1

Authors: Patino, Vogel, Alvarado, Rose

Introduction

The regional geochemical studies of volcanic rocks in Central America

have been focused on the chemical variation of lavas and ophiolites. In contrast, with

some exceptions, studies of the ignimbrites have not been common. The reasons for this

lack of study includes many factors, some of which are: the modern arc was thought to

consist predominantly of orogenic andesistes; many early workers thought that the

volcanic edifices were younger than the ignimbrites; the silicic rocks were thought to all

be derived from mantle derived mafic magmas; the caldera forming events were not

considered important; mineral exploration was not focused on the ignimbrites; and the

early studies of ignimbrites elsewhere were more focused on understanding ignimbrites

as instantaneous samples of magma chambers (and therefore magma chamber processes),

rather than including them in a regional tectonic setting. Understanding the regional and

spatial geochemical variation of ignimbrites along the recent and old volcanic fronts is

important. These studies are critical to better constrain the evolution of the arc and

consequent the evolution of the continental crust. This understanding will lead to better

predictive models for this dangerous type of volcanic eruptions.

There are no studies integrating the variation of silicic volcanic deposits along the

whole of Central America. In this paper we focus mainly on silicic ignimbrites of

Miocene to Pleistocene age along the Central American volcanic front from Guatemala to

Panamá – in Belize there are no reports. We review what is known about their

occurrence, age, distribution and chemical characteristics and place them in a regional

tectonic framework, and explore the origin of these silicic magmas.

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Central America has been divided into two main tectonic blocks of different

crustal origins: the northern Chortis block and the southern Chorotega block (see other

chapters for figure). An important contrast in Central America is that the Chortis block

consists of a basement of crystalline Paleozoic rocks, whereas in the Chorotega block

there is no crystalline Paleozoic basement. The Chorotega block is made up of an over-

thickened oceanic crust, the Caribbean Large Igneous Province (CLIP), that was

emplaced in the Cretaceous (see other chpt for details). There is no consensus on the

location of the boundary between the Chortis and the Chorotega blocks (see chpt).

However, what is important for this study is that the northern part of the Central

American is underlain by old continental crust and the southern part of the arc contains

no old continental crust. We explore in this chapter the effect of this changing nature of

the crust on the chemical variation of the silicic volcanic rocks. The Central American

volcanic front is an excellent locality in which to study the effect of variation of basement

rocks on the origin and evolution of silicic magmas.

Geographic distribution

Ignimbrites occur from Guatamala to Panamá. Although most Quaternary

volcanism has been concentrated near the active arc, Tertiary ignimbrites cover a wide

area of the interior, especially in central Honduras and northeastern Nicaragua (Williams

and McBirney,UC Pubs in Geol. Sci.). Figure ** shows the geographic and age

distribution of the ignimbrites of Central America (Guillermo will make this map).

Age distribution – Strat columns (See table at the of document)

There have been two periods of intensive volcanic activity in Central America in

the last 10 Ma (Carr, 1982). The first period occurred between 6 and 3 Ma, and the

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second period occurred between 1 Ma and the present with the volcanic products ranging

in composition from basalt to rhyolite. However, neither of these periods can compare

with the volume or extent of volcanic activity that occurred during the middle Miocene,

peaking at 14 Ma (Carr et al., 1982). Large ignimbrite deposits cover the early

Quaternary volcanic structures in Guatemala and El Salvador, and the younger volcanic

centers sit on top of a silicic pyroclastic stratigraphic marker dated at 84 Ka (Los

Chocoyos ignimbrite). However, in the southern part of the volcanic front, ignimbrites

underlay the Quaternary volcanoes (Carr et al., 1982).

