroxene removal but not

.magnetite from a more primitive basaltic liquid.Variations in the ratios of elements differently com-patible in plagioclase, olivine, pyroxene, can be usedto infer the changes in the liquidus mineral assem-

Ž .blage. In the SrrZr vs. Ba diagram Fig. 11 , frac-tionation vectors show a plagioclase-dominated ini-tial fractionating assemblage that changes to pyrox-ene-dominated assemblage in the most evolved rocks.The positive Sr and Eu anomalies in a pre-caldera

Ž ) .high-alumina basalt sample 98: EurEu 1.14 , cou-pled with a high Al O content, is convincing evi-2 3

dence for plagioclase accumulation. According toŽ .Crawford et al. 1987 , 30% of plagioclase accumu-

Fig. 11. SrrZr vs. Ba with fractionation vectors. They wereconstructed using the mean of D-values reported in Table 7. Thefigure shows a plg-dominated trend for less evolved pre-calderalavas, px-dominated for post-caldera ones. Rock types for pre-caldera lavas: and, andesites; ba, basaltic andesites; dac, dacite;hab, high-alumina basalts; rhy, rhyolites. Symbols as in Fig. 6.


lation is needed to rise EurEu) up to 1.15 andAl O up to 23 wt.%.2 3

Pre- and post-caldera lavas show also very similarisotopic compositions, thus, landing further supportto a common mantle source hypothesis. The narrowrange of Sr and Nd ratios indicates either similarisotopic mantlercrustal components or a well-mixedmagma with constant proportions of isotopically dif-ferent sources. In fact, the highest recorded 87Srr86Sr

Ž .isotopic ratio 0.70395 shows that crustal assimila-tion is not quantitatively significant, unless the crusthas the same isotopic signatures. AFC modellingalso suggests that crustal assimilation is not capableof producing the observed range of 87Srr86Sr and143Ndr144 Nd ratios, bearing in mind the lack of anycorrelation between SiO and isotopic ratios.2

Isotopic data points are plotted close to the mantlearray in a field coinciding with Hawaiian magmasŽ .Zindler and Hart, 1984 , thereby implying sourcesthat evolved with the contribution of depleted andenriched mantle components. Because Sr–Nd iso-topic compositions appear to be decoupled from eachother and from elemental variations, the existence ofa long-lived component in their mantle source can beexcluded.

The broad correlation between 87Rbr86Sr and87Srr86Sr might reflect a mixing process that couldinvolve different end-members: subducted slab, man-tle wedge, lower and upper crust. The sub-horizontal

Ž .alignment of isotopic data inset in Fig. 10 could beascribed to processes involving sea water interaction.The similarity of geochemical patterns between pre-

Žand post-caldera lavas i.e., LILE enrichment, HFSEdepletion, Ce negative anomaly as well as high

.BarNb and BarLa ratios could reflect the signatureof the source region. Particularly, they could mirrorthe contribution of pelagic terrigenous sediments ofthe down-going slab, and the interaction of slab-de-

Žrived hydrous melts with the mantle wedge Hickey.et al., 1986 . These are generally characterized by

Sr–Nd isotopic compositions of f0.704 and fŽ0.5131 White and Dupre, 1986; Stern and Kilian,´

.1996 .

6.2. Crystal fractionation models

The occurrence of high-alumina basalts closelyassociated with andesites is important in understand-

ing evolutionary trends. Even though the origin ofŽhigh-alumina basalts is still controversial Sisson and

.Grove, 1993b , the effect of water on medium pres-Žsure phase equilibria seems important Sisson and

.Grove, 1993a . If pressure and water content arehigh enough during the ascent of a basaltic liquid,

Žthen plagioclase nucleation is suppressed Eggler,.1972; Sekine et al., 1979 and olivine plus clinopy-

roxene become the liquidus phases. This causesAl O enrichment in the liquid and consequently the2 3

generation of low-MgO high-alumina basalts.Regardless of their exact origin, we used the

pre-caldera high alumina basalts as a starting pointfor the modelling of the evolutionary paths of theChichontepec volcanic centre. A step-by-step modelof the liquid line of descent for closed-system frac-tional crystallization was superimposed onto ob-served major elements on Harker diagrams, starting

Žfrom the most primitive pre-caldera sample sample98: SiO s50.49; Mgas49.2; see details in the2

