provenance of plutonic detritus in cover sandstones of nicoya complex, costa rica...

For permission to copy, contact [email protected] 2003 Geological Society of America832

GSA Bulletin; July 2003; v. 115; no. 7; p. 832–844; 10 figures; 3 tables; Data Respository item 2003095.

Provenance of plutonic detritus in cover sandstones of NicoyaComplex, Costa Rica: Cretaceous unroofing history of a

Mesozoic ophiolite sequence

Claudio Calvo†

Anna-Peters-Strasse 51/C, 70597 Stuttgart, Germany

ABSTRACT

This study presents new petrologic andsedimentologic data from northwestern Cos-ta Rica concerning the provenance of Cre-taceous forearc sandstones that contain plu-tonic detritus. Plutonic rock fragments areimportant accessory particles in pyroxene-bearing sandstones overlying the ophiolitenamed the Nicoya Complex. Through theuse of modal analysis of the frameworkgrains, I studied three sandstone suites ofthe El Viejo and Rivas Formations that in-clude both shallow- and deep-water depos-its, ranging from late Campanian to Maas-trichtian in age. In terms of primaryframework components, the sandstones re-semble those derived from magmatic arcs.Two modal parameters are introduced toevaluate detrital plutonic contributions andaffinity of source rocks: the ratio of pluton-ic to total lithic fragments [(Lp 1 iQF)/Lt],and the ratio of uralitized pyroxene to totalpyroxene grains (uralPx/Px). Modal valuesfor (Lp 1 iQF)/Lt indicate that plutonicfragments comprise up to 9% of total lithicfragments. A strong correlation betweenthese two parameters suggests that uraliti-zed pyroxene grains were also derived fromintrusive rocks of probably basic and inter-mediate compositions. In particular, signif-icant concentrations of lithic fragments ex-hibiting micrographic textures anduralitized pyroxene grains are interpretedto have been derived predominantly fromeroded plagiogranites. Sandstone suitescontaining plutonic detritus signal an un-roofing of deeper levels of the Mesozoicophiolitic sequence as a consequence ofstrong uplift of the Costa Rican arc in lateSenonian time. This tectonic event began in

†E-mail: [email protected].

the Campanian at ca. 75 Ma, ;9 m.y. afterthe intrusive magmatic activity on NicoyaPeninsula, and is consistent with the onsetof the Laramide orogeny.

Keywords: provenance, modal analysis,plagiogranites, ophiolite complex, CostaRica.

INTRODUCTION

The ophiolitic basement of northwesternCosta Rica and its sedimentary cover strataconstitute the most thoroughly studied on-landrock assemblage of the southern CentralAmerican forearc. This study focuses on theorigin and provenance of plutonic detritus inboth deep-sea channel and shallow-water bas-al sandstones from the Cretaceous cover unitsoverlying the ophiolite named the NicoyaComplex. In these forearc sandstones, detritalgrains of igneous plutonic origin are importantaccessory constituents, making up as much as9% of total lithic framework grains.

Previous studies on petrology and prove-nance of forearc sandstones from northwesternCosta Rica have suggested—on the basis ofprimary framework modes—that Cretaceoussandstones were derived from a magmatic arc(Lundberg, 1991; Calvo, 1998). Composition-ally, both those sandstones within the Nicoyaophiolite complex and many of the uncon-formably overlying sandstones are commonlyreferred to as basaltic sandstones, derivedfrom erosion of basaltic basement (e.g., Kuij-pers, 1979; Baumgartner et al., 1984; Gursky,1989; Lundberg, 1991). Although basalticsandstones dominate in some sequences, thereare also volcaniclastic sandstones that containabundant nonbasaltic framework components.For example, volcaniclastic wackes from theLoma Chumico Formation of the upper Ni-coya Complex exhibit a bimodal composition

consisting of basaltic fragments and differen-tiated pyroclastic materials (Calvo and Bolz,1994). Similarly, Cretaceous cover sandstonesof the Nicoya Complex contain a wide spec-trum of framework grains from other sources:radiolarites (including radiolarian cherts), pe-lagic limestones, shallow-marine carbonates,andesitic lavas, differentiated ejecta, and ig-neous intrusive rocks (Calvo, 1998). Thisspectrum of grain types suggests that theseforearc sandstones were derived predominant-ly from shallow levels of an intraoceanic arc,but it also reflects a significant contributionfrom uplifted ophiolitic basement areas partlyfringed by shallow-water carbonate deposits(Calvo, 1998). The plutonic detritus particu-larly records an ophiolitic provenance anddeeper unroofing of the ophiolitic basementthan previous workers have suggested.

The main objectives of this study are (1) toanalyze mineral composition and textural fea-tures of plutonic detritus in order to determinelikely source rocks and provenance relation-ships; (2) to establish detrital plutonic contri-butions and affinity of source rocks by usingmodal analysis of secondary framework pa-rameters; and (3) to integrate these new datainto a model for the Cretaceous unroofing his-tory of the ophiolitic sequence of southernCentral America.

TECTONIC SETTING

Southern Central America (Costa Rica andPanama) represents an island arc that formedover an intraoceanic subduction zone situatedat the western margin of the Caribbean plate(e.g., Kuijpers, 1979; Lundberg, 1982; Wild-berg, 1984; Calvo and Bolz, 1994). At thepresent time, the Cocos plate is being sub-ducted beneath Costa Rica at 90 mm/yr (Min-ster and Jordan, 1978). Bathymetric swathmapping of the convergent margin offshore

Geological Society of America Bulletin, July 2003 833

CRETACEOUS UNROOFING HISTORY OF A MESOZOIC OPHIOLITE SEQUENCE, COSTA RICA

Costa Rica shows an oceanic plate coveredwith numerous seamounts (von Huene et al.,1995). The most prominent feature is the Co-cos Ridge subducting opposite the Osa Pen-insula (Fig. 1).

The study area is located in northwesternCosta Rica. This region is characterized bythree major morphotectonic elements that re-flect an evolved arc edifice (Fig. 1): (1) theemerged outer arc, forming the Nicoya andSanta Elena Peninsulas; (2) the inner forearctrough (Tempisque basin), comprising theGulf of Nicoya and lowlands of the Tem-pisque River; and (3) the active volcanic cor-dilleras of Guanacaste and Tilaran, represent-ing the inner magmatic arc. Behind thecordilleras are the Guatusos lowlands, repre-senting the southern terminus of the exten-sional Nicaraguan Depression.

