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doi: 10.1111/j.1365-3121.2006.00688.x The early interaction between the Caribbean Plateau and the NW South American Plate Cristian Vallejo,1 Richard A. Spikings,2 Leonard Luzieux,1 Wilfried Winkler,1 David Chew2 and Laurence Page3 1 Geological Institute, ETH Zürich, CH-8092 Zürich, Switzerland; 2Department of Mineralogy, University of Geneva, CH-1205 Geneva, Switzerland; 3Department of Geology, Lund University, Sölvegatan 12, 223 62 Lund, Sweden ABSTRACT 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 Introduction There is a consensus that a majority of thickened, allochthonous oceanic mafic material exposed in the western part of the Northern Andes in Ecuador and Colombia represents relict fragments of the Caribbean Plateau, which erupted above an oceanic hotspot. Several studies have proposed that the Caribbean Plateau erupted above the Galápagos hotspot (e.g. Duncan and Hargraves, 1984; Ross and Scotese, 1988; Hoernle et al., 2004), whereas other authors suggest it may have erupted above more than one hotspot, some of which may have a southern Pacific origin (e.g. Reynaud et al., 1999; Kerr and Tarney, 2005). Knowledge of the timing of accretion of the plateau with the continental margin is paramount to any plate reconstruction model of the Caribbean Plate, as it temporally constrains the onset of its fragmentation. Previous estimates of the timing of collision between the Caribbean Plateau and the Ecuadorian continental margin cluster at either 85–65 Ma Correspondence: Dr Cristian Vallejo, Geologisches Institut, ETH Zentrum HAD, Haldenbachstrasse 44, 8092 Zürich, Switzerland. Tel.: +41 1 632 6129; fax: +41 4 4632 1422; e-mail: cristian.vallejo@erdw. ethz.ch 264 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 (2r) 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 (e.g. Lebrat et al., 1987; Aspden et al., 1992; Kerr et al., 2002; Spikings et al., 2005) or the Late Campanian–Maastrichtian (e.g. 75– 65 Ma; Spikings et al., 2001; Hughes and Pilatasig, 2002; Jaillard et al., 2004). We present new geochronological, geochemical and isotopic evidence from the accreted oceanic rocks, which, when combined with previous sedimentological and thermochronological analyses, argues against previous models of accretion during 85–80 Ma and strongly supports a model of Late Campanian–Maastrichtian (75–65 Ma) collision between the Caribbean Plate and the Ecuadorian continental margin. The age and origin of the mafic oceanic basement in western Ecuador Basaltic lavas and hyaloclastites of the Pallatanga Unit, which form the basement of the Pallatanga Terrane, represent fragments of the Caribbean Plateau that collided against the South American Plate (Kerr et al., 2002). These rocks are exposed in fault-bound slivers along the eastern margin of the Western Cordillera (Fig. 1). Their major oxides, trace elements and isotope geochemistry suggest that they erupted in an oceanic plateau setting (Reynaud et al., 1999; Lapierre et al., 2000; Kerr et al., 2002; Mamberti et al., 2003), with geochemical similarities with mafic rocks of the Caribbean– Colombian Oceanic Plateau. Ultramafic cumulates and gabbros of the San Juan Unit are in faulted contact with the Pallatanga Unit and are considered to represent the ultramafic root component of an oceanic plateau sequence (Cosma et al., 1998; Lapierre et al., 2000; Mamberti et al., 2004). Radiometric ages from the mapped San Juan Unit range between 123 and 87 Ma, suggesting that the lithologically defined unit may comprise unrelated rock sequences. Lapierre et al. (2000) report an Early Cretaceous amphibole–plagioclase–whole rock internal Sm/Nd isochron age of 123 ± 13 Ma (2r) from a gabbro (their sample 98SJ13) in the San Juan Unit. Mamberti et al. (2004) presented a weighted mean 40Ar/39Ar (hornblende) age of 99.2 ± 1.3 Ma (2r) from a gabbro, which is mapped as part of the San Juan Unit. This 40Ar/39Ar age is derived from the youngest steps of a saddle-shaped age spectrum, which is typical of mineral phases that contain excess 40Ar, and their age should be considered as a maximum age (e.g. Harrison and McDougall, 1981). Finally, zircons extracted from a layered gabbro of the San Juan Unit yield a weighted mean U/Pb (SHRIMP) age 2006 Blackwell Publishing Ltd Terra Nova, Vol 18, No. 4, 264–269 C. Vallejo et al. • Caribbean Plateau–South American Plate collision ............................................................................................................................................................. Fig. 1 Simplified geology of Ecuador showing the juxtaposition of oceanic mafic rocks of the Pallatanga Terrane (which includes the San Juan Unit, Pujilı́ Mélange, Yunguilla Unit and the Rio Cala Arc sequence) and continental rocks of the Eastern Cordillera and the Amotape Terrane. of 87.10 ± 1.66 Ma (2r; Fig. 2). Lapierre et al. (2000) interpreted their Early Cretaceous Sm/Nd age as evidence for a distinct, oceanic plateau sequence that predates the Pallatanga Unit (e.g. Jaillard et al., 2004), whereas we interpret our Late Cretaceous U/Pb age as the time of crystallization of mafic components of the Pallatanga Unit. Consequently, we suggest that the San Juan unit is inaccurately mapped, and both Early and Late Cretaceous ultramafic and mafic rocks define the basement sequence in the easternmost Western Cordillera. The U/Pb (zircon) age obtained in this study overlaps with the peak of ages (92–88 Ma) obtained from basalts of the present day Caribbean Plateau using the 40Ar/39Ar method (Sinton et al., 1998; Kerr et al., 2003). The U/ Pb age is also indistinguishable from a plateau hornblende 40Ar/39Ar age of 88.8 ± 1.6 Ma (2r), obtained from geochemically defined oceanic plateau rocks of the Piñón Fm. in the coastal 2006 Blackwell Publishing Ltd forearc of Ecuador (Luzieux et al., 2005), suggesting that both units may comprise parts of the same plateau sequence. Palaeomagnetic data from both the Piñon Fm. and its sedimentary cover rocks imply that they crystallized at latitudes between 0 and 5S and experienced a clockwise rotation event during the Campanian (Luzieux et al., 2005). The Early Cretaceous rock sequences reported by Lapierre et al. (2000) may represent detached fragments of the Early Cretaceous ultramafic–mafic Peltetec Unit, which yields island arc geochemical affinities, and accreted to the South American continent during the Aptian (Litherland et al., 1994). This interpretation is also supported by a negative Nb anomaly in some of the rocks included in the San Juan Unit (Mamberti et al., 2004), typical for rocks formed in a subduction zone. Zircons from a granitic block (Pujilı́ Granite) entrained in a tectonized zone of the Pallatanga Unit (Pujilı́ Mélange; Fig. 1) yield a weighted mean U/Pb (SHRIMP) age of 85.5 ± 1.4 (2r) Ma. White mica from the same granite yields an indistinguishable plateau 40Ar/39Ar age of 86 ± 1 Ma (Spikings et al., 2005). These ages, combined with positive (juvenile) eNdi values (+6.9), high concentrations of LILE elements (Ba, Sr), and negative Nb anomalies (Fig. 3A,B) indicate a subduction-related origin for the granite in an intraoceanic setting. The U/Pb age of the Pujilı́ Granite is indistinguishable from the Pallatanga Unit, and hence may have formed as a fractionated intrusive fragment produced during the initiation of westward subduction beneath the Caribbean Plateau, shortly after the eruption of the plateau, resulting in the formation of part of the Great Arc of the Caribbean. Earlier interpretations, which proposed that the Pujilı́ Granite represents fragments of Triassic plutons that are currently exposed in the Eastern Cordillera, and became incorporated into the mélange zone during ocean–continent collision (Hughes and Pilatasig, 2002; Spikings et al., 2005) are no longer plausible. The association of Late Cretaceous granitic rocks with island arc signatures and Late Cretaceous mafic oceanic plateau rocks is also described on the Island of Aruba. White et al. (1999) interpreted the Aruba Batholith as being produced by partial melting of the Caribbean Plateau above an incipient subduction zone with an anomalously hot mantle wedge. The Aruba Batholith (c. 85 Ma, 40Ar/39Ar hornblende; White et al., 1999) and the Pujilı́ Granite share a similar crystallization age (c. 85 Ma, U/Pb zircon), and very distinctive trace element distribution (e.g. high LILE, high La/Yb, high Sr/Y and HREE depletion) and isotopic geochemistry (eNdi approximately 7), inferring that both sequences may have formed in the same subduction system. The Totoras amphibolite sequence is located in the central Western Cordillera (Fig. 1). The amphibolites yield an oceanic plateau geochemical affinity (e.g. Jaillard et al., 2004; Beaudon et al., 2005) and are juxtaposed against unmetamorphosed mafic rocks of the Pallatanga Unit. Beaudon et al. (2005) suggested the 265 Caribbean Plateau–South American Plate collision • C. Vallejo et al. Terra Nova, Vol 18, No. 4, 264–269 ............................................................................................................................................................. (A) Gabbro (B) Granite Hornblende, Totoras Amphibolite Biotite, Triassic migmatite (Figs 1 and 2B), which