Hernández W. , Erban V. , Rapprich V. , Janoušek V. , Holub F.V

1 Geochemistry and genesis of lavas

Servicio Nacional behind volcanic front in El Salvador project no. 205-06-1811 de Estudios Teritoriales Hernández W.1 , Erban V. 2,3 , Rapprich V. 2,3 , Janoušek V. 2,3 , Holub F.V. 3 2 3 1 - Servicio Nacional de Estudios Teritoriales, San Salvador, El Salvador Gulf of 2 - Czech Geological Survey, Prague, Czech Republic Mexico

3 - Faculty of Science, Charles University in Prague, Czech Republic Belize Mexico Caribbean Honduras Sea + Guatemala Nica- Pacific ragua + Ocean Panama ecent volcanic activity in El Salvador is concentrated at the subduction-related + Costa ++ El Salvador Rvolcanic front, which predominantly consists of andesitic to rhyolitic +++ Rica

Columbia stratovolcanoes and calderas. Apart of it, several areas of cinder cones, lava fields ++ +++ and small shield volcanoes formed behind the volcanic front (BVF). ++ ++ + Differentiated rocks are scarce in this setting. +++ + In order to investigate the spatial variability of BVF products, we sam- + + + + pled two cross sections from the volcanic front landwards. One is lo- + cated in the Cinotepeque range, north of San Salvador Volcano, the sec- ond one in the Salvadorian part of the Ipala Graben, north of Santa Ana - Izalco volcanic complex.

1mm1 mm Cinotepeque range Ipala Graben, Salvadorian part San Salvador Volcano (Boquerón) Santa Ana - Izalco volcanic complex Data for Santa Ana - Izalco from: Carr and Pontier, 1981; Feigenson et al., 1986; Noll et al., 1996

1mm1 mm

10 Phono- he Salvadorian BVF tephrite Trachydacite lavas were classified as Trachy- T he basaltic lavas consist 8 andesite Basaltic subalkaline basalts to mostly of plagioclase and trachy- andesite basaltic andesites, with rare T Basanite

olivine pheno-crysts, enclosed in 6 Trachy- Rhyolite rhyolitic occurrences. Mafic plg-ol-px-mt matrix (below). Some basalt members are fairly very fine-grained varieties do not 2 fractionated, though the

4 have phenocrysts. Pyroxenes are Dacite

2 outcrops of the primitive

Andesite

Na O+K O (wt%)

Basaltic

andesite Basalt lavas are rare (SiO2 = 2 49.1–– 55.5 %, mg# = 28 63, 500m m Cr = 61– 252 ppm and Ni

0 =14– 113 ppm). The position 45 50 55 60 65 70 75 80 represented by both: ortho- and SiO2 (wt%) in the AFM diagram clinopyroxene. Occasionally, a cli- demonstrates its calc- nopyroxene rim is mantling alkaline affinity. This is in sharp contrast with clearly tholeiitic lavas orthopyroxene core. Cores of Mg- of the volcanic front Boquerón Volcano. A characteristic feature of the rich olivines enclose chromite BVF lavas is a sharp increase in K2O with increasing differentiation inclusions (a bove). (SiO ). 250m m 2 There is a significant decrease in fluid-mobile vs. incompatible ratios (i.e. Ba/La) with increasing distance from the volcanic front. This indicates decreasing role of the slab -related fluid during magma generation. Cpx Plg En Wo Fs An 44 42.5 12 50 F

Pecerillo & Taylor, 1976 16 Px Irvine& Baragar matrix En Wo Fs Ti-mt 1971 34.5 22.5 38.5 Shoshonite High-K CA Tholeiite series series An37-56 series En Wo Fs 32 41.5 23.5 Plg 2 CA

host rock

Cpx KO series Th/Ta En Wo Fs An Fo 44.5 42 12.5 70-90 70 1cm1 cm rim Ol Calc Alkaline (CA) series Tholeiite series 2468 12

01234

phenocrysts Fo

core 77 avas of the San Diego Volcano (NW El A M 45 50 55 60 010 30 50km Salvador) contains abundant chromite- SiO2

rim An L 72 Plagioclase

60 olivine-plagioclase glomerocrysts, reflecting An 350 complex magmatic evolution of erupted 85

lavas. We assume that deeply accumulated 250

144

Olivine 0.51295 Fo

olivine was captured by a portion of more 80-81,5 40

Ba/Nb

Ba/La

Nd/ Nd

glomerocryst

150

143 evolved magma and served as a nucleus for core Chromite

rapid heterogenous nucleation of plagioclase. 0.51285

010 30 50km 0103050km 0103050km Data sources: Carr M. J., Pontier N. K. Evolution of a young parasitic cone towards a mature central vent; Izalco and Santa Ana volcanoes in El Salvador, Central; America; J. Volcanol. Geotherm. Res. 11, 1981, 277-292 Feigenson M. D., Carr M. J., Maharaji S. V., Juliano S., Bolge L. L. ; Lead isotope composition of Central American volcanoes: Influence of the Galapagos plume. Geochemistry Geophysics Geosystems 5 (2003GC0006), 2004 Noll P. D. JR., Newsom H. E., Leeman W. P., Ryan J. G. ; The role of hydrothermal fluids in in the production of subduction zone magmas: Evidence from siderophile and chalcophyle trace elements and boron. Geochim. Cosmochim. Acta 60, 1996, 587-611 Sofield D. Eruptive history and volcanic hazards Volcan San Salvador. In: Natural Hazards in El Salvador , edited by William I. Rose, J. J. Bommer, D. L. Lopez, M. J. Carr, and J. J. Major, 2004, p. 147-158.