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.
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.
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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. cussed in the geochemistry section. GENERAL DESCRIPTION Catherine center. Pyroclastic flow deposits are well preserved inisitu only in association Miocene to Holocene The island of Grenada is roughly oval, 35 with the Mount St. Catherine center, but a by 20 km, and elongate in a northeast- considerable proportion of the fragmented Northern Domes Center. Several southwest direction. The topography is deposits may be derived from this type of episodes of extrusion of basalt and andesite rugged, rising to a maximum height of 840 source material. flows and intrusion of andesite domes oc- m (Mount St. Catherine). A chain of moun- Grenada field geology is similar in all re- curred in the Northern Domes center. The tains strikes almost the length of the island spects to volcanic islands to the north in the range of rock types is from alkalic basalt to but is offset toward the western coast. Con- Lesser Antilles (Baker, 1963; Tomblin, dacite.
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Approximate center boundary
Hot Spring Leverà I ¿Green Is Mountain summit
Probable geological boundary & à & à A Pyroclast flows
Andesitic lava flows and domes Crayfish
|x X X | Basalt lava flows
Undifferentiated South East s Mt. volcanics
|v V vj Scoria and Ash • Reworked volcanics 12°10'N
Tufton Hall Formation
Volcanic Centers Dothan
5 Mt.St. Catherine
Mt. Granby - U Fedon's Camp
3 Mt. Maitland
South East 2 12°05'N
1 Northern Domes
Figure 2. Outline geological map of Grenada. Rock units are present in most centers and are not in any stratigraphic sequence in the key. Re- 12°00'N worked volcanic rocks form most terrain bordering coast and may be de- rived from one or more centers of activity. Pleistocene limestones are pre- served near Mount Alexander in north of island. (Spelling on figure fol- Prickly lows the 1962 Department of Overseas Surveys map of Grenada.) Point 6Tk5' 6i°kcr
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Figure 3. Preliminary strat- UJ of Dominica are at least 1,500 m thick z UJ igraphie column for Grenada (Wills, 1974). The presence of andesitic and o SUBMARINE ERUPTIONS o based on faunal age determina- dacitic pyroclast flows, domes, and lava o tions of Martin-Kaye (1969), X flows are typical of the erupted products of EXPLOSION CRATERS Lewis and Robinson (1975), other islands in the Lesser Antilles and of and unpublished K-Ar dates island arcs in general. However, the pres- UJ LOCAL LIMESTONE DEPOSITION (Briden and others, in prep.). Z Submarine eruptions of Kick- ence of rocks of strongly silica-under- ( > saturated basaltic composition appears to O * MT. ST. CATHERINE em-Jenny volcano, 8 km north h- be a unique feature of the island of Grenada z SOUTH EAST ACTIVITY •o UJ alkalic basalt, 16 percent; andesite, 38 per- o LOCAL LIMESTONE DEPOSITION 16.0 o cent; and dacite, 17 percent. a> < 3t NORTHERN DOMES ACTIVITY o 22 5 GEOCHEMISTRY OF THE CE UJ GRENADA VOLCANIC SUITE UJ DEFORMATION a 3 UJ o 30 0 D The Grenada volcanic suite ranges from O LOCAL LIMESTONE DEPOSITION strongly silica-undersaturated rocks with % _J O o more than 10 percent normative nepheline _l 37.5 to rocks containing as much as 26 percent UJ DEFORMATION QC z normative quartz and occasionally modal UJ UJ quartz. The following nomenclature has Û. a TUFTON HALL FORMATION 3 o been used to classify the variety of rock UJ compositions and is based on the systems of Green (1969), Chayes (1966), and Mid- Southeast Grenada Center. Basanitoid Explosion Craters. Morphologically dlemost (1973): basanitoid, normative and alkalic basalt flows are radially distrib- well-preserved craters occur at and near St. nepheline >5 percent but no modal neph- uted around South East Mountain. Ande- Georges, Grand Etang, Lake Antoine, the eline; alkalic basalt, normative nepheline site domes (at Mount Lebanon) and lava Punchbowl, Green Island, and lie de Caille <5 percent or normative hypersthene plus flows (near Munich) are also present and (2 and 7 km north of Grenada, respec- olivine with hypersthene <5 percent; sub- pass distally into reworked detritus. The tively). Basanitoid, basaltic, and andesitic alkalic basalt, normative olivine with range of rock types is from basanitoid to scoria and flows are present in the craters. hypersthene (>5 percent) or normative dacite. Morphologies vary from maars to tuff rings hypersthene plus quartz but <53.5 percent Mount Maitland Center. Basanitoid, and may be dependent on the amount of Si02 (<2 percent normative quartz); ande- alkalic, and subalkalic flows compose most ground water present during eruption (Lo- site, 53.5 to 62 percent Si02 (2 to 15 per- of the surface outcrop around Mount Mait- renz, 1973). cent normative