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The determination of accurate and precise ages for the timing of collision between oceanic plateaus and continental crust requires an understanding of how the indenting and buttressing plates respond to the collision. We present geochronological, thermochronological, geochemical and isotopic analyses of magmatic rocks from the Ecuadorian Andes, which relate to the collision of the Late Cretaceous Caribbean Plateau and Great Arc sequence with NW South America. The cessation of subduction magmatism during 65–64 Ma beneath the eastern edge of Caribbean Plateau was synchronous with accelerated surface uplift and exhumation within the buttressing continental margin during 75–65 Ma. We interpret this as the collision of the leading edge of the Caribbean Plateau and arc sequence with the South American Plate at 75–65 Ma. A U/Pb (zircon) SHRIMP age of 87.10 ± 1.66 (2σ) Ma, yielded by an accreted fragment of the plateau, precludes previous estimates of collision at 85–80 Ma if the plateau erupted above the Galápagos hotspot. Terra Nova, 18, 264–269, 2006

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Ion microprobe analytical techniqueZircons were separated from several kilograms of sample by conventional means. The sub-300 µm fraction was processed using a Wilfey table, and then the Wilfey heavies were passed through a Frantz magnetic separator at 1 A. The non-paramagnetic portion was then placed in a filter funnel with di iodomethane. The resulting heavy fraction was then passed again through the Frantz magnetic separator at full current. All zircons were hand picked in ethanol using a binocular microscope. Zircons were mounted in a resin disk along with the zircon standard and polished to reveal the grain interiors. The mounts were gold-coated and imaged with a Hitachi S-4300 scanning electron microscope (SEM), using a cathodoluminescence probe (CL) to image internal structures, overgrowths and zonation. Secondary electron mode (SE) imaging was employed to detect fractures and inclusions within the grains. U-Th-Pb zircon analyses were performed on a Cameca IMS 1270 ion-microprobe following methods described by Whitehouse & Kamber (2005) which were modified from Whitehouse et al. (1999). U/Pb ratio calibration was based on analyses of the Geostandards zircon 91500, which has an age of 1065.4 ± 0.3 Ma and U and Pb concentrations of 80 and 15 ppm, respectively (Wiedenbeck et al., 1995). Replicate analyses of the same domain within a single zircon were used to independently assess the validity of the calibration. Data reduction employed Excel macros developed by Whitehouse at the Swedish Natural History Museum, Stockholm. Age calculations were made using Isoplot version 3.02 (Ludwig, 2003). U-Pb data are plotted as 2σ error ellipses. All age errors quoted in the text are 2σ unless specifically stated otherwise. Common lead corrections were only applied to samples which exhibited significant levels of 204 Pb, and where applied are indicated in the data tables. They assume a modern day average terrestrial common Pb composition (Stacey and Kramers, 1975), i.e., 207 Pb/ 206 Pb = 0.83. A detailed rationale for choosing present day Pb as a contaminant is given by Zeck and Whitehouse (1999). LA-MC-ICPMS analytical techniqueA c. 12kg sample of Tyrone Central Inlier gneiss (JTP-210) was was crushed and sieved using standard mineral preparation procedures. Heavy minerals were concentrated using a