Geology and geochemistry of the alkali basalt—andesite association of Grenada, Lesser Antilles island arc RICHARD J. ARCULUS* Department of Geological Sciences, University of Durham, Durham, England ABSTRACT frontal trench is present only to the east of American continent (Fig. 1, after Andrew the northern half of the Lesser Antilles. and others, 1970). Despite the absence of a Basanitoids and alkalic basalts that are However, the belt of negative gravity ano- trench east of the southern Lesser Antilles, strongly undersaturated in silica occur on maly associated with the Puerto Rico active subduction of the American plate the island of Grenada in the Lesser Antilles. Trench continues southward along the belt appears to be taking place (Molnar and Several volcanic centers have erupted basic of deformed sediments of the Barbados Sykes, 1969; Westbrook and others, 1973). lava of these compositions together with Ridge before bifurcating near the South A Benioff zone is present beneath the Lesser subalkalic basalt, andesite, and dacite from Miocene to Holocene time. The volcanic rocks overlie a folded volcanic-sedimentary formation of Eocene to Miocene age. Tuff rings and maars of explosive origin are present. Andesite and dacite are less signifi- cant volumetrically on Grenada in compari- son with other islands in the Lesser Antilles. The variable trace-element geochemistry of the basanitoids and alkalic basalts is re- lated, on the basis of rare-earth-element data, to a model of variable degrees of par- tial melting of an upper-mantle garnet peridotite source. It is suggested that frac- tional crystallization of olivine, clino- pyroxene, and spinel, observed as the phenocryst assemblage in the basanitoids and alkalic basalts, takes place at high temperatures; at lower temperatures, these phenocrysts are joined by amphibole and plagioclase. A trend toward increased silica saturation is the result of this frac- tional crystallization process. The presence of alkalic lava rocks together with variable trace-element abundances and Sr isotope ratios are unusual features of the volcanic- ity. Key words: volcanology, island arc, geochemistry. INTRODUCTION Aspects of the unusual association of silica-undersaturated alkalic basalt, calc- alkalic andesite, and dacite on the island of Grenada, Lesser Antilles island arc, have been discussed (Arculus and Curran, 1972; Sigurdsson and others, 1973; Cawthorn and others, 1973). In this account, the field occurrence and major- and trace-element geochemistry of the Grenada suite of vol- canic rocks is described. Discussion of the petrography and mineralogy of the suite are presented in separate publications (Arculus, 1974; in prep.). Grenada is the southernmost volcanic is- land of the Lesser Antilles (Fig. 1). A deep * Present address: Department of Geology, Rice Uni- Figure 1. Index map of Lesser Antilles island arc. Pre-Miocene arc corresponds to Limestone versity, Houston, Texas 77001 Caribbees; post-Miocene arc corresponds to Volcanic Caribbees of Martin-Kaye (1969). Geological Society of America Bulletin, v. 87, p. 612-624, 9 figs., April 1976, Doc. no. 60414. 612 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/87/4/612/3444105/i0016-7606-87-4-612.pdf by guest on 25 September 2021 ALKALI BASALT-ANDESITE ASSOCIATION OF GRENADA, LESSER ANTILLES ISLAND ARC 613 Antilles island arc at a depth of about 110 sequently, the island is asymmetric in 1964; Rea, 1970). However, the eight or to 120 km beneath Grenada and dipping profile, the western side being considerably possibly nine explosion craters on Grenada approximately 30°W (Sigurdsson and steeper and more incised by deep valleys are unusual (compare with Robson and others, 1973). Farther north in the arc, the than the eastern. From north to south the Tomblin, 1966). dip of the Benioff zone increases to 50°W high ground is composed of the massifs and is present at approximately 160 km be- of Mount St. Catherine, Mount Granby— GEOLOGIC HISTORY neath Dominica (Sigurdsson and others, Fedon's Camp (764 m), and South East 1973). Mountains—Sinai—Mount Maitland (715 Eocene to Oligocene The occurrence of nepheline-normative m) (Fig. 2, after Arculus, 1973). rock compositions within island arcs has Northward-facing scarp faces define the The Miocene to Holocene volcanic rocks commonly been associated with great dis- northern topographic limits of the high of Grenada are underlain by a tectonically tances (400 km) from the trench and great ground from South East Mountain to disturbed series of volcanic-sedimentary depths (600 km) to the Benioff zone (for Mount Maitland (Fig. 2). It is possible that units ranging from Eocene to Miocene in example, Kuno, 1966). However, the ap- recent step faulting was primarily responsi- age (Fig. 3). No pre-Tertiary horizons have pearance of basanitoid and alkalic basalt ble for the development of these features. been discovered. Martin-Kaye (1969) lava rocks on Grenada 150 km from the The approximately