For permission to copy, contact editing@geosociety.org © 2007 Geological Society of America 697 ABSTRACT We investigated the Neoproterozoic–early Paleozoic evolution of the Gondwanan mar- gin of the north-central Andes by employing U-Pb zircon geochronology in the Eastern Cordilleras of Peru and Ecuador using a combination of laser-ablation–inductively coupled plasma–mass spectrometry detrital zircon analysis and dating of syn- and post- tectonic intrusive rocks by thermal ionization mass spectrometry and ion microprobe. The majority of detrital zircon samples exhibits prominent peaks in the ranges 0.45–0.65 Ga and 0.9–1.3 Ga, with minimal older detri- tus from the Amazonian craton. These data imply that the Famatinian-Pampean and Grenville (= Sunsas) orogenies were available to supply detritus to the Paleozoic sequences of the north-central Andes, and these oro- genic belts are interpreted to be either buried underneath the present-day Andean chain or adjacent foreland sediments. There is evi- dence of a subduction-related magmatic belt (474–442 Ma) in the Eastern Cordillera of Peru and regional orogenic events that pre- and postdate this phase of magmatism. These are confirmed by ion-microprobe dating of zircon overgrowths from amphibolite-facies schists, which reveals metamorphic events at ca. 478 and ca. 312 Ma and refutes the previously assumed Neoproterozoic age for orogeny in the Peruvian Eastern Cordillera. The presence of an Ordovician magmatic and metamorphic belt in the north-central Andes demonstrates that Famatinian meta- morphism and subduction-related mag- matism were continuous from Patagonia through northern Argentina to Venezuela. The evolution of this extremely long Ordo- vician active margin on western Gondwana is very similar to the Taconic orogenic cycle of the eastern margin of Laurentia, and our findings support models that show these two active margins facing each other during the Ordovician. Keywords: Gondwana, Andes, Peru, geochro- nology, zircon, Paleozoic. INTRODUCTION The Andes represent the locus of continued plate convergence through much of the Pha- nerozoic. Whereas Andean deformation and magmatism have been extensively studied, the early evolution of much of the proto-Andean margin remains poorly understood. The main reason is because exposures of pre-Andean basement rocks are in many places extremely limited because they are either obscured by later tectonic events along the convergent margin or buried by the ubiquitous volcanic cover. This problem is particularly acute in the north-central Andes, where Precambrian base- ment is not exposed for over 2000 km along strike, from 15°S in Peru to 2°S in Colombia. This corresponds to the distance between the northern extent of the Arequipa-Antofalla base- ment (Fig. 1), a Proterozoic crustal block that experienced 0.9–1.2 Ga Grenville metamor- phism (Loewy et al., 2004; Wasteneys et al., 1995), and the southernmost basement expo- sures in Colombia, the Proterozoic Garzón inlier (Restrepo-Pace et al., 1997; Cordani et al., 2005). This zone is also characterized by sub- stantial development of Andean foreland sedi- ments to the east, so that the basement geology peripheral to the orogen is not known with any degree of certainty. However, in the Eastern Cordilleras of Peru and Ecuador, Paleozoic metasedimentary sequences are well exposed. Most of these sequences, including the Marañon Complex in Peru (Fig. 2) and the Isimanchi and Chiguinda Units of the Cordillera Real in Ecuador (Fig. 2), are considered to be autochthonous with respect to the Gondwanan margin (Hae- berlin, 2002; Pratt et al., 2005). Hence, their heavy mineral assemblages, and, in particular, their detrital zircon populations, should reveal information regarding their source areas—in U-Pb geochronologic evidence for the evolution of the Gondwanan margin of the north-central Andes David M. Chew † Urs Schaltegger Department of Mineralogy, University of Geneva, Rue des Maraîchers 13, 1205 Geneva, Switzerland Jan Košler Department of Earth Science, University of Bergen, Allegaten 41, N-5007 Bergen, Norway Martin J. Whitehouse Laboratory for Isotope Geology, Swedish Museum of Natural History, S-104 05 Stockholm, Sweden Marcus Gutjahr Institute for Isotope Geology and Mineral Resources, ETH-Zentrum, Clausiusstrasse 25, CH-8092 Zürich, Switzerland Richard A. Spikings Aleksandar Miškovi´ c Department of Mineralogy, University of Geneva, Rue des Maraîchers 13, 1205 Geneva, Switzerland † Present address: Department of Geology, Trinity College Dublin, Dublin 2, Ireland; e-mail: chewd@ tcd.ie. GSA Bulletin; May/June 2007; v. 119; no. 5/6; p. 697–711; doi: 10.1130/B26080.1; 8 figures; Data Repository item 2007110.