Abstract Northern (nuclear) Central America is conveniently divided into the Maya (sometimes called Yucatán) and Chortis blocks (Fig. 1). The division between the two blocks is the Motagua suture zone, which follows the Motagua River in east and central Guatemala, but whose western extension is buried beneath Tertiary volcanic cover in western Guatemala. The Maya block includes Guatemala north of the Motagua suture zone, Belize, the Yucatán Peninsula, and Mexico west to the Isthmus of Tehuantepec. The Chortis block consists of southern Guatemala, El Salvador, Honduras, an indeterminate part of northern Nicaragua, and the water-covered Nicaraguan Rise. The boundary between these blocks along the Motagua suture zone of central Guatemala marks the locus of interblock suturing in latest Cretaceous time. This chapter emphasizes the older geologic history of northern Central America. Certain geologic topics relevant to this area will appear in other chapters of this volume, especially seismicity, neotectonics, magmatism, volcanism, paleomagnetism, mineral deposits, and energy. Our discussion of the Maya block is limited to Belize and Guatemala north of the Motagua Valley; adjacent Mexican portions will be described in volumes of the Geology of North America covering Mexico and the Gulf of Mexico. Some Mexican rock occurrences which are especially pertinent to the interpretation of Guatemalan units are, however, included here. The Motagua suture zone between the Maya and Chortis blocks is discussed in this subchapter. The Chortis block subchapter emphasizes the geology of Honduras, where most of the older rocks of this block occur. The Nicaraguan Rise is discussed

Abstract The Caribbean area has a rich and varied igneous history, especially in the Late Cretaceous and Cenozoic. The emphasis of this chapter will be on that portion of the igneous history that began with Mesozoic separation of South and North America and has continued to the Holocene. Pre-Jurassic magmatism is recorded in northern Central America, Venezuela, and Colombia, and will be discussed briefly in other chapters of this volume. The aim of this chapter will be to describe igneous rock associations according to magmatic styles that characterize certain time intervals of the Caribbean and to suggest how these styles relate to contemporary tectonics. A recurring theme will be the close relationship among widespread igneous rock occurrences which might have prompted special interpretations in the past. Because the volcanic units that form the backbone of this account are mainly named stratigraphic units, much of this account will refer to these units. Further discussion may be found in other chapters of this volume.

Abstract The Caribbean area as defined here includes the Greater Antilles, Lesser Antilles, the northern boundary of South America, and Central America (Fig. 1); it spans approximately 7,800 km in an east-west direction and 3,900 km in a north-south direction. Although the geology of this region should be considered along with that of Mexico and the Gulf of Mexico, these areas form separate chapters in this book. This chapter is in part a condensation of numerous contributions prepared for the synthesis volume on the Caribbean region by Case and Dengo (1989). Further details are available in that book. Modern geological interest in the Caribbean has centered on its Cretaceous to Recent orogenic belts that resulted from plate interactions between North and South America. The Caribbean is the site of America’s most extensive Cretaceous and Cenozoic oceanic-continental tectonic zone and has (along with the Aleutians) its only real island arcs. It has the majority of the active volcanic centers of the New World and a major share of the destructive earthquakes. The goal of the Caribbean geologist is to reconstruct the history of a minor “plate” whose extensive internal deformation belies the strict application of this term. This “plate” has been broken and twisted within the jaws of three major plates (Farallon, North America, and South America), whose relative motions have changed dramatically from Jurassic to Recent time. However, the average motion among the three plates since the middle Cretaceous has been one of roughly east-west compression, and the aim of this paper is to place the geologic history in the context of these changing major plate motions.

The Water Island and Louisenhoj Formations of the U.S. Virgin Islands represent a mid-Cretaceous transition from deep-water to shallow-water and subaerial volcanic eruption of andesitic magmas. The upward increase in porphyritic texture, evidences of explosive eruption, segregation vesicles, and pumice and hyaloclastic fragments all suggest the dominating effect of ambient water pressure at the eruptive vent, which in turn controls the rapidity of separation of intrinsic volatiles during solidification.

