HR: 0830h
AN: V31C-0942    [PDF]
TI: Chemical Variation and Origin of Pleistocene Silicic Pyroclastic Flows from Central American Volcanic Arc
AU: * Patino, L C
EM: patinoL@msu.edu
AF: Department of Geological Sciences, Michigan State University, 206 Natural Sciences Bldg, East Lansing, MI 48824 United States
AU: Vogel, T A
EM: vogel@msu.edu
AF: Department of Geological Sciences, Michigan State University, 206 Natural Sciences Bldg, East Lansing, MI 48824 United States
AU: Alvarado, G E
EM: GAlvaradoI@ice.go.cr
AF: Escuela Centroamericana de GeologĦa, Universidad de Costa Rica, Apdo. 35, San Jos‚, Costa Rica, San Jose, 00000 Costa Rica
AU: Rose, W I
EM: raman@mtu.edu
AF: Department of Geological Engineering and Sciences, Michigan Technological University, 207 Dow Environmental Sciences Bldg, 1400 Townsend Drive, Houghton, MI 49931 United States
AU: Viray, E L
EM: virayela@msu.edu
AF: Department of Geological Sciences, Michigan State University, 206 Natural Sciences Bldg, East Lansing, MI 48824 United States
AB: Silicic pyroclastic flows have occurred along the Central American volcanic arc during the Pleistocene. The volcanic products from these eruptions range in composition have SiO${2}$ mode of 70 wt.%. In the latest part of the Pleistocene ($<$100 Ky), ash-flow tuffs are common in Guatemala, El Salvador and Nicaragua, but absent from Costa Rica. Phenocryst phases of the silicic pyroclastic flows along Central America are generally very similar. Plagioclase dominates as phenocryst and in the groundmass. Sub-calcic augite, hypersthene are common in dacites. Amphibole as well as biotite are common in the rhyolites and rhyodacites in Guatemala and El Salvador, but these minerals are absent from all units in Nicaragua, and from the youngest unit in Costa Rica. In addition, Ti-oxides, apatite, quartz, are common. It is worth noting that the silicic rocks from Central America contain very little alkali feldspar. There are some chemical variations in the composition of ash-flow tuffs along the arc. The most primitive materials in ignimbrites from Guatemala and Costa Rica are basaltic andesites and andesites; but in Nicaragua and El Salvador, basaltic scorias are present. The ignimbrites from Central Costa Rica have higher alkali content, than the rest of the Pleistocene ash-flow tuffs from Central America, a pattern similar to that observed in lavas. Furthermore, some trace element and trace element ratios also vary along the arc in a pattern similar to that observed in the lavas. The ignimbrites from Nicaragua tend to have the lowest concentrations of Zr ($<$100 ppm) and the highest concentration of Rb ($>$200 ppm); Ba/Th, U/Th are highest and La/Yb is the lowest in Nicaragua.Fractional crystallization has played a role in the evolution of ash-flow sheets in the Central American volcanic arc. Some samples from Guatemala and El Salvador have Eu/Eu*$<$0.7 and Sr$<$200 ppm. These compositions can be interpreted as the result of plagioclase fractionation. However, this process alone does not explain the wide range of compositions observed in the ignimbrites. We argue that variations in the composition of the parent material and in the conditions of melt generation played an important role in the generation of the diverse compositions of Pleistocene ash-flow tuffs. Furthermore, we propose that that the basement rocks along the arc, Paleozoic continental crust in the north and Caribbean large igneous province in the south, are not possible sources for the Pleistocene silicic pyroclastic flows. Based on the similar key trace-element ratios of the ignimbrites and the arc lavas, we concluded that remelting of the calc-alkaline plutons, emplaced at depth, are the likely source of the ash-flow tuffs.
DE: 1010 Chemical evolution
DE: 1020 Composition of the crust
DE: 3640 Igneous petrology
DE: 3655 Major element composition
DE: 3670 Minor and trace element composition
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting