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1.
Recent, fresh, volcanic rocks of the intra-oceanic Mariana and Volcano Arcs were analyzed for O and Sr isotopic compositions in order to determine the source of these magmas. Fresh, non-arc, volcanic rocks from the regions surrounding the Mariana-Volcano Arcs and some DSDP sediments were also analyzed for comparison. The oxygen isotopic ratios of the arc lavas (5.5–6.8‰) exhibited a small inter-island variation that cannot be entirely explained by fractional crystallization. The Sr isotopic composition of the arc lavas is remarkably uniform (0.70332–0.70394 for the Marianas). Three models are considered in order to explain the observed isotopic characteristics: (1) bulk mixing and melting of MORB-type mantle with (a) subducted sediments, and (b) subducted oceanic crust (excluding sediments); (2) melting of a mixture of sediment-derived fluids and MORB-type mantle; and (3) melting of a mixture of sediment-derived fluids and oceanic island or “hot-spot” type mantle. The last model fits the data best. The conclusion that very small, and variable, amounts of sediment-derived fluid ( 1%) are required to explain the observed inter-island O isotopic variation, is consistent with that of other workers who used different isotopic and trace element methods. The generation of magmas in the Mariana-Volcano Arcs involves very little sediment and the source region of Mariana lavas is isotopically indistinguishable from that of hot-spot basalts.  相似文献   

2.
The Gangdese magmatic belt is located in the southern margin of the Lhasa terrane, south Tibet. Here zircon U–Pb ages and Hf isotopic data, as well as whole‐rock geochemistry and Sr–Nd isotopes on andesites from the Bima Formation with a view to evaluating the history of the Gangdese magmatism and the evolution of the Neotethys Ocean. Zircon U–Pb dating yields an age of ca 170 Ma from six samples, representing the eruptive time of these volcanic rocks. Zircon Hf isotopes show highly positive εHf(t) values of +13 to +16 with a mean of +15.2. Whole‐rock geochemical and Sr–Nd isotopic results suggest that the magma source of these andesites was controlled by partial melting of a depleted mantle source with addition of continental‐derived sediments, similar to those in the southern arcs of the Lesser Antilles arc belt. In combination with published data, the volcanic rocks of the Bima Formation are proposed to have been generated in an intra‐oceanic arc system, closely associated with northward subduction of the Neotethyan oceanic lithosphere.  相似文献   

3.
High-pressure experiments on a natural pelite have been conducted at 2–11-GPa pressures in order to evaluate contributions of subducted sediments to arc and ocean island magmatism. Obtained phase relations suggest that, at least in modern subduction zones, subsolidus dehydration of chlorite and phengitic muscovite in the subducted sediments, rather than partial melting, is a predominant process in overprinting sediment components onto the magma source region. Trace element compositions of sediment-derived fluids are estimated based on dehydration experiments at 5.5 GPa and 900/1300°C. Pb is effectively transported by fluids relative to other elements. This results in the Pb enrichment for arc basalts by fluids, generated by the dehydration of subducted sediments, together with altered mid-ocean ridge basalt (MORB), and complementary depletion of Pb in subducted sediments. Inferred arc magma compositions obtained by model calculations based on the present experimental results agree well with a natural primitive arc basalt composition. A large increase in the U/Pb ratio in the subducted sediments at deeper levels than major dehydration depths results in a high Pb isotopic ratio through radioactive decay after long periods of isolation. Combined with other isotopic ratios such as Sr and Nd, it is possible to produce the EM II source, one of the enriched geochemical reservoirs for ocean island basalt magmas, by mixing of a small amount of subducted sediments with depleted or primitive mantle.  相似文献   

