Tectonic and magmatic evolution of the active volcanic ... · 370 S. Agostini et al. / Geothermics 35 (2006) 368–408 Fig. 2. Schematic tectonic map of El Salvador. Dashed columns

Geothermics 35 (2006) 368–408

Tectonic and magmatic evolution of the activevolcanic front in El Salvador:

insight into the Berlın and Ahuachapangeothermal areas

Samuele Agostini a,∗, Giacomo Corti a, Carlo Doglioni b,Eugenio Carminati b, Fabrizio Innocenti a,c, Sonia Tonarini a,

Piero Manetti a, Gianfranco Di Vincenzo a, Domenico Montanari a

a Istituto di Geoscienze e Georisorse (CNR), Via G. Moruzzi 1, 56124 Pisa, Italyb Dipartimento di Scienze della Terra, Universita di Roma, La Sapienza,

Piazzale Aldo Moro 5, 00185 Roma, Italyc Dipartimento di Scienze della Terra, Universita di Pisa, Via S. Maria 53, 56126 Pisa, Italy

Received 28 February 2006; accepted 30 May 2006Available online 11 September 2006

Abstract

In El Salvador, Central America, active deformation takes the form of a major dextral strike-slip faultsystem, the El Salvador Fault Zone, resulting from the oblique subduction of the Cocos Plate. The faultsystem is laterally discontinuous, being subdivided into different major en-echelon segments that partiallyoverlap to form pull-apart structures. Volcanic activity is spatially confined to the fault segments and absentin the intervening pull-apart basins; no significant temporal gap exists in the erupted products, at least duringthe Plio-Quaternary. Detailed analyses within the geothermal fields of Berlın and Ahuachapan have revealedimportant volcano-structural and petrologic differences between the two areas. In the Berlın area activedeformation is controlled by the regional transcurrent stress field, resulting in the development of systemsof right-lateral E–W-trending strike-slip faults. Conversely, the structural setting of the Ahuachapan areais more complex, reflecting an interaction among different stress fields. Berlın products exhibit a markedgeochemical and isotopic homogeneity indicating the presence of a single magmatic system. At Ahuachapan,on the other hand, the rocks display significant variations in both Sr isotopes and the LILE/HFSE ratios:this area is characterized by multiple volcanic centres, fed by different magma batches that reach the surfacewithout reciprocal interactions in shallow reservoirs. Thus, the characteristics of the volcanic products at

∗ Corresponding author. Tel.: +39 050 3152266; fax: +39 050 3153280.E-mail address: [email protected] (S. Agostini).

0375-6505/$30.00 © 2006 Published by Elsevier Ltd on behalf of CNR.doi:10.1016/j.geothermics.2006.05.003

mailto:[email protected]

dx.doi.org/10.1016/j.geothermics.2006.05.003

S. Agostini et al. / Geothermics 35 (2006) 368–408 369

Berlın and Ahuachapan reflect their different tectonic settings, with important implications for geothermalinvestigations.© 2006 Published by Elsevier Ltd on behalf of CNR.

Keywords: Geothermal exploration; Structural geology; Petrology; Geochemistry; Berlın; Ahuachapan; El Salvador

1. Introduction

El Salvador is located within the Central American Volcanic Front (CAVF), a volcanic chainextending for more than 1000 km from Guatemala to Costa Rica, and related to the subductionof the relatively young lithosphere (age <25 Ma) of the Cocos Plate beneath the Caribbean Plate(DeMets, 2001) (Fig. 1). The CAVF is divided into several distinct structural segments with dif-ferent geological and geophysical features (Carr and Stoiber, 1990). The El Salvador segment ischaracterized by strong tectonic and volcanic activity. The latter has been continuous since at leastthe early Neogene; during the Holocene 21 volcanoes were active, 6 of which erupted in historicaltimes (Siebert and Simkin, 2002). The widespread magmatic activity has resulted in a relativelyhigh heat flow that has created areas of shallow thermal anomaly whose exploitation is econom-ically feasible. In El Salvador, 22% of the electric energy produced annually is currently beinggenerated by geothermal plants (Bertani, 2005). New geological, geochronological, geochemicaland petrological data on the El Salvador volcanic front are presented here, with particular attentionpaid to the geothermal fields of Berlın and Ahuachapan (Fig. 2). A detailed description is givenof the volcano-tectonic evolution of the front, which could lead to more reliable evaluations ofthe real potential of El Salvador’s geothermal resources.

2. Geodynamic setting

El Salvador is located on the Caribbean Plate close to its western and northern margins (Fig. 1).To the north, this plate interacts with the North American Plate along the Cayman-Motagua sinis-tral transcurrent fault. In continental Central America, the contact between the two plates occursalong the complex Motagua fault system, which is located about 100 km from the northwestern

Fig. 1. Geodynamic setting of Central America (after DeMets, 2001; Corti et al., 2005a). Dashed white line indicates thevolcanic front.

370 S. Agostini et al. / Geothermics 35 (2006) 368–408

Fig. 2. Schematic tectonic map of El Salvador. Dashed columns show the main N–S grabens. Arrows indicate the activestress field (black arrows: the axes of maximum compression; white arrows: the axes of maximum extension); ESFZ, ElSalvador Fault Zone.

border of El Salvador (Fig. 2). The relative motion associated with this fault system is ∼20 mm/yr(Fig. 1).

To the west, the oceanic Cocos Plate is being subducted beneath the Caribbean Plate alongthe Middle America Trench, at rates of 73–84 mm/yr (Fig. 1) (DeMets, 2001). Several studiessuggest that the convergence between the two plates is oblique and partitioned between trench-orthogonal compression and strike-slip deformation parallel to the volcanic front, associated withthe northwestward transport of a fore-arc sliver relative to the Caribbean Plate (Fig. 1) (Harlowand White, 1985; White, 1991; DeMets, 2001). The transcurrent component of the movementis estimated to be ∼14 mm/yr (Fig. 1). This complex kinematics is reflected in seismic activity,characterised by earthquakes of two distinct types, subduction-related and strike-slip-related (e.g.Dewey et al., 2004; Martınez-Dıaz et al., 2004).

Fig. 3. Historical destructive earthquakes and focal mechanisms of crustal-depth earthquakes available for the El Salvadorarea (numbers: year and magnitude of the events). The thick black line indicates the inferred trace of the El Salvador FaultZone (ESFZ) (modified after Bosse et al., 1978; Martınez-Dıaz et al., 2004; Corti et al., 2005a). RL, Rıo Lempa; SM, SanMiguel; SS, San Salvador.


The transcurrent component of the movement between the Cocos and the Caribbean platesis probably accommodated onland by slip along a major E–W trending dextral transcurrentfault system (El Salvador Fault Zone, ESFZ; Martınez-Dıaz et al., 2004) running along thevolcanic front and representing the source of the strong strike-slip seismicity (Figs. 2 and 3;see Section 2.1). Between the dextral ESFZ and the sinistral Motagua fault system, a series ofN–S trending grabens testify to the existence of a broad zone of nearly E–W extension (Fig. 2)(Guzman-Speziale, 2001; see Section 3). Historical and recorded seismicity constrain the exten-

Fig. 4. (a) Segmentation of the El Salvador Fault Zone (ESFZ) east of San Salvador. BS, Berlın segment; IA, interveningarea; IL, Lake Ilopango; SMS, San Miguel segment; SVS, San Vicente segment. (b) Landsat image. (c) Line drawing ofmajor structures in the area west of San Salvador, with the hypothesised pull-apart structure linking two major segmentsof the ESFZ.


sional rates along the grabens of northern Central America to ∼8 mm/yr (Guzman-Speziale,2001).

2.1. Strike-slip deformation in El Salvador

Geological field work carried out in continental El Salvador suggests that the main structuresbelonging to the ESFZ have an approximate E–W strike, and are characterized by right-lateralkinematics. These faults running sub-parallel to, and north of, the volcanic front affect LatePleistocene and Holocene deposits and present a strong morpho-tectonic signature testifyingto very recent activity (Fig. 2) (Corti et al., 2005a). The active tectonics of the ESFZ is furthercorroborated by the strong transcurrent crustal seismicity associated with this fault zone (Martınez-Dıaz et al., 2004). In particular, the major upper crustal seismic events that have taken place since1912 in El Salvador occur parallel to, and north of, the volcanic front (e.g. Martınez-Dıaz et al.,2004). Reliable focal mechanisms indicate strike-slip events with one of the planes oriented E–W(Fig. 3). At least six of the major destructive earthquakes (M ≥ 6) along the volcanic front seemto be related to slip along the ESFZ segment located between Rıo Lempa and Lago Ilopango(Fig. 4a) (Martınez-Dıaz et al., 2004).

Geological and seismological analyses suggest that the ESFZ is laterally discontinuous, anddivided into large segments that are particularly evident east of the city of San Salvador (Fig. 4a)(Martınez-Dıaz et al., 2004; Corti et al., 2005a). Between the city and San Miguel, two majorsegments have been identified, the first of which extends from the Ilopango Caldera to the SanVicente Volcano-Rıo Lempa area (San Vicente segment), and the second from Rıo Lempa east-wards (Berlın segment; Fig. 4a). The two segments overlap in a dextral en-echelon style with theformation of an intervening pull-apart basin. East of San Miguel, the strain may be transferredto a third segment (San Miguel segment) that is, however, poorly known as yet. West of SanSalvador the existence of another major segment of the ESFZ can be hypothesized on the basisof strike-slip focal mechanisms (e.g. Martınez-Dıaz et al., 2004). The structural pattern obtainedthrough analysis of satellite images and aerial photos suggests that this segment can be linked tothe Ilopango-San Vicente sector of the ESFZ through another major pull-apart basin, extendingbetween Santa Ana and San Salvador (Fig. 4b).

3. The volcanic front: structure and stratigraphy

Subduction of the oceanic Cocos Plate under the Caribbean Plate resulted in the developmentof a volcanic front that, in El Salvador, comprises 21 active volcanoes, 3 of which (Santa Ana,San Salvador and San Miguel) erupted after 1900 (Siebert and Simkin, 2002). The CAVF runsfrom southern Mexico to Costa Rica, with gaps in which active volcanism is absent. Carr etal. (1982, 2004) recognized a number of breaks separating seven segments that are assumedto be tectonically controlled. The active volcanic front in El Salvador constitutes one of thesesegments, located between those of Guatemala and Nicaragua. In El Salvador, detailed structuralinvestigations have, however, led to the identification of at least three E–W segments arrangeden-echelon (Fig. 5); again, this architecture is assumed to be controlled by regional tectonics.

The El Salvador area is almost entirely made up of Cenozoic to Recent volcanic rocks andreworked volcanic material. The sedimentary basement is exposed sporadically in the NW cornerof the region, where the Metapan Formation (Jurassic-Cretaceous) outcrops. This formation con-sists of quartz-rich conglomerates, limestones and sandstones with rare intercalations of volcanicrocks (Bosse et al., 1978).


Fig. 5. Schematic representation of the main segments of the volcanic front (dotted ellipses) within El Salvador. AH,Ahuachapan; B, Berlın; SA, Santa Ana Volcano; SM, San Miguel Volcano; SS, San Salvador.