we interpret as being indicative of rapid cooling through 380–330 C during the Late Maastrichtian. Aspden et al. (1992) report widespread resetting of K/Ar ages acquired from Jurassic–Lower Cretaceous rocks in the Eastern Cordillera during 85–65 Ma. However, it is impossible to assess whether any individual age is partially or fully reset and consequently these data cannot be used to distinguish between Santonian–Early Campanian and Late Campanian– Maastrichtian cooling. Consequently, there is no thermochronological support from the buttressing continental margin for reactivation by collision during 85–80 Ma. White mica, Triassic migmatite Evidence from sedimentary rocks that span the age of collision Pyroxene, Natividad Unit Fig. 2 (A) Tera-Wasserburg concordia diagrams for U/Pb (zircon) data yielded by the San Juan Unit (UTM: 759961/9967463) and the Pujili Unit (UTM: 755300/ 9898600) using SHRIMP analysis. Errors are given at the 2r level. (B) 40Ar/39Ar age spectra (25W CO2-IR laser; 2r errors) from the Totoras amphibolite (UTM: 730223/ 9809289), the Triassic migmatites exposed in the southern Eastern Cordillera (UTM: 758038/9669978) and basaltic lavas of the Natividad Unit (UTM: 771610/9996629). Analytical details and data tables are stored in the data repository. UTM coordinates are indicated in the Prov. S. Amer. 56 system. amphibolites were formed by metamorphism of an oceanic plateau at 800–850 C and 6–9 kbar. We obtained a hornblende 40Ar/39Ar plateau age of 84.69 ± 2.23 (2r) Ma from the amphibolite (Fig. 2B). Considering an estimated thickness of the Caribbean Plateau of approximately 20 km (Sinton et al., 1998; Revillon et al., 2000) and a geothermal gradient of 40 C km)1, the peak temperature and pressure conditions acquired from the amphibolites could have been reached at the base of the plateau. The high geothermal gradient was probably supported by the same mantle plume that produced the oceanic plateau. Retrogression of the amphibolites through approximately 500 C at 84.69 ± 2.23 Ma is probably a consequence of thermal relaxation of the oceanic plateau, subsequent to drift away from the hotspot. 266 Thermochronological constraints from the buttressing Late Cretaceous continental margin (Eastern Cordillera) Palaeozoic–Early Cretaceous metasedimentary and intrusive rocks of the Eastern Cordillera and the Amotape Complex (Fig. 1) define the Late Cretaceous continental margin (i.e. prior to collision). The earliest significant cooling and exhumation event detected along the continental margin by 40Ar/39Ar (white mica, biotite) and fission track (zircon, apatite) thermochronology occurred during 75– 65 Ma (Spikings et al., 2001, 2005). This has been confirmed by new indistinguishable plateau 40Ar/39Ar ages from Triassic migmatites (U/Pb zircon age of 227 ± 2 Ma; Litherland et al., 1994) of 68.5 ± 0.4 (2r; white mica) and 68.6 ± 0.5 (2r; biotite) Ma from the southern Eastern Cordillera Prominent Late Cretaceous sedimentary formations to the east and west of the developing contemporaneous Andean chain provide a further record of the accretion of the oceanic plateau in the forearc. During the Maastrichtian, erosion of the Napo Group marine sequence in the developing Amazon Foreland Basin (Fig. 1) was followed by deposition of continental redbeds of the Tena Formation (Aspden and Litherland, 1992; Jaillard, 1997). Sedimentary rocks of the Tena Formation are the oldest within the foreland basin to host a significant assemblage of metamorphic mineral grains derived from the Eastern Cordillera (Ruiz et al., 2004). Furthermore, fissiontrack ages of detrital zircons in the same rocks are indistinguishable from their stratigraphic age, which is indicative of extremely rapid exhumation in the supplying Eastern Cordillera (Ruiz et al., 2004). At the same time, detrital supply from the Guyana Shield diminished. In the forearc, the coeval (Late Campanian–Maastrichtian) turbiditic Yunguilla Unit was being deposited in a basin partly floored by the Pallatanga Unit (e.g. Jaillard et al., 2004; Fig. 1). The turbidites were partially sourced from metamorphic rocks of the Eastern Cordillera and from mafic volcanic rocks. These sedimentary sequences attest to the topographic growth of the Eastern Cordillera, located proximal to the zone of ocean–continent collision, during the Latest Campanian–Maastrichtian. 2006 Blackwell Publishing Ltd C. Vallejo et al. • Caribbean Plateau–South American Plate collision Terra Nova, Vol 18, No. 4, 264–269 ............................................................................................................................................................. Rock/primitive mantle (A) (B) Fig. 3 (A) Primitive mantle normalized (Sun and McDonough, 1989) multi-element diagram for the Pallatanga Unit and the Rio Cala Arc sequence. The subduction nature from the Rio Cala Arc sequence is evident by the relative enrichment in LILE elements and the Nb anomaly, the latter not observed in rocks from the Pallatanga Unit (i.e. whole-rock data shown in grey field; Hughes and Pilatasig, 2002). (B) eNdi– eSri correlation diagram for the Rio Cala Arc sequence (Rio Cala, Pilatón, Mulaute and Natividad units), including data from Cosma et al. (1998) and Mamberti et al. (2004). The eNdi ratios of these rocks (+6 to +9) imply an intra-oceanic setting and may be a continuation of the Great Arc of the Caribbean (e.g. Burke, 1988). The shift towards higher eSri values in the Pujilı́ Granite can be accounted for by hydrothermal alteration, possibly associated with serpentinization of the host mafic rocks. The lifespan of the Rio Cala Arc sequence The Rio Cala Arc sequence, exposed in the northern Western Cordillera (Fig. 1), is defined here as a series of volcanoclastic turbidites (Natividad, Pilatón and Mulaute units), with intercalated and juxtaposing, faultbounded sequences of basaltic lavas (e.g. the Rio Cala and La Portada units). Primitive mantle-normalized multielement plots of these volcanic rock sequences (Fig. 3A) indicate high concentrations of LILE elements (Sr and Ba) and distinctive negative Nb anomalies, typical of rocks formed via subduction. High eNdi values of (+6 to +9) have been obtained from the Rio Cala Arc sequence, which overlap with those acquired from basalts in the present-day Caribbean region (e.g. Thompson et al., 2003). The chemical composition of clinopyroxenes extracted from both basalts and sandstones in the Rio Cala arc sequence indicates a tholeiitic island arc setting (data repository item). 2006 Blackwell Publishing Ltd Kerr et al. (2002) proposed that volcanic rocks of the Rio Cala Unit were produced by eastward subduction below an already accreted oceanic plateau in a continental arc setting. This interpretation was based on the more evolved nature of rocks from the Rio Cala Unit, as shown by an enrichment of LREE concentrations, despite relatively high MgO contents (approximately 8%). However, geochemical evidence led Allibon et al. (2005) to suggest that the lavas of the Rio Cala Unit originated from subduction beneath thickened oceanic crust in an intra-oceanic arc. The LREE enrichments are accounted for by the assimilation of oceanic plateau rocks. Similarly, initial Nd and Pb isotope ratios indicate that the rocks of the Rio Cala Unit result from the mixing of Pacific MORB mantle, subducted pelagic sediments and an oceanic plateau component (Allibon et al., 2005), which is consistent with an intra-oceanic island arc setting. Furthermore, heavy mineral assemblages (Vallejo et al., 2003) within turbidites intercalated with volcanic rocks of the Rio Cala Unit, indicate they were derived exclusively from a mafic volcanic source region. Collectively, these data support the hypothesis that the Rio Cala Arc developed in an intraoceanic setting, and has a pre-accretionary origin. Sedimentary rocks intercalated within boninitic pillow basalts of the La Portada Unit (Van Thournout et al., 1992; Kerr et al., 2002) yield a Santonian–Campanian biostratigraphic age (Boland et al., 2000). A prevailing interpretation for boninitic rocks is that they define the early stage of an island arc (Pearce et al., 1992; Stern and Bloomer, 1992), formed beneath very young and hot oceanic crust (Stern et al., 1991). Lavas intercalated in the Natividad Unit yield a plateau 40Ar/39Ar (clinopyroxene) age of 64.3 ± 0.4 Ma (2r; Fig. 2B) and volcanoclastic strata of the Natividad Unit yield Campanian to Maastrichtian microfossils (Boland et al., 2000). We interpret the Rio Cala Arc sequence to have originated by west dipping subduction of the protoCaribbean oceanic crust beneath the relatively buoyant Caribbean Plateau (e.g. Burke, 1988) during the Santonian–Early Campanian (Fig. 4). The termination of the arc in the Maastrichtian–Danian period corresponds to clogging of the subduction zone by collision between the Caribbean Plateau and the South American Plate. The absence of regional igneous activity on the continental margin during 85–65 Ma may be attributable to a cessation of subduction beneath the continental margin as a consequence of ocean basin closure being solely accommodated by westward subduction beneath the Caribbean Plateau. A significant quantity of material derived from the Eastern Cordillera occurs in the