Striking Local Distinctions in Basaltic Melts within Nicaraguan Cross-arc Lineaments

Abstract:

The Nejapa-Miraflores (NM) and Granada (G) lineaments which cut across the Central American volcanic front (CAVF) host numerous monogenetic vents which have erupted diverse basaltic magmas (e.g., Walker, 1984). As previously shown by Walker (1984), the basaltic magmas loosely fall into two groups: a high Ti, low K group which are reminiscent of MORB or BABB; and a low Ti, high K group which are more typical of subduction zones worldwide. Major element data obtained from over 200 olivine-hosted melt inclusions found within NM and G tephras from six separate monogenetic vents confirm this unusual compositional dichotomy. Melt inclusions from four of the six monogenetic vents are exclusively high- or low-Ti, while two of the volcanoes have both high- and low-Ti melt inclusions.

New volatile and trace element data on over 40 of the NM and G melt inclusions has yielded additional compositional distinctions between the high- and low-Ti groups. Least degassed high-Ti melts tend to have lower water contents than their low-Ti counterparts. The high-Ti Inclusions also have lower concentrations of U, Th, Pb, Ba and Cs and lower La/Yb ratios. In addition, there are subtle HFSE variations between the two types of basalts.

The overall geochemical differences between the high- and low-Ti groups suggest that the mantle wedge source of the latter contains a greater slab-derived (hemipelagic) sediment melt component than the former linked to a larger flux of hydrous fluids from deeper in the subducting Cocos plate. What is particularly significant is that the contrasting mafic emanations from these monogenetic volcano lineaments demonstrate that transport of fluids, volatiles and basaltic melts in subduction zones can be quite variable and complex on a very localized scale.