.caption of Fig. 7 .The whole path of the liquid line of descent for

pre-caldera lavas was then checked by mass-balanceŽ .Stormer and Nichols, 1978 , choosing as parent–daughter pairs those which showed either high orlow silica differences. Rim compositions of phe-nocrysts from the inferred parent were subtractedfrom the parent in order to derive a daughter magma.The sum of squared residualss0.7 was chosen asthe upper limit of acceptability; runs involving the

Ž .addition of phases were rejected Table 6 .The best results for major elements were then

Žsubjected to tests involving the trace elements V,.Cr, Rb, Sr, Ba, Ce , using the previously calculated

Žconditions step size, percentage of fractionation,.phase abundance . The wide variation of existing

crystalrliquid partition coefficients for calc-alkalinemagmas introduces considerable uncertainty. Wehave thus chosen two sets of D-values for each

Ž .model: high and low Ds Table 7 . The two series ofresults which were obtained for high and low Dswere considered appropriate if the observed abun-dance of the given trace element in the daughtermagma fell within this highrlow range. The graphi-

Ž .cal result Fig. 7 shows a quite good fit for thepre-caldera lavas. The post-caldera trend instead,although scattered, matches a model liquid descentline, only if plagioclase is lowered from 70–75% to


Table 6Ž . Ž .Selected mass balance calculations Stormer and Nichols, 1978 for pre-caldera 98: high-alumina basalt; 73: dacite and post-caldera lavas

Ž .117: basaltic andesite; 131: andesite

Ž . Ž .Pre-caldera: from sample 98 to 73 residuals Post-caldera: from sample 117 to 131 residuals

SiO 0.00 0.012

TiO 0.14 0.122

Al O y0.02 0.052 3

FeO) y0.02 y0.02MnO 0.05 0.01MgO 0.01 0.00CaO 0.00 0.00Na O 0.10 y0.242

K O y0.15 0.152

P O 0.00 0.002 5

Sum of squared residuals 0.06 0.10

RemoÕed mineralsPlg 58.4 27.9Ol 9.6Cpx 3.0 6.3Opx 3.3Mt 4.8 3.4Ap 0.2 0.1Weight percent of residual liquid 24.0 59.0

Residuals: observed minus calculated values for each component.Mineral abbreviations as in Table 2; Mt: magnetite; Ap: apatite.

55–60% and orthopyroxene is increased from 0 to20%.

However, the derivation from the most mafic toŽ .the most acid sample i.e., sample 98 to 73 is

possible for the pre-caldera series. Conversely, post-caldera group gives very few solutions and only for a

Žvery limited compositional range best results in.Table 6 . No solution was obtained for a derivation

of post-caldera samples from a pre-caldera parentŽ .and a derivation of rhyolites Nahuistepe dome from

a pre-caldera parent was not possible due to an

overenrichment in Ba and Sr and underenrichment inCe in rhyolites.

6.3. Some remarks on the occurrence of Al-richspinel, anorthite and reÕerse zoning

Al-rich and Cr-poor spinel is quite rare in arcŽ .rocks Della Pasqua et al., 1995 . The rounded and

Ž .lobate Al-rich spinel Al O s50 to 55 wt.% , which2 3

occurs in pre-caldera high-alumina basalts, is clearly

Table 7High and low partition coefficients used in the FC model

Olivine Cpx Opx Mt Plg Ap

V 0.04 1.1–5 0.5–2.2 8.7–54 -0.01 -0.01Cr 2–34 10–43 7–21 6–10 0.02–0.25 -0.01Ni 10–30 4–9 0.8–24 9–18 -0.01 -0.01Co 2–7 0.9–3 3–15 4–25 0.04–0.10 -0.01Rb 0.01–0.08 0.01–0.1 0.01–0.03 0.01 0.02–0.19 -0.01Sr 0.03 0.05–0.1 0.01–0.1 -0.01 1.8–3 0.01–2Ba 0.01 0.02–0.05 0.03 0.12 0.08–0.36 -0.01Ce 0.01 0.04–0.51 0.03–0.33 0.06–0.82 0.06–0.30 15–30

Ž . Ž . Ž .D-values from Luhr and Carmichael 1980, 1985 , Gill 1981 , De Pieri et al. 1984 .


in disequilibrium with the host liquid. In our opinion,Al-spinel is a relic of an earlier crystallization stage

