www.elsevier.com/locate/epsl

Earth and Planetary Science Le

Silicic ignimbrites within the Costa Rican volcanic front: evidence

for the formation of continental crust

Thomas A. Vogela,*, Lina C. Patinoa,1, Guillermo E. Alvaradob,2, Phillip B. Gansc,3

aDepartment of Geological Sciences, Michigan State University, East Lansing, MI 48824-1115, United StatesbArea de Amenazas y Auscultacion Sısmica y Volcanica, Instituto Costarricense de Electricidad, Escuela Centroamericana de Geologıa,

Universidad de Costa Rica, Apdo. 35, San Jose, Costa RicacDepartment of Geological Sciences, University of California, Santa Barbara, CA 93106, United States

Received 13 January 2004; received in revised form 8 July 2004; accepted 8 July 2004

Editor: R.D. van der Hilst

Abstract

The origin of silicic magmas (greater than 65% silica) in areas without old, evolved continental crust has long been an

enigma. In oceanic arcs, silicic volcanic deposits are not common, which has generally been attributed to subduction-related

magmas rising, without stalling or fractionating, through a relatively thin, mafic oceanic crust. The volcanic arc in Costa Rica is

built on a thick (~40 km) oceanic plateau, and extensive silicic magmatism, dominated by silicic ignimbrites, has occurred in the

region in the absence of old, evolved continental crust. The silicic deposits display the common large ion lithophile element

(LILE) enrichment and high field strength element (HFSE) depletion observed in magmas generated by subduction processes,

which are not characteristic of melts generated from oceanic plateaus. Key trace element ratios in the ignimbrites vary with

position along the volcanic front and this variation is similar to that of the volcanic front lavas. Ignimbrites contain unique

cumulative frequency distributions of incompatible trace element ratios that demonstrate different magma batches for each unit.

We propose that the Caribbean oceanic plateau in Costa Rica is being converted to continental crust, in large part, by the

emplacement of silicic magmas in this area. The origin of these silicic magmas is due to reprocessing, juvenile, mantle derived,

subduction related magmas that have ponded in the crust.

D 2004 Elsevier B.V. All rights reserved.

Keywords: silic magmas; continental crust growth; mature island arcs; trace elements; Costa Rica; Central America

0012-821X/$ - s

doi:10.1016/j.ep

* Correspon

E-mail addr

gans@geol.ucsb1 Tel.: +1 52 Tel.: +5063 Tel.: +1 8

tters 226 (2004) 149–159

ee front matter D 2004 Elsevier B.V. All rights reserved.

sl.2004.07.013

ding author. Tel.: +1 517 353 9029; fax: +1 517 353 8787.

esses: vogel@msu.edu (T.A. Vogel)8 patinoL@msu.edu (L.C. Patino)8 GAlvaradoI@ice.go.cr (G.E. Alvarado)8

.edu (P.B. Gans).

17 432 5522; fax: +1 517 353 8787.

220 7741; fax: +506 220 8212.

05 893 2642; fax: +1 805 893 2314.

T.A. Vogel et al. / Earth and Planetary Science Letters 226 (2004) 149–159150

1. Introduction

It is widely accepted that subduction processes at

converging plate boundaries play an important role in

adding juvenile material to the crust, but even though

there has been over 30 years of work on crustal

growth models, our understanding of this process is

still obscure [1]. In settings without old and evolved

continental crust, the production of silicic magmas has

long been an enigma (we refer here to bsilicicQmagmas as those with greater than 65 wt.% silica).

In these settings, large volumes of high-silica magmas

have only recently been recognized. Until recently,

large volumes of high-silica magmas were thought to

be rare in oceanic arc settings, a situation that is

generally attributed to the absence of continental

crust. However, abundant silicic volcanic deposits

have been recently recognized in these settings-in the

Izu-Bonin arc [2,3] and in the Kermadec arc [4].

Various combinations of two endmember models have

been proposed to explain the generation of silicic

magmas in non-continental arc settings. One is the

prolonged fractional crystallization of basalt or

basaltic andesite parent ([5] and references therein).

The other is melting of, or melt extraction from

previously emplaced, mantle-derived melts (anatexis)

([3,4,6] and references therein). In the second model,

mantle-derived magmas can stall (crystallize) in the

crust because they are water saturated ([3] and

references therein), or the magmas can stall at a level

where they reach neutral buoyancy ([4] and references

therein). In continental arcs, the lighter, more evolved,

continental crust forms density traps. In non-con-

tinental arcs, as the crust evolves with time, influx of

magma causes the crust to thicken. The more evolved,

low-density magmas concentrate in the upper crust

and the depth of neutral buoyancy increases [7]. In

this way non-continental arcs can evolve to

bquasicontinentalQ [8] arc.Melting of oceanic plateau has also been proposed

as a method of generating silicic melts in non-

continental settings (for example, see [1] and refer-

ences therein). However from energy considerations

this would be inefficient because mantle melts would

not be very effective in melting mafic/ultramafic

cumulates, gabbros and amphibolites that are thought

to make up the lower portions of oceanic plateaus.

Melting of mafic material has been suggested for the

generation of silicic magmas and K-rich intermediate

magmas in the Central Andes and in New Zealand

[9,10].

Our preferred model for producing silicic melts,

where evolved continental crust is absent, is either by

remelting of, or residual melt extraction from,

subduction related, stalled, hot crystallized (or parti-

ally crystallized) magmas in the crust [3–6]. Water

saturated magmas can stall in the lower crust [3]

because of neutral buoyancy [4]. Remelting is due to

heat transfer from the emplacement of other mantle

derived magmas. The thick oceanic plateau may only

act as a thermal insulator [11] inhibiting the diffusion

of heat, and not contributing any material to new

batch of melt. Remelting of hot plutons or residual

melt extraction is energy efficient and is consistent

with the data presented below. Residual melt extrac-

tion of silicic liquid from a magma body with greater

than 45–50% crystals (rheologically a solid) has

recently been evaluated [6]. Regardless of the exact

process, the production of silicic melts leads to the

development of continental crust, providing that the

mafic residues become part of the mantle.

Volcanism in Costa Rica represents a mature island

arc that has evolved at the margin of the Caribbean

Large Igneous Province. Large volumes of silicic

ignimbrites occur in the central and northern part of

Costa Rica, and granitoid plutonic rocks occur in the

central and southern part of the country, in the

Cordillera de Tilaran and Cordillera de Talamanca.

We propose that silicic igneous rocks in Costa Rica

are far more common than is generally recognized and

reflect the ongoing conversion of the Costa Rican

crust from that of an oceanic plateau to young

continental crust (or bquazicontinentalQ crust [8]).

This is a consequence of normal subduction processes

that have modified an oceanic plateau due to the

emplacement of melts. In this paper we document the

geochemical characteristics and ages of silicic vol-

canic deposits (dominantly ignimbrites) in central and

northern Costa Rica, and place them in a regional

context to better understand the formation of con-

tinental crust in an evolving arc environment where

old, evolved continental crust is absent. The Miocene

Talamanca Intrusive Suite of granitoids [12–14] may

be the plutonic equivalent of the more recent volcanic

deposits. Similar plutons occur elsewhere in Costa

Rica [15,16] and these record the emplacement of

T.A. Vogel et al. / Earth and Planetary Science Letters 226 (2004) 149–159 151

subduction related magmas into an oceanic plateau,

which results in production of continental crust [12].

2. Tectonic setting

Central America has been divided into two main

tectonic blocks of different crustal origins: the Chortis

block in the north and the Chorotega block in the

south (Fig. 1). The Chortis block consists of a

basement of crystalline Paleozoic rocks (evolved

continental basement), whereas in the Chorotega

block there is no crystalline Paleozoic basement

[17–19]. There is no consensus on the location of

the boundary between the Chortis and the Chorotega

blocks [17–20]. The Chorotega block consists of the

Caribbean Large Igneous Province (CLIP), which was

emplaced in the Jurassic and it has a thickness of ~40

km [18,19]. Overlying this thickened crust, there are

Fig. 1. Tectonic setting of Central America showing plates, Cocos crust p

(CNS), triple junction trace (heavy dots), and Middle America Trench (M

Guatemala (G), El Salvador (S), Honduras (H), Nicaragua (N), Costa Ric

tholeiitic basalts that are typical of an island arc

setting [17], not related to the present day subduction

of the Cocos plate, but rather to the Farallon plate [20]

(Fig. 1). It was not until the Eocene–Oligocene that

the present form of the Central American volcanic arc

had begun building as a result of the unification of the

Chortis and the Chorotega blocks [19,20].

The crust of northern Costa Rica is ~40 km thick

and its velocity structure is as follows: 5.3–5.7 km/s

for the upper 4 km; 6.2–6.5 km/s for the underlying

mid crust ~13 km; and 6.9–7.3 km/s for the lower 20

km [21,22]. The mean velocities for the entire crust—

especially the middle and upper crust are much lower

than typical oceanic plateaus [21], raising important

questions as to how much of the crust is the original

oceanic plateau and how much is new material that

has been added and modified by subduction related

magmatism and/or underplating. It is our thesis that

the upper and middle crust in Costa Rica has been

roduced at East Pacific Rise (EPR) Cocos-Nazca spreading center

AT). The countries are labeled as follows: Mexico (M), Belize (B),

a (CR) and Panama (P). Map modified from [42].

T.A. Vogel et al. / Earth and Planetary Science Letters 226 (2004) 149–159152

profoundly modified by emplacement of silicic melts,

as evidenced by the record of voluminous silicic

magmatism.

3. Previous petrologic work in Costa Rica

Heretofore, most petrological work in Costa Rica

has focused on understanding of the generation of

basaltic and andesitic lavas along the modern arc ([23]

and references therein; [24,25]), yet few studies have

addressed the silicic igneous rocks. There have been

however numerous studies of silicic volcanic products

from the northern part of the Central American arc

(Guatemala and El Salvador), where Paleozoic con-

Fig. 2. Geologic Map of Costa Rica, modified from [43]. Insets of histogra

Cordillera de Tilaran and Cordillera de Central are compared to the lavas

Talamanca. GR, Tr, Cr and TaR are Guanacaste region, Cordillera de Tilar

tinental crust is present [26]. In addition, the Neogene

intrusive rocks of Costa Rica have been surveyed [15]

and there have been recent petrologic studies of the

intrusive and volcanic rocks of the deeply dissected

Cordillera de Talamanca, Costa Rica [12–14,27].

Drummond et al. proposed that the processes that

formed the granitoid suite in the Cordillera de

Talamanca represent the b. . .evolutionary change

from oceanic into continental crustQ ([12] p. 912).The volcanic products in Costa Rica are chemically

bimodal—with the ignimbrites dominated by silicic

compositions (66–75 wt.% SiO2) and the lavas

dominated by more mafic compositions (52–58

wt.% SiO2) (Fig. 2, insets). The relatively large-

volume silicic ignimbrites (ash-flow sheets) and air-

ms for SiO2 are shown for the ignimbrites for the Guanacaste region,

from the modern arc and the intrusive rocks from the Cordillera de

an, Cordillera de Central and Cordillera de Talamanca, respectively.

T.A. Vogel et al. / Earth and Planetary Science Letters 226 (2004) 149–159 153

fall deposits are the focus of this study. The recent arc

lavas are dominated by basaltic to andesitic flows

have been described in numerous review papers in the

last two decades [23–25]. The SiO2 frequency

diagrams for the Talamanca Intrusive Suite [12–14]

are shown in Fig. 2 inset for comparison with the

volcanic rock frequency diagrams.

3.1. Silicic volcanic rocks in Costa Rica

A recently completed comprehensive geochrono-

logic investigation of volcanic rocks in Costa Rica

Table 1

Silicic igneous deposits of Costa Rica

Formation Unit Age

(Ma)

Cordillera de Central

Tiribı 0.324 [28]

Cordillera de Tilaran

Alto Palomo

Palmito 0.54 [28]

Lajas

Palomo 0.57 [28]

Guanacaste Region

Guayabo La Ese 0.665–0.890

Alcantaro-Guachapilın Buena Vista 1.31 [28]

Salitral 1.36 [28]

Rıo Liberia 1.47 [28]

Canas 2.06 [28]

Sandillal 4.1–4.15 [28

Bagaces b4.1 [28]