STRATIGRAPHIC FRAMEWORK

The forearc basement is exposed along thePacific coast of Costa Rica and comprises aMesozoic ophiolitic sequence, the NicoyaComplex (Dengo, 1962). It is principally com-posed of peridotites, basalts (massive and pil-lowed flows), basaltic breccias, dolerites, ra-diolarites, limestones, volcaniclastic rocks,and igneous intrusive rocks (plagiogranitesand gabbros). Differentiated pyroclastic de-posits of Albian to Campanian age within theNicoya Complex record the earliest volcanicactivity of the Costa Rican arc orogen (Calvoand Bolz, 1994; Calvo, 1998). Overlying theNicoya Complex is the sedimentary cover thatincludes both shallow- and deep-marine strataranging from Upper Cretaceous through Pli-ocene age. The forearc rock succession is part-ly covered by Quaternary volcanic rockserupted from stratovolcanoes of the north-western cordilleras (Fig. 1). Seismic reflectiondata from the Pacific margin offshore CostaRica suggest the seaward continuation of theNicoya Complex to the middle slope (Hinz etal., 1996).

The cover sandstone suites studied are strat-igraphically grouped into the El Viejo and Ri-vas Formations (Table 1). The El Viejo For-mation (late Campanian–Maastrichtian) is ashallow-marine clastic and related carbonatereef unit a few meters thick consisting of rud-istid framestones, bioclastic grainstones, andsandstones that rest unconformably on rocksof the Nicoya Complex (Schmidt-Effing,1975; Ulloa, 1977; Seyfried and Sprechmann,1985; Calvo, 1987). The Rivas Formation(late Campanian–Paleocene) comprises a tur-biditic slope sequence ;1500 m thick com-posed of volcaniclastic sandstones, mud-

stones, and minor conglomerates and breccias(Dengo, 1962; Protti, 1981; Lundberg, 1982;Rivier, 1983; Baumgartner et al., 1984; Calvo,1998). This unit overlies conformably hemi-pelagic limestones and volcaniclastic rocks ofthe Sabana Grande Formation of Campanianage.

MATERIALS AND METHODS

Detritus derived from igneous intrusiverocks in Cretaceous cover sandstones was firstdetected during analysis of main frameworkgrains (Calvo, 1998). On the basis of deposi-tional environment and age, 20 sandstonesamples were selected for this study. Theywere collected from three different locationsin Cretaceous sandstone suites of the innerforearc: La Tigra, Quebrada Pilas, and CalleCodornices (Fig. 1). The sampled units formpart of the sedimentary cover of Nicoya Com-plex, recording shallow- and deep-water clas-tic sedimentation in the forearc region ofnorthwestern Costa Rica during the late Sen-onian. Their principal stratigraphic and sedi-mentologic characteristics are summarized inTable 1. Sandstone ages were determined onthe basis of their contained foraminiferal as-semblages. Samples were examined petro-graphically in thin section, cut normal to bed-ding. For analysis of sandstone frameworks,an average of 500 grain points were countedin each section. Significant concentrations ofplutonic detritus in these sandstones makepossible the modal analysis of detrital plutoniccontributions using the Gazzi-Dickinsonpoint-counting method (Gazzi, 1966; Dickin-son, 1970). Point-count results are listed in Ta-ble DR1.1 Plutonic lithic fragments and maficmineral grains were counted as separatedgrains. In order to establish plutonic contri-butions, fragments of plutonic rock were in-cluded in tabulations of total lithic grains. Be-cause of the scarcity of total counts of maficmineral grains, additional 100 point counts fortotal pyroxene grains were necessary to im-prove the statistical reliability of the values forthe uralitized pyroxene ratio. Petrologically,the general correspondence of ratios calculat-ed here is considered to be significant becauseit reflects compositional tendencies observedon the basis of the detailed optical analysis offramework grains, made prior to the pointcounts. Following Dickinson’s (1970) conven-tion, carbonate fragments were not included in

1GSA Data Repository item 2003095, sedimen-tary petrography of forearc sandstones from north-western Costa Rica, is available on the Web at http://www.geosociety.org/pubs/ft2003.htm. Requestsmay also be sent to [email protected].

the modal analysis, because of their possibleintrabasinal origin and generally minor im-portance for constraining ophiolitic prove-nance. Sandstone suites derived from differentdepositional environments were also analyzedto evaluate dispersal patterns of detrital sedi-ment in the marine forearc basin.

In this study, the timing of unroofing of theophiolitic sequence was established from therelative ages of foraminiferal assemblagesidentified in sandstone samples. Radiometricage determinations of intrusive rocks from theNicoya Peninsula by Sinton et al. (1997), cou-pled with the biostratigraphic data presentedhere, constrain the age of crystallization andearliest erosion of Nicoya Complex intrusiverocks in northwestern Costa Rica.

RESULTS

Detailed petrographic examination offramework grains reveals that Cretaceousforearc sandstones from northwestern CostaRica contain abundant ophiolitic grains, rang-ing from basic volcanic fragments and igneousintrusive-derived detritus to cherty rock frag-ments of sedimentary origin (Fig. 2). Thisstudy focuses on plutonic rock fragments andradiolarian chert grains, which are thought torepresent useful distinctive provenance indi-cators reflecting unroofing of deeper levels ofthe ophiolitic sequence. Petrographically, an-alyzed sandstones comprise dominantly lithicand feldspatholithic wackes, litharenites, andhybrid arenites as defined by Zuffa (1980), in-cluding arenites bearing Sulcoperculina.

Plutonic Grains

Detrital grains of plutonic origin comprisea diverse assortment of grain types, includingboth lithic fragments and monocrystallinegrains. Figures 2A through 2D show photo-micrographs of representative plutonic frame-work grains found in Cretaceous cover sand-stones of the Nicoya Complex. A unique graintype consists of crystalline lithic grains thatexhibit micrographic textures distinctive of ig-neous intrusive rocks (Troger, 1967). Thesemicrographic grains are composed of plagio-clase feldspar (albite) and quartz intergrowths(Figs. 2A, 2C). Because of the instability offeldspars in the sedimentary environment rel-ative to quartz, micrographic intergrowthsgenerally exhibit differential weathering: Inplane-polarized light, albite is seen to havebeen altered to clay minerals, whereas quartzremained essentially unaltered. Other rockfragments include polycrystalline quartzgrains and crystalline lithic fragments. Poly-

834 Geological Society of America Bulletin, July 2003

C. CALVO



TABLE 1. STRATIGRAPHIC AND SEDIMENTOLOGIC CHARACTERISTICS OF THREE CRETACEOUS FOREARC-SANDSTONE SUITES FROM NORTHWESTERNCOSTA RICA

Sandstone suite La Tigra Quebrada Pilas Calle Codornices

Formation Rivas Rivas El ViejoAge Late Campanian to early Maastrichtian Maastrichtian† Late Campanian to early MaastrichtianForaminiferal assemblage Sulcoperculina dickersoni (Palmer);

Sulcoperculina globosa de Cizancourt;Pseudorbitoides sp.; Sulcorbitoides pardoi

Bronnimann

Gansserina gansseri (Bolli); Orbitocyclinaminima (H. Douville); Sulcoperculina sp.

Sulcoperculina globosa de Cizancourt;Sulcoperculina sp.; Pseudorbitoides cf.

israelsky Vaughan & Cole

Thickness ;150 m ;30 m ;5 mType of facies Channel sandstones Channel sandstones Basal sandstonesDepositional environment Inner forearc–trough (Tempisque Basin): Island-arc platform:

Lowstand submarine slope Arc platform–related submarine slope Transgressive bioclastic shoals

†Age also supported by paleontologic determinations of macrofauna (Fischer and Aguilar, 1994)

Figure 1. Geologic map of northwestern Costa Rica showing locations of studied Cretaceous forearc sandstone suites: La Tigra, QuebradaPilas, and Calle Codornices. Place names discussed in text are also included. From Calvo and Bolz (1994). Inset map shows the regionaltectonic framework of the study area (MPF—Motagua-Polochic Fault; MAT—Middle America Trench; ND—Nicaraguan Depression;PFZ—Panama Fracture Zone).N

crystalline quartz grains resemble those de-rived from granitoid source rocks, becausethey are composed of relatively few crystalsof equant shape and have few or no intracrys-talline sutured contacts. Crystalline lithic frag-ments containing polysynthetically twinnedplagioclase and uralitized clinopyroxene arealso present (Fig. 2B). Some of them displaysubophitic textures. Monocrystalline grainsinclude plagioclase, quartz, and pyroxene.Both plagioclase and quartz grains are, how-ever, difficult to interpret, because in manycases they do not exhibit diagnostic petro-graphic features of a plutonic or volcanic or-igin. The most important mafic mineral grainis pyroxene, which often appears partly ural-itized (Fig. 2B).

The predominance of coarse-grained plu-tonic fragments showing little evidence ofchemical weathering, in conjunction with an-gular grain textures exhibiting remains of in-tercrystalline boundaries (Fig. 2A), indicatesthat intrusive rocks in the source area werebroken mechanically along intercrystallineboundaries and that detrital grain size is, inpart, controlled by original textures of coarse-grained source rocks. Although tropicalweathering conditions prevailed during denu-dation, as suggested by alterite grains identi-fied in samples analyzed, the spectrum of plu-tonic detritus apparently reflects originalcompositions of igneous source rocks, an in-terpretation that implies significant relief inthe source area.

Cherty Rock Fragments

Siliceous rock fragments of sedimentary or-igin, including radiolarian mudstones and

cherts, are typical framework grains foundwith plutonic detritus in studied Cretaceousforearc sandstones (Fig. 2). In thin section, ra-diolarite grains exhibit a dark rusty red tobrown color and bear silica-filled blebs thatare likely microfossil (radiolaria) remains(Fig. 2E). Radiolarian chert grains are poly-crystalline quartz fragments, commonly ex-hibiting hematitic clusters and vestiges of ra-diolaria (Fig. 2F). Such grains are of particularinterest for provenance of cover sandstones,because they probably result from erosion ofribbon cherts within the Nicoya Complex thatare interpreted to form part of a remnant ac-cretionary unit (Baumgartner, 1990), i.e., thePunta Conchal Formation (Gursky andSchmidt-Effing, 1983).

Modal Analysis of Plutonic Detritus

In terms of primary framework grains(QFL, Qp-Lvm-Lsm), Cretaceous sandstonesfrom northwestern Costa Rica containing plu-tonic detritus have a composition similar tothose derived from magmatic arcs (Fig. 3;Calvo, 1998). Ophiolitic and especially plu-tonic provenances are commonly obscured inthese sandstones by abundant volcanic frag-ments, but both sources are recognizable inchannel sandstones and transgressive basalsandstones from the slope and neritic sequenc-es, respectively, selected for this analysis.

This study focused on the semiquantitativemodal analysis of detrital parameters sensitiveto plutonic provenance, defined in Table 2.Recalculated modal values are summarized inTable 3 and presented graphically in Figures4 and 5. Original point data for primary andsecondary parameters are available in Table

DR1. In order to establish detrital plutoniccontributions and affinity of source rocks, twonew modal parameters are introduced. Theyare defined as the ratio of plutonic to total lith-ic fragments [(Lp 1 iQF)/Lt] and the ratio ofuralitized pyroxene to total pyroxene grains(uralPx/Px). Uralitization, the alteration of cli-nopyroxene to amphibole, is characteristic ofplutonic terranes (Troger, 1967). Modal valuesfor (Lp 1 iQF)/Lt indicate that plutonic frag-ments average up to 9% of total lithic frag-ments. Values between 4% and 9% are onlyfound in sandstones of the Calle Codornicesand La Tigra suites (Table 3). Modal contentof plutonic materials calculated with thismethod represents only a minimum contribu-tion, because (Lp 1 iQF)/Lt provides dataonly on the contribution of lithic fragments.Quartz and feldspar grains were not includedin tabulations of plutonic grains, because theirprovenance remains in many cases unclear.Because many monocrystalline grains are in-dependent of lithic fragments, but some weresurely derived from plutonic rocks, it is notpossible to calculate the true total contribu-tions of detritus from plutonic rocks.

