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

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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

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Michael Abratis*   Geochemisches Institut, Universita¨t Go¨ttingen, Goldschmidtstrasse 1, 37077 Go¨ttingen, 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 opening 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 ␮, window formed beneath southeastern Costa Rica and western Panama where ␮ϭ238U/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 cessation 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 Ϯ 0.40 16.87 ؎ 0.27 17.46 Ϯ 1.05 17.36 Ϯ 0.37 Ð BRI 25 7.42 Ϯ 0.34 Ð 5.64 Ϯ 0.58 5.82 ؎ 0.09 5.88 Ϯ 0.23 GUA 28 4.54 Ϯ 0.15 4.39 ؎ 0.14 3.77 Ϯ 0.61 3.92 Ϯ 0.11 Ð TAL 104 1.92 Ϯ 0.10 Ð 1.89 Ϯ 0.93 1.90 ؎ 0.17 Ð Note: Ages are in Ma. Age differences result from different analytical methods and interpretation methods. Uncertainties are 2 ␴ 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.

᭧ 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 radiogenic 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 Ϯ 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 Ϯ 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 Ϯ 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 Ϯ 0.27 Ma for tholeiitic rocks rocks indicate the magma sources since Cocos Ridge collision. Nb/Zr (ALT 161; Table 1) from southeastern Costa Rica. Whereas arc mag- ratios (Fig. 2) are a good parameter to characterize enrichment and matism is continuous in northern and central Costa Rica, there are no depletion of mantle magma sources because they should be insensitive dated `normal' calc-alkalic arc rocks (i.e., derived from melting the to partial melting and largely unaffected by the addition to their source of mobile components from slab dehydration. Ba/La ratios in arc mag- 1GSA Data repository item 200116, Age data, geochemistry, and isotopes, is available on request from Documents Secretary, GSA, P.O. Box 9140, Boul- ma sources, by contrast, are highly susceptible to contributions from a der, CO 80301±9140, [email protected], or at www.geosociety.org/pubs/ dehydrating subducted slab. ft2001.htm. Intrusive and extrusive rocks older than 8 Ma in the Cordillera de

128 GEOLOGY, February 2001 Talamanca are mostly calc-alkalic in composition, followed in volume tle (backarc alkalic magmas), and formation of melts from the sub- by still older tholeiitic arc rocks. Their high Ba/La ratios and low Nb/ ducting oceanic slab (adakites). Zr ratios (0.03±0.09) suggest derivation from a ¯uid-modi®ed, deplet- A model for the magmatic and plate tectonic evolution in Costa ed-mantle wedge, as is typical for `normal' arc magmatism. Rica thus has to explain the following observations: normal calc-alkalic Minor centers of alkalic volcanic and intrusive rocks occur scat- magmatism until 8 Ma and absence thereafter in southern Costa Rica; tered in the Costa Rican backarc and in one location at the forearc (5.8 strong uplift of the Cordillera de Talamanca since 5 Ma; the slab angle Ma to Holocene). They are clearly different from the older arc lavas shallowing below the Cordillera de Talamanca from ϳ60Њ in the north and intrusive rocks, although erupted in spatial and temporal proximity. to 35Њ±40Њ in the south (Carr et al., 1990); sudden shift to Pb isotope These rocks have only a subtle arc signature (Ba/La ϭ 13±19) and composition (206Pb/204Pb Ͼ 19.05) similar only to the Galapagos their high Nb/Zr ratio (0.17±0.46) indicates derivation from partial plume signature, and an enriched source for alkalic magmas behind melting of enriched-mantle material typical of an OIB source. the arc starting from 5.8 Ma; migration with time of backarc alkalic Adakites were previously known from western Panama (Defant et magmatism from 5.8 Ma in the southwest to Holocene in the northeast; al., 1991a, 1991b) and also occur in relatively large volumes at the evidence for enriched OIB-type sources in arc rocks in central Costa southern slope of the Cordillera de Talamanca, west of the Panamanian Rica; and the formation of adakites, which also have a Galapagos Pb border (Fig. 1). They are characterized by intermediate silica contents isotope signature. (SiO2 ϭ 55%±60%), high Mg# [Mg/(Mg ϩ Fe) ϭ 60±70], and frac- In order to explain these observations, we built our model on the tionated heavy rare earth element (REE) patterns. Their geochemical evolution of a slab window below southeastern Costa Rica as proposed signature (La/Yb ϭ 23±66, Sr/Y ϭ 73±116, Sr ϭ to 1500 ppm) dem- by Johnston and Thorkelson (1997). The hypothesis of slab-window onstrates the existence of residual garnet in the source consistent with formation is also supported by seismicity data for central Costa Rica melting of metamorphosed oceanic crust at pressures of at least 15 (Protti et al., 1995), which document the absence of shallow seismicity GPa (Peacock et al., 1994; Defant and Drummond, 1990). Adakites in the slab-window area. This slab window formed subsequent to the occur under unusually hot conditions within a subduction zone where Cocos Ridge±trench collision and the subduction of the Galapagos young oceanic crust is subducted at low convergence rates (Kay et al., spreading center, possibly aided by the presence of the Panama Fracture 1993; Peacock et al., 1994; Yogodzinski et al., 1995). Zone (Figs. 1 and 4; Johnston and Thorkelson 1997; Meschede et al., 1998, van der Lee and Nolet, 1997). The consequences of this slab CHANGING Pb ISOTOPIC COMPOSITIONS THROUGH window for magmatic evolution were as follows. TIME 1. Collision between the Cocos Ridge and trench, shallow sub- Post-5.8 Ma and thus syncollision to postcollision lavas from the duction, and the formation of the slab window caused normal calc- Cordillera de Talamanca and central Costa Rica show, with no excep- alkalic magmatism to stop, which explains the volcanic gap in the tion, higher 206Pb/204Pb ratios (19.06±19.28) compared with all older region of the Cordillera de Talamanca. Collision and slab-window for- calc-alkalic and tholeiitic arc rocks (18.72±18.87) and products of the mation may both have caused uplift of that mountain range by a com- northern parts of the currently active Central American arc (18.50± bination of tectonic and thermal processes. 18.65; Fig. 3). This change in Pb isotopes is coincident with the change 2. The evolution of a slab window explains the sudden appearance in magma sources indicated by the trace element data. It correlates in of alkalic magmatism, which derives from a completely different man- timing with the onset of the magmatic gap and uplift of the Cordillera tle source than previously recorded in Central America. The enriched de Talamanca (Fig. 3A). This new magma source emerged rather sud- Pb isotope composition (high 206Pb/204Pb ratios) of these lavas is char- denly (after ca. 6 Ma) and spatially close to (alkalic rocks) or even acteristic of only those mantle sources related to the Galapagos plume superimposed onto the former arc (adakites). The only magma source in the Paci®c. We therefore propose that Paci®c mantle enters through in this part of the world that has this distinctive radiogenic Pb isotope the slab window into the Caribbean realm. The observation that such composition is the plume component of the Galapagos hotspot (White enriched-mantle sources are today observed in Holocene backarc mag- et al., 1993; Fig. 3B). matism as well as in the active volcanic arc front (only) of central Costa Rica suggests that the mantle ¯ow has entered the Caribbean DISCUSSION mantle wedge to the north and is spreading parallel to the subducting Southern Costa Rica is the location of ridge-trench collision, where plate. A ¯ux of Paci®c asthenosphere into the Caribbean realm is thus the aseismic Cocos Ridge is currently being subducted beneath Costa documented, which increasingly replaces and mixes with the mantle Rica. Present continued convergence at a rate of 7±9 cm/yr (Kellogg et wedge of the Central American arc. The ®rst evidence for Paci®c man- al., 1995) and the Pliocene-Pleistocene uplift of the Talamanca region argue for shallow subduction of the Cocos Ridge, consistent with seismic data (Protti et al., 1995). If Cocos Ridge collision is responsible for cessation of normal arc volcanism since 8 Ma, then the collision must have occurred at that time. Collins et al. (1996) also proposed an effec- tive biogeographic barrier between Paci®c and Caribbean surface waters as early as 6±8 Ma. Krawinkel et al. (2000) observed the ®rst uncon- formities in forearc sedimentary rocks in the upper Miocene part of the stratigraphic section. However, it is clear from the sedimentological, pa- leogeographical, and paleontological records that the actual uplift and formation of the Cordillera de Talamanca started later, between 4.5 and 3.5 Ma (von Eynatten et al., 1993; Meschede et al., 1998; Krawinkel et al., 2000; Haug and Tiedemann, 1998). Figure 4. Schematic model of subduction zone beneath Costa Rica At that time, major changes in the melting regime beneath south- showing disruption of Cocos plate as consequence of collision and ern Costa Rica had occurred, as documented in the volumes and com- subduction of extinct spreading ridge (Cocos Ridge). Flux of Gala- pagos plume-modi®ed asthenosphere into mantle wedge of south- position of erupted lavas. These changes comprise the cessation of ern Costa Rica occurs through this slab window while leading edge normal, ¯uid-triggered, mantle-wedge melting (i.e., no more calc-al- of Cocos Ridge is heated and melts to form adakite magmas. PFZÐ kalic magmatism), a change to melting of a deep-seated, enriched man- Panama Fracture Zone.

