Ophiolite and island-arc volcanism in Costa Rica

CARLOS GALLI-OLIVIER* Central American School of Geology, University of Costa Rica, Apartado Postal 35, Ciudad Universitaria "Rodrigo Facio," Costa Rica

ABSTRACT belt there are active volcanic chains, many oceanic trenches, and most deep- and intermediate-focus earthquakes. The southern part The Pacific margin of Costa Rica is a very deformed basement of Central America is the result of subduction that took place ferrane of ophiolite composed of pillowed and massive basalt, within the oceanic realm, mafic and ultramafic plutonic rocks, volcanic breccia, hyaloclastite, The Central American isthmus is located on the southwestern radiolarian chert, and limestone. The ophiolite underwent changes edge of one of the smaller plates, the Caribbean plate. The bound- through burial metamorphism after subduction of the Cocos plate ary of the Caribbean plate and the Cocos plate, situated to the west, under the southwestern margin of the Caribbean plate. In some is marked by the Middle America Trench, a long topographic de- areas the ophiolite is a mélange. While the age of the emplacement pression where the two plates collide in such a fashion that the of the ophiolite in northwestern Costa Rica is late Santonian to Cocos plate is consumed by its plunging downward along a sub- early Campanian, the period of accumulation of the ophiolite duction zone into the asthenosphere below the Caribbean plate. seems to be very long, possibly extending from middle Tithonian to The boundary of the Cocos and Caribbean plates, however, is not a late Santonian. New age determinations based on foraminifera and place where there was a Total loss of Pacific oceanic crust. On the radiolaria support the previous dating. A relative scarcity of turbi- contrary, material was transferred on the border of the Caribbean dite in the ophiolite of Costa Rica, compared to other similar ter- plate. Part of the material was added by magmatism to the margi- ranes of the Pacific margin, has been associated with the intra- nal belt through formation of magmas from rocks of the descend- oceanic origin of the southern Central American arc. ing slab, which then built the volcanic arc. Other portions of the The island-arc suite clastic rocks first unconformably covered the material were accreted to the Caribbean plate by the scraping of ophiolite in Costa Rica in early Campanian time. Their origin is trench and oceanic sediments and slices of oceanic crust and mantle closely related to the intrusive and volcanic activity of a plutonic- from the downgoing slab (Fig. 2). The final result is a thickened volcanic arc located between South and North America since early crust in which the rocks pass from a truly oceanic crust to a crust of Campanian time. The clastic rocks of the island-arc suite are low- continental type. Pichler and Weyl (1975) have shown that the porosity volcanogenic types. Limestone has accumulated from Cre- main trend since Late Cretaceous time is characterized by a con- taceous time to the present, in some areas forming porous bioher- tinual increase of the silica level of the magmatic products that mal bodies. Vertical tectonics originated marginal- and intra-arc compose the present Central American arc. The last episode was sedimentary basins intermittently throughout the evolution of the the welding of the ends of the intra-oceanic arc to the South and arc. A fourfold geotectonic division of Costa Rica is proposed. North American continents to form a continuous land bridge since Pliocene time. INTRODUCTION Lloyd (1963), Dengo (1962a, 1962b, 1973), Malfait and Dinkelman (1972), Weyl (1973), Pichler and Weyl (1973, 1975, The Central American subcontinent is a continuous and narrow 1976), Pichler and others (1974), del Giudice (1973), Case (1974), tongue of land that, in its southern part, evolved from an old United Nations (1975), Schmidt-Effing (1976), Galli-Olivier and intra-oceanic island arc. The southern part forms an isthmus Schmidt-Effing (1977), and Galli-Olivier (1977) have made sig- unique on Earth because it connects two large continents, South nificant contributions on the paleogeography, evolution, petrology, and North America, separating completely two major oceans and and paleontology of the orogen. acting as a barrier to the world-wide circulation of water (Fig. 1). Geologists studying the basement rocks of Costa Rica and Like other ancient and modern arcs, it is an individual entity, with Panama, in this paper referred to as ophiolite or ophiolitic suite, its own peculiarities. have had to contend with its complex petrology and structural Recently developed ideas of global tectonics suggest a framework style. Some of the obstacles have been the unfossiliferous nature of within which to consider the origin of the southern part of the Cen- most of the rocks, the almost total lack of radiometric age determi- tral American isthmus. According to these ideas, the crust and part nations, the lack of guide beds, the lenticular stratigraphy, and the of the upper mantle are divided into a small number of nearly rigid metamorphic changes. A particularly troublesome problem has slabs, or plates in motion relative to one another. The circum- been that ideas on the petrotectonic significance of the ophiolite as- Pacific region is the locus of belts of deformed Phanerozoic rocks semblage and its complex structural evolution within a framework known collectively as the circum-Pacific orogenic belt. Within this of plates in motion evolved only during the past 15 years. Fortu- nately, Dengo (1962a, 1962b) and his co-workers realized very early that the basement terrane was an assemblage of different * Present address: Ciencias Marinas, Apartado Postal 453, Universidad rocks with intricate structural relations deserving the name of Autónoma de Baja California, Ensenada, Baja California, México. "complex" years before this term was proposed and adopted as

Geological Society of America Bulletin, Part I, v. 90, p. 444-452, 4 figs., May 1979, Doc. no. 90507.

444

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part of the formal name for the lithologically and structurally simi- PETROTECTONIC ASSEMBLAGES lar Franciscan Complex (Berkland and others, 1972) of North America. Ophiolitic Suite This paper describes the petrotectonic assemblages of the Mesozoic basement terranes and the base of its sedimentary cover The western edge of the Caribbean plate in Costa Rica (Fig. 3) is in the southern part of Central America. Geotectonic interpretation partly formed by the ophiolite, a folded, faulted, and stratally dis- of their lithology and structural styles is presented. New age de- rupted basement terrane of the Pacific margin which is composed of terminations and conclusions that extend the results of other au- mafic to intermediate volcanic rocks, mafic and ultramafic plutonic thors are also given. rocks, volcanic breccia, hyaloclastite, radiolarian chert, limestone,

Figure 1. Index map of Central America, showing schematically the main geographic and political regions and lo- cation of profile of Fig- ure 2. Other geographic localities mentioned in text: 1, Santa Elena Pen- insula; 2, Sardinal; 3, Punta Salinas; 4, Brasilito; 5, Punta Gorda; 6, El Bolsón; 7, Junquillal; 8, Colorado de Abangares; 9, Tem-

pisque River; 10, Nicoya CARIBBEAN Peninsula; 11, Gulf of SEA Nicoya; 12, Punta Cón- cavas; 13, Montezuma; 14, Jaco; 15, El General valley; 16, Talamanca Range; 17, Coto Brus valley; 18, Osa Penin- sula; 19, Burica valley; 20, Bocas del Toro; 21, Azuero; 22, Gulf of Par- ita; 23, San Blas; 24, Darién Province; 25, Colombia.

Figure 2. Schematic cross section along line A-A' of Figure 1, show- ing accretionary prism (1) and other geotectonic belts of northwestern Costa Rica, based on Karig and Sharman (1975).

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and their metamorphic equivalents. A part of this suite is the tinguished on geologic maps. The ophiolite underlies an angular Nicoya Complex of Dengo (1962a, 1962b). A similar lithologic as- unconformity and was eroded and faulted before the deposition of sociation forms the basement terrane of several regions of Panama clastic rocks and limestone of an Upper Cretaceous island-arc suite (Case, 1974), Colombia, and Ecuador (Pichler and others, 1974). that is associated with an entirely different geotectonic episode. The ophiolite has a deep oceanic and pelagic origin. Pichler and The ophiolitic suite is composed of rocks that may have origi- Weyl (1975) pointed out that the rocks of the ophiolite of Costa nated in the upper mantle and oceanic crust. The upper mantle and Rica are quite different, in mineralogical and petrochemical re- level 3 of the crust may be represented by peridotite, amphibole- rich dike swarms, gabbro, diabase, and diorite; level 2 by tholeiitic spects, from all the other magmatic rocks of Central America. The basalt; and level 1 by pelagic sedimentary rocks. Level 2 and level 1 structure, however, is as characteristic as the petrology. Ophiolite rocks are intercalated in most areas, and plutonic rocks penetrated terranes are formed by both melanges and coherent units. Although peridotite as well as tholeiitic basalt. rocks with ages probably ranging from middle Tithonian (E. Pes- Igneous Rocks. The Santa Elena Peninsula (Fig. 1, loc. 1) pres- sagno, 1976, written commun.) to late Santonian (Galli-Olivier and ents the largest and northernmost outcrops of level 3 and upper- Schmidt-Effing, 1977) have been found, neither the stratigraph- mantle rocks. Dondoli (1965) discovered the ultramafic nature of ically higher part nor the base of the ophiolite have ever been dis- ss0 85° 84° 83°

CARIBBEAN SEA

10°

MUDFLOW, ALLUVIUM, AND MARINE DEPOSITS ( QUATERNARY )

VOLCANIC ROCKS (TERTIARY AND QUATERNARY)

9" PLUTONIC ROCKS (MAINLY MIOCENE)

PELAGIC AND VOLCANICL ASTIC ROCKS (LATE CRETACEOUS-PLIOCENE)

OPHIOLITIC SUITE (MESOZOIC)

INTERNATIONAL BOUNDARY

10 0 50 Km

Figure 3. Generalized geologic map of Costa Rica, simplified from Mapa geológico de Costa Rica (Dirección de Geología, Minas y Petróleo, scale 1:700,000, 1968).