The spacing of the rhyolitic volcanic centers in Guatemala and El Salvador

averages 90 km, however this contrasts with the spacing of 25 km for the basaltic-

andesitic centers (Carr et al., 1982) – in Costa Rica this spacing is 50 km (this is from

Guillermo). These rhyolite volcanic centers (calderas) are offset from the volcanic front,

to the north and east (Rose et al., ???). Wadge (1984) estimates that for Central America

volcanic production rate is of 31-62 km3/Ma/km-arc. Rose et al. (1999) estimated that

volume of the silicic pyroclastic deposits, which were less than 200 Ka, were similar in

volume of the lavas. The earliest records of large silicic eruptions in Central America in

marine basins are based on data from ODP cores. The Ocean Drilling Program (ODP)

have recorded two of the largest silicic eruptions in Central America with volumes

estimated to be > 100 km3 (Siggurdsson et al., 2000). The first was mid-Eocene (~46

Ma) to early Oligocene (~32), which are correlated with the Matagalpa and Morazán

Fms. in Guatemala, El Salvador, Honduras and Nicaragua (Ehrenborg, 1996). The second

was Oligocene/Miocene (~23 Ma) to the mid-Miocene, which are correlated with the

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Chalatenango, Padre Miguel and Coyol Fms. in Guatemala – El Salvador, Honduras and

Nicaragua respectively (Ehrenborg, 1996).

Tertiary Ignimbrites

Early Tertiary stratigraphy and geochemistry of ignimbrites in Central America

are poorly known. Weisemann (1975) described the Morazán Fm. as the Oligocene to

Middle Miocene volcanics in El Salvador. Reynolds (1980, 1987) summarized the

stratigraphy of the late Tertiary ignimbrite units in northern Central America. He notes

two formations, one from the Middle to Upper Miocene (Chalatenango) and a younger

one from the Pliocene (Cuscatlán). The older unit, Chalatenango formation consists of

rhyolitic tuffs and lavas. The source of large welded tuffs from the Chalatenango Fm.

originated from the Santa Rosa de Lima caldera (Reynolds, 1987). This unit is

contemporanous to the lowest formation in the Padre Miguel Group, in Honduras, and the

Coyol Group in Nicaragua (Ehrenborg, 1996).

Tertiary ignimbrites in Honduras are widespread but poorly studied and are best

known through the work of Williams and McBirney(UC pub) . Quaternary ignimbrites

are absent in Honduras, but Miocene and Pliocene are very abundant. These have been

roughly divided into two groups, an older, more voluminous group of Miocene age,

which occupy a large portion of the central highlands. A younger group, probably of

Miocene to Pliocene age, that occurs in the region bordering the Gulf of Fonseca (modify

this with more data). Ignimbrites of the older group tend to form extensive plateaus,

whereas those of the younger group tend to form the caps of mesas and cuestas adjacent

to the Gulf of Fonseca. Ignimbrites from Honduras are dominated by rhyolite, but

rhyodacites are nearly as abundant (Williams and McBirney). The mineralogy of the

ignimbrites is dominated by quartz, sanidine , plagioclase and biotite. In most rhyolitic

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units , phenocrysts of quartz and alkali feldspar are more abundant than plagioclase. In

some units pyroxene and hornblende are common. Many of the ignimbrites reported by

(Williams and McBirney) are crystal rich, containing twenty-five percent or more

phenocrysts. Only 14 chemical analyses are available for the silicic ignimbrites (W and

M, tables 3 and 5). Rhyolites are the most abundant composition in this group. There are

no studies that identify the caldera sources for the ignimbrites.

In Nicaragua there are only limited data about the Tertiary ignimbrites. One of

the sources of the ignimbrites from the Coyol Group is El Limon caldera in Nicaragua

(Ehrenborg, 1996), and like the Santa Rosa de Lima caldera in Honduras, it is located

behind the Neogene volcanic front. The younger unit, Cuscatlán Fm., has rhyolitic tuffs

and volcanic sediments overlain by rhyolitic and basaltic lavas. This formation is

contemporaneous with the upper formations of the Padre Miguel Group in Honduras and

with the deposits of Las Sierras in Nicaragua (Weyl, 1980). The only unit described in

the literature from this time frame is Las Sierras (Weyl, 1980). This unit includes ash-

flows of Plio-Pleistocene age located to the west of the modern volcanic front. Viray

(2003) has shown that these units have distinct compositions. Detail radiogenic dating

and fieldwork is needed to estimate the age and volume of the different units.

The Tertiary ignimbrites of Costa Rica are the best studied in Central America.