uppermost Mulaute Unit (Spikings et al., 2005), which conformably overlies Maastrichtian to Palaeocene volcanic-derived turbidites of the Pilatón Unit. This data suggest that the island arc system was close to the continent during the Palaeocene. Summary and conclusions A new zircon U/Pb (SHRIMP) age of 87.10 ± 1.66 Ma from basement 267 Caribbean Plateau–South American Plate collision • C. Vallejo et al. Terra Nova, Vol 18, No. 4, 264–269 ............................................................................................................................................................. Hughes, regarding accretionary events in western Ecuador and the Caribbean region. John Aspden and Andrew Kerr provided constructive reviews of the manuscript. Field sampling benefited from the assistance and knowledge of Peter Hochuli, Friedrich Heller, Efraı́n Montenegro, William Lugo and Diego Villagomez. This work was supported by the Swiss National Science Foundation (projects 2-77193-02 and 2-77504-04). Fig. 4 Reconstruction of the Caribbean Plateau during the Santonian–Maastrichtian (modified and simplified from Ross and Scotese, 1988). Schematic postulated extensions to the Great Arc of the Caribbean have been made to include the Rio Cala Arc sequence. The Pallatanga Terrane is shown as a fragment of the Caribbean Plateau. GHS: Galápagos hot spot. mafic rocks of the previously mapped Early Cretaceous San Juan Unit, within the eastern Western Cordillera, suggests that the San Juan Unit is inaccurately mapped and the Early Cretaceous rocks may only exist as small-scale (e.g. several metres wide), fault-bounded slivers. Our new Late Cretaceous U/Pb age is the first radiometric age from mafic basement rocks of the Pallatanga Unit, which, when combined with its geochemical characteristics, supports a derivation from the coeval Caribbean Plateau. The U/Pb age is consistent with volcanism associated with the Caribbean Plateau during 92–88 Ma (Sinton et al., 1998; Kerr et al., 2003) and an 40Ar/39Ar age of 88 ± 1.6 Ma from oceanic plateau rocks of the Piñón Fm., located along present-day coastal Ecuador (Luzieux et al., 2005). Palaeomagnetic analyses of the Piñón Fm. (Luzieux et al., 2005) strongly indicate that these oceanic plateau remnants were derived from an oceanic plateau formed at equatorial palaeolatitude, which is consistent with a Galapagos hotspot origin (Fig. 4). The initiation of west-dipping subduction at the leading edge of the plateau, was probably responsible for the generation of the ocean island-arc related Pujilı́ Granite at c. 85.5 Ma, via partial melting of the plateau, similar to what has been proposed by White et al. (1999) for the Aruba Batholith. Furthermore, Santonian–Campanian boninites of the La Portada Unit (Rio Cala Arc sequence) were also produced during the early stages of arc magmatism via subduction beneath 268 the Caribbean Plateau, and may be partly coeval with the Pujilı́ Granite. The time-span between the eruption of oceanic plateau basalts and the islandarc related Pujilı́ Granite and boninites of the Rio Cala Arc sequence suggests that migration of the Caribbean Plateau and subsequent initiation of westward subduction below the plateau occurred c. 2 Myr after the eruption of the plateau. The oceanic plateau and overlying island arc subsequently drifted to the northeast and collided with the South American continental margin during the Campanian, although subductionrelated magmatism terminated at the Maastrichtian–Danian transition, suggesting that the collision may have been oblique. It is likely that the collision between the South American Plate and the Caribbean Plateau resulted in rapid surface uplift in the Eastern Cordillera, with erosion and deposition in the fore- and backarc. Furthermore, the onset of rapid exhumation throughout the Late Cretaceous continental margin at 75–65 Ma temporally corroborates the onset of clastic sedimentation derived from the continental margin during the Late Campanian–Maastrichtian. Collectively, this evidence strongly favours the hypothesis that the collision between the Caribbean Plateau and the Ecuadorian margin occurred in the Late Campanian–Maastrichtian (75–65 Ma). Acknowledgements The authors would like to express their thanks to Arturo Egüez and Richard References Allibon, J., Monjoie, P., Lapierre, H., Jaillard, E., Bussy, F., Bosch, D., 2005. High Mg-basalts in the Western Cordillera of Ecuador: evidence of plateau root melting during Late Cretaceous arcmagmatism. 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Lithos, 46, 43–68. Received 15 March 2006; revised version accepted 17 May 2006 269