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the Santa Elena rocks in 1949. Later, Harrison (1953), Dengo generally red, hematite-rich, and rhythmically bedded due to var- (1962a, 1962b), and Tournon (1970) described these rocks in de- iations in the clay content. Radiolaria are everywhere present in tail. Dengo (1962b) pointed out the presence of harzburgite, and unmetamorphosed or slightly metamorphosed chert, but their pres- Tournon (1970) found that most of the peridotite of Santa Elena is ervation varies broadly from deformed, recrystallized chalcedonic partially or totally serpentinized lherzolite composed of olivine, specimens to well-preserved forms of Sphaerellaria and Nassellaria. clinopyroxene, orthopyroxene, and picotite, the latter being the The thickest known sequence of chert is about 70 m. The geometry only fresh mineral in many serpentinized areas. Serpentinite is as- of the bodies of chert varies from lenticular masses as much as 2 km sociated with chromite and asbestos (Dondoli, 1965). The lherzo- long to inclusions of all shapes and sizes injected between pillows lite has parallel, north-dipping banding due to concentrations of or in fault and fracture planes, or engulfed by lava flows. Chert was pyroxene (Dengo, 1962a). Tournon (1970) emphasized the pres- also injected into lavas as igneouslike dikes several metres long, or ence of amphibole-rich dikes, approximately north-trending, in- is present in melanges as tectonic blocks of variable size and shape. truded in the lherzolite every 20 to 50 m in places; the nearly verti- Chert is associated with manganese ore in many places (Roberts, cal dikes are between 1 and 10 m thick and show sharp contacts. 1944). Manganese ore was emplaced as veins, rounded inclusions a The westernmost outcrops of the Santa Elena Peninsula contain few milimetres in diameter, or irregular masses several metres in large masses of amphibole-rich rock, locally with pegmatitic tex- diameter. In Punta Gorda (Fig. 1, loc. 5) chert and pillow lava are ture. The mineralogical composition of the dikes and masses is associated with a massive sulfide body (Flores, 1976) in a manner primarily basic plagioclase and green hornblende. In the Nicoya similar to that reported from Troodos, Cyprus (Constantinou and and Osa Peninsulas (Fig. 1, Iocs. 10 and 18) localized outcrops of Govett, 1973) and the northern Apennines (Bonatti and others, ultramafic rocks have been reported (Romanes, 1912; Sears, 1919; 1976). Case and others (1971) also reported chert associated with Webber, 1942; Manuel Brenes, 1976, oral commun.; Eric Kuijpers, basic rocks in the Darien province of Panama, and in the Pacific 1976, oral commun.). In 1976 Jorge Laguna and I found porphyri- coast and the Cordillera Occidental of Colombia (Fig. 1, Iocs. 24 tic harzburgite in the Colorado de Abangares area (Fig. 1, loc. 8). and 25). Bandy and Casey (1973) dated cherts and correlated them G. Metti and G. Recchi (del Giudice, 1973) suggested that in with reflector horizons in eastern Panama. Panama ultramafic rocks of two ages crop out. The older rocks are Limestone varies from biomicrite with few igneous crystals to re- metamorphosed. sedimented, well-graded, turbiditic lithic limestone rich in Gabbro, diabase, and diorite are the main coarse-grained intru- volcanic-rock and crystal fragments. It is possible that some lithic sive rocks emplaced in the Santa Elena rocks as well as in the limestone is redeposited peperite. Most radiolaria and foraminifera tholeiitic basalt elsewhere. Olivine-augite gabbro, augite- are well preserved even in intensely sheared tectonic limestone hypersthene gabbro, and ilmenite-magnetite-rich gabbro have been blocks of melanges. The thickest known limestone bed is ap- recognized. All these rocks are possibly parts of stratiform sheets proximately 7 m, and the overall shape of the deposit is lenticular. and homogeneous dikes. Small inclusions of