Fig. 6. General stratigraphic section and nomenclature for Upper Cenozoic formations in El Salvador region (modifiedfrom Donnelly et al., 1990).


The Cenozoic volcanic products are the result of the trenchward (i.e. southward) migrationof the volcanic front (e.g. La Femina et al., 2002). Several volcanic formations, ranging fromPaleocene to Recent, have been distinguished on the geological map of El Salvador (Bosse et al.,1978; Donnelly et al., 1990); a generalized stratigraphic section of the outcropping products isreported in Fig. 6. The oldest (Paleocene-Miocene) formations (Morazan and Chalatenango) areexposed in the northern inner part of the region, close to the border with Honduras. Pleistocene-Holocene products, including the active volcanoes, are comprehensively grouped within the SanSalvador Formation, which, for the most part, developed over the Miocene-Pliocene BalsamoFormation, formed by an assemblage of volcanoclastics and volcanic breccias with intercalatedlavas and scorias. Locally, a relatively thin layer of Plio-Quaternary volcanics known as theCuscatlan Formation (e.g. Donnelly et al., 1990) may be present between the Balsamo and theSan Salvador Formations.

4. Geology of the Berlın and Ahuachapan geothermal areas

4.1. Berlın area

The volcanic sequence in the Berlın area is characterized by a local basement corresponding tothe Balsamo Formation. Over the basement there are remnants of an old (Pleistocene) stratovol-cano, covered by a sequence of ignimbrite layers and pumice fall deposits that are likely relatedto the formation of a caldera structure; the pyroclastics have an age around 0.1 Ma (D’Amore andMejia, 1999). The youngest activity is represented by basaltic and basaltic-andesitic lava flows.The geothermal area lies within the ESFZ, close to the pull-apart structure linking the Berlın andSan Vicente segments (Fig. 2). The structural analysis carried out in this area reveals that the E–Wstrike-slip faults of the ESFZ are associated with minor structures that can be assigned to threedifferent sets according to their fault orientation (Fig. 7) (Corti et al., 2005a): (1) WNW–ESE(∼N110◦E); (2) NW–SE (∼N130◦E); (3) NNW–SSE to N–S (N170◦E to N180◦E). As observedin the classical strike-slip physical experiments (e.g. Tchalenko, 1970), these different fault setscan be related to an E–W dextral shear couple, with the E–W structures representing the maindeformation zone (Y shears) and the subordinated fault trends representing Riedel (WNW–ESE),antithetic Riedel (NNW–SSE to N–S), shear and tension (NW–SE) fractures. Fault-slip analysison different structural stations along the main structures indicates a dominant strike-slip stressfield, with σ1 and σ3 oriented ∼NNW–SSE (∼N155◦E) and ∼ENE–WSW (∼N65◦E), respec-tively (Fig. 7) (Corti et al., 2005a).

At volcanic edifices such as the Berlın one, tension fractures related to an extensional stressfield may locally predominate and create complex interactions with the strike-slip structures, assuggested by small-scale analogue models of magma intrusion during transcurrence (Fig. 8; Cortiet al., 2005b).

4.2. Ahuachapan-Cuyanasul area

In the Ahuachapan region, four main volcanic stages were identified by geological mapping(Gonzalez Partida et al., 1997). Several stratovolcanoes formed over the local Balsamo basement(e.g. Cuyanausul, Cerro de Apaneca, Cerro Empalizada); they were dissected by a caldera eventassociated with abundant pyroclastic products; the structure is known as Concepcion de AtacoCaldera (Fig. 9). The post-caldera activity took the form of small mixed volcanoes (Volcan delas Ninfas, Laguna Verde, Hoyo de Cuajuste) and domes (Cerro San Lazaro). The Ahuachapan-


Fig. 7. (A and B) Digital elevation model and map of active structures forming the El Salvador Fault Zone (ESFZ) betweenIlopango Caldera and San Miguel. Inset in (A) shows the location of the study area. Digital Elevation Model data andLandsat Enhanced Thematic Mapper 7 satellite mosaic images courtesy of the University of Maryland, Global Land CoverFacility. IL, Ilopango; RL, Rıo Lempa. Inset in (B) shows a schematic interpretation of the fault pattern. (C) Stereonetsof fault-slip data collected along the ESFZ (Wulff net, lower hemisphere). The large black arrows indicate the directionof compression and extension (after Corti et al., 2005a). The localities of structural sites are reported in (B).

Cuyanausul area is characterised by a complex tectonics since the region is located close to thewestern end of the Motagua and El Salvador fault systems and to the N–S grabens associatedwith the intraplate deformation of the Caribbean Plate (Fig. 9a). Detailed field work revealed thepresence of faults of different strikes that can be assigned to four main groups (Fig. 9b): (a) N–S,(b) NW–SE, (c) NE–SW, and (d) E–W. The N–S and NW–SE trending structures predominate.Most of these faults are normal with a minor component of oblique motion (either sinistralor dextral). All the analyzed structural trends are very young, as they affect Late Pleistocene-Holocene volcanic rocks. The structures with the most conspicuous morphological evidenceare the NE–SW oriented faults that, in the area west of the geothermal power plant, seem toaccommodate the active deformation. These structures cut both the N–S and the NW–SE faults.


Fig. 8. Top: fault pattern at the Berlın Caldera. Bottom: small-scale model of magma intrusion along a strike-slip fault(after Corti et al., 2005b) showing a remarkable similarity with faulting at the Berlın geothermal field.

However, the occurrence of very recent N–S faults and grabens in the NE part of the study area(e.g. Fig. 9a) and the analysis of local seismicity suggest an active stress field characterised byE–W extension (Guzman-Speziale, 2001).

At a regional scale, the N–S structures seem to be more or less contemporaneous with theNE–SW faults. At the outcrop scale, there appear to be no temporal relationships between theE–W striking fault structures and the above-mentioned trends. On the basis of regional (large-scale) tectonics, the E–W trend could be related to the dextral strike-slip fault system of theESFZ, hidden below the volcanic edifices. These different structures could therefore testify tothe interaction among different stress fields at a regional scale. In particular, the NW–SE and theE–W trends could be associated with the transcurrent stress field responsible for the developmentof the ESFZ, whereas the N–S structures are likely generated by the E–W extensional stresses thatcause intraplate deformation and the development of the N–S graben systems. The NE–SW trendcould be related to the Motagua stress field or to reactivation of a pre-existing fabric generatedby this major fault system.

4.3. Geochronology

There is some uncertainty with regard to the geochronological constraints of the Salvadoreanvolcanic formations. In the Ahuachapan area, Gonzalez Partida et al. (1997) claimed, on the basisof K–Ar datings, that the Balsamo Formation could be as young as the Pleistocene. In the Berlınregion, on the other hand, the local volcanic basement represented by the Balsamo is consideredto be at least 2 Ma old (Anderson et al., 1994).


Fig. 9. (a) Landsat Thematic Mapper satellite image of the Ahuachapan area with detail of N–S normal faults, indicating aroughly E–W extension. (b) Graphic representation (Stereonets) of the structures measured at outcrop scale superimposedon a digital elevation model, showing the main faults of the Ahuachapan area. Black dots evidence rim of Concepcion deAtaco Caldera.

In order to better constrain the stratigraphic sequence at Berlın and Ahuachapan, we selectedsix samples for 39Ar–40Ar isotopic age determinations (see Table 1 for their location). The threefrom Berlın constitute a sequence ranging from local basement (Balsamo, ES 46) to the youngestproducts (ES 8-scoria, and ES 27-lava). Likewise, the Ahuachapan samples encompass the age

Table 1Location of the collected samples

Sample Place Rock type Latitude (N) Longitude (W) Formation

BE 1 Berlın Pumice 13◦30′26′′ 88◦32′13′′ San SalvadorBE 2 Berlın Lava flow 13◦31′00′′ 88◦31′42′′ BalsamoBER 1 Berlın Lava flow – – BalsamoBER 3 Berlın Lava fragments 13◦35′04′′ 88◦34′11′′ BalsamoES 1 Volcan San Salvador (El Boqueron) Lava block 13◦44′07′′ 89◦16′48′′ San SalvadorES 2 Volcan San Salvador (El Boqueron) Lava 13◦45′29′′ 89◦16′12′′ San SalvadorES 3 Volcan San Salvador Lava 13◦48′26′′ 89◦19′35′′ San SalvadorES 4 Coatepeque Dyke 13◦53′37′′ 89◦31′49′′ San SalvadorES 5 Ilopango Lava dome 13◦42′23′′ 89◦03′50′′ CuscatlanES 6 Ilopango Lava dome 13◦42′32′′ 89◦04′05′′ CuscatlanES 7 Cojutepeque Lava 13◦43′44′′ 88◦55′59′′ BalsamoES 8 Berlın Scoria 13◦31′39′′ 88◦29′44′′ San SalvadorES 9 Berlın Pumice 13◦31′39′′ 88◦29′44′′ San Salvador


Table 1 (Continued )

