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Ophiolite and Island-Arc Volcanism in Costa Rica

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Ophiolite and Island-Arc Volcanism in Costa Rica 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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/90/5/444/3418852/i0016-7606-90-5-444.pdf by guest on 26 September 2021 OPHIOLITE AND ISLAND-ARC VOLCANISM IN COSTA RICA 445 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). Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/90/5/444/3418852/i0016-7606-90-5-444.pdf by guest on 26 September 2021 446 C. GALLI-OLIVIER 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.
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