carbonate in lavas are less common than those In most areas of the western edge of the Caribbean plate in Costa of chert. Some have not been affected by thermometamorphism, Rica and in parts of Panama, the tholeiitic basalts (level 2?) pre- but others, between pillows, are completely recrystallized. Dolo- dominate; these have been studied by Dengo (1962a, 1962b), mitic crystalline veins may be related to pelagic limestone deposits. Henningsen and Weyl (1967), Pichler and Weyl (1973, 1975), and Very minor amounts of volcarenite, possibly resedimented ba- Case (1974). Case and others (1971) reported similar rocks from saltic ash, is known in places. Angular plagioclase, augite, volcanic the Pacific coast of northwestern Colombia. Goossens and Rose rock fragments, and glass, partly altered to chlorite; sericite; and (1973) and Pichler and others (1974) pointed out that a Basic Igne- iron oxides, moderately sorted and laminated, are characteristic. In ous Complex extends from Costa Rica to Ecuador and contains samples from Junquillal (Fig. 1, loc. 7) radiolaria (Sphaerellaria) oceanic basalt. Pichler and Weyl (1975) reported that the tholeiitic are common, but Globotruncana lapparenti s.l. is rare (Axel von basalt and the serpentinized lherzolite of Santa Elena are pet- Hillebrandt, 1975, oral commun.) rochemically very similar. Although massive basalt is common, pil- Metamorphic Changes. The ophiolitic suite was affected by low lava structures are characteristic, with pillow diameters rang- burial metamorphism on a regional scale and, locally, by disloca- ing from 20 cm to 1 m, surrounded by darker, in part fragmented, tion and thermal metamorphism. Beatriz Levi (1974, written com- vitreous crusts of younger lava injected between the pillows. It is mun.) studied the regional metamorphic effects and found low- estimated that more than 90% of the ophiolite is formed of basalt. temperature mineral associations in the rocks of the suite filling White veins of zeolite and carbonate crisscross the basalt almost cavities, replacing primary minerals, and forming cements. She did everywhere, filling fractures that pervade the rock. In many areas, not find textural changes but, rather, found high-temperature min- basalt is intercalated with massive volcanic breccia and well- erals partially or totally changed into low-temperature secondary bedded hyaloclastite. The volcanic breccia consists of angular ba- minerals. The ophiolitic suite is characterized by the following sec- salt, glass, and jasper fragments in a matrix of altered glass contain- ondary associations: chlorite-calcite, prehnite-zeolite, zeolite- ing plagioclase and augite crystals. The chloritized hyaloclastite calcite, and zeolite-chlorite. Levi pointed out that these metamor- contains very angular fragments of dark glass with interstitial phic changes are also present in rocks affected by burial light-colored glass. Graded bedding is well developed and suggests metamorphism of zeolite to prehnite-pumpellyite facies in the con- that turbidity currents occurred after subaqueous pyroclastic flows, tinental Pacific margins of New Zealand, Japan, Chile, the north- which were formed by debris settling from submarine eruptions. western United States, and other regions. In Panama, outcrops of Sedimentary Rocks. The sedimentary rocks of the ophiolitic rocks of the greenschist facies are known. Common mineralogical suite are mainly radiolarian chert and biomicritic limestone or their associations of this facies are chlorite-actinolite, actinolite-albite, metamorphosed equivalents. These rocks have a pelagic origin and actinolite-epidote, and serpentine-actinolite (del Giudice, 1973). were deposited in localized areas, below or above the carbonate Thermal metamorphic changes are shown in chert and limestone compensation depth, between submarine volcanic events. Chert is in contact with tholeiitic lavas. G. Recchi (del Giudice, 1973) men-

