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Ridge Collision, Slab-Window Formation, and the Flux of Pacific

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Ridge Collision, Slab-Window Formation, and the Flux of Pacific See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/249520792 Ridge collision, slab-window formation, and the flux of Pacific asthenosphere into the Caribbean realm Article in Geology · February 2001 DOI: 10.1130/0091-7613(2001)029<0127:RCSWFA>2.0.CO;2 CITATIONS READS 178 393 2 authors: M. Abratis Gerhard Wörner Friedrich Schiller University Jena Georg-August-Universität Göttingen 45 PUBLICATIONS 531 CITATIONS 235 PUBLICATIONS 6,499 CITATIONS SEE PROFILE SEE PROFILE Some of the authors of this publication are also working on these related projects: Trace element determination via high sensitivity Laser Ablation Sector Field ICP-MS View project Geology of the central andes in northern Chile View project All content following this page was uploaded by Gerhard Wörner on 14 August 2014. The user has requested enhancement of the downloaded file. Ridge collision, slab-window formation, and the ¯ux of Paci®c asthenosphere into the Caribbean realm Michael Abratis* Geochemisches Institut, UniversitaÈt GoÈttingen, Goldschmidtstrasse 1, 37077 GoÈttingen, Germany Gerhard WoÈrner ABSTRACT Mantle wedge±derived arc volcanism ceased in southern Costa Rica after ca. 8 Ma because of subduction of the aseismic Cocos Ridge beneath the Central American arc and the subsequent open- ing of a slab window. Geochemical and isotopic compositions of small volumes of adakitic and alkalic backarc lavas erupted between 5.8 and 2 Ma identify a source derived from the Galapagos plume. The presence of this source is explained by an in¯ux of Paci®c upper mantle into the Caribbean mantle wedge through a slab window, where the alkalic rocks form by melting of the upwelling mantle and the adakites result from melting of the leading edge of the subducted Cocos Ridge. By using geochemical and isotopic signatures, we trace this upper mantle ¯ow beneath Central America from southern Cos- ta Rica northward at a rate of 40 mm/yr. Keywords: Costa Rica, Cocos Ridge, subduction zone, geochemistry, isotope ratios, adakite. INTRODUCTION Many studies on Central American volcanism have pointed out differences in volumes, composition, and plate con®guration in central and southern Costa Rica as compared to the remainder of the arc (Carr, 1984; Feigenson and Carr, 1986; Carr et al., 1990; de Boer et al., 1995; Figure 1. Plate tectonic con®guration of Central America and Gala- Drummond et al., 1995). The arc in southeastern Costa Rica is char- pagos area (after Gutscher et al., 1999). Holocene stratovolcanoes acterized by (1) the involvement of an enriched-mantle source, (2) a are missing in this segment of Central American volcanic arc, where volcanic gap in the region of the strongly uplifted Cordillera de Tala- aseismic Cocos Ridge is being subducted. Inset map shows sample manca in southeastern Costa Rica, and (3) the eruption of alkalic rocks, locations in area of Cordillera de Talamanca in southern Costa Rica. mainly in the backarc, and adakitic lavas. PFZÐPanama Fracture Zone; RSBÐrough±smooth boundary. The enriched component was identi®ed only in arc rocks just to the north of the volcanic gap (Carr, 1984; Feigenson and Carr, 1986; al. (1995), i.e., trench-parallel mantle ¯ow bringing the enriched com- Reagan and Gill, 1989; Carr et al., 1990; Chan et al., 1999). Radiogenic ponent from central South America. isotope studies by Feigenson et al. (1996) helped to de®ne the ocean- Johnston and Thorkelson (1997) introduced the idea that a slab island basalt (OIB) type mantle component as HIMU mantle (high m, window formed beneath southeastern Costa Rica and western Panama where m5238U/204Pb). because of the subduction of an extinct segment of the Galapagos Observed trace element and isotope variations in central Costa spreading center (Fig. 1). Their conclusion was supported by the ex- Rican volcanic rocks are explained by mixing of an enriched-mantle istence of adakites in Panama (de Boer et al., 1991; Drummond et al., component into a depleted-mantle wedge with a slab component that 1995) because such products of slab melting occur where spreading is decreasing volumetrically with time (Carr et al., 1990). The cause ridges are subducted (e.g., Kay et al., 1993; Stern and Kilian, 1996). for mixing these components and their temporal and spatial distribution We present geochronological, trace element, and isotopic data on remained unclear. Carr et al. (1990) postulated that the subarc mantle a suite of magmatic rocks from the Cordillera de Talamanca as well might consist of enriched domains (enriched mid-ocean ridge basalt, as the forearc and backarc regions in southeastern Costa Rica (Table