The earliest Miocene record of silicic volcanism in northern Costa Rica is the voluminous

Carbonal dacitic lava flow (7.8 Ma, Gillot et al., 1994). An extensive ignimbrite sequence

(ca. 2400 km²) overlies and underlies the Carbonal lava and occurs in the Pacific water-

sheds of the Guanacaste range, flanking the stratovolcanoes and exposed in fluvial

valleys. It is known as the Santa Rosa plateau (Bagaces Formation) and is of Upper

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Miocene to Late Pliocene age. It consists of a series of pyroclastic flow deposits with

minor interbedded lava flows and terrigenous sediments (Gillot et al., 1994, Tournon,

1984; Chiesa et al., 1987, 1992). Some of the oldest pyroclastic deposits in northern

Costa Rica are included in the Bagaces Fm., a unit that incorporates numerous deposits of

highly welded to unwelded deposits. Dating of these deposits is in progress and

preliminary dates indicate that they may be less than 8 Ma (Alvarado et al., 1992). The

volume of the Bagaces ignimbrite sequence is more than 100±40 km³, DRE of silicic

magma . A second and third episode of ignimbrite eruption occurred at 2.06 Ma and 4.1

Ma (Vogel et al., in press), both with unknown caldera sources and informally named the

Sandillal and Montano units respectively.

Quaternary Ignimbrites

In Central America the Quaternary ignimbrites are almost always related to large

calderas. In Guatemala and El Salvador, five major silicic centers occur behind the active

arc, and ash-flow deposits cover large areas in these countries (Hahn et al., 1979; Hart,

1993). The total volume of all these silicic deposits (< 200ka) have been conservatively

estimated to be between 300 and 500 km3, which is similar to the volume of lavas erupted

from the youngest generation of volcanic front volcanoes (Rose et al., 1999).

Rose et al. (1999) presented high-resolution stratigraphy of silicic volcanism

during the late Quaternary (<200 ka) from 5 calderas from Guatemala and El Salvador:

the Atitlán, Amatitlán, Ayarza, Coatepeque, and Ilopango (Table ?). Atitlán caldera

formed by episodic eruptions within the last 12 Ma (Newhall, 1980, 1987). The most

voluminous unit of recent eruptions, Los Chocoyos (270 km3 DRE), is a stratigraphic

marker for almost all of Central America. The youngest silicic eruption (AD 260)

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reported by Rose et al. (1999) is from Ilopango caldera in El Salvador, which consists of

air falls and pyroclastic flows with a volume estimated between 15 and 20 km3 (DRE) of

material.

In Nicaragua there are three ignimbrite shields (de Vries). The northern

Malpaisillo shield has little topographic expression, whereas the southern mots Las

Sierras shield is nearly 1 km above sea level. The Malpaisillo ignimbrite field outcrops

around the Monte Galan – Momotombo area. The most recent ignimbrites come from the

Monte Galan caldera and older ones probably from the Malpaisillo and San Fenando

structures (de Vries). A large ignimbrite shield volcano dominates the volcanic front

south of Momotomibito with the main caldera enclosing the Masaya volcano.

Ignimbrites from this caldera occur 50 km from the center, and have filled valleys near

Las Maderas in the Nicaraguan Depression. The upper layer of this ignimbrite, near

Diramba, has been dated from a carbon sample at 29,500 years BP (Sussman, 1985).

In Nigaragua the most recent silicic eruption reported is from Apoyo, with 11 km3

erupted from Apoyo 23,000 years BP (Sussman, 1985). The Apoyoque ignimbrite has

recently been studied by **** and erupted *** years ago with a volume of ****. The

Masaya volcano lies within the Las Sierras caldera with the present active volcano is

enclosed with the 2,00-4,000 year old Masaya caldera, which was formed during an 8

km3 ignimbrite and surge producing eruption (Williams, 1983 from de Vries)

In Costa Rica the youngest silicic deposit in the north is La Ese formation (0.66-

0.89 Ma) and is associated with the Guayabo caldera, the precursor to Miravalles

volcano. It is a crystal-poor unit (Plag-Qz-Cpx-Hb+/- Ol) Three other episodes of

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ignimbrite eruptions in northern Costa Rica have been recognized from field relationships

and dated. The first is an unnamed silicic flow dated at 1.18 Ma. The next unit is 1.31 to

1.47 Ma tentatively associated with the Guachieplín-Alcantara caldera a precursor to

Rincón de La Vieja volcano (Kempter et al., 1997; Zamora et al. in press –see ref