4.
Analytical results of the relative and absolute abundance of LIL-incompatible trace elements (K, Rb, Cs, Sr, and Ba) and isotopic compositions ( , , and ) are summarized for fresh samples from active and dormant volcanoes of the Volcano and Mariana island arcs. The presence of thickened oceanic crust (T 15–20 km) beneath the arc indicates that while hybridization processes resulting in the modification of primitive magmas by anatectic mixing at shallow crustal levels cannot be neglected, the extent and effects of these processes on this arc's magmas are minimized. All components of the subducted plate disappear at the trench. This observation is used to reconstruct the composition of the crust in the Wadati-Benioff zone by estimating proportions of various lithologies in the crust of the subducted plate coupled with analyses from DSDP sites. Over 90% of the mass of the subducted crust consists of basaltic Layers II and III. Sediments and seamounts, containing the bulk of the incompatible elements, make up the rest. Bulk Western Pacific seafloor has , δ 18O +7.2, K/Rb 510, K/Ba 46, and K/Cs 13,500. Consideration of trace-element data and combined systematics limits the participation of sediments in magmagenesis to less than 1%, in accord with the earlier results of Pb-isotopic studies. Combined data indicate little, if any, involvement of altered basaltic seafloor in magmagenesis. Perhaps more important than mean isotopic and LIL-element ratios is the restricted range for lavas from along over 1000 km of this arc. Mixtures of mantle with either the subducted crust or derivative fluids should result in strong heterogeneities in the sources of individual volcanoes along the arc. Such heterogeneities would be due to: (1) gross variations of crustal materials supplied to the subduction zone; and (2) lesser efficiency of mixing processes accompanying induced convection between arc segments (parallel to the arc) as compared to that perpendicular to the arc. The absence of these heterogeneities indicates that either some process exists for the efficient mixing of mantle and subducted material parallel to the arc or that subducted materials play a negligible role in the generation of Mariana-Volcano arc melts.Consideration of plausible sources in the mantle indicates that (1) an unmodified MORB-like mantle cannot have generated the observed trace-element and isotopic composition of this arc's magmas, while (2) a mantle similar to that which has produced alkali-olivine basalts (AOB) of north Pacific “hot spot” chains is indistinguishable in many respects spects from the source of these arc lavas.  相似文献   

5.
himu, em i andem ii are three of the main geochemical mantle components that give rise to oceanic island basalts [1]. They represent the end members that produce the extreme isotopic compositions measured on intraplate volcanics. In French Polynesia, all three mantle components are represented in volcanic rocks. The characteristichimu signature is found in Tubuai, Mangaia and Rurutu,em i is present in the source of Rarotonga and Pitcairn volcanics andem ii dominates the composition of most Society Islands. Intermediate values between the three end members are found on most islands.We suggest that the three components are not independent but are physically related in the mantle. Thehimu component is thought to be recycled oceanic crust that lost part of its Pb through hydrothermal processes prior to and during subduction.em i andem ii are believed to acquire their isotopic and trace element characteristics through entrainment of sediments that were subducted together with the oceanic crust.The trace element pattern and the isotopic composition ofhimu lavas can be quantitatively modelled using a mixture of 25% old recycledmorb crust and 75% mantle peridotite. The extreme Pb composition is modelled assuming that Pb was lost from oceanic crust when hydrothermal alteration at the ridge leached Pb from the basalt to redeposit it as sulphides on top of and throughout the crust, followed by preferential dissolution of sulphides during dehydration in the subduction zone. These processes led to a drastic increase of theU/Pb ratio of the subducted material which evolved over 2 Ga to very radiogenic Pb isotopic compositions. Pb isotopic compositions similar to those ofem i andem ii are modelled assuming that sediments with average crustal Pb isotopic compositions were subducted and recycled into the mantle together with the underlyingmorb oceanic crust. Pelagic sediments (μ 5 andκ 6) account for the Pb isotopic composition ofem i whereas terrigenous sediments (μ 10 andκ 4.5) evolve towards theem ii end member. A few percent of sediment in the recycled crust-sediment mixture will destroy the characteristic Pb isotopic signature of thehimu component. This, together with the low probability of isolating oceanic crust in the mantle for 2 Ga, explains why the extremehimu composition, as seen on Tubuai and St Helena, is sampled so rarely by oceanic volcanism.  相似文献   

