The geochemistry and petrogenesis of the volcanic rocks of Carriacou,Grenadine Islands,West Indies

Carriacou is small volcanic island located near the southern end of the Lesser Antillean chain. Field relationships, petrography and geochemistry of the Tertiary lavas, outcropping in the southern half of the island, are used to identify the rocks present and to determine their petrogenesis and assess their significance within the island arc.Six main volcanic units are present. From oldest to youngest, these are the clinopyroxene-phyric basalt (CPB) sequence, the amphibole-phyric andesite (APA) sequence, the clinopyroxene-megaphyric basalt (CMB) sequence, the olivine-microphyric basalt (OMB) sequence, the clinopyroxene phyric andesite (CPA) sequence, and the amphibole-megaphyric andesite (AMA) sequence. Volcaniclastic deposits are associated with the APA, CMB, and AMA sequences. The APA sequence is calcalkaline, whereas the other five sequences are tholeiitic.Sr isotope and rare earth element (REE) data suggest that these volcanic rocks were derived from partial melts of garnet-peridotite generated deep within the mantle. The OMB lavas have the highest temperature assemblages of intratelluric minerals and the least evolved chemical characteristics, and are considered to be closest in composition to a parental melt. Phenocryst assemblages and chemical variation suggest that the andesite sequences have been derived from the mafic melts by low pressure fractional crystallization of approximately 20% clinopyroxene and 20% olivine, plus smaller amounts of plagioclase and amphibole. The high concentrations of incompatible and compatible elements and the high⁸⁷Sr/⁸⁶Sr ratios may indicate that subduction is slower in the southern part of the arc, and fluids released during slab dehydration rich in Incompatible trace elements, in Radiogenic strontium, and in Silica (IRS), have modified the parental melts.     

Petrology of the volcanic rocks of martinique,West Indies

Volcanological differences between the old and the recent lavas from Martinique, Lesser Antilles, are presented, showing that two volcanic series exist in this island:

Dash

a high-alumina basalt series generally mafic, line-grained, partly pillowed, with clinopyroxene-rich lavas which show iron enrichment tendancies en an A.F.M. plot;

a calc-alkaline (slightly potassic) series much more siliceous as a group, porphyric, predominantly sub-aerially erupted with orthopyroxene-rich lavas which show no iron enrichment.

The high-alumina basalt series is considered as having originated from a differentiation trend by fractionation of olivine, clinopyroxene and plagioclase. Lavas range from olivine basalt to tridymite-rich dacite. The calc-alkaline series probably derives from the contamination of the first suite but the occurence of hornblende-rich cumulates indicates the process of fractionation takes place too. Lavas range from orthopyroxene andesite and hornblende andesite to quartz-hornblende dacite and quartz-biotite dacite.     

Petrology of the volcanic rocks of saba,West Indies

Saba is composed of basic and intermediate members of the calc alkaline series with a broad resemblance to volcanic rocks on other islands of the northern Lesser Antilles. The most abundant rock-type is hornblende andesite with relatively high potassium and associated elements. Notable petrographic features are the persistence of magnesian olivine into relatively siliceous whole-rock composition and the prevalence of hornblende even in the more basic rocks. Quartz xenocrysts also occur in the basalts Clinopyroxene phenocrysts are of an gite composition but groundmass grains are sometimes in the range subcalcic augite to magnesian pigeonite. Disequilibrium features in the petrography, together with anomalies in the chemical trends may be due to incorporation of xenocrysts or perhaps in extreme instances to the hybridisation of contrasting magmas.     

Volcanoes and volcanic rocks in central America and the west Indies

It is the purpose of this paper to point out the similarity of the magmatic products in the circum-Caribbean region, and to show the geologic objections to the hypothesis of the production of sialic magmas by melting of the root of a sialic crust. These objections were presented only from the viewpoint of a geologist. They may show the necessity of other hypotheses as have been developed byEwing, Gorshkov, Hess, Donnelly, Wilson and others.     

Capping volcanic rocks: West central Nevada

Cenozoic capping volcanic rocks in the Nevada portion of the Basin and Range Province of the western United States belong to the high alumina calcalkaline igneous series. Varying proportions of plagioclase (An=85 to 45 percent), pyroxene (augite, pigeonite, and hypersthene), olivine, magnetite, biotite, and oxyhornblende indicate a modal range from olivine basalt to andesite. Major element analyses made on randomly collected samples, as well as on samples from systematically measured stratigraphic sections in localities of minimum erosion show ranges in Al₂O₃ (from 17.5 to 22.5 percent); SiO₂ (from 44.0 to 54.0 percent); MgO (from 3.47 to 8.20 percent) and CaO (from 7.19 to 11.90 percent). Na₂O/K₂O is always greater than 1.0. Ba⁺⁺ and Sr⁺⁺ abundances for some of the rocks are in agreement with those suggested, by workers in the field, for average basalt and andesite derived by melting of mantle or lower crust, but for many of the samples the values found are considerably higher. Although the presence of biotite in the earlier flows and oxyhornblende in the later ones along with the presence of much magnetite in all the rocks examined suggests that in part these rocks were derived by crystallization of a melt under conditions of high partial pressure of oxygen, the available trace element data indicates that contamination of the magma with crustal material was also a factor in their developmental history. The case for a parent magma, subsequently fractionally crystallized and contaminated in part, is strengthened by the occurrence of crystal cumulates and highly altered xenoliths in some of the flows.     

Geochemistry of the E-MORB type ophiolite and related volcanic rocks from the Wushan area,West Qinling

The mafic-ultramafic assemblages, which thrustthrust into the Wushan-Tangzang boundary fault as some blocks and outcropped in the Yuanyangzhen, Lijiahe, Lubangou and Gaojiahe area, consist mainly of meta-peridotites, gabbros and basalts. The meta-peridotites are characterized by high SiO₂ and MgO contents, low ΣREE, as well as their chondrite-normalized rare earth element patterns show some similarities to that of middle oceanic meta-peridotite. The basalts from the Yuanyangzhen, Lijiahe and Lubangou area are characterized by relatively high TiO₂ content, low Al₂O₃ content and Na₂O>>K₂O. Above all, it is the slight enrichment or flat REE distribution patterns and the unfractionated in HFS elements in the primitive-normalized trace elements distribution patterns that indicate these basalts are similar to that of the typical E-MORB. In comparison, the basalts from the Gaojiahe section are featured by depletion in Nb and Ta contents and enrichment in Th content which show that these were derived from an island-arc setting. From studies of the regional geology, petrology, geochemistry, geo-chronology and all above evidence, it can be suggested that the mafic-ultramafic rocks from the Wushan area are mainly dismembered E-MORB type ophiolite, which represent the fragments of the lithosphere of the Early-Paleozoic Qinling ocean. It is preferred that these rocks were formed in an initial mid-ocean ridge setting during the beginning stage of the oceanic basin spreading. This ophiolite together with the Gaojiahe island-arc basalts shows that there exists an ophiolitic mélange along the Wushan-Tangzang boundary fault, and marks the suture zone after the closure of the Qinling ocean in early Paleozoic.

     

Strontium isotope composition in volcanic rocks from the Northeast Japan arc

In the Northeast Japan arc, a number of Quaternary volcanoes form a long, narrow belt, parallel to the Japan Trench. ⁸⁷Sr/⁸⁶Sr ratios were determined in 52 specimens of volcanic rocks from 27 volcanoes in the Northeast Japan arc area. The results reveal that the ratios change systematically in space. Decreasing ⁸⁷Sr/⁸⁶Sr ratios across the arc were confirmed over a wide area of Northeast Japan. In the same direction, increases in both Rb and Sr contents were also found. The regular trends are considered to be a strong constraint for elucidation of subduction-originated magma genesis at the Eurasia plate vs. Pacific plate boundary. In the northern region of the Northeast Japan arc, ⁸⁷Sr/⁸⁶Sr ratios in volcanic rocks along the volcanic front were almost constant (0.7038–0.7045) and slightly higher than those from the Izu-Ogasawara arc (0.7032–0.7038). This suggests that “interactions” between the Eurasia plate and the Pacific plate, and those between the Philippine Sea plate and the Pacific plate are slightly different. The southern region of the Northeast Japan arc, where the direction of the volcanic front bends from southward to westward, showed anomalously high ⁸⁷Sr/⁸⁶Sr ratios, reaching to 0.7077. This region coincides with the triple junction of the Eurasia, Pacific and Philippine Sea plates, suggesting “anomalous interaction” at the triple junction.     

Crustal attenuation for Jamaica, West Indies

The S and coda wave spectra of small earthquakes on the island of Jamaica were used to determine the near surface and coda Q attenuation, and Q _c,respectively. Q _c determined by the single-station method was found in the range of 1 to 10 Hertz to be given by the relationship, Q _c= 60 ± 5f ^0.87±0.05. This suggests that the Jamaican crust is highly attenuating which is further supported by the observation of rapid intensity fall-off with distance for earthquakes that have affected the island in the past. , determined from S-wave spectra with short travel times was found to be 0.058 ± 0.012 on the central crustal block, which makes up nearly two-thirds of the island, and 0.080 ± 0.014 in surrounding belt sub-regions. The pattern of values seems to fit with the surface geology in that the central block has areas of exposed outcrops of older and harder rock than the belts, which are characterized by thicker sedimentary sequences as well as intense fracturing and faulting.Atkinson and Boore (1998) and Atkinson(2001) presented an alternative method to stochastic modelling for ground motion in Eastern North America, whereby California attenuation relationships were modified to account for crustal differences invelocity-depth profile, Q and between both regions. Following their example, the California spectral attenuation relation of Boore, Joyner and Fumal (1997) was modified to account for differences between the California and Jamaica crust, resulting in an attenuation relation that is deemed to be more appropriate for Jamaica. Spectral accelerations for Jamaica when compared to California, are especially reduced beyond 20 km from the source and at high frequencies, f 1 hertz.The study concludes that the Jamaican crust, although having an oceanic composition is highly attenuating, which may be a result of intensive tectonic processes, whereas is consistent with near-rock conditions on the central block and soft rock conditions elsewhere on the island.     

Fluid geochemistry of Montserrat Island,West Indies

 Two geochemical surveys carried out in March 1991 and September 1992 revealed the existence of a hydrothermal system in the southern portion of Montserrat Island, below Soufrière Hills Volcano. This conclusion is supported by the presence of: (a) the thermal springs of Plymouth which are fed by deep Na–Cl waters (Cl concentration ∼25 000 mg/kg, temperature ca. 250  °C) mixed with shallow steam-heated waters; (b) the four fumarolic fields of Galway's Soufrière, Gages Upper Soufrière, Gages Lower Soufrière, and Tar River Soufrière, where acid to neutral, steam-heated waters are present together with several fumarolic vents, discharging vapors formed through boiling of hydrothermal aqueous solutions. Involvement of magmatic fluids in the recharge of the hydrothermal aquifers is suggested by: (a) the high ³He/⁴He ratios of fumarolic fluids, i.e., 8.2 R_A at Galway's Soufrière and 5.9 R_A at Gages Lower Soufrière; (b) the δD and δ¹⁸O values of Na–Cl thermal springs and steam condensates, indicating the involvement of arc-type magmatic water in the formation of deep geothermal liquids; and (c) the CH₄/CO₂ ratios of fumarolic fluids, which are lower than expected for equilibrium with the FeO–FeO_1.5 hydrothermal rock buffer, but being shifted towards the SO₂–H₂S magmatic gas buffer. Received: 26 March 1996 / Accepted: 19 July 1996     

Isotope composition of helium in ultrabasic xenoliths from volcanic rocks of Kamchatka

The purpose of this work is to refine our knowledge about the nature of helium with a high abundance of the rare isotope³He(³He/⁴He= 10^?5) discovered in terrestrial volcanic gases in 1968.We will discuss here the results of isotope analyses of helium released by step-wise heating of ultrabasic xenoliths and some volcanic rocks. On the basis of these results, possible sources of³He in the earth due to fission and nuclear reactions are considered critically. The most probable source of the high abundance of³He is shown to be due to the capture and trapping of primordial He by the earth during its formation (primordial helium³He/⁴He= 3 × 10^?4), a small but significant fraction of which has been retained to the present time.     

The origin ofBe in island-arc volcanic rocks

The presence of cosmogenic¹⁰Be (t_1/2 = 1.5 × 10⁶ y) in island-arc volcanic rocks has been interpreted as indicating that sediments are subducted to the depths of island-arc magma genesis [1,2]. We have measured¹⁰Be and⁹Be (the stable common isotope) in phenocryst and groundmass fractions separated from four Aleutian volcanic rocks.¹⁰Be and⁹Be covary linearly in three of the rocks proving that the¹⁰Be was incorporated prior to the formation of phenocrysts. In the other rock¹⁰Be and⁹Be do not covary; the lack of covariation indicates either that the rock incorporated¹⁰Be during weathering or that⁹Be-rich xenocrysts were incorporated by mechanical mixing. Our results strongly support the hypothesis of Brown et al. [1] and Tera et al. [2] that sediments containing¹⁰Be are an integral part of island-arc magmatic systems. As we cannot identify the depth at which the¹⁰Be was incorporated we still cannot completely rule out the possibility of assimilation of sediments or altered crust near the Earth's surface as a source of¹⁰Be.     

On the lateral variations in chemical composition and volume of quaternary volcanic rocks across Japanese arcs

The Fe/Mg+Fe) ratios (X_Fe) of the Quaternary basalts (SiO₂ < 53 wt.%) in the Japanese arcs were examined. The X_XFe of relatively magnesian basalts decreases from the volcanic front toward the Japan Sea across the arcs. Based on the partition coefficient of Mg-Fe²⁺ between olivine and liquid, it is suggested that all the basalts near the volcanic front, which are mostly tholeiitic basalts, are significantly fractionated, whereas many basalts near the Japan Sea, which are mostly alkali basalts, are little fractionated. The K₂ O content in the primary basalt magmas increases toward the Japan Sea. Combining the X_Fe and K₂ O data, it is suggested that relatively large amounts of tholeiitic magmas are produced near the volcanic front, but they fractionate during their ascent, whereas smaller amounts of alkali basalt magmas are formed near the Japan Sea, but they can ascend with less fractionation. The density of primary tholeiite magma is significantly larger than that of primary alkali basalt magmas. It is most likely that primary tholeiite magmas cannot ascend beyond the upper crust and would fractionate to produce less dense tholeiitic magmas near the volcanic front, whereas primary alkali basalt magmas can ascend through the upper crust without fractionation, as far as buoyancy is the principal ascending force. In the Japanese arcs, the stress field may be less compressional near the Japan Sea than near the volcanic front, so that magmas can ascend more rapidly in the latter region than in the former. These two factors may be responsible for the above mentioned chemical variations of basalt magmas across the arcs. The variation in volume of the Quaternary volcanic rocks across the arcs can be explained by the presence of a melt-rich zone above but nearly parallel to the subducted slab.     

K-Ar geochronology of the Limestone Caribbees and Martinique,Lesser Antilles,West Indies

Twenty-seven K-Ar ages have been measured on igneous rocks from islands of the Limestone Caribbees (St. Martin, St. Bartholomew, Antigua, and Grande Terre, Guadeloupe) and Martinique in the Lesser Antilles arc. Earlier paleontological data indicated that rocks on several of these islands are Eocene in age and among the oldest known in the arc.The oldest igneous activity on the islands studied is 35–40 m.y. Bifurcation of the Lesser Antilles arc north of Guadeloupe took place between 7 and 20 m.y. ago. A distinct physical overlap of older and younger volcanic arcs exists on Martinique.Our results do not support previous suggestions that the pre-Tertiary basement of the Greater Antilles extends through the Limestone Caribbees to the anomalously old, Jurassic-Cretaceous, igneous rocks of La Desirade.     

Seismicity and earthquake hazard in Trinidad and Tobago,West Indies

The two-island Republic of Trinidad and Tobago in the eastern Caribbean has enjoyed increased physical and industrial development in recent years; however, these islands have been subjected to damaging earthquakes during their history and an up-to-date risk assessment is needed. We examine two approaches to quantifying this problem: (a) the risk estimated probabilistically using recent instrumental data and (b) the hazard inferred from regional tectonic movements. The probabilistic approach indicates that peak ground accelerations with a probability of exceedance of 10 per cent in 50 years could range from 0–23g in Tobago to 0–36g in North-West Trinidad. Tectonic considerations suggest that a maximum-moment earthquake occurring directly under either land mass could generate accelerations as high as 0–6g; the probability of occurrence of such an event is estimated to be about 2 per cent in 50 years for Trinidad and about a tenth this risk for Tobago. This level of hazard would be significant for critical facilities such as LNG plants.