Carbonal (lava) b10.7 [28]

Cordillera de Talamanca

Talamanca

Intrusive Series

1–31 [14,40

Main phase

8–12 [14,40

[28] has lead to a greatly improved understanding of

the time-space-composition patterns of Neogene and

younger rocks (see Table 1 for ages and composi-

tions). Known ignimbrites in Costa Rica range from

late-Miocene to Pleistocene and occur in three main

areas: Guanacaste region, Cordillera de Tilaran and

Cordillera de Central (Fig. 2). Ignimbrite composi-

tions range from basaltic andesite to rhyolite, but are

dominated by rhyolite (Fig. 2). In addition, silicic

lavas and domes as old as 17 Ma have been identified

in the San Carlos region of north-central Costa Rica,

northeast of the modern volcanic front. In the

Dimensions SiO2 wt.%

(normalized

anhydrous)

785 km2 [29,30] 55.4–68.4 [29]

22 km3 [29,30] Bimodal [29]

~3500 km2 [37]

~123 km3 [36,37]

Unknown 59.2–74.2

Unimodal

Unknown

Unknown Unimodal

[28] 0.1 km3 [34,35,46] 62.3–77.2

Unimodal

4.86 km3 70.3–74.2

Unimodal

5.6 km3 67.9–72.9

Unimodal

3500 km2 [34,35,46] 58.7–78.7

25 km3 Unimodal

Unknown 58.2–75.1

Unimodal

] Unknown 64.3–77.4

Unimodal

Unknown 66.8–71

Unimodal

Unknown 65.7–68.9

Unimodal

] Unknown 48.9–78.3

[12,13,14]

]

Bimodal

[12,13,14]

T.A. Vogel et al. / Earth and Planetary Science Letters 226 (2004) 149–159154

Guanacaste region, there are vast ignimbrites, which

form a broad plateau. The 40Ar/39Ar ages of these

silicic volcanic products range from ~6 to 0.652 Ma

[28]. In the Cordillera de Tilaran, explosive silicic

eruptions are an integral part of the evolution of the

modern composite volcanoes, with known ignimbrite-

forming eruptions at 6.0, 1.5, 0.92, 0.57, and 0.44 Ma

[28]. The Tiribı ignimbrite in Cordillera Central is

325F1 ka and is one of the largest in Costa Rica. The

age, location, and composition of the different

ignimbrite units for which we have geochemical data

are shown in Table 1. The three regions sampled in

this study include most of the known silicic volcanic

deposits in Costa Rica and cover a large time range

(~6–0.320 Ma) as well as a large distance along the

volcanic arc (~200 km) (Fig. 2). Of these, detailed

geochemical and volcanological studies have only

been completed on the ignimbrites from the Cordillera

Central [29,30]. Detailed studies of the other ignim-

brites are in progress [31–33]. Extensive physical

volcanology studies have been published on different

ignimbrite units in the Guanacaste region [34,35].

There are differences in mineralogical composition

among the ignimbrites of Costa Rica. The major

mineralogical difference is that some units are

anhydrous with only clinopyroxene and orthopyrox-

ene as the mafic phase, whereas other units contain

abundant biotite and amphibole. All units contain

abundant plagioclase, and quartz is present, but not

common in most units. Magnetite is present in all

units and ilmenite is not common. K-feldspar is rare in

all units.

There are also differences in chemical composition

among the ignimbrites. In general the silicic ignim-

brites from the Guanacaste region are the most silicic

with pumice fragments up to 78.7 SiO2wt.% (all values

normalized to 100 wt.% anhydrous), whereas the

maximum silica content of pumice fragments from

the Cordillera Central and Cordillera de Tilaran is 68.4

and 74.2 wt.% SiO2 respectively. The Tiribı ignimbrite

of the Cordillera Central has bimodal pumice compo-

sitions with high- and low-silica modes, whereas those

of the Cordillera de Tilaran and Guanacaste region

contain only a unimodal, high-silica population. The

ignimbrites from Cordillera Central are more alkaline

than the other ignimbrites in Costa Rica.