Žat medium–high pressure i.e., PG10 kbar, John-.ston and Draper, 1992 from an Al O -rich, Cr O2 3 2 3

poor, evolved liquid. Another possibility is that anor-thite crystals, associated with Al-spinel in pre-calderahigh-alumina basalts, grew from the same alumina-rich liquid from which Al-spinel had been precipi-tated. But the evidence from anorthositic enclavesŽ .Section 3.2 is that anorthite crystals coexist with a

Ž .low-Al spinel Al O s11 to 12 wt.% . We specu-2 3

late that the association of anorthite with low-Alspinel could represent a lower pressure assemblage,the pressure dependence of Al in spinel having been

Žwell-documented Osborn and Arculus, 1975; Bartels.et al., 1991; Thy, 1991; Johnston and Draper, 1992 .

Orthopyroxene reverse zoning, as occurs in somepost-caldera lavas, may imply mixing with a more

Ž .mafic magma Nixon, 1988 , but it may also be dueto decompression during the ascent or to an increase

Žin f Kontak et al., 1984; Luhr and Carmichael,O 2

.1980 . Considering its association with the reverselyzoned plagioclase, we favour magma mixing. Almostconstant quantities of Cr, Ni, Co and V means thatthis mixing process was limited, and that the maficend-member was already depleted in all compatibleelements.

7. Conclusion

Tholeiitic and calc-alkaline rocks in the Chichon-tepec volcanic complex are temporally and spatiallyconnected. Pre-caldera tholeiitic to transitional lavaswere separated from post-caldera calc-alkaline rocksby a caldera-forming plinian eruption.

Mineralogical, textural and petrochemical datasupport the possibility that the low-MgO high-alumina basalts of the pre-caldera series were gener-ated at medium pressure from a basaltic liquid,which had already experienced the removal of olivineplus pyroxene. An Al-rich, Cr-poor spinel consti-tuted one liquidus phase of this liquid, that wasAl O enriched owing to suppressed plagioclase nu-2 3

Žcleation. Subsequently, and at lower pressures 5 to 6.kbar , anorthite began to crystallize, causing deple-

tion of alumina in the liquid. The significant Sr andEu positive anomalies of high-alumina basalts cou-

pled with their highly porphyritic nature indicate thatthey suffered some plagioclase accumulation.

Pre-eruptive temperature estimates range from1025–10758C in high-alumina basalts, 920–9808C inandesites and 780–8008C in rhyolites.

Some features of post-caldera lavas can now beŽ .outlined: 1 they have a clearly calc-alkaline charac-

ter, compared to pre-caldera rocks that are mildlyŽ .tholeiitic; 2 they contain textural evidence for open

system crystallization: frequent reverse zoning inpyroxene and plagioclase, dusty or glass-sieved pla-

Ž .gioclase and augite moats around orthopyroxene; 3they lack any evidence of polybaric evolution. Fur-thermore, the overall homogeneities, the high vol-ume of emitted lavas, and the frequently mingled

Žmafic microgranular enclaves or cumulates floor.cumulates? , all suggest the presence of a shallow,

high volume—and vigorously convecting—magmachamber.

Petrochemical and Sr–Nd isotopic data suggest acommon mantle source for both series and the smallvariation in isotopic composition, in light of theobserved negative Ce anomalies, indicate the influ-ence of a ‘sea-water’ component from the down-going slab.

A simple crystal fractionation model could ex-plain magma evolution in the Chichontepec volcano,whereby pre- and post-caldera magmas were gener-ated through differentiation from a high-aluminatholeiitic basalt. To produce the calc-alkaline mag-mas in the post-caldera stage from a tholeiitic pre-cursor requires an increase in pyroxene, at the ex-pense of plagioclase, in the fractionation assemblagethat prevents iron-enrichment in derivative liquids.

Acknowledgements

We would sincerely like to thank the SalvadorianŽ .State Agency for Electric Power CEL for their

valuable cooperation and logistic support given to usduring the field work. S.G.R. is indebted to F.Barberi for advice and support throughout this re-search. We also wish to acknowledge the contribu-tion of two anonymous reviewers and T.H. Druitt, allof whom have greatly improved the manuscript. Wewould also like to thank M. Coltorti and M. Gaetafor their helpful comments on an early version of


this paper and A. Aiuppa, M. Barbieri, F. Parello,and M. Valenza for their help, which was given inmany different ways. Thanks are due to C. Gioia forher drawings and C. Garbarino, F. Olmi and G.Vaggelli who gave generous assistance during themicroprobe work.

