Geochemistry and genesis of lavasbehind volcanic front in El Salvador
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project no. 205-06-1811Servicio Nacional
de Estudios Teritoriales
Hernández W. , Erban V. , Rapprich V. , Janoušek V. , Holub F.V.1 2,3 2,3 2,3 3
2
1
Mexico
Colu
mbia
Honduras
Guate
mala
Nica-ragua
Belize
CostaRica
Panama
El S
alv
ador
Gulf ofMexico
CaribbeanSea
PacificOcean
Basalt
Basaltic
andesite
Andesite
Dacite
Rhyolite
Trachydacite
Trachy-andesite
Basaltictrachy-
andesite
Trachy-basalt
Basanite
Phono-tephrite
45 50 55 60 65 70 75 80
02
46
810
SiO (wt%)2
Na
O+
KO
(wt%
)2
2
1 - Servicio Nacional de Estudios Teritoriales, San Salvador, El Salvador2 - Czech Geological Survey, Prague, Czech Republic3 - Faculty of Science, Charles University in Prague, Czech Republic
ecent volcanic activity in El Salvador is concentrated at the subduction-relatedvolcanic front, which predominantly consists of andesitic to rhyolitic
stratovolcanoes and calderas. Apart of it, several areas of cinder cones, lava fieldsand 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 SantaAna - Izalco volcanic complex.
Chromite
OlivineFo
glo
mero
cry
st
co
re
rim
80-81,5
PlagioclaseAn
An85
72
hostro
ck
co
re
rim
Fo
Fo
Ol
70
77
phenocry
sts
matr
ix
An
An
Plg
37-56
70-90
En Wo Fs
CpxEn Wo Fs
44.5 42 12.5
32 41.5 23.5
CpxEn Wo Fs
44 42.5 12
PxEn Wo Fs
34.5 22.5 38.5
PlgAn
50
Ti-mt
Lavas of the San Diego Volcano (NW ElSalvador) contains abundant chromite-
olivine-plagioclase glomerocrysts, reflectingcomplex magmatic evolution of eruptedlavas. We assume that deeply accumulatedolivine was captured by a portion of moreevolved magma and served as a nucleus forrapid heterogenous nucleation of plagioclase.
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
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, 277-292Feigenson 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), 2004Noll 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, 587-611Sofield 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.
The basaltic lavas consistmostly of plagioclase and
olivine pheno-crysts, enclosed inplg-ol-px-mt matrix (below). Somevery fine-grained varieties do nothave phenocrysts. Pyroxenes are
represented by both: ortho- andclinopyroxene. Occasionally, a cli-nopyroxene rim is mantlingorthopyroxene core. Cores of Mg-rich olivines enclose chromiteinclusions ( bove).a
The Salvadorian BVFlavas were classified as
subalkaline basalts tobasaltic andesites, with rarerhyolitic occurrences. Maficmembers are fairlyfractionated, though theoutcrops of the primitivelavas are rare (SiO2 =49.1 55.5 %, mg# = 28 63,Cr = 61 252 ppm and Ni=14 113 ppm). The positionin the AFM diagramdemonstrates its calc-
alkaline affinity. This is in sharp contrast with clearly tholeiitic lavasof the volcanic front Boquerón Volcano. A characteristic feature of theBVF lavas is a sharp increase in K2O with increasing differentiation(SiO2).There is a significant decrease in fluid-mobile vs. incompatible ratios(i.e. Ba/La) with increasing distance from the volcanic front. Thisindicates decreasing role of the slab -related fluid during magmageneration.
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–
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0 10 30 50 km
30
40
50
60
Ba/L
a
0 10 30 50 km
50
15
02
50
35
0
Ba/N
b
0 10 30 50 km
24
68
12
16
Th/T
a
0 10 30 50 km
0.5
12
85
0.5
12
95
Nd/
Nd
143
144
SiO2
Tholeiiteseries
Calc Alkaline (CA) series
A
F
M
Irvine Baragar1971
&
45 50 55 60
01
23
4
KO
2
Shoshoniteseries
High-K CAseries
CAseries
Tholeiite series
Pecerillo & Taylor, 1976
3
R
1 cm1 cm
1 mm1 mm
1 mm1 mm
250 m�250 m�
500 m�500 m�
