Tectonic and magmatic evolution of the active volcanic front in El

Transcription

Geothermics 35 (2006) 368–408
Tectonic and magmatic evolution of the active
volcanic front in El Salvador:
insight into the Berlı́n and Ahuachapán
geothermal 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
c
Istituto di Geoscienze e Georisorse (CNR), Via G. Moruzzi 1, 56124 Pisa, Italy
b Dipartimento di Scienze della Terra, Università di Roma, La Sapienza,
Piazzale Aldo Moro 5, 00185 Roma, Italy
Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria 53, 56126 Pisa, Italy
Received 28 February 2006; accepted 30 May 2006
Available online 11 September 2006
Abstract
In El Salvador, Central America, active deformation takes the form of a major dextral strike-slip fault
system, the El Salvador Fault Zone, resulting from the oblique subduction of the Cocos Plate. The fault
system is laterally discontinuous, being subdivided into different major en-echelon segments that partially
overlap to form pull-apart structures. Volcanic activity is spatially confined to the fault segments and absent
in the intervening pull-apart basins; no significant temporal gap exists in the erupted products, at least during
the Plio-Quaternary. Detailed analyses within the geothermal fields of Berlı́n and Ahuachapán have revealed
important volcano-structural and petrologic differences between the two areas. In the Berlı́n area active
deformation is controlled by the regional transcurrent stress field, resulting in the development of systems
of right-lateral E–W-trending strike-slip faults. Conversely, the structural setting of the Ahuachapán area
is more complex, reflecting an interaction among different stress fields. Berlı́n products exhibit a marked
geochemical and isotopic homogeneity indicating the presence of a single magmatic system. At Ahuachapán,
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 surface
without 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: s.agostini@igg.cnr.it (S. Agostini).
0375-6505/$30.00 © 2006 Published by Elsevier Ltd on behalf of CNR.
doi:10.1016/j.geothermics.2006.05.003
S. Agostini et al. / Geothermics 35 (2006) 368–408
369
Berlı́n and Ahuachapán reflect their different tectonic settings, with important implications for geothermal
investigations.
© 2006 Published by Elsevier Ltd on behalf of CNR.
Keywords: Geothermal exploration; Structural geology; Petrology; Geochemistry; Berlı́n; Ahuachapán; El Salvador
1. Introduction
El Salvador is located within the Central American Volcanic Front (CAVF), a volcanic chain
extending for more than 1000 km from Guatemala to Costa Rica, and related to the subduction
of 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 different geological and geophysical features (Carr and Stoiber, 1990). The El Salvador segment is
characterized by strong tectonic and volcanic activity. The latter has been continuous since at least
the early Neogene; during the Holocene 21 volcanoes were active, 6 of which erupted in historical
times (Siebert and Simkin, 2002). The widespread magmatic activity has resulted in a relatively
high heat flow that has created areas of shallow thermal anomaly whose exploitation is economically feasible. In El Salvador, 22% of the electric energy produced annually is currently being
generated by geothermal plants (Bertani, 2005). New geological, geochronological, geochemical
and petrological data on the El Salvador volcanic front are presented here, with particular attention
paid to the geothermal fields of Berlı́n and Ahuachapán (Fig. 2). A detailed description is given
of the volcano-tectonic evolution of the front, which could lead to more reliable evaluations of
the 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 sinistral transcurrent fault. In continental Central America, the contact between the two plates occurs
along 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 the
volcanic front.
370
Fig. 2. Schematic tectonic map of El Salvador. Dashed columns show the main N–S grabens. Arrows indicate the active
stress field (black arrows: the axes of maximum compression; white arrows: the axes of maximum extension); ESFZ, El
Salvador 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 along
the Middle America Trench, at rates of 73–84 mm/yr (Fig. 1) (DeMets, 2001). Several studies
suggest that the convergence between the two plates is oblique and partitioned between trenchorthogonal compression and strike-slip deformation parallel to the volcanic front, associated with
the northwestward transport of a fore-arc sliver relative to the Caribbean Plate (Fig. 1) (Harlow
and White, 1985; White, 1991; DeMets, 2001). The transcurrent component of the movement
is 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; Martı́nez-Dı́az et al., 2004).
Fig. 3. Historical destructive earthquakes and focal mechanisms of crustal-depth earthquakes available for the El Salvador
area (numbers: year and magnitude of the events). The thick black line indicates the inferred trace of the El Salvador Fault
Zone (ESFZ) (modified after Bosse et al., 1978; Martı́nez-Dı́az et al., 2004; Corti et al., 2005a). RL, Rı́o Lempa; SM, San
Miguel; SS, San Salvador.
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The transcurrent component of the movement between the Cocos and the Caribbean plates
is probably accommodated onland by slip along a major E–W trending dextral transcurrent
fault system (El Salvador Fault Zone, ESFZ; Martı́nez-Dı́az et al., 2004) running along the
volcanic 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 of
N–S trending grabens testify to the existence of a broad zone of nearly E–W extension (Fig. 2)
(Guzmán-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, Berlı́n segment; IA, intervening
area; IL, Lake Ilopango; SMS, San Miguel segment; SVS, San Vicente segment. (b) Landsat image. (c) Line drawing of
major structures in the area west of San Salvador, with the hypothesised pull-apart structure linking two major segments
of the ESFZ.
372
sional rates along the grabens of northern Central America to ∼8 mm/yr (Guzmán-Speziale,
2001).
2.1. Strike-slip deformation in El Salvador
Geological field work carried out in continental El Salvador suggests that the main structures
belonging to the ESFZ have an approximate E–W strike, and are characterized by right-lateral
kinematics. These faults running sub-parallel to, and north of, the volcanic front affect Late
Pleistocene and Holocene deposits and present a strong morpho-tectonic signature testifying
to very recent activity (Fig. 2) (Corti et al., 2005a). The active tectonics of the ESFZ is further
corroborated by the strong transcurrent crustal seismicity associated with this fault zone (Martı́nezDı́az et al., 2004). In particular, the major upper crustal seismic events that have taken place since
1912 in El Salvador occur parallel to, and north of, the volcanic front (e.g. Martı́nez-Dı́az 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 seem
to be related to slip along the ESFZ segment located between Rı́o Lempa and Lago Ilopango
(Fig. 4a) (Martı́nez-Dı́az et al., 2004).
Geological and seismological analyses suggest that the ESFZ is laterally discontinuous, and
divided into large segments that are particularly evident east of the city of San Salvador (Fig. 4a)
(Martı́nez-Dı́az et al., 2004; Corti et al., 2005a). Between the city and San Miguel, two major
segments have been identified, the first of which extends from the Ilopango Caldera to the San
Vicente Volcano-Rı́o Lempa area (San Vicente segment), and the second from Rı́o Lempa eastwards (Berlı́n segment; Fig. 4a). The two segments overlap in a dextral en-echelon style with the
formation of an intervening pull-apart basin. East of San Miguel, the strain may be transferred
to a third segment (San Miguel segment) that is, however, poorly known as yet. West of San
Salvador the existence of another major segment of the ESFZ can be hypothesized on the basis
of strike-slip focal mechanisms (e.g. Martı́nez-Dı́az et al., 2004). The structural pattern obtained
through analysis of satellite images and aerial photos suggests that this segment can be linked to
the Ilopango-San Vicente sector of the ESFZ through another major pull-apart basin, extending
between 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 development
of 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 runs
from southern Mexico to Costa Rica, with gaps in which active volcanism is absent. Carr et
al. (1982, 2004) recognized a number of breaks separating seven segments that are assumed
to be tectonically controlled. The active volcanic front in El Salvador constitutes one of these
segments, located between those of Guatemala and Nicaragua. In El Salvador, detailed structural
investigations have, however, led to the identification of at least three E–W segments arranged
en-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 and
reworked volcanic material. The sedimentary basement is exposed sporadically in the NW corner
of the region, where the Metapán Formation (Jurassic-Cretaceous) outcrops. This formation consists of quartz-rich conglomerates, limestones and sandstones with rare intercalations of volcanic
rocks (Bosse et al., 1978).
373
Fig. 5. Schematic representation of the main segments of the volcanic front (dotted ellipses) within El Salvador. AH,
Ahuachapán; B, Berlı́n; 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 (modified
from Donnelly et al., 1990).
374
The Cenozoic volcanic products are the result of the trenchward (i.e. southward) migration
of the volcanic front (e.g. La Femina et al., 2002). Several volcanic formations, ranging from
Paleocene 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 is
reported in Fig. 6. The oldest (Paleocene-Miocene) formations (Morazán and Chalatenango) are
exposed in the northern inner part of the region, close to the border with Honduras. PleistoceneHolocene products, including the active volcanoes, are comprehensively grouped within the San
Salvador Formation, which, for the most part, developed over the Miocene-Pliocene Bálsamo
Formation, formed by an assemblage of volcanoclastics and volcanic breccias with intercalated
lavas and scorias. Locally, a relatively thin layer of Plio-Quaternary volcanics known as the
Cuscatlán Formation (e.g. Donnelly et al., 1990) may be present between the Bálsamo and the
San Salvador Formations.
4. Geology of the Berlı́n and Ahuachapán geothermal areas
4.1. Berlı́n area
The volcanic sequence in the Berlı́n area is characterized by a local basement corresponding to
the Bálsamo Formation. Over the basement there are remnants of an old (Pleistocene) stratovolcano, covered by a sequence of ignimbrite layers and pumice fall deposits that are likely related
to the formation of a caldera structure; the pyroclastics have an age around 0.1 Ma (D’Amore and
Mejia, 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 Berlı́n and
San Vicente segments (Fig. 2). The structural analysis carried out in this area reveals that the E–W
strike-slip faults of the ESFZ are associated with minor structures that can be assigned to three
different 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 observed
in the classical strike-slip physical experiments (e.g. Tchalenko, 1970), these different fault sets
can be related to an E–W dextral shear couple, with the E–W structures representing the main
deformation 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 analysis
on different structural stations along the main structures indicates a dominant strike-slip stress
field, with σ 1 and σ 3 oriented ∼NNW–SSE (∼N155◦ E) and ∼ENE–WSW (∼N65◦ E), respectively (Fig. 7) (Corti et al., 2005a).
At volcanic edifices such as the Berlı́n one, tension fractures related to an extensional stress
field may locally predominate and create complex interactions with the strike-slip structures, as
suggested by small-scale analogue models of magma intrusion during transcurrence (Fig. 8; Corti
et al., 2005b).
4.2. Ahuachapán-Cuyanasul area
In the Ahuachapán region, four main volcanic stages were identified by geological mapping
(González Partida et al., 1997). Several stratovolcanoes formed over the local Bálsamo basement
(e.g. Cuyanausul, Cerro de Apaneca, Cerro Empalizada); they were dissected by a caldera event
associated with abundant pyroclastic products; the structure is known as Concepción de Ataco
Caldera (Fig. 9). The post-caldera activity took the form of small mixed volcanoes (Volcán de
las Ninfas, Laguna Verde, Hoyo de Cuajuste) and domes (Cerro San Lázaro). The Ahuachapán-
375
Fig. 7. (A and B) Digital elevation model and map of active structures forming the El Salvador Fault Zone (ESFZ) between
Ilopango Caldera and San Miguel. Inset in (A) shows the location of the study area. Digital Elevation Model data and
Landsat Enhanced Thematic Mapper 7 satellite mosaic images courtesy of the University of Maryland, Global Land Cover
Facility. IL, Ilopango; RL, Rı́o Lempa. Inset in (B) shows a schematic interpretation of the fault pattern. (C) Stereonets
of fault-slip data collected along the ESFZ (Wulff net, lower hemisphere). The large black arrows indicate the direction
of 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 the
western end of the Motagua and El Salvador fault systems and to the N–S grabens associated
with the intraplate deformation of the Caribbean Plate (Fig. 9a). Detailed field work revealed the
presence 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 sinistral
or dextral). All the analyzed structural trends are very young, as they affect Late PleistoceneHolocene volcanic rocks. The structures with the most conspicuous morphological evidence
are the NE–SW oriented faults that, in the area west of the geothermal power plant, seem to
accommodate the active deformation. These structures cut both the N–S and the NW–SE faults.
376
Fig. 8. Top: fault pattern at the Berlı́n 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 Berlı́n 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 by
E–W extension (Guzmán-Speziale, 2001).
At a regional scale, the N–S structures seem to be more or less contemporaneous with the
NE–SW faults. At the outcrop scale, there appear to be no temporal relationships between the
E–W striking fault structures and the above-mentioned trends. On the basis of regional (largescale) tectonics, the E–W trend could be related to the dextral strike-slip fault system of the
ESFZ, hidden below the volcanic edifices. These different structures could therefore testify to
the interaction among different stress fields at a regional scale. In particular, the NW–SE and the
E–W trends could be associated with the transcurrent stress field responsible for the development
of the ESFZ, whereas the N–S structures are likely generated by the E–W extensional stresses that
cause intraplate deformation and the development of the N–S graben systems. The NE–SW trend
could be related to the Motagua stress field or to reactivation of a pre-existing fabric generated
by this major fault system.
4.3. Geochronology
There is some uncertainty with regard to the geochronological constraints of the Salvadorean
volcanic formations. In the Ahuachapán area, González Partida et al. (1997) claimed, on the basis
of K–Ar datings, that the Bálsamo Formation could be as young as the Pleistocene. In the Berlı́n
region, on the other hand, the local volcanic basement represented by the Bálsamo is considered
to be at least 2 Ma old (Anderson et al., 1994).
377
Fig. 9. (a) Landsat Thematic Mapper satellite image of the Ahuachapán area with detail of N–S normal faults, indicating a
roughly E–W extension. (b) Graphic representation (Stereonets) of the structures measured at outcrop scale superimposed
on a digital elevation model, showing the main faults of the Ahuachapán area. Black dots evidence rim of Concepción de
Ataco Caldera.
In order to better constrain the stratigraphic sequence at Berlı́n and Ahuachapán, we selected
six samples for 39 Ar–40 Ar isotopic age determinations (see Table 1 for their location). The three
from Berlı́n constitute a sequence ranging from local basement (Bálsamo, ES 46) to the youngest
products (ES 8-scoria, and ES 27-lava). Likewise, the Ahuachapán samples encompass the age
Table 1
Location of the collected samples
Sample
Place
Rock type
Latitude (N)
Longitude (W)
Formation
BE 1
BE 2
BER 1
BER 3
ES 1
ES 2
ES 3
ES 4
ES 5
ES 6
ES 7
ES 8
ES 9
Berlı́n
Berlı́n
Berlı́n
Berlı́n
Volcán San Salvador (El Boquerón)
Volcán San Salvador (El Boquerón)
Volcán San Salvador
Coatepeque
Ilopango
Ilopango
Cojutepeque
Berlı́n
Berlı́n
Pumice
Lava flow
Lava flow
Lava fragments
Lava block
Lava
Lava
Dyke
Lava dome
Lava dome
Lava
Scoria
Pumice
13◦ 30 26
13◦ 31 00
–
13◦ 35 04
13◦ 44 07
13◦ 45 29
13◦ 48 26
13◦ 53 37
13◦ 42 23
13◦ 42 32
13◦ 43 44
13◦ 31 39
13◦ 31 39
88◦ 32 13
88◦ 31 42
–
88◦ 34 11
89◦ 16 48
89◦ 16 12
89◦ 19 35
89◦ 31 49
89◦ 03 50
89◦ 04 05
88◦ 55 59
88◦ 29 44
88◦ 29 44
San Salvador
Bálsamo
Bálsamo
Bálsamo
San Salvador
San Salvador
San Salvador
San Salvador
Cuscatlán
Cuscatlán
Bálsamo
San Salvador
San Salvador
378
Table 1 (Continued )
Sample
ES 10
ES 16
ES 23
ES 27
ES 28
ES 29
ES 30
ES 31
ES 32
ES 35
ES 39
ES 40
ES 41
ES 42
ES 43
ES 44
ES 45
ES 46
ES 47
ES 48
ES 50
ES 51
ES 52
ES 53
ES 54a
ES 54b
ES 55
ES 56
ES 57
ES 58
ES 59
ES 60
ES 61
ES 62
ES 63
ES 64
ES 65
ES 66
ES 67
ES 68
ES 69
ES 70
Place
Berlı́n
San Francisco Gotera
Berlı́n (Alegria)
Berlı́n (Agua Caliente)
Berlı́n (TR 18)
CA-4 Cerro Nejapa
CA-4 Guazapa
CA-4 Cerro Rico
Nueva Concepción
Ahuachapán (Cuyanausul)
Ahuachapán (Hoyo de Cuajuste)
Ahuachapán (Poza del Diablo)
Ahuachapán (Juayua)
Ahuachapán (Cerro de Apaneca)
Ahuachapán (Concepción de Ataco)
Izalco
CA-1 Puente Rı́o Lempa
CA-1 Puente Rı́o Lempa
San Miguel
Conchagua
Ahuachapán (Las Chinamas)
Ahuachapán (Cerro San Lázaro)
Ahuachapán (Rı́o Frı́o)
Ahuachapán (Rı́o Frı́o)
Ahuachapán (Chalchuapa)
Ahuachapán (La Magdalena)
Ahuachapán (La Magdalena)
Ahuachapán (San Isidro)
Ahuachapán (Turı́n)
Ahuachapán (Cuyanausul)
Concepción de Ataco
Ahuachapán (Atiquizaia)
Ciudad del Triunfo
Ciudad del Triunfo
Rı́o Lempa
Berlı́n (Lomo Los Capules)
Rock type
Latitude (N)
Longitude (W)
Formation
Glass fragments
Perlitic lava
Lava
Lava
Lava
Lava
Lava
Neck
Lava
Lava
Lava
Altered lava
Lava block
Lava
Lava
Lava
Dyke
Lava
Lava
Lava
Lava
Lava
Lava
Lava
Ignimbrite
Ignimbrite
Lava
Lava
Lava
Lava
Lava
Lava
Lava
Dyke
Lava
Lava
Lava
Pumice
Lava
Lava
Lava
Lava
13◦ 31 39
88◦ 29 44
San Salvador
Chalatenango
San Salvador
San Salvador
San Salvador
Bálsamo
Bálsamo
Bálsamo
Bálsamo
San Salvador
San Salvador
San Salvador
San Salvador
San Salvador
Bálsamo
San Salvador
Bálsamo
Bálsamo
San Salvador
San Salvador
Bálsamo
San Salvador
San Salvador
San Salvador
Bálsamo
Bálsamo
San Salvador
San Salvador
San Salvador
Bálsamo
San Salvador
San Salvador
San Salvador
Bálsamo
Bálsamo
Bálsamo
Bálsamo
San Salvador
Bálsamo
Bálsamo
Bálsamo
Bálsamo
13◦ 40 07
13◦ 31 00
13◦ 31 31
13◦ 30 07
13◦ 48 42
13◦ 50 36
14◦ 06 16
14◦ 09 11
13◦ 54 18
13◦ 52 57
13◦ 56 03
13◦ 51 25
13◦ 50 27
13◦ 53 08
13◦ 44 42
13◦ 37 00
13◦ 39 25
13◦ 30 36
13◦ 17 58
14◦ 01 16.5
13◦ 56 25.6
13◦ 56 25.6
13◦ 56 15.7
13◦ 59 40.5
13◦ 59 40.5
14◦ 02 39.4
14◦ 02 56.1
14◦ 03 34.9
14◦ 05 57.9
13◦ 57 50.9
13◦ 57 14.8
13◦ 54 41.2
13◦ 51 39.9
13◦ 49 51.5
13◦ 49 33.3
13◦ 48 50.2
13◦ 57 51.9
13◦ 33 00.1
13◦ 32 46.4
13◦ 35 14.4
13◦ 34 05.1
88◦ 05 36
88◦ 30 29
88◦ 30 13
88◦ 31 13
89◦ 11 43
89◦ 09 29
89◦ 14 04
89◦ 18 19
89◦ 46 11
89◦ 47 49
89◦ 47 21
89◦ 44 20
89◦ 48 35
89◦ 51 20
89◦ 37 39
88◦ 33 56
88◦ 42 37
88◦ 14 50
87◦ 48 54
89◦ 54 12.5
89◦ 46 36.9
89◦ 46 36.9
89◦ 46 47.6
89◦ 49 32.8
89◦ 49 32.8
89◦ 41 59.1
89◦ 42 11.3
89◦ 42 21.1
89◦ 42 24.6
89◦ 45 33.4
89◦ 47 15.5
89◦ 45 42.9
89◦ 50 33.9
89◦ 50 33.9
89◦ 51 22.6
89◦ 51 18.6
89◦ 44 28.0
88◦ 21 57.4
88◦ 20 09.0
88◦ 35 42.4
88◦ 31 10.6
All samples labeled ES were collected in El Salvador by the authors. The samples labeled BE and BER were taken in the
Berlı́n 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 the
Cuayanausul Caldera (ES 35), and the post-caldera activity (ES 39). The results are reported in
Table 2. The analytical procedure and results are described in detail in Appendix A.
The geochronological data obtained indicate that the exposed volcanic sequence in Berlı́n
and Ahuachapán is of Quaternary age. Thus, in both areas the top of the Bálsamo Formation
was formed during the Pleistocene, further supporting geological observations on a temporal
Table 2
40 Ar–39 Ar isotope ages for Berlı́n and Ahuachapán rocks
Locality
Berlı́n area
ES 8
Berlı́n-Mercedes Humana
ES 27 Berlı́n-Agua Caliente
ES 46 Puente Rı́o Lempa
Ahuachapán area
ES 39 Cuyanausul-Hoyo de
Cuajuste (post-caldera)
ES 35 Cuyanausul (pre-caldera)
ES 43 Concepción de Ataco
a
b
c
d
e
f
g
Formation
No. of
stepsa
Total gas
age (ka)
40 Ar*
SS
SS
Ba
11
8
8
62 ± 53
50 ± 35
414 ± 29
SS
8
SS
Ba
8
8
%b
Plateau
age (ka)c
No. of steps
plateaud
Isochron
age (ka)
MSWDe
(40 Ar/36 Ar)i f
Preferred
age (ka)g
1.2
2.6
27.7
55 ± 36
57 ± 35
411 ± 17
11
8
8
65 ± 72
106 ± 68
414 ± 27
0.26
1.53
0.33
294.8 ± 9.9
288 ± 20
293.8 ± 8.3
≤115
≤85
415 ± 27
58 ± 27
1.4
55 ± 15
8
54 ± 21
0.85
295.6 ± 2.4
≤75
129 ± 31
248 ± 67
4.6
5.9
125 ± 19
239 ± 43
8
8
127 ± 20
272 ± 169
0.49
0.36
295.1 ± 3.0
293 ± 17
127 ± 20
248 ± 67
See Appendix A for details of heating steps.
Percent of radiogenic 40 Ar.
Average age resulting from gas released in different heating steps showing similar apparent age.
Number of heating steps used to calculate plateau age.
Mean square weighted deviate.
Initial 40 Ar/36 Ar ratio.
See Appendix A for criteria used. Errors are given at 2σ. SS, San Salvador Formation; Ba, Bálsamo Formation.
Sample
379
380
Fig. 10. Total alkali vs. silica classification diagram (Le Maitre, 2002) for El Salvador volcanic rocks. Literature samples from active volcanic front (gray dots) from Carr’s database (http://www-rci.rutgers.edu/∼carr/index.html). SS in
parentheses indicates samples from Ahuachapán and Berlı́n geothermal areas belonging to the San Salvador Formation.
continuity between the Bálsamo products and the youngest volcanism. It is also worth noting
that the current volcanic edifices in the Berlı́n and Ahuachapán-Concepción de Ataco areas were
formed 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 Berlı́n,
Ahuachapán and the El Salvador volcanic front are given in Table 3 (see Table 1 for sample
location; rock samples are labeled ES, BE and BER). Analyses of 28 borehole samples from
geothermal wells drilled in the Berlı́n and Ahuachapán areas are also reported (Table 4; borehole
samples from Berlı́n are labeled TR, and those from Ahuachapán are labeled AH and TO). The
data are plotted in TAS (total alkali silica) and K2 O versus SiO2 classification diagrams together
with data on volcanic rocks taken from the literature (Figs. 10 and 11). Overall, the studied rocks
form a subalkaline association, ranging in composition from basalts to rhyolites with a predominance of relatively less evolved rocks, although the association displays different degrees of alkali
enrichment. In the K2 O versus silica diagram most of the samples fall in the field of a typical
calc-alkaline series; the few rocks exhibiting higher K2 O content lie astride the boundary line
between calc-alkaline and high-K calc-alkaline associations (Fig. 11). A wide dispersion of K2 O
values is observed in the basaltic rocks where, at comparable silica abundance, K2 O ranges from
0.60% (ES 67, SiO2 = 51.6%) to 1.55% (BER 3, SiO2 = 51.4%) (Table 3). It is noteworthy that the
two samples with lowest and highest K2 O content belong to the Bálsamo Formation. The most
evolved rocks exhibit similar potassium dispersions, and two distinct groups of rhyolites with
high (≈5%) and low (≈2.2%) potassium abundance can be distinguished (Fig. 11).
381
Table 3
Major and trace elements of El Salvador volcanic rocks
Bálsamo Formation
ES 45
(Berlı́n)
ES 69
(Berlı́n)
BE 2
(Berlı́n)
SiO2 (wt.%)
TiO2
Al2 O3
Fe2 O3
FeO
MnO
MgO
CaO
Na2 O
K2 O
P2 O5
L.O.I.
47.63
1.07
18.58
5.84
6.05
0.19
4.31
10.60
2.63
0.67
0.24
0.82
50.03
1.12
17.48
3.39
7.50
0.19
3.91
9.54
2.87
1.14
0.34
1.28
50.68
1.22
17.92
12.10
51.19
1.14
17.55
11.70
0.20
4.27
9.51
2.73
1.39
0.32
−0.21
Total
98.63
98.79
100.13
Be (ppm)
Sc
V
Cr
Co
Ni
Cu
Ga
Rb
Sr
Y
Zr
Nb
Cs
Ba
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Hf
Ta
Tl
Pb
Th
U
0.55
35
320
8
32
10
156
18.2
12.6
688
19.6
48.0
1.21
0.49
397
7.5
17.1
2.70
13.1
3.44
1.28
3.67
0.58
3.39
0.71
1.85
0.28
1.71
0.24
1.44
0.07
0.03
3.3
0.67
0.37
BER 3
(Berlı́n)
ES 70
(Berlı́n)
BER 1
(Berlı́n)
ES 46
(Berlı́n)
60.49
1.03
15.61
8.84
0.20
3.99
9.24
2.78
1.54
0.34
0.07
58.14
0.99
16.70
8.25
0.60
0.20
2.08
5.23
4.29
2.21
0.41
0.31
99.74
99.41
281
11
32
5
84
0
20
0
24
466
32
129
4
39
427
42
210
5
698
16
33
1278
24
39
ES 43
(Ahuac.)
ES 58
(Ahuac.)
ES 50
(Ahuac.)
0.19
1.23
4.14
3.93
2.79
0.44
0.83
60.82
0.70
17.46
3.81
2.72
0.13
1.61
5.15
4.24
1.71
0.20
1.03
50.03
0.91
18.53
3.25
6.70
0.18
4.56
9.59
2.84
1.20
0.21
1.01
52.43
1.09
17.58
1.87
6.66
0.16
4.96
7.96
3.64
1.28
0.42
0.88
57.42
0.59
18.04
3.20
3.40
0.14
3.01
6.91
3.45
1.43
0.18
1.31
99.52
99.58
99.01
98.93
99.08
1.11
16
103
1
10
3
7
16.8
29.4
483
25.1
114.9
2.65
0.53
893
13.3
23.8
4.18
18.8
4.56
1.28
4.47
0.71
4.17
0.86
2.39
0.38
2.46
0.38
3.08
0.20
0.09
4.9
1.95
0.98
0.82
31
253
51
31
30
121
17.9
27.3
592
23.4
75.2
1.50
1.18
477
10.4
23.0
3.49
15.6
3.92
1.29
4.05
0.66
3.89
0.83
2.19
0.33
2.10
0.31
2.11
0.10
0.12
3.5
1.64
0.75
181
83
27
40
123
7
20
8
19
558
28
166
6
21
494
16
102
2
628
21
43
604
8
22
382
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 del
Triunfo)
SiO2 (wt.%)
TiO2
Al2 O3
Fe2 O3
FeO
MnO
MgO
CaO
Na2 O
K2 O
P2 O5
L.O.I.
57.45
0.73
17.61
4.00
3.64
0.14
2.59
6.72
3.71
1.41
0.14
0.74
58.05
0.79
17.50
4.11
3.97
0.11
2.71
6.07
3.75
1.38
0.13
1.26
58.74
0.82
17.54
3.49
3.66
0.12
1.98
5.62
4.35
1.41
0.20
0.84
59.71
0.72
17.29
3.04
3.42
0.15
1.86
5.17
4.67
1.80
0.19
0.62
63.78
0.69
16.25
0.99
3.93
0.15
1.32
3.57
5.17
2.42
0.25
0.48
64.19
0.59
16.37
2.96
2.08
0.11
0.92
3.06
5.25
2.52
0.15
0.65
49.78
1.07
19.35
2.01
8.25
0.20
3.49
9.59
2.82
1.16
0.23
1.03
50.16
1.07
17.28
2.63
6.58
0.15
6.48
8.77
2.91
1.12
0.29
1.66
50.40
1.06
18.66
2.51
6.96
0.16
5.19
8.77
3.02
1.04
0.26
0.63
50.96
0.94
21.04
4.20
4.79
0.16
2.84
9.90
3.20
0.59
0.18
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)
Sc
V
Cr
Co
Ni
Cu
Ga
Rb
Sr
Y
Zr
Nb
Cs
Ba
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Hf
Ta
Tl
Pb
Th
U
154
7
21
4
174
4
22
5
130
1
17
3
100
4
16
4
42
0
10
4
48
0
8
4
267
7
22
3
34
347
31
124
3
29
360
24
111
3
31
349
38
141
4
39
348
30
147
3
54
285
37
213
3
58
293
32
213
6
9
621
19
59
3
624
7
22
602
10
18
707
12
18
880
12
32
978
18
31
1067
18
31
399
5
15
383
Table 3 (Continued)
ES 7
(Cojutepeque)
ES 68
(Ciudad del
Triunfo)
ES 32
(Nueva
Concepción)
ES 16
(San Francisco
Gotera)
ES 28
(Berlı́n)
ES 23
(Berlı́n)
ES 27
(Berlı́n)
ES 8
(Berlı́n)
ES 9
(Berlı́n)
ES 10
(Berlı́n)
SiO2 (wt.%)
TiO2
Al2 O3
Fe2 O3
FeO
MnO
MgO
CaO
Na2 O
K2 O
P2 O5
L.O.I.
51.35
0.85
17.78
9.93
0.15
4.26
9.47
2.50
1.42
0.19
0.70
52.51
0.99
17.87
3.37
6.92
0.18
3.91
8.30
3.27
0.83
0.22
0.90
52.80
1.06
17.23
2.76
5.67
0.15
4.14
8.19
3.06
1.57
0.45
1.74
70.45
0.16
14.04
0.92
0.78
0.10
0.35
1.05
2.72
5.18
0.04
4.14
54.15
0.76
17.77
3.60
5.32
0.17
3.91
8.00
2.96
1.56
0.19
0.45
55.99
0.75
17.46
3.08
5.14
0.16
3.49
7.43
3.15
1.87
0.17
0.62
56.58
0.71
17.29
2.62
4.93
0.16
3.04
6.85
3.20
2.05
0.19
1.13
57.97
0.90
17.28
2.43
5.07
0.22
2.43
5.75
4.49
1.56
0.34
0.85
61.89
0.59
16.25
1.19
3.66
0.15
1.46
3.84
4.04
2.31
0.17
3.51
63.90
0.66
16.57
1.19
3.50
0.19
1.32
3.48
5.11
2.32
0.22
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)
Sc
V
Cr
Co
Ni
Cu
Ga
Rb
Sr
Y
Zr
Nb
Cs
Ba
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Hf
Ta
Tl
Pb
Th
U
0.71
30
236
10
30
17
82
17.5
30.3
468
22.5
103.7
2.23
0.55
580
10.0
21.8
3.27
14.6
3.74
0.94
3.88
0.62
3.73
0.78
2.16
0.32
2.01
0.30
2.88
0.17
0.10
4.8
2.29
1.12
260
6
30
4
17
507
37
82
3
596
10
18
0.82
25
209
4
27
11
221
16.8
30.2
467
23.5
109.3
2.53
0.73
712
10.4
23.2
3.44
15.4
3.74
0.91
3.95
0.64
3.78
0.80
2.27
0.35
2.28
0.34
3.04
0.17
0.16
31.4
2.06
1.08
0.98
24
106
2
42
3
18
18.0
28.5
513
33.9
108.1
2.48
1.54
870
12.2
27.5
4.43
20.8
5.51
1.57
5.99
0.95
5.62
1.20
3.28
0.48
3.03
0.46
3.32
0.18
0.08
4.6
1.98
1.04
1.32
17
35
1
5
2
9
17.2
44.3
363
37.4
153.6
3.38
2.35
1135
14.4
32.8
4.97
22.4
5.73
1.38
6.07
1.00
6.04
1.31
3.68
0.56
3.63
0.56
4.54
0.23
0.26
7.8
2.91
1.56
384
Table 3 (Continued)
BE 1
(Berlı́n)
SiO2 (wt.%)
TiO2
Al2 O3
Fe2 O3
FeO
MnO
MgO
CaO
Na2 O
K2 O
P2 O5
L.O.I.
Total
Be (ppm)
Sc
V
Cr
Co
Ni
Cu
Ga
Rb
Sr
Y
Zr
Nb
Cs
Ba
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Hf
Ta
Tl
Pb
Th
U
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.)
0.11
1.22
2.34
4.11
2.36
0.16
2.62
47.72
1.08
17.29
4.35
7.74
0.19
6.91
10.10
2.62
0.60
0.15
0.65
48.41
1.31
16.64
6.38
3.05
0.15
8.27
9.42
3.09
1.08
0.32
0.65
49.62
0.92
18.28
3.18
6.76
0.18
4.61
9.57
2.84
1.17
0.21
0.75
49.64
1.25
17.30
1.27
7.56
0.15
7.32
8.95
3.36
1.20
0.33
0.59
55.39
0.84
18.10
2.19
6.09
0.16
3.22
6.95
3.83
1.48
0.21
0.42
55.74
0.84
17.94
2.05
5.98
0.15
3.29
7.00
3.69
1.61
0.19
0.90
57.31
0.85
17.47
3.56
3.81
0.14
3.03
5.73
3.97
1.92
0.22
1.07
58.31
0.74
17.72
7.00
0.33
0.07
1.74
5.61
3.71
1.57
0.15
1.92
58.92
0.77
18.18
3.21
3.29
0.14
2.15
5.83
4.30
2.01
0.26
0.48
59.13
0.70
17.89
1.65
4.97
0.14
2.41
5.57
3.85
2.13
0.17
1.40
59.95
0.62
16.88
1.55
4.65
0.14
2.16
5.62
3.75
2.25
0.19
1.45
100.42
99.40
98.77
98.09
98.92
98.88
99.38
99.08
98.87
99.54
100.01
99.21
65.36
0.70
16.50
4.94
0.48
45
343
26
46
23
109
18.0
6.0
393
21.6
56.1
1.30
0.17
309
5.6
12.4
2.03
9.8
2.89
1.08
3.38
0.57
3.53
0.76
2.12
0.31
2.01
0.27
1.66
0.10
0.03
2.1
0.98
0.37
ES 60
(Ahuac.)
ES 52
(Ahuac.)
215
292
38
134
213
227
35
111
173
2
24
6
176
2
24
6
140
7
24
10
121
1
18
5
97
1
17
5
20
541
22
142
9
19
577
23
150
8
34
400
25
130
3
37
391
25
132
2
35
401
116
98
4
50
329
29
166
3
52
348
25
160
4
278
11
29
380
13
29
650
13
32
629
15
29
824
89
53
802
12
32
711
12
36
385
Table 3 (Continued)
ES 53
ES 61
ES 66
ES 47
ES 48
ES 2
ES 44 ES 1
ES 4
ES 3
ES 5
ES 6
(Ahuac.) (Ahuac.) (Ahuac.) (San Miguel) (Conchagua) (Boquerón) (Izalco) (Boquerón) (Coatepeque) (Boquerón) (Ilopango) (Ilopango)
SiO2 (wt.%) 60.30
TiO2
0.62
Al2 O3
17.03
Fe2 O3
1.73
FeO
4.62
MnO
0.14
MgO
2.17
CaO
5.56
Na2 O
3.95
K2 O
2.02
P2 O5
0.20
L.O.I.
0.65
65.09
0.51
15.72
1.93
2.67
0.11
1.26
3.30
4.71
2.76
0.12
0.55
69.47
0.13
14.34
0.41
1.82
0.11
0.36
1.47
3.66
4.79
0.04
3.91
49.74
0.95
19.68
4.05
6.10
0.16
4.14
10.50
2.46
0.72
0.18
0.48
51.19
0.83
19.09
3.53
6.49
0.18
4.58
9.15
2.70
0.78
0.18
0.64
52.30
1.12
18.33
2.31
7.90
0.19
3.68
8.81
3.18
1.18
0.23
0.53
52.60
1.06
18.08
4.59
4.99
0.17
3.64
7.98
3.24
1.83
0.31
0.63
53.37
1.23
16.39
3.82
7.03
0.21
3.26
7.37
3.54
1.42
0.28
0.52
54.09
0.82
19.73
2.97
4.88
0.14
2.51
8.01
3.23
1.77
0.24
0.66
57.71
1.17
15.15
0.17
9.27
0.22
2.16
5.45
3.89
2.22
0.38
0.55
68.59
0.35
15.29
1.18
1.87
0.11
0.84
3.08
4.13
2.26
0.12
2.07
70.83
0.33
15.11
1.36
1.29
0.11
0.69
2.54
4.43
2.27
0.11
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
97
0
17
3
66
0
11
5
8
0
2
5
48
341
24
156
4
65
232
34
218
4
166
115
34
198
7
729
17
36
1046
14
32
782
34
64
Be (ppm)
Sc
V
Cr
Co
Ni
Cu
Ga
Rb
Sr
Y
Zr
Nb
Cs
Ba
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Hf
Ta
Tl
Pb
Th
U
See Table 1 for description and exact location of samples collected. See Appendix A for description of analytical procedures. Trace element values in italics are XRF data. Ahuac., Ahuachapán. L.O.I., loss on ignition.
386
Table 4
Major and trace elements in rock samples from Berlı́n and Ahuachapán boreholes
Berlı́n
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
51.20
0.97
19.55
9.29
0.14
3.23
9.65
2.30
0.96
0.23
2.81
51.16
0.93
18.35
8.08
0.18
3.10
8.43
2.21
1.33
0.38
5.70
52.86
0.88
19.93
8.60
0.15
3.89
4.82
3.85
1.70
0.17
2.98
57.38
0.56
19.37
5.98
0.15
2.37
6.71
4.42
0.89
0.13
2.04
47.79
0.95
19.29
9.89
0.15
4.54
8.96
3.21
1.38
0.12
3.61
45.66
0.98
16.26
11.53
0.21
3.33
11.27
2.35
0.80
0.20
6.54
58.88
0.86
15.01
8.53
0.19
2.88
6.34
3.75
0.23
0.30
2.58
41.59
1.12
19.77
13.81
0.27
4.65
13.97
1.94
0.03
0.28
3.93
100.33
99.85
99.83
100.00
99.89
99.13
99.55
101.36
SiO2 (wt.%)
TiO2
Al2 O3
Fe2 O3T a
MnO
MgO
CaO
Na2 O
K2 O
P2 O5
L.O.I.