6.
The geology, petrology, and petrogenesis of Saba Island, Lesser Antilles   总被引：1，自引：0，他引：1  
Saba is the northernmost volcano along the Lesser Antilles island-arc chain. The Lesser Antilles arc results from the west-northwest subduction of the Atlantic lithosphere beneath the Caribbean Plate. Sediment thickness along the trench decreases northward away from sediment sources on the continent of South America. We focused our attention on Saba precisely because it is the furthest away from documented geochemical effects in the southern arc volcanics of the large sediment thicknesses — normally attributed to both source or upper level contamination (i.e. assimilation).Field mapping, petrology, mineralogy, K–Ar dating, and geochemical analyses (major and trace element) indicate a complex history of magma petrogenesis including crystal fractionation, magma mixing, and, surprisingly, crustal assimilation. This is the first time assimilation has been documented in the northern section of the Lesser Antilles arc. Magma mixing shows up in the field as banded pumice and petrographically and mineralogically as complex zoning in phenocrysts (such as reverse zoning in plagioclase), disequilibrium mineral assemblages (e.g. quartz and olivine), and disequilibrium between minerals and whole-rock compositions (e.g. forsterite content of olivine). Mass-balance modeling of major and trace elements support our contention that crystal fractionation (including amphibole) played an important role in magma evolution. However, various geochemical trends can only be explained by assimilation-fractional crystallization based on the fact that the trends of various trace elements and trace-element ratios vary with increasing silica. Finally, we could find no evidence of sediment source contamination in the most mafic rocks. It may exist but is overprinted by the later assimilation effects.  相似文献   

7.
Late Miocene (7–9 Ma) basaltic rocks from the Monbetsu‐Kamishihoro graben in northeast Hokkaido have chemical affinities to certain back‐arc basin basalts (referred to herein as Hokkaido BABB). Pb‐, Nd‐ and Sr‐isotopic compositions of the Hokkaido BABB and arc‐type volcanic rocks (11–13 Ma and 4–4.5 Ma) from the nearby region indicate mixing between the depleted mantle and an EM II‐like enriched component (e.g. subducted pelagic sediment) in the magma generation. At a given 87Sr/86Sr, Hokkaido BABB have slightly lower 143Nd/144Nd and slightly less radiogenic 206Pb/204Pb compared with associated arc‐type lavas, but both these suites are difficult to distinguish solely on the basis of isotopic compositions. These isotopic data indicate that while generation of the Hokkaido BABB involves smaller amounts of the EM II‐like enriched component than do associated arc lavas, Hokkaido BABB are isotopically distinct from basalts produced at normal back‐arc basin spreading centers. Instead, northeast Hokkaido BABB are more similar to basalts erupted during the initial rifting stage of back‐arc basins. The Monbetsu‐Kamishihoro graben may have developed in association with extension that formed the Kurile Basin, suggesting that opening of the basin continued until late Miocene (7–9 Ma).  相似文献   

8.
New lead isotope data for calc-alkaline volcanic rocks from New Zealand and the Lesser Antilles, combined with published data for Japan and the Andes, show that the spread of isotopic composition in each volcanic arc region is small (2–4% range in Pb206/Pb204) compared to the range of values observed (8%). Pb207 and Pb206 increase systematically from Japan to the Andes to New Zealand to the Caribbean. Likewise Pb208 and Pb206 are positively correlated, but there is evidence of long term (108 m.y.) differences of Th/U between the regions studied. The apparent U/Pb ratios of Peruvian, New Zealand and Caribbean calc-alkaline volcanics do not differ greatly from the apparent ratio for the single stage growth curve for stratiform Pb ores. In contrast the apparent U/Pb ratios for Japanese calc-alkaline volcanics are distinctly lower. Although the Japanese Pb has model ages near zero, the other volcanic arcs have negative (future) model ages, the Caribbean samples being most extreme in this respect. Published oceanic volcanic and sediment lead isotopic composition data and the new results are consistent with a model of volcanic arc evolution in which oceanic sediments are dragged into the mantle, mixed to some degree with mantle material, and partially melted to form calc-alkaline magmas. Either constant continental volume or continental growth are compatible with this process. The mixing of two separate « frequently mixed » leads is the minimum complexity required to explain volcanic are leads. Mathematically there are probably no single-stage leads but isotopic homogenization during earth history has caused lead isotopes to closely approximate a single stage growth. The use of lead isotopic composition as a « tracer » suggests that mantle — crust geochemical evolution involves an exchange of material and is not simply a one-way process. The Pb isotopic composition of the Auckland, New Zealand alkali basalts is apparently the result of incomplete mixing of two leads to give a linear array of Pb207/Pb204-Pb206/Pb204 data with negative slope.  相似文献   

9.
This paper is concerned with the islands of Montserrat Nevis, St. Kitts, St. Eustatius and Saba, which lie on the inner volcanic are at the northern of the Lesser Antilles. Andesites greatly predominate over basalts and dacites in this part of the arc. Generally the lavas from the northern Lesser Antilles contain low abundances of Ni, Cr and residual trace elements but lavas from Saba are enriched in these elements compared with the other islands in the group. The most important petrogenetic process in this part of the Lesser Antilles is probably partial melting of subducted oceanie tholeiite and this process accounts satisfactorily for the chemistry (especially the low Ni, Cr) and large volumes of the erupted andesites. Some andesites have, however, been produced by fractional crystallisation of basaltic magma and magma mixing probably accounts for some of the peculiar chemical and petrographic properties of the Saba andesites. The rocks from the Northern Lesser Antilles are different from those in the central part of the arc (more acid rocks, higher residual trace elements) and the southern islands have much higher proportions of basalt, some of it undersaturated and alkaline. It is thought that partial melting of mantle peridotite may be the predominant petrogenetic process at the southern end of the Lesser Antilles whereas partial melting of subducted oceanic crust is more important in the north.  相似文献   

10.
Lithium isotopes have been identified as a promising tracer of subducted materials in arc lavas due to the observable variations in related reservoirs such as subducting sediments and altered oceanic crust. The Tonga–Kermadec arc–Lau back‐arc provides an end‐member of subduction zones with the coldest thermal structure on Earth. Reported here are Li isotope data for 14 lavas from the arc front and 7 back‐arc lavas as well as 12 pelagic and volcaniclastic sediments along a profile through the sedimentary sequence at DSDP Site 204. The arc and back‐arc lavas range from basalts to dacites in composition with SiO2 = 48.3–65.3 wt% over which Li concentrations increase from 2 ppm to 16 ppm. Li/Y ratios range from 0.08 to 0.77 and from 0.24 to 0.65 in the arc and back‐arc lavas, respectively. The majority of the lavas have δ7Li that ranges from 2.5 ‰ to 5.0 ‰ with an average of (3.6 ±0.7) ‰, similar to that reported from other arcs and there is no distinction between the arc front and back‐arc lavas. The pelagic sediments have variable Li concentrations (33–133 ppm) and δ7Li that ranges from 1.2 ‰ to 10.2 ‰ while the volcaniclastic sediments have an even greater range of Li concentrations (3.6–165 ppm) and generally higher δ7Li values (8–14 ‰). However, δ7Li in the lavas does not correlate with commonly used trace element ratio or isotope signatures indicative of slab‐derived fluids or the sediments. This is probably because the range of δ7Li in the lavas and sediments overlap. Calculated sediment mass‐balance models require significantly more sediment than previous estimates based on Th–Nd–Be isotopes. This may indicate that a sizeable proportion of the total Li budget in the lavas is provided by Li‐enriched fluids from the subducting sediments and/or altered oceanic crust.  相似文献   

11.
Summary In 1976 and 1977, seismic profiles were carried out in Guadeloupe. Two profiles were established in the area of La Soufriére volcano and one profile through the northern part of Guadeloupe and southern part of Grande Terre. The two first profiles were occupied from 1 to 30 km and the third profile between 5 and 50 km.The interpretation shows that the superficial structures are characterized by a three-layers model: the compressional velocity is about 2.7 to 3.0 km/s down to a depth from 1 to 3 km. Below this, the velocity is between 4.0 and 4.5 km/s in a layer whose thickness varies from 1 to 2.5 km. Under this layer we find a 6.0–6.1 km/s layer which is one of the two known crustal layer under Lesser Antilles. The boundary between the old and new are which form the Lesser Antilles arc, is marked by a thicker layer of sediments on the eastern flank of recent volcanic chain.
  相似文献   

12.
Pb, Nd and Sr isotope compositions of oceanic basalts have been used to identify recycled components of continent derivation in the mantle. The isotopic compositions of Sr, Nd and Pb, together with U, Pb, Sm, Nd, Rb, and Sr abundances have been determined for back-arc basalt glasses from the Scotia Sea and Parece Vela and West Philippine Basins, in addition to basalts from South Sandwich Islands, Ascension, St. Helena and Tristan da Cunha. Comparisons made between the isotopic compositions of South Sandwich Islands basalts and Atlantic MORB glasses permit the identification of recycled components of continent derivation in the source of the island arc basalts. Recycled Sr of continent derivation is also recognisable in back-arc basalt glasses from the Scotia Sea and Parece Vela and West Philippine Basins. However, contemporary reinjection of material with the isotopic structures similar to those identified as a component of island arc and back-arc regions cannot be the sole or dominant influence on the fine structure observed in MORB glasses from the Atlantic Ocean, nor the isotopic compositions of Tristan da Cunha, St. Helena and Ascension basalts. Recycled materials are likely to have been responsible for the generation of these heterogeneities only if they have been stored in the mantle for periods of time exceeding 109 years.  相似文献   