Ternary DiagramsThe ternary diagrams of Figure 4 display

the detrital compositions of sandstones interms of their populations of lithic fragmentsand mafic mineral grains. The [(Lp 1 iQF)/Lt]–Lvm–(rC 1 Lsm) diagram shows modalvalues of plutonic, volcanic, and sedimentaryrock fragments. Radiolarian chert grains areincluded in tabulations of lithic grains of sed-imentary origin. This diagram demonstratesthat the lithic grain fraction in all analyzedsamples is strongly dominated by volcanic


C. CALVO

Figure 2. Photomicrographs of ophiolitic detritus found in Cretaceous forearc sandstones from northwestern Costa Rica, including (A–D) plutonic fragments and (E and F) cherty rock fragments. (A) Lithic fragment composed of monocrystalline quartz (Qm) and micro-graphic intergrowth of quartz and albite (iQF); albite at extinction. Irregular grain outline (right) is interpreted to be a relict inter-crystalline contact. Calle Codornices sandstone, cross-polarized light. (B) Uralitized pyroxene grain (urPx) showing cores of unalteredpyroxene (Px). Calle Codornices sandstone, plane-polarized light. (C) Lithic fragment composed of altered plagioclase (P) and inter-growth of quartz and plagioclase exhibiting micrographic texture (iQF); quartz at extinction. Quebrada Pilas arenite, cross-polarizedlight. (D) Outsized feldspar grain (F) of probably plutonic origin showing differential weathering and thin coatings of hematite. It iscomposed of fresh albite and altered anorthite-rich zones, replaced by calcite and sericite minerals. Calle Codornices sandstone, plane-polarized light. (E) Radiolarite fragment (Rf), bearing microfossil remains (radiolaria) filled with silica. La Tigra sandstone, cross-polarized light. (F) Radiolarian chert grains (rC) showing vestiges of radiolarian microfossils and hematitic clusters; tests of Sulcoper-culina sp. (S), a late Senonian larger foraminifera species, also occur. Calle Codornices sandstone, plane-polarized light.



Figure 3. QFL and Qp-Lvm-Lsm plots of mean framework values and their standarddeviations for analyzed Cretaceous sandstone suites from northwestern Costa Rica con-taining plutonic detritus. Provenance fields from Dickinson and Suczek (1979) and Dick-inson (1985). Mean values and standard deviations are represented by distinctive symbolsand polygons, respectively. Magmatic arc provenance field on diagram QFL is dividedinto dissected arc (P . V) and undissected arc segments (P , V) (P—plutonic detritus,V—volcanic detritus); n—number of samples. Because plutonic fragments are accessoryconstituents in cover sandstones, plutonic contributions cannot be reflected directly by theprimary framework parameters on both diagrams. Note that the QFL plot for the Que-brada Pilas sandstone suite within the P . V subfield, in particular, is caused by strongcontents of radiolarian chert fragments in this suite (Table DR1). This origin is demon-strated by a second plot (depicted by a dashed polygon) where these rock fragments areadded to the lithic grain populations L and Lsm. The detailed petrographic examinationreveals that the Quebrada Pilas sandstones have relatively low contents of identifiableplutonic detritus. This example shows that the use of secondary parameters is indispens-able to determine the real provenance relationships in cover sandstones. In general, allthree cover sandstone suites record volcanic sources derived from the Cretaceous arcactivity, in addition to an ophiolitic provenance derived from exposed plagiogranite bodiesand radiolarian chert units of the Nicoya Complex.

TABLE 2. MODAL PARAMETERS AND CLASSIFICATION OF FRAMEWORK GRAINS

Primary parameters (after Dickinson and Suczek, 1979)Q 5 total quartz grainsF 5 total feldspar grainsLt 5 total lithic fragments (including polycrystalline grains, Qp)Lvm 5 volcanic and metavolcanic rock fragmentsLsm 5 sedimentary and metasedimentary rock fragments

Secondary parameters sensitive to plutonic provenance (defined in this study)rC 5 radiolarian chert grainsLp 5 plutonic lithic fragments exhibiting granular and subophitic textures, composed of plagioclase, quartz,

and pyroxeneiQF 5 grains showing micrographic intergrowth of quartz and plagioclase feldsparHbl 5 hornblende grainsPx 5 total pyroxene grainsBt 5 biotite grainsRatios:uralPx/Px, where uralPx 5 uralitized pyroxene grains(Lp 1 iQF)/Lt, where (Lp 1 iQF) 5 total lithic grains demonstrably of plutonic origin

fragments. Sandstone suites of the Rivas For-mation display, however, divergent trends.Sandstones from the La Tigra suite show sig-nificant values of plutonic and volcanic frag-ments, very similar to those for Calle Codorn-ices arenites. Arenites from Quebrada Pilasare a particular petrofacies, because they are

characterized by extraordinarily high values ofsedimentary lithic fragments and especially ofradiolarian chert grains. In some samples,these dominate the lithic grain fraction. Thepredominance of radiolarian chert materials inthis forearc suite is interpreted to suggest asubduction-complex provenance, in addition

to a magmatic arc source (Calvo, 1998). Onthe QFL diagram of Figure 3, in particular,such a compositional trend is expressed by amean plot within the subfield for dissected-arcprovenance. In contrast, identifiable plutonicdetritus in these sandstones occurs only intrace amounts.

The Bt-Hbl-Px diagram of Figure 4 dis-plays modal values of the mafic mineral grainpopulation. Two main types of sandstonescan be distinguished: pyroxene- and pyroxene-hornblende–bearing sandstones. Most of thesesamples contain clinopyroxene and greenhornblende. Arenites from La Tigra are pre-dominantly pyroxene-rich, whereas in sand-stones from Quebrada Pilas, hornblende dom-inates. The Sulcoperculina-bearing sandstonesfrom Quebrada Pilas are extraordinarily richin hornblende, which accounts for 87% to93% of the mafic mineral grain fraction. Onevolcaniclastic sandstone sample from La Tigracontains a similar high abundance of horn-blende. Arenites from Calle Codornices con-tain exclusively clinopyroxene, partly urali-tized, and are quite similar to most of La Tigrasandstones. The enrichment of detrital pyrox-ene grains is strongly related to significantcontents of plutonic fragments within the lith-ic grain fraction (Fig. 4). Biotite grains arerare in most sandstones. Only a few samplesfrom Quebrada Pilas and one sample fromCalle Codornices contain trace amounts of bi-otite fragments.

Plutonic Lithic Fragments vs. PyroxeneGrains

The (Lp 1 iQF)/Lt vs. uralPx/Px diagramshows that abundance of uralitized pyroxenegrains can be correlated with increasingamounts of plutonic rock fragments (Fig. 5).In sandstone samples from Calle Codornicesand La Tigra that contain .4% of identifiableplutonic lithic fragments, these two detritalparameters increase in parallel. This observa-tion allows the following conclusions. First, itindicates that uralitized pyroxene grains alsoare derived from eroded intrusive rocks. Sec-ond, because both uralitized pyroxene and in-tergrowths of albite and quartz are character-istic mineral phases and textures ofplagiogranites (Wildberg, 1984), the strongcorrelation between these parameters recordsa plutonic provenance, probably resultingfrom erosion of plagiogranitic bodies. It alsodocuments the predominantly intermediatecomposition of igneous source rocks (Wild-berg, 1987; Sinton et al., 1997).

This relationship can also be recognized in-directly, by comparing modal values of rockfragments with those of mafic mineral grains


C. CALVO

Figure 4. Ternary diagrams displaying the lithic and mafic mineral grain populations offorearc sandstones from the El Viejo and Rivas Formations (upper Campanian–Maastrichtian). Mean values indicated by distinctive symbols; n—number of samples.

(Table 3; Fig. 4). Note that significant valuesof plutonic lithic fragments only occur inthose sandstones that contain a mafic mineralgrain population strongly dominated by py-roxene grains. This statement is true for bothLa Tigra and Calle Codornices sandstonesuites. In samples from Calle Codornices, plu-tonic contributions are, in part, documented byprimary detrital modes (Table DR1). Somemodal QmPK values fall in Dickinson’s(1985) volcanoplutonic suite of circum-Pacificsandstones (Calvo, 1998).

DISCUSSION

Source Rocks

The stratigraphic context and frameworkgrain compositions of the analyzed sandstonesindicate that the source of plutonic materiallikely lies within the underlying Nicoya Com-plex. Plutonic rocks occur in the lower part ofthe ophiolitic sequence, where they generallyhave intruded massive basaltic flows (Ku-ijpers, 1979; Wildberg, 1987). The transgres-sive sandstones from Calle Codornices pro-vide unequivocal sedimentologic evidence forthis interpretation, because they were depos-ited directly on exposed basement. Thus, non-carbonate detritus reworked during the trans-gression was derived from basement rocksand/or from volcanic eruptions. The compo-sition of framework grains that include radi-olarian mudstones and cherts, pelagic lime-stones, and basalts records basement sourcelithologies that are all found in the ophioliticsequence. Nonophiolitic grains are biogeniccarbonate fragments and volcanic grains, prin-cipally andesitic grains and ejecta fragmentsincluding glass shards and fresh individualcrystals. On the other hand, the compositionof plutonic fragments clearly indicates that thedominant plutonic source was basic to inter-mediate in composition. Conglomerate depos-its within the La Tigra sequence containinggabbro and dolerite clasts support this inter-pretation. Plagiogranites in the Nicoya Com-plex constitute a likely specific source rock ofplutonic coarse-grained lithic fragments andrelated pyroxene grains.

Plagiogranitic Source RocksAccording to Wildberg (1987), plagiogran-

ites on the Nicoya Peninsula occur within in-trusions of isotropic gabbros and dolerites.They form dikes up to several meters in thick-ness and intrusive bodies of considerable sizewith exposures several hundred meters across.On the basis of the extent of outcrops, plagio-granites probably constitute no more than 5

vol% of the Nicoya Complex. Plagiogranitesare composed of plagioclase, quartz, pyrox-ene, opaque minerals, apatite, and zircon.Most of them display subhedral granular tex-tures. Graphic intergrowth of plagioclase andquartz is a further typical feature (Figs. 6A,6C). Secondary amphibole resulting fromcomplete or partial uralitization of clinopyrox-ene is often present (Fig. 6C). Gabbros, dol-erites, and diorites also contain accessory min-eral phases of uralitized pyroxene andmicrographic intergrowths (Kussmaul, 1980)(Fig. 6D). However, the detrital abundance ofmicrographic grains of up to 9% of total lithic

fragments in some sandstone samples requiresigneous source rocks to contain abundant mi-crographic intergrowth textures, rather thanonly accessory amounts. Such sources are theplagiogranites. Kuijpers (1979) called atten-tion to the abundance of micrographic inter-growths of albite and quartz in some plagio-granites of the Nicoya Complex (e.g., PlayaEl Ocotal; Fig. 1). Petrographic examinationof these rocks confirms a wide occurrence ofmicrographic textures with intergrowths of upto 10 mm across, displaying differentialweathering recognizable in plane-polarizedlight (Fig. 6A).



Figure 5. Correspondence of proportion of lithic fragments that are plutonic and propor-tion of clinopyroxene grains that are uralitized in pyroxene-bearing sandstones from theEl Viejo and Rivas Formations (upper Campanian–Maastrichtian); n—number ofsamples.

TABLE 3. RECALCULATED MODAL VALUES OF DETRITAL PARAMETERS FOR 20 FOREARCSANDSTONES OF EL VIEJO AND RIVAS FORMATIONS (UPPER CRETACEOUS) FROM

NORTHWESTERN COSTA RICA

Lithic fragments† Mafic mineral grains†‡ Ratios†§

Sample Lvm Lp 1iQF

rC 1 Lsm Px Hbl Bt (Lp 1iQF)/Lt

uralPx/Px

Calle Codornices sandstonesBB-14/1 81 6 13 96 2 2 5.5 9BB-14/2 81 5 14 100 0 0 4.4 3BB-14/4 80 5 15 100 0 0 5.0 5BB-1414a 86 6 8 100 0 0 5.1 12BB-14/7 78 9 13 100 0 0 9.0 25La Tigra sandstonesLT-1/1 86 8 6 100 0 0 8.2 22LT-1/4 87 5 8 97 0 3 5.6 15LT-1/5 80 6 14 100 0 0 6.0 8LT-2/2 94 1 5 21 79 0 0.9 0LT-2/6 83 3 14 100 0 0 3.4 0LT-4/1 83 ,0.5 17 67 33 0 0.3 0LT-4/1a 89 1 10 100 0 0 0.6 0Quebrada Pilas sandstonesBB-4/1b 43 3 54 33 62 5 2.2 0BB-4/2b 49 2 49 53 47 0 0.9 0BB-4/5b 56 0 44 33 67 0 0 0BB-4/1a 49 ,0.5 51 40 57 3 0.3 0BB-4/2a 42 ,0.5 58 63 37 0 0.5 0BB-4/5a 47 1 52 31 54 15 1.2 0BB-4/6b 70 0 30 5 93 2 0 0BB-4/6a 56 2 42 13 87 0 2.0 0

†Values in percent plotted on diagrams of Figure 4 and Figure 5.‡Includes independent monomineralic grains only.§Values for uralPx/Px based on additional 100 point counts for total pyroxene.

Only very minor amounts of both potassi-um feldspar and biotite grains in analyzedsandstones argue against granitic source rocks.This inference is consistent with the fact thatno older rocks of continental affinity havebeen identified in southern Central America(Lundberg, 1991). The predominance of so-dium-rich plagioclase feldspar within micro-graphic grains and the strong correlation be-tween uralitized pyroxene grains and plutoniclithic fragments (Fig. 5) also require a plutonicsource rock of basic to intermediate compo-sition, preferably a plagiogranitic source.

Geochemical studies of potential plagio-granitic sources (Wildberg, 1987; Sinton et al.,1997) suggest that plutonic rock fragments incover sandstones and megabreccia deposits onSanta Elena Peninsula (Tournon and Azema,1980) were derived from intrusive rocks ofboth island arc– and oceanic plateau–relatedorigin. Of particular interest is the occurrenceof arc-derived plutonic grains containing po-tassium feldspar in Pliocene sandstones fromthe Montezuma Formation of the southern Ni-coya Peninsula (Lundberg, 1991) (Fig. 1).They probably record the erosion of differ-

entiated intrusive rocks displaying a matureisland-arc affinity, previously detected in thisregion by Wildberg (1984).

Provenance Areas

The geographic distribution of the sand-stone suites records very different provenanceareas of plutonic detritus in northwestern Cos-ta Rica. On the other hand, the common oc-currence of angular, coarse sand-sized plutonicparticles requires limited transport and depo-sition near the source area. The sandstonesuite from La Tigra, located in the eastern Ni-coya Peninsula, illustrates the relationship be-tween the source area and site of deposition,because this region exposes several intrusivebodies. In the Quebrada Cuajiniquil, near thelocality of La Tigra where the clastic sequenceis exposed, Protti (1981) found a plagiogran-itic sill. This rock obviously represents a po-tential source of plutonic detritus present inthe Cretaceous turbidite sandstones. In thiscase, the distance from source rock to site ofdeposition is only a few kilometers (Fig. 1).Similarly, sandstones from Calle Codornicesand Quebrada Pilas record source areas situ-ated on the northeastern margin of the Gulf ofNicoya (Fig. 1). The intrusive rocks of CerroBarbudal, located 2.5 and 10 km from the out-crops of Quebrada Pilas and Calle Codornices,respectively, are considered to be possiblesources. These intrusive rocks contain acces-sory mineral phases including uralitized cli-nopyroxene as well as myrmekitic and micro-graphic intergrowths of plagioclase and quartz(Calvo, 1998). Trace amounts of plutonic lith-ic fragments are also present in Maastrichtianbioclastic limestones within the rudistid reeffacies of Cerro Barbudal (Fig. 1). The lime-stones rest unconformably on eroded basalticflows laterally penetrated by the intrusions.

Detrital Dispersal Patterns

Sandstone suites from northwestern CostaRica record the dispersion of plutonic rockfragments into both shallow- and deep-waterenvironments during Late Cretaceous time.However, sandstones deposited in the two dif-ferent environments exhibit very divergentdispersal patterns. The detrital dispersion ap-pears to have been controlled mainly by acombination of location and areal extent ofexposed source rocks, sedimentary environ-ment, and relative sea-level changes.

In the shallow-water clastic rocks (CalleCodornices sandstones) and related carbonatereef deposits of the El Viejo Formation, plu-tonic detritus commonly appears in lithofacies


C. CALVO

Figure 6. Photomicrographs of representative intrusive rocks from the Nicoya ophiolite complex. (A) Plagiogranite exhibiting charac-teristic granophyric texture. The radiating intergrowth of quartz and albite (iQF) is arranged about a euhedral plagioclase crystal (P);albite at extinction. Rock sample from Playa El Ocotal, cross-polarized light. (B) Dolerite with subhedral texture consisting mainly ofplagioclase laths and clinopyroxene crystals. Rock sample from Cerro Barbudal, plane-polarized light. (C) Plagiogranite exhibitinggraphic intergrowth of quartz and albite (at the right of the photograph), clinopyroxene (Px), partly uralitized (urPx), and plagioclasephenocryst (P). Rock sample from Playa El Ocotal, cross-polarized light. (D) Accessory micrographic intergrowth of plagioclase andquartz (iQF), albite at extinction, and zoned plagioclase crystals (P) in dolerite from Cerro Barbudal, cross-polarized light.

at the base of the sequence. Significant con-centrations are found in clastic deposits thatimmediately overlie the basement rocks (Fig.7). Up section, the relative content of plutonicdetritus rapidly decreases in the reef frame-work and in grainstone facies overlying basalstrata. In terms of sequence stratigraphy, thesefacies represent deposits of a transgressivesystems tract. Moreover, it is clear from thepresence of both plutonic detritus and weath-ered basaltic rocks that the unconformity atthe base of the Calle Codornices transgressivesequence represents a subaerial erosional sur-face, a type 1 unconformity following theclassification of Posamentier and Vail (1988).

Transgressive systems tracts are depositedduring rapid rises in relative sea level whenlittle sediment is delivered to the shelf (Po-samentier and Vail, 1988). This scenario co-incides with the accumulation of basement-derived detritus at the base of the CalleCodornices sequence, proximal to the sourcearea, indicating moderate reworking duringthe transgression. These observations suggestthat plutonic detritus was deposited in shallow-water environments during rapid marine in-cursions on exposed and previously emergentsource terranes of the Nicoya Complex.