Sample Place Rock type Latitude (N) Longitude (W) Formation

ES 10 Berlın Glass fragments 13◦31′39′′ 88◦29′44′′ San SalvadorES 16 San Francisco Gotera Perlitic lava 13◦40′07′′ 88◦05′36′′ ChalatenangoES 23 Berlın (Alegria) Lava 13◦31′00′′ 88◦30′29′′ San SalvadorES 27 Berlın (Agua Caliente) Lava 13◦31′31′′ 88◦30′13′′ San SalvadorES 28 Berlın (TR 18) Lava 13◦30′07′′ 88◦31′13′′ San SalvadorES 29 CA-4 Cerro Nejapa Lava 13◦48′42′′ 89◦11′43′′ BalsamoES 30 CA-4 Guazapa Lava 13◦50′36′′ 89◦09′29′′ BalsamoES 31 CA-4 Cerro Rico Neck 14◦06′16′′ 89◦14′04′′ BalsamoES 32 Nueva Concepcion Lava 14◦09′11′′ 89◦18′19′′ BalsamoES 35 Ahuachapan (Cuyanausul) Lava 13◦54′18′′ 89◦46′11′′ San SalvadorES 39 Ahuachapan (Hoyo de Cuajuste) Lava 13◦52′57′′ 89◦47′49′′ San SalvadorES 40 Ahuachapan (Poza del Diablo) Altered lava 13◦56′03′′ 89◦47′21′′ San SalvadorES 41 Ahuachapan (Juayua) Lava block 13◦51′25′′ 89◦44′20′′ San SalvadorES 42 Ahuachapan (Cerro de Apaneca) Lava 13◦50′27′′ 89◦48′35′′ San SalvadorES 43 Ahuachapan (Concepcion de Ataco) Lava 13◦53′08′′ 89◦51′20′′ BalsamoES 44 Izalco Lava 13◦44′42′′ 89◦37′39′′ San SalvadorES 45 CA-1 Puente Rıo Lempa Dyke 13◦37′00′′ 88◦33′56′′ BalsamoES 46 CA-1 Puente Rıo Lempa Lava 13◦39′25′′ 88◦42′37′′ BalsamoES 47 San Miguel Lava 13◦30′36′′ 88◦14′50′′ San SalvadorES 48 Conchagua Lava 13◦17′58′′ 87◦48′54′′ San SalvadorES 50 Ahuachapan (Las Chinamas) Lava 14◦01′16.5′′ 89◦54′12.5′′ BalsamoES 51 Ahuachapan (Cerro San Lazaro) Lava 13◦56′25.6′′ 89◦46′36.9′′ San SalvadorES 52 Ahuachapan (Cerro San Lazaro) Lava 13◦56′25.6′′ 89◦46′36.9′′ San SalvadorES 53 Ahuachapan (Cerro San Lazaro) Lava 13◦56′15.7′′ 89◦46′47.6′′ San SalvadorES 54a Ahuachapan (Rıo Frıo) Ignimbrite 13◦59′40.5′′ 89◦49′32.8′′ BalsamoES 54b Ahuachapan (Rıo Frıo) Ignimbrite 13◦59′40.5′′ 89◦49′32.8′′ BalsamoES 55 Ahuachapan (Chalchuapa) Lava 14◦02′39.4′′ 89◦41′59.1′′ San SalvadorES 56 Ahuachapan (La Magdalena) Lava 14◦02′56.1′′ 89◦42′11.3′′ San SalvadorES 57 Ahuachapan (La Magdalena) Lava 14◦03′34.9′′ 89◦42′21.1′′ San SalvadorES 58 Ahuachapan (San Isidro) Lava 14◦05′57.9′′ 89◦42′24.6′′ BalsamoES 59 Ahuachapan (Turın) Lava 13◦57′50.9′′ 89◦45′33.4′′ San SalvadorES 60 Ahuachapan (Cerro San Lazaro) Lava 13◦57′14.8′′ 89◦47′15.5′′ San SalvadorES 61 Ahuachapan (Cuyanausul) Lava 13◦54′41.2′′ 89◦45′42.9′′ San SalvadorES 62 Concepcion de Ataco Dyke 13◦51′39.9′′ 89◦50′33.9′′ BalsamoES 63 Concepcion de Ataco Lava 13◦49′51.5′′ 89◦50′33.9′′ BalsamoES 64 Concepcion de Ataco Lava 13◦49′33.3′′ 89◦51′22.6′′ BalsamoES 65 Concepcion de Ataco Lava 13◦48′50.2′′ 89◦51′18.6′′ BalsamoES 66 Ahuachapan (Atiquizaia) Pumice 13◦57′51.9′′ 89◦44′28.0′′ San SalvadorES 67 Ciudad del Triunfo Lava 13◦33′00.1′′ 88◦21′57.4′′ BalsamoES 68 Ciudad del Triunfo Lava 13◦32′46.4′′ 88◦20′09.0′′ BalsamoES 69 Rıo Lempa Lava 13◦35′14.4′′ 88◦35′42.4′′ BalsamoES 70 Berlın (Lomo Los Capules) Lava 13◦34′05.1′′ 88◦31′10.6′′ Balsamo

All samples labeled ES were collected in El Salvador by the authors. The samples labeled BE and BER were taken in theBerlın area by other researchers but analyzed in the IGG-CNR laboratories.

interval from the local basement (ES 43) to the volcanic rocks preceding the formation of theCuayanausul Caldera (ES 35), and the post-caldera activity (ES 39). The results are reported inTable 2. The analytical procedure and results are described in detail in Appendix A.

The geochronological data obtained indicate that the exposed volcanic sequence in Berlınand Ahuachapan is of Quaternary age. Thus, in both areas the top of the Balsamo Formationwas formed during the Pleistocene, further supporting geological observations on a temporal

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Table 240Ar–39Ar isotope ages for Berlın and Ahuachapan rocks

Sample Locality Formation No. ofstepsa

Total gasage (ka)

40Ar* %b Plateauage (ka)c

No. of stepsplateaud

Isochronage (ka)

MSWDe (40Ar/36Ar)if Preferred

age (ka)g

Berlın areaES 8 Berlın-Mercedes Humana SS 11 62 ± 53 1.2 55 ± 36 11 65 ± 72 0.26 294.8 ± 9.9 ≤115ES 27 Berlın-Agua Caliente SS 8 50 ± 35 2.6 57 ± 35 8 106 ± 68 1.53 288 ± 20 ≤85ES 46 Puente Rıo Lempa Ba 8 414 ± 29 27.7 411 ± 17 8 414 ± 27 0.33 293.8 ± 8.3 415 ± 27

Ahuachapan areaES 39 Cuyanausul-Hoyo de

Cuajuste (post-caldera)SS 8 58 ± 27 1.4 55 ± 15 8 54 ± 21 0.85 295.6 ± 2.4 ≤75

ES 35 Cuyanausul (pre-caldera) SS 8 129 ± 31 4.6 125 ± 19 8 127 ± 20 0.49 295.1 ± 3.0 127 ± 20ES 43 Concepcion de Ataco Ba 8 248 ± 67 5.9 239 ± 43 8 272 ± 169 0.36 293 ± 17 248 ± 67

a See Appendix A for details of heating steps.b Percent of radiogenic 40Ar.c Average age resulting from gas released in different heating steps showing similar apparent age.d Number of heating steps used to calculate plateau age.e Mean square weighted deviate.f Initial 40Ar/36Ar ratio.g See Appendix A for criteria used. Errors are given at 2σ. SS, San Salvador Formation; Ba, Balsamo Formation.


Fig. 10. Total alkali vs. silica classification diagram (Le Maitre, 2002) for El Salvador volcanic rocks. Literature sam-ples from active volcanic front (gray dots) from Carr’s database (http://www-rci.rutgers.edu/∼carr/index.html). SS inparentheses indicates samples from Ahuachapan and Berlın geothermal areas belonging to the San Salvador Formation.

continuity between the Balsamo products and the youngest volcanism. It is also worth notingthat the current volcanic edifices in the Berlın and Ahuachapan-Concepcion de Ataco areas wereformed essentially during the last 100 ka.

5. Petrography and chemistry

5.1. Classification

Whole-rock major and trace-element analyses of 54 representative samples from Berlın,Ahuachapan and the El Salvador volcanic front are given in Table 3 (see Table 1 for samplelocation; rock samples are labeled ES, BE and BER). Analyses of 28 borehole samples fromgeothermal wells drilled in the Berlın and Ahuachapan areas are also reported (Table 4; boreholesamples from Berlın are labeled TR, and those from Ahuachapan are labeled AH and TO). Thedata are plotted in TAS (total alkali silica) and K2O versus SiO2 classification diagrams togetherwith data on volcanic rocks taken from the literature (Figs. 10 and 11). Overall, the studied rocksform a subalkaline association, ranging in composition from basalts to rhyolites with a predomi-nance of relatively less evolved rocks, although the association displays different degrees of alkalienrichment. In the K2O versus silica diagram most of the samples fall in the field of a typicalcalc-alkaline series; the few rocks exhibiting higher K2O content lie astride the boundary linebetween calc-alkaline and high-K calc-alkaline associations (Fig. 11). A wide dispersion of K2Ovalues is observed in the basaltic rocks where, at comparable silica abundance, K2O ranges from0.60% (ES 67, SiO2 = 51.6%) to 1.55% (BER 3, SiO2 = 51.4%) (Table 3). It is noteworthy that thetwo samples with lowest and highest K2O content belong to the Balsamo Formation. The mostevolved rocks exhibit similar potassium dispersions, and two distinct groups of rhyolites withhigh (≈5%) and low (≈2.2%) potassium abundance can be distinguished (Fig. 11).

http://www-rci.rutgers.edu/~carr/index.html


Table 3Major and trace elements of El Salvador volcanic rocksBalsamo Formation

ES 45(Berlın)

ES 69(Berlın)

BE 2(Berlın)

BER 3(Berlın)

ES 70(Berlın)

BER 1(Berlın)

ES 46(Berlın)

ES 43(Ahuac.)

ES 58(Ahuac.)

ES 50(Ahuac.)

SiO2 (wt.%) 47.63 50.03 50.68 51.19 58.14 60.49 60.82 50.03 52.43 57.42TiO2 1.07 1.12 1.22 1.14 0.99 1.03 0.70 0.91 1.09 0.59Al2O3 18.58 17.48 17.92 17.55 16.70 15.61 17.46 18.53 17.58 18.04Fe2O3 5.84 3.39 12.10 11.70 8.25 8.84 3.81 3.25 1.87 3.20FeO 6.05 7.50 0.60 2.72 6.70 6.66 3.40MnO 0.19 0.19 0.20 0.20 0.20 0.19 0.13 0.18 0.16 0.14MgO 4.31 3.91 4.27 3.99 2.08 1.23 1.61 4.56 4.96 3.01CaO 10.60 9.54 9.51 9.24 5.23 4.14 5.15 9.59 7.96 6.91Na2O 2.63 2.87 2.73 2.78 4.29 3.93 4.24 2.84 3.64 3.45K2O 0.67 1.14 1.39 1.54 2.21 2.79 1.71 1.20 1.28 1.43P2O5 0.24 0.34 0.32 0.34 0.41 0.44 0.20 0.21 0.42 0.18L.O.I. 0.82 1.28 −0.21 0.07 0.31 0.83 1.03 1.01 0.88 1.31

Total 98.63 98.79 100.13 99.74 99.41 99.52 99.58 99.01 98.93 99.08

Be (ppm) 0.55 1.11 0.82Sc 35 16 31V 320 281 84 103 253 181 123Cr 8 11 0 1 51 83 7Co 32 32 20 10 31 27 20Ni 10 5 0 3 30 40 8Cu 156 7 121Ga 18.2 16.8 17.9Rb 12.6 24 39 29.4 27.3 19 21Sr 688 466 427 483 592 558 494Y 19.6 32 42 25.1 23.4 28 16Zr 48.0 129 210 114.9 75.2 166 102Nb 1.21 4 5 2.65 1.50 6 2Cs 0.49 0.53 1.18Ba 397 698 1278 893 477 628 604La 7.5 16 24 13.3 10.4 21 8Ce 17.1 33 39 23.8 23.0 43 22Pr 2.70 4.18 3.49Nd 13.1 18.8 15.6Sm 3.44 4.56 3.92Eu 1.28 1.28 1.29Gd 3.67 4.47 4.05Tb 0.58 0.71 0.66Dy 3.39 4.17 3.89Ho 0.71 0.86 0.83Er 1.85 2.39 2.19Tm 0.28 0.38 0.33Yb 1.71 2.46 2.10Lu 0.24 0.38 0.31Hf 1.44 3.08 2.11Ta 0.07 0.20 0.10Tl 0.03 0.09 0.12Pb 3.3 4.9 3.5Th 0.67 1.95 1.64U 0.37 0.98 0.75


Table 3 (Continued)San Salvador Formation

ES 64(Ahuac.)

ES 65(Ahuac.)

ES 63(Ahuac.)

ES 54B(Ahuac.)

ES 62(Ahuac.)

ES 54(Ahuac.)

