Nicoya Peninsula, Costa Rica: a Single Suite of Caribbean Oceanic Plateau Magmas Christopher W
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University of Rhode Island DigitalCommons@URI Graduate School of Oceanography Faculty Graduate School of Oceanography Publications 1997 Nicoya Peninsula, Costa Rica: A single suite of Caribbean oceanic plateau magmas Christopher W. Sinton University of Rhode Island Robert A. Duncan See next page for additional authors Follow this and additional works at: https://digitalcommons.uri.edu/gsofacpubs Terms of Use All rights reserved under copyright. Citation/Publisher Attribution Sinton, C. W., R. A. Duncan, and P. Denyer (1997), Nicoya Peninsula, Costa Rica: A single suite of Caribbean oceanic plateau magmas, J. Geophys. Res., 102(B7), 15507–15520, doi: 10.1029/97JB00681. Available at: https://doi.org/10.1029/97JB00681 This Article is brought to you for free and open access by the Graduate School of Oceanography at DigitalCommons@URI. It has been accepted for inclusion in Graduate School of Oceanography Faculty Publications by an authorized administrator of DigitalCommons@URI. For more information, please contact [email protected]. Authors Christopher W. Sinton, Robert A. Duncan, and Percy Denyer This article is available at DigitalCommons@URI: https://digitalcommons.uri.edu/gsofacpubs/254 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 102, NO. B7, PAGES 15,507-15,520,JULY 10, 1997 Nicoya Peninsula, Costa Rica: A single suite of Caribbean oceanic plateau magmas ChristopherW. Sinton1 andRobert A. Duncan Collegeof Oceanicand Atmospheric Sciences, Oregon State University, Corvallis Percy Denyer EscuelaCentroamericana de Geologia,Universidad de CostaRica, SanJose, Costa Rica Abstract. The pre-Tertiaryoceanic crust exposed on the westcoast of CostaRica hasbeen broadlyreferred to asthe Nicoya Complex. This study was designed to determinethe age of the NicoyaComplex in theNicoya Peninsula, Playa Jac6, and the Quepos Peninsula using 4øAr-39Ar radiometricdating and to assessthe petrologic relationships between the different localities using majorelement, trace element, and Sr, Nd, Pb isotopicdata. Radiometricages of basaltsand diabasesfrom theNicoya Peninsula are 88-90 Ma (with a weightedmean of 88.5 Ma), andthose of two intrusiverocks (a gabbroand plagiogranite) are both 83-84 Ma. The combinedgeochemical dataindicate that the sampled Nicoya Peninsula rocks belong to a singlesuite related by fractional crystallizationof similarparental magmas. Nd andPb isotopic ratios indicate a commonmantle sourcedistinct from thatof mid-oceanridge basalts. Both the ageand composition of the Nicoya rocksare consistent with theidea that they are a partof the CaribbeanCretaceous oceanic plateau [Donnelly,1994]. TheJac6 lavas are geochemically similar to theNicoya Peninsula suite, and a singleage of 84 Ma is identicalto theage of theNicoya Peninsula intrusives. The one analyzed Queposbasalt has a radiometricage of ~64Ma, andit is enrichedin incompatibleelements relative to theNicoya rocks. Similarities in Nd andPb isotopicratios indicate that the Quepos and Nicoya/Jac6lavas were derived from a similarmantle source to thatwhich produced the Nicoya rocks,possibly the Galapagosplume. Introduction was described as normal oceanic crust of Jurassic to Early Cretaceous age, and the upper unit was subdivided, based on Obductedoceanic crust exposed on the west coastof Costa composition, into a suite of Late Cretaceous oceanic rocks Rica is broadly referred to as the Nicoya Complex [Dengo, associatedwith the Caribbeanoceanic plateau and a primitive 1962] (Figure 1). Theserocks are composedof intermingled arc series. However, our re-examination of contacts between basalts (massive and pillowed flows, dikes, and breccias), chert and basalt demonstrated only intrusive (basalt into gabbros and plagiograniteswith interspersedpieces of sediment) rather than depositional (sediment on basalt) radiolarianchert [e.g., Kuijpers, 1980;Alvarado et al., 1993; relationships,so the sedimentage cannot be used to constrain Denyer and Arias , 1993]. Outcropsare generallylimited to the basalt age. The division of the igneous rocks into two coastalpeninsulas with the most extensiveexposures on the seriesof different ages is thereforeopen to question. Nicoya Peninsula.The full extentof the obductedcrust is not The Caribbean oceanic plateau (also called the Caribbean preciselyknown, but it is possiblethat the Nicoya Complex Cretaceous Basalt Province by Donnelly et al. [1990]) is is part of a belt of accretedMesozoic oceaniccrust that composedof the thick oceaniccrust that makesup the core of extendssoutheastward through Panama into westernColombia the Caribbeanplate and numerouspieces of obductedoceanic and Ecuador[e.g., Escalante, 1990]. crust along the plate margin [Donnelly et al., 1990]. On the