13.
At two locations in the Atlantic Ocean (DSDP Sites 367 and 530) early to middle Cretaceous organic-carbon-rich beds (“black shales”) were found to have significantly lower δ15N values (lower15N/14N ratios) than adjacent organic-carbon-poor beds (white limestones or green claystones). While these lithologies are of marine origin, the black strata in particular have °15N values that are significantly lower than those previously found in the marine sediment record and most contemporary marine nitrogen pools. In contrast, black, organic-carbon-rich beds at a third site (DSDP Site 603) contain predominantly terrestrial organic matter and have C- and N-isotopic compositions similar to organic matter of modern terrestrial origin.The recurring15N depletion in the marine-derived Cretaceous sequences prove that the nitrogen they contain is the end result of an episodic and atypical biogeochemistry. Existing isotopic and other data indicate that the low15N relative abundance is the consequence of pelagic rather than post-depositional processes. Reduced ocean circulation, increased denitrification, and, hence, reduced euphotic zone nitrate availability may have led to Cretaceous phytoplankton assemblages that were periodically dominated by N2-fixing blue-green algae, a possible source of this sediment15N-depletion. Lack of parallel isotopic shifts in Cretaceous terrestrially-derived nitrogen (Site 603) argues that the above change in nitrogen cycling during this period did not extend beyond the marine environment.  相似文献   

14.
Two extensive marine tephra layers recovered by piston coring in the western equatorial Atlantic and eastern Caribbean have been correlated by electron microprobe analyses of glass shards and mineral phases to the Pleistocene Roseau tuff on Dominica in the Lesser Antilles arc. Tephra deposition and transport to the deep sea was primarily controlled by two processes related to two different styles of eruptive activity: a plinian airfall phase and a pyroclastic flow phase. A plinian phase produced a relatively thin (1–8 cm) airfall ash layer in the western Atlantic, covering an area of 3.0 × 105 km2 with a volume of 13 km3 (tephra). The majority of the airfall tephra was transported by antitrade winds at altitudes of 6–17 km. Aeolian fractionation of crystals and glass occurred during transport resulting in an airfall deposit enriched in crystals relative to the source. Mass balance calculation based on crystal/glass fractionation indicates an additional 12 km3 of airfall tephra was deposited outside the observed fall-out envelope as dispersed ash.Discharge of pyroclastic flows into the sea along the west coast of Dominica initiated subaqueous pyroclastic debris flows which descended the steep western submarine flanks of the island. 30 km3 of tephra were deposited by this process on the floor of the Grenada Basin up to 250 km from source. The Roseau event represents the largest explosive eruption in the Lesser Antilles in the last 200,000 years and illustrates the complexity of primary volcanogenic sedimentation associated with a major explosive eruption within an island arc environment.  相似文献   

15.
In order to understand the role of the subducted lithosphere in producing the geochemical characteristics of arc magmas, major- and trace-element along with Sr- and Nd-isotope compositions have been determined for Quaternary volcanic rocks from the Izu-Bonin intra-oceanic arc. 87Sr/86Sr and 143Nd/144Nd ratios decrease away from the volcanic front of this arc and lie on mixing lines between the assumed isotopic compositions of fluid phases mainly derived from the basalt layer of the subducted lithosphere and upper-mantle materials in the sub-arc wedge. This across-arc variation can be explained through a simple sequence of processes involving initial release of fluid phases from the subducted oceanic crust to produce hydrous peridotite at the base of the mantle wedge. This hydrous peridotite is dragged downward with the slab and releases a second-stage metasomatizing fluid beneath the volcanic arc. The higher concentrations of both Sr and Nd in the fluid beneath the volcanic front than those beneath the back-arc side may be a possible cause of the observed across-arc variation in Sr-Nd isotopic ratios. The difference in compositions of fluid phases is attributed to the different hydrous phases which decompose in the hydrous peridotite layer; amphibole beneath the volcanic front and phlogopite beneath the back-arc side of the volcanic arc. The mineralogically controlled fluid addition may also be responsible for the across-arc variation in Rb/K and Rb/Zr ratios, increasing away from the volcanic front.  相似文献   

16.
Lower Carboniferous lavas from the Midland Valley and adjacent regions of Scotland are mildly alkaline and intraplate in nature. The sequence is dominated by basalt and hawaiite, although mugearite, benmoreite, trachyte and rhyolite are also present. Basic volcanic rocks display the LIL element and LREE enrichment typical of intraplate alkali basalt terrains. Low initial87Sr/86Sr (0.7029–0.7046), high εNd (−0.4 to +5.6) and moderately radiogenic206Pb/204Pb (17.77–18.89) ratios are also comparable with alkali basalts from other continental rifts and oceanic islands.When the Carboniferous lavas are compared with subduction-related lavas of Old Red Sandstone age, erupted in and around the Midland Valley ca. 50 Ma earlier (at 410 Ma) remarkable similarities are apparent. Significant overlap occurs in Nd and Pb isotopic compositions. Sr isotopic compositions are, however, more radiogenic in the older subduction-related lavas. This, combined with high K and Rb concentrations in ORS lavas may be explained by the incorporation of a sediment component derived from the subducted slab, which by Lower Carboniferous times had been lost from the mantle source region by convection. A pronounced negative Nb anomaly in the ORS subduction-related lavas may be explained by the retention of a Nb-bearing phase in the mantle during hydrous melting of the mantle wedge above the subduction zone.Allowing for the effects of the added component from the subducted slab, there appears to be no necessity to invoke separate mantle source regions for the two suites of lavas: both may have been derived from chemically similar portions of mantle. If volcanic arc lavas are derived from the mantle wedge, the implication is that such a source lies at relatively shallow depth within the upper mantle: the same may therefore apply to the Carboniferous continental rift basalts. This evidence, combined with the fact that there is no evident hot-spot trail across the Midland Valley despite a long period of within-plate volcanism and rapid plate movements during the Carboniferous, suggests that the alkali basalt magmatism is not the product of a deep-seated mantle plume. Rather, the volcanism appears to owe more to passive rifting and to diapiric upwelling from a source region within the uppermost mantle.  相似文献   

17.
We have relocated the twenty-eight largest magnitude (4.3M s 7.3) historical (1922–1963) earthquakes of the southeastern Caribbean. We also present new focal mechanisms for seven of these events. The relocations are based on reported ISSP andS arrival times that we analyzed using generalized linear inversion techniques. The new focal mechanisms were constrained by first motionP polarities as reported by the ISS and as picked by us where records were available, and by the polarities and ratios ofSH andsSH, andSV andsSV arrivals that we determined from seismograms. The results of the relocations are commensurate with the distribution of seismicity observed in the recent era: hypocenters are shallow and intermediate in depth (0–200 km), and the events occur almost exclusively in areas known to be currently seismic. The frequent seismic activity in the vicinity of the Paria Peninsula, Venezuela, is clearly a persistent feature of the regional earthquake pattern; intermediate depth earthquakes indicative of subduction beneath the Caribbean plate occur here and along the Lesser Antilles arc. The Grenadines seismic gap is confirmed as an area of low seismic moment release throughout the historical era. Trinidad and the eastern Gulf of Paria were also largely quiescent.The new focal mechanisms, despite being a sparse data set, give significant insight into both subduction processes along the Lesser Antilles arc and into the shallow deformation of the Caribbean-South America plate boundary zone. The largest earthquake to have occurred in this region, the 19 March 1953 event (M m =7.01), is a Lesser Antilles slab deformation event, and another earthquake in this region of the Lesser Antilles is probably a rarely-observed interplate thrust event. Shallow deformation in the plate boundary zone is complex and, near the Paria Penninsula, involves mixed southeastward thrusting and dextral strike-slip on east-striking faults, and secondarily, normal faulting. Bending of the subducting Atlantic-South American plate also seems to generate seisms. The rather high ratio of intraplate deformation to interplate deformation observed along the Lesser Antilles subduction zone in the more recent era seems to have been operative in the historical era as well.  相似文献   