ES 30(Cerro Guazapa)

ES 31(Cerro Rico)

ES 29(Cerro Nejapa)

ES 67(Ciudad delTriunfo)

SiO2 (wt.%) 57.45 58.05 58.74 59.71 63.78 64.19 49.78 50.16 50.40 50.96TiO2 0.73 0.79 0.82 0.72 0.69 0.59 1.07 1.07 1.06 0.94Al2O3 17.61 17.50 17.54 17.29 16.25 16.37 19.35 17.28 18.66 21.04Fe2O3 4.00 4.11 3.49 3.04 0.99 2.96 2.01 2.63 2.51 4.20FeO 3.64 3.97 3.66 3.42 3.93 2.08 8.25 6.58 6.96 4.79MnO 0.14 0.11 0.12 0.15 0.15 0.11 0.20 0.15 0.16 0.16MgO 2.59 2.71 1.98 1.86 1.32 0.92 3.49 6.48 5.19 2.84CaO 6.72 6.07 5.62 5.17 3.57 3.06 9.59 8.77 8.77 9.90Na2O 3.71 3.75 4.35 4.67 5.17 5.25 2.82 2.91 3.02 3.20K2O 1.41 1.38 1.41 1.80 2.42 2.52 1.16 1.12 1.04 0.59P2O5 0.14 0.13 0.20 0.19 0.25 0.15 0.23 0.29 0.26 0.18L.O.I. 0.74 1.26 0.84 0.62 0.48 0.65 1.03 1.66 0.63 0.43

Total 98.88 99.83 98.77 98.64 99.00 98.85 98.98 99.10 98.66 99.23

Be (ppm)ScV 154 174 130 100 42 48 267Cr 7 4 1 4 0 0 7Co 21 22 17 16 10 8 22Ni 4 5 3 4 4 4 3CuGaRb 34 29 31 39 54 58 9Sr 347 360 349 348 285 293 621Y 31 24 38 30 37 32 19Zr 124 111 141 147 213 213 59Nb 3 3 4 3 3 6 3CsBa 624 602 707 880 978 1067 399La 7 10 12 12 18 18 5Ce 22 18 18 32 31 31 15PrNdSmEuGdTbDyHoErTmYbLuHfTaTlPbThU



ES 7(Cojutepeque)

ES 68(Ciudad delTriunfo)

ES 32(NuevaConcepcion)

ES 16(San FranciscoGotera)

ES 28(Berlın)

ES 23(Berlın)

ES 27(Berlın)

ES 8(Berlın)

ES 9(Berlın)

ES 10(Berlın)

SiO2 (wt.%) 51.35 52.51 52.80 70.45 54.15 55.99 56.58 57.97 61.89 63.90TiO2 0.85 0.99 1.06 0.16 0.76 0.75 0.71 0.90 0.59 0.66Al2O3 17.78 17.87 17.23 14.04 17.77 17.46 17.29 17.28 16.25 16.57Fe2O3 9.93 3.37 2.76 0.92 3.60 3.08 2.62 2.43 1.19 1.19FeO 6.92 5.67 0.78 5.32 5.14 4.93 5.07 3.66 3.50MnO 0.15 0.18 0.15 0.10 0.17 0.16 0.16 0.22 0.15 0.19MgO 4.26 3.91 4.14 0.35 3.91 3.49 3.04 2.43 1.46 1.32CaO 9.47 8.30 8.19 1.05 8.00 7.43 6.85 5.75 3.84 3.48Na2O 2.50 3.27 3.06 2.72 2.96 3.15 3.20 4.49 4.04 5.11K2O 1.42 0.83 1.57 5.18 1.56 1.87 2.05 1.56 2.31 2.32P2O5 0.19 0.22 0.45 0.04 0.19 0.17 0.19 0.34 0.17 0.22L.O.I. 0.70 0.90 1.74 4.14 0.45 0.62 1.13 0.85 3.51 0.75

Total 98.60 99.27 98.82 99.93 98.84 99.31 98.75 99.29 99.06 99.21

Be (ppm) 0.71 0.82 0.98 1.32Sc 30 25 24 17V 236 260 209 106 35Cr 10 6 4 2 1Co 30 30 27 42 5Ni 17 4 11 3 2Cu 82 221 18 9Ga 17.5 16.8 18.0 17.2Rb 30.3 17 30.2 28.5 44.3Sr 468 507 467 513 363Y 22.5 37 23.5 33.9 37.4Zr 103.7 82 109.3 108.1 153.6Nb 2.23 3 2.53 2.48 3.38Cs 0.55 0.73 1.54 2.35Ba 580 596 712 870 1135La 10.0 10 10.4 12.2 14.4Ce 21.8 18 23.2 27.5 32.8Pr 3.27 3.44 4.43 4.97Nd 14.6 15.4 20.8 22.4Sm 3.74 3.74 5.51 5.73Eu 0.94 0.91 1.57 1.38Gd 3.88 3.95 5.99 6.07Tb 0.62 0.64 0.95 1.00Dy 3.73 3.78 5.62 6.04Ho 0.78 0.80 1.20 1.31Er 2.16 2.27 3.28 3.68Tm 0.32 0.35 0.48 0.56Yb 2.01 2.28 3.03 3.63Lu 0.30 0.34 0.46 0.56Hf 2.88 3.04 3.32 4.54Ta 0.17 0.17 0.18 0.23Tl 0.10 0.16 0.08 0.26Pb 4.8 31.4 4.6 7.8Th 2.29 2.06 1.98 2.91U 1.12 1.08 1.04 1.56



BE 1(Berlın)

ES 42(Ahuac.)

ES 57(Ahuac.)

ES 41(Ahuac.)

ES 55(Ahuac.)

ES 51(Ahuac.)

ES 59(Ahuac.)

ES 35(Ahuac.)

ES 56(Ahuac.)

ES 39(Ahuac.)

ES 60(Ahuac.)

ES 52(Ahuac.)

SiO2 (wt.%) 65.36 47.72 48.41 49.62 49.64 55.39 55.74 57.31 58.31 58.92 59.13 59.95TiO2 0.70 1.08 1.31 0.92 1.25 0.84 0.84 0.85 0.74 0.77 0.70 0.62Al2O3 16.50 17.29 16.64 18.28 17.30 18.10 17.94 17.47 17.72 18.18 17.89 16.88Fe2O3 4.94 4.35 6.38 3.18 1.27 2.19 2.05 3.56 7.00 3.21 1.65 1.55FeO 7.74 3.05 6.76 7.56 6.09 5.98 3.81 0.33 3.29 4.97 4.65MnO 0.11 0.19 0.15 0.18 0.15 0.16 0.15 0.14 0.07 0.14 0.14 0.14MgO 1.22 6.91 8.27 4.61 7.32 3.22 3.29 3.03 1.74 2.15 2.41 2.16CaO 2.34 10.10 9.42 9.57 8.95 6.95 7.00 5.73 5.61 5.83 5.57 5.62Na2O 4.11 2.62 3.09 2.84 3.36 3.83 3.69 3.97 3.71 4.30 3.85 3.75K2O 2.36 0.60 1.08 1.17 1.20 1.48 1.61 1.92 1.57 2.01 2.13 2.25P2O5 0.16 0.15 0.32 0.21 0.33 0.21 0.19 0.22 0.15 0.26 0.17 0.19L.O.I. 2.62 0.65 0.65 0.75 0.59 0.42 0.90 1.07 1.92 0.48 1.40 1.45

Total 100.42 99.40 98.77 98.09 98.92 98.88 99.38 99.08 98.87 99.54 100.01 99.21

Be (ppm) 0.48Sc 45V 343 215 213 173 176 140 121 97Cr 26 292 227 2 2 7 1 1Co 46 38 35 24 24 24 18 17Ni 23 134 111 6 6 10 5 5Cu 109Ga 18.0Rb 6.0 20 19 34 37 35 50 52Sr 393 541 577 400 391 401 329 348Y 21.6 22 23 25 25 116 29 25Zr 56.1 142 150 130 132 98 166 160Nb 1.30 9 8 3 2 4 3 4Cs 0.17Ba 309 278 380 650 629 824 802 711La 5.6 11 13 13 15 89 12 12Ce 12.4 29 29 32 29 53 32 36Pr 2.03Nd 9.8Sm 2.89Eu 1.08Gd 3.38Tb 0.57Dy 3.53Ho 0.76Er 2.12Tm 0.31Yb 2.01Lu 0.27Hf 1.66Ta 0.10Tl 0.03Pb 2.1Th 0.98U 0.37



ES 53(Ahuac.)

ES 61(Ahuac.)

ES 66(Ahuac.)

ES 47(San Miguel)

ES 48(Conchagua)

ES 2(Boqueron)

ES 44(Izalco)

ES 1(Boqueron)

ES 4(Coatepeque)

ES 3(Boqueron)

ES 5(Ilopango)

ES 6(Ilopango)

SiO2 (wt.%) 60.30 65.09 69.47 49.74 51.19 52.30 52.60 53.37 54.09 57.71 68.59 70.83TiO2 0.62 0.51 0.13 0.95 0.83 1.12 1.06 1.23 0.82 1.17 0.35 0.33Al2O3 17.03 15.72 14.34 19.68 19.09 18.33 18.08 16.39 19.73 15.15 15.29 15.11Fe2O3 1.73 1.93 0.41 4.05 3.53 2.31 4.59 3.82 2.97 0.17 1.18 1.36FeO 4.62 2.67 1.82 6.10 6.49 7.90 4.99 7.03 4.88 9.27 1.87 1.29MnO 0.14 0.11 0.11 0.16 0.18 0.19 0.17 0.21 0.14 0.22 0.11 0.11MgO 2.17 1.26 0.36 4.14 4.58 3.68 3.64 3.26 2.51 2.16 0.84 0.69CaO 5.56 3.30 1.47 10.50 9.15 8.81 7.98 7.37 8.01 5.45 3.08 2.54Na2O 3.95 4.71 3.66 2.46 2.70 3.18 3.24 3.54 3.23 3.89 4.13 4.43K2O 2.02 2.76 4.79 0.72 0.78 1.18 1.83 1.42 1.77 2.22 2.26 2.27P2O5 0.20 0.12 0.04 0.18 0.18 0.23 0.31 0.28 0.24 0.38 0.12 0.11L.O.I. 0.65 0.55 3.91 0.48 0.64 0.53 0.63 0.52 0.66 0.55 2.07 0.51

Total 98.99 98.73 100.51 99.16 99.34 99.76 99.12 98.44 99.05 98.34 99.89 99.58

Be (ppm)ScV 97 66 8Cr 0 0 0Co 17 11 2Ni 3 5 5CuGaRb 48 65 166Sr 341 232 115Y 24 34 34Zr 156 218 198Nb 4 4 7CsBa 729 1046 782La 17 14 34Ce 36 32 64PrNdSmEuGdTbDyHoErTmYbLuHfTaTlPbThU

See Table 1 for description and exact location of samples collected. See Appendix A for description of analytical proce-dures. Trace element values in italics are XRF data. Ahuac., Ahuachapan. L.O.I., loss on ignition.


