Provenance of Plutonic Detritus in Cover Sandstones of Nicoya Complex, Costa Rica: Cretaceous Unroo®Ng History of a Mesozoic Ophiolite Sequence
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Provenance of plutonic detritus in cover sandstones of Nicoya Complex, Costa Rica: Cretaceous unroo®ng history of a Mesozoic ophiolite sequence Claudio Calvo² Anna-Peters-Strasse 51/C, 70597 Stuttgart, Germany ABSTRACT the Campanian at ca. 75 Ma, ;9 m.y. after consisting of basaltic fragments and differen- the intrusive magmatic activity on Nicoya tiated pyroclastic materials (Calvo and Bolz, This study presents new petrologic and Peninsula, and is consistent with the onset 1994). Similarly, Cretaceous cover sandstones sedimentologic data from northwestern Cos- of the Laramide orogeny. of the Nicoya Complex contain a wide spec- ta Rica concerning the provenance of Cre- trum of framework grains from other sources: taceous forearc sandstones that contain plu- Keywords: provenance, modal analysis, radiolarites (including radiolarian cherts), pe- tonic detritus. Plutonic rock fragments are plagiogranites, ophiolite complex, Costa lagic limestones, shallow-marine carbonates, important accessory particles in pyroxene- Rica. andesitic lavas, differentiated ejecta, and ig- bearing sandstones overlying the ophiolite neous intrusive rocks (Calvo, 1998). This named the Nicoya Complex. Through the INTRODUCTION spectrum of grain types suggests that these use of modal analysis of the framework forearc sandstones were derived predominant- grains, I studied three sandstone suites of The ophiolitic basement of northwestern ly from shallow levels of an intraoceanic arc, the El Viejo and Rivas Formations that in- Costa Rica and its sedimentary cover strata but it also re¯ects a signi®cant contribution clude both shallow- and deep-water depos- constitute the most thoroughly studied on-land from uplifted ophiolitic basement areas partly its, ranging from late Campanian to Maas- rock assemblage of the southern Central fringed by shallow-water carbonate deposits trichtian in age. In terms of primary American forearc. This study focuses on the (Calvo, 1998). The plutonic detritus particu- framework components, the sandstones re- origin and provenance of plutonic detritus in larly records an ophiolitic provenance and semble those derived from magmatic arcs. both deep-sea channel and shallow-water bas- deeper unroo®ng of the ophiolitic basement Two modal parameters are introduced to al sandstones from the Cretaceous cover units than previous workers have suggested. evaluate detrital plutonic contributions and overlying the ophiolite named the Nicoya The main objectives of this study are (1) to af®nity of source rocks: the ratio of pluton- Complex. In these forearc sandstones, detrital analyze mineral composition and textural fea- ic to total lithic fragments [(Lp 1 iQF)/Lt], grains of igneous plutonic origin are important tures of plutonic detritus in order to determine and the ratio of uralitized pyroxene to total accessory constituents, making up as much as likely source rocks and provenance relation- pyroxene grains (uralPx/Px). Modal values 9% of total lithic framework grains. ships; (2) to establish detrital plutonic contri- for (Lp 1 iQF)/Lt indicate that plutonic Previous studies on petrology and prove- butions and af®nity of source rocks by using fragments comprise up to 9% of total lithic nance of forearc sandstones from northwestern modal analysis of secondary framework pa- fragments. A strong correlation between Costa Rica have suggestedÐon the basis of rameters; and (3) to integrate these new data these two parameters suggests that uraliti- primary framework modesÐthat Cretaceous into a model for the Cretaceous unroo®ng his- zed pyroxene grains were also derived from sandstones were derived from a magmatic arc tory of the ophiolitic sequence of southern intrusive rocks of probably basic and inter- (Lundberg, 1991; Calvo, 1998). Composition- Central America. mediate compositions. In particular, signif- ally, both those sandstones within the Nicoya icant concentrations of lithic fragments ex- ophiolite complex and many of the uncon- TECTONIC SETTING hibiting micrographic textures and formably overlying sandstones are commonly uralitized pyroxene grains are interpreted referred to as basaltic sandstones, derived Southern Central America (Costa Rica and to have been derived predominantly from from erosion of basaltic basement (e.g., Kuij- Panama) represents an island arc that formed eroded plagiogranites. Sandstone suites pers, 1979; Baumgartner et al., 1984; Gursky, over an intraoceanic subduction zone situated containing plutonic detritus signal an un- 1989; Lundberg, 1991). Although basaltic at the western margin of the Caribbean plate roo®ng of deeper levels of the Mesozoic sandstones dominate in some sequences, there (e.g., Kuijpers, 1979; Lundberg, 1982; Wild- ophiolitic sequence as a consequence of are also volcaniclastic sandstones that contain berg, 1984; Calvo and Bolz, 1994). At the strong uplift of the Costa Rican arc in late abundant nonbasaltic framework components. present time, the Cocos plate is being sub- Senonian time. This tectonic event began in For example, volcaniclastic wackes from the ducted beneath Costa Rica at 90 mm/yr (Min- Loma Chumico Formation of the upper Ni- ster and Jordan, 1978). Bathymetric swath ²E-mail: [email protected]. coya Complex exhibit a bimodal composition mapping of the convergent margin offshore GSA Bulletin; July 2003; v. 115; no. 7; p. 832±844; 10 ®gures; 3 tables; Data Respository item 2003095. For permission to copy, contact [email protected] 832 q 2003 Geological Society of America CRETACEOUS UNROOFING HISTORY OF A MESOZOIC OPHIOLITE SEQUENCE, COSTA RICA Costa Rica shows an oceanic plate covered stones, and minor conglomerates and breccias the modal analysis, because of their possible with numerous seamounts (von Huene et al., (Dengo, 1962; Protti, 1981; Lundberg, 1982; intrabasinal origin and generally minor im- 1995). The most prominent feature is the Co- Rivier, 1983; Baumgartner et al., 1984; Calvo, portance for constraining ophiolitic prove- cos Ridge subducting opposite the Osa Pen- 1998). This unit overlies conformably hemi- nance. Sandstone suites derived from different insula (Fig. 1). pelagic limestones and volcaniclastic rocks of depositional environments were also analyzed The study area is located in northwestern the Sabana Grande Formation of Campanian to evaluate dispersal patterns of detrital sedi- Costa Rica. This region is characterized by age. ment in the marine forearc basin. three major morphotectonic elements that re- In this study, the timing of unroo®ng of the ¯ect an evolved arc edi®ce (Fig. 1): (1) the MATERIALS AND METHODS ophiolitic sequence was established from the emerged outer arc, forming the Nicoya and relative ages of foraminiferal assemblages Santa Elena Peninsulas; (2) the inner forearc Detritus derived from igneous intrusive identi®ed in sandstone samples. Radiometric trough (Tempisque basin), comprising the rocks in Cretaceous cover sandstones was ®rst age determinations of intrusive rocks from the Gulf of Nicoya and lowlands of the Tem- detected during analysis of main framework Nicoya Peninsula by Sinton et al. (1997), cou- pisque River; and (3) the active volcanic cor- grains (Calvo, 1998). On the basis of deposi- pled with the biostratigraphic data presented dilleras of Guanacaste and TilaraÂn, represent- tional environment and age, 20 sandstone here, constrain the age of crystallization and ing the inner magmatic arc. Behind the samples were selected for this study. They earliest erosion of Nicoya Complex intrusive cordilleras are the Guatusos lowlands, repre- were collected from three different locations rocks in northwestern Costa Rica. senting the southern terminus of the exten- in Cretaceous sandstone suites of the inner sional Nicaraguan Depression. forearc: La Tigra, Quebrada Pilas, and Calle RESULTS Codornices (Fig. 1). The sampled units form STRATIGRAPHIC FRAMEWORK part of the sedimentary cover of Nicoya Com- Detailed petrographic examination of plex, recording shallow- and deep-water clas- framework grains reveals that Cretaceous The forearc basement is exposed along the tic sedimentation in the forearc region of forearc sandstones from northwestern Costa Paci®c coast of Costa Rica and comprises a northwestern Costa Rica during the late Sen- Rica contain abundant ophiolitic grains, rang- Mesozoic ophiolitic sequence, the Nicoya onian. Their principal stratigraphic and sedi- ing from basic volcanic fragments and igneous Complex (Dengo, 1962). It is principally com- mentologic characteristics are summarized in intrusive-derived detritus to cherty rock frag- posed of peridotites, basalts (massive and pil- Table 1. Sandstone ages were determined on ments of sedimentary origin (Fig. 2). This lowed ¯ows), basaltic breccias, dolerites, ra- the basis of their contained foraminiferal as- study focuses on plutonic rock fragments and diolarites, limestones, volcaniclastic rocks, semblages. Samples were examined petro- radiolarian chert grains, which are thought to and igneous intrusive rocks (plagiogranites graphically in thin section, cut normal to bed- represent useful distinctive provenance indi- and gabbros). Differentiated pyroclastic de- ding. For analysis of sandstone frameworks, cators re¯ecting unroo®ng of deeper levels of posits of Albian to Campanian age within the an average of 500 grain points were counted the ophiolitic sequence. Petrographically, an- Nicoya Complex record the earliest volcanic in each section. Signi®cant concentrations of alyzed sandstones comprise dominantly lithic activity of the Costa Rican arc