On the western side of the Guanacaste region, there

are extensive rhyolitic and dacitic ignimbrite flows

and lava flows, with small volume of andesitic

pyroclastic flows, which form the broad Guanacaste

ignimbrite plateau. These deposits represent the most

voluminous (N200 km3) silicic volcanic deposits in

Costa Rica (Fig. 2) and were emplaced from the

Miocene to Pleistocene. We have identified and dated

seven ignimbrites and one voluminous silicic lava

flow. Pumice fragments in these silicic ignimbrites

range of from 59 wt.% to over 79 wt.% SiO2.

Pumice fragments in the silicic ignimbrites from

the Cordillera de Tilaran, range from 59 to 74 wt.%

SiO2 [36] (and this study). The maximum extension of

these ignimbrites is between 3500 and 4000 km2 and

with an estimated maximum volume of 123 km3 [37],

although more detailed studies are needed to verify

these estimates.

In Cordillera de Central (Fig. 2) during the

Pleistocene, silicic magmas (the Tiribı Tuff) were

erupted along the western flank of Barva volcano

[29,30]. This tuff covers approximately 785 km2 and

has a volume of about 22 km3, and is exposed in deep

river valleys of the Central Valley and on meandering

channels; exposures are found as far as the Pacific

coast [29,38]. The Tiribı Tuff is a chemically diverse

ignimbrite where abundant low and high-silica pum-

ice fragments (55–68 wt.% SiO2) are found within the

same depositional unit [29].

The chemical differences among the ignimbrites

are best illustrated by comparing frequency distribu-

tions of ratios of incompatible trace elements.

Niobium (Nb) and tantalum (Ta) are chosen because

they are similarly incompatible with all phases in the

silicic magmas. However, other incompatible ratios

also have unique distributions for different ignim-

brites. The means of Nb/Ta ratios for three units, with

abundant biotite, from the Rio Liberia Tuff erupted

from the Guachapilın caldera (Table 1) in northern

Costa Rica are 4.3 (F0.65), 10.2 (F1.52), and 14.6

(F0.68). The Tiribı Tuff, for comparison, has mean

Nb/Ta of 19.8 (F1.72) (numbers in brackets are one

standard deviation). These data are best examined

using normal probability (Fig. 3), which are just a

quantitative method of analyzing frequency distribu-

tions (cumulative frequencies are plotted on the

vertical access on a log scale). Assuming that our

sampling is not biased, the frequency distributions

should represent normal distributions (a straight line

on a normal probability plot) if a single process (such

Fig. 3. Normal probability plots of Nb/Ta for some silicic

ignimbrites (see text for explanation). Three Liberia Tuff units are

shown that contain large differences in Nb/Ta even though they

erupted from the same caldera. The Tiribı Tuff is also shown for

comparison. The distinct Nb/Ta ratios can be produced by melting

different sources or by different amounts of melting of the same

source.

T.A. Vogel et al. / Earth and Planetary Science Letters 226 (2004) 149–159 155

as crystal fractionation, partial melting with similar

melt fractions, magma mixing) relates the samples

within an ignimbrite. If the source rocks for different

Fig. 4. Variation of Ce/Pb and Ba/Th with distance along the arc in Centr

lavas (lower diagrams). Political boundaries are noted by a dashed line. N

similar pattern to those from basaltic to andesitic lavas from the modern a

mantle source for the lavas, whereas high Ba/Th ratios have been interpre

ignimbrites were different, the normal probability

plots would be different. In addition, if melt fractions

are small, different amounts of melting will produce

different incompatible trace element ratios. On a

normal probability plot the slope of the line represents

the dispersion of the data (the line becomes vertical as

the standard deviation approaches zero). The normal

probability distributions of Nb/Ta from pumice from

closely related erupted units are distinct and represent

different populations (Fig. 3). Fractional crystalliza-

tion could not have produced this variation among the

units, but partial melting is consistent with these

chemical variations.

Another important observation is that the patterns

of variation of some key trace-element ratios from

these silicic ignimbrite deposits (such as Ce/Pb and

Ba/Th), which are indicative of subduction-related

magmatism parameters, are similar to the trace

element ratios of basaltic to andesitic deposits from

the volcanic front (Fig. 4). It has been well docu-

mented that these trace element ratios from the

youngest lavas in the modern arc in Costa Rica as

well as all of Central America form two distinct

groups that reflect different mantle compositions and

different degrees of mantle metasomatism ([39] and

al America for silicic ignimbrites (upper diagrams) and modern arc

ote how these trace-element ratios of the silicic ignimbrites have a

rc. High Ce/Pb ratios have been interpreted to indicate an enriched

ted to indicate a high slab input for the lavas [39].

Fig. 5. Spider diagram (normalized to normal mid-ocean ridge

basalt [44] for the Rıo Liberia Biotite ignimbrite (upper panel)

compared to granitoids from the Talamanca Intrusive Suite (lower

panel) and estimates of the upper continental crust (star symbol)

[45] shown on each panel. Note the overall similarity in pattern

particularly Nb, Pb, P, and Ti for the ignimbrite, granitoids and

continental crust. The ignimbrites and granitoids are slightly

enriched in the most incompatible elements (Rb, Ba, Th and U)

compare to the continental crust.

Fig. 6. Rare earth element plot (normalized to chondrite [44]), for

the Rıo Liberia Biotit ignimbrite (upper panel) compared to

granitoids from the Talamanca Intrusive Suite (lower panel) and

estimates of the upper continental crust (star symbol) [43] shown on

each panel. The overall patterns are similar except for the sligh

depletion of middle rare earth elements for the ignimbrites

compared to the upper continental crust. Note that there is no Eu

anomaly in the ignimbrite and only a slight anomaly in the

granitoids.

T.A. Vogel et al. / Earth and Planetary Science Letters 226 (2004) 149–159156

references therein). The fact that these trace element

ratios of the silicic ignimbrites are similar to those of

lavas from the modern arc suggests to us that the

ignimbrites are genetically related to the arc lavas,

which will be discussed further below.

3.2. Silicic plutonic rocks in Costa Rica

In contrast to the abundant occurrence of silicic

volcanic rocks in Costa Rica, silicic plutonic rocks,

with few exceptions, are limited to those exposed in

the Cordillera de Talamanca. A few researchers [12–

14] have conducted petrologic studies of the plutonic

rocks from the Cordillera de Talamanca. Earlier

studies have been reconnaissance in nature with few

chemical analyses ([15] and references therein). Ages

of the major silicic intrusive events in the Talamancas

occurred predominantly between 8 and 12 Ma

[12,14,40] with intrusive activity perhaps continuing

longer in the northwestern part of the Cordillera (7–4

Ma) [14]. The analyses (46 samples) from published

studies [12,14] can be used to compare the variation

of these intrusive rocks with the silicic volcanic rocks

in Costa Rica. If these are representative of the

distribution of plutonic rock types in the Cordillera de

Talamanca, then the intrusive complex is bimodal and

composed of 12% gabbro, 30% diorite, 6% tonalite,

28% granodiorite, 18% quartz monzonite and 6%

t

T.A. Vogel et al. / Earth and Planetary Science Letters 226 (2004) 149–159 157

granite [12]. Silica content ranges from 49 to 74 wt.%.

The granitoid rocks of the series contain abundant

plagioclase, K-feldspar and quartz. The mafic miner-

als in the granitoids are predominantly hornblende and

biotite with subordinate amounts of augite and rare

orthopyroxene [12].

In comparing the trace-element pattern of the

Talamanca Intrusive suite with the silicic ignimbrites,

it is instructive to compare spider diagrams (normal-

ized to normal mid-ocean ridge basalts) using trace

elements that change compatibility from left to right

(Fig. 5). The Rıo Liberia Tuff (biotite unit) from the

Guanacaste region is use as an example of silicic

ignimbrites. Both the ignimbrite and granitoids plots

are relatively similar to the continental crust. There is

more scatter in the granitoid data (most likely due to

different granitoids being sampled). They are slightly

enriched in the most incompatible elements (Rb, Ba,

Th and U) and depleted in P and Ti. Rare earth

element plots (Fig. 6) of the ignimbrites and

granitoids (normalized to chondrites) are similar to

the continental crust, with a slight depletion of the

middle rare earth elements (Dy, Ho, and Er) for the

ignimbrites, which may reflect amphibole in the

source. It is important to note that in the ignimbrites

there is no depletion of Sr (Fig. 5) or Eu (Fig. 6)

relative to their neighboring elements, which would

be expected if plagioclase fractionation were an

important process for the formation of these silicic

units. There is a slight depletion of Eu in the

granitoids (Fig. 6).

4. Discussion

The major conclusions from the geochemical and

chronological study of silicic volcanic rocks in Costa

Rica are: (a) Silicic volcanic deposits are abundant in

central and northern Costa Rica and range in age from

0.325 to ~6 Ma. (b) The volcanic arc in Costa Rica

developed on an oceanic plateau, which has been

modified by subduction processes. The silicic samples

display the common LILE enrichment and HFSE

depletion (Fig. 5) observed in magmas generated by

subduction processes, which are not characteristic of

ocean plateaus. (c) Key trace element ratios in the

ignimbrites from Costa Rica vary with position along

the arc—this chemical variation is similar to the

variation that has been observed in the Quaternary

basaltic and basaltic andesite lavas in this portion of

the arc (Fig. 4). Because of the similarity of key trace

element ratios in the ignimbrites and the modern arc

lavas, we infer that the silicic melts and the

Quaternary magmas are genetically related. (d) Each

ignimbrite unit is independent, reflecting either differ-

ent source areas or different amounts of melting

(magma batches) from a similar source. In examining

the frequency distributions of incompatible trace

element ratios (Fig. 3) for the ignimbrites, each unit

has a unique distribution reflecting these different

magma batches. (e) The Talamanca Intrusive Series,

which occurs in the deeply dissected Cordillera de

Talamanca, contains intrusive rocks that are chemi-

cally equivalents to some of the more recent

ignimbrite deposits (Figs. 5 and 6).

Because of the similarity in key trace element

ratios (such as Ce/Pb and Ba/Th) of the silicic

magmas and the modern arc lavas, the composition

of the source rocks for the silicic magmas is related to

the modern arc lavas. We conclude that melting of

previously emplaced calc-alkaline plutons or extrac-

tion of residual liquid from mostly solidified plutons

produced the silicic magmas. The distinct incompat-

ible trace-element ratios for many ignimbrite units

(Fig. 3) are consistent with independent magma

batches. The variation in major element compositions

observed in the different silicic units may be produced

by differences in the source mineralogy, water content

and/or melting at different pressure and temperature

conditions. Detailed experimental petrology is needed

to constrain the melting parameters (source composi-

tion, P, T) that can produce silicic magmas of

compositions similar to those silicic deposits in Costa

Rica.

Tamura et al. [11] have discussed the problems of

producing silicic melts by heating of the crust.

Because it is the most energy efficient melting

process, we prefer producing silicic melts in this

environment from hot, stalled crystallized (or partially

crystallized) magmas in the crust either by heat

transfer from the emplacement of other mantle derived

magmas [2]; or by extraction of residual melt from a

partially crystallized magma [6]. Each would produce

similar results.

It is our thesis that the addition of these silicic

magmas to the crust in Costa Rica, perhaps in concert

T.A. Vogel et al. / Earth and Planetary Science Letters 226 (2004) 149–159158

with the removal of the mafic/ultramafic residues and

cumulates from the base of the crust, have lead to the

conversion of this crust over time to a more continental

character [7]. Pichler and Weyl [41] almost 30 years

ago suggested that basaltic crust in Costa Rica has

been gradually transformed to continental crust, by the

emplacement of silicic magmas. This is consistent with

the observation that the mean upper and mid-crustal

velocities of northern Costa Rica [21,22] are lower,

and the crustal thickness higher than would be

expected for an oceanic plateau—closer to velocities

and thickness of continental crust.

Acknowledgments

We thank Wendy Perez, David Szymanski, Jennifer

Wade and Adrian Villegas for their help in the field

and for discussions. David Szymanski, Karen Tefend

and Ela Viray helped with data collection and

analyses. John Valley and Katherine Fishburn are

thanked for their reviews of the manuscript. We

acknowledge support from U.S. National Science

Foundation grants the National Science Foundation

NSF INT-9819236 (to Vogel and Patino) and NSF-

EAR 9975339 (to Gans).

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