Table 4Major and trace elements in rock samples from Berlın and Ahuachapan boreholes

Berlın

Sample TR 2 TR 2 TR 2 TR 2 TR 8A TR 5B TR 17 TR 17

Depth (m) 1250 1350 1600 1650 1779 n.a. 2000 2417

SiO2 (wt.%) 51.20 51.16 52.86 57.38 47.79 45.66 58.88 41.59TiO2 0.97 0.93 0.88 0.56 0.95 0.98 0.86 1.12Al2O3 19.55 18.35 19.93 19.37 19.29 16.26 15.01 19.77Fe2O3T

a 9.29 8.08 8.60 5.98 9.89 11.53 8.53 13.81MnO 0.14 0.18 0.15 0.15 0.15 0.21 0.19 0.27MgO 3.23 3.10 3.89 2.37 4.54 3.33 2.88 4.65CaO 9.65 8.43 4.82 6.71 8.96 11.27 6.34 13.97Na2O 2.30 2.21 3.85 4.42 3.21 2.35 3.75 1.94K2O 0.96 1.33 1.70 0.89 1.38 0.80 0.23 0.03P2O5 0.23 0.38 0.17 0.13 0.12 0.20 0.30 0.28L.O.I. 2.81 5.70 2.98 2.04 3.61 6.54 2.58 3.93

Total 100.33 99.85 99.83 100.00 99.89 99.13 99.55 101.36

Nb (ppm)ZrYSrRbCeBaLaNiCrVCo

Berlın Ahuachapan

Sample TR 18 TR 2 TR 2 AH 34-2 AH 34-3 AH 8-1 TO 1 TO 1

Depth (m) 1053 1450 1502 n.a. n.a. n.a. 40 50

SiO2 (wt.%) 53.78 52.40 55.54 56.34 56.81 60.71 62.58 65.81TiO2 0.74 0.77 0.70 0.80 0.61 0.80 0.77 0.81Al2O3 18.15 16.71 16.82 16.84 17.74 16.72 16.83 19.08Fe2O3T

a 7.96 9.65 8.41 8.27 8.04 6.35 10.84 7.16MnO 0.17 0.18 0.18 0.15 0.15 0.11 0.12 0.09MgO 2.85 2.75 3.11 3.40 2.87 2.04 3.44 2.79CaO 7.46 7.78 6.55 6.54 5.32 3.18 2.46 1.88Na2O 4.36 2.73 2.84 2.32 3.47 1.73 1.62 0.90K2O 1.88 0.77 1.04 1.62 1.20 1.76 1.17 1.41P2O5 0.23 0.23 0.20 0.13 0.19 0.17 0.17 0.08L.O.I. 2.14 5.30 3.54 2.85 2.54 5.21 – –

Total 99.72 99.27 98.93 99.26 98.94 98.78 98.78 98.78

Nb (ppm) 7 7Zr 178 190Y 31 52Sr 200 175Rb 28 39


Table 4 (Continued)

Berlın Ahuachapan

Sample TR 18 TR 2 TR 2 AH 34-2 AH 34-3 AH 8-1 TO 1 TO 1

Depth (m) 1053 1450 1502 n.a. n.a. n.a. 40 50

Ce 33 37Ba 582 635La 13 17Ni 3 8Cr 0 3V 123 188Co 21 29

Ahuachapan

Sample TO 1 TO 1 TO 1 TO 1 TO 1 TO 1 TO 1 TO 1 TO 1

Depth (m) 55 225 230 375 380 400 441 700 1500

SiO2 (wt.%) 69.23 60.18 60.55 59.60 59.06 57.54 60.91 54.53 54.13TiO2 0.72 0.91 0.85 0.71 0.73 0.79 0.74 0.85 0.86Al2O3 19.81 16.94 16.79 17.86 16.90 18.23 17.29 16.51 16.75Fe2O3T

a 5.49 7.36 6.57 6.36 6.58 6.73 6.68 10.20 10.48MnO 0.06 0.16 0.14 0.13 0.15 0.13 0.13 0.18 0.20MgO 1.57 2.92 2.95 3.06 4.38 3.79 2.54 5.17 5.12CaO 1.28 6.18 6.04 7.01 7.02 7.56 5.49 8.44 8.34Na2O 0.99 3.83 3.82 3.46 3.09 3.25 3.99 2.78 2.84K2O 0.77 1.29 2.06 1.64 1.90 1.81 2.06 1.14 1.10P2O5 0.08 0.22 0.23 0.16 0.18 0.17 0.17 0.18 0.19L.O.I. – – – – – – – – –

Total 98.78 98.78 98.78 98.78 98.78 98.78 98.78 98.78 98.78

Nb (ppm) 6 5 3 2 2 3 3 2 3Zr 209 158 148 133 133 109 147 116 115Y 50 29 29 24 25 20 26 25 27Sr 162 392 381 403 385 440 347 407 406Rb 35 47 36 31 36 33 42 11 11Ce 39 35 29 32 25 18 28 18 18Ba 813 684 652 725 745 631 745 587 585La 14 17 12 13 15 10 15 10 10Ni 8 7 5 5 5 7 4 9 9Cr 7 47 5 12 6 4 2 16 18V 281 141 130 135 141 176 142 226 231Co 34 19 18 18 20 18 19 31 33

n.a.: not available.TR, Tronador; AH, Ahuachapan; TO, Tortuguero; L.O.I., loss on ignition. See Appendix A for description of analyticalprocedures. Trace element values in italics are XRF data.

a Fe2O3T is total iron expressed as Fe2O3.

In Fig. 12 the rocks are plotted in the AFM diagram, which shows the boundary line betweencalc-alkaline and tholeiitic series (Irvine and Baragar, 1971). The rocks display modest Fe enrich-ment; some slightly evolved samples show relatively high FeO*/MgO ratios falling in the tholeiiticfield, although their K2O content is not typical of tholeiitic arc lavas (Fig. 11). The Berlınrocks (from basaltic andesites to dacites) do not show any Fe enrichment. High values of theFeO*/MgO ratio and a mildly tholeiitic affinity have also been observed in recent volcanics of


Fig. 11. K2O vs. SiO2 classification diagram (after Peccerillo and Taylor, 1976) for Ahuachapan, Berlın and other volcanicrocks from the El Salvador volcanic front. SS in parentheses indicates samples from Ahuachapan and Berlın geothermalareas belonging to the San Salvador Formation.

Fig. 12. AFM (Alkali–FeO–MgO) diagram for Ahuachapan, Berlın and other volcanic rocks from El Salvador volcanicfront; boundary line between calc-alkaline and tholeiitic series taken from Irvine and Baragar (1971). SS in parenthesesindicates samples from Ahuachapan and Berlın geothermal areas belonging to the San Salvador Formation.


the Boqueron (Fairbrothers et al., 1978) and Chichontepeque Volcanoes (Rotolo and Castorina,1998).

5.2. Petrography

Basalts are relatively abundant in the Ahuachapan area and in the products of the BalsamoFormation, but are missing in the Berlın volcanic system where basaltic andesites are the mostprimitive rocks. Petrographically, the basalts and basaltic andesites have porphyritic to suba-phyric textures. The basalts usually exhibit a phenocryst assemblage made up of abundant augiticclinopyroxene (cpx) and plagioclase (pl; up to 2–3 cm in size), and smaller olivine (ol). Only sam-ple ES 55 (Ahuachapan) exhibits Mg-rich olivine (Fo86–77) as unique phenocryst, with augiticcpx present as microphenocryst and in the groundmass. The basalts typically have modal cpx > ol;the pyroxene is Mg-rich and displays Mg# [that is, Mg/(Mg + Fe2+) ratio] in the range 83–71,greater than the Mg# of olivine (Fo70–60), suggesting an earlier segregation of augite with respectto olivine. The plagioclases are generally zoned, with Ca-rich cores (An90–82) and rims, usu-ally reaching labradoritic composition (An ≈ 60). Ore minerals are present in the groundmass,together with pl + cpx ± ol ± glass, and are essentially represented by Ti–magnetite (mt); thereare rare needles of ilmenite (ilm). In the basaltic andesites, as well as the cpx + ol + pl assem-blage, orthopyroxene may be present, with compositions ranging from En65 to En50. Augiteis weakly zoned, with slight Fe-enrichment in the rims; again their Mg# is much greater thanthe Fo content of zoned olivine (Fo67–53). Plagioclases are usually labradoritic, and may showoscillatory zoning. The matrices are generally crystalline for the most part, with abundantglass in a few samples; the groundmass mineral phases include plagioclase, two pyroxenes andTi–magnetite.

Intermediate rocks are represented by two-pyroxene andesites with predominant plagioclase,ranging in composition from Ca-rich bytownite (cores) to labradorite (rims). Embayed olivinemay be present, mainly in low-silica andesites (e.g. ES 8, Berlın). Scattered opacitized amphiboleoccurs as megacrysts in sample ES 50 (Balsamo Formation). Opaques are represented mainly bymicrophenocrysts of Ti–magnetite and ilmenite in very minor amounts.

The evolved rocks consist of dacites and rhyolites, occurring either as lava flows or pyroclas-tics (ignimbrites and fall-out deposits). The dacites are slightly porphyritic, often with abundantglass in the matrix. The phenocryst assemblage is formed by andesinic plagioclase, ortho- andclinopyroxene with opx/cpx ratio > 1. Quartz and sanidine are sporadically present. In additionto quartz, feldspars (plagioclase and sanidine) and orthopyroxene, amphibole and/or biotite aresometimes found in the rhyolites.

5.3. Geothermal boreholes

In the Berlın area, the borehole samples came from the El Tronador wells (TR 2–TR 18).The samples exhibit widespread alteration with parageneses that include mainly quartz-calcite-clay minerals, along with diffuse albite, epidote, titanite, pyrite and, rarely, adularia. Zeolites,represented mainly by wairakite, are often present in veins together with calcite and phyllosili-cates.

In the Ahuachapan area, the Tortuguero field samples (well TO 1) show the occurrence ofa widespread zeolite alteration zone in the upper part of the well (down to ≈400 m depth): thezeolites are represented by heulandite-clinoptinolite and stilbite-epistilbite; wairakite also occurssporadically. The middle and lower part of this well is characterized by an association of quartz,


clay minerals (mainly chlorite) and variable amounts of calcite. Epidote is observed at depth onlysporadically in the calc-alumina-silicate zone. By contrast, the rocks from other Ahuachapanboreholes (wells AH) present extensive epidote-wairakite-actinolite alteration, as also observedin the nearby Chipilapa wells (Aumento et al., 1982).

5.4. Major element chemistry

The data points display a good negative correlation of CaO, FeOtot, MgO and, to a lesserextent, TiO2 with silica (Fig. 13). Positive correlations are observed for Na2O and K2O, althougha relative dispersion is present in the alkali abundances of the less evolved samples. A negativecorrelation between MgO and silica is observed, plotting as a convex curve. The most primitiveBalsamo and Ahuachapan basalts (MgO 6–9%) plot along a steeper curve, whereas, from themore evolved basalts (MgO < 5%) up to the most SiO2-rich samples, the correlation flattens outand is almost linear. The Al2O3–SiO2 plot shows a wide dispersion for the basaltic rocks; at thesame silica content (≈50%), alumina ranges from 16.6 to 21.0%, whereas a well-defined negativetrend appears in the basaltic andesites-rhyolites range.