Appendix A. Analytical methods

Mineral analyses were carried out using an ARLSEMQ electron microprobe at the University ofCagliari, and a JEOL JXA-8600 Superprobe at theUniversity of Florence. Operating conditions com-prised an accelerating voltage of 15 kV and a samplecurrent of 20 nA on brass. The beam diameter was30–40 mm for groundmass analyses, 10 mm forplagioclase and 2–3 mm for all other minerals. ZAFcorrection was made using the MAGIC IV programŽ .Colby, 1971 .

Whole-rock samples were analyzed for major andtrace elements by XRF at Pisa University, following

Ž .the analytical procedures of Franzini et al. 1975 .LOI was determined after the samples had been keptat 10508C for 2 h; resulting losses were corrected forFeO oxidation. Na O and MgO were determined by2

AES and AAS, respectively, and FeO by titrationwith KMnO after acid digestion under inert atmo-4

sphere. REE were determined by ICP-MS.Sr–Nd isotopic compositions were measured by

VG-54E ISOMASS single collector and FinniganMAT 262RPQ multicollector mass spectrometers. Srand Nd were analyzed after separation by matrixfollowing standard procedures. Repeated analyses of

Ž .standards gave averages and errors 2s as follows:NBS 987, 87Srr86Srs0.71024"1, 86Srr88Sr nor-malized to 0.1194; La Jolla, 143Ndr144 Nds0.51185"1, 146 Ndr144 Nd normalized to 0.7219.

References

Aiuppa, A., Carapezza, M.L., Parello, F., 1997. Fluid geochem-Ž .istry of the San Vicente geothermal field El Salvador .

Geothermics 26, 83–97.Anders, E., Ebihara, M., 1982. Solar system abundance of ele-

ments. Geochim. Cosmochim. Acta 46, 2363–2380.Aramaki, S., Katsura, T., 1973. Petrology and liquidus tempera-

ture of the magma of the 1970 eruption of Akita-KomagatakeVolcano, northeastern Japan. J. Assoc. Min. Petrol. Econ.Geol. 68, 101–124.

Bacon, C.R., Hirschmann, M.M., 1988. MgrMn partitioning as atest for equilibrium between coexisting Fe–Ti oxides. Am.Mineral. 73, 57–71.

Baker, D.R., Eggler, D.H., 1983. Fractionation paths of AtkaŽ .Aleutians high-alumina basalts: constraints from phase rela-tions. J. Volcanol. Geotherm. Res. 18, 334–387.

Baker, M.B., Grove, T.L., Price, R., 1994. Primitive basalts andandesites from the Mt. Shasta region, N. California: productsof varying melt fraction and water content. Contrib. Mineral.Petrol. 118, 111–129.

Bartels, K.S., Kinzler, R.J., Grove, T., 1991. High pressure phaserelations of primitive high-alumina basalts from MedicineLake volcano, northern California. Contrib. Mineral. Petrol.108, 253–270.

Brophy, J.G., 1986. The Cold Bay Volcanic center, aleutianvolcanic arc: I. Implications for the origin of hi-alumina arcbasalt. Contrib. Mineral. Petrol. 93, 368–380.

Buddington, A.F., Lindsley, D.H., 1964. Iron–titanium mineralsand synthetic equivalents. J. Petrol. 5, 310–357.

Carmichael, I.S.E., Nicholls, J., Spera, F.J., Wood, B.J., Nelson,S.A., 1977. High-temperature properties of silicate liquids:applications to the equilibration and ascent of basic magma.Philos. Trans. R. Soc. London 286, 341–373.

Carr, M.J., 1976. Underthrusting and quaternary faulting in Cen-tral America. Geol. Soc. Am. Bull. 84, 2917–2930.

Carr, M.J., Mayfield, D.G., Walker, J.A., 1981. Relation of lavacompositions to volcano size and structure in El Salvador. J.Volcanol. Geotherm. Res. 10, 35–48.

Carr, M.J., Feigenson, M.D., Bennett, E.A., 1990. Incompatibleelement and isotopic evidence for tectonic control of sourcemixing and melt extraction along the Central American arc.Contrib. Mineral. Petrol. 105, 369–380.Ž .CEL Comision Ejecutiva Hidroelectrica del Rio Lempa , 1992.

