共查询到20条相似文献,搜索用时 10 毫秒
1.
87Sr/ 86Sr ratios in the island-arc tholeiite series from the South Sandwich Islands cluster about a value of 0.7040. There is no apparent correlation of strontium isotope values with any major chemical component or with Rb/Sr ratios. The uniformity of the 87Sr/ 86Sr values is consistent with the fractional crystallization relationship previously proposed for this suite. Though higher than values reported for ocean-floor tholeiites they are not significantly different from ratios reported for calc-alkaline island-arcs. 相似文献
2.
The Pampean Ranges of northwest Argentina are a basin-and-range tectonic province with a late Precambrian to Paleozoic basement and extensive Miocene-Recent calc-alkaline volcanism. The volcanoes include the large resurgent Cerro Galan caldera, and Recent scoria cones and lava flows. Miocene-Recent volcanic rocks of basalt to dacite composition from the Cerro Galan area exhibit a range of Rb/Sr ratios of 0.043–1.092 and initial 87Sr/ 86Sr ratios of 0.7057–0.7115 with a clear positive correlation between 87Sr/ 86Sr and 87Rb/ 86Sr, indicating an apparent age of ca. 130 Ma. This relationship is interpreted to indicate that the Sr isotope variation in the Cerro Galan volcanic rocks results from mixing of a mantle-derived component with low 87Sr/ 86Sr (<0.7057) and high Sr (>700 ppm) with a crustal component characterized by higher 87Sr/ 86Sr (>0.7115) and lower Sr (<240 ppm). It is concluded that the mixing is best explained as a result of a small degree of selective crustal Sr contamination (ca. 10%) of a range of subsequently erupted magmas produced largely by fractional crystallization within the continental crust. We propose that the mantle-derived end-member is derived by partial melting of sub-Andean mantle with an 87Sr/ 86Sr ratio of ca. 0.704, and that such an Sr isotope ratio characterizes the source region for calc-alkaline volcanic rocks throughout the Andes. 相似文献
3.
The Aegean volcanic arc formed in response to northeasterly subduction of the Mediterranean sea floor beneath the Aegean Sea. The active arc lies over 250 km from the Hellenic Trench in a region which has suffered considerable extension and subsidence since the mid-Tertiary. Suites of samples from the different volcanic centres making up the arc have been studied geochemically in order to assess lateral variations and to constrain the contribution of crustal contamination and sediment subduction in their petrogenesis.Lavas from all the major volcanic centres exhibit typical calc-alkaline major-element characteristics, and show enrichment in light REE and LIL elements but low contents of HFS elements. The enrichment in light REE is greater in the eastern (Nisyros, Kos) and western (Milos, Poros, Methana, Aegina) sectors of the arc (Ce n/Yb n=4) than in the central Santorini sector (Ce n/Yb n=2). All lavas have significant negative Eu anomalies and many have slight negative Ce anomalies. Less coherence is observed in the abundances and ratios of the other LIL elements, compared with the REE, along the island chain.Whereas the effects of crystal fractionation are evident in the trace-element patterns of lavas from individual islands, and are particularly well marked for Santorini, it is clear that there are consistent differences in trace-element abundances and ratios in the lavas of the various islands which reflect compositional differences in the mantle source and/or in melting conditions. Lavas from the eastern and western sectors have much higher levels of Ba and Sr but relatively lower Th, K and Rb than those from Santorini. Although some geochemical features could be explained through involvement of a component of subducted sediment in the source regions of the volcanoes, other element abundances and ratios indicate that this component must be very small. Detailed consideration of the inter-island geochemical variations suggests a complex make-up of the underlying lithosphere, resulting from a long history of subduction. In the region of Santorini, where crustal stretching is greatest, the underlying asthenosphere may be involved in magma production. 相似文献
4.
Abstract Recent advanced chronological studies for the Tertiary volcanic rocks from the Northeast (NE) Japan arc revealed three volcanic fronts which differed in temporal and spatial distribution. These fronts were (i) the Matsumae-Shizukuishi-Shiogama line of 22–25 Ma which is obliquely across the Quaternary volcanic front (QVF); (ii) the Tomari-Shiogama line of 13–16 Ma which exists 30–50 km east of the QVF and (iii) a line of 0–8 Ma which is the same as the QVF. The first shifting of the 22–25 Ma line to the 13–16 Ma one was due to the counterclockwise rotation of the NE Japan arc during 20–12 Ma as proposed by Otofuji et al . (1985), and the second shifting of the 13–16 Ma line to the 0–8 Ma line could have contributed to a decrease in the dip of the slab of the Pacific plate which subducted beneath the NE Japan arc during 13–8 Ma. 相似文献
5.