As a whole, major element variations are compatible with a crystal fractionation processinvolving the mineral phases forming the observed parageneses. However, plagioclase and maficminerals may play different roles during the fractionation process, as evidenced by the Al2O3 ver-sus MgO diagram (Fig. 14). In the most basic rocks, MgO and alumina are negatively correlated (upto ≈5% MgO), whilst in the more evolved products these two elements coherently decrease. Thesevariations suggest that the first stages of crystallization involved mainly mafic phases (clinopy-roxene ± olivine), whereas plagioclase becomes the dominant removed phase during successivestages of rock evolution. At about 3–5% MgO, the maximum Al2O3 contents indicate the presenceof plagioclase-cumulus phenomena, as evidenced by the strongly plagioclase-phyric textures.

Borehole samples from Berlın and Ahuachapan exhibit basically the same geochemical andpetrographic characters and largely overlap the variation range of the outcropping rocks (Fig. 13).The borehole samples from Berlın are generally less evolved than the exposed rocks, being mainlybasalts and basaltic andesites. The samples most affected by hydrothermal alteration are depletedin Na2O and enriched in Al2O3 (Fig. 13).

5.5. Trace elements and isotope geochemistry

Trace element data are presented in Table 3 and selected trace elements versus silica areplotted in Fig. 15. The abundances of compatible elements (e.g. V, Fig. 15; Co, Ni and Cr,not shown) are generally low, with the exception of the most primitive samples from theAhuachapan area, e.g. ES 55 and ES 57, with 100 < Ni < 150 ppm. Taking into account allthe samples, Ni and Cr display a negative hyperbolic correlation with silica, whereas V andCo exhibit a good negative linear correlation with r2 = 0.88 and 0.75, respectively. The largeion lithophile elements (LILE) show a generally positive correlation with an increase in evo-lution degree; a noteworthy exception is Sr (Fig. 15), which decreases with increasing silica.High field-strength elements (HFSE: Zr, Fig. 15; Nb and Y, not shown) are positively correlatedwith silica, although the data points are more scattered. Overall, the scattering of trace elementabundances is generally greater in the most primitive rocks, matching the behaviour of majorelements.

Trace-element distribution in the primordial mantle-normalized, multi-element patterns(Fig. 16) shows the typical features of volcanic arc rocks, with high LILE/HFSE ratios, markedly


Fig. 13. Major element (anhydrous) variation diagrams plotted against SiO2 for Ahuachapan, Berlın and other volcanicrocks from El Salvador volcanic front (all units in wt.%). SS in parentheses indicates samples from Ahuachapan andBerlın geothermal areas belonging to the San Salvador Formation.


Fig. 14. Al2O3 vs. MgO variation diagram for Ahuachapan, Berlın and other volcanic rocks from El Salvador volcanicfront. Thick grey lines delineate the path of crystal fractionation starting from basalt ES 42 to dacites, through high-Albasalts and andesites. ol, olivine; px, pyroxene; pl, plagioclase. The negative trend results from the removal of a solidassemblage in which mafic minerals plagioclase; conversely, plagioclase dominates fractionation in the final part ofthe trend, producing a positive correlation. SS in parentheses indicates samples from Ahuachapan and Berlın geothermalareas belonging to the San Salvador Formation.

negative Nb, Ta, Ti and, to a lesser extent P, anomalies along with positive spikes of Ba, K and Sr;the latter are particularly evident in the Ahuachapan rocks (e.g. ES 42), where negative Hf andZr anomalies are also observed.

Rare earth elements (REE) are variably fractionated, with La/YbN ranging from ≈1.5 to 4. TheBerlın samples in particular show higher REE contents, in agreement with their higher degreeof evolution and patterns of a similar trend; a significant Eu negative anomaly is also observed(Eu/Eu* ≈ 0.7–0.8; Fig. 16). By contrast, the Balsamo samples from Berlın show a large variabilityof LREE contents and patterns with variable slope. Basalt ES 42 from Ahuachapan is characterizedby the lowest LREE/HREE ratio measured in the data set. A significant Ce negative anomaly isobserved for the whole analyzed samples; this could be related with the involvement a slab-derivedcomponent depleted in cerium.

Strontium and neodymium isotope ratios are reported in Table 5 and Fig. 17: Nd isotope ratiosare close to a value of 0.5130 and show very small variations, between 0.51297 and 0.51301. Onthe other hand, the Sr isotopic composition ranges between 0.70357 and 0.70390. In the classicSr–Nd diagram there is a shift toward higher Sr isotopic composition with respect to the mantlearray (inset of Fig. 17).

The 87Sr/86Sr and 143Nd/144Nd ratios in the Berlın borehole samples are very close to thoseof surface lavas, suggesting that water–rock interaction processes did not significantly modifytheir isotope features. The isotope composition of the drilled and outcropping rocks does notchange from the surface down to bottomhole depth (2500 m), indicating a marked homogeneity


Fig. 15. Trace element (Rb, Sr, Zr and V) variation diagrams plotted against wt.% SiO2 (anhydrous) for Ahuachapan,Berlın and other volcanic rocks from the El Salvador volcanic front. Symbols as in Fig. 14.

for the whole volcanic sequence, including its local volcanic basement. Ahuachapan boreholeshave higher 87Sr/86Sr values than those measured in outcropping rocks; it is not clear from thesedata alone whether this difference is due to water–rock alteration processes or to intrinsic isotopicheterogeneities between basement and more recent rocks.

5.6. Estimates of magmatic intensive parameters

Different geothermometers were used to estimate crystallization temperatures in the stud-ied rocks. In basalts, where clinopyroxene phenocrysts coexist with olivine, the olivine–augiteFe–Mg-exchange geothermometer of Loucks (1996) was applied; the standard error in this caseis ±6 ◦C. Just one basalt sample (ES 55, Ahuachapan) contains olivine phenocrysts only: thecrystallization temperature in this rock was thus assessed through the Fe–Mg partition betweenolivine and whole rock (Beattie et al., 1991).

In more evolved rocks, where two pyroxenes occur, the temperature of crystallization of thephenocryst assemblage was calculated by equilibrium enstatite–augite pairs (Wells, 1977). Oxy-gen fugacity was calculated by the QUILF code (Andersen et al., 1993, updated in 1995). Most of


Fig. 16. Primitive Mantle (PM)-normalized trace-element variation diagrams (left side) and chondrite-normalized RareEarth Element (REE) diagrams for volcanic rocks from Ahuachapan and Berlın geothermal areas. Normalization valuesfrom McDonough and Sun (1995). Bas And, Basaltic Andesite.

the rocks display Ti–magnetite as microphenocrysts; ilmenite is generally confined to the ground-mass as needles that are too small to be analyzed with any reliability. In only three samples itwas possible to use ilmenite–magnetite pairs (ES 55, ES 46 and ES 10); for the other rocks thetemperature was constrained by mafic assemblage equilibria.


Table 5Sr and Nd isotope data of the Berlın and Ahuachapan geothermal areas

Sample 87Sr/86Sr 2σ 143Nd/144Nd 2σ

BerlınES 7 (Balsamo) 0.703850 ± 6 0.512999 ± 7ES 45 (Balsamo) 0.703805 ± 7 0.512999 ± 6ES 10 0.703867 ± 7 0.512997 ± 7ES 28 0.703898 ± 10 0.513002 ± 8

AhuachapanES 50 (Balsamo) 0.703582 ± 8 0.512997 ± 8ES 42 0.703666 ± 8 0.513013 ± 8ES 61 0.703574 ± 7 0.512997 ± 10

Borehole BerlınTR 8A 1779 0.703915 ± 17 0.512999 ± 10TR 17 2000 0.703872 ± 12 0.512988 ± 9TR 17 2417 0.703881 ± 7 0.512990 ± 8

Borehole AhuachapanTO 1 700 0.703778 ± 7 0.512986 ± 11TO 1 1500 0.703829 ± 10 0.512993 ± 12

The bulk samples were leached with hot HCl 6.6N. Sr and Nd isotope compositions were determined using a Finnigan MAT262V multi-collector mass spectrometer following separation of Sr and Nd using conventional ion-exchange procedures.Measured 87Sr/86Sr ratios have been normalized to 86Sr/88Sr = 0.1194. During collection of isotopic data, 14 replicateanalyses of SRM-NIST 987 (SrCO3) standard gave an average value of 0.710243 ± 0.000011 (2 standard deviations).Measured 143Nd/144Nd ratios have been normalized to 146Nd/144Nd = 0.7219. Replicate analyses of La Jolla standardgave an average 143Nd/144Nd of 0.511847 ± 0.000007 (2 standard deviations).

Crystallization pressure (i.e. depth) was estimated using the thermobarometer of Putirka et al.(2003), which assesses temperature and pressure on the basis of equilibrium between clinopy-roxene and liquid. Equilibrium is evaluated assuming a value of the Fe–Mg partition coefficientbetween cpx and liquid, K

cpx-lqD (Fe–Mg) = 0.27 (Kcpx-lq

D (Fe–Mg)) = (XcpxFe /X

lqFe)(Xlq

Mg/XcpxMg),

where XFe and XMg are cation fractions of Fe and Mg in the superscripted phases, respectively;the liquid equilibrium composition was evaluated by readjusting the whole-rock chemistry, addingequilibrium olivine and clinopyroxene in cotectic proportions to reach the pre-defined KD value.This calculation was performed by a code described in Armienti et al. (in press). The results arereported in Table 6. The temperature of crystallization of mafic phenocrysts in basalts rangesbetween 1200 and 1050 ◦C irrespective of the geothermometer applied. Overall, a negative cor-relation is observed between silica content and temperature, shifting from basalts to dacites. Thetemperatures of the groundmasses are generally 100–150 ◦C lower than in the phenocrysts, exceptfor sample ES 8 (andesite from Berlın) for which an anomalous high groundmass temperaturewas obtained; in this sample the mt–ilm equilibrium was probably not preserved.

The values of fO2 are usually between +1 and +2 ΔFMQ; no significant variations areobserved with respect to the degree of evolution. The clinopyroxene crystallization depth variesfrom shallow level (pressure < 0.1 GPa) to levels corresponding to the intermediate crust (pres-sure ≈ 0.5 GPa). It is worth noting that the variations detected in basalt ES 42 (Ahuachapan), oneof the most primitive rocks of the studied data set, show a good P–T correlation (r2 = 0.97). Thecontinuous variations of pressure between 0.49 and 0.28 GPa suggest an uninterrupted segregationof pyroxene during magma ascent, starting from the middle crust where magma ponding probablyoccurred for the first time.


Fig. 17. 143N144Nd vs. 87Sr/86Sr diagram for volcanic rocks from Ahuachapan and Berlın geothermal areas.The inset reports the “Mantle Array” and literature data for Central America volcanics (http://www.rci.rutgers.edu/∼carr/index.html). Dashed field encloses samples from Berlın area. SS in parentheses indicates samplesfrom Ahuachapan and Berlın geothermal areas belonging to the San Salvador Formation. DMM, depleted MORB mantle;BSE, bulk silicate earth.

Estimates of the water content of the magmas were constrained by applying two procedures tothe more primitive and evolved rocks. In the basalts and basaltic andesites, the sequence of crystalprecipitation, suggested from petrographic observations, is cpx ⇒ mt ⇒ ol ⇒ plg; this anhydrousassemblage and the crystallization sequence can be reproduced at a pressure between 0.3 and0.5 GPa, with an H2O content of around 3% if we consider the phase diagram for the basalt ES42, produced by the MELT code (Ghiorso et al., 2002).

In the dacitic rocks, water abundance was assessed by the experimental geohygrometer ofMerzbacher and Eggler (1984), following the calculations of projection parameters proposed bySchmitt and De Silva (2000). The results are plotted in Fig. 18, where the data points generallyfall along the line of 4% H2O content. Only three glass-rich dacites (ES 54 and ES 62 from theBalsamo Formation, and ES 10 from Berlın) fall outside the main trend, showing higher watercontents, and within the field of amphibole stability, in spite of the anhydrous paragenesis of thephenocrysts.

The basalt-dacite transition through crystal fractionation implies the removal of about 80%of solids (see below). The amount of water in the dacitic magma should therefore be very

http://www.rci.rutgers.edu/~carr/index.html

http://www.rci.rutgers.edu/~carr/index.html


Table 6Estimate of intensive parameters of Ahuachapan and Berlın rocks

Sample Rock type T (◦C) T (◦C)cpx/lq

P (GPa)cpx/lq

TQ (◦C) log fO2 ΔFMQ

ES 55Basalt 1200 (ol–lq) −7 ± 0.8 +1.3Groundmass 981 0.1 1025 (a) −9.8 ± 3.5 +0.7

ES 42 Basalt 1206–1092 (ol–cpx)

1111 0.49

−7.2 +2.4

1101 0.441098 0.411092 0.361083 0.341078 0.311068 0.28

ES 43 Basalt 1166–1123 (ol–cpx)

1076 0.07

−7.06 +2.11068 0.141059 0.02

BER 3 Basalt 1123 0.29

ES 28Basaltic andesite 1179–1040 (ol–cpx) −8.5 ± 0.3 +1.8Groundmass 963 ± 41 −9.5 ± 0.7 +2.0

ES 50 Andesite 1041–1022 (ol–cpx)

ES 8Andesite 1077–980 (ol–cpx)Groundmass 1205 ± 10 −7.23 +0.96

ES 46Andesite 1055–956 (opx–cpx)Groundmass 773 ± 84 (a) −13.7 ± 1 +1.4

ES 10Dacite 1214 (opx–cpx)Groundmass 823 ± 35 (a) −13.5 ± 0.4 +0.7

Notes: ol–lq after Beattie et al. (1991); ol–cpx after Loucks (1996); opx–cpx after Wells (1977); cpx–lq after Putirka etal. (2003). TQ indicates estimates using the QUILF code (Andersen et al., 1993). (a) Ilmenite–magnetite pairs. ΔFMQ isoxygen fugacity expressed as the difference between estimated log fO2 and log fO2 of fayalite–magnetite–quartz bufferat TQ. ol, olivine; lq, liquid; cpx, clinopyroxene; opx, orthopyroxene.

high (>10%) if the process occurred in a closed system. The estimated average H2O contentof around 4% suggests that the magmas suffered a significant water loss during magmatic differ-entiation.

6. Discussion

6.1. Magmatic constraints

The volcanic products erupted along the El Salvador volcanic front constitute a typical calc-alkaline association, ranging in composition from basalts to rhyolites. A small-scale geochemicalvariability is observed in the most primitive samples, where two groups of rocks can be distin-guished on the basis of differences in 87Sr/86Sr (≈0.7036 and ≈0.70385, respectively), along withdistinct LILE contents and LILE/HFSE ratios (Table 5; Fig. 19).

In the Berlın volcanic system all the samples belong to the most 87Sr-enriched group;Ahuachapan area samples are less 87Sr-enriched, and, despite their limited number, are more


Fig. 18. Whole-rock andesites and dacites plotted in the plagioclase (pl)–orthopyroxene (opx)–quartz + orthoclase (Q + or)system using the Schmitt and De Silva (2000) projection scheme. Lines represent pl–opx cotectic lines for water contentof 2 and 4 wt.%. Grey area is the field of amphibole stability (Merzbacher and Eggler, 1984).

variable. In contrast to the Sr isotopes, the 143Nd/144Nd ratios have almost constant values(0.51299–0.51301).

The isotopic and geochemical variability observed in the studied rocks could mirror sourceheterogeneity and/or magma interaction with shallow crustal material. The lack of any cor-

Fig. 19. 87Sr/86Sr vs. SiO2 (a) and Ba/Zr (b) for volcanic rocks from Ahuachapan and Berlın geothermal areas. Dashedfields enclose samples from Berlın area.


relation between evolution degree (e.g. SiO2 content; Fig. 19) and 87Sr/86Sr ratios, coupledwith the occurrence of the relatively high isotope and geochemical variability in the basalticrocks, strongly suggest that contamination processes played a minor role in the petrogene-sis of this volcanic association. Hence, the geochemistry of the basaltic rocks is consideredto be linked to the geochemical variability of the source. It is generally acknowledged that,in a subduction context, mantle characteristics reflect the addition of subduction-related com-ponents, different in their amounts and/or geochemical signatures (e.g. Tatsumi and Eggins,1995). Many authors have described this mechanism for the Central America front lavas usingtrace elements and isotope features (e.g. Carr et al., 2004; Eiler et al., 2005). Consideringour data set, the subduction component cannot be strictly constrained. However, the relativeenrichment in 87Sr and fluid mobile elements such as Rb, Ba, Cs, Sr and, to a lesser extent,LREEs, does suggest that the metasomatic agent was a hydrous fluid derived from the dehydra-tion of the subducting oceanic lithosphere. Furthermore, the high uniform 143Nd/144Nd ratios(≈0.51300) indicate that the magma source, before the addition of the subduction compo-nent, had an Sm/Nd ratio close to the value observed in Mid-Ocean Ridge Basalts (MORB)sources.

The variation of major and trace elements suggests that the main process occurring in therock association was fractional crystallization, without significant interaction with crustal mate-rial. In order to evaluate this process we modeled, by major element mass balance calculations(Stormer and Nicholls, 1978), the transition basalt-andesite-dacite and basaltic andesite-dacitein the Ahuachapan and Berlın rocks, respectively. The results are reported in Table 7; the lowvalues of the sum of square residuals between calculated and observed compositions suggestthat removal of gabbroic assemblages is a viable mechanism to produce the studied rocks. Tofurther check the model, we applied the Rayleigh equation for trace element crystal fractiona-tion, using the amount and modal composition of removed solid as derived from major elementmass balance calculations. The difference between calculated and measured contents is generallylower than 25% (Table 8), suggesting that the model is consistent. In some cases, however, thediscrepancies reached 50%, mainly for compatible elements, which may be a consequence of theuncertainty with regard to their solid/liquid partition coefficients. In the transition basalt-andesitein Ahuachapan, the strong enrichment in Rb of the andesite (ES 60) cannot be explained by themodel.

6.2. Tectonic constraints

The structural analyses carried out in our study lend support to the hypothesis that the activedeformation resulting from oblique subduction of the Cocos Plate under the Caribbean Plate isaccommodated within continental El Salvador by strike-slip movement along the E–W trending ElSalvador Fault Zone (ESFZ). In particular, the kinematics and deformation rates of the major faultsystem, as well as the absence of large-scale compressional deformation, corroborate the modelof strain partitioning between trench-orthogonal compression and trench-parallel transcurrentdeformation at the Cocos-Caribbean plate boundary (Harlow and White, 1985; DeMets, 2001;Corti et al., 2005a).

Analysis of the regional architecture of the ESFZ reveals the discontinuous nature of this faultsystem, with main segments separated by pull-apart structures (Fig. 20) (Corti et al., 2005a). Themain fault segments contain active volcanism, which is more or less absent in the intervening pull-apart basins. This coincidence between the segmentation of the ESFZ and that of the volcanic

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Table 7Differentiation fractionation model for Ahuachapan and Berlın rocks

Starting composition SiO2 (%) Total removedsolids (%)

SSR cpx ol mt pl opx Final composition SiO2 (%)

AhuachapanES 42 Basalt 48.32 −69.9 0.42 −16.3 −10.8 −6.3 −36.5 – ES 60 Andesite 59.96ES 60 Andesite 59.96 −30.4 0.34 −0.6 – −2.9 −21.5 −5.4 ES 61 Dacite 66.3

BerlınES 28 Basaltic andesite 55.24 −47.7 0.05 −11.2 −4.6 −4.3 −27.7 – ES 10 Dacite 64.98

SSR, sum of square residuals; cpx, clinopyroxene; ol, olivine; mt, magnetite; opx, orthopyroxene.

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Table 8Trace element fractionation model for Ahuachapan and Berlın rocks

Berlın Ahuachapan Ahuachapan

ES 28 Bulk KD Model ES 10 ES 42 Bulk KD Model ES 60 ES 60 Bulk KD Model ES 61

Sc (ppm) 25 2.61 8.6 17 45 1.55 23.1 – – 0.71 – –V 209 3.51 40 35 343 1.86 122 121 121 3.04 59 66Cr 3.7 3.39 0.8 0.7 25.8 4.02 0.7 1.0 1.0 2.90 0.5 –Co 27 3.16 6.6 4.8 46 1.78 18.1 18.0 18.0 2.74 9.7 11Ni 10.9 3.64 1.9 1.8 23.2 2.17 5.7 5.0 5.0 1.44 4.3 5Rb 30.2 0.13 53.3 44.3 6.0 0.10 17.8 50.0 50.0 0.15 67.8 65Sr 467 1.08 443 363 393 1.08 357 329 329 2.03 227 232Y 23.5 0.14 41.2 37.4 21.6 0.60 35.1 29.0 29.0 0.17 39.0 34Zr 109 0.15 191 154 56 0.15 156 166 166 0.05 233 218Nb 2.53 0.69 3.10 3.38 1.30 0.23 3.28 3.00 3.00 0.03 4.25 4Ba 712 0.14 1249 1135 309 0.21 803 802 802 0.27 1040 1046La 10.4 0.27 16.7 14.4 5.6 0.25 13.8 12.0 12.0 0.25 15.7 14Ce 23.2 0.25 37.9 32.8 12.4 0.28 29.4 32.0 32.0 0.26 41.6 32

Bulk KD for rocks derived from Kd of mineral phases; Kd values for minerals involved in the crystal fractionation process are taken from http://earthref.org. The amount ofremoved phases corresponds to the results of the model reported in Table 7.

http://earthref.org/


Fig. 20. Schematic hypothetical regional segmentation of El Salvador Fault Zone (ESFZ). Grey elliptic areas indicate thesegments of the volcanic front; large black dots represent some of the main active volcanoes located at the boundaries ofthe volcanic segments close to the pull-apart basins. Dashed lines represent main segments of the ESFZ. A, Ahuachapan;B, Berlın.

front suggests a genetic relationship between the two processes of faulting (lithospheric-scaledeformation) and volcanism; the strike-slip fault system in particular acts as a preferential pathwayfor magma to ascend to the surface. However, more detailed analyses are needed to better definethese relationships and to explain the absence of volcanic activity within pull-apart basins, whereextensional tectonics is normally considered to favor volcanism.

