HR: 10:35h
AN: U52A-02 INVITED     [Abstracts]
TI: U-Pb Geochronologic Evidence for the Evolution of the Gondwanan Margin of the North- Central Andes
AU: * Chew, D
EM: chewd@tcd.ie
AF: Trinity College, Dublin, Dublin 2, Ireland
AU: Schaltegger, U
EM: urs.schaltegger@terre.unige.ch
AF: Department of Mineralogy, University of Geneva, CH-1205 Geneva, Switzerland
AU: Košler, J
EM: jan.kosler@geo.uib.no
AF: Department of Earth Science, University of Bergen, Allegaten 41, N-5007 Bergen, Norway
AU: Whitehouse, M
EM: martin.whitehouse@nrm.se
AF: Laboratory for Isotope Geology, Swedish Museum of Natural History, S-104 05 Stockholm, Sweden
AU: Gutjahr, M
EM: gutjahr@erdw.ethz.ch
AF: Institute for Isotope Geology and Mineral Resources, ETH-Zentrum, Clausiusstrasse 25, CH-8092 Zürich, Switzerland
AU: Spikings, R
EM: richard.spikings@terre.unige.ch
AF: Institute for Isotope Geology and Mineral Resources, ETH-Zentrum, Clausiusstrasse 25, CH-8092 Zürich, Switzerland
AU: Miškovic, A
EM: aleksandar.miskovic@terre.unige.ch
AF: Institute for Isotope Geology and Mineral Resources, ETH-Zentrum, Clausiusstrasse 25, CH-8092 Zürich, Switzerland
AB: The Neoproterozoic - Early Paleozoic evolution of the Gondwanan margin of the north-central Andes has been investigated by a U-Pb zircon geochronology study in the Eastern Cordilleras of Peru and Ecuador, combining LA- ICPMS detrital zircon analysis with dating of syn- and post-tectonic intrusives by TIMS and ion microprobe. The majority of detrital zircon samples exhibit prominent peaks in the ranges 0.45 - 0.65 Ga and 0.9 - 1.3 Ga, with minimal older detritus from the Amazonian craton. The detrital zircon data demonstrate that the basement to the western Gondwanan margin was likely composed of a metamorphic belt of Grenvillian age, upon which an Early Paleozoic magmatic belt was situated in a similar way to the Sierra Pampeanas and Famatina Terranes of northern Argentina. These two orogenic belts are interpreted to be either buried underneath the present-day Andean chain or adjacent foreland sediments. Plutons associated with the Early Paleozoic subduction-related magmatic belt have been identified in the Eastern Cordillera of Peru, and have been dated by U-Pb zircon TIMS and ion microprobe to 474 - 442 Ma. This is in close agreement with the ages of subduction-related magmatism in the Arequipa - Antofalla Basement (e.g. Loewy et al., 2004). This Early Paleozoic arc is clearly not linear as it jumps from a coastal location in the Arequipa - Antofalla Basement to several hundred kilometers inland in the Eastern Cordillera further to the north. This is interpreted as an embayment on the Proto-Andean margin at the time the arc was initiated; if this is the case the northern termination of the Arequipa-Antofalla Basement in the vicinity of Lima is an Ordovician or older feature. The arc magmatism pre- and post dates phases of regional metamorphism in the Eastern Cordillera of Peru. U- Pb zircon ion microprobe dating of zircon overgrowths in high-grade leucosomes demonstrates that the presence of a metamorphic event at c. 478 Ma, and refutes the previously-assumed Neoproterozoic age for orogeny in the Peruvian Eastern Cordillera. The presence of an Early - Middle Ordovician age magmatic and metamorphic belt in the north-central Andes demonstrates that Famatinian metamorphism and subduction-related magmatism was continuous from Patagonia (Pankhurst et al., 2006) through northern Argentina and Chile to as far north as Colombia and Venezuela, a distance of nearly seven thousand kilometers. The presence of an extremely long Early - Middle Ordovician active margin on western Gondwana invites comparison with the Taconic - Grampian orogenic cycle of the eastern Laurentia margin (which is of similar age and strike length) and supports models which have these two active margins facing each other during the Ordovician. U-Pb zircon ion microprobe dating of zircon overgrowths in migmatites yields ages of c. 312 Ma, and represent a previously unreported high-grade Gondwanide event which has affected the Peruvian segment of the Proto- Andean margin. The pattern of crustal growth in the north-central Andes implies that it was dominated by a series of progressive crustal accretion events, which results in a series of age domains that young away from an old Amazonian core.
UR: http:www.tcd.ie/Geology/Staff/chewd/publications.php
DE: 1115 Radioisotope geochronology
DE: 8104 Continental margins: convergent
DE: 8185 Volcanic arcs
DE: 9360 South America
SC: Union [U]
MN: 2007 Joint Assembly