HR: 12:05h
AN: T42A-08    [Abstracts]
TI: Crustal Recycling in Accretionary Orogens: LA-ICP-MS Geochronology and Hf Isotope Evidence of Detrital Zircons in Late Paleozoic Turbidite Units of the Southern Central Andes
AU: * Bahlburg, H
EM: bahlbur@uni-muenster.de
AF: Geologisch-Palaeontologisches Institut, Muenster University, Muenster, 48149, Germany
AU: Vervoort, J D
EM: vervoort@wsu.edu
AF: School of Earth & Environmental Sciences, Washington State University, Pullman, WA 99164-2812, United States
AB: Active margin accretionary orogens are considered major sites of formation of juvenile continental crust. In the southern central Andes, Nd model ages of the Proterozoic metamorphic basement, the Neoproterozoic and Paleozoic sedimentary cover and the coeval magmatic rocks cluster between 1.8 and 1.4 Ga and vary between 2.2 and 1.2 Ga, respectively. This indicates a dominant involvement of Meso- and Paleoproterozoic crustal material in crustal cycling. We applied LA-ICP-MS to obtain U-Pb age spectra of detrital zircons and to analyze individual dated zircons for their Hf isotope systematics. We selected 7 samples of Late Devonian to Permian turbidite units of northern Chile because their detritus most likely reflects the provenance evolution of this margin prior to the onset of the Andean active margin cycle. The studied rocks are parts of an accretionary wedge at Gondwana's late Paleozoic active margin in the Central Andes. The units overstep the boundary between the enigmatic Chilenia terrane of alleged Laurentian origin in the south and coeval autochthonous regions of Gondwana to the north. Zircon populations of overstepping units deposited on and derived from Chilenia should ideally demonstrate the Laurentian heritage of the terrane by an absence of geochronologic events typical of Gondwana and absent from Laurentia, i.e. the Brasiliano orogenic cycle. Of each sample we analysed 140 to 145 grains for their U-Pb systematics. We only considered grains falling within 10% of concordia, i.e. between 99 and 118 grains per samples. Subsequently c. 20 dated grains per sample were analyzed for Hf isotopes. The U-Pb data of all samples reflect the same distribution of main events, the Rhondonian orogeny of the southwestern Amazon craton between 1.5 and 1.2 Ga, the "Grenvillian" orogenic cycle between 1.2 and 0.9 Ga, the Brasiliano cycle between 0.8 and 0.55 Ga, and the early Paleozoic Famatiniano cycle between 0.55 and 0.42 Ga. The younger, Late Carboniferous-Permian turbidites also register a cluster of ages between 0.33 and 0.25 Ga marking the re-initiation of subduction magmatism after a period of magmatic, tectonic and metamorphic quiescence in the Late Silurian and Devonian. Evidence of the Gondwanan Brasiliano cycle in all samples casts doubt on a Laurentian origin of the Chilenia terrane. The Hf data demonstrate that all analyzed sedimentary rocks contain a significant number of juvenile zircons with positive epsilon Hf values. These grains, however, mostly fall in the 1.5 to 0.9 Ga age bracket and indicate formation of their protoliths during the Rhondonian and "Grenvillian" orogenic cycles. Grains with negative epsilon Hf values can also be traced back to the same juvenile Mesoproterozoic crustal domains. In conclusion, the available data indicate that formation of juvenile crust mainly took place between 1.5 and 0.9 Ga. During all subsequent orogenic cycles affecting the region of the southern central Andes between 0.9 and 0.25 Ga, the crustal evolution of this accretionary orogen seems to have been characterized by repeated recycling of the continental crust originally formed in the Mesoproterozoic.
DE: 1020 Composition of the continental crust
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1115 Radioisotope geochronology
DE: 8104 Continental margins: convergent
DE: 8169 Sedimentary basin processes
SC: Tectonophysics [T]
MN: 2007 Fall Meeting