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tioned garnet hornfels derived from Mesozoic rocks of the base- One set of subparallel surfaces in places may be associated with ment terrane of Panama. Recrystallization, chloritization, hematiti- other intersecting sets, forming complicated systems of planar sur- zation, and color changes are the main effects of thermal faces. These seem to be the tectonites without flow, or SF- metamorphism. Chert was recrystallized to a fine, very uniform tectonites, of Raymond (1975), which characterize large areas in mosaic, in which radiolaria appear as recrystallized spherical or the low-temperature metamorphic belt of Costa Rica. elongated areas or have completely disappeared. Some chert was recrystallized to chalcedony spherulites. Quartz veins are Basal Island-arc Suite everywhere associated with recrystallized chert and include veins of microscopic size to masses in which milky quartz veins form most Large areas of Nicaragua, Costa Rica, and Panama are underlain of the rock, breaking the chert apart. Calcite with finely to medium by dominantly Upper Cretaceous-Cenozoic clastic wedges derived crystalline mosaics, forming inclusions in lava, probably is the from the uplifted tectonic and volcanic land masses of the arc. As- thermometamorphic derivative of pelagic limestone of the ophidi- sociated with limestone, they form an island-arc suite separated ne suite. Crystalline dolomite veins may also be a product of ther- from the subjacent ophiolitic suite by an angular unconformity. mally altered limestone. Fossiliferous limestone xenoliths, on the Volcanic detritus in the clastic rock strongly suggests the nearby other hand, may be virtually unaltered amidst pillow lava. presence of the growing island arc. On the Pacific side, these mate- Although chlorite may be present in some altered calcite inclu- rials accumulated in the sediment traps of the upper slope trough or sions, chloritization is a thermal effect rather common in chert on downthrown blocks between faults cutting the accretionary xenoliths. Field relations suggest hematitic chert replacement by prism. On the Caribbean side, they were deposited on the simpler chlorite from the borders of the xenoliths toward the central part oceanic margins of the back-arc area. The simplicity of the struc- which, in certain places, is not chloritized. Chalcedony and quartz tural style contrasts with the underlying melanges and broken for- veins were formed after the chloritization. Color changes seem to mations of the opholitic suite. The oldest known beds of the be closely related to distances from the contact with the basalt, to island-arc suite in Costa Rica are of early Campanian age (Galli- chloritization, hematitization, and bleaching. Chert inclusions be- Olivier and Schmidt-Effing, 1977). come earthy looking and deep red by hematitization. Radiolaria The clastic wedge of the island-arc suite is composed of both may be almost completely replaced by hematite, which causes the dominantly shallow-marine, transgressive detrital units, underlain development of a colloform texture. In places, the color of the by a basal conglomerate and interdigitated with carbonate accumu- xenoliths varies in short distances from dark greenish-black outside lations, and by deeper water, flyschlike formations of rhythmic, in to light yellowish-gray inside. These differences in thermal parts graded and sole-marked, lutite and arenite. Pelagic limestone metamorphic effects suggest differences in the cooling histories of and argillite are known in places. Turbidity currents were impor- the basalt, as well as in the physical conditions of the sediments at tant agents of transportation and deposition of terrigenous vol- the time of volcanic activity. carenitic and detrital limestone materials, in places running parallel Mélange. Melange zones are known from several localities of to the longitudinal axis of the upper slope trough. The clastic the Nicoya Peninsula and Jacó (Fig. 1, Iocs. 10 and 14). Nicoya wedge transgressed across an erosion surface on the ophiolitic suite Complex mélanges are bodies of rock characterized by the inclu- at different times during the Cretaceous and Tertiary; the resultant sion of fragments and blocks of all sizes embedded in a dark, frag- rocks, although of different ages, appear very similar and contain mented, and sheared matrix of rock comminuted to a paste. It has large amounts of volcanic detritus in the framework. For the most not been determined if all embedded fragments and blocks are na- part, the volcarenites are first-cycle, little-weathered, and angular tive. Faults divide these bodies of rock into segments of many and have a detrital framework of as much as one-third volcanic metres to fractions of a millimetre in width. In the latter instance, rock fragments, as much as three-fourths plagioclase, and com- parallel and penetrative cleavage has obliterated older structures paratively large amounts of pyroxene and amphibole; this indicates (for example, pillows in lavas). Some fault zones are brecciated, a predominantly basaltic and andesitic intra-arc source, short others are not. Fault planes may be folded in complicated patterns. transportation, and rapid burial. Quartz is rare, and most of it Mylonites and protomylonites contain a sheared chloritized paste shows embayments, planar crystal outlines, straight extinction, and with or without embedded fragments, in part with boudins and almost no inclusions. Its origin is volcanic. Other grains, some of disharmonically folded phacoids. Tholeiitic basalt is more suscep- which show recycling, were derived from the underlying ophiolitic tible to deformation than chert or limestone. Very closely spaced suite, particularly chert, magnetite, and ilmenite. Feldspar crystals shear planes in basalt produce a tectonite easily broken with a are dominantly andesine and labradorite. A large proportion of the hammer. Chert may form 5 cm to 30 m or more, yellow to red, feldspar is zoned. Volcanic-rock fragments are perhaps the most elongated phacoids in a mass of dark-green sheared basalt. While variable in composition and degree of alteration. Porosity is low. the longer sides are generally parallel to the bedding planes of the Basal conglomerate of the wedge has well-rounded clasts, but in chert, the extremities are rounded or faulted. Larger chert masses (1 places it is texturaljy immature, suggesting rapid burial of debris- km or more), forming coherent units, like those near Sardinal or flow materials at the foot of scarps in the ophiolites. In areas of the Brasilito (Fig. 1, Iocs. 2 and 4), are strongly folded and, in places, Tempisque-Nicoya interdeep (Fig. 2), shallow-marine conditions pervaded by shear planes. The tectonic blocks of limestone are pink seem to have prevailed from Campanian to at least Eocene time, as to brown, sheared, microfolded masses of rock, in which recrystal- suggested by reefs and nearshore sandstone and conglomerate. In lization along certain shear planes has taken place. Most radiolaria contrast, in Late Cretaceous time in Panama, as well as during and foraminifera in the limestone are not deformed. some of the Tertiary in Costa Rica, volcaniclastic flysch was depo- Coherent units of tholeiitic basalt are characterized by fabric sited in deeper basins of the Pacific marg:;n. Fine- and medium- elements produced by fracture and/or shear along a pervasive set of grained clastic rocks of the wedge contain Globotruncana elevata, subparallel surfaces, commonly slickensided and almost every- G. calcarata, G. gansseri, and Pseudokossmaticeras sp. of Campa- where filled with white zeolite and minor amounts of carbonate. nian and Maestrichtian ages (Galli-Olivier and Schmidt-Effing,

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1977) and G. conica and G. stuarti of Maestrichtian age (R. date of the ophiolitic basement is then confined to a very short span Fischer, 1978, oral commun.). of a few million years, from late Santonian to early Campanian. Limestone was formed by biohermal, clastic, or pelagic accumu- Earlier attempts to date the ophiolitic suite were based on the lations. Although in most places the island-arc suite is underlain by radiometric age of its basalt (Barr and Escalante, 1969) or were on clastic rocks, in others (for example, El Bolson; Fig. 1, loc. 6) trans- foraminifera present in the pelagic rocks of the suite (Henningsen gressive reefs cover the ophiolitic suite (Galli-Olivier and and Weyl, 1967). Both methods indicated a somewhat younger age Schmidt-Effing, 1977). Rudists, stromatoporoids, and corals com- (Campanian to Maestrichtian). pose most of the bioherms, some of which are porous. Calcarenite and calcirudite, in part deposited by turbidity currents and mixed GEOTECTONIC INTERPRETATION with volcaniclastic detritus, are widespread. Dense pelagic lime- stone, with variable amounts of volcanic glass and crystals, con- The petrotectonic assemblages of Costa Rica were formed in dif- tains radiolarians and planktonic foraminifera. These rocks crop ferent geotectonic environments. Although deformation and out in several areas of the Pacific margin of Costa Rica. metamorphism considerably changed many of the rocks, they are well enough preserved to be identified as a crust-forming rifted Ages of Ophiolitic and Basal Island-arc Suites zone of complicated topography, abyssal plains, a subduction zone, and sedimentary basins developed during the evolution from an Emile Pessagno (1976, written commun.) could identify intra-oceanic island arc to a continuous land bridge. radiolaria from only one of several samples of chert from the The majority of the plutonic and volcanic rocks of the ophiolitic ophiolitic suite of Brasilito (Fig. 1, loc. 4), where the preservation of suite may have been derived from magma generated beneath an the specimens was good. He identified Parvacingula sp., Pantanel- oceanic ridge, flanks of the ridges, and volcanoes on the elevated lium riedeli n. sp., Pantanellium n. sp. (figured by Riedel and rims and scarps of the fractured zones (see Hutchison, 1975, Fig. Sanfilippo from the Tithonian to Berriasian of Sicily), Archaeodic- 2A). Mantle lherzolite is envisioned to have risen into the axial tyomitra sp., and "Pseudodictyomitra" sp. (genus not known to part of the ridges and to have melted to produce a liquid phase occur below middle Tithonian). According to Pessagno (1976, (basalt) and a residuate phase (harzburgite) (Engel and Fisher, written commun.), the biostratigraphic determination is middle 1975). The widespread occurrence of amphibole-rich dikes in the Tithonian to Valanginian, zone 5, subzone 5A, with a high proba- lherzolite of the Santa Elena Peninsula (Fig. l,loc. 1) is indicative of bility of being restricted to the interval of late Tithonian to Berria- both a water-rich higher crustal level and a rifted zone under ten- sian. It is my opinion that future investigations of the pelagic rocks sional stress. Basalt is extruded as pillow basalt. Beneath it are of the ophiolite of other regions of Costa Rica will prove ages even masses of ultramafic rocks, gabbro, diabase, and diorite. Hyalo- older than Tithonian. clastite and massive volcanic breccia may be related to the volcanoes The ophiolitic accumulation ages indicate a long interval of time close to the ridge. Transportation of volcanogenic clastic material during which an oceanic lithosphere formed. It is generally ac- by sediment gravity flows produced turbidites similar to the cepted (Hutchison, 1975) that during this period there was original turbidity-current—generated, graded-bedded volcarenite of Punta igneous crystallization or metamorphic recrystallization within and Concavas, or a pebbly sandstone facies such as the debris-flow under an oceanic ridge, where a crust is formed, and deposition of units of very angular volcanic and chert fragments of Montezuma pelagic sediments in the expanding deep ocean bottom, in its way (Fig. 1, Iocs. 12 and 13). Metal sulfide mineralization in the ophio- toward a subduction zone, where lithosphere is lost. This period of lite (for example, Punta Gorda; Fig. 1, loc. 5) suggests hydrother- time is known as the "accumulation period," which in Costa Rica mal circulation within the oceanic crust at a spreading center seems to be very long. On the basis of the age determinations made (Bonatti and others, 1976). by Pessagno (1976, written commun.), it can be concluded that the Although sedimentation of pelagic rocks on graben between minimum accumulation period of the suite extended possibly from mid-oceanic ridges has been reported (Garrison, 1974; Rabinowitz middle Tithonian to late Santonian, a span of approximately 70 and Melson, 1976), most sediments probably formed later as the m.y. oceanic crust rode away from the rifted zone. Pelagic chert and According to Hutchison (1975), the age of an ophiolitic belt is limestone suggest a long interval of deposition, a very low rate of defined by the time of its emplacement in the zone of continental sedimentation, and a relatively tranquil abyssal-plain geotectonic and oceanic plate contact or the time of plate collision ("emplace- environment while the spreading ocean floor moved slowly toward ment age"). That age can be deduced only, according to Hutchison, the present Caribbean region, as hypothesized by Dengo (1973). by the stratigraphy of the clastic sedimentary envelope of the belt. However, volcanic activity in places interrupted and disturbed In Costa Rica, on the other hand, an exceptional stratigraphic rela- sedimentation on deep abyssal plains, as indicated by soft pelagic tion permits the dating of rocks above and below the angular un- calcareous ooze and lava breccia contacts, by chert and limestone conformity that separates the ophiolite from its cover. The recog- squeezed between pillows and injected in fractures of lavas while in nition of foraminiferal biozones in both the ophiolitic suite and the a gelatinous state, or by the incorporation of chert by advancing unconformable sedimentary envelope permits a very precise age de- pillow lava flows. First-cycle hyaloclastite and basaltic ash, in part termination of the emplacement of the ophiolite. Globotruncana with graded bedding, are the products of short-lived volcanic ac- carinata, of late Santonian age, has been found in radiolarite of the tivity coeval with pelagic sedimentation. ophiolitic suite of Punta Gorda (Fig. 1, loc. 5; Galli-Olivier and Thick, monotonous sequences of rhythmic chert, as much as 70 Schmidt-Effing, 1977). This is the oldest known planktonic m in Punta Salinas (Fig. 1, loc. 3) indicate a long period of quies- foraminifer found in Costa Rica. We also reported Globotruncana cence. The apparent absence of foraminifera, coccoliths, and other elevata, of early Campanian age, from clastic sedimentary deposits calcareous organisms in chert strongly suggests deposition below of the island-arc suite cover, above the unconformity. At least for the carbonate compensation depth, possibly on an abyssal plain this part of the Pacific margin of Central America, the emplacement deeper than 5,000 m. Some limestone may have formed on sub-

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marine plateaus or ridges above that depth. The deep-ocean pelagic slope trough (Rodolfo Madrigal, 1977, oral commun.). Faulting, nature and the age of the sediments of the Costa Rican ophiolite longitudinal and transverse to the geotectonic belts of the arc, took support the conclusion that oceanic conditions existed in the south- place in different areas and at different times (Stibane and others, ern Central America area from Early Jurassic to Late Cretaceous 1977). Today they are represented by the Gulf of Nicoya, or the time. Tempisque, El General, and Coto Brus valleys (Fig. 1, Iocs. 1, 9,11, These oceanic lithofacies, transported by the conveyor belt of the 15, and 17), in Costa Rica, or the Burica valley and Gulf of Parita spreading Pacific Ocean, underthrusted the evolving arc complex (Fig. 1, Iocs. 19 and 22), in Panama, all with active Quaternary bordering the Caribbean plate during Late Cretaceous time. At the sedimentation. Tournon (1972) and Pichler and Weyl (1976) convergent juncture with the Caribbean plate, subduction pro- showed that the present distribution of volcanoes and the lateral duced penetratively sheared tectonic zones, mixing of oceanic and changes in the calc-alkalic characteristics of the volcanic rocks rep- other materials extruded from the pristine volcanic arc, and mech- resent one of the last episodes of plate tectonics in this region. On anisms for carrying such components to depths where moderately the basis of this briefly outlined geotectonic evolution, I propose a high-pressure, low-temperature metamorphism prevailed. Evidence fourfold geotectonic division of Costa Rica, based upon the ideas of blueschist facies metamorphism has not yet been found. Thus, of Karig and Sharman (1975; Figs. 2 and 4 here). rock masses not originally formed in the same time or space were accumulated in the relatively restricted area of the Caribbean plate CONCLUSIONS margin to form the basement complex terranes of Costa Rica and Panama. Age differences of approximately 70 m.y. are now recog- The ancestral Central American trench was intra-oceanic. As a nized in the basement (Galli-Olivier, 1977). result of the subduction of the Cocos plate, an ophiolitic accretion When subduction commenced in latest Santonian or earliest prism with slabs composed of mantle and crustal oceanic igneous Campanian time, sheets of oceanic crustal material from the con- and pelagic rocks, with a low rate of turbidite influx, was formed verging Pacific plate accreted to the edge of the Caribbean plate on the southwestern margin of the Caribbean plate. The subducted (Fig. 2). After accretion, the material became a mélange with a rocks were highly deformed, underwent burial metamorphism, and characteristic tectonic style in which shear fractures replaced other now constitute, in parts of Central America, a gigantic tectonic partings as the dominant structural feature. Because the Central zone that continues south to Colombia and Ecuador. American arc was an intra-oceanic arc system, the turbidity influx The ophiolite in Costa Rica is the result of a tectonic process of was small. Large continental masses likely to supply huge clastic accretion that formed pseudostratigraphic sequences. Thus, the wedges were too far from the trench (Dengo, 1962a), and igneous rock record of the prism would be inherently incomplete and would rocks and pelagic sediments largely constructed a prism of accre- reflect neither the thickness nor the stratigraphic sequence of the tion. original rocks formed in any particular area of the Pacific Ocean. The ultramafic Santa Elena block of the ophiolite is a window The period of accumulation of the ophiolite in different parts of into the deeper parts of the ocean crust and mantle. Its emplace- an older Pacific basin is close to 70 m.y., possibly between middle ment in the outer arc may be due to two causes: (1) vertical faulting Tithonian and late Santonian time. New paleontological determi- perpendicular to the arc polarity after accretion of the ophiolitic nations refine previous estimates of the age of emplacement of the prism; the overall shape of the block suggests vertical ("yo-yo") ophiolitic suite at the Caribbean plate margin to a time between tectonics, as proposed by Coleman (1975); (2) accretion of a slice late Santonian and early Campanian, at least for the ophiolite that contained blocks of basalt and peridotite already faulted at a accretion prism of the Nicoya Peninsula. Pacific ridge system, similar to what Engel and Fisher (1975) found The relative scarcity of turbidites in the ophiolite of Costa Rica, at the Indian Ocean ridge system. These authors pointed out that compared to other "Franciscan" terranes, may be explained by the gravity stratiform bodies of mafic to ultramafic composition — like absence of island arcs or cratonic land masses within the general the Santa Elena block — are widespread features of the oceanic region where the ancestral Central American arc was later formed. crust. Other faults transverse the outer arc, with an average strike Deep oceanic conditions prevailed in the region from Early Jurassic of N60°E (United Nations, 1975). to Late Cretaceous time. A genetic relation between subduction and the origin of a The origin of the island-arc clastic wedge, resting unconformably plutonic-volcanic arc is one of the most important postulates of on the ophiolite, is related to the formation of the Central Ameri- plate tectonics. It is assumed that andesitic eruptive suites and can plutonic-volcanic arc since early Campanian time, which, in granitic intrusive suites are roughly contemporaneous (Williams turn, is the result of subduction. Similar genetic relations have been and McBirney, 1969) and have shed large amounts of volcaniclastic material into the adjacent basins of the Central American arc. In Panama, quartz-dioritic, granodioritic, and other silicic to inter- mediate Cretaceous to Holocene plutons have been recognized in the San Bias, Darién, Bocas del Toro, and Azuero regions (Fig. 1, Figure 4. Schematic Iocs. 20, 21, 23, and 24), among others (Miranda, 1976). In Costa fourfold geotectonic Rica, predominantly Miocene quartz diorite, granodiorite, and division of Costa Rica: subordinate granite intrusions were emplaced in the Talamanca 1, ophiolitic accretio- Range region, but the first indication of activity at the volcanic arc nary prism; 2, frontal is the early Campanian volcaniclastic arenite trapped in the arc; 3, plutonic-vol- sedimentary basins formed on top of the ophiolitic basement fer- canic arc; 4, back arc. rane of the accretion prism (Galli-Olivier and Schmidt-Effing, 1977). Continuous accretion beneath the prism may have caused the uplift of the accreted western edge and subsidence of the upper

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proposed for the Great Valley Sequence and the Nevadan orogeny Case, J. E., 1974, Oceanic crust forms basement of eastern Panama: in North America (Pessagno, 1973; Jones, 1975). Geological Society of America Bulletin, v. 85, p. 645-652. Case, J. E., Durán S., L. G., López R., Alfonso, and others, 1971, Tectonic The southern Central American arc is mostly composed of investigations in western Colombia and eastern Panama: Geological "eugeosynclinal" rocks associated with volcanism. "Miogeosyncli- Society of America Bulletin, v. 82, p. 2685-2712. nal" rock sequences are lacking because the island arc, later to be- Coleman, Patrick J., 1975, On island arcs: Earth Science Reviews, v. 11, come an isthmus, was not close to a stable foreland region at the p. 47-80. edge of a craton or platform. Orthoquartzite or other quartz-rich Constantinou, G., and Govett, G.J.S., 1973, Geology, geochemistry and genesis of the Cyprus sulfide deposits: Economic Geology, v. 68, arenites are rare and local in Costa Rica and Panama. Instead, p. 843-858. first-cycle, little-weathered medium-grained clastic rocks such as Contescu, Lorin R., 1975, Source areas for flysch sediments in light of plagioclase-rich or volcanic-fragment-rich arenite with low global tectonics, in Tectonique et sedimentation: International Con- porosity are widespread. Limestone has formed from Cretaceous gress of Sedimentology, 9th, Nice 1975, v. 1, p. 67-71. time to the present. Locally, limestone is biohermal and porous. del Giüdice, Daniele, 1973, Características geológicas de la Repüblica de Panama: Guatemala, Instituto Centroamericano de Investigación y While the best known and largest flysch deposits of the world Tecnología Industrial (ICAITI) Open-File Report. have a minor contribution of volcaniclastic material, the Central Dengo, Gabriel, 1962a, Tectonic-igneous sequence in Costa Rica, in Engel, American island-arc flysch is for the most part composed of clasts A.E.J., and others, eds., Petrologic studies: Boulder, Colo., Geological derived from volcanic-plutonic sources. It represents a type of Society of America, p. 133-161. 1962b, Estudio geológico de la región de Guanacaste, Costa Rica: San circum-Pacific flysch (Contescu, 1975) without cratonic influences. José, Instituto Geográfico de Costa Rica, 113 p. Pelagic limestone and flysch, changing vertically to shallow- 1973, Estructura geológica, historia tectónica y morfología de América facies rocks, suggest that the depth of the ocean and the location of Central: Guatemala, Instituto Centroamericano de Investigación y the coastline changed repeatedly within the region of arc build-up. Tecnología Industrial (ICAITI), 52 p. Dóndoli B., César, 1965, Información general geológica-petrográfica y Vertical tectonics was important particularly in the interdeep, in mineralógica sobre Costa Rica: Costa Rica Dirección de Geología, parts of the frontal arc, and in the back-arc area, since the begin- Minas y Petróleo, Informes Técnicos y Notas Geológicas, Año 4, no. ning of the ancestral Central American arc. Subsidence of marginal- 13, 21 p. and intra-arc basin floors caused development of several sedimen- Engel, Celeste G., and Fisher, Robert L., 1975, Granitic to ultramafic rock tary basins. complexes of the Indian Ocean ridge system, western Indian Ocean: Geological Society of America Bulletin, v. 86, p. 1553-1578. Block faulting and vertical movements in parts of the outer arc Flores R., Wilmer S., 1976, Estudio geológico relacionado con una took place at different times. For example, the lowermost deposits mineralización de sulfures en Punta Gorda, Nicoya, Costa Rica of Campanian age of the island-arc clastic wedge are younger in the [Licenciado thesis]: San José, Universidad de Costa Rica, 35 p. Santa Elena Peninsula and older in the south of the Nicoya Penin- Galli-Olivier, Carlos, 1977, Edad de emplazamiento y período de acumula- sula (Galli-Olivier and Schmidt-Effing, 1977). In the southern part ción de la ofiolita de Costa Rica: Ciencia y Tecnología, Journal of University of Costa Rica, v. 1, no. 1, p. 81-86. of the Nicoya Peninsula the first island-arc suite deposits to trans- Galli-Olivier, Carlos, and Schmidt-Effing, Reinhard, 1977, Estratigrafía de gress a faulted block are Miocene. la cubierta sedimentaria supra-ofiolítica cretácica de Costa Rica: A fourfold geotectonic division of Costa Rica — ophiolitic ac- Ciencia y Tecnología, Journal of University of Costa Rica, v. 1, no. 1, cretionary prism, frontal arc, plutonic-volcanic arc, and back-arc p. 87-96. Garrison, Robert E., 1974, Radiolarian cherts, pelagic limestones, and — is proposed. igneous rocks in eugeosynclinal assemblages, in Hsú, Kenneth J., and Jenkyns, Hugh C., eds., Pelagic sediments: On land and under the sea: ACKNOWLEDGMENTS International Association of Sedimentologists Special Publication no. 1, p. 367-399. Emile A. Pessagno, Jr., identified radiolaria, and Reinhard Goossens, Pierre J., and Rose, William I., Jr., 1973, Chemical composition and age determination of tholeiitic rocks in the Basic Igneous Com- Schmidt-Effing and Axel von Hillebrandt identified foraminifera plex, Ecuador: Geological Society of America Bulletin, v. 84, from the ophiolitic suite. Discussions with Percy Denyer, César p. 1043-1052. Dóndoli, Gerald Edwards, Rudolf Fischer, José M. Fúster, Eric Harrison, J. V., 1953, The geology of the Santa Elena Peninsula in Costa Kuijpers, Siegfried Kussmaul, Rodolfo Madrigal, Arístides J. B. Rica, Central America: Pacific Science Congress, 7th, Proceedings, v. 2, p. 102-114. Romero, Reinhard Schmidt-Effing, and Fritz Stibane proved most Henningsen, Dierk, and Weyl, Richard, 1967, Ozeanische Kruste im useful. This work was supported by the University of Costa Rica. Nicoya-Komplex von Costa Rica (Mittelamerika): Geologischen Rundschau, v. 57, p. 33-47. 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