basis of the fossil ages of the radiolarian chert Debatable aspects of the Caribbean plateau include its associatedwith the igneous rocks, most previous workers formation location (Pacific or in situ) and the plate tectonic have divided the oceanic igneous rocks of the Nicoya setting of origin (i.e., intraplate, ridge centered, back arc). Peninsulainto an upper and a lower unit [Schrnidt-Effing, The preexistingoceanic lithospherewhich, based on seismic 1979; Kuijpers, 1980; Bourgoiset al., 1984; Gurskyet al., reflection profiles [Diebold et al., 1981] and plate 1984; Wildberg, 1984; Frisch et al., 1992]. The lower unit reconstructions[Duncan and Hargraves, 1984], purportedly lies beneath the plateau has not yet been definitively identified in any of the on-land exposures nor has it been 1Now at GraduateSchool of Oceanography,University of Rhode sampledby deep sea drilling. The exposedoceanic crust in Island, Narragansett. western Costa Rica may play a very important role in understandingthe nature of the plateauif, indeed, somepart of Copyright1997 by theAmerican Geophysical Union. the Nicoya Complex representsthis preexistingoceanic crust. Papernumber 97JB00681. This possibility can be evaluatedby determiningthe age and 0148-0227/97/97 JB-00681 $09.00 compositionof the lavas. 15,507 15,508 SINTONET AL.' THE NICOYA PENINSULA 83.2 _+1.3 Peninsula 92.5 _+ 5.4 10030 ' .2 Nico' 1el0 • 90.3_+3.2 Peninsula PI. Jac6! ;00 PI. Herradura 89.7 + 1.7: 19 / •-• 18 83.8_+1.1 88.0_+0.7 84.2 _+ 1.8 1-33 63.9 _+ 0.5 Sta. Cruz 22 21 Nicoya 89.9 _+ 2.8 10 ø 23 Carmona 85 ø 30' Figure 1. Schematicmap of the Nicoya Peninsulawith inset showinglocation of Playa Jac6 and Quepos. Solid circlesare sample locations. The 4øAr-39Ar plateau ages of basaltsand diabases are given, with the ages of the intrusive rocks in italics. Thin lines on the Nicoya Peninsulaare major roads. This projectwas designedto (1) determinethe radiometric similar mantle source and that all can be related by fractional age of rocksfrom the NicoyaComplex, (2) testthe idea of an crystallizationof similar parental melts. Nd and Pb isotopic upper and lower series within the Nicoya Peninsulausing ratios from the Nicoya Peninsula,Playa Jac6, and Queposare 4øAr-39Ardating and geochemistry, and (3) betterconstrain similar but are distinct from normal mid-ocean ridge basalts the origin of the Nicoya rockswith an improvedgeochemical eruptedon the East Pacific Rise. Altogether,these data do not data set. In addition, some samples were collected from support the division of the Nicoya Peninsula igneous rocks coastalexposures of oceanicbasalts at Playa Jac6 and the into two or more suites; rather, the rocks appear to be QueposPeninsula for comparisonto the Nicoya rocks. We genetically related and are part of the Caribbean oceanic report4øAr-39Ar radiometric ages as well as majorelement, plateau. trace element, and Sr, Nd, and Pb isotopic data from these three localities. Our data show that rocks from the Nicoya Previous Work Peninsula erupted at 88-90 Ma, which is contemporaneous with other on-land Caribbeanplateau sites in Haiti, Curaqao, Donnelly [1994] summarizedthe previous studiesof the and western Colombia [Sinton, 1996]. The geochemicaldata, Nicoya Complex and the reader is referred to his paper for a in conjunctionwith previouslypublished data, show that all more thorough description of these. The majority of the of the rocks collectedfrom the Nicoya Peninsulacome from a exposuresof igneousoceanic rocks and oceanicsediments are SINTONET AL.:THE NICOYAPENINSULA 15,509 on the Nicoya Peninsula, and smaller exposuresare found [Gradstein et al., 1994]) Sabana Grande sediments along the coast to the north, at the Santa Elena Peninsula,and conformably overlie the igneous rocks [Bourgois et al., to the south, at Playa Jac6/PlayaHerradura, and the Quepos 1984], placing a minimum age on eruptionand crystallization Peninsula(Figure 1). For clarity, the rocks will be referredto of the magmas.This formationis a deepwater assemblage of by their locality, so the term Nicoya in this paper will refer to a sedimentarybasalt/radiolarian chert breccia base overlain by the Nicoya Peninsula(samples designated NC in Table 1). thin siliceousmudstones grading into limestone[Bourgois et In general, previous authors have subdividedthe Nicoya al. , 1984]. Peninsula igneous rocks into two general units, termed the There are no reportedradiometric or fossil ages for the Matapalo and Esperanza units by Kuijpers [1980] and Playa Jac6/Herradurarocks. The basalts at Quepos are Bourgoiset al. [1984]. This division was originally basedon interbeddedwith Paleoceneforaminifera and are overlainby fossil age and field observations [Schrnidt-Effing, 1979; early Eocenesediments [Schmidt-Effing, 1979; Baumgartner Baurngartner et al., 1984], although no contact between the et al., 1984],