quartz); and dacite, >62 land, but andesite and dacite fragments are In summary, the repeated eruptions of percent Si02 (>15 percent normative present in interlayered detritus. Plutonic silica-undersaturated basalt magma during quartz). Robson and Tomblin (1966) sug- cumulate blocks occur in reworked vol- the Miocene to Holocene evolution of gested that the term "andesite" should canic rocks in the vicinity of the Prickly Grenada is the most important feature of apply to rocks in the Lesser Antilles within Point andesite plug. the volcanicity of the island. In addition, the range 55 to 63 percent Si02. The use of Mount Granby-Fedon's Camp Center. there is an intimate field relationship of the Middlemost's (1973) divisions in this ac- A series of lava flows ranging from silica-undersaturated lava rocks with the count makes comparison easier with other basanitoid through andesite in composition silica-saturated calc-alkalic andesite and provinces. were erupted from several centers that dacite. The geochemical association of al- shifted southward from Mount Granby in kalic basalt and calc-alkalic compositions is Analytical Techniques time. Andesite domes were intruded near discussed in the following section. centers of eruption. Plutonic cumulate The total volume of erupted volcanic The major elements Si, Al, total Fe, Mg, blocks are present in reworked volcanic products appears to be considerably less Ca, Na, Ti, K, and P were determined using rocks at Dothan. than in the other major volcanic islands of x-ray fluorescence spectrometry on com- Mount St. Catherine Center. Lava and the Lesser Antilles island arc. The undis- pressed powder briquettes. A Philips pyroclastic flows surround a breached cra- turbed volcanic deposits overlying the PW1212 automatic spectrometer equipped ter that contains a central andesite dome. lower Tertiary basement of Grenada are with a Cr-anode and evacuated x-ray path Several hot springs are active in the center nowhere greater than 900 m thick at pres- was used. Mn was determined separately (Fig. 2). The range of rock types is from ent. In comparison, the undisturbed upper with a W-anode. International standards basanitoid through dacite. Tertiary and Quaternary volcanic deposits and wet-chemical-analyzed Grenada sam-
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TABLE 1A. MAJOR- AND TRACE-ELEMENT ANALYSES OF SOME GRENADA BASANITOIDS AND ALKALIC AND SUBALKALIC BASALTS
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Weight percent oxides 47.34 46.78 47.11 46.82 Si02 45.14 45.60 46.74 45.98 46.39 46.46 46.95 46.45 46.50 44.70 44.63 16.66 18.21 16.53 17.62 AI2O3 16.08 15.91 17.62 16.55 16.66 16.45 17.60 17.27 16.31 14.76 15.30 9.28 9.07 9.22 9.52 Fe203* 10.16 9.62 9.06 9.71 9.50 9.77 9.59 9.91 10.31 10.36 10.10 MgO 10.80 10.95 10.68 12.07 10.90 12.19 11.59 11.67 13.00 11.81 14.27 10.17 9.11 12.06 9.59 CaO 12.91 13.50 11.28 11.34 12.06 11.61 10.28 11.19 10.50 11.99 11.36 10.79 11.49 10.72 11.86 2.85 2.50 NaaO 2.40 1.88 2.99 2.58 2.47 1.96 2.22 1.80 1.85 3.40 2.46 3.10 2.85 1.08 0.37 0.71 K2O 0.92 1.01 0.42 0.49 0.53 0.40 0.59 0.48 0.33 1.27 0.65 1.19 0.86 0.79 0.89 TiOz 1.03 0.95 0.82 0.80 1.00 0.82 0.83 0.91 0.88 1.16 0.82 1.01 0.34 0.15 0.27 p2o5 0.37 0.40 0.19 0.31 0.31 0.16 0.14 0.15 0.09 0.32 0.18 0.25 MnO 0.18 0.18 0.21 0.17 0.17 0.17 0.21 0.17 0.22 0.24 0.22 0.21 0.21 0.21 0.22
Cationic norms Or 5.40 5.94 2.45 2.86 3.11 2.34 3.45 2.82 1.93 7.38 3.76 6.95 6.33 2.15 4.18 Ab 8.25 8.39 18.54 16.01 16.63 17.43 19.71 16.05 16.45 3.30 8.14 17.06 16.16 20.34 18.11 An 30.18 31.83 32.92 31.67 32.51 34.58 35.92 37.38 34.89 20.89 28.19 27.68 33.40 30.68 34.57 Ne 7.89 5.04 4.74 4.09 3.20 0.00 0.00 0.00 0.00 16.01 8.09 6.25 5.51 2.88 2.52 Cpx 24.82 25.77 16.79 17.42 19.82 17.15 10.89 13.44 12.89 28.40 20.68 18.85 16.71 16.49 17.75 Opx 0.00 0.00 0.00 0.00 0.00 1.08 3.53 6.09 7.75 0.00 0.00 0.00 0.00 0.00 0.00 Ol 18.63 18.37 20.71 23.71 20.24 23.43 22.58 20.08 22.02 19.11 27.05 18.91 17.65 23.69 18.60 Mt 2.64 2.51 2.34 2.50 2.47 2.53 2.48 2.58 2.67 2.66 2.59 2.39 2.35 2.37 2.47 11 1.42 1.32 1.13 1.10 1.38 1.13 1.14 1.26 1.21 1.59 1.12 1.39 1.19 1.08 1.23 0.31 0.56 Ap 0.77 0.83 0.39 0.64 0.64 0.33 0.29 0.31 0.19 0.66 0.37 0.52 0.71
Trace-element abundances (ppm) Ba 414 345 500 177 299 181 192 114 108 640 170 509 538 135 474 Nb 11 9 10 2 12 2 8 3 7 25 9 19 13 7 14 Zr 126 123 73 68 109 68 73 64 62 176 75 138 170 65 102 Y 20 20 18 18 30 20 18 16 17 120 17 16 21 20 16 Sr 933 902 542 396 596 398 484 287 327 973 617 786 749 402 829 Rb 12 23 14 12 5 6 12 11 7 40 17 38 29 7 27 Zn 75 69 78 76 88 78 80 73 81 81 73 76 75 74 87 Cu 158 90 86 84 11 101 96 78 62 128 86 86 119 96 65 Ni 254 170 301 369 277 358 495 384 485 292 434 263 267 371 260
* Total Fe as Fe203. Normative compositions calculated assuming a fixed oxidation of Fe0/Fe203 = 3:1 equivalent to the least oxidized Grenada basalts (Sigurdsson and others, 1973).
pies were used as standards. The analytical TABLE IB. EXPLANATION OF COLUMN NUMBERS IN TABLE 1A data were corrected for mass absorption Latitude Longitude differences between standards and un- Sample no. Rock type Center* (N) (W) knowns by an iterative computer procedure Column described by Holland and Brindle (1966) 1 531 Basanitoid SEG 12°05.9' 61°40.2' and Reeves (1971). Representative analyses 2 503 Basanitoid MM 12°01.4' 61°46.6' from the 250 samples completed during the 3 449 Alkalic basalt MM 12°05.7' 61°45.3' study are presented, normalized to 100 per- 4 43 Alkalic basalt MSC 12°11.9' 61°36.9' 61°43.9' cent on a volatile-free basis, in Table 1 and 5 505 Alkalic basalt MM 12°04.8' MM 12°05.2' 61°43.1' supplementary material.1 6 507 Alkalic basalt 7 24 Alkalic basalt MSC 12°12.6' 61°38.6' The trace elements Ba, Nb, Zr, Y, Sr, Rb, 8 500 Subalkalic basalt MM 12°03.5' 61°46.9' Zn, Cu, and Ni were determined with the 9 474 Subalkalic basalt HS 12°05.0' 61°45.5' same Philips spectrometer using a W-anode 10 483 Basanitoid MM 12°05.3' 61°43.r and evacuated x-ray path. Correction for 11 61 Basanitoidf SI 12°13.4' 61°35.3' 12°12.8' 61°38.3' blank-contamination and K0 interference 12 33 Basanitoid MSC (in the case of Nb, Zr, and Y) was made 13 373 Alkalic basalt MSC 12°10.6' 61°39.1' with the computer program TRATIO (Gill, 14 202 Alkalic basalt MSC 12°12.0' 61°36.8' 12°11.3' 61°39.0' 1972). Synthetic spiked glasses prepared by 15 286 Alkalic basalt MSC the Pilkington Research Laboratory * MGF = Mount Granby-Fedon's Camp; MM = Mount Maitland; MSC = Mount St. Catherine; (Lathom, England) were used as standards SEG = South East Grenada; HS = Holocene scoria (explosion craters); SI = Sandy Island, (Brpwn and others, 1970). f Termed "oceanite" by Martin-Kaye (1969). Selected analyses of Grenada basaltic compositions are projected as normative plots in Figure 4 (representative composi- Further discussion of the petrogenesis of the tions listed in Table 1). The feature to note suite of Grenada basaltic compositions "Copies of GSA supplementary material 76-10 on Figure 4 is the transgressive nature of the must be preceded by a more detailed ex- (Major- and trace-element analyses of the Mount St. compositional variation from silica- amination of the nature of the chemical var- Catherine volcanic center) may be ordered from Docu- undersaturated, nepheline-normative basalt iation of the most mafic members of the ments Secretary, Geological Society of America, 3300 Penrose Place, Boulder, Colorado 80301. to hypersthene-normative compositions. suite.
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Figure 4. Normative (cationic) compositions of basanitoids and alkalic and sub- alkalic basalts projected in Ol'-Ne'-Q' (upper), Ne-Di-Ol-Hy (bottom right), and
Ca(Mg,Fe)O2-(Mg,Fe)O-SiO2 diagrams. Named fields after Coombs (1963); par- tially dashed lines are discriminants proposed by Chayes (1966) to distinguish al- kalic and subalkalic basalts (subscripted c); nomenclature in figure as given by Coombs (1963) and Chayes (1966). Ol'-Ne'-Q' projection based on method of Irvine
and Baragar (1971) and Ca (Mg,Fe)02-(Mg,Fe)0-Si02 projection based on method of Irvine (1974).
PETROGENESIS OF THE ALKALIC ROCKS
It has been suggested that two divergent basaltic groups are present on the island (Sigurdsson and others, 1973). The possible effects of either variable degrees of partial melting of a hydrous pyrolite upper mantle
Ca[Mg,Fel02 at pressures of 20 to 25 kbar or variable degrees of fractionation of olivine plus aluminous orthopyroxene and clinopyrox- ene were examined to account for the vari- able major- and trace-element geochemistry of the basaltic compositions. The results of experimental studies by Green and co- workers (for example, Green, 1969) were extensively used in the discussion. Additional field sampling and laboratory studies have been completed (Arculus, 1973; Arculus and Shimizu, 1974), and some revision of the previous account by
(Mg.FelO Sigurdsson and others (1973) is now possi- ble. Within the range of basaltic composi- tions there is variation in the abundance of trace elements such as Ba, Nb, and Sr up to a factor of 4 (Table 1). In general, the great- er abundances of these elements and of Rb are associated with higher K and P con- tents, higher Ca/Al ratios, and higher con- tent of normative clinopyroxene. Theoret- ical considerations (Gast, 1968) indicate that enrichment of the incompatible trace elements in residual liquids to the degree observed in the Grenada samples is not pos- sible without extensive solidification (for example, 70 to 80 percent) of the initial melt. The similarity of Mg/(Mg -I- SFe) ratios of the basanitoids and alkalic basalts listed in Table 1 does not indicate extensive fractionation of any one phase or combina- tion of phases such as garnet, clinopyrox- ene, olivine, or orthopyroxene. Nine of the basanitoids and alkalic and subalkalic basalts were selected for rare- earth-element (REE) analysis (Arculus and Shimizu, 1974). In Figure 5, the REE con- centrations normalized to chondrite are re- produced. Within the range of basaltic compositions chosen for analysis (Table 1, cols. 1 to 9), the enrichment of the light REE relative to chondrite varies continu- ously up to a factor of 5.5, from 17 to 92. In contrast, the enrichment of the heavy REE concentrations (Dy to Yb) remains nearly constant at 8 to 12 times the chondritic Ce Pr Nd IPm) Sm Eu Gd Tb Dy Ho Er Tm Yb Lu abundance. This constancy, within experi- Figure 5. Chondrite-normalized rare-earth-element concentrations in Grenada basanitoid and mental determination, of the abundance of alkali basalt compositions. Sample numbers refer to analyses listed in Table 2. the heavy REE implies that garnet was in-
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TABLE 2A. LEAST-SQUARES APPROXIMATION TO AVERAGE BASANITOID test possible fractional crystallization schemes, average compositions at given Observed Calculated Weight fraction percent Si02 content of all the Grenada lava (%) (%) Variable (g) and pyroclastic rocks were computed and a employed in least-squares linear mixing calculations (Bryan and others, 1969). In Si02 45.23 45.38 Alkalic basalt 0.8685 AI2O3 15.10 14.84 Olivine 0.0380 this section the results of the fractionation Fe203* 10.30 10.42 Calcic augite 0.0628 of analyzed phenocrysts of olivine, calcic MgO 12.52 12.34 Spinel 0.0192 augite, and spinel (Arculus, 1974; in prep.) CaO 12.08 11.90 from basanitoid magma are presented Na20 2.35 2.34 (Table 2). Fractional crystallization models K2O 0.77 0.75 for subalkalic basalt, andesite, and dacite 2 Ti02 0.92 0.90 2R = 0.1692 are presented in the following section. b The fractionation of olivine, calcic au-
Si02 45.23 45.26 Alkalic basalt 0.7678 gite, and spinel from basanitoid magma re- AI2O3 15.10 15.04 Amphibole 0.1377 sults in a trend toward increasing SiO;>, FezO/ 10.30 10.31 Olivine 0.0199 A1203, and alkali content and decreasing MgO 12.52 12.49 Spinel 0.0179 CaO and Mg/(Mg + 2Fe) ratio in the re- CaO 12.08 12.04 Calcic augite 0.0536 sidual melts (Table 2). The dominant pro- NazO 2.35 2.39 portion of clinopyroxene and spinel relative K2O 0.77 0.73 to olivine required in the crystal extract for 0.92 2 Ti02 1.06 £R = 0.0306 a match in major-element geochemistry is Note: Top half of table (a) is approximation calculated as a linear combination of average alkalic seen in Table 2. The approximation of nor- basalt plus clinopyroxene, olivine, and spinel phenocrysts observed in the basanitoids (Arculus, malizing the spinel composition to 100 per- 1973, 1974, in prep.)- Bottom half of table (b) represents the same phenocryst compositions with the cent without Cr203 was made. The addition of amphibole (Arculus, 1973, in prep.) as a possible fractionating phase. basanitoids and alkalic basalts of Grenada * Total Fe as Fe203. contain as much as 1,400 ppm Cr (Sigurdsson and others, 1973). Removal of TABLE 2B. COMPOSITION OF VARIABLES IN TABLE 4A Cr-bearing spinel in the proportion given in Table 2 would result in rapid depletion of Alkalic basalt Olivine Calcic augite Spinel Amphibole the residual magma in Cr, a feature that is (%) (%) (%) (%) (%) observed in the Grenada suite (G. M. Brown and J. G. Holland, 1973, oral com- Si02 46.86 41.00 49.80 •• 42.10 A1203 16.38 3.70 19.90 13.90 mun.). However, the proportion of spinel Fe203* 9.65 15.40 3.90 62.88 9.20 relative to olivine appears unrealistically MgO 10.73 46.30 16.00 13.55 16.20 high in the calculation. Some of the difficul- CaO 11.96 24.10 •• 11.40 ties may be associated with the procedure of Na20 2.66 0.40 2.40 selection of average compositions. The K20 0.86 0.50 spectrum of major-element variation in the Ti02 0.90 0.60 4.40 1.90 Grenada suite (Fig. 6) may conceal a more
* Total Fe as Fe203. complex derivation of magma than a direct averaging of compositions is capable of re- vealing. volved in the petrogenesis of these basaltic zoned from Fo90 to approximately Fo80, melts (Arculus and Shimizu, 1974). Several calcite augite (core compositions with 100 A closer approximation is achieved if possible explanations of the REE distribu- x Mg/(Mg + SFe) = 88) (Arculus, in amphibole is included as a possible frac- tion patterns were examined: (1) partial prep.), and spinel microphenocrysts (Ar- tionating phase (Table 2). However, the melting of different source materials; (2) culus, 1974). Plagioclase also occurs as a amphibole-bearing assemblages so far selective wall-rock contamination of the microphenocryst phase in some samples analyzed (Arculus, 1973; in prep.) are melts en route to the surface; (3) some frac- and is present in addition to olivine, calcic characterized by lower Mg/(Mg + 2Fe) tional crystallization of the basaltic melts, augite, spinel, and glass in the groundmass ratios than olivine-clinopyroxene pairs in involving separation of one or more phases; of all samples. the basanitoids and alkalic basalts. Thus, and (4) variable degrees of partial melting In the same volcanic centers as the while it is believed that amphibole is an im- of some upper-mantle source material. Ar- basanitoids and alkalic basalts, high- portant fractionating phase in the Grenada culus and Shimizu (1974) examined these alumina (17 to 21 percent A1203) subal- suite (Cawthorn and others, 1973), I sug- possibilities and demonstrated that the REE kalic basalts are present. Additional alkalic gest that fractional crystallization processes distribution in the Grenada basanitoids and basalts are also present containing pheno- at temperatures higher than the upper sta- alkalic basalts could be accounted for either cryst olivine and calcic augite with lower bility limit of amphibole (>1100°C) may be by explanation (4) or (1), whereas explana- 100 x Mg/(Mg + 2Fe) ratios (<80; Ar- dominated by the assemblage olivine, tions (2) and (3) were not found to be satis- culus, in prep.) than within the samples in- clinopyroxene, and spinel observed in the factory mechanisms. The samples chosen cluded for REE analysis. It is unlikely that basanitoids and alkalic basalts. for the REE distribution study were rep- these represent equilibrium melts of an In some alkalic and subalkalic basalt resentative of most of the range of incom- upper-mantle peridotite source (Green, compositions, the order of crystallization patible trace-element abundances found in 1969). It is possible that these compositions inferred for the phenocryst phases is spinel, microphyric basaltic compositions. Further are derived by fractional crystallizations of olivine, clinopyroxene, and then plagioclase studies are in progress (Arculus and others, more mafic [in terms of Mg/(Mg + £Fe) before amphibole. However, plagioclase- in prep.). ratio] mineral assemblages observed in free but amphibole-bearing cumulate All the samples contain olivine normally basanitoid and alkalic basalts. In order to blocks (Arculus, 1973; in prep.) suggest
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Ca(Mg.Fel02 It is possible to predict the effect of frac- and the phase compositions listed in Table tional crystallization of olivine, clinopyrox- 3 is obtained. ene, and spinel on the abundance of trace In the preceding discussion, two main elements such as Ni and Sr, assuming calcu- processes have been suggested to account lated partition coefficients (Hakli and for the variation in major- and trace- Wright, 1967; Philpotts and others, 1972). element composition of the mafic lava At least a tenfold depletion in the abun- rocks. The variation in REE and incompat- dance of Ni and a one-third increase in the ible trace elements within basanitoids and abundance of Sr may result from fractiona- selected alkalic basalts is explicable as the tion of olivine, clinopyroxene, and spinel in result of variable partial melting processes the proportions given in Table 2. The varia- within the upper mantle. Thus, a variety of tion in Sr and Ni observed in the most mafic potential parental magma compositions members of the Grenada suite (45 to 47 was present during the evolution of the is- percent Si02; Figs. 7, 8) is of the correct land. The second process discussed is frac magnitude, but as previously suggested, tional crystallization. It is suggested that th< there is a considerable divergence in the trend toward increased silica saturation anc abundance of these elements in the suite. It high-alumina alkalic and subalkalic basalts Figure 6. Suggested relationships between basanitoid, alkalic, and subalkalic basalt mag- is possible that the divergence between is initially generated by the fractional crys- mas. Normative compositions are taken from "high-Sr" and "low-Sr" trends (described tallization of olivine, calcic augite, ani Figure 4. Broad arrow represents sequence in the following section), inherited initially spinel from parental basanitoid and alkalic basanitoid to subalkalic basalt derived by vari- from mafic lava with variable abundances basalt magmas. Fractionation of plagio- able degrees of partial melting of a peridotite of Sr and Ni, reflects an increased propor- clase from nepheline-normative composi- source. Two lower dashed arrows represent tion of fractionated olivine relative to tions would prevent further development of trends resulting from fractional crystallization clinopyroxene in the high-Sr relative to the the trend toward silica saturation (Yoder mechanisms outlined in text. Upper dashed low-Sr series. and Tilley, 1962). However, the appear- arrow represents dominant olivine plus spinel ance of amphibole nearly contemporaneous fractionation from basanitoid magma as a possi- The results of fractional crystallization of ble mechanism to account for augite-phyric al- olivine, calcic augite, plagioclase, magnet- with plagioclase in the Grenada suite (Ar- kalic and subalkalic basalts of the high-Sr series. ite, and amphibole from alkalic basalt culus, in prep.) suggests that amphibole magma are presented in Table 3. The first fractionation is the important factor pro- four phases are present within all alkalic viding a breach of the low-pressure thermal variation in the activity of volatile compo- basalt compositions, and amphibole is pre- divide that separates nepheline-normative nents (Holloway and Burnham, 1972; sent in a few samples. Variable proportions from hypersthene-normative basaltic com- Mysen, 1973). It appears that the variation of these phases are present within plutonic positions (Cawthorn and others, 1973). in pressure and temperature of crystalliza- cumulate blocks (Arculus, in prep.). A close The processes envisaged are schematically tion of the Grenada basalts resulted in the match between alkalic basalt and a linear outlined in Figure 6. Experimental studies variation in order of crystallization of these combination of average subalkalic basalt in progress (Arculus, 1974; in prep.) may two phases. Nevertheless, the trend of TABLE 3 A. LEAST-SQUARES APPROXIMATION TO AVERAGE ALKALIC BASALT A1203 content in the Grenada suite from gradual increase to decrease of from 48 to Observed Calculated Weight fraction 51 percent Si0 (Fig. 6) suggests that frac- 2 (%) (%) Variable tionation of plagioclase is significant at this (g) stage in the evolution of the suite. Si02 46.86 46.90 Subalkalic basalt 0.5420 The fractionation of spinel from nephe- AI2O3 16.38 16.37 Olivine 0.0552 line-normative basaltic composition is one Fe203* 9.65 9.64 Calcic augite 0.0819 MgO 10.73 10.70 Magnetite mechanism by which a breach of the low- 0.0166 CaO 11.96 11.90 Amphibole 0.2264 pressure thermal divide (olivine-clino- NazO 2.66 2.38 Plagioclase 0.0769 pyroxene-plagioclase) of the normative K20 0.86 0.73 2 basalt tetrahedron (Yoder and Tilley, 1962) TiOz 0.90 1.11 SR = 0.0764 may take place (Osborn and Tait, 1952; O'Hara and Schairer, 1963). Arculus Note: Approximation calculated as a linear combination of average subalkalic basalt plus olivine, clinopyroxene, magnetite, amphibole, and plagioclase (Arculus, 1973 and in prep.). (1974) described the extensive spinel com- * Total Fe as Fe 0 . positional range in the Grenada basanitoids 2 3 and alkalic basalts, which varies from pleonaste through chromite to titaniferous TABLE 3B. COMPOSITION OF VARIABLES IN TABLE 3A magnetite within individual rock samples. Euhedral spinels included in olivine, Subalkalic Calcic clinopyroxene, amphibole, and plagioclase basalt Olivine augite Magnetite Amphibole Plagioclase (%) suggest that spinel was a stable fractionat- (%) (%) (%) (%) (%)
ing phase throughout the evolution of the Si02 50.51 41.00 50.10 42.60 45.60
suite. AI2O3 18.57 4.30 3.89 14.50 33.90 The clinopyroxene in the basanitoids and Fe203* 8.91 15.40 5.30 90.23 8.80 0.40 MgO 6.09 46.30 16.20 0.10 alkalic basalts shows strong zoning, but 15.30 CaO 10.80 23.20 12.00 18.60 rare calcic augite megacrysts (as large as 1 Na20 3.14 0.30 2.40 1.40 cm in diameter) within these lava flows K2O 1.14 0.50 (Sigurdsson and others, 1973; Arculus, Ti02 0.84 0.60 5.34 2.30 1973; in prep.) are possible examples of 1 high T and P cumulus compositions. Total Fe as Fe203.
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ANDESITE-DACITE ASSOCIATION WITH ALKALIC ROCKS
The variation of major oxides with in- creasing silica content is presented in Figure 7. Analyses of fresh samples from all types of volcanic units and centers are included. Apart from the presence of very mafic com- positions, the general trends of oxide varia- tion are similar to previously described calc-alkalic suites of the Lesser Antilles (P. Baker, 1968; Rea, 1970). The derivation of the calc-alkalic suites of the islands north of Grenada by fractional crystallization of high-alumina basalt magma has been sug- gested by Baker (1968), Rea (1970), and Lewis (1971). In detail, however, there is a considerable degree of variability of a given oxide at a specified percent Si02 content in the Grenada volcanic suite. Interpretation of the geochemical data is simplified by ex- amination of rock suites from individual volcanic centers. Samples from the Mount St. Catherine and Mount Granby-Fedon's Camp complexes have been selected to il- lustrate the variation of major- and trace- element geochemistry observed (Figs. 8, 9), and representative analyses from the Mount St. Catherine center are included in supplementary material (see footnote 1). The variation of Sr and Ni with increas- ing percent Si02 are shown in Figures 8 and 9. There is a considerable degree of varia- tion in the abundance of trace elements such as Sr and Ni at a given percent SiOz. It has proved possible to delineate arbitrarily at least two different rock series within each of the volcanic centers figured. In order to distinguish these series, the terms "high-Sr" and "low-Sr" are applied, since the be- havior of Sr is particularly striking. Analyses of samples from the different series have been identified by contrasting symbols in Figures 8 and 9. Use of these symbols is to aid identification of the char- acteristic geochemical variation displayed by each series. Thus, for example, the samples described as the high-Sr series of the Mount Granby-Fedon's Camp complex are associated with a variation in the abun- dance of Sr from approximately 600 to 900 ppm in basanitoids to 1,500 ppm in alkalic and subalkalic basalts. In the same sense of increasing percent Si02, there is a decrease in the abundance of Ni from 300 to 30 ppm. In contrast, the variation in abun- dance of Sr in the low-Sr series is from 300 ppm in alkalic basalt to 900 ppm in ande-
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500r 300 200 Ni ZR ZN
400h • x X X
60 70 40 50
45 60 65 70
Figure 8. Trace-element variation with 200
weight percent Si02 in Mount St. Catherine vol- CU canic center. Filled circles correspond to mem- bers of high-Sr series and crosses to low-Sr series. zo site and dacite. The abundance of Ni is gen- erally higher in the low-Sr series than in the >S< X high-Sr series at a given percent Si02. Vari- ation in MgO in the two series is similar to
ure 7, would reveal a broad spectrum of • X • trace-element variation that includes the ex- . A amples of Mount Granby-Fedon's Camp % • „ and Mount St. Catherine described here. In • v ** other words, a gradation in trace-element geochemistry from the strongly variable abundances in the basanitoids and alkalic is the low-Sr series. At similar percent Si0 , Some alkalic basalts of the high-Sr series basalts is observed through the subalkalic 2 alkalic and subalkalic basalts of the low-Sr with abundant, oscillatory-zoned plagio- basalt, andesite, and dacite of the island. series contain a greater proportion (as clase and oscillatory-zoned and sector- Mineralogically, the high-Sr series of the much as 20 percent) of modal olivine than zoned clinopyroxene may be cumulus en- island is characterized by a greater modal do the high-Sr series. These features were riched in these minerals. Some scatter in the abundance of clinopyroxene and plagio- evident in laboratory examination of the trace-element variation may be the result of clase feldspar (as much as 35 percent) than rocks and were not a basis for classification. such cumulus enrichment in phases. In gen-
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300 600 r 200 Ni ZR ZN
500 200 Jl * 100 400 • XW ^ • X 100 * • V • X X * 300
40 60 70 40 60 200
CU x><* 45 50 55 60 65 Figure 9. Trace-element variation with \ X X • • weight percent Si02 in Mount Granby-Fedon's * 1 Camp volcanic center. For symbols see Figure 8. •r x
these minerals has occurred (Arculus, 1973; V in prep.). (3) There is a diversity in abun- dance of trace elements in andesite and da- cite that is to be expected as a result of 40 50 60 70 40 50 60 70 differentiation of parental mafic magma 20 00 • 16 00 also characterized by a considerable degree BA SR of variation in abundance of trace elements. In Table 4, the results of fractional crys- 1200 tallization of calcic augite, plagioclase, am- phibole, and magnetite from subalkalic basalt are presented. This four-phase as- I 0 00 8 00 semblage is predominant within the Gren- X X ada cumulate blocks and on the island of St. Vincent (Lewis, 1964). A close match is ob- ** x* A tained with average andesite composition. >y. » 400
STRONTIUM ISOTOPE RESULTS 60 70 40 60 The major- and trace-element geochemis- try of the Grenada basanitoids and alkalic basalts is consistent with derivation by par- 200 tial melting of upper-mantle, garnet perido- Nb Rb tite source material (Arculus and Curran, 1972; Arculus and Shimizu, 1974). Pre- liminary strontium isotope data (Arculus and others, in prep.) indicate a range of 87 86 . xx*. Sr /Sr ratios from 0.70430 to 0.70501 (± «C X • X 100 5 and ± 2) in the Grenada basanitoids and xlfc basalts. Ratios measured for andesite and 10 dacite are in a similar range and do not X • « • « suggest any crustal or subducted sediment xx • X .XX*- • contribution to the melts. This suggests that melting of sediments is not an important factor in the Grenada situation. However, 70 40 initiation of melting in the garnet peridotite overlying the subducted lithospheric plate eral, however, the andesite and dacite units within volcanic series of individual centers may be the result of the release of volatile of Grenada are believed to be derived from of activity. (2) In plutonic cumulate blocks, materials by dehydration reactions in the basaltic melts by fractional crystallization mafic and feldspathic minerals are of simi- plate (for example, Boettcher, 1973) or, al- processes. The evidence for this may be lar composition to the phenocryst phases ternatively, disturbance of the thermal re- summarized as follows: (1) There is a found in erupted volcanic rocks, suggesting gime of the upper mantle during the sub- smooth trend of major-element variation that crystallization and some settling of duction process (Toksoz and others, 1971).