linear northeast trend proposed that the well-bedded sequences of trench — where the typical island-arc as- of the explosion craters may be related to calcareous shale, siltstone, and sandstone sociation is high-alumina basalt, andesite, the presence of the same fault zone exposed in the northern half of the island be and dacite as in the islands to the north in (Martin-Kaye, 1969). called the Tufton Hall Formation (Fig. 2). the Lesser Antilles — is an unusual feature. Exposure is poor due to the mantle of Tuffaceous horizons are present within the S. E. DeLong and others (in prep.) have tropical vegetation. However, deep erosion formation and become prominent in strata suggested that tectonic dislocation of the of the terrain has exposed some fresh rock of lower Oligocene age. In thin section, the upper mantle related to discontinuities surfaces, and unaltered samples are availa- prominent features of these sedimentary within the subducted plate is associated ble at all but the highest elevations. A units are the abundance of detrital carbon- with the Grenada and other similar occur- characteristic feature of Grenada geology is ate fragments, microfossils, and carbonate rences. the large surface area occupied by second- cement. Minerals of volcanic origin, such as ary or reworked volcanic detritus. Heavy oscillatory-zoned clinopyroxene and PREVIOUS WORK rainfall on unconsolidated ash and pyro- plagioclase feldspar, are also present in var- clastic flows redistributes the material to ying stages of preservation of form and de- A summary of the geologic history of the lower altitudes and to the sea. Anderson grees of alteration. Lesser Antilles island arc was presented by (1908) observed these reworking processes Almost all the outcrops of the Tufton Martin-Kaye (1969). The earliest account on the island of St. Vincent after the 1902 Hall Formation are folded or faulted, and of the geology of Grenada was by Harrison eruption of the Soufrière volcano. boudinage features are developed in some (1896), who listed the first chemical Lava flows form erosion-resistant units, localities. The axes of the folds strike pre- analyses of the silica-undersaturated basalt. and inversions of topography are common, dominantly east-west. At Levera Bay in the Earle (1924) commented on the folded with the flows now capping ridges. A de- northern part of the island, a dike of lower Tertiary basement of the island and termination of center of activity has often augite-phyric basalt has been folded to- also recognized the occurrence of augite- been based on the radial distribution of lava gether with the intruded sediments. How- phyric basalt and what would now be flows. The proximal ends of many of the ever, unfolded basalt dikes occur at the termed plutonic cumulate blocks. The flows have been eroded in the higher parts south end of Grenada Bay on the east coast. lower Tertiary basement was extensively of the island, leaving butt-ended escarp- Martin-Kaye (1969) suggested that at least studied by Martin-Kaye (1969), who de- ments. The degree of dissection of a center two phases of deformation took place termined the stratigraphic relationships and has proved to be a useful indicator of rela- (Fig. 3). ages of the units by means of the fossil tive age, and in combination with unpub- Igneous activity is an important feature fauna. A brief account of the overlying vol- lished K-Ar ages determined by J. M. Bri- of the lower and middle Tertiary sequence canic rocks was given by Robson and den and D. C. Rex (Briden and others, in of Grenada and is contemporaneous with Tomblin (1966), who mentioned the un- prep.), a probable order of volcanic activity the activity described by Christman (1953) usually mafic nature of the volcanic prod- on the island has been proposed (Fig. 3). in the Limestone Caribbees northward in ucts. A gravity survey of the island was Most of the basalt lava flows are aa or the arc (Fig. 1). Small outcrops of marine completed by Andrew and others (1970). block flows (for example, MacDonald, limestone are preserved 1 km east and 4 km Positive Bouguer gravity anomalies of 140 1972). Andesite flows are as much as 3 km southeast of St. Georges and 3 km east of to 160 mgal correspond approximately to in length and 30 to 40 m thick. Andesitic Belvidere. These are of Oligocene and the loci of the volcanic centers. A gravity and dacitic dome intrusions, frequently Miocene age (Martin-Kaye, 1969). The low, however, is associated with the Lake mantled by scree deposits, are the dominant Miocene to Holocene volcanic history is Antoine maar (Fig. 2), which is chiefly con- landform in the northern part of the island briefly summarized below in order of de- structed of loosely consolidated pyroclastic and are present at or near the center of creasing age as given in Figure 2. The ter- deposits at the surface. eruption of the Mount Granby-Fedon's minology used in rock classification is dis- Camp volcanic complex and the Mount St.