Strontium isotopic analyses are reported for ophiolitic and associated rocks of Mesozoic and Tertiary age from the Drocea Mountains. The samples and their average Sr 87/Sr 86 and Rb/Sr ratios, in order, include: ( a) ultramafics (partly serpentinized or uralitized peridotite, peridotite-melagabbro). 0.7043, 0.106; ( b) gabbros, dolerites and anemasites (including magnetite-bearing, quartz dolerite, hornblende and normal gabbros), 0.7032, 0.021; ( c) basalts (amygdaloidal, hyalopyroclasite), 0.7030, 0.040; and ( d) granophyric and albitic vein rocks, 0.7046, 0.058. Also analyzed were ( e) basalt-spilites of a younger intrusive cycle 0.7042, 0.046 ( f) banatites, 0.7064, 0.542. Two Quaternary volcanics were analyzed from outside the Drocea Mountains: ( g) augite-hypersthene andesite from Mt. Gut?i, 0.7083, 0.247, and olivine basalt from Raco?, 0.7043, 0.056. The data for the ophiolite suite show highest Sr 87/Sr 86 and Rb/Sr ratios in the ultramafics which suggests a two-stage origin with the ultramafics derived from a more primitive mantle than the later gabbros and basalts. Initial Sr 87/Sr 86 ratios range from 0.7021 to 0.7045 in gabbro and basalt and 0.7035 to 0.7056 in peridotite which is well within the limits found in oceanic tholeiites and suggests an origin for the complex as a spreading oceanic ridge. Cross-cutting felsic granophyric and albititic rocks as well as the late-stage basalts ( a) have relatively low Sr 87/Sr 86 and Rb/Sr ratios, ( b) represent a small volume, and ( c) are intimately related to the ophiolites. They appear to have developed largely by late-stage differentiation and fractional crystallization of a tholeiitic magma. The higher ratios for the banatites and andesite from Mt. Gut?i suggest that significant amounts of sialic crustal material were involved in their formation. The basalt from Raco? is from the vicinity of a deep fracture zone; its relatively low Sr 87/Sr 86 ratio suggests a direct link to a mantle source with little or no crustal contamination. 相似文献
6.
Analyses of fifty-one rock samples from three stratovolcanoes in Central Japan revealed that K and Rb contents vary in a saw-toothed fashion with the growth of these volcanoes. Peaks and valleys of the saw-toothed variation pattern of Rb (and also K) increase at first and then gradually converge on constant values. This variation trend is also shown by the Rb/Sr ratio. The convergent Rb/Sr ratio (0.23–0.24) at the peaks coincides with recent estimates of the average value for continental crust. These geochemical features are well explained by the batch fractionation model. In this model, the magma reservoir lying at the top of the mantle is periodically supplied with a batch of parental magma, while the magma in it undergoes continuous crystallization and the cumulate is continuously removed by the divergent movement of the mantle. This model, working under physical conditions in the crust-mantle structure of an island arc, not only accounts for the above geochemical features, but also gives insight into the genesis of the calc-alkaline rock series and of the continental crust. 相似文献
7.
3He/4He ratios have been measured for olivine and clinopyroxene phenocrysts in 7–15 m.y. old basaltic lavas from the island of St. Helena. Magmatic helium was effectively resolved from post-eruptive radiogenic helium by employing various extraction techniques, including in vacuo crushing, and stepwise heating or fusion of the powders following crushing. The inherited 3He/4He ratio at St. Helena is 4.3–5.9 R A. Helium isotope disequilibrium is present within the phenocrysts, with lower 3He/4He upon heating and fusion of the powders following crushing, due to radiogenic ingrowth or to -particle implantation from the surrounding (U + Th)-rich lavas. A single crushing analysis for clinopyroxene in a basalt from Tubuaii gave3He/4He= 7.1 RA.3He/4He ratios at St. Helena and Tubuaii (HIMU hotspots characterized by radiogenic Pb isotope signatures) are similar to3He/4He ratios previously measured at Tristan da Cunha and Gough Island (EM hotspots characterized by low206Pb/204Pb). Overall, the HeSrPb isotope systematics at these islands are consistent with a mantle origin as contiguous, heterogeneous materials, such as recycled crust and/or lithosphere.3He/4He ratios at HIMU hotspots are similar to mantle xenoliths which display nearly the entire range of Pb isotope compositions found at ocean islands, and are only slightly less than values found in mid-ocean ridge basalts (7–9 RA). This suggests that the recycled materials were injected into the mantle within the last 109 yrs. 相似文献
8.
