HR: 1340h
AN: T13C-1468    [Abstracts]
TI: Late Cretaceous Arc Initiation on the Edge of an Oceanic Plateau (Southern Central America)
AU: * Buchs, D M
EM: david.buchs@unil.ch
AF: University of Lausanne, Institut de Geologie et Paleontologie, Anthropole, Lausanne, 1015, Switzerland
AU: Baumgartner, P O
EM: peter.baumgartner@unil.ch
AF: University of Lausanne, Institut de Geologie et Paleontologie, Anthropole, Lausanne, 1015, Switzerland
AU: Arculus, R
EM: rarculus@ems.anu.edu.au
AF: Australian National University, Department of Earth and Marine Sciences, DEMS Building 047, Canberra, ACT 0200, Australia
AB: The Caribbean Plate comprises one or several late Cretaceous oceanic plateaus imbricated between the Northern and Southern Americas. Uplifted portions of plateau(s) along plate boundaries have been recognized in many sites, including that underlying the south Central American Volcanic Arc. We provide new constraints for the role of the plateau in the evolution of this arc obtained by mapping of the uplifted forearc area between southern Costa Rica and western Panama. An oceanic plateau, accreted seamounts and arc rocks were identified, and a new tectono-stratigraphy defined. The arc basement is composed of a Coniacian oceanic plateau. In the outer margin, late Cretaceous-Eocene accreted seamounts are in contact with the plateau along tectonic mélanges and active faults. Campanian-Maastrichtian primitive arc rocks are found 40-110 km to the trench on the top of -or as dykes within- the plateau. The location of these rocks correlates to previous observations and indicates that the arc front migrated away from the trench during the late Cretaceous, potentially in response to subduction erosion or slab flattening [Lissinna et al., EGU 2006]. The first island arc lavas were deposited under sea level, over a broad area. They were quickly followed by more evolved intrusives and lavas, which were emplaced along a volcanic front during the late Cretaceous-Paleocene. Detrital and volcanic records along the Central American isthmus indicate that a continuous volcanic arc extended between eastern Panama and northern Costa Rica in this time. In southern Costa Rica (Golfito complex) and western Panama (Sona-Azuero-Coiba complex), the oceanic plateau consists mainly of pillowed and massive low Fe (tholeiitic) basalts. These rocks have a highly consistent geochemistry characterized by flat, primitive upper mantle-normalized incompatible element patterns with low Pb and high Nb-Ti contents. Primitive arc igneous samples are low-medium Fe basalts to trachyandesites found as pillow lavas, massive lava flows and intrusives. These rocks have incompatible element patterns similar to plateau basalts. However, whole rock and unaltered clinopyroxene compositions indicate variable Pb enrichments and Nb-Ti depletions that are due to slab-derived fluids in the source. Differentiation trends of the primitive island arc differ compared with the plateau in terms of Si, Al, Fe, Mg, Ca and alkalis. The source of the primitive arc magmas corresponds to the lithospheric mantle initially associated with the plateau. Beginning of melting is linked to slab-derived fluids and was likely facilitated by high thermal gradients under the plateau. As a consequence, incompatible elements of the primitive arc lavas mimic plateau affinities and show variable arc signature overprints. In some cases, the resemblance between the early arc and plateau rocks is very high, making a distinction between the two types of rocks almost impossible.
DE: 1065 Major and trace element geochemistry
DE: 3640 Igneous petrology
DE: 8038 Regional crustal structure
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
DE: 8486 Field relationships (1090, 3690)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting