Volcanic history of western Nicaragua and geochemical evolution of the Central American volcanic front.

摘要

As part of a long term study on the geochemical evolution of the Central American Volcanic Front led by my advisor, Michael Carr and colleagues at Rutgers University, my dissertation work centers around the development of a better understanding of the duration and scope of a Central America volcanic lull to better constrain and interpret possible causation. To so, I have attempted to improve our knowledge of the volcanic record during this interval by attempting to locate samples that might have erupted during this time period. This effort yielded ages on volcanic samples ranging from 1.1 to 3.6 Ma and, thus, substantially reducing the volcanic gap in Nicaragua. Thus, it may be that while Nicaraguan volcanism was reduced in volume between 7 and 3.6 Ma, activity was still occurring albeit at a substantially reduced rate.;Results of this effort, has given a more complete record that permits more precise analysis of how the volcanic front has evolved since the Miocene. Geochemical analyses of the active and Miocene volcanic fronts document that U/Th values increased by nearly threefold following the Central America, Pacific Coast "carbonate crash" that has been documented at about 10 Ma, related to the closing of the Isthmus of Panama. Although this transition was considered rather abrupt, data from this study indicate that the transition was somewhat more gradual, occurring over the last 7 myr. An increase in La/Yb Since the Miocene and with increasing distance from the trench, is perhaps due to backward migration of the arc towards the trench. A systematic increase in Zr/Nb towards the trench shows no correlation with age is indicative of a stronger subduction signal near the trench.;Geochemical analysis of Tertiary volcanic rocks from El Salvador to Costa Rica combined with available 40Ar/39Ar ages indicate that U/Th values along the Nicaraguan segment of the front have increased since the Miocene, while Ba/La values have not. These values have both remained the same towards the edges of the front.