HR: 0830h
AN: S41D-0123    [PDF]
TI: Termination of a Central Andean arc: The Chemical Evolution of the Bonete-Incapillo Volcanic Complex, Argentina
AU: * Goss, A R
EM: arg32@cornell.edu
AF: INSTOC, Snee Hall Cornell Univ., Ithaca, NY 14853 United States
AU: Kay, S M
EM: smk16@cornell.edu
AF: INSTOC, Snee Hall Cornell Univ., Ithaca, NY 14853 United States
AB: The Bonete-Incapillo Volcanic Complex is situated near 28$\deg$S at the southern termination of the Central Volcanic Zone (CVZ), where the more steeply-dipping segment of the subducting Nazca Plate to the north intersects the amagmatic flatslab to the south. Dominated by the Pissis (6882m), Bonete Chico (6759m), and Bonete Grande (6436m) centers and the Incapillo Caldera, this complex includes some of the highest Plio-Pleistocene volcanoes on Earth. K-Ar ages indicate that volcanism in this region began at 6.5 to 3.5 Ma as the Miocene magmatic arc to the west broadened and eventually died out. During this period, the subducting Nazca Plate shallowed, initiating eastward broadening of the arc. The arc front migrated 50km to the east before terminating at this latitude with the formation of the Incapillo Caldera at 2.9 to 1.1 Ma. Nearly 80 major and trace element analyses from the Bonete Complex and the associated Pircas Negras andesitic lavas combined with 35 new $^{87}$Sr/$^{86}$Sr analyses allow for interpretation of the regional geochemical evolution. Prior to arc migration, the Miocene Do\~{n}a Ana Formation lavas that erupted in this region have $^{87}$Sr/$^{86}$Sr ratios (0.7047-0.7056) that positively correlate with SiO$_{2}$ content (56-74 %), suggesting that MASH-style crustal contamination was a dominant process in magma evolution. In contrast, the Late Miocene to Pliocene magmas from the Bonete Complex and Pircas Negras units show a range of $^{87}$Sr/$^{86}$Sr ratios (0.7057-0.7073) in samples with uniform SiO$_{2}$ contents. Unlike the older Do\~{n}a Ana magmas and $<$3 Ma CVZ magmas further north, these migration-stage magmas exhibit steep `adakitic' REE patterns (La/Yb = 40-70) indicating equilibration with garnet-bearing crust. These steep REE patterns and elevated $^{87}$Sr/$^{86}$Sr ratios are consistent with source region contamination by forearc subduction erosion that resulted in transport of the missing Mesozoic igneous and forearc suites along the Chilean coast into the melt zone of the mantle wedge. A model invoking only lower crustal contamination cannot explain flatter REE patterns (La/Yb $<$35) in CVZ lavas to the north, as there is no obvious way to decrease crustal thicknesses beneath the CVZ arc. As magmatism waned, the Incapillo ignimbrites and domes (SiO$_{2}$ = 67-71; $^{87}$Sr/$^{86}$Sr = 0.7064-0.7067; La/Yb = 40-80) represent the final stages of a residual source-enriched magma that equilibrated with garnet-bearing thickened lower crust.
DE: 1040 Isotopic composition/chemistry
DE: 1065 Trace elements (3670)
DE: 3640 Igneous petrology
DE: 8105 Continental margins and sedimentary basins
DE: 9360 South America
SC: Seismology [S]
MN: 2003 Fall Meeting