In comparison with the shallow-water se-quence, the slope sequences of La Tigra and

Quebrada Pilas of the Rivas Formation (Table1) have an extensive vertical distribution ofplutonic detritus, but in generally lower con-centrations. Identifiable plutonic fragmentscommonly appear in levels of these sequencesthat contain medium to coarse-grained channelsandstones (Fig. 7). Texturally, they comprisewell-sorted, graded and massive sandstoneswith grain support and without or little matrixcontents, showing erosional basal contacts.These sedimentologic features indicate thatplutonic detritus in this case was redistributedinto the deep-water environment by turbiditycurrents and/or sandy debris flows. Typicalfining-upward trends in both sequences sug-



Figure 7. Model suggested here for the dispersion of plutonic detritus in the marine en-vironment during relative sea-level changes, based on detrital dispersal patterns observedin (A) the neritic sequence of Calle Codornices, and (B) the slope sequences of La Tigraand Quebrada Pilas. Dispersion of plutonic detritus succeeded periods of significant upliftand subaerial erosion that likely cut deep into the basement sequence to expose the plu-tonic bodies (e.g., plagiogranites). U—Campanian unconformity representing an erosionsurface on exposed basement areas. See text for discussion.

Figure 8. Comparing crystallization and earliest erosion ages of intrusive rocks from theNicoya ophiolite complex of northwestern Costa Rica. Radiometric age determinations ofNicoya Peninsula intrusive rocks from Sinton et al. (1997); the two rocks come fromoutcrops located in the northwestern part of the peninsula. Biostratigraphic ages of sand-stones bearing plutonic detritus established on the basis of their foraminiferal assemblages.Time scale and planktic foraminiferal zones from Gradstein et al. (1994) and Erba et al.(1995), respectively. See text for discussion.

gest, moreover, a grain transport via subma-rine channels, probably during periods of rel-ative sea-level lowstands.

Unroofing History of the OphioliteSequence and Tectonic Implications

Basaltic sandstones directly overlying theNicoya Complex were derived from erodedbasaltic rocks (massive and pillowed flows).Cover sandstones bearing plutonic detritus re-cord the erosion of deeper levels of the ophio-litic sequence. Biostratigraphic age determi-nations of sandstones based on theirforaminiferal assemblages indicate that theearliest unroofing of these levels took placeduring late Campanian and Maastrichtian time(Fig. 8). This finding implies that intrusivebodies of the Nicoya Complex must have in-truded before the late Campanian (ca. 75–71Ma, Gradstein et al., 1994). In fact, radiomet-ric ages of two intrusive rocks (a gabbro anda plagiogranite) from the Nicoya Peninsula asdetermined by the 40Ar-39Ar method indicatethat intrusive magmatic activity occurred atca. 84 Ma (Sinton et al., 1997). The unroofingof plutonic rocks probably required long pe-riods of subaerial exposure and concomitantdenudation of the forearc ophiolite complexduring evolution of the Costa Rican orogen,as suggested by a marked Campanian uncon-formity. Relationships between crystallizationand earliest erosion ages of Nicoya Complexintrusions suggest that unroofing had been ef-fectively accomplished ;9 m.y. after the ca.84 Ma intrusive activity on northern NicoyaPeninsula (Fig. 8). On the other hand, geo-chemical studies of these plutonic rocks (e.g.,Wildberg, 1987) suggest that plutonic detri-tus originated, in part, from intermediate arc-related intrusions. Campanian sandstones andtuff interbeds of the Sabana Grande Formationin southern Nicoya Peninsula (Lundberg,1982), as well as Albian to Campanian pyro-clastic rocks of the Loma Chumico Formationoccurring within the Nicoya Complex (Calvoand Bolz, 1994), indicate that the arc was ac-tive at the time of intrusive activity.

Like late Senonian carbonate reefs and plat-forms, which grew on both mafic and ultra-mafic substrates in northwestern Costa Rica(Ulloa, 1977; Seyfried and Sprechmann, 1985;Calvo, 1987), plutonic and radiolarian chertdetritus in cover sandstones points to stronguplift of the Costa Rican orogen, which in-cluded the formation of emergent terranes ofophiolitic basement (Fig. 9). Therefore bothgrain types in these forearc sandstones can beconsidered as detrital paleotectonic indicatorsof the regional Campanian unconformity—the

most important Cretaceous unconformity insouthern Central America and one that marksthe boundary between the ophiolitic basementrocks and their sedimentary cover strata. Thistectonic uplift is coeval with the onset of theLaramide orogeny in the Late Cretaceous. Interms of relative plate motions, the onset ofthe Laramide orogeny at ca. 75 Ma coincideswith the beginning of rapid convergence

(.100 km/m.y.) of the Farallon plate with re-spect to North America (Engebretson et al.,1985).

Two volcanic rock units of note are: (1) thevesicular basaltic lavas intercalated within theupper Campanian carbonate slope sequence ofBahıa Santa Elena, located in northern SantaElena Peninsula (Baumgartner et al., 1984),and (2) the basaltic lavas and breccias inti-


C. CALVO

Figure 9. Cartoon depicting model of evolution for the Mesozoic ophiolite sequence ofCosta Rican arc orogen during the late Senonian, including (A) intrusive magmatic activ-ity, probably arc-related, in the Santonian–early Campanian (at ca. 84 Ma, Sinton et al.,1997), and (B) uplift and subsequent erosion in the late Campanian, resulting in unroofingof ophiolitic intrusive bodies: dolerites, gabbros, and plagiogranites. Both episodes wereaccompanied by andesitic arc and basic forearc volcanism. In late Campanian and Maas-trichtian times, transgressive shallow-marine carbonate reef and platform deposits of theEl Viejo Formation prograded over exposed basement. (MAT—Middle America Trench).

mately commingled with hemipelagic lime-stones of Maastrichtian age, exposed on thewestern coast of Nicoya Peninsula, betweenGarza and Puerto Carrillo (Schmidt-Effing,1979) (Fig. 1). These two volcanic units in-dicate that both uplift and unroofing of theophiolitic sequence in the late Senonian wereaccompanied by mafic forearc volcanic activ-ity (Fig. 9). These processes together attest tocoeval Cretaceous subduction and erosion,probably related to rapid convergence of theFarallon plate in southern Central Americaalso. In addition, the presence of alteritegrains, indicative of tropical weathering en-vironments (Johnsson, 1990), in cover sand-stones provides strong evidence for the resi-dence of the Costa Rican arc in low latitudesnear the equator during Cretaceous time, asconstrained by paleomagnetic data (de Boer,1979; Sick, 1989; Frisch et al., 1992).