One important finding of our study is the difference in structural setting between the Berlın andAhuachapan geothermal fields. The first field is located in correspondence to an important segmentof the ESFZ. The major and minor structures affecting the Berlın area are presumably related to atranscurrent stress field, with a NNW-directed axis of maximum compression and an ENE-directedaxis of maximum extension. Interaction of this stress field with the volcanic edifices has given riseto a local dominance of NNW- to NW-directed extensional structures within the Berlın geothermalarea. Conversely, the Ahuachapan area is structurally less straightforward, being characterisedby different groups of structures that cannot be so easily attributed to a dominant regional stressfield. In this case, the complex fault arrangement possibly results from the interaction betweenthree stress fields. Indeed, the E–W and NW–SE trending faults may be related to the transcurrentstress field associated with the El Salvador Fault Zone. The N–S dilatational joints and normalfaults are presumably the result of the intraplate extension of the Caribbean Plate, whereas theinfluence of the Motagua fault system is possibly expressed by the NE–SW directed normal faultswest of Ahuachapan.

7. Conclusions

The tectonic and petrologic study of the El Salvador active volcanic front and its local vol-canic basement has evidenced a strong link between active deformation and volcanic activity. Incontinental El Salvador, strike-slip deformation gives rise to a major dextral transcurrent fault sys-tem, the El Salvador Fault Zone, which is laterally discontinuous, being subdivided into differentmajor en-echelon segments that partially overlap to form pull-apart structures. The present-dayvolcanic activity is more or less confined to the fault segments and is practically non-existent inthe intervening pull-apart basins.

The studied volcanic rocks were erupted during the Quaternary. The most recent composite vol-canoes grew on a local volcanic basement with no recognizable temporal gap: indeed, the top of theBalsamo Formation reaches the upper Pleistocene. No significant geochemical or petrological dif-ferences were found between the volcanics of the Balsamo Formation and the most recent products.


Our study reveals the important volcano-structural and petrologic differences between theeastern (Berlın) and western (Ahuachapan) areas of El Salvador. The marked geochemical andisotopic homogeneity found in Berlın, where basalts are lacking, points to the presence of asingle magmatic system. By contrast, the Ahuachapan rocks show significant variations in bothSr isotopes and the LILE/HFSE ratios. The multiple volcanic centres characterizing this area werefed by magma batches with different Sr isotopes and trace-element contents, inherited from theirmantle source.

The relative abundance of basaltic products reveals that in Ahuachapan most of the risingmagmas were not intercepted by the shallow reservoirs linked to the small volcanic centres thaterupted more evolved products. The geochemical homogeneity in the Berlın products reflects the“simple” tectonic framework of the area whose active deformation is controlled by the regionaltranscurrent stress field, resulting in the development of systems of right-lateral E–W-trendingstrike-slip faults and associated secondary Riedel and tensional shears. Conversely, the structuralsetting of the Ahuachapan area is more complex, characterized by different fault systems that mayreflect an interaction among the ESFZ-dominated stress field, the sinistral Motagua transform fault,and the active E–W extension responsible for the intraplate deformation of the Caribbean Plate; themore complex and pervasive fault pattern of the Ahuachapan area facilitated the development ofmultiple smaller magma chambers, and the creation of pathways for the ascent of basaltic magma.

Considering the different settings of the two areas, further geothermal exploration at Berlın andAhuachapan may take different approaches. At Berlın, for example, prospecting should accountfor the existence of a regional geothermal anomaly magnified by the presence of a shallow magmachamber. On the other hand, at Ahuachapan the geothermal system appears more complex, andlinked to local magma ponds that are probably limited in volume, as inferred by the small volcaniccentres. This volcanological framework, coupled with the widespread and complex faulting pat-tern, has probably led to the occurrence of small-scale shallow geothermal anomalies. Overall, theresults of this study show that a combined tectonic and petrological–geochemical approach mayprove an important tool for selecting potential areas of geothermal interest, before proceedingwith further more expensive geothermal exploration work.

Acknowledgments

The authors would like to thank ENEL and LaGeo for their support during the fieldwork,laboratory investigations and scientific discussions. They are indebted to Armando Ceccarelli andMarvyn O. Garcia for the stimulating discussions in the field. Reviews by R.M. Prol-Ledesmaand M.J. Carr, as well as editorial comments by M.J. Lippmann, have significantly improved thequality of the manuscript.

Appendix A. 40Ar–39Ar dating of selected samples

A.1. Experimental procedures

Six volcanic rock samples (ES 8, ES 27, ES 35, ES 39, ES 43, and ES 46) were selected for40Ar–39Ar incremental laser-heating analyses. The groundmass was concentrated from the 250to 300 �m grain size using standard separation techniques and purified by hand-picking undera binocular microscope. The groundmass separates were further cleaned by acid leaching in anultrasonic bath (1 h at ∼50 ◦C in 3.5N HCl and 1N HNO3). The final product was wrapped inaluminum foil and irradiated for 2 h in the TRIGA reactor (University of Pavia, Italy) along


with the dating standard FCT-3 biotite (age 27.95 Ma; Baksi et al., 1996). After irradiation, thesamples (∼40 mg) were loaded into the 9-mm diameter holes of a copper holder, placed in anultra high-vacuum laser port and baked overnight at ∼200 ◦C.

Incremental laser-heating experiments were carried out using an infrared laser beam generatedby a diode-pumped CW Nd–YAG laser (maximum power 15 W), which was defocussed to ∼2 mmspot. The laser beam was homogenized by passing it through a lens that produces a flat-powerdistribution. Homogenous heating was achieved by slowly rastering the laser beam by a computer-controlled x–y stage. The heating steps were carried out at increasing laser powers until completemelting occurred. The gas extracted was purified by two SAES AP10 getters (at 400 ◦C) and oneSAES GP50 getter (at room temperature) and, after 15 min (including ∼6 min of laser heating),equilibrated via automated valves into an MAP215-50 noble gas mass-spectrometer fitted witha Balzers SEV 217 secondary electron multiplier. The data (Table A1) were corrected for post-irradiation decay, mass discrimination effects, isotopes derived from interfering neutron reactionsand blanks. Errors are given at 2σ.

A.2. Results

Data from the six samples are presented in the form of age spectra in Fig. A1 and the com-plete analytical results are listed in Table A1. Analysis of these samples was extremely difficultbecause of their young age, low potassium content and the material analyzed (groundmass), whichresulted in severe contamination by atmospheric argon (radiogenic argon �10% in most cases; seeTable A1). Results from regression calculations in an isochron plot (36Ar/40Ar versus 39Ar/40Ardiagram), and the total gas, plateau and preferred ages are summarized in Table 2.

In samples ES 8, ES 27, and ES 39, the ages measured at each heating step are close to or overlapwith the zero age (Table A1, Fig. A1). For these samples, the data only allow us to set a lower (older)age limit. From Table 2 it is evident that samples ES 8, ES 27 and ES 39 should be conservativelyconsidered coeval and younger than ∼100 ka, whereas the remaining samples are older.

Sample ES 35 yielded a weighted mean (plateau age) of 125 ± 19 ka, which is in agreementwith that of total gas (129 ± 31 ka). In an isochron plot, all eight heating steps gave a lineararray with an intercept age of 127 ± 20 ka and an initial 40Ar/36Ar ratio of 295.1 ± 3.0, whichis indistinguishable from that of modern atmospheric argon (295.5). The 127 ± 20 ka estimate isconsidered the best for sample ES 35.

Fig. A1. Age release spectra of groundmass samples.

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Table A140Ar–39Ar laser step-heating data

Step 36Ar(atm)37Ar(Ca)

38Ar(Cl)39Ar(K)

40Ar(Tot) Age (ka) ±2σ 40Ar* % 39Ar(K) % Ca/K ±2σ

Groundmass ES 8 (37.4 mg), J = 0.0001849 ± 0.00000231 0.00751 0.03016 0.00234 0.04054 2.264 380 11677 2.0 0.2 1.40 0.392 0.1262 0.8402 0.07276 1.153 37.42 37 421 0.3 5.0 1.38 0.053 0.1626 1.713 0.11935 1.980 48.12 11 180 0.1 8.6 1.63 0.054 0.1875 3.291 0.1669 3.095 56.14 79 164 1.3 13.4 2.01 0.065 0.1569 4.816 0.1936 3.518 47.65 121 166 2.7 15.2 2.58 0.076 0.1003 4.052 0.1329 2.451 29.95 41 98 1.0 10.6 3.12 0.097 0.07063 3.297 0.08571 1.700 21.37 97 144 2.3 7.3 3.66 0.108 0.06291 3.538 0.07979 1.517 18.67 16 96 0.4 6.6 4.40 0.139 0.06979 3.857 0.1579 1.830 21.02 72 97 1.9 7.9 3.98 0.1110 0.1047 6.232 0.2775 3.035 31.23 33 112 0.9 13.1 3.87 0.1111 0.1239 5.864 0.2576 2.821 37.12 62 67 1.4 12.2 3.92 0.11

Total gas 62 53 3.060 0.030

Groundmass ES 27 (39.3 mg), J = 0.0001860 ± 0.00000221 0.2204 1.963 0.6265 6.540 65.60 24 122 0.7 14.4 0.566 0.0192 0.3066 5.891 1.399 14.12 92.53 46 69 2.1 31.2 0.787 0.0273 0.2177 8.733 1.441 15.02 65.54 27 42 1.8 33.1 1.097 0.0314 0.05606 4.289 0.4237 4.558 18.15 117 101 8.7 10.1 1.775 0.0535 0.02866 3.890 0.2044 2.198 9.337 133 88 9.3 4.8 3.339 0.0916 0.01256 3.324 0.1181 1.098 4.273 172 132 13.1 2.4 5.71 0.167 0.00826 1.992 0.07095 0.8103 2.519 33 392 3.1 1.8 4.64 0.168 0.00774 2.112 0.08928 0.9825 2.387 35 325 4.2 2.2 4.06 0.14

Total gas 50 35 1.340 0.016


Total gas 129 31 1.555 0.018

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Table A1 (Continued)

Step 36Ar(atm)37Ar(Ca)

38Ar(Cl)39Ar(K)

40Ar(Tot) Age (ka) ±2σ 40Ar* % 39Ar(K) % Ca/K ±2σ


Total gas 58 27 1.124 0.014


Total gas 248 67 4.037 0.053


Total gas 414 29 2.427 0.032

Argon isotope concentrations are expressed in ×10−15 mol. Errors are given as 2s. J, irradiation parameter.


Sample ES 43 yielded a plateau age of 239 ± 43 ka, which agrees with that of total gas(248 ± 67 ka). The isochron age is not well defined. Because of the large uncertainties in allthe measurements, the preferred value is that of total gas (248 ± 67 ka).

The oldest value was that obtained for sample ES 46, with a plateau age of 411 ± 17 ka and atotal gas age of 414 ± 29 ka. All eight heating steps yielded an isochron age of 415 ± 27 (initial40Ar/36Ar ratio = 293.8 ± 8.3), which is considered the best estimate for sample ES 43.

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Tectonic and magmatic evolution of the active volcanic ... · 370 S. Agostini et al. / Geothermics 35 (2006) 368–408 Fig. 2. Schematic tectonic map of El Salvador. Dashed columns