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TABLE 4A. LEAST-SQUARES APPROXIMATION TO AVERAGE SUBALKALIC BASALT Carnegie Inst. Washington Year Book 73, p. 322-327. Observed Calculated Weight fraction Arculus, R. J., and Curran, E. B., 1972, The (%) (%) Variable (8) genesis of the calc-alkaline rock suite: Earth and Planetary Sci. Letters, v. 15, p. Si02 50.51 50.52 Andesite 0.4408 255-262. AI2O3 18.57 18.54 Calcic augite 0.0671 Arculus, R. J., and Shimizu, N., 1974, Rare earth Fe A,* 8.91 8.91 Magnetite 0.0348 elements in a suite of basanitoids and alkali MgO 6.09 6.08 Amphibole 0.2168 olivine basalts from Grenada, Lesser Antil- CaO 10.80 10.78 Plagioclase 0.2444 les: Carnegie Inst. Washington Year Book NazO 3.14 3.23 73, p. 553-560. K2O 1.14 0.88 Baker, I., 1968, Intermediate oceanic volcanic 2 Ti02 0.84 0.95 2R = 0.0876 rocks and the Daly gap: Earth and Plane- tary Sci. Letters, v. 4, p. 103-106. Note: Approximation calculated as a linear combination of average andesite plus clinopyroxene, Baker, P. E., 1963, The geology of Mt. Misery magnetite, amphibole, and plagioclase (Arculus, 1973, in prep.) volcano, St. Kitts [Ph.D. thesis]: Oxford, 4 Total Fe as Fe 0 . 2 3 England, Univ. Oxford. 1968, Petrology of Mt. Misery volcano, St. TABLE 4B. COMPOSITION OF VARIABLES IN TABLE 4A Kitts, West Indies: Lithos, v. 1, p. 124-150. Berggren, W. A., 1971, A Cenozoic time scale — Andesite Calcic augite Magnetite Amphibole Plagioclase Some implications for regional geology and (%) (%) (%) (%) (%) palaeobiogeography: Lethaia, v. 5, p. 195-215. 49.30 42.10 53.40 Si02 56.79 Boettcher, A. L., 1973, Volcanism and orogenic AI O 17.93 3.80 3.89 13.90 29.60 2 3 belts — The origin of andesites: Tec- 7.30 90.23 9.20 0.30 Fei)/ 7.27 tonophysics, v. 17, p. 223-240. MgO 3.24 15.10 2.80 16.20 0.10 Brown, G. M., Emeleus, C. H., Holland, J. G., CaO 8.87 22.60 11.40 11.80 and Phillips, R., 1970, Petrographic, Na20 3.60 0.30 2.40 4.50 mineralogic and x-ray fluorescence analysis K O 0.50 0.30 2 1.59 of lunar igneous-type rocks and spherules: Ti0 5.34 1.90 2 0.71 0.60 Science, v. 167, p. 599-601. * Total Fe as Fe^^ Bryan, W. B., Finger, L. W., and Chayes, F., 1969, Estimating proportions in petro- graphic mixing equations by least-squares A significant feature of Grenada volcanic- active volcanic centers of the Lesser Antilles approximation: Science, v. 163, p. ity is the occurrence within a restricted (Baker, 1963; Tomblin, 1964; Rea, 1970) 426-427. geographic area of rock associations of con- suggest that upper-mantle chemical Cawthorn, R. G., Curran, E. B., and Arculus, trasting trace-element abundances and heterogeneity may be an additional factor. R. J., 1973, A petrogenetic model for the strontium isotope ratios. Hedge and Lewis origin of the calc-alkaline suite of Grenada, 87 86 Lesser Antilles: Jour. Petrology, v. 14, p. (1971) reported Sr /Sr ratios for three ACKNOWLEDGMENTS 327-338. volcanic centers in the Lesser Antilles (St. Chayes, F., 1966, Alkaline and subalkaline Kitts, St. Vincent, and Carriacou). Within I gratefully acknowledge the award of a basalts: Am. Jour. Sci., v. 264, p. 128-145. 87 86 each center, a consistent Sr /Sr ratio was Natural Environment Research Council re- Christman, R., 1953, Geology of St. Barth- discovered, but the ratio differed between search studentship and the facilities made olomew, St. Martin and Anguilla, Lesser centers (see Powell and DeLong, 1966, for a available at the University of Durham by Antilles: Geol. Soc. America Bull., v. 64, p. similar situation). Lateral heterogeneity of Prof. M.H.P. Bott. The support and en- 65-96. the upper mantle was postulated by Hedge couragement of my supervisor G. M. Coombs, D. S., 1963, Trends and affinities of and Lewis (1971) to account for this inter- Brown and discussions with E. B. Curran, basaltic magmas and pyroxenes as illus- center variability. Gunn and others (1974) R. G. Cawthorn, R. K. O'Nions, R. Powell, trated on the diopside-olivine-silica dia- gram: Mineralog. Soc. America Spec. Paper reported variability between volcanic cen- and K.J.A. Wills are appreciated. The 1, p. 227-250. ters of Martinique in trace-element abun- manuscript was reviewed by G. M. Brown, Earle, K. W., 1924, Geological survey of Gren- dances. However, the volcanic centers of W. B. Bryan, and S. E. DeLong. ada and the (Grenada) Grenadines: St. Grenada are individually characterized by George, Grenada, Govt. Printing Office, strongly variable trace-element abun- 9 p. dances. As pointed out above, the total vol- REFERENCES CITED Gast, P. W., 1968, Trace element fractionation ume of erupted products on Grenada is and the origin of tholeiitic and alkaline considerably less than in the major volcanic Anderson, T., 1908, Report on the eruptions of magma types: Geochim et Cosmochim. islands northward in the Lesser Antilles. It the Soufrière in St. Vincent in 1902, and on Acta, v. 32, p. 1057-1086. appears that each of the volcanic episodes a visit to Montagne Pelée in Martinique: Gill, R.C.O., 1972, The geochemistry of the Royal Soc. London Philos. Trans., ser. A, v. Gronnedal-Ika alkaline complex, South on Grenada has been associated with indi- 208, p. 275-332. Greenland [Ph.D. thesis]: Durham, Eng- vidual melting events in the upper mantle, Andrew, E. M., Masson-Smith, D„ and Robson, land, Univ. Durham. which resulted in variable trace-element G. R., 1970, Gravity anomalies in the Les- Green, D. H., 1969, The origin of basaltic and geochemistry. A single parental magma ser Antilles: Natl. Environment Research nephelinitic magmas in the earth's mantle: type obviously did not exist in Grenada. Council Inst. Geol. Sci. Geophys. Paper, no. Tectonophysics, v. 7, p. 409-422. 5, 25 p. Homogeneity of major-element, trace- Gunn, B. M., Roobol, M. J., and Smith, A. L., element, and isotope geochemistry within Arculus, R. J., 1973, The alkali basalt, andesite 1974, Petrochemistry of the Pelean-type association of Grenada, Lesser Antilles volcanoes of Martinique: Geol. Soc. volcanic centers of the other islands of the [Ph.D. thesis]: Durham, England, Univ. America Bull., v. 85, p. 1023-1030. Lesser Antilles presumably indicates uni- Durham. Hakli, T. A., and Wright, T. 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