Informe de prefactibilidad del campo geotermico de San Vi-cente, San Salvador, pp. 1–67.

Colby, J., W., 1971. MAGIC IV. A computer program for quanti-tative electron microprobe analysis. Bell Telephone Lab., Al-lentown, PA.

Crawford, A.J., Falloon, T.J., Eggins, S., 1987. The origin ofisland arc high-alumina basalts. Contrib. Mineral. Petrol. 97,417–430.

Della Pasqua, F.N., Kamenetsky, V.S., Gasparon, M., Crawford,A.J., Varne, R., 1995. Al-spinels in primitive arc volcanics.Mineral. Petrol. 53, 1–26.

De Pieri, R., Gregnanin, A., Stella, R., Ganzerli-Valentini, M.T.,1984. Coefficienti di distribuzione cristallo–liquido dei miner-ali nelle trachiti e rioliti dei colli Euganei Italia settentrionale.Memorie di Scienze Geologiche dell’ Universita di Padova 36,`461–498.

Eggler, D.H., 1972. Water saturated and undersaturated meltingrelations in a Paricutan andesite and an estimate of watercontent in the natural andesite. Contrib. Mineral. Petrol. 34,261–271.

Eggler, D.H., Burnham, C.W., 1973. Crystallization and fractiona-


tion trends in the system andesite–H O–CO –O at pressures2 2 2

to 10 kbar. Geol. Soc. Am. Bull. 84, 2517–2532.Fairbrothers, G.E., Carr, M.J., Mayfield, D.J., 1978. Temporal

magmatic variation at Boqueron volcano, El Salvador. Con-trib. Mineral. Petrol. 67, 1–9.

Franzini, M., Leoni, L., Saitta, M., 1975. Revisione di unametodologia analitica per fluorescenza X basata sulla cor-rezione completa degli effetti matrice. Rend. Soc. It. Mineral.Petrol. 31, 365–378.

Ghiorso, M., Sack, R.O., 1991. Fe–Ti oxide geothermometry:thermodynamic formulation and the estimation of intensivevariables in silicic magmas. Contrib. Mineral. Petrol. 108,485–510.

Gill, J.B., 1981. Orogenic Andesites and Plate Tectonics. Springer,Berlin.

Grove, T.L., Baker, M.B., 1984. Phase equilibrium control on thetholeiitic versus calc-alkaline differentiation trends. J. Geo-phys. Res. 89, 3253–3274.

Hickey, R.L., Frey, F.A., Gerlach, D.C., Lopez-Escobar, L., 1986.`Multiple sources for basaltic arc rocks from central southChile: trace element and isotopic evidence for contributionsfrom subducted oceanic crust, mantle, and continental crust. J.Geophys. Res. 91, 5963–5983.

Irvine, T.N., 1982. Terminology for layered intrusions. J. Petrol.23, 127–162.

Johnston, A.D., Draper, D.S., 1992. Near-liquidus phase relationsof an anhydrous high-magnesia basalt from the Aletian Is-lands: implications for arc magma genesis and ascent. J.Volcanol. Geotherm. Res. 52, 27–41.

Johnson, J.W., Oelkers, E.H., Helgeson, H.C., 1992. A softwarepackage for calculating the standard molal properties of miner-als, gases, aqueous species, and reaction from 1 to 10008C.Computer Geosci. 18, 899–947.

Kerrick, D.M., Darken, L.S., 1975. Statistical thermodynamicmodels for ideal oxide and silicate solid solutions, with appli-cation to plagioclase. Geochim. Cosmochim. Acta 39, 1431–1442.

Kontak, D.J., Clark, A.H., Pearce, T.H., 1984. Recognition ofsimple and complex zoning in olivine and orthopyroxenephenocrysts using laser interference microscopy. Miner. Mag.48, 547–550.

Kudo, A.M., Weill, D.F., 1970. An igneous plagioclase ther-mometer. Contrib. Mineral. Petrol. 25, 52–65.

Kuno, H., 1959. Origin of Cenozoic petrographic provinces ofJapan and surrounding areas. Bull. Volcanol. 20, 37–76.

Luhr, J.F., Carmichael, I.S.E., 1980. The Colima volcanic com-plex: I. Post-caldera andesites from Volcan Colima. Contrib.`Mineral. Petrol. 71, 343–372.

