Revista Geológica de América Central, 21: 25-36, 1998 AN INTRODUCTION TO SAN VICENTE (CHICHONTEPEC) VOLCANO, EL SALVADOR Rotolo S.G.*1, Aiuppa A.1, Pullinger C. R.2,Parello F.1, Tenorio-Mejia J.3 1) Dipartimento di Chimica e Fisica della Terra (C.F.T.A.), Via Archirafi 36, 90123 Palermo, ITALY 2) Department of Geological Engineering and Sciences, Michigan Technological University, 1400 Townsend Drive, Houghton, Michigan 49931, U.S.A. 3) Comision Ejecutiva Hidroeléctrica del Rio Lempa (CEL), 49 Avenida Norte, San Salvador, El Salvador * Corresponding Author; e-mail: [email protected] (Recibido 30/3/1998; Aceptado 29/5/1998) ABSTRACT: San Vicente is a composite volcano whose oldest volcanic events (2.2 Ma) are recorded in the western sector, where the remnants of the older La Carbonera edifice outcrop. This volcanic center, which produced mildly tholeiitic to transitional lavas, collapsed during a plinian eruption, giving rise to the formation of a partially preserved caldera (La Carbonera caldera). The renewal of volcanic activity started with clearly calc-alkaline lavas, mostly two- pyroxene andesites, that built up the San Vicente volcanic edifice. Crystal fractionation had primary control on magma evolution. The event that triggered the caldera forming plinian eruption is still uncertain; we have some textural evidences of mixing/mingling with more mafic magmas, but this process seems to be delimited to few samples. A major role could have been played by the tectonic instability related to the first stages of central graben opening. RESUMEN: Durante 1993 un equipo de investigadores italianos y salvadoreños realizó investigaciones en el volcán de San Vicente, El Salvador, con el objetivo de definir la evolución petrológica y volcánica de dicho centro volcánico. Estos estudios formaron parte de estudios preliminares de geotermia en el área. Los productos volcánicos más antiguos (2,2 M.a.) se encuentran en el sector occidental del volcán de San Vicente, en donde afloran los depósitos de la estructura volcánica La Carbonera. Este centro volcánico, el cuál produjo lavas medianamente toleíticas a transicionales, colapsó posterior a una erupción pliniana y formó una caldera preservada parcialmente y definida como La Carbonera. La actividad volcánica se reanuda con la producción de lavas calco-alcalinas, en su mayoria andesitas de dos piroxenos. El proceso magmático principal es el de fraccionamiento de cristales. El evento que causó la erupción pliniana es todavia desconocido. Tenemos cierta evidencia textural de “mixing/mingling” de magmas, pero esto sólo se observa en algunas muestras. La inestabilidad tectónica del área debido a la formación del graben central de El Salvador pudo haber tenido un papel muy importante como causa de la erupción. 26 REVISTA GEOLÓGICA DE AMÉRICA CENTRAL INTRODUCTION El Salvador in a WNW-ESE direction. The graben cuts through a Tertiary volcanic basement During 1993 efforts were made by an which locally consists of basalt and andesite italian-salvadorian team to investigate San lavas, tuffs, agglomerates, and diatomite Vicente volcano, El Salvador. Geothermal deposits that reflect a prehistoric lake (Carr, exploration in the San Vicente geothermal field 1976; Carr & Stoiber 1977; Carr et al. 1981; started in 1965 with a reconnaissance geological Pichler & Weyl, 1973; Wiesemann, 1975; survey done by CEL. After the first promising Williams & Meyer-Abich, 1955). results, research continued with the drilling of a Chichontepec is a densely vegetated deep exploration well (SV-1) and two shallow paired stratovolcano with summit craters aligned ones (PESV-1 and PESV-2). The area seemed to roughly in an E-W direction. Based on be particularly promising for exploitation and morphology, the eastern crater (elevation 2180 energy production, since a hot (T>250 °C) m) appears to be the most recent, while the permeable layer was drilled through at a western crater (elevation 2105 m) appears older. relatively low depth (1200-1300 m). Recent Thick (~20 m), lobate lava flows which probably studies on fluid geochemistry (CEL, 1992; were emitted from a central vent are observed on Aiuppa et al.,1997) confirmed the hydraulic the north and south flanks. The eastern flank is continuity between the superficial thermal also characterized by thick lobate lava flows manifestations and the geothermal reservoir, which were emitted from a parasitic vent aligned allowing also a better evaluation of its extension. in an almost E-W direction with both summit The area had been previously described craters. There is no evidence of historical by Meyer-Abich (1956) and in an earlier study eruptions. However, its youngest products are Williams & Meyer-Abich (1955) studied the covered by the AD 260 Tierra Blanca tephra Apastepeque volcanic field just northeast of the emitted from the nearby Ilopango volcano (Hart San Vicente volcanic field. & Steen-Mcintyre, 1983). Its present day state is The aim of this