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