Geochemical differences between along-arc and across-arc volcanics in west-central Nicaragua

The La Paz Centro - Malpaisillo Lineament (LPML) in west-central Nicaragua is a north-south striking, 20km long chain of maars and cinder cones, which intersects the northwest-southeast striking main volcanic front. A tectonic control of LPML volcanism is likely but only evident for the Malpaisillo fissure at the northern end of the LPML. Previous work demonstrated geochemical variations implying changes in mantle-source composition (i.e., added slab components) along the Central American Volcanic Arc at spatial scales of some 10's of kilometers. Our study of the LPML shows that minor but systematic changes also occur across the arc within 20km distance. Variations in trace element ratios such as Zr/Nb, Ba/Th, Ba/La, Th/Zr, U/La and La/Yb along the LPML, i.e. across the volcanic front indicate little change in the degree of partial melting but an increase particularly in the hemipelagic sediment component in the mantle source from the fore arc towards the arc front, followed by a decrease behind the arc. Interestingly, the slab component is most prominent just in front of the arc. About 60km southeast of the LPML, the Nejapa-Miraflores volcanic and tectonic lineament, which marks a 20km north-south offset in the arc, differs substantially from the LPML. There is a wide scatter in incompatible trace element ratios indicating a heterogeneous mantle source at small spatial scales (c.1km). This mantle heterogeneity may represent vertical rather than across-arc variations and is probably related to the arc offset, because in the absence of such offset at the LPML mantle source conditions vary much less but more systematically.