E-MORB, or OIB source) embedded in a matrix of depleted mantle, 1; Figs. 1±3). These rocks were emplaced since 17 Ma; they document creating a ``plum pudding±type'' mantle wedge. An alternative expla- changing magma sources through time. We develop a model that relates nation for this upper mantle enrichment was proposed by Herrstrom et the formation and consequences of a slab window with (1) the cessa- tion of volcanism in southeastern Costa Rica, (2) the in¯ux of enriched- *Present address: Abteilung fuÈr Vulkanologie und Petrologie, GEOMAR, mantle material into the mantle wedge beneath the Central American Wischhofstrasse 1±3, 24148 Kiel, Germany. arc, and (3) the origin and isotopic composition of backarc alkalic rocks TABLE 1. NEW Ar/Ar AGE DETERMINATIONS FOR MAGMATIC ROCKS FROM THE CORDILLERA DE TALAMANCA AREA Sample FurnaceÐtotal fusion FurnaceÐlargest step Laser mean age Laser weighted mean Inverse isochron ALT 16 18.33 6 0.40 16.87 6 0.27 17.46 6 1.05 17.36 6 0.37 Ð BRI 25 7.42 6 0.34 Ð 5.64 6 0.58 5.82 6 0.09 5.88 6 0.23 GUA 28 4.54 6 0.15 4.39 6 0.14 3.77 6 0.61 3.92 6 0.11 Ð TAL 104 1.92 6 0.10 Ð 1.89 6 0.93 1.90 6 0.17 Ð Note: Ages are in Ma. Age differences result from different analytical methods and interpretation methods. Uncertainties are 2 s of the mean. Boldface entries are the most reliable age values on the basis of an evaluation of the different methods; see Abratis (1998) for details. q 2001 Geological Society of America. For permission to copy, contact Copyright Clearance Center at www.copyright.com or (978) 750-8400. Geology; February 2001; v. 29; no. 2; p. 127±130; 4 ®gures; 1 table; Data Repository item 200116. 127 Figure 3. A: Pb isotope composition of southeastern Costa Rica magmatic rocks through time. Note sudden change to more radio- genic compositions after 6 Ma. B: Pb isotope ratios of south Costa Rican samples and reference ®elds (EPRÐEast Paci®c Rise, GSCÐ Galapagos spreading center, GIÐGalapagos Islands) are from White et al. (1993); sediment ®eld is from Ben Othmann et al. (1989). Shad- ed ®elds from central and northern Costa Rica are from Feigenson et al. (1996) and M. Carr (2000, personal commun.). Note grouping of samples into two distinct ®elds, near compositions of Galapagos Figure 2. A: Distribution and age of different rock types in Cordillera plume source (high 206Pb/204Pb ratios) and EPR-, and GSC, mid- de Talamanca. B: Geochemical parameters (Nb/Zr, Ba/La) that char- ocean ridge basalt, respectively. acterize mantle-wedge composition show distinct change in space and time across Cordillera de Talamanca. Radiometric ages for Tal- amanca were either determined on our samples or taken from ref- mantle wedge above the slab) younger than 8 Ma from the Cordillera erences cited in text. de Talamanca (Alvarado et al., 1992; de Boer et al., 1995). This lack of younger arc rocks de®nes the magmatic gap in the arc in that region. Apparently since 8 Ma, the Cocos Ridge collided with Central Amer- and of slab melts in that region. The enriched-mantle material has the ica, causing uplift and the formation of the Cordillera de Talamanca Pb isotope composition of the Galapagos plume component; its tem- between 4.5 and 3.5 Ma (Krawinkel et al., 2000; Meschede et al., poral and spatial distribution de®nes mantle ¯ow through, and the evo- 1998). Alkalic backarc volcanism also occurred about this time. This lution of, the slab window. backarc magmatic activity shows a trend of younging toward the north- ARC EVOLUTION IN SOUTHERN COSTA RICA THROUGH west as evidenced by the rock suite of Bribri near the Panamanian TIME border, dated here as 5.82 6 0.09 Ma (BRI 25; Table 1), the Guayacan The Farallon plate broke up to form the Cocos and Nazca plates suite in central Costa Rica of Pliocene age (GUA 28: 4.39 6 0.14 Ma; to the south and the Juan de Fuca plate in the north at 28 Ma (Hey, Table 1), and backarc suites farther north, which are Pleistocene (1.2 1977); spreading of ridges within the Cocos and Nazca plates com- Ma) to Holocene in age (Milionis et al., 1986). Adakites erupted be- menced at 23 Ma (Meschede et al., 1998). The volcanic arc in southern tween 3.5 Ma (de Boer et al., 1995) and 1.9 6 0.17 Ma (Table 1) in Costa Rica formed subsequently, thus closing the Central American the Cordillera de Talamanca and on its southwestern slope (Fig. 1). landbridge (Seyfried et al., 1991). With increasing arc maturity from GEOCHEMISTRY OF ERUPTED LAVAS AND INTRUSIVE 23 Ma to the present, magmatic products changed from tholeiitic arc ROCKS to calc-alkalic compositions (Alvarado et al., 1992). This change is Trace elements (Fig. 2) and isotopic compositions (Fig. 3) of these consistent with our older age of 16.87 6 0.27 Ma for tholeiitic rocks rocks indicate the magma sources since Cocos Ridge collision.
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