Guillermo e-mailed). This is a crystal-rich unit (Plag-Qz-Hb-Cpx-Opx) The oldest unit

(1.47 Ma) is a biotite rich flow that is a marker unit and it often directly overlies the

Bagaces sequence. This unit is one of the largest known ignimbrite eruptions in the

Quaternary time in Guanacaste (Chiesa, 1991). It is a crytal-rich pyroclastic flow deposit

(Qz-Biot-Hb-Opx-Plag An30-35) covering an area of ca. 4000 km2, which corresponds to

34 km3 of ignimbrite erupted volume (about 25 km3 of rhyolitic magma).

In the Central Valley region of Costa Rica, the Tiribí Formation represents the

latest example of silicic volcanism in the Costa Rica and has been dated at 0.324 Ma and

is the best-studied ignimbrite in Costa Rica (Hannah et al., 2002). It is chemically zoned

from 55.4 wt. % to 68.4 wt. % SiO2. Crystal content varies based on silica content. The

lowest silica samples contain 28% plagioclase (An76), 4% clinopyroxene (Wo44 En47 Fs9)

and less than 1% olivine (Fo70-73) and opaques (magnetite and ilmenite). The high-silica

samples are crystal poor rhyolites, and are nearly aphyric, with 0 to 2% crystals

consisting of plagioclase (An34 to An49), clinopyroxene (Wo47 En44 Fs9), orthopyroxene

(Wo3-6 En69-75 Fs 22-29), rare alkali feldspar (Ab48 Or43 An8) and opaques dominated by

magnetite. It covers about 500 km2 and has a volume of about 25 km3 DRE.

The Alto Palomo Unit unit consists of a series of poorly welded, dacitic to

rhyolitic ignimbrites that have been dated at 0.57 Ma. The Alto Palomo Unit consist of at

least two formations. The Alto Palomo Fm. (Plag-Cpx-Hb) and is characterized by

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mingling of basaltic magma as small streaks and clots in the pumice samples. The

Palmito Fm. (Plag-Hb-Bt-Qz) is very poorly known, which the source. Other ignimbrites

from the Central Valley and vicinity for which we have data have been dated at 6.0, 1.5,

0.92, and 0.44 Ma (Vogel et al., in review).

In Panama there has been only one reference to silicic pyroclastic deposits and

this occurs at the El Valle Volcano (Defant et al. 1991). The ages are 0.9-0.2 Ma and are

of dacitic composition with higher concentrations of Sr and lower concentrations of Y

than any other silicic samples in Central America in our data collection. Defant et al.

(1991) interpreted these to be melts of the hot, young subducting slab.

Table xAge and estimated volume of different formations in Northern Costa Rica

Name Formation

Age (Ma)

Volume (DRE, km³)

Guayabo 0.65-1.18

15.5±(increase)

Guachiepilín 1.31- 1.47

25±2 (increase)

Bagaces 8 -16

100±40

References: Chiesa (1991), Chiesa (1992), Gillot et al. (1004), Vogel et al. (in review)

Variation along the arc

The most common assemblage of ignimbrites is dominated by plagioclase both as

phenocrysts phase and in the groundmass, with lesser amounts of sub-calcic augite, and

hypersthene. A less common assemblages contains amphibole as well. In some assemblages only

hydrous ferromagnesium minerals are present such as amphibole or biotite , or both . These

hydrous minerals are absent all units studied in Nicaragua (Viray 2003), and from the Tiribí Fm,

Costa Rica, the youngest unit in Costa Rica (Hannah et al., 2002). In addition, Fe-Ti oxides,

apatite, quartz, are common in most ignimbrites. Alkali feldspars are rare in all ignimbrites in

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Central America (Carr et al., 1982; Hannah et al., 2002; Viray, 2003) with the exception that they

have been reported to be common in Honduras (Williams and McBirney).