Cretaceous Erosion ProfileProvenance relationships recorded by detri-

tal compositions of cover sandstones allow the

reconstruction of a Cretaceous erosion profilethrough the ophiolitic sequence (Fig. 10). Onthe basis of the earliest assemblage of pseu-dorbitoidal foraminifera present in analyzedsamples, the late Campanian (ca. 75 Ma) isassumed for this reconstruction. As can be de-duced from framework grain compositions,erosion at that time affected both the upperand lower parts of the Nicoya ophiolite com-plex. Because intrusive rocks appear in thelower part of the Nicoya Complex, plutonicrock fragments in cover sandstones record theunroofing of the Lower Nicoya Complex innorthwestern Costa Rica since at least ca. 75Ma. Such unroofing is also well constrainedby the Maastrichtian cherty sandstones fromthe Quebrada Pilas suite, whose detrital modes(Q23F50L27 and Qp41Lvm46Lsm13; Table DR2)plot close to Dickinson’s dissected-arc andsubduction-complex provenance fields, re-spectively (Calvo, 1998) (Fig. 3). Consideringthe minimum depth of formation of 4 km ob-served for MORB-related intrusions (S. Foley,2002, personal commun.), the unroofing of

plagiogranites implies significant strong upliftand subsequent deep erosion of the ophiolitesequence. The Cretaceous megabreccia depos-its containing plagiogranite boulders (Tournonand Azema, 1980) suggest a similar profile oferosion on Santa Elena Peninsula. In contrast,framework grains derived from shallow levelsof the Nicoya ophiolitic sequence principallyinclude basaltic and tachylite grains (partlyshowing vesicular textures) as well as radio-larian mudstone, pelagic limestone, and tuff-aceous lithic fragments. In general, the spec-trum of ophiolitic grains in cover sandstonespoints to an ophiolitic source-rock assemblagethat does not differ from that of the exposedmodern forearc basement (Fig. 10).

SUMMARY AND CONCLUSIONS

Detrital plutonic grains, previously over-looked in earlier studies, comprise accessoryframework components of Cretaceous forearcsandstones in northwestern Costa Rica. Inte-grated petrographic, sedimentologic, strati-graphic, and field evidence clearly indicatethat plutonic detritus was derived from erodedintrusive rocks of the Nicoya ophiolite com-plex. Moreover, the plutonic detritus corre-sponds, compositionally and texturally, tosource rocks of basic and intermediate com-positions. An important new petrologic resultis the parallel increase of plutonic lithic frag-ments [(Lp 1 iQF)/Lt] and uralitized pyrox-ene grains (uralPx/Px) identified in the frame-work population of pyroxene-bearing arenites.This observation indicates that uralitized py-roxene grains probably also resulted fromeroded intrusive rocks; the observation alsoreflects more clearly a predominantly inter-mediate composition of plutonic source rocks.In particular, significant concentrations of lith-ic fragments exhibiting micrographic texturesand uralitized pyroxene grains are interpretedto be predominantly derived from eroded pla-giogranitic intrusions. This study demon-strates, moreover, that both newly introducedparameters—i.e., (Lp 1 iQF)/Lt and uralPx/Px—can be used as semiquantitative modalparameters to determine affinity of sourcerocks and detrital plutonic contributions insandstones derived from ophiolitic sequences.

Sedimentologically, dispersal patterns ofdetrital sediment suggest that plutonic frag-ments, and all ophiolitic grains in general,were deposited in shallow-water environmentsduring rapid marine transgressions on exposedbasement areas. The detritus was apparentlydistributed into the slope and deep-water en-vironments by turbidity currents and sandy de-bris flows, principally during relative sea-level



Figure 10. Cartoon showing a hypothetical profile of erosion through the ophiolitic se-quence of Nicoya Complex in the Late Cretaceous (at ca. 75 Ma), deduced from frameworkcompositions and biostratigraphic ages of forearc sandstones of the Rivas and El ViejoFormations (upper Campanian–Maastrichtian). This ophiolitic source-rock assemblage isvery similar to that currently exposed in the forearc region of northwestern Costa Rica.The erosion unroofed deeper levels of the sequence, where intrusions occur. The erosionalsurface corresponds with the Campanian unconformity. No scale implied.

lowstands. Provenance areas were apparentlylocated on Nicoya Peninsula as well as in thearea of Cerro Barbudal, north of the Gulf ofNicoya.

Tectonically, Cretaceous sandstone suitescontaining plutonic detritus provide evidencefor the unroofing of deeper levels of the Mes-ozoic ophiolitic sequence of Costa Rica in lateSenonian time. Sandstone ages show that theearliest erosion of Nicoya Complex intrusiverocks began at least by late Campanian time(ca. 75 Ma), ;9 m.y. after cessation of theintrusive magmatic activity on northern Ni-coya Peninsula. Geochemical signatures ofpotential plutonic sources (Wildberg, 1987;Sinton et al., 1997) suggest coeval erosion ofboth island arc– and oceanic plateau–relatedintrusive rocks. The unroofing of the ophiolit-ic sequence concomitant with the beginning ofneritic carbonate sedimentation records stronguplift of the Costa Rican orogen, including theformation of emergent ophiolitic basement ar-eas in the forearc. This tectonic event marksthe onset of the Laramide orogeny in LateCretaceous time. In this context, plutonic andassociated radiolarian chert grains in Creta-ceous cover sandstones are considered as de-trital paleotectonic indicators of the prominentCampanian unconformity separating theophiolitic basement from its sedimentary cov-er strata.

ACKNOWLEDGMENTS

I thank A. Bolz for determination of larger fo-raminifera in thin section and S. Kussmaul for pro-viding representative samples of the plagiogranitesfrom Playa El Ocotal. M. Meschede and S. Foleyprovided additional data on plutonic rocks. Reviewsby J. Mezger, A. Bolz, and H.-J. Gursky improvedthe early manuscript version. I am also indebted toR.J. Dorsey, N. Lundberg, and G.H. Girty for help-ful reviews of the submitted manuscript.

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