GEOLOGY, February 2001 129 tle in the source of Costa Rican lavas is revealed in 5.8 Ma rocks. This amanca), in Mann, P., ed., Geologic and tectonic development of the Caribbean plate boundary in southern Central America: Geological Society of America Special Paper in¯ux is mirrored by the age progression (from southeast to northwest) 295, p. 35±55. in the eruption of alkalic OIB-type magmas with Galapagos-type Pb Defant, M.J., and Drummond, M.S., 1990, Derivation of some modern arc magmas by isotope signatures, suggesting a rate of movement of 40 mm/yr. melting of young subducted lithosphere. Nature, v. 347, p. 662±665. ϳ Defant, M.J., Richerson, P.M., de Boer, J.Z., Stewart, R.H., Maury, R.C., Bellon, H., Drum- 3. Adakite magmatism started after the slab window had formed. mond, M.S., Feigenson, M.D., and Jackson, T.E., 1991a, Dacite genesis via both slab The subducted oceanic crust of the Galapagos spreading center, even melting and differentiation: Petrogenesis of La Yeguada volcanic complex, Panama: Journal of Petrology, v. 32, p. 1101±1142. being rather young, could not have been the source for the adakites (as Defant, M.J., Clark, L.F., Stewart, R.H., Drummond, M.S., de Boer, J.Z., Maury, R.C., proposed by Johnston and Thorkelson, 1997), because their Pb isotope Bellon, H., Jackson, T.E., and Restrepo, J.-F., 1991b, Andesite and dacite genesis via composition is different from that of the Galapagos spreading center, contrasting processes: The geology and geochemistry of El Valle volcano, Panama: Contributions to Mineralogy and Petrology, v. 106, p. 309±324. more resembling that of the Galapagos plume (Fig. 3). Therefore, these Drummond, M.S., Bordelon, M., de Boer, J.Z., Defant, M.J., Bellon, H., and Feigenson, magmas may have formed by melting at the leading edge of the sub- M.D., 1995, Igneous petrogenesis and tectonic setting of plutonic and volcanic rocks ducting Cocos Ridge (Fig. 3), which is derived from the Galapagos of the Cordillera de Talamanca, Costa Rica±Panama, Central American arc: American Journal of Science, v. 295, p. 875±919. hotspot (Meschede et al., 1998). The subducted Cocos Ridge, however, Feigenson, M.D., and Carr, M.J., 1986, Positively correlated Nd and Sr isotope ratios of would have been too old (15 Ma) and too fast (7±9 cm/yr) to melt lavas from the Central American volcanic front: Geology, v. 14, p. 79±82. Feigenson, M.D., Carr, M.J., Patino, L.C., Maharaj, S., and Juliano, S., 1996, Isotopic (Peacock et al., 1994). The upwelling of Paci®c mantle through the identi®cation of distinct mantle domains beneath Central America: Geological So- slab window may therefore have facilitated Cocos Ridge melting by ciety of America Abstracts with Programs, v. 28, no. 7, p. 380. heating the slab edge. This additional heat source may be dispensable Gutscher, M.-A., Malavieille, J., Lallemand, S., and Collot, J.