The calderas that produced the large volume of silicic deposts in Central America erupt

material that ranges in composition from basalt to rhyolite. Early workers (Pushkar et al., 1968;

Carr and Stoiber, 1982?) thought that the ignimbrites become more dacitic as you move

southward along the arc. However, recent work has shown that rhyolitic ignimbrites are common

in Nicaragua and Costa Rica (Chiesa, 1991; Chiesa et al., 1992; Alvarado and Carr, 1993;

Kempter et al., 1996; Viray, 2003; Vogel et al., in press and references therein). In Guatemala and

Costa Rica the most primitive materials from the calderas are basaltic andesites and andesites,

respectively. In the other regions, basaltic scoria is present. Another regional characteristic is that

the Tiribí Fm. from Central Costa Rica and the Coyol Fm. from Nicaragua have a higher alkali

content than the rest of the samples from Central America.

Chemical trends along the arc

Carr et al. (1990) and others have shown that there are geochemical variations along the

arc in the modern lavas from Central America (see chpt). These variations have been explained

by changes in the crustal thickness and slab input (Carr et al., 2003?). Similar geochemical

variations occur along the arc in the silicic ignimbrites (Viray, 2003; AGU abstract). This

observation was previously noticed by Pushkar et al. (1968), Weyl (1980), Carr et al. (1982) on a

more limited set of geochemical parameters. Many of the trace element variations in the silicic

volcanic products in Central America resembles the modern basaltic-andesitic lavas. The

similarity in composition between the modern lavas and the silicic pyroclastic material is a key in

understanding the origin of the silicic volcanic rocks in Central America and will be discussed

more below.

The data set for the ignimbrites from Costa Rica and Nicaragua cover the widest time

range (0.023 Ma to ~12 Ma) that we have complete geochemistry for, and the geochemical data

set for Guatemala and El Salvador cover the most recent events (< 0.2 Ma). Thus, we are

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restricted from making inferences on temporal relationships of composition variations along the

arc.

Sources of the magmas and involvement of sediments from the subducting slab have

been inferred from trace element ratios of recent lavas (cf. Patino, 2000; Plank et al., 2003). For

example in southeastern Nicaragua and northwestern Costa Rica samples from the modern arc

(basalt to andesite) have low Ce/Pb, and high Ba/Nb, high Ba/La and high U/Th ratios, indicating

a large input from slab fluids. In Central Costa Rica samples from the modern arc (basalt to

andesite) have high Ce/Pb and low Ba/Nb and low Ba/La ratios indicating a smaller input from

the slab. In the silicic pyroclastic rocks, these ratios mimic those of the recent lavas (see Figures

**** Ce/Pb vs distance, Ba/Nb vs distance, Ba/La vs distance, Ce/Pb vs Ba/La – all figures to

have lava samples as background). In Nicaragua, Plank et al. (2003) interpreted the change in

U/Th ratios form Miocene to recent in the volcanics as reflecting the changes in the nature of

organic matter content in the sediment input. The slab contribution for the older magmas had low

U content, reflecting low organic matter in the sediments. For the younger magmas, the U in the

slab component increases due to the higher organic matter conent in the sediments. Our data for

U/Th are also consistent with Plank’s et al., 2003 data with the oldest ignimbrites contain the

lowest U/Th ratios and the youngest ignimbrites mimicking the modern arc. These trace element

ratios, which have been used to infer sources of the recent arc lavas, show the same trends in the

silicic pyroclastic rocks as the recent arc lavas.

However there are important exceptions to the similarities in trends of the ignimbrites

with the modern arc lavas. The Nicaraguan ignimbrites are very different with some specific

trace elements and trace element ratios. Specifically Zr, Zr/Hf and K2O/Rb show the lowest

values in the Nicaraguan ignimbrites whereas in the modern arc lavas there is little change in

Nicaragua compared to the ignimbrites in adjacent countries. (Figs ***). Thus there must be a

fundamental difference between the origin and evolution of the silicic magmas in Nicaragua,

compared to those in adjacent areas.

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Petrogenic Models

Models for the origin of the silicic deposits can be evaluated by examining the spatial

distribution of the data. There is no doubt that fractionation of minerals has played a role in the

evolution of silicic melts in the Central American volcanic arc. For example, some samples from

Guatemala and El Salvador have Eu/Eu* < 0.7 and Sr < 200 ppm. The occurrence of a Eu

anomaly, along with low Sr concentrations, can be used to support fractionation of plagioclase.

However, the absence of a Eu anomaly cannot be used to argue against plagioclase fractionation

because the oxidation of Eu+2 to Eu+3 would prevent Eu from being partitioned to plagioclase.