Total
Nb (ppm)
Zr
Y
Sr
Rb
Ce
Ba
La
Ni
Cr
V
Co
Berlı́n
Ahuachapán
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.%)
TiO2
Al2 O3
Fe2 O3T a
MnO
MgO
CaO
Na2 O
K2 O
P2 O5
L.O.I.
53.78
0.74
18.15
7.96
0.17
2.85
7.46
4.36
1.88
0.23
2.14
52.40
0.77
16.71
9.65
0.18
2.75
7.78
2.73
0.77
0.23
5.30
55.54
0.70
16.82
8.41
0.18
3.11
6.55
2.84
1.04
0.20
3.54
56.34
0.80
16.84
8.27
0.15
3.40
6.54
2.32
1.62
0.13
2.85
56.81
0.61
17.74
8.04
0.15
2.87
5.32
3.47
1.20
0.19
2.54
60.71
0.80
16.72
6.35
0.11
2.04
3.18
1.73
1.76
0.17
5.21
62.58
0.77
16.83
10.84
0.12
3.44
2.46
1.62
1.17
0.17
–
65.81
0.81
19.08
7.16
0.09
2.79
1.88
0.90
1.41
0.08
–
Total
99.72
99.27
98.93
99.26
98.94
98.78
98.78
98.78
Nb (ppm)
Zr
Y
Sr
Rb
7
178
31
200
28
7
190
52
175
39
387
Table 4 (Continued)
Berlı́n
Ahuachapán
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
33
582
13
3
0
123
21
37
635
17
8
3
188
29
Ce
Ba
La
Ni
Cr
V
Co
Ahuachapán
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.%)
TiO2
Al2 O3
Fe2 O3T a
MnO
MgO
CaO
Na2 O
K2 O
P2 O5
L.O.I.
69.23
0.72
19.81
5.49
0.06
1.57
1.28
0.99
0.77
0.08
–
60.18
0.91
16.94
7.36
0.16
2.92
6.18
3.83
1.29
0.22
–
60.55
0.85
16.79
6.57
0.14
2.95
6.04
3.82
2.06
0.23
–
59.60
0.71
17.86
6.36
0.13
3.06
7.01
3.46
1.64
0.16
–
59.06
0.73
16.90
6.58
0.15
4.38
7.02
3.09
1.90
0.18
–
57.54
0.79
18.23
6.73
0.13
3.79
7.56
3.25
1.81
0.17
–
60.91
0.74
17.29
6.68
0.13
2.54
5.49
3.99
2.06
0.17
–
54.53
0.85
16.51
10.20
0.18
5.17
8.44
2.78
1.14
0.18
–
54.13
0.86
16.75
10.48
0.20
5.12
8.34
2.84
1.10
0.19
–
Total
98.78
98.78
98.78
98.78
98.78
98.78
98.78
98.78
98.78
Nb (ppm)
Zr
Y
Sr
Rb
Ce
Ba
La
Ni
Cr
V
Co
6
209
50
162
35
39
813
14
8
7
281
34
5
158
29
392
47
35
684
17
7
47
141
19
3
148
29
381
36
29
652
12
5
5
130
18
2
133
24
403
31
32
725
13
5
12
135
18
2
133
25
385
36
25
745
15
5
6
141
20
3
109
20
440
33
18
631
10
7
4
176
18
3
147
26
347
42
28
745
15
4
2
142
19
2
116
25
407
11
18
587
10
9
16
226
31
3
115
27
406
11
18
585
10
9
18
231
33
n.a.: not available.
TR, Tronador; AH, Ahuachapán; TO, Tortuguero; L.O.I., loss on ignition. See Appendix A for description of analytical
procedures. Trace element values in italics are XRF data.
a Fe O
2 3T is total iron expressed as Fe2 O3 .
In Fig. 12 the rocks are plotted in the AFM diagram, which shows the boundary line between
calc-alkaline and tholeiitic series (Irvine and Baragar, 1971). The rocks display modest Fe enrichment; some slightly evolved samples show relatively high FeO*/MgO ratios falling in the tholeiitic
field, although their K2 O content is not typical of tholeiitic arc lavas (Fig. 11). The Berlı́n
rocks (from basaltic andesites to dacites) do not show any Fe enrichment. High values of the
FeO*/MgO ratio and a mildly tholeiitic affinity have also been observed in recent volcanics of
388
Fig. 11. K2 O vs. SiO2 classification diagram (after Peccerillo and Taylor, 1976) for Ahuachapán, Berlı́n and other volcanic
rocks from the El Salvador volcanic front. SS in parentheses indicates samples from Ahuachapán and Berlı́n geothermal
areas belonging to the San Salvador Formation.
Fig. 12. AFM (Alkali–FeO–MgO) diagram for Ahuachapán, Berlı́n and other volcanic rocks from El Salvador volcanic
front; boundary line between calc-alkaline and tholeiitic series taken from Irvine and Baragar (1971). SS in parentheses
indicates samples from Ahuachapán and Berlı́n geothermal areas belonging to the San Salvador Formation.
389
the Boquerón (Fairbrothers et al., 1978) and Chichontepeque Volcanoes (Rotolo and Castorina,
1998).
5.2. Petrography
Basalts are relatively abundant in the Ahuachapán area and in the products of the Bálsamo
Formation, but are missing in the Berlı́n volcanic system where basaltic andesites are the most
primitive rocks. Petrographically, the basalts and basaltic andesites have porphyritic to subaphyric textures. The basalts usually exhibit a phenocryst assemblage made up of abundant augitic
clinopyroxene (cpx) and plagioclase (pl; up to 2–3 cm in size), and smaller olivine (ol). Only sample ES 55 (Ahuachapán) exhibits Mg-rich olivine (Fo86–77 ) as unique phenocryst, with augitic
cpx 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 respect
to olivine. The plagioclases are generally zoned, with Ca-rich cores (An90–82 ) and rims, usually 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); there
are rare needles of ilmenite (ilm). In the basaltic andesites, as well as the cpx + ol + pl assemblage, orthopyroxene may be present, with compositions ranging from En65 to En50 . Augite
is weakly zoned, with slight Fe-enrichment in the rims; again their Mg# is much greater than
the Fo content of zoned olivine (Fo67–53 ). Plagioclases are usually labradoritic, and may show
oscillatory zoning. The matrices are generally crystalline for the most part, with abundant
glass in a few samples; the groundmass mineral phases include plagioclase, two pyroxenes and
Ti–magnetite.
Intermediate rocks are represented by two-pyroxene andesites with predominant plagioclase,
ranging in composition from Ca-rich bytownite (cores) to labradorite (rims). Embayed olivine
may be present, mainly in low-silica andesites (e.g. ES 8, Berlı́n). Scattered opacitized amphibole
occurs as megacrysts in sample ES 50 (Bálsamo Formation). Opaques are represented mainly by
microphenocrysts of Ti–magnetite and ilmenite in very minor amounts.
The evolved rocks consist of dacites and rhyolites, occurring either as lava flows or pyroclastics (ignimbrites and fall-out deposits). The dacites are slightly porphyritic, often with abundant
glass in the matrix. The phenocryst assemblage is formed by andesinic plagioclase, ortho- and
clinopyroxene with opx/cpx ratio > 1. Quartz and sanidine are sporadically present. In addition
to quartz, feldspars (plagioclase and sanidine) and orthopyroxene, amphibole and/or biotite are
sometimes found in the rhyolites.
5.3. Geothermal boreholes
In the Berlı́n 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-calciteclay 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 phyllosilicates.
In the Ahuachapán area, the Tortuguero field samples (well TO 1) show the occurrence of
a widespread zeolite alteration zone in the upper part of the well (down to ≈400 m depth): the
zeolites are represented by heulandite-clinoptinolite and stilbite-epistilbite; wairakite also occurs
sporadically. The middle and lower part of this well is characterized by an association of quartz,
390
clay minerals (mainly chlorite) and variable amounts of calcite. Epidote is observed at depth only
sporadically in the calc-alumina-silicate zone. By contrast, the rocks from other Ahuachapán
boreholes (wells AH) present extensive epidote-wairakite-actinolite alteration, as also observed
in 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 lesser
extent, TiO2 with silica (Fig. 13). Positive correlations are observed for Na2 O and K2 O, although
a relative dispersion is present in the alkali abundances of the less evolved samples. A negative
correlation between MgO and silica is observed, plotting as a convex curve. The most primitive
Bálsamo and Ahuachapán basalts (MgO 6–9%) plot along a steeper curve, whereas, from the
more evolved basalts (MgO < 5%) up to the most SiO2 -rich samples, the correlation flattens out
and is almost linear. The Al2 O3 –SiO2 plot shows a wide dispersion for the basaltic rocks; at the
same silica content (≈50%), alumina ranges from 16.6 to 21.0%, whereas a well-defined negative
trend appears in the basaltic andesites-rhyolites range.
As a whole, major element variations are compatible with a crystal fractionation process
involving the mineral phases forming the observed parageneses. However, plagioclase and mafic
minerals may play different roles during the fractionation process, as evidenced by the Al2 O3 versus MgO diagram (Fig. 14). In the most basic rocks, MgO and alumina are negatively correlated (up
to ≈5% MgO), whilst in the more evolved products these two elements coherently decrease. These
variations suggest that the first stages of crystallization involved mainly mafic phases (clinopyroxene ± olivine), whereas plagioclase becomes the dominant removed phase during successive
stages of rock evolution. At about 3–5% MgO, the maximum Al2 O3 contents indicate the presence
of plagioclase-cumulus phenomena, as evidenced by the strongly plagioclase-phyric textures.
Borehole samples from Berlı́n and Ahuachapán exhibit basically the same geochemical and
petrographic characters and largely overlap the variation range of the outcropping rocks (Fig. 13).
The borehole samples from Berlı́n are generally less evolved than the exposed rocks, being mainly
basalts and basaltic andesites. The samples most affected by hydrothermal alteration are depleted
in Na2 O and enriched in Al2 O3 (Fig. 13).