-Y., 1999, Tectonic segmen- tation of the North Andean margin: Impact of the Carnegie Ridge collision: Earth for slab melting in Panama since subduction is highly oblique and at and Planetery Science Letters, v. 168, p. 255±270. low convergence rates at that arc segment. Haug, G.H., and Tiedemann, R., 1998, Effect of the formation of the Isthmus of Panama on Atlantic Ocean thermohaline circulation: Nature, v. 393, p. 673±676. Herrstrom, E.A., Reagan, M.K., and Morris, J.D., 1995, Variations in lava composition CONCLUSIONS associated with ¯ow of asthenosphere beneath southern Central America: Geology, The geochemical and isotopic compositions of Pliocene-Pleisto- v. 23, p. 617±620. cene magmatic rocks from southern Costa Rica and of Holocene arc Hey, R., 1977, Tectonic evolution of the Cocos-Nazca spreading center: Geological Society of America Bulletin, v. 88, p. 1404±1420. magmatic rocks from central to northern Costa Rica involve an en- Johnston, S.T., and Thorkelson, D.J., 1997, Cocos-Nazca slab window beneath Central riched, OIB-type mantle source, similar in isotopic composition to the America: Earth and Planetary Science Letters, v. 146, p. 465±474. Kay, S.M., Ramos, V.A., and Marquez, M., 1993, Evidence in Cerro Pampa volcanic rocks Galapagos plume. for slab-melting prior to ridge-trench collision in southern South America: Journal The opening of a slab window in the subducting lithosphere en- of Geology, v. 101, p. 703±714. abled the in¯ux of Paci®c upper mantle into the Caribbean mantle Kellogg, J.N., Vega, V., Stallings, T.C., Aiken, C.L.V., and Kellogg, J.N., 1995, Tectonic development of Panama, Costa Rica, and the Colombian Andes: Constraints from wedge. Asthenospheric upwelling through the slab window by decom- Global Positioning System geodetic studies and gravity, in Mann, P., ed., Geologic pression of this Paci®c mantle induced partial melting and OIB-type and tectonic development of the Caribbean plate boundary in southern Central Amer- magmatism. The superposition of hot asthenosphere to the slab edges ica: Geological Society of America Special Paper 295, p. 75±90. Krawinkel, H., Seyfried, H., Calvo, C., and Astorga, A., 2000, Origin and inversion of resulted in a high geothermal gradient beneath the arc that eventually sedimentary basins in southern Central America: Zeitschrift fuÈr Angewandte Geo- caused partial melting of the subducted Cocos Ridge. Heating by up- logie, Sonderheft 1, p. 71±77. Meschede, M., Barckhausen, U., and Worm, H.-U., 1998, Extinct spreading on the Cocos welling mantle could also have contributed to uplift of the Cordillera Ridge: Terra Nova, v. 10, p. 211±216. de Talamanca, in addition to ample evidence of crustal shortening. Milionis, P.N., Feigenson, M.D., Carr, M.J., and Alvarado, G.E., 1986, Constraints on the The age progression of these OIB magmas of Paci®c-Galapagos source of central Costa Rican alkalic lavas [abs.]: Eos (Transactions, American Geo- physical Union), v. 67, p. 1280. ¯avor to the north and involvement of similar sources in Holocene Peacock, S.M., Rushmer, T., and Thompson, A.B., 1994, Partial melting of subducting Central American arc magmatism suggest subduction-parallel north- oceanic crust: Earth and Planetary Science Letters, v. 121, p. 227±244. ward ¯ow at a rate of 40 mm/yr. Protti, M., GuÈendel, F., and McNally, K., 1995, Correlation between the age of the subducting Cocos plate and the geometry of the Wadati-Benioff zone under Nicaragua and Costa Rica, in Mann, P., ed., Geologic and tectonic development of the Caribbean ACKNOWLEDGMENTS plate boundary in southern Central America: Geological Society of America Special We are grateful to G.E. Alvarado, F. 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