Variations in the composition of the parent material have a great influence on the

composition of melts (Smith et al., 2003; Beard and Lofgren 1991). Smith et al. (2003) infer that

the source for silicic melts in arc settings is very heterogeneous, and this heterogeneity can be at a

small vertical and horizontal scale. The silicic melts from Central America display a wide range

of compositions. There is a wide range in the degree of alkalinity of the different units. For

example, at SiO2 of 70 wt.%, Na2O+K2O ranges from 4 to 8. In addition, the southern most

ignimbrites, those from El Valle Central in Costa Rica, follow a more alkalic trend than any of

the other units. The composition of trace element also display wide ranges. For example Sr varies

from 100 to 500 ppm at SiO2 70 wt.%. The ignimbrites from Nicaragua tend to have the lowest

concentrations of Zr (<100 ppm for SiO2 of 70wt%). Nicaragua has the highest concentration of

Rb (>200 ppm), yet the K2O concentration is similar to other silicic rocks in Central America

(Fig.**** K2O vs Rb). A phase is needed in the source for the Nicaraguan ignimbrites that has a

much lower partition coefficient for K2O than for Rb.

Variation in source composition is one factor in the variation of the compositions of the

silicic melts. Another factor is the conditions of melt generations (P-T and water content). Beard

and Lofgren (1991) observed that in melting experiments of amphibole rich metamorphic rocks,

K2O concentration in the silicic melt of an amphibole rich source is affected by the amount of

water in the system because at higher water concentrations, the stability field of amphibole is

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expanded. As a consequence there will be more amphibole in the residue and plagioclase and

quartz will melt. If the temperature of the system increases, higher degrees of melting occur and

the concentration of elements such as K2O will decrease.

Based on radiogenic isotope compositions, Carr et al. (1982) conclude that the

composition of Quaternary volcanic products is not significantly affected by interaction with

continental crust. They interpret the large volume of silicic rocks in Guatemala and El Salvador

as the result of increased fractional crystallization of mantle melts due to thicker crust. However

few isotopic analyses were available at that time. A recent compilation by Carr (see Chpt and

Centam File) contains more data for the modern arc lavas (Fig **** Nd vs Sr lavas and

Ignimbrites). There is a clear continental crustal signature of the lavas from Guatemala, but little

crustal signature in the rest of the arc lavas in Central America.

There are few radiogenic isotope analyses of ignimbrites from Central America (Pushkar

et al 1968). The samples from Guatemala, Honduras and Nicaragua range from 0.7035 to 0.7175,

higher than for calc-alkaline lavas from the region. The largest variation was observed in the

ignimbrites from Honduras, 0.7045 to 0.7175. The ignimbrites from Guatemala, 0.7044 to

0.7070, and Nicaragua, 0.7035 to 0.7053, have similar ranges, but with the difference that those

from Guatemala have higher ratios. Kempter reported on three(?) analyses of ignimbrite from

Guanacaste province with values ranging from (0.70386 – 0.70394, 0.51301 – 0.51304) for Sr

and Nd isotopes respectively. Hannah (2002) reported on four samples from the Tiribi formation,

Central Costa Rica (0.70372 – 0.70374, 0.512946 – 0.512950) for Sr and Nd isotopes

respectively - values similar to the isotope ratios of Quaternary that range from 0.7035 to 0.7046

(Carr et al., 1990). These samples form two trends when Sr and Nd isotopes are combined

(Fig***Sr vs Nd). The majority of the mafic samples from the volcanic front show a positive

correlation between these two isotope systems, which has been explained by Carr (1990) as the

result of mantle metasomatism with fluids derived from the slab(See chpt). The other trend is

formed by samples from the northwestern part of the arc, in Guatemala, and display a negative

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relationship between Sr and Nd isotopes, which has been interpreted as evidence of crustal

contamination. Pushkar et al. (1968) also report Sr isotope ratios for the basement rocks in the

northern part of the arc and found a wide range (0.7037 – 0.7481), as expected from a large

diversity of rock types and ages. Magmas derived from the basement rocks from the southern

section of Central America should not be enriched because of the young age and mafic

composition of these rocks.