5.5. Trace elements and isotope geochemistry
Trace element data are presented in Table 3 and selected trace elements versus silica are
plotted 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 the
Ahuachapán area, e.g. ES 55 and ES 57, with 100 < Ni < 150 ppm. Taking into account all
the samples, Ni and Cr display a negative hyperbolic correlation with silica, whereas V and
Co exhibit a good negative linear correlation with r2 = 0.88 and 0.75, respectively. The large
ion lithophile elements (LILE) show a generally positive correlation with an increase in evolution 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 correlated
with silica, although the data points are more scattered. Overall, the scattering of trace element
abundances is generally greater in the most primitive rocks, matching the behaviour of major
elements.
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
391
Fig. 13. Major element (anhydrous) variation diagrams plotted against SiO2 for Ahuachapán, Berlı́n and other volcanic
rocks from El Salvador volcanic front (all units in wt.%). SS in parentheses indicates samples from Ahuachapán and
Berlı́n geothermal areas belonging to the San Salvador Formation.
392
Fig. 14. Al2 O3 vs. MgO variation diagram for Ahuachapán, Berlı́n and other volcanic rocks from El Salvador volcanic
front. Thick grey lines delineate the path of crystal fractionation starting from basalt ES 42 to dacites, through high-Al
basalts and andesites. ol, olivine; px, pyroxene; pl, plagioclase. The negative trend results from the removal of a solid
assemblage in which mafic minerals plagioclase; conversely, plagioclase dominates fractionation in the final part of
the trend, producing a positive correlation. SS in parentheses indicates samples from Ahuachapán and Berlı́n geothermal
areas 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 Ahuachapán rocks (e.g. ES 42), where negative Hf and
Zr anomalies are also observed.
Rare earth elements (REE) are variably fractionated, with La/YbN ranging from ≈1.5 to 4. The
Berlı́n samples in particular show higher REE contents, in agreement with their higher degree
of 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 Bálsamo samples from Berlı́n show a large variability
of LREE contents and patterns with variable slope. Basalt ES 42 from Ahuachapán is characterized
by the lowest LREE/HREE ratio measured in the data set. A significant Ce negative anomaly is
observed for the whole analyzed samples; this could be related with the involvement a slab-derived
component depleted in cerium.
Strontium and neodymium isotope ratios are reported in Table 5 and Fig. 17: Nd isotope ratios
are close to a value of 0.5130 and show very small variations, between 0.51297 and 0.51301. On
the other hand, the Sr isotopic composition ranges between 0.70357 and 0.70390. In the classic
Sr–Nd diagram there is a shift toward higher Sr isotopic composition with respect to the mantle
array (inset of Fig. 17).
The 87 Sr/86 Sr and 143 Nd/144 Nd ratios in the Berlı́n borehole samples are very close to those
of surface lavas, suggesting that water–rock interaction processes did not significantly modify
their isotope features. The isotope composition of the drilled and outcropping rocks does not
change from the surface down to bottomhole depth (2500 m), indicating a marked homogeneity
393
Fig. 15. Trace element (Rb, Sr, Zr and V) variation diagrams plotted against wt.% SiO2 (anhydrous) for Ahuachapán,
Berlı́n 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. Ahuachapán boreholes
have higher 87 Sr/86 Sr values than those measured in outcropping rocks; it is not clear from these
data alone whether this difference is due to water–rock alteration processes or to intrinsic isotopic
heterogeneities between basement and more recent rocks.
5.6. Estimates of magmatic intensive parameters
Different geothermometers were used to estimate crystallization temperatures in the studied rocks. In basalts, where clinopyroxene phenocrysts coexist with olivine, the olivine–augite
Fe–Mg-exchange geothermometer of Loucks (1996) was applied; the standard error in this case
is ±6 ◦ C. Just one basalt sample (ES 55, Ahuachapán) contains olivine phenocrysts only: the
crystallization temperature in this rock was thus assessed through the Fe–Mg partition between
olivine and whole rock (Beattie et al., 1991).
In more evolved rocks, where two pyroxenes occur, the temperature of crystallization of the
phenocryst assemblage was calculated by equilibrium enstatite–augite pairs (Wells, 1977). Oxygen fugacity was calculated by the QUILF code (Andersen et al., 1993, updated in 1995). Most of
394
Fig. 16. Primitive Mantle (PM)-normalized trace-element variation diagrams (left side) and chondrite-normalized Rare
Earth Element (REE) diagrams for volcanic rocks from Ahuachapán and Berlı́n geothermal areas. Normalization values
from McDonough and Sun (1995). Bas And, Basaltic Andesite.
the rocks display Ti–magnetite as microphenocrysts; ilmenite is generally confined to the groundmass as needles that are too small to be analyzed with any reliability. In only three samples it
was possible to use ilmenite–magnetite pairs (ES 55, ES 46 and ES 10); for the other rocks the
temperature was constrained by mafic assemblage equilibria.
395
Table 5
Sr and Nd isotope data of the Berlı́n and Ahuachapán geothermal areas
Sample
87 Sr/86 Sr
2σ
143 Nd/144 Nd
2σ
Berlı́n
ES 7 (Bálsamo)
ES 45 (Bálsamo)
ES 10
ES 28
0.703850
0.703805
0.703867
0.703898
±6
±7
±7
± 10
0.512999
0.512999
0.512997
0.513002
±7
±6
±7
±8
Ahuachapán
ES 50 (Bálsamo)
ES 42
ES 61
0.703582
0.703666
0.703574
±8
±8
±7
0.512997
0.513013
0.512997
±8
±8
± 10
Borehole Berlı́n
TR 8A 1779
TR 17 2000
TR 17 2417
0.703915
0.703872
0.703881
± 17
± 12
±7
0.512999
0.512988
0.512990
± 10
±9
±8
Borehole Ahuachapán
TO 1 700
TO 1 1500
0.703778
0.703829
±7
± 10
0.512986
0.512993
± 11
± 12
The bulk samples were leached with hot HCl 6.6N. Sr and Nd isotope compositions were determined using a Finnigan MAT
262V multi-collector mass spectrometer following separation of Sr and Nd using conventional ion-exchange procedures.
Measured 87 Sr/86 Sr ratios have been normalized to 86 Sr/88 Sr = 0.1194. During collection of isotopic data, 14 replicate
analyses of SRM-NIST 987 (SrCO3 ) standard gave an average value of 0.710243 ± 0.000011 (2 standard deviations).
Measured 143 Nd/144 Nd ratios have been normalized to 146 Nd/144 Nd = 0.7219. Replicate analyses of La Jolla standard
gave an average 143 Nd/144 Nd 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 clinopyroxene and liquid. Equilibrium is evaluated assuming a value of the Fe–Mg partition coefficient
cpx-lq
cpx-lq
cpx
lq
lq
cpx
between cpx and liquid, KD
(Fe–Mg) = 0.27 (KD (Fe–Mg)) = (XFe /XFe )(XMg /XMg ),
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, adding
equilibrium 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 are
reported in Table 6. The temperature of crystallization of mafic phenocrysts in basalts ranges
between 1200 and 1050 ◦ C irrespective of the geothermometer applied. Overall, a negative correlation is observed between silica content and temperature, shifting from basalts to dacites. The
temperatures of the groundmasses are generally 100–150 ◦ C lower than in the phenocrysts, except
for sample ES 8 (andesite from Berlı́n) for which an anomalous high groundmass temperature
was 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 are
observed with respect to the degree of evolution. The clinopyroxene crystallization depth varies
from shallow level (pressure < 0.1 GPa) to levels corresponding to the intermediate crust (pressure ≈ 0.5 GPa). It is worth noting that the variations detected in basalt ES 42 (Ahuachapán), one
of the most primitive rocks of the studied data set, show a good P–T correlation (r2 = 0.97). The
continuous variations of pressure between 0.49 and 0.28 GPa suggest an uninterrupted segregation
of pyroxene during magma ascent, starting from the middle crust where magma ponding probably
occurred for the first time.
396
Fig. 17. 143 N144 Nd vs. 87 Sr/86 Sr diagram for volcanic rocks from Ahuachapán and Berlı́n 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 Berlı́n area. SS in parentheses indicates samples
from Ahuachapán and Berlı́n 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 to
the more primitive and evolved rocks. In the basalts and basaltic andesites, the sequence of crystal
precipitation, suggested from petrographic observations, is cpx ⇒ mt ⇒ ol ⇒ plg; this anhydrous
assemblage and the crystallization sequence can be reproduced at a pressure between 0.3 and
0.5 GPa, with an H2 O content of around 3% if we consider the phase diagram for the basalt ES
42, produced by the MELT code (Ghiorso et al., 2002).
In the dacitic rocks, water abundance was assessed by the experimental geohygrometer of
Merzbacher and Eggler (1984), following the calculations of projection parameters proposed by
Schmitt and De Silva (2000). The results are plotted in Fig. 18, where the data points generally
fall along the line of 4% H2 O content. Only three glass-rich dacites (ES 54 and ES 62 from the
Bálsamo Formation, and ES 10 from Berlı́n) fall outside the main trend, showing higher water
contents, and within the field of amphibole stability, in spite of the anhydrous paragenesis of the
phenocrysts.
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
397
Table 6
Estimate of intensive parameters of Ahuachapán and Berlı́n rocks
Sample Rock type
ES 55
ES 42
Basalt
Groundmass
Basalt
T (◦ C)
T (◦ C)
cpx/lq
P (GPa)
cpx/lq
TQ (◦ C)
log fO2
ΔFMQ
981
0.1
1025 (a)
−7 ± 0.8
−9.8 ± 3.5
+1.3
+0.7
1111
1101
1098
1092
1083
1078
1068
0.49
0.44
0.41
0.36
0.34
0.31
0.28
−7.2
+2.4
1076
1068
1059
0.07
0.14
0.02
−7.06
+2.1
1123
0.29
963 ± 41
−8.5 ± 0.3
−9.5 ± 0.7
+1.8
+2.0
1205 ± 10
−7.23
+0.96
1200 (ol–lq)
1206–1092 (ol–cpx)
1166–1123 (ol–cpx)
ES 43
Basalt
BER 3
Basalt
ES 28
Basaltic andesite
Groundmass
1179–1040 (ol–cpx)
ES 50
Andesite
1041–1022 (ol–cpx)
ES 8
Andesite
Groundmass
1077–980 (ol–cpx)
ES 46
Andesite
Groundmass
1055–956 (opx–cpx)
ES 10
Dacite
Groundmass
1214 (opx–cpx)
773 ± 84 (a) −13.7 ± 1
+1.4
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 et
al. (2003). TQ indicates estimates using the QUILF code (Andersen et al., 1993). (a) Ilmenite–magnetite pairs. ΔFMQ is
oxygen fugacity expressed as the difference between estimated log fO2 and log fO2 of fayalite–magnetite–quartz buffer
at TQ . ol, olivine; lq, liquid; cpx, clinopyroxene; opx, orthopyroxene.
high (>10%) if the process occurred in a closed system. The estimated average H2 O content
of around 4% suggests that the magmas suffered a significant water loss during magmatic differentiation.
6. Discussion
6.1. Magmatic constraints
The volcanic products erupted along the El Salvador volcanic front constitute a typical calcalkaline association, ranging in composition from basalts to rhyolites. A small-scale geochemical
variability is observed in the most primitive samples, where two groups of rocks can be distinguished on the basis of differences in 87 Sr/86 Sr (≈0.7036 and ≈0.70385, respectively), along with
distinct LILE contents and LILE/HFSE ratios (Table 5; Fig. 19).
In the Berlı́n volcanic system all the samples belong to the most 87 Sr-enriched group;
Ahuachapán area samples are less 87 Sr-enriched, and, despite their limited number, are more
398
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 content
of 2 and 4 wt.%. Grey area is the field of amphibole stability (Merzbacher and Eggler, 1984).
variable. In contrast to the Sr isotopes, the 143 Nd/144 Nd ratios have almost constant values
(0.51299–0.51301).
The isotopic and geochemical variability observed in the studied rocks could mirror source
heterogeneity and/or magma interaction with shallow crustal material. The lack of any cor-
Fig. 19. 87 Sr/86 Sr vs. SiO2 (a) and Ba/Zr (b) for volcanic rocks from Ahuachapán and Berlı́n geothermal areas. Dashed
fields enclose samples from Berlı́n area.
399
relation between evolution degree (e.g. SiO2 content; Fig. 19) and 87 Sr/86 Sr ratios, coupled
with the occurrence of the relatively high isotope and geochemical variability in the basaltic
rocks, strongly suggest that contamination processes played a minor role in the petrogenesis of this volcanic association. Hence, the geochemistry of the basaltic rocks is considered
to 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 components, 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 using
trace elements and isotope features (e.g. Carr et al., 2004; Eiler et al., 2005). Considering
our data set, the subduction component cannot be strictly constrained. However, the relative
enrichment in 87 Sr 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 dehydration of the subducting oceanic lithosphere. Furthermore, the high uniform 143 Nd/144 Nd ratios
(≈0.51300) indicate that the magma source, before the addition of the subduction component, 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 the
rock association was fractional crystallization, without significant interaction with crustal material. 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-dacite
in the Ahuachapán and Berlı́n rocks, respectively. The results are reported in Table 7; the low
values of the sum of square residuals between calculated and observed compositions suggest
that removal of gabbroic assemblages is a viable mechanism to produce the studied rocks. To
further check the model, we applied the Rayleigh equation for trace element crystal fractionation, using the amount and modal composition of removed solid as derived from major element
mass balance calculations. The difference between calculated and measured contents is generally
lower than 25% (Table 8), suggesting that the model is consistent. In some cases, however, the
discrepancies reached 50%, mainly for compatible elements, which may be a consequence of the
uncertainty with regard to their solid/liquid partition coefficients. In the transition basalt-andesite
in Ahuachapán, the strong enrichment in Rb of the andesite (ES 60) cannot be explained by the
model.
6.2. Tectonic constraints
The structural analyses carried out in our study lend support to the hypothesis that the active
deformation resulting from oblique subduction of the Cocos Plate under the Caribbean Plate is
accommodated within continental El Salvador by strike-slip movement along the E–W trending El
Salvador Fault Zone (ESFZ). In particular, the kinematics and deformation rates of the major fault
system, as well as the absence of large-scale compressional deformation, corroborate the model
of strain partitioning between trench-orthogonal compression and trench-parallel transcurrent
deformation 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 fault
system, with main segments separated by pull-apart structures (Fig. 20) (Corti et al., 2005a). The
main fault segments contain active volcanism, which is more or less absent in the intervening pullapart basins. This coincidence between the segmentation of the ESFZ and that of the volcanic
400
Starting composition
SiO2 (%)
Total removed
solids (%)
SSR
cpx
ol
mt
pl
opx
Final composition
SiO2 (%)
Ahuachapán
ES 42 Basalt
ES 60 Andesite
48.32
59.96
−69.9
−30.4
0.42
0.34
−16.3
−0.6
−10.8
–
−6.3
−2.9
−36.5
−21.5
–
−5.4
ES 60 Andesite
ES 61 Dacite
59.96
66.3
Berlı́n
ES 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.
Table 7
Differentiation fractionation model for Ahuachapán and Berlı́n rocks
Table 8
Trace element fractionation model for Ahuachapán and Berlı́n rocks
ES 28
Sc (ppm)
V
Cr
Co
Ni
Rb
Sr
Y
Zr
Nb
Ba
La
Ce
25
209
3.7
27
10.9
30.2
467
23.5
109
2.53
712
10.4
23.2
Ahuachapán
Bulk KD
2.61
3.51
3.39
3.16
3.64
0.13
1.08
0.14
0.15
0.69
0.14
0.27
0.25
Model
8.6
40
0.8
6.6
1.9
53.3
443
41.2
191
3.10
1249
16.7
37.9
ES 10
ES 42
17
35
0.7
4.8
1.8
44.3
363
37.4
154
3.38
1135
14.4
32.8
45
343
25.8
46
23.2
6.0
393
21.6
56
1.30
309
5.6
12.4
Ahuachapán
Bulk KD
1.55
1.86
4.02
1.78
2.17
0.10
1.08
0.60
0.15
0.23
0.21
0.25
0.28
Model
23.1
122
0.7
18.1
5.7
17.8
357
35.1
156
3.28
803
13.8
29.4
ES 60
ES 60
–
121
1.0
18.0
5.0
50.0
329
29.0
166
3.00
802
12.0
32.0
–
121
1.0
18.0
5.0
50.0
329
29.0
166
3.00
802
12.0
32.0
Bulk KD
0.71
3.04
2.90
2.74
1.44
0.15
2.03
0.17
0.05
0.03
0.27
0.25
0.26
Model
–
59
0.5
9.7
4.3
67.8
227
39.0
233
4.25
1040
15.7
41.6
ES 61
–
66
–
11
5
65
232
34
218
4
1046
14
32
Berlı́n
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 of
removed phases corresponds to the results of the model reported in Table 7.
401
402
Fig. 20. Schematic hypothetical regional segmentation of El Salvador Fault Zone (ESFZ). Grey elliptic areas indicate the
segments of the volcanic front; large black dots represent some of the main active volcanoes located at the boundaries of
the volcanic segments close to the pull-apart basins. Dashed lines represent main segments of the ESFZ. A, Ahuachapán;
B, Berlı́n.
front suggests a genetic relationship between the two processes of faulting (lithospheric-scale
deformation) and volcanism; the strike-slip fault system in particular acts as a preferential pathway
for magma to ascend to the surface. However, more detailed analyses are needed to better define
these relationships and to explain the absence of volcanic activity within pull-apart basins, where
extensional tectonics is normally considered to favor volcanism.
One important finding of our study is the difference in structural setting between the Berlı́n and
Ahuachapán geothermal fields. The first field is located in correspondence to an important segment
of the ESFZ. The major and minor structures affecting the Berlı́n area are presumably related to a
transcurrent stress field, with a NNW-directed axis of maximum compression and an ENE-directed
axis of maximum extension. Interaction of this stress field with the volcanic edifices has given rise
to a local dominance of NNW- to NW-directed extensional structures within the Berlı́n geothermal
area. Conversely, the Ahuachapán area is structurally less straightforward, being characterised
by different groups of structures that cannot be so easily attributed to a dominant regional stress
field. In this case, the complex fault arrangement possibly results from the interaction between
three stress fields. Indeed, the E–W and NW–SE trending faults may be related to the transcurrent
stress field associated with the El Salvador Fault Zone. The N–S dilatational joints and normal
faults are presumably the result of the intraplate extension of the Caribbean Plate, whereas the
influence of the Motagua fault system is possibly expressed by the NE–SW directed normal faults
west of Ahuachapán.
7. Conclusions
The tectonic and petrologic study of the El Salvador active volcanic front and its local volcanic basement has evidenced a strong link between active deformation and volcanic activity. In
continental El Salvador, strike-slip deformation gives rise to a major dextral transcurrent fault system, the El Salvador Fault Zone, which is laterally discontinuous, being subdivided into different
major en-echelon segments that partially overlap to form pull-apart structures. The present-day
volcanic activity is more or less confined to the fault segments and is practically non-existent in
the intervening pull-apart basins.
The studied volcanic rocks were erupted during the Quaternary. The most recent composite volcanoes grew on a local volcanic basement with no recognizable temporal gap: indeed, the top of the
Bálsamo Formation reaches the upper Pleistocene. No significant geochemical or petrological differences were found between the volcanics of the Bálsamo Formation and the most recent products.
403
Our study reveals the important volcano-structural and petrologic differences between the
eastern (Berlı́n) and western (Ahuachapán) areas of El Salvador. The marked geochemical and
isotopic homogeneity found in Berlı́n, where basalts are lacking, points to the presence of a
single magmatic system. By contrast, the Ahuachapán rocks show significant variations in both
Sr isotopes and the LILE/HFSE ratios. The multiple volcanic centres characterizing this area were
fed by magma batches with different Sr isotopes and trace-element contents, inherited from their
mantle source.
The relative abundance of basaltic products reveals that in Ahuachapán most of the rising
magmas were not intercepted by the shallow reservoirs linked to the small volcanic centres that
erupted more evolved products. The geochemical homogeneity in the Berlı́n products reflects the
“simple” tectonic framework of the area whose active deformation is controlled by the regional
transcurrent stress field, resulting in the development of systems of right-lateral E–W-trending
strike-slip faults and associated secondary Riedel and tensional shears. Conversely, the structural
setting of the Ahuachapán area is more complex, characterized by different fault systems that may
reflect 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; the
more complex and pervasive fault pattern of the Ahuachapán area facilitated the development of
multiple 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 Berlı́n and
Ahuachapán may take different approaches. At Berlı́n, for example, prospecting should account
for the existence of a regional geothermal anomaly magnified by the presence of a shallow magma
chamber. On the other hand, at Ahuachapán the geothermal system appears more complex, and
linked to local magma ponds that are probably limited in volume, as inferred by the small volcanic
centres. This volcanological framework, coupled with the widespread and complex faulting pattern, has probably led to the occurrence of small-scale shallow geothermal anomalies. Overall, the
results of this study show that a combined tectonic and petrological–geochemical approach may
prove an important tool for selecting potential areas of geothermal interest, before proceeding
with 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 and
Marvyn O. Garcia for the stimulating discussions in the field. Reviews by R.M. Prol-Ledesma
and M.J. Carr, as well as editorial comments by M.J. Lippmann, have significantly improved the
quality of the manuscript.
Appendix A.
40 Ar–39 Ar
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 for
incremental laser-heating analyses. The groundmass was concentrated from the 250
to 300 ␮m grain size using standard separation techniques and purified by hand-picking under
a binocular microscope. The groundmass separates were further cleaned by acid leaching in an
ultrasonic bath (1 h at ∼50 ◦ C in 3.5N HCl and 1N HNO3 ). The final product was wrapped in
aluminum foil and irradiated for 2 h in the TRIGA reactor (University of Pavia, Italy) along
40 Ar–39 Ar
404
with the dating standard FCT-3 biotite (age 27.95 Ma; Baksi et al., 1996). After irradiation, the
samples (∼40 mg) were loaded into the 9-mm diameter holes of a copper holder, placed in an
ultra high-vacuum laser port and baked overnight at ∼200 ◦ C.
Incremental laser-heating experiments were carried out using an infrared laser beam generated
by a diode-pumped CW Nd–YAG laser (maximum power 15 W), which was defocussed to ∼2 mm
spot. The laser beam was homogenized by passing it through a lens that produces a flat-power
distribution. Homogenous heating was achieved by slowly rastering the laser beam by a computercontrolled x–y stage. The heating steps were carried out at increasing laser powers until complete
melting occurred. The gas extracted was purified by two SAES AP10 getters (at 400 ◦ C) and one
SAES 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 with
a Balzers SEV 217 secondary electron multiplier. The data (Table A1) were corrected for postirradiation decay, mass discrimination effects, isotopes derived from interfering neutron reactions
and 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 complete analytical results are listed in Table A1. Analysis of these samples was extremely difficult
because of their young age, low potassium content and the material analyzed (groundmass), which
resulted in severe contamination by atmospheric argon (radiogenic argon 10% in most cases; see
Table A1). Results from regression calculations in an isochron plot (36 Ar/40 Ar versus 39 Ar/40 Ar
diagram), 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 overlap
with 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 conservatively
considered 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 agreement
with that of total gas (129 ± 31 ka). In an isochron plot, all eight heating steps gave a linear
array with an intercept age of 127 ± 20 ka and an initial 40 Ar/36 Ar ratio of 295.1 ± 3.0, which
is indistinguishable from that of modern atmospheric argon (295.5). The 127 ± 20 ka estimate is
considered the best for sample ES 35.
Fig. A1. Age release spectra of groundmass samples.
Table A1
40 Ar–39 Ar laser step-heating data
Step
36
Ar(atm)
37
Ar(Ca)
38
Ar(Cl)
Ar(K)
0.04054
1.153
1.980
3.095
3.518
2.451
1.700
1.517
1.830
3.035
2.821
40
2.264
37.42
48.12
56.14
47.65
29.95
21.37
18.67
21.02
31.23
37.12
380
37
11
79
121
41
97
16
72
33
62
11677
421
180
164
166
98
144
96
97
112
67
62
53
6.540
14.12
15.02
4.558
2.198
1.098
0.8103
0.9825
24
46
27
117
133
172
33
35
122
69
42
101
88
132
392
325
50
35
223
145
127
115
88
219
78
109
194
63
25
40
94
340
340
133
129
31
65.60
92.53
65.54
18.15
9.337
4.273
2.519
2.387
Total gas
Groundmass ES 35 (35.0 mg), J = 0.0001876 ± 0.0000021
1
0.05993
0.7921
0.01312
2
0.04204
4.847
0.03042
3
0.02173
5.593
0.04009
4
0.03817
3.869
0.05572
5
0.09676
3.944
0.08869
6
0.06209
1.697
0.02416
7
0.1240
2.098
0.04467
8
0.4344
4.236
0.1278
Total gas
1.909
7.856
8.959
5.123
3.453
0.7308
1.122
3.706
18.97
15.80
9.791
13.02
29.49
18.82
36.90
129.6
40
Ar* %
2.0
0.3
0.1
1.3
2.7
1.0
2.3
0.4
1.9
0.9
1.4
0.7
2.1
1.8
8.7
9.3
13.1
3.1
4.2
6.6
21.3
34.2
13.3
3.0
2.5
0.7
0.9
39
Ar(K) %
0.2
5.0
8.6
13.4
15.2
10.6
7.3
6.6
7.9
13.1
12.2
14.4
31.2
33.1
10.1
4.8
2.4
1.8
2.2
5.8
23.9
27.3
15.6
10.5
2.2
3.4
11.3
Ca/K
±2σ
1.40
1.38
1.63
2.01
2.58
3.12
3.66
4.40
3.98
3.87
3.92
0.39
0.05
0.05
0.06
0.07
0.09
0.10
0.13
0.11
0.11
0.11
3.060
0.030
0.566
0.787
1.097
1.775
3.339
5.71
4.64
4.06
0.019
0.027
0.031
0.053
0.091
0.16
0.16
0.14
1.340
0.016
0.783
1.164
1.178
1.425
2.155
4.38
3.53
2.157
0.031
0.036
0.036
0.041
0.064
0.15
0.11
0.061
1.555
0.018
405
Age (ka)
Total gas
1
0.2204
1.963
0.6265
2
0.3066
5.891
1.399
3
0.2177
8.733
1.441
4
0.05606
4.289
0.4237
5
0.02866
3.890
0.2044
6
0.01256
3.324
0.1181
7
0.00826
1.992
0.07095
8
0.00774
2.112
0.08928
±2σ
Ar(Tot)
1
0.00751
0.03016
0.00234
2
0.1262
0.8402
0.07276
3
0.1626
1.713
0.11935
4
0.1875
3.291
0.1669
5
0.1569
4.816
0.1936
6
0.1003
4.052
0.1329
7
0.07063
3.297
0.08571
8
0.06291
3.538
0.07979
9
0.06979
3.857
0.1579
10
0.1047
6.232
0.2775
11
0.1239
5.864
0.2576
39
406
Table A1 (Continued)
Step
36 Ar
(atm)
37 Ar
(Ca)
38 Ar
(Cl)
40 Ar
(Tot)
1.700
5.391
10.59
7.563
3.289
1.575
1.330
6.104
21.41
26.55
38.15
51.02
57.77
56.39
36.29
153.8
Total gas
1
0.1616
1.234
0.2326
2
0.3652
7.435
0.8010
3
0.1763
9.608
0.3407
4
0.08561
5.695
0.07166
5
0.08543
8.209
0.0694
6
0.09474
13.22
0.1367
7
0.02208
3.122
0.04114
8
0.04880
8.174
0.1053
2.516
8.740
6.597
2.199
1.528
2.535
0.7300
1.655
49.90
114.5
57.12
26.25
26.56
29.43
7.126
15.58
Total gas
1
0.05429
0.9434
0.01339
2
0.05514
4.140
0.02935
3
0.04230
11.48
0.04496
4
0.02147
6.399
0.03770
5
0.03877
4.062
0.05531
6
0.05428
4.116
0.07147
7
0.02194
2.429
0.04945
8
0.02700
4.384
0.09843
Total gas
1.165
3.585
9.935
5.389
3.274
2.308
1.345
2.500
17.25
21.14
24.70
12.67
15.32
18.76
8.038
10.96
±2σ
40 Ar*
25
65
65
31
75
24
84
74
127
28
25
27
185
326
235
62
0.6
3.9
5.3
1.3
1.3
0.2
0.9
0.9
58
27
289
258
260
147
294
193
280
239
352
132
66
260
387
88
277
102
248
67
355
464
421
402
404
404
396
409
468
133
26
36
42
76
71
46
414
29
Age (ka)
Argon isotope concentrations are expressed in ×10−15 mol. Errors are given as 2s. J, irradiation parameter.
4.3
5.8
8.8
3.6
5.0
4.9
8.4
7.4
7.0
22.9
49.2
49.7
25.1
14.5
19.3
27.2
%
39 Ar
4.5
14.4
28.2
20.1
8.8
4.2
3.5
16.3
9.5
33.0
24.9
8.3
5.8
9.6
2.8
6.2
3.9
12.2
33.7
18.3
11.1
7.8
4.6
8.5
(K)
%
Ca/K
±2σ
0.691
0.839
0.888
0.998
1.312
2.206
1.870
1.522
0.026
0.027
0.026
0.029
0.041
0.074
0.062
0.047
1.124
0.014
0.925
1.605
2.748
4.89
10.14
9.84
8.07
9.32
0.037
0.064
0.089
0.17
0.36
0.30
0.25
0.28
4.037
0.053
1.53
2.18
2.18
2.24
2.34
3.37
3.41
3.31
0.12
0.08
0.07
0.07
0.07
0.10
0.11
0.10
2.427
0.032
1
0.07205
0.6223
0.03904
2
0.08633
2.398
0.1285
3
0.1223
4.982
0.2302
4
0.1703
3.999
0.2214
5
0.1930
2.286
0.1403
6
0.1905
1.842
0.08904
7
0.1217
1.318
0.07362
8
0.5162
4.924
0.3370
39 Ar
(K)
407
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 all
the 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 a
total gas age of 414 ± 29 ka. All eight heating steps yielded an isochron age of 415 ± 27 (initial
40 Ar/36 Ar 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 front in El

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