Marine geophysical measurements in Unare Bay, Venezuela, reveal a system of east-west-striking normal faults. The normal faulting ranges upward in scale from slump faults bounding 1 to 2 km broad terraces on Tortuga-Margarita Bank, through large growth faults in the sedimentary fill of the Cariaco Basin, to the faults bounding the horsts and grabens that control the island and strait topography on the south boundary of the Caribbean. It is believed that the slump faulting can trigger gravity slides that in turn could result in thrust sheets such as those seen onshore adjacent to the marine study area. There is indication from magnetic measurements of a belt of basic igneous intrusives extending across Unare Bay from north of Cabo Cordera to the straits between Araya and Cubagua. A segment of this belt, west of Araya, is offset to the south. This may possibly be due to north-south strike-slip offset. Gravity observations show a lack of correlation of gravity maxima with the locus of basic intrusives. This is probably due to the intrusive bodies underlying sediment-filled structural lows. Igneous or metamorphic rocks occur at shallow depths (less than 1 km) under most of Tortuga-Margarita Bank. The south boundary of the Caribbean is considered to be a type of continental margin. The system of flattened normal faults bounding tilted blocks is a result of crustal extension associated with the origin of the Caribbean Sea. Smaller slump faults and gravity slides are due to the tilting of blocks on the larger normal faults. The relative lack of seismicity of the Caribbean's south boundary with apparent simultaneous welding of the South American Continent to both the Caribbean and Atlantic can be explained by invoking left-lateral strike-slip motion in the Atlantic floor that is taken up by underthrusting in the Lesser Antilles island arc. Geometric reconstruction of Atlantic spreading reveals a shear requirement that matches in sense and magnitude the left-lateral shear indicated by offsets on the Mid-Atlantic Ridge. From this it follows that ridge offsets are true transcurrent rather than transform faults, and that these faults, coupled with spreading, are the mechanism for opening the Caribbean Sea.

A bathymetric, magnetic, and seismic profiler survey of the Caribbean Sea north of Colombia revealed four natural geologic systems: (1) a Quaternary erosional system of continental shelf modification and submarine canyon formation; (2) a sedimentological system of seaward progradation of the continental shelf and slope and of turbidite deposition in basins; (3) a deformational system with a fold belt flanking the continent and faults marking the coast; and (4) the ocean-continent transition. The deformation system severely modifies the sedimentational system and is penecontemporaneous. The fold belt acts as a dam to a marginal, turbidite-filled basin. The east-west magnetic pattern of the Colombian basin contrasts with the less intense anomaly pattern paralleling and associated with the continental margin. The geological evidence indicates that the continent is decoupled from the Colombian basin, and that a relative east-west movement exists.

Small lenses of detrital serpentinite, completely surrounded by massive serpentinite, occur within a sequence of Mesozoic phyllites and metasiltstones in the Cerros de Parashi area, Guajira Peninsula. These lenses, which are as much as 30 m thick, consist of poorly sorted breccias that grade irregularly upward into bedded serpentinite sandstone and shale. Graded bedding is well developed in the sandstones, and channel structures and cross-bedding are also common. Two chemical analyses of serpentinite sandstone and shale show them to be of normal serpentinite composition. The original detrital serpentine minerals in these rocks were lizardite and chrysotile, but they have been partially replaced by antigorite during a period of upper greenschist facies metamorphism. The detrital serpentinites were formed by large submarine turbidity currents or mudflows, and the massive serpentinites surrounding them are believed to have been emplaced by gravity sliding.

The three ranges of the Colombian Andes have distinct geologic characteristics. The Central Cordillera is an uplifted zone of crystalline rocks, mostly pre-Mesozoic in age. The Western and Eastern Cordilleras are composed, respectively, of eugeosynclinal and miogeosynclinal rocks, deposited during the Mesozoic, and deformed in Late Cretaceous and Tertiary time. Although the Andes as a topographic feature end at 8° N. lat, pre-Tertiary rocks are found to the northeast in the Santa Marta-Perijá region and the Guajira Peninsula. Recent work has shown that a three-fold structure, similar to that of the Andes, characterizes these regions as well, suggesting that the Andean mobile belt was more extensive than the present day topographic Andes.