The similarity in the isotope composition of the ignimbrites and the Quaternary mafic

lavas has lead several authors to conclude a generic relationship between these two volcanic

products. This relationship can be such that the silicic ignimbrites originated by fractional

crystallization of the mafic melts, or the silicic rocks were generated by partially melting

previously emplaced mafic magmas at the base of the crust. A similar model was proposed by

Costa and Singer (2002) who concluded that an Andean dacite is not the product of fractional

crystallization, even though the Sr isotope composition is relatively low (0.70399), but of partial

melting of relatively young (<Miocene) gabbroic rocks. Feely et al. (1998) also concluded that

dacites from El Guadal volcanic region, in the Andes Southern Volcanic Zone, were produced by

partial melting of gabbroic crustal rocks. These rocks are the precursor or cogenetic to TSPC

basaltic intrusions and are heated by new batches of solidifying basalt. Small volume of basaltic

magma are emplaced at the mid-crust and fractionate to form andesites. However, when large

volumes of basaltic material are emplaced, the solidifying basalt produces enough heat to

partially melt the more evolved previously emplaced andesites.

In Costa Rica major conclusions based on a large number the geochemical analyses and

age determinations of silicic rocks in Costa Rica (Vogel et al., in review) are that melting of

previously emplaced calc-alkaline plutons produced the silicic magmas. Independent sources for

the silicic ignimbrites are demonstrated by their distinct incompatible trace-element ratios. These

authors prefer Tamura and Tatsumi’s (2002) model for producing silicic melts in this

environment by remelting hot, stalled crystallized magmas in the crust due to heat transfer from

15

the emplacement of other mantle derived magmas, bcause it is the most energy efficient melting

process.

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Table

CountryUnit Source Volume/Area Age SiO2

rangeGuatemala

I2-I5 fall f Atitlan 7 km3 >40 ka 67.5-73.4

Los Chocoyos – H fall

Atitlán 270-280 Km3 DRE

84,000 yr 74.8-77.5

W fall and pf Atitlán 5-10 km3 158 ka 52.76-75.84

? Atitlán ? 12 Ma

9 units (L, Z1-Z5, T, E)

Amatitlan 60-80 km3DRE

<23 Ka to 191 Ka

68.2-73.8

Piños - Tapala Ayarza 2 km3 (DRE) 23,000 yrMixta Ayarza <1 km3 27,000 yr

Custcatlán (~ upper Padre Miguel)

? ? 4 Ma

Chalatenango Fm (~ lower Padre Miguel

and Coyol)

Sant Rosa de Lima

? 15.7-9.4 Ma

Matagalpa Oligocene

El Salvador

Congo Coatepeque 6 km3 56.9 kaArce Coatepeque 17 km3 72,000 yr

Bellavista Coatepeque 0.4 km3 77 kaEmpalizada 0.35 Ma

TBJ (Tierra Blanca Joven)

Ilopango 15-20 km3 DRE

260 yr (AD)

TB2-TB4 Ilopango ? < 56 KaMorazan

(~Matagalpa)Oligocene

mid-Miocene

Honduras Upper Formations, Padre Miguel

Group

Pliocene

Lower Formation, Padre Miguel

Group

? ? Middle – Upper Miocene

Nicaragua Apoyo Apoyo 10.7 km3

DRE23,000 yr

Apoyeque Apoyeque ? 23,000 yrLas Sierras ? ? Pleistocene

(Weyl)?

San Rafael ? ? ?Ostocal ? ? ?

Guanacaste ? ? ?Monte Galan ? ? ?Las Banderas ? ? ?Las Maderas El Limon

(Ehrenborg)? 12.3-18.4

Ma (Ehrenborg)Coyol Calderas near

Condega (Fig. 107 Weyl)? 12.3-18.4

Ma (Ehrenborg)Matagalpa Oligocene

Costa Rica

Tiribí Barva 25 km3 DRE 0.320 Ma

17

Alto Palomo 125 km3DRE 0.565 MaGuayabo (La

Ese)Guayabo

(near Mirivalles)0.665 Ma

Guachapilín Guachapielín (near Rincon de la Vieja)

1.31 – 1.47 Ma

Montano 2.06Sandillal 4.15 MaBagaces > 4 MaCarbonal < 10.7 Ma 65.7-

68.9