18.
Comparisons are made between the Lesser Antilles and the South Sandwich Islands, the recent volcanic island chains at the eastern margins of the Caribbean and Scotia arcs. Although situated in similar geological and structural environments there are differences in the type of volcanic activity which prevails in these two arcs and in the petrography and chemistry of the lavas emitted. There is good evidence that the South Sandwich Islands are in general appreciably younger than the islands of the Lesser Antilles. Basaltic rocks predominate in the South Sandwich Islands whereas andesite is the dominant rock-type of the Lesser Antilles. Many of the lavas of the South Sandwich Islands, including the andesites and dacites are aphyric whereas those of the Lesser Antilles are almost invariably porphyritic. The basalts of the South Sandwich Islands are of tholeiitic type and the series shows more pronounced iron enrichment than does that of the Lesser Antilles. Basalts of the South Sandwich Islands have a lower Fe2O3/FeO ratio, contain lower concentrations of K, Sr and Ba and higher Cr, Co and Ni than the basalts of the West Indies. It is thought that the South Sandwich Islands may represent a volcanic island-arc in the early stages of development and the Lesser Antilles a later stage.  相似文献   

19.
143Nd/144Nd,87Sr/86Sr and REE results are reported on volcanic rocks from the islands of Dominica and St. Kitts in the Lesser Antilles. Particular attention is given to the lavas and xenoliths of the Foundland (basalt-andesite) and the Plat Pays (andesite-dacite) volcanic centres on Dominica. Combined major and trace element [2] and isotope results suggest that the bulk of the andesites and dacites on Dominica, and by analogy in the rest of the arc, are produced by fractional crystallisation of basaltic magma. The differences in the erupted products of the two volcanoes do not appear to be related to any significant differences in the source rocks of the magmas.Along the arc87Sr/86Sr ratios range from 0.7037 on St. Kitts, to 0.7041–0.7047 on Dominica, and 0.7039–0.7058 on Grenada [5], and these are accompanied by a parallel increase in K, Sr, Ba and the light REE's. Moreover, compared with LIL-element-enriched and -depleted rocks from MOR and intraplate environments, the basic rocks from the Lesser Antilles are preferentially enriched in alkaline elements (K, Ba, Rb, Sr) relative to less mobile elements such as the rare earths.143Nd/144Nd varies from 0.51308 on St. Kitts, to 0.51286 on Dominica, and 0.51264–0.51308 on Grenada [5], and all these samples have relatively high87Sr/86Sr ratios compared with the main trend of Nd and Sr isotopes for most mantle-derived volcanic rocks. Alkaline elements and87Sr appear to have been introduced from the subducted ocean crust, but the results on other, less mobile elements are more ambiguous — island arc tholeiites (as on St. Kitts) do not appear to contain significant amounts of REE's, Zr, Y, etc., from the subducted oceanic crust, but such a contribution may be present in more LIL-element-enriched calc-alkaline rock types.  相似文献   

20.
In an earlier study of Mesozoic and Cenozoic mineralization in Central America and the Caribbean region, we found that lead isotopic compositions of deposits in northern Central America, which is underlain by a pre-Mesozoic craton, ranged to higher206Pb/204Pb and207Pb/204Pb compositions than did deposits from elsewhere in the region, where the basement is Mesozoic oceanic material. Using 16 analyses for 12 new deposits, as well as new analyses for 11 of the samples studied previously, we have found that lead isotopic compositions correlate closely with crustal type but show little or no correlation with depth to the M-discontinuity. The deposits are divisible into three main groups including (in order of increasing207Pb/204Pb and208Pb/204Pb ratio): (1) deposits in southern Central America and all deposits in the Greater Antilles except Cuba; (2) all deposits in northern Central America; and (3) the Cuban deposits. Southern Central American and Caribbean lead is higher in207Pb/204Pb and208Pb/204Pb than most mid-ocean ridge basalts but could have been derived directly or indirectly from undepleted mantle. Northern Central America can be divided into the Maya block, which belongs to the Americas plate, and the Chortis block, which belongs to the Caribbean plate. Maya block deposits fall along a linear array whereas those of the Chortis block (except the Monte Cristo deposit) form a cluster. These results suggest that the Maya block is underlain by crust or mantle with a large range of U/Pb and Th/U ratios, whereas the Chortis block basement is more homogeneous. Two-stage model calculations indicate an age of about 2280±310 m.y. for the Maya block basement, although no such rocks are known in the region. Comparison of the Chortis block data to our recently published lead isotopic analyses of Mexican deposits shows considerable similarities suggesting that the Chortis block could have been derived from Mexico.  相似文献   

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