project, was to develop a that of solfatara-type activity, characterized by deeper knowledge of the geological structure of low temperature fumaroles and hot springs, the volcano in order to contribute to CEL’s which are located on the northern and western geothermal exploration and development flanks of the volcano (Aiuppa et al. 1997). The projects in the area. In this paper, a preliminary summit craters do not present any fumarolic account of the volcanological history, petrology, activity. West of the main volcanic edifice is a and magmatic evolution of the San Vicente semicircular structure, which in this work will be volcanic complex is given. designated La Carbonera. It consists of a series of pronounced hills arranged in a semicircle. Small hills, west and south of the main GEOLOGY AND STRATIGRAPHY Chichontepec volcanic edifice represent uncovered remnants of tertiary basement rocks. San Vicente, also known as Chichontepec, The Nahuistepe domes which lie approximately is the second most voluminous volcano in El 12 km southwest of the Chichontepec summit Salvador (130 km3), after Santa Ana volcano represent an older magmatic period probably which has a volume of 265 km3 (Carr et al., unrelated to the present one. 1981). It is located approximately 50 km east of Evidence of faulting in the San Vicente the capital city, San Salvador, and shadows the volcanic complex is limited to those areas that city of San Vicente (population 100,000) which have not been covered by the San Vicente lies on its northeastern slopes. The edifice (Fig. volcano and Ilopango caldera recent deposits. 1) rises inside the Central Graben structural The most pronounced faulting is observed just depression, a late-Pliocene extensional structure north of the volcano, where a series of E-W (20 to 30 km wide) that runs along the length of trending faults form a large scarp with ROTOLO et al.: San Vicente volcano, El Salvador 27 Lahars and debris flows Central Graben fault scarps Chichontepec post-caldera lavas Rim of La Carbonera caldera Pyroclastic sequence related Faults to La Carbonera caldera SV-1 well Nahuistepe rhyolitic dome La Carbonera pre-caldera lavas Fig 1: Generalized map of the San Vicente volcanic area. displacements of up to 300 m. These faults Previous field and petrological studies represent the northern margin of the Central recognized the existence of a multistage Graben, while the southern margin is evolution of the San Vicente volcanic area represented by a series of faults that extend (Aiuppa et al., in preparation; Barberi et al., eastwards from lake Ilopango through La 1995; Rotolo & Castorina, in press). The most Carbonera and in the process aligns the summit important stages and events in the history of the craters. A series of NW-SE faults are also volcano are described below. observed in older deposits west of Chichontepec volcano and in Cerro Ramirez, 1 Pre-caldera stage: La Carbonera km southwest of the city of San Vicente. (CARB) lava flows Northerly trending faults control the location of This stage is characterized by the effusive the major fumarolic areas on the northern flank activity of a volcanic center, whose remnants of Chichontepec. actually form an annular structure of cinder 28 REVISTA GEOLÓGICA DE AMÉRICA CENTRAL cones and lava flows, outcropping around La opening of the Central Graben. This deposit is Carbonera hill (El Cimarron, La Carbonera and composed of a brownish yellow, poorly welded, El Volcancito hills; Fig. 3). chaotic mix of ash matrix, with pumice During this phase, mildly tholeiitic to fragments and less abundant lithic lapilli. transitional lavas, ranging from high alumina The maximum measured thickness is 10 basalts to andesites, were produced. Preliminary m. BI unit is separated from the overlying K/Ar ages (Table 1) give a quite wide time span, deposits by a two meter thick brown-colored from 2.2 to 1.2 Ma, indicating that this volcanic paleosol, containing volcanoclastic fragments. It centre was active long before the opening of the indicates that a considerable time span occurred Central Graben. Table 1 K/Ar age determinations on whole rock and plagioclase fractions Sample Rock Group Material K (wt%) 40 Ar* Age (Ma) type SVN 14 hab CARB w.r. 0.448 0.90 1.2 ± 0.2 SVN 98 hab CARB w.r. 0.448 1.72 2.2 ± 0.4 SVN 85 rhy NAU plg 0.378 1.12 1.7 ± 0.3 Decay constants: λε+λ’ε = 0.581 *10-10 yr-1; λβ= 4.962 * 10-10 yr-1 40 -4 40 K/Ktotal = 1.193 * 10 ; Ar* refers to radiogenic Argon (10-12 mol/g). hab = high alumina basalt; rhy = rhyolite; w.r.= whole rock; plg = plagioclase. Caldera stage: La Carbonera plinian activity CARB effusive activity was abruptly interrupted by an explosive phase that produced a pyroclastic sequence that outcrops mostly in the northwestern sector of San Vicente volcanic area (Fig. 1). In the southwestern sector the few sections that have been found are highly condensed and heavily reworked by post- fine-grained ashes depositional processes.