Puerto Rico is part of a geanticlinal ridge on the ocean floor, built by deposition of sedimentary and volcanic rocks in and around a volcanic framework. Shallow-water sediments and volcanic rocks formed rock sequences on this ridge during the Late Cretaceous and early Tertiary. The basement beneath the volcanic succession, exposed in southwestern Puerto Rico, is probably largely serpentinized peridotite. The style and intensity of deformation of the volcanic ridge and its covering sediments seem to be controlled by the thickness of sedimentary and volcanic rocks present above this basement. Where thin rock sequences occur above basement, as in southwestern Puerto Rico, folding is tight and folds are narrow; where thick rock sequences occur, as in central and eastern Puerto Rico, folding is gentle and folds are broad. Nearness to basement thus seems to be correlated with the intensity of deformation. This may be the direct result of the mobility of serpentinized peridotite. This mobility is demonstrated by the diapiric upward movement of serpentinite in the centers of anticlines in areas where the overlying rock sequences are thin.

Palinspastic reconstructions of the central part of the Venezuelan Coast Ranges combined with sedimentary data indicate that deep water marine deposits accumulated in east-west-trending troughs which were located successively farther south through time. From north to south, four such troughs are recognized. Regional metamorphism accompanied downbuckling in the two northern troughs, and allochthonous material slid southward into the two southern troughs. The emplacement of the allochthonous sequences is thought to be due to gravity sliding promoted by uplift in the north and downwarping in the south. Metamorphic grade and estimated sedimentary thicknesses suggest that the amount of downwarping decreased through time as the troughs migrated southward. In the eastern part of the East Venezuela Basin, this trough migration is reflected in the southward displacement of depositional axes between Late Cretaceous and Pliocene time. The structure of the Venezuelan Coast Ranges is interpreted as resulting from a linear downwarped zone, followed by an upwarped zone immediately to the north, migrating southward across the northern edge of the Guayana Shield at a rate of 1 to 2 km per m.y. from Jurassic until latest Tertiary time.

The Upper Cretaceous (?) Colombier volcanic sequence, which is the oldest exposed rock in the Terre-Neuve Mountains, is unconformably overlain by Upper Cretaceous sediments, and both of these rocks have been intruded by the 66.2 m.y. old Terre-Neuve quartz monzonite stock. This entire sequence is overlain unconformably by Paleocene and Eocene limestone, which has been intruded by a small, hypabyssal basalt pluton. Left-lateral, strike-slip faulting, previously unrecognized in northern Haiti, has offset parts of the area by more than 2 km. Although the Terre-Neuve stock and Colombier volcanic rocks are closely associated in space and time, subtraction diagrams suggest that they are not directly related by crystal-melt equilibria and are therefore not strictly consanguineous. Plots of Na 2 O versus K 2 O and plagioclase versus K-feldspar for intrusive rocks of the Greater Antilles indicate that the Terre-Neuve stock is among the most granitic of this group. Also, the Terre-Neuve igneous series (including the Colombier volcanic rocks) contains substantially less A1 2 O 3 than rock series from both oceanic and continental orogenic zones, as illustrated by differentiation index diagrams. The age and mineral content of the igneous rocks of northern Haiti plotted on a Qz-Cpx-Ol-Ne tetrahedron indicate that these rocks have become increasingly subsilicic and alkalic with time. This range of rock compositions could have resulted from mantle-generated crystal-melt diapirs that lost most of their crystals (thereby fixing the melt compositions) at the crust-mantle boundary, which migrated downward with time. The spacial variation in magma composition in Hispaniola suggests that the Americas plate originally underthrust the Caribbean plate from the north rather than from the east as presently observed.

Calc-alkaline volcanic suites from circum-oceanic island-arc regions should be treated separately from the volcanic and plutonic assemblages of continental margins when developing petrogenetic hypotheses. Chemical plots for volcanic suites from the West Indies and Solomon Islands indicate that crystal fractionation of high-alumina basalt magmas could account for the basalt-andesitedacite suite. The separated minerals responsible for this control are likely to differ from those (chiefly magnesian olivine) in the tholeiitic suite. Experimental studies on a variety of calc-alkaline volcanic rocks in vacuo have shown unexpectedly high liquidus temperatures for the andesites and dacites (> 1250° C), and a general irregularity in the relationships between melting temperatures and usually accepted chemical-composition parameters, despite measures taken to eliminate residual grains and to achieve nucleation from the glasses. Reconnaissance runs at P H2O = 2 kb have helped to resolve some of these problems. They lead to the conclusion that the calc-alkaline volcanic suite differs from the tholeiitic and alkaline chiefly in that dissolved water is needed to produce calc-alkaline andesitic and dacitic melts at reasonable temperatures. Plutonic nodules from the West Indies lavas carry an assemblage of cumulate crystals notably rich in amphiboles and anorthite, with interstitial vesicular basalt. This supports the concept that magma was generated by partial melting of either a locally hydrous layer of the upper mantle, or a depressed wedge of hydrous basaltic rock. Fractional crystallization under conditions in which P H2O and oxygen fugacity were constant or increased thus led to derivative andesitic and dacitic liquids. The removal of amphibole is more likely than the removal of iron oxides in producing such derivative liquids.

Basaltic rocks occurring in the Lesser Antilles are directly comparable in chemical composition and mineralogy with basalts from other calc-alkaline suites of the circum-oceanic islands and from orogenic belts on the continental margins. Basalts of this type, characterized by a high alumina and low alkali content, are distinct from olivine tholeiites of the ocean basins and from alkali basalts of the intraoceanic islands. Available geochemical and geophysical data place certain limitations on the composition of the magma-source area (?upper mantle). The slightly higher values for K, Rb, and Sr, the higher Rb/K, Rb/Sr, Fe/Mg, Co/Ni and Sr 87 /Sr 86 ratios, and the plagioclase enrichment found in circum-oceanic basalts, compared with oceanic tholeiites, indicate that circum-oceanic basalts are derivative types. It is suggested that the common circum-oceanic basalts are fractionation products of olivine tholeiite, or represent partial melts from a basic-ultrabasic zone with the appropriate chemistry developed beneath the arc and corresponding with the anomalous low velocity zone. Variations in the composition of the aluminous circum-oceanic basalts may reflect physico-chemical heterogeneities within the source area related to the tectonic or chemical evolution of the arc, or both. Present data indicate that Lesser Antillean basalts have probably gone through at least two and possibly three types or stages of fractionation at different levels and under different physical conditions. The primary liquid, derived by melting of a peridotite mantle, probably has the composition of an olivine tholeiite. The first stage of fractionation takes place at depths of 30 km, where forsterite and possibly enstatite separate, increasing the A1 2 O 3 content and the Fe/Mg and Co/Ni composition ratios. Alternatively, the primary magma was generated from a source approximating basalt with comparatively high Fe/Mg and Co/Ni ratios, in which case fractionation would commence with stage two. The second stage occurs at depths of about 6 to 8 km and involves the separation of anorthite-olivine-salite-amphibole-magnetite cumulates under high-water vapor pressure. Fractionation of this type enriches the liquid in silica, giving andesitic magmas. The third and final stage, occurring at shallow depths of 2 to 3 km, involves the separation of the assemblage labradorite-olivine-augite-hypersthene-titanomagnetite, that is, the dominant phenocrystic assemblage found in the surface basalts and basaltic andesites. Accumulation of one or more of the phases will give rise to accumulative basalts and andesites.

A suite of West Indian igneous rocks ranging in age from pre-Middle Cretaceous to Recent has been analyzed for major elements, Th, U, and Sr and Pb isotopes. Two major groups of magmas are revealed by the Th, U, and Pb isotopes. The first of these is a “chemically primitive” group generated prior and briefly subsequent to the major unconformity of the Middle Cretaceous, which is in turn related to initial rupture along the Benioff Zone and the initiation of expression of trench-island platform topography. A “chemically more evolved” group was dominant from the Middle Cretaceous to the Recent. Two explanations for the evolution in radioactive elements and isotope con-tents are suggested: (1) the crustal and upper mantle transported down the Benioff Zone to the locus of fusion varied with time, reflecting either original vertical or lateral variations; or (2) increasing transport of more radioactive sedimentary materials to the locus of fusion during island-arc evolution resulted in increased contamination. Positive evidence for the latter explanation during the Tertiary is presented, but the former effect is also suggested.