Petrological evolution of the Tertiary island arc in the Izu-Mariana region has been accompanied by the development of three different volcanic suites: 1) oceanridge basalt now exposed as the metamorphic basement on Yap; 2) island-arc tholeiites of Eocene to early Oligocene age characterized by low contents of incompatible elements at all levels of silica enrichment; and 3) calc-alkalic rocks of late Oligocene to early Miocene age showing higher contents of silica and incompatible elements. All these three suites have primitive, undifferentiated basalts or andesites (boninites) characterized by high Mg/Fe, Cr, and Ni, suggesting that they have been derived from an upper mantle peridotite at relatively high temperatures. The earliest volcanism appears to have occurred at a spreading ridge. Later, as subduction proceeded, the island-arc tholeiite magma may have been produced by the introduction of a smaller amount of water into the locus of fusion from the subducted oceanic crust. An increasingly larger amount of water introduced into the same region could have led to the development of the more siliceous, calc-alkalic magma, as represented typically by the boninite. 相似文献
9.
Measurements of stable isotope compositions and water contents of boninite series volcanic rocks from the island of Chichi-jima, Bonin Islands, Japan, confirm that a large amount (1.6–2.4 wt.%) of primary water was present in these unusual magmas. An enrichment of 0.6‰ in 18O during differentiation is explained by crystallization of 18O-depleted mafic phases. Silicic glasses have elevated δ18O values and relatively low δD values indicating that they were modified by low-temperature alteration and hydration processes. Mafic glasses, on the other hand, have for the most part retained their primary isotopic signatures since Eocene time. Primary δD values of −53 for boninite glasses are higher than those of MORB and suggest that the water was derived from subducted oceanic lithosphere. 相似文献
10.
Two older granitic rocks and some selected Quaternary volcanic rocks from the Japanese Islands were analyzed in a reconnaissance study for the purpose of examining the relationships between Nd and Sr isotopic abundances and the megatectonic structure around the Japanese Islands. Model ages of ~0.9 AE were determined by the Nd and Sr methods on a Paleozoic gneiss which confirms that a relatively ancient acidic basement exists in the Japanese Islands. The Nd and Sr isotopic data show that the Cretaceous granodiorite is the result of partial melting of older crust.The Nd of tholeiitic rocks from the Izu arc gives ε Nd ranging from 8.3 to 9.3 and with the corresponding ε Sr from ?14.5 to ?18.5. These results are identical to those found for the Mariana arc. These values are distinctly lower than typical MORB by around 1~2 εu. This difference in ε Nd between arcs and MORB is attributed to the contribution of oceanic sediments to the partial melts produced during subduction of oceanic crust. The Hakone volcano is clearly confirmed as belonging to an oceanic source by Nd isotopic results.ε Sr-ε Nd values of the volcanics from a section along the Fossa Magna show a clear indication that they are a blend of oceanic mantle material and continental crustal material. The crustal component clearly increases in going from south to north. Volcanics across the Northeast Japan arc also show a distinct correlation of ε Sr-ε Nd related to the position relative to the active subduction zone but with the opposite trend. These relationships of the present isotopic pattern and the zonal arrangement relative to the subduction zone suggest the former existence of a local spreading center in the Japan Sea.In general there appear to be regular isotopic relationships between the Izu-Mariana oceanic island arc and the continental island arc of Japan which indicates that partially melted or assimilated older continental basement is admixed with young rising oceanic arc magmas. 相似文献
12.
The purpose of this work is to refine our knowledge about the nature of helium with a high abundance of the rare isotope 3He( 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 of 3He in the earth due to fission and nuclear reactions are considered critically. The most probable source of the high abundance of 3He is shown to be due to the capture and trapping of primordial He by the earth during its formation (primordial helium 3He/ 4He= 3 × 10 ?4), a small but significant fraction of which has been retained to the present time. 相似文献
14.
Eruptive suites from Tonga (tholeiitic), Raoul Island (tholeiitic) and Macauley Island (high-alumina) are characterised by low alkalis, an absence of andesites in the range 56–65% silica, and restricted acidity for minor glassy differentiates ( SiO2=65–68 %). These volcanics form a chain of islands overlying a seismic zone which extends from Tonga to the central volcanic region of North Island, New Zealand where a calc-alkaline series contains basaltic, andesitic and rhyolitic members in that order of increasing abundance. Within this continental suite, tholeiitic and high-alumina phases are recognised as closely similar to the intra-oceanic Tonga-Kermadec magma types and show petrochemical gradation into the medium-silica andesites, apparently by sialic assimilation. 相似文献
15.
The Cenozoic volcanic rocks of the southern Andes are characterized by low 87Sr/ 86Sr ratios (0.7040–0.7045), which are consistent with an origin in the downgoