HR: 1340h
AN: V13B-1484    [Abstracts]
TI: Ar/Ar ages and Chemistry of Miocene "Adakites" East of the Chile Triple Junction: An Association between Slab-melts of the Nazca Plate and Patagonian Plateau Basalts
AU: * Kay, S M
EM: smk16@cornell.edu
AF: INSTOC, Cornell Univ., Snee Hall, Ithaca, NY 14853 United States
AU: Ramos, V A
EM: andes@gl.fcen.uba.ar
AF: Univ. Buenos Aires, Laboratorio de Tectonica Andina, Buenos Aires, C1428EHA Argentina
AU: Singer, B S
EM: bsinger@geology.wisc.edu
AF: Univ. Wisconsin, Dept. of Geology and Geophysics, Madison, WI 53706 United States
AB: The origin of many 'adakites' ascribed to slab melting has been questioned with their formation being attributed to melting of thickened continental crust or forearc subduction erosion. Some of the best remaining candidates for a slab-melt origin are the Cerro Pampa 'adakites' ($\sim$48$\deg$S) in Patagonia, erupted east of the inactive volcanic arc as the Chile Ridge collided with the Chile Trench at ca. 12 Ma. Support for a slab melt origin for these hornblende-bearing dacites comes from high Sr (1330-2300 ppm) at 63-68% SiO$_{2}$, MORB-like $^{87}$Sr/$^{86}$Sr (0.7028-0.7031) and $^{143}$Nd/$^{144}$Nd $>$0.5129, and steep (heavy depleted) REE patterns (La/Yb = 30-37) in an area of thin crust. Two other 'adakite' localities to the southwest, also east of where the Chile ridge collided at ca. 12 Ma, have been recognized (65-66% SiO$_{2}$, 1370-1440 ppm Sr, $^{87}$Sr/$^{86}$Sr = 0.7032-7033, $^{143}$Nd/$^{144}$Nd $\sim$0.51289, La/Yb = 28-30). These 'adakites' at Puesto Neuvo ($\sim$49$\deg$S) and Chalt\'{e}n north of Lago Viedma ($\sim$49.5$\deg$S) lie east of young Austral Volcanic Zone 'adakites'. $^{40}$Ar/$^{39}$Ar plateau ages obtained via laser incremental heating on hornblendes show that the Chalt\'{e}n, Puesto Nuevo and Cerro Pampa 'adakites' decrease in age (14.50$\pm$0.29, 13.12$\pm$0.55, 11.39$\pm$0.61 Ma) consistent with melting of the trailing edge of the Nazca plate as ridge collision progressed to the north. Their chemistry contrasts with non-adakitic Miocene (?) backarc andesite/dacites further south, east of where the ridge collided at ca. 13 to 14 Ma. A major difference in the later region is the absence of late Miocene plateau basalts related to a slab window between the Nazca and Antarctic plates. This association suggests that Miocene slab melting in Patagonia required a thermal input from the asthenospheric mantle as well as a young, hot subducting plate. A similar case can be made for Austral Volcanic Zone "adakites" and Pliocene/Recent plateau basalts.
DE: 9360 South America
DE: 8110 Continental tectonics--general (0905)
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
DE: 1065 Trace elements (3670)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting