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U-Pb spot dating for detrital single zircon grains collected from the mouth of the Amazon River with help of LA-ICP-MS and continental crust growth history of the Amazon River Basin.

Motoki, A.*; Orihashi, Y.**; Hirata, D.***; Haller, M.J.****; Ramos, V.A.*****; Schilling, M.******; Iwano, H.*******; Cario, F.D.********; Anma, R.*********

* Rio de Janeiro State University, Brazil.
** University of Tokyo, Japan.
*** Kanagawa Prefecture Museum, Japan.
**** National Patagonian centre, Argentina.
***** Buenos Aires University, Argentina.
****** Chile University, Chile.
******* Kyoto Fission Track, Japan.
******** Federal Fluminense University, Brazil.
********* Tsukuba University, Japan.

Short Papers of VI South American Symposium on Isotopic Geology, 1, 108-110. August, 2003, Salvador, Bahia, Brazil. Download full texto (3 pgs). View poster.

The Patagonian adakitic volcanoes take place in the arc region of the southern end of South America Continent, forming the Austral Volcanic Zone, and are attributed to slab melting caused by on-going active Chile Ridge subduction below South America continental plate (Stern and Killian, 1996). On the other hand, in the back-arc region, two small dacitic adakite bodies, the Cerro Pampa (Ramos, et al., 1991; Kay, et al, 1993) and the Puesto Nuevo (Ramos, et al, 1994) are exposed (Fig. 1). Their peculiar occurrences are of a great interest to understand the tectonic setting of adakitic magma generation.

The previous works show that the K-Ar radiometric age for the Cerro Pampa (Ramos, et al., 1991; Kay, et al, 1993) and the Puesto Nuevo (Ramos, et al, 1994) were widely different. The Cerro Pampa body has Middle Miocene age (12.10.7Ma and 12.00.7Ma, whole rock), obtained by the Sernageomin, Chile (Ramos, et al., 1991). The Puesto Nuevo body has late Cretaceous ages (84.56 Ma and 76.75Ma Ma, in hornblende) obtained in the late seventies in the Cambridge University, UK. Because of these Mesozoic ages, the Puesto Nuevo adakitic body was attributed to possible existence of an old ridge subduction, and interpreted to have no relation to on-going Chile ridge subduction.

The authors performed U-Pb dating from spot (pinpoint) analyses of single zircon crystal grains for both adakitic bodies with help of laser ablation-ICP Mass Spectrometry at the Earthquake Research Institute of the University of Tokyo. The laser ablation system utilises a frequency-quadrupled Nd-YAG UV laser (266 nm wavelength) and produces an ablation pit size of 30-35 mm. Detail of the analytical procedure is described by Orihashi & Hirata (2003). No statistic correction to improve apparent dating precision is applied.

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