Shallow open-system evolution of basaltic magma beneath a subduction zone volcano: The Masaya Caldera Complex, Nicaragua

Journal of Volcanology and Geothermal Research, 56 ( 1993 ) 379-400

379

Elsevier Science Publishers B.V., Amsterdam

Shallow open-system evolution of basaltic magma beneath a subduction zone volcano: the Masaya Caldera Complex, Nicaragua James A. Walker ~, Stanley N. Williams b'*, Ruth I. Kalamarides a and Mark D. Feigenson c aDepartment of Geology, Northern Illinois University, DeKalb, IL 60115, USA bDepartment of Earth Sciences, Dartmouth College, Hanover, NH 03755, USA CDepartmentof Geological Sciences, Rutgers University, New Brunswick, NJ 08903, USA (Received August 7, 1991; revised version accepted March 9, 1993 )

ABSTRACT Lavas and pyroclastic material from the Masaya Caldera Complex have a number of distinctive geochemical features: relative compositional homogeneity, low A1203 and high FeO contents, a tholeitic differentiation trend, and elevated, large-ion-lithophile ( LIL )-element concentrations (e.g., Ba ~ 800 ppm ). On CMAS projections their compositions always fall on or near low-pressure cotectics. In addition, the basalts of Masaya have unusually high 87Sr/86Sr and l°Be. Masaya has exhibited medium-term compositional cycles, best exhibited by the sawtoothed changes in TiO2 and FeO*/MgO. There are also a number of longer-term compositional changes which are abrupt and generally coincide with caldera formation. Many of the geochemical characteristics of Masaya, coupled with a number of volcanological observations, indicate Masaya is underlain by a large, shallow, open-system magma chamber, perhaps on the order of 10 km 3 in size. Although fractional crystallization is a significant magmatic process in Masaya's open-system chamber, magma mixing/contamination is equally important. Magma mixing is necessary to explain the discontinuous stratigraphic changes in magma composition observed at Masaya, and crustal contamination is necessary to explain their generally elevated STSr/86Srand LIL-element concentrations. Two components, therefore, have been admixed into the magma chamber of Masaya: a LILpoor basaltic component such has been erupted from the nearby Nejapa and Granada cinder cones; and a LIL-rich acidic component such has been erupted from the nearby calderas of Apoyo and Apoyeque. Admixtures of the former have dominated. Admixtures of the latter ended with caldera formation. Ironically, open-system behavior has exerted fundamental control on the maintenance of relative compositional homogeneity.

Introduction It is becoming increasingly apparent that multiple open-system magmatic processes operate beneath many large subduction zone volcanoes (Grove et al., 1982, 1988; Reagan et al., 1987; Davidson et al., 1988; Nixon, 1988; WGrner et al., 1988; Huijsmans and Barton, 1989; Ferguson et al., 1992). Open-system magmatic processes operating at intra-crustal *Present address: Department of Geology, Arizona State University, Tempe, AZ 85287, USA.

or Moho levels may include: mixing of magmas with either disparate or similar physicochemical characteristics; fractional crystallization; assimilation of country rock; and partial magmatic withdrawal (e.g., O'Hara, 1977). The importance of fractional crystallization in magma genesis in subduction zones has been clearly established by Gill (1981). Although relegated to secondary petrogenetic roles by Gill ( 1981 ), magma mixing and crustal contamination can be of equal importance to compositional variation, or lack thereof, at subduction zone volcanoes (Barton and

0377-0273/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.

380

Huijsmans, 1986; Nixon, 1988; Huijsmans and Barton, 1989; Norman and Leeman, 1990; Brophy, 1990; Ferguson et al., 1992). At the same time, the dynamics of magma chambers, in which mixing of magmas and crustal contamination are integral processes, are becoming better constrained (e.g., Huppert and Sparks, 1985; Huppert et al., 1986; Campbell and Turner, 1987; Rosing et al., 1989; Lesher, 1990). Development of such constraints are facilitated by comprehensive studies of compositional variation at individual volcanoes, particularly where some temporal control is available (e.g., Barton and Huijsmans, 1986; Ferguson et al., 1992). This paper uses major- and trace-element and isotopic data with temporal control to demonstrate the operation and significance of opensystem magmatic differentiation, particularly magma mixing and crustal contamination, in a relatively large, shallow chamber beneath a single subduction zone volcano, the Masaya Caldera Complex. Walker (1989) showed previously that Masaya is one of the few volcanoes in Central America that exhibits strong iron enrichment. Beneath subduction zone volcanoes, open-system processes often combine to accentuate the suppression of iron enrichment and the development of a calc-alkaline differentiation trend (Grove et al., 1982; Grove and Kinzler, 1986; Defant and Nielson, 1990). Nevertheless, this paper will also show that iron enrichment and open-system behavior are not necessarily mutually exclusive in a subduction zone setting (see also Grove et al., 1988).

Volcanological background The Masaya Caldera Complex lies on the Quaternary volcanic front of Central America about 20-25 km southeast of Managua, the capital of Nicaragua (Fig. 1 ). The volcanic front is the major volcanic manifestation of plate convergence between the Cocos and Caribbean plates (Carr et al., 1982). Masaya lies

J.A. WALKER ET AL.

on or near the northwest termination of the eastern Nicaragua segment of the volcanic front (Carr, 1984 ) and is between the Nejapa cinder cones and Apoyeque caldera to the northwest (Walker, 1984; Bice, 1985) and Apoyo caldera to the southeast (Fig. 1; Sussman, 1985). Masaya has had frequent historic activity with twelve eruptions since 1940 (Simkin et al., 1981). The caldera is large, shallow and elongate (6 km X 11.5 km ) with scalloped margins created by interactions between regionally-defined joint systems (Williams, 1983b ). Its long axis is oriented northwest, parallel to the volcanic front (Figs. 1 and 2). It had been regarded as the type example of a caldera produced by repeated passive collapse due to underground magma withdrawal (Williams and McBirney, 1979 ). Detailed field study has revealed, however, that the caldera is actually the product of a series of explosive eruptions, all of basaltic composition, including plinian and ignimbrite eruptions (Williams, 1983a,b; Bice, 1985) and has been redefined as the mafic analogue of a Krakatau-type caldera (Williams and Stoiber, 1983). Pyroclastic rocks erupted from Masaya are briefly described below (and in more detail by Williams 1983a,b, and Bice, 1985). The nearby calderas, Apoyo and Apoyeque, in contrast, are more typical and were produced by explosive eruption of dacitic magma (Sussman, 1985; Bice, 1985 ). Masaya is distinctive not only for its explosive basaltic activity, but also for a number of the other reasons. For one, pre-caldera activity resulted in the construction of a low, shield volcano with a form quite unlike the composite cones which dominate the Nicaraguan volcanic front. Second, the post-caldera presently active crater, Santiago, is floored by a lava lake, encrusted with basalt. Last, since its inception in 1850-1853, Santiago has experienced regular and repeated periods of strong gas emission not associated with major eruptions. These passive gas emissions last anywhere for 4-12

BASALTIC MAGMA EVOLUTION BENEATH A SUBDUCTION ZONE VOLCANO: MASAYA CALDERA, NICARAGUA I

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years and have a recurrence interval of ~ 25 years (Stoiber et al., 1986 ). The last degassing episode began in September 1979 and ended in 1988 (Van Wyk de Vries et al., 1988). Volcanic stratigraphy The following summarizes the relevant map unit descriptions of Williams (1983b). The oldest lavas and pyroclastic rocks that crop out around Masaya are part of the Plio-Pleistocene

Las Sierras formation. This unit is regional and very complex in lithology. Within the caldera, the Las Sierras formation occurs as a massive lithic-rich lahar or pyroclastic flow deposit. It is found at the base of the exposed section in the eastern end of the caldera and is unconformably overlain by the pre-caldera lavas of Masaya. The thickness and distribution of the Las Sierras suggest that the deposits may have been derived from a pre-caldera, proto-Masaya, although at present this is unclear. Re-

382

J.A. WALKER ET AL

gardless, some of the "proto-Masaya" eruptions were quite explosive as the J1 marine tephralayer from Pacific Ocean cores adjacent to Central America (Ledbetter, 1985) can be geochemically correlated with the Las Sierras (Table 1 ). The J 1 tephra has an age of 135,000 years (Ledbetter, 1985) consistent with a "proto-Masaya" origin. The caldera wall lavas of Masaya are thin ( 1-5 m) pahoehoe flows, often with no flow rubble. Pre-caldera activity also included violent explosive events as suggested by multiple, bedded airfall deposits best exposed in the western part of the caldera and

in outcrops in and around Managua (Williams, 1983a; Bice, 1985). The oldest of these pyroclastic deposits is locally known as the Fontana Lapilli [Masaya Lapilli Bed of Bice (1985)] with an estimated age of 25,00035,000 y.B.P. (Bice, 1985). The youngest major pre-caldera pyroclastic deposits are locally known as the San Judas Formation [Masaya Triple Layer of Bice (1985) ] and probably erupted between 7,000 and 9,000 years ago (Bice, 1985 ). Both the Masaya Lapilli Bed and Masaya Triple Layer are scoria-fall deposits laid down during plinian eruptions (Williams,

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Masaya Caldera Complex, Nicaragua age

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lava lake of 1965

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aa lava flow and associated fissure-erupted ash and scoria of 1772

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aa lava flow and lava lake of 1670

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pahoehoe lava flow of Oviedo

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pahoehoe lava flow with abundant lava tubes

Qa 24

acoria and ash (probably from site of Nindiri crater)

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blocky lava and spatter erupted from fissure on NW flank of San Pedro crater

QI 22

lava probably erupted from San Pedro crater

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lava flow exposed below Vantarron

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Nindiricomplex-cinder cones, lava flows and lava lakes

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aa lava flow with possible compound nature

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aa lava flow probably from fissure N of Montoso

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blocky lava flow and lava lake

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Santiago Crater collapse 1953

Nindid Crater Collapse pre-1524

Maseya Crater collapse

scoria and ash, locally agglutinated

Q114

aa lava flow with abundant vesicles, ovedain at head by scoria and ash, locally agglutinated

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pahoehoe lava flow

QI 12

Case Vieja lava flow

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Maseya Crater Complex-cinder cones, lava flows, and lava lakes; Santiago Crater Complex-cinder cones and lava flows

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Lava and scoria and ash deposits (oldest of caldem flow, correlation and exact stmtigraphic position uncertain)

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Railroad flow-massive lava erupted from N caldara margin

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lavas and scoria and ash deposits erupted after caldem collapse from caldera-bounding faults or on flanks

San Femando Crater collapse

Caldem coflapse 2,250-6,500 y.b.p. Qs 4

pyroclastic surge deposits \

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pyroclastic flow deposits J

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lavas of caldera wall (includes minor ashfall and pyroslastic flow deposits)

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scoria and ashfall deposits of caldem wall (includes Maseya Triple Layer and Maseya Lapilli plinian deposits (Williams, 1983 and Bice,1985)

PLIOPLEISTOCENE

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Las Sierras Formation-pyroclastic flow or mudflow deposits exposed in area of Lake Maseya historically active volcanic vent vent

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fault or eruptive fissure; concealed contact; location approximate

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35 991 434 330

53.70 0.97 15.60 10.71 0.21 3.67 8.70 3.06 1.44 0.27

6.3 0.704186 0.513083

3.48 7.07 4.37 1.47

54.82 0.92 15.67 12.19

6.8 0.704157

23 820 443 354 43 34

50.60 1.24 16.00 11.50 0.21 4.38 10.40 2.86 1.14 0.27

18 775 442 378 44 30 133 45

49.70 t.26 16.40 11.70 0.21 4.81 10.60 2.72 1.01 0.25

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FeO* - total Fe as FeO. - not determined. J 1 from Ledbetter ( 1985 ). Complete analyses are available from the first author on request.

6180 87Sr/86Sr 143Nd/t44Nd

ppm Rb Ba Sr V Cr Ni Zr Sc

wt.% SiO2 TiO2 A1203 FeO* MnO MgO CaO Na20 K20 P2Os

Qlw2 MS-8

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Pre-caldera lavas

Las Sierras Marine Formation tephra

Representative analyses of volcanic rocks of Masaya

TABLE I

6.5 0.704216

117 42

22 882 494 344

50.50 1.05 16.30 11.40 0.22 3.75 10.00 2.83 1.14 0.25

6.3 0.704121 0.513024

24 865 483 398

49.60 1.08 15.60 11.60 0.22 5.54 10.20 2.69 1.10 0.26

5.9 0.704207 0.513067

12 780 497 341 45 27

50.00 0.99 16.10 11.10 0.22 5.35 10.30 2.89 1.04 0.16

CalderaPost-caldera lavas forming pyroclastics Qs 4 QI6 QI23 MA-70 LM-IO MS-14

6.4 0.704186

24 818 478 346 45 30

50.70 1.02 16.40 11.00 0.21 4.72 10.50 2.86 1.07 0.22

Q127 MS-I 1

24 870 454 372 39 26

50.90 1.14 14.80 11.90 0.23 5.26 9.90 2.82 1.16 0.26

Q128 MS-4

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BASALTIC MAGMA EVOLUTION BENEATH A SUBDUCT1ON ZONE VOLCANO: MASAYA CALDERA, NICARAGUA

1983a,b). Available K-Ar ages, stratigraphic relationships with pre-caldera ashes, and chemical evidence presented later indicate that the pre-caldera lavas outcropping in the eastern end of the caldera (near the towns of Nindiri and Venecia) are older than those exposed in the western walls of the caldera. Williams (1983b) recognized pyroclastic fall, flow and surge deposits associated with the eruptions which caused caldera collapse (see also Krusi and Schultz, 1979). Bice (1985) referred to the fall deposits as the Masaya Tuff. Collapse is thought to have occurred between 2,250 and 6,500 years ago (Williams, 1983b; Bice, 1985). Some of the oldest post-caldera lavas emanated from a series of vents on or near the caldera-bounding faults, particularly on the northern and southern rims of the caldera (Fig. 2 ). The youngest of these "moat" lavas, the so-called Railroad flow, erupted from a vent on the northern caldera-bounding fault and flowed more than 8 km to the north. The vent for the Railroad flow lies along a well-defined NNW-trending fissure or series of vents which includes San Fernando, Masaya and San Juan craters and which may be the eastern edge of the transverse break in the volcanic front. All remaining post-caldera lavas were erupted from vents within the caldera. Post-caldera activity has been overwhelmingly effusive with only small-volume scoria and ash deposits occurring interlayered with post-caldera lavas (Williams, 1983b). The stratigraphy of the post-caldera lavas has been established by Williams (1983b). The most recent eruption of a significant volume of lava occurred in 1772 from a fissure vent or vents colinear with the vent of the Railroad flow and the aforementioned craters (Fig. 2 ).

Sampling, petrography and analytical methods Sixteen pre-caldera and thirty-eight postcaldera lavas from the Masaya Caldera Complex have been collected and subsequently analyzed (representative analyses are given in

38 5

Table 1 ). The caldera-forming magma is represented by five analyses of juvenile scoria in pyroclastic flow and surge deposits (Table 1 ). In addition, five samples of fresh scoria from the Masaya pyroclastic units, and seven samples of lava and scoria from the regional Las Sierras formation have been analyzed to completely document the chemical evolution of Masaya (Table 1 ). Detailed sample locations are available on request. Lavas range texturally from porphyritic to aphyric. Plagioclase is the most abundant phenocryst mineral followed by olivine and clinopyroxene (augite). Magnetite occurs as a microphenocryst in about one-third of the lavas and is ubiquitous as a groundmass constituent. The g~oundmass is generally intergranular containing plagioclase, pyroxene, and olivine, in addition to magnetite. Olivine, however, is only present in the groundmass of pre-caldera lavas. In addition, all of the precaldera lavas have olivine phenocrysts or microphenocrysts, whereas four post-caldera lavas appear to lack olivine altogether, although this may reflect phenocryst sorting. The textures suggest the following order of crystallization: (1) olivine; (2) plagioclase; (3) clinopyroxene; and finally, (4) magnetite. Major and trace elements were mainly analyzed by direct-current plasma multi-element atomic emission spectrometry (DCP-MAES) at Rutgers University (Feigenson and Carr, 1985 ). Some major elements were determined by X-ray fluorescence (XRF) at X-ray Assay Laboratories of Toronto, Canada. Agreement between results from DCP-MAC and XRF is within 2% except for Na20 which is within 4%. Strontium and neodymium isotopes were measured on a VG sector thermal ionization mass spectrometer at Rutgers University, using standard separation techniques (Feigenson and Carr, 1986). Oxygen isotope ratios were measured on CO2 extracted from whole rock samples using the BrF2 method (Clayton and Mayeda, 1963) by a Varian-MAT 250 at

386

J.A. W A L K E R E T A L .

Northern Illinois University (Varekamp and Kalamarides, 1989 ). Results

General compositional character The volcanic rocks from Masaya have some striking geochemical features, particularly in light of the volcanological distinctions outlined above. First of all, volcanic rocks from Masaya, except for the Las Sierras samples, are all basalts or basaltic andesites showing surprisingly little overall compositional variation as compared to other Central American vol-

canoes (Fig. 3; see figs. 5 and 10 of Walker, 1989). Even explosively fragmented magmas were relatively basic, although slightly more differentiated (Table 1). Moreover, relative compositional homogeneity has lasted at least 30,000 years, unusual for subduction zone stratovolcanoes (e.g., Gill, 1981). As an example, Masaya has an age (>t 30,000 years) and a volume ( ~ 60 km 3) similar to Asama Volcano in Japan (Wadge, 1982). In contrast to Masaya's compositional homogeneity, Asama has erupted basic andesites, acid andesites and more SiO2-rich lavas (Appendix of Gill, 1981 ). Another distinguishing feature of Masaya's

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387

BASALTIC M A G M A E V O L U T I O N B E N E A T H A S U B D U C T I O N Z O N E V O L C A N O : MASAYA CALDERA, N I C A R A G U A

basic rocks are their relatively low A1203 and high FeO* concentrations in comparison to other Nicaraguan basalts (Fig. 3 ). In addition, the lavas of Masaya are one of the few Central America volcanoes to exhibit a tholeiitic differentiation trend (Walker, 1989). Basalts from Masaya also have very high contents of large-ion lithophile (LIL)-elements, again in contrast to other Nicaraguan basalts (Fig. 3 ). These large contents of LIL elements are particularly noteworthy in central Nicaragua as mafic basalts with unusually low concentrations of LIL elements have been erupted from the flanking Nejapa and Granada cinder cones (Walker, 1984; Walker et al., 1990). Also, no matter the projection or projection method, the basalts of Masaya fall quite near low-pressure ( 1 atmosphere ) cotectics (Fig. 4). In Figure 4, most other Central American basic suites define "trends" parallel to Masaya's, but displaced toward the olivine apex (Carr, 1984), and hence, approximate higher pressure cotectics (e.g., Baker and Eggler, 1987 ). Lastly, the basalts of Masaya have the highest 87Sr/a6Sr

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388

J.A. WALKER

(Fig. 5 ) and ~°Be values of any Central American basalts (Morris and others, 1990). The latter distinction, however, is based on only one analysis.

Stratigraphic compositionalchanges Although there is little overall compositional variation amongst the post-Las Sierras volcanic rocks of Masaya, some medium- and long-term stratigraphic changes in magma chemistry are evident and, it turns out, petrogenetically important. Older lavas have generally higher TiO2 and FeO*/MgO and generally lower MgO, Sr and K20/TiO2 than younger la-

vas (Figs. 6 and 7 ). The changes in MgO, TiO2 and FeO*/MgO appear to correlate with caldera formation, while the changes in Sr and K20/TiOe may have occurred somewhat earlier (Figs. 6 and 7 ). On the shorter term, close inspection of Figure 6 shows that variations in TiO2 and FeO*/MgO are sawtoothed and crudely cyclic. Periodically, the TiO2, and to a lesser extent FeO*, contents of lavas increase with time, hence high TiO: generally correlates with high FeO*. These periodic increases are followed by drops in TiO2 and FeO*/Mg0, which have been used to tentatively break the stratigraphy into five cycles (Fig. 6). Since these cycles last on the order of years to thou-

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389

BASALTIC MAGMA EVOLUTION BENEATH A SUBDUCTION ZONE VOLCANO: MASAYACALDERA, NICARAGUA

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.

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.

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.

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.

.

.

.

.

o °

I~

.

.

.

.

.

.

.

.

.

.

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[3 [] ~ [] Caldera x- . . . . . ~ . . . . . . . . . . . . . . . . . . . . . . . . . . -(~)-

..................................

_~ n,-

.

®

•

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.

++ ...............................................

X_~

× X

X

............................

x

.......................

®

× X )~ ............................................

X

X

X

(~ I

0.5

1.0

1.5

K20/"l'iO 2

Fig. 7. R e l a t i v e p o s i t i o n in M a s a y a s t r a t i g r a p h y v e r s u s K 2 0 / T i O 2 for M a s a y a s a m p l e s as in Fig. 6. Symbols: • = p o s t c a l d e r a lavas; X = p r e - c a i d e r a lavas; [] = c a l d e r a - f o r m i n g scoria; + = p r e - c a l d e r a scoria.

sands of years, they represent medium-term compositional variability which is usually a reflection of post-melting differentiation processes (e.g., Newhall, 1979). The Ba and K20 contents of erupted lavas also change sympathetically with TiO2, however, earliest precaldera lavas, those exposed in the eastern walls of the caldera, are dearly distinct in K20/TiO2, as is the Fontana Lapilli or Masaya LapiUi Bed (Fig. 7). This lack of geochemical coherence between TiO2 and LIL elements is reminiscent of that shown for the volcanic rocks of the Nejapa and Granada cinder cones although on a much reduced scale (Walker, 1984; Walker et al., 1990). Discussion

Origin of general compositional character Appropriate Pearce-element diagrams (Russell et al., 1990 ) indicate that much of the compositional variability and distinctiveness of Masaya can be attributed to fractional crys-

tallization of olivine, plagioclase and clinopyroxene (Fig. 8 ). Mass balance calculations indicate that the maximum amount of fractional crystallization is less than 20% (Table 2 ) and that occasionally compositions can be related by fractionation of olivine and plagioclase alone (Table 3) or olivine and plagioclase (Table 4). Carr (1984) concludes that much of the fractional crystallization recorded by Central American basalts is the result of magmatic differentiation at or near the base of the crust. The inference is, therefore, that subsequent upper crustal storage and differentiation is not overly significant beneath the basic-intermediate volcanoes of the Central American volcanic front. Masaya, however, is a notable exception. As discussed by Walker (1989), the iron enrichment and elevated CaO/AI:O3 of the lavas of Masaya indicate fractional crystallization at low pressure and/or low magmatic water contents. "Shallow" differentiation pressures are supported by the consistent positioning of Masaya compositions near low pressure cotectics

390

J.A. WALKER ET AL. 80

12. -IX Q..

(D

60-

+

§ O~uD

× j,/J 40

I 40

60

80

$i/Ti

Fig. 8. Pearce element diagram for all Masaya samples except Las Sierras. Symbolsas in Fig. 4. Compositionshave been recast in cation proportions. (OI+Cpx+PI) in the abscissa= 1.5 Ca+2.75 Na +0.5 [Fe+Mg] +0.25 A1. Magma compositions reflectingolivine and plagioclaseand clinopyroxenecontrol should fall along a straight line with a slope of 1.0 (Russell et al., 1990). Referenceline shown has a slope of 1.0.

on CMAS projections (Fig. 4). Finally, the regular episodes of passive degassing at Masaya indicate a large, near-surface source of magmatic volatiles (Stoiber et al., 1986). Moreover, the huge amount of sulfur non-explosively released from Masaya also indicates a shallow system as sulfur exsolution is insignificant below 5 0 0 m (Gerlach, 1986). The magma chamber, therefore, resides in the uppermost crust. However, unless the magmatic system underneath Masaya is unusually volatile-rich to begin with, periodic degassing also bespeaks periodic fractional crystallization at relatively low volatile contents. Hence, at Masaya fractional crystallization at relatively low pressure is probably coupled with evolution at relatively low volatile (water) contents. Strong iron-enrichment, among other geochemical characteristics, results (Eggler, 1972; Grove and Baker, 1984; Walker, 1989). The shallow, relatively dry magma system or

chamber beneath Masaya is likely of considerable size. On the basis of the gas flux during the most recent degassing period and estimates of magmatic SO2 and HC1, Stoiber et al. ( 1986 ) calculated that approximately 1.2 km 3 of magma must have been completely degassed to provide the observed SO2 flux at Masaya between 1979 and 1985 and perhaps 10 km 3 has been degassed in the past 100 years. These figures likely represent m i n i m u m estimates of the magma chamber volume. The total volume of pyroclastic material erupted from Masaya during caldera formation is about 8-9 km 3 (Williams, 1983b), again likely an underestimate of the magma chamber volume, particularly if draw-up heights for basaltic systems are less than those for rhyolitic systems (Blake and Ivey, 1986). A total magma chamber volume of perhaps 10 km 3 places Masaya on a par with larger composite cones such as Krakatoa and Mr. Mazama, which, like Masaya, experienced

391

BASALTICMAGMA EVOLUTION BENEATH A SUBDUCTION ZONE VOLCANO: MASAYACALDERA, NICARAGUA

TABLE2

TABLE4

Results of least-squares modelling for Masaya pre-caldera lavas (QIw 2), MA-59 to MA-60

Results of least-squares modelling for Masaya post-caldera lavas, MA-34B (Q125) to MA-16 (Q127)

Fractions

Variables

Fractions

Variables

0.010 0.147 0.022 0.819

Olivine Plagioclase Clinopyroxene MA-60

0.025 0.047 0.926

Olivine Plagioclase MA- 16

Calculated parent MA-34

Observed parent MA-34A

50.81 0.96 16.36 11.58 0.20 5.31 10.73 2.52 1.12 0.23

50.91 1.02 16.32 11.46 0.22 5.37 10.74 2.70 1.01 0.26

Results (wt.%)

Results (wt.%)

SiO2 TiO2 A1203 FeO MnO MgO CaO Na20 K20 P205

Calculated parent MA-59

Observed parent MA-59

50.35 1.13 17.02 11.45 0.21 4.62 10.81 2.94 1.02 0.27

50.27 1.19 17.03 11.46 0.21 4.62 10.84 2.94 1.15 0.29

Sum of squares of weighted residuals = 0.02.

TABLE3

Results of least-squares modelling for Masaya post-caldera lavas, MA-34A (Q127) to MS4 (Q128) Fractions

Variables

0.085 0.914

Plagioclase MS4

Calculated parent MA-34A

Observed parent MA-34A

51.22 1.06 16.56 11.13 0.21 4.89 10.76 2.73 1.08 0.24

51.02 1.07 16.59 11.23 0.22 4.90 10.82 2.80 1.09 0.26

Results (wt.%)

SiO2 TiO2 A1203 FeO MnO MgO CaO Na20 K20 P205

Sum of squares of weighted residuals = 0.03.

SiO2 TiO2 A1203 FeO MnO MgO CaO Na20 K20 P205

Sum of squares of weighted residuals = 0.07.

large explosive eruptions and caldera formation (Smith, 1979). According to the eruptive model of Carey and Sigurdsson (1989), the sheer existence of a large caldera produced during high-intensity eruptions requires large ( >/l 0 km3?) magma chamber volumes. Again, all evidence suggests that this considerable magma chamber resides at relatively shallow, upper-crustal depths and contains magma that is relatively volatile poor. Although fractional crystallization is of consequence beneath Masaya, it has not resulted in major amounts of differentiation given the relative compositional homogeneity of erupted magmas through time (Fig. 3 ). Compositional (and, by inference, thermal) homogeneity would be facilitated by the following: ( 1 ) vigorous convection; and (2) periodic influx of hot, basic magma. According to Carrigan (1987), Martin et al. (1987), and Martin and Nokes ( 1989 ), a basaltic magma body of moderate to large size, such as is presumed beneath Masaya, should undergo time-dependent (i.e.,

392

turbulent) convection. In addition, there is no evidence for the development of a low-density, silica-rich cap on Masaya's chamber, which, if present, would drive double-diffusive, as opposed to chamber-wide, convection (e.g., Huppert and Sparks, 1984). Periodic influx of less-differentiated magma is strongly indicated by the stratigraphic compositional changes at Masaya (Fig. 6; see discussion below). Finally, Stoiber and others (1986) indicate huge amounts of sulfur are released from Masaya during periods with few surface eruptions. This observation necessitates convection or repeated intrusion of undegassed magma (e.g., Andres and others, 1989). Therefore, both convection and replenishment facilitate compositional homogeneity in the magma chamber beneath Masaya. The final geochemical distinctions of the basalts of Masaya are their elevated LIL-element concentrations and their elevated 87Sr/86Sr and l°Be. Such distinctions in subduction zone basalts are generally linked to processes in the magmatic source region (Ellam and Hawkesworth, 1988; Walker, 1989; Carr et al., 1990; Morris et al., 1990). Specifically, such geochemical distinctions would necessitate robust source contributions from the subducting plate, including subducting sediments (Pearce, 1983, 1983; Ellam and Hawkesworth, 1988; Walker et al., 1990; Morris et al., 1990). This would have to be true in a local as well as a regional sense, in that Masaya's basalts are locally anomalous (Figs. 3 and 5 ). One likely consequence of strong-source contributions from the subducting plate, likely via hydrous fluids (e.g., Tatsumi et al., 1986), are relatively high degrees of source melting (Walker, 1989; Cart et al., 1990; Morris et al., 1990). Resulting magmas should have relatively low absolute concentrations of incompatible elements, such as the LIL elements (e.g., Walker, 1989). However, Masaya's basalts are locally high, not low, in LIL-element contents (Fig. 3 ). Therefore, the only way increased contributions from the subducting Co-

J.A. WALKERET AL.

cos plate can explain all of the remaining geochemical distinctions of Masaya's basalts is if LIL-element enrichment via slab fluids is faster than LIL-element depletion via increased source melting (Stolper and Newman, 1991 ). So with that important caveat, some of Masaya's geochemical distinction could be inherited from the magmatic source. If so, significant local variability in fluid flux from the subduction Cocos plate, and hence in the subduction zone source component, occurs throughout central Nicaragua (Walker et al., 1990 ). Alternatively, the final geochemical distinctions of Masaya's basalts could have developed through combined assimilation and fractional crystallization (or AFC) from more mafic parental basalts. The likeliest parental magmas to Masaya are the so-called NejapaGranada (or NG) basalts (Table 5 ) given their close and encircling proximity, identical age (Quaternary) and generally more mafic comTABLE 5 Comparison of average compositions of Masaya and NejapaGranada basalts Masaya

wt.% MgO

4.68

ppm Rb Ba K La Ce Sr P Zr Ti Y V

24 852 9379 11.5" 26.8* 477 1091 107 6535 28 360

Ba/Zr La/Y STSr/S6Sr ~43Nd/144Nd dlsO

8 0.4 0.70417 0.513068 6.4

*preliminary data on four samples.

NejapaGranada High-Ti

8.36

3.5 111 1245 3.4 10.0 289 480 52 7074 21 268 2 0.2 0.70378 0.513024 6.1

NejapaGranada Low-Ti

7.02

9.3 373 3818 5.8 13.4 408 655 46 4436 17 286 8 0.3 0.70402 0.513064 6.1

393

BASALTIC MAGMA EVOLUTION BENEATH A SUBDUCTION ZONE VOLCANO: MASAYACALDERA, NICARAGUA

position (Walker, 1984; Sussman, 1985; Walker et al., 1990). In terms of incompatible-element and isotopic ratios, the basalts of Masaya are most similar to NG basalts with relatively low TiO2 and high LIL-element concentrations (Table 5). Based on a plot of 87Sr/86Sr versus 1/Sr, about 15-30% AFC can produce Masaya-like characteristics starting from a NG low-TiO2 basalt and assuming a contaminant represented by a dacite from nearby Apoyo caldera. Acidic compositions erupted from Apoyo and Apoyeque (see Fig. 1 ) have very high LIL-element concentrations (Ba contents from 1,000 to 1,500 ppm, for example; Sussman, 1985; Bice, 1985) and relatively high 875r/86Sr

(0.7043-0.7053; Pushkar et al., 1972), making them an ideal source for the final geochemical distinctions of Masaya, except perhaps lOBe. Of the two alternatives, source versus (crustal) wallrock enrichment, the latter is favored since the basalts of Masaya are nowhere near primary compositions (Mg #'s 0.39-0.51 ) and crustal contamination can also explain the stratigraphic variability in K20/TiO2 (see below). However, if Masaya truly represents a local l°Be anomaly, crustal contamination would have to include assimilation of nearsurface sediment (e.g., Morris and Tera, 1989 ). In summary, the peculiar compositional character of the basalts of Masaya relative to

TABLE 6

Open-system modelling results Parental compositions

Calculated compostions

Observed compositions

TiO2

TiO2

K20

TiO2

K20

1.36 1.19

1.35 1.29

1.3-1.4 1.151.25

1.2-1.5 1.2-1.4

1.17 1.03

1.08

~1.2

0.97

1.0-1.1 0.95-1.05

K20

A. Intervals dominated by fractional crystallization 1 ) Cycles 1 a n d 2 2) Cycles 4 a n d 5

1.18 0.98

1.16 1.05

B. Intervals dominated by mixing with low-Ti basalt 3) pre-caldera 1.36 1.35 4) post-caldera 1.19 1.29

0.9-1.0

TABLE 7

Open-system modelling parameters K20, wt.%

TiO2, wt.%

Interval

r,

rz

r3

OK

DTi

Co

Ci

CA

Co

Ci

CA

1

0

2 3 4

0 0.3 0

0.1 0.1 2.0 1.0

0.1 0.1 5.0 10.0

0.01 0.01 0.01 0.01

0.1 0.1 0.1 0.1

1.16 1.05 1.35 1.29

0.46 0.46 0.46 0.46

1.8 -

1.18 0.98 1.36 1.19

0.74 0.74 0.74 0.74

0.5

rl = rate of assimilation / rate of crystallization. r2 = rate of extrusion / rate of crystallization. r 3 = rate of"intrusion" / rate of crystallization. D = bulk distribution coefficient. Co = initial elemental concentration in magma. Ci = elemental concentration in "intruded" magma. CA = elemental concentration in assimilant See Reagan et al. ( 1987 ) for details.

-

394

other Central American and Nicaraguan basalts appear to be the result of: ( 1 ) fractional crystallization in a large, shallow and, at times, degassed magma chamber; (2) vigorous convection in said chamber; (3) periodic influx of hotter, probably more primitive magma (i.e., magma mixing); and (4) crustal contamination. The latter must occur prior to entry into the shallow chamber of Masaya or have occurred during the initial stages of chamber formation. Given the noteworthy compositional homogeneity of Masaya, the latter seems more likely. In addition, as will be demonstrated next, the overall importance of crustal assimilation/contamination has diminished with time.

Origin of stratigraphic compositional changes Most of the stratigraphic compositional variability at Masaya can be explained by the alternating dominance of one of two open-system magmatic processes in Masaya's chamber: (1) fractional crystallization so that TiO2, FeO*/MgO, K20 and Ba all increase sympathetically; and (2) mixing with low-Ti, NG-like basaltic magma so that TiO2, K20 and Ba all fall sympathetically (Fig. 6 ). The longer-term compositional trends toward lower TiO2 and higher MgO (Fig. 6) suggest a longer-term control by (2). For example, Table 6 illustrates how these two processes could produce some of the significant stratigraphic changes in K20 and TiO2. The illustrations utilize the open-system equations of Reagan et al. ( 1987 ), in which assimilation, intrusion (or input into the system) and extrusion may occur with crystallization. Parameters for the four models are given in Table 7. Table 7 also indicates that a pre-caldera interval of mixing with low-TiO2 basalt could be accompanied by contamination with K20-rich, TiOz-poor magma or wall rock. Contamination, either through assimilation or mixing, is consistent with other trace-element variability

J.A. WALKERET AL. TABLE 8 Results of least-squares modelling for producing Masaya Lapilli (LM-23A, Qaw 1) from mixing Masaya basalt (MA7-82, Qlw 2) and Apoyo dacite (NA-108)* Fractions

Variables

0.763 0.237

MA-7-82 NA-168

Calculated LM-23A

Observed LM-23A

54.26 1.15 15.13 12.20 0.24 3.84 8.84 2.88 1.54 0.30

53.55 1.22 15.21 12.23 0.23 3.88 8.73 3.01 1.59 0.33

Results

wt.%

SiO2 TiO2 AI203 FeO MnO MgO CaO Na20 K20 P20s ppm

Rb Ba Sr V Zr

35 1070 440 360 135

37 1127 443 412 140

Sum of squares of weighted residuals = 0.07. *Analysis of NA- 16 from Sussman ( 1985 ).

(not shown), the slight variations in K20/TiO2 within pre-caldera magmas (Fig. 7), and is necessary to explain the K20/TiO2 distinctions between the earliest pre-caldera (or Nindiri-Venecia, NV) lavas, the Masaya Lapilli Bed, and all of the remaining lava and clast compositions (Fig. 7 ). The latter could also be explained by crystallization and separation of respectable amounts of magnetite from an NV parent composition, thus raising KEO/TiO2. However, magnetite is not an observed phenocryst phase, nor are there complementary changes in V, as would be expected (Table 1 ). Therefore, as illustrated in Figure 10, contamination with acidic compositions erupted from nearby Apoyo and Apoyeque calderas can explain the high KEO/TiO2 of all but one of

395

BASALTIC MAGMA EVOLUTION BENEATH A SUBDUCTION ZONE VOLCANO: MASAYA CALDERA, NICARAGUA

0.705

0.704

-

co

Iow-Ti

high-Ti

0.703 0.001

I 0.002

I 0.003

0.004

l/Sr

Fig. 9. 1/Sr versus 87Sr/a6Srfor lavas from Masaya ( • = post-caldera lavas; × = pre-caldera lavas) and low-Tiand highTi from the Nejapa-Granada cinder cones. Solid line and tic marks show 10-30% fractionation for AFC model discussed in text. the pre-caldera lavas, of all pyroclastic sampies, and even of the early, Los Sierras sampies. This is supported by mass balance calculations (Table 8). Most of the post-caldera lavas, on the other hand, show limited variability in K20/TiO2 and hence, we suggest they have experienced little to no crustal contamination. This view is supported by the higher Sr and MgO concentrations of post-caldera lavas (Fig. 6). The post-caldera lavas, however, do have uniformly higher K20/TiO2 than the pre-caldera lavas. These higher ratios are attributed not to mixing with a siliceous magma, but to mixing between an evolved caldera-forming magma and relatively unevolved low-Ti NGlike basalt as also illustrated in Figure 9. In other words, we envisage a relatively large mixing event, sometimes preceding caldera formation, involving a magma already enriched in K20/TiO2 and a more mafic, NG-

like parent. This mixing event set the post-

caldera K20/TiO2 values. Caldera formation, therefore, represents an important petrological discontinuity, as has been observed in large acidic magmatic systems (e.g., Hildreth et al., 1984; Broxton et al., 1989). In this case, the discontinuity coincides with a relatively large mixing event and marks the end of recognizable contamination with dacitic magmas or some high-KzO/TiO2 crustal component.

Implications for future activity Detailed stratigraphic studies at other volcanoes have shown that hypothesized mixing episodes are coincident with or followed by large pyroclastic eruptions or volcanic "crises" (Newhall, 1979; Huijsmans and Barton, 1989; Luhr and Carmichael, 1990). Similarly, at Masaya, the first three of the four completed

396

J.A. WALKER E T AL.

Apoyo, Apoyeque

Mixing Lines

oJ

o o

~Z \

•

\

\

• \

\

I 4

8

12

MgO, wt.%

Fig. 10. K20/TiO2 versus MgO for all Masaya samples, high- and low-Ti; lavas from the Nejapa and Granada cinder cones; and samples from Apoyo and Apoyeque calderas. Oand Oshow exact samples used as acidic and basic end-members for mixing calculations discussed in text. Las Sierras samples are shown by ~. Other Masaya symbols: • = postcaldera lavas; × = pre-caldera lavas; [] = caldera-formingscoria; + = pre-caldera scoria. Apoyo and Apoyequedata from Sussman ( 1985 ) and unpublished results. Nejapa-Granada data from Walker (1984). Mixing lines discussed in text. compositional cycles tentatively proposed end with or are coincident with large pyroclastic eruptions, although the exact stratigraphic position of the pre-caldera tephra deposits is conjectural. The end o f the hypothesized fourth cycle, although not coincident with a large explosive eruption, is marked by the eruption o f perhaps the petrographically most unusual post-caldera lava, the Casa Vieja flow, a smallvolume flow with abundant plagioclase phenocrysts. The consistently close association of explosive volcanism and subvolcanic mixing events here and elsewhere highlights the need for petrological monitoring of frequently active subduction zone volcanoes as previously pointed out by Luhr and Carmichael (1990). At Masaya, TiO2, FeO*/MgO and KaO compositions are again attaining m a x i m u m values (Figs. 9 and 10) perhaps prefiguring a large explosive eruption in the future. However,

post-caldera activity to date has been overwhelmingly non-explosive and the periodic degassing episodes since 1850-1853 may be preventing explosive devolatilization. In addition, the apparent lack of recognizable input of acidic magma into Masaya's chamber after caldera formation may be inhibiting explosive activity.

Conclusions Compositional variation and evolution at the Masaya Caldera Complex is the result o f opensystem magmatic differentiation in a large, shallow magma chamber. The two dominant processes operating in the chamber are fractional crystallization and mixing with lessevolved basaltic magmas, like those erupted from neighboring cinder cones. Mixing also serves to thermally sustain and composition-

BASALTIC MAGMA EVOLUTION BENEATH A SUBDUCTION ZONE VOLCANO: MASAYA CALDERA, NICARAGUA

ally homogenize the chamber. Crustal assimilation/contamination was also a fundamental magmatic process during the early history of the chamber but has not played a role in magmagenesis since caldera formation. Crustal contamination could involve mixing with acidic magmas, again like those erupted from neighboring volcanoes. Overall, therefore, mixing and complete homogenization of magmas is suggested to be the fundamental magmatic process beneath Masaya. Magma mixing has both established and maintained the high-LIL, high-Fe magma chamber of Masaya (compare Barton and Huijsmans, 1986). The Masaya Caldera Complex serves to illustrate that open-system magmatic differentiation can dominate at moderate-volume basaltic volcanoes, sustain a tholeiitic differentiation trend, and totally shield the identity of the parent magmas. Subduction zone volcanoes with well-developed calderas and erupting relatively Fe-rich lavas (e.g., Dixon and Stern, 1983) are also likely dominated by shallow open-system processes. In addition, Masaya may indicate that the magmatic systems beneath neighboring subduction zone volcanoes, particularly in Central America where volcano spacing is small (Carr, 1984), may often be quite interconnected. Surprisingly, therefore, many of the petrological/geological distinctions of Masaya which make it stand out so clearly from its neighbors are, quite possibly, inherited from the magmas of those very neighbors.

Acknowledgements Support was provided by: NSF Grant EAR80-25719 to R.E. Stoiber; NSF Grant EAR80-07424 to M.J. Carr; NSF Grants EAR86-06986 and EAR 88-03546 to M.J. Carr and M.D. Feigenson; Dartmouth College; and Northern Illinois University. Many people assisted in Nicaragua, including A. Rodriquez and R. Argenal of INETER, J. Jenkins, E. Cedeno, V. Cedeno of IRENA, R. Sandino, D. Fa-

397

jardo, A. Arburto, J. Garayar, and, of course, M.J. Carr and R.E. Stoiber. Many thanks also to L. Paulson for drafting the figures, A. Polzin and P. Turczyn for typing the manuscript and D. Sussman, R.E. Stoiber, W.I. Rose, Jr., and M.J. Carr for their thoughts. B.D. Marsh and an anonymous reviewer helped greatly in clarifying the ideas presented.

References Baker, D.R. and Eggler, D.H., 1983. Fractionation paths of Atka (Aleutians) high-alumina basalts: constraints from phase relations. J. Volcanol. Geotherm. Res., 18: 387-404. Baker, D.R. and Eggler, D.H., 1987. Compositions of anhydrous and hydrous melts coexisting with plagioclase, augite and olivine or low-Ca pyroxene from 1 atm to 8 kbar: Application to the Aleutian volcanic center of Atka. Am. Mineral., 72:12-28. • Barton, M. and Huijsmans, J.P.P., 1986. Post-caldera dacites from the Santorini volcanic complex, Aegean Sea, Greece: an example of the eruption of lavas of near-constant composition over a 2,200 year period. Contrib. Mineral. Petrol., 94: 472-495. Bice, D.C., 1985. Quaternary volcanic stratigraphy of Managua, Nicaragua: Correlation and source assignment for multiple overlapping plinian deposits. Geol. Soc. Am. Bull., 96: 553-566. Blake, S. and Ivey, G.N., 1986. Magma-mixing and the dynamics of withdrawal from stratified reservoirs. J. Volcanol. Geotherm. Res., 27:153-178. Brophy, J.G., 1990. Andesites from northeastern Kanaga Island, Aleutians. Implications for calc-alkaline fractionation mechanisms and magma chamber development. Contrib. Mineral. Petrol., 104:568-581. Broxton, D.E., Warren, R.G., Byers, F.M. and Scott, R.B., 1989. Chemical and mineralogic trends within the Timber Mountain-Oasis Valley Caldera Complex, Nevada: evidence for multiple cycles of chemical evolution in a long-lived silicic magma system. J. Geophys. Res., 94: 5961-5985. Carey, S. and Sigurdsson, H., 1989. The intensity of plinian eruptions. Bull. Volcanol., 51: 28-40. Campbell, I.H. and Turner, J.S., 1987. A laboratory investigation of assimilation at the top of a basaltic magma chamber. J. Geol., 95: 155-172. Carr, M.J., 1984. Symmetrical and segmented variation of physical and geochemical characteristics of the Central American volcanic front. J. Volcanol. Geotherm. Res., 20: 231-252. Carr, M.J., Rose, W.I. and Stoiber, R.E., 1982. Central America. In: R.S. Thorpe (Editor), Andesites: Oro-

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