Luhr, J.F., Carmichael, I.S.E., 1985. Jorullo Volcano, Michoacan,Mexico 1759–1774: the earliest stages of fractionation incalc-alkaline magmas. Contrib. Mineral. Petrol. 90, 142–161.

Miyashiro, A., 1974. Volcanic rock series in island arcs and activecontinental margins. Am. J. Sci. 274, 321–355.

Nixon, G.T., 1988. Petrology of the younger andesites and dacitesat Iztaccihuatl Volcano, Mexico: I. Disequilibrium phenocrystsassemblages as indicators of magma chamber processes. J.Petrol. 29, 213–264.

Osborn, E.F., Arculus, R.J., 1975. Phase relations in the systemMg SiO –Iron oxide–CaAl Si O –SiO at 10 kb and their2 4 2 2 8 2

bearing on the origin of andesite. Carnegie Inst. WashingtonGeophys. Lab. Yearb. 74, 503–507.

Pichler, H., Weyl, R., 1973. Petrochemical aspects of CentralAmerican magmatism. Geol. Rundsch. 62, 357–396.

Rutherford, M.J., Devine, J.D., 1988. The May 18, 1980 eruptionof Mount St. Helens: 3. Stability and chemistry of amphibolein the magma chamber. J. Geophys. Res. 93, 11949–11959.

Sekine, T., Katsura, T., Aramaki, S., 1979. Water saturated phaserelations of some andesites with application to the estimationof the initial temperature and water pressure at the time oferuption. Geochim. Cosmochim. Acta 43, 1367–1376.

Sisson, T.W., Grove, T.L., 1993a. Experimental investigations ofthe role of H O in calc-alkaline differentiation and subduction2

zone magmatism. Contrib. Mineral. Petrol. 113, 143–166.Sisson, T.W., Grove, T.L., 1993b. Temperatures and H O con-2

tents of low-MgO high-alumina basalts. Contrib. Mineral.Petrol. 113, 167–184.

Stern, C.R., Kilian, R., 1996. Role of the subducted slab, mantlewedge and continental crust in the generation of adakites fromthe Andean Austral Volcanic Zone. Contrib. Mineral. Petrol.123, 263–281.

Stormer, J.C., Nichols, D.J., 1978. XL-FRAC: a program forinteractive testing of magmatic differentiation models. Com-puter Geosci. 4, 143–159.

Sun, S.S., Mc Donough, W.F., 1989. Chemical and isotopicsystematics of oceanic basalts: implications for mantle compo-

Ž .sition and processes. In: Saunders, A.D., Norry, M.J. Eds. ,Magmatism in the Ocean Basins. Geol. Soc. London Spec.Publ. 42, 313–346.

Thy, P., 1991. High and low pressure phase equilibria of a mildlyalkalic lava from the 1965 Surtsey eruption: experimentalresults. Lithos 26, 223–243.

Ui, T., 1972. Recent volcanism in the Masaya–Granada area,Nicaragua. Bull. Volcanol. 36, 174–190.

Wallace, P.J., Carmichael, I.S.E., 1994. Petrology of VolcanTequila, Jalisco, Mexico: disequilibrium phenocryst assem-blage and evolution of the subvolcanic magma system. Con-trib. Mineral. Petrol. 117, 345–361.

Wells, P.R.A., 1977. Pyroxene thermometry in simple and com-plex systems. Contrib. Mineral. Petrol. 62, 129–139.

White, W.M., Dupre, B., 1986. Sediment subduction and magma´genesis in the Lesser Antilles: isotopic and trace elmentconstraints. J. Geophys. Res. 91, 5927–5941.

White, W.M., Patchett, J., 1984. Hf–Nd–Sr isotopes and incom-patible element abundances in island arcs: implications formagma origins and crust–mantle evolution. Earth Planet. Sci.Lett. 67, 167–185.

Wiesemann, G., 1975. Remarks on the geologic structure of theRepublic of El Salvador, Central America. Mitt. Geol. Paleon-tol. Inst. Univ. Hamburg 44, 557–574.

Williams, H., Meyer-Abich, H., 1955. Volcanism in the southernpart of El Salvador. Univ. Calif. Publ. Geol. Sci. 32, 1–64,Berkeley and Los Angeles.

Zindler, A., Hart, S.R., 1984. Chemical geodynamics. Annu. Rev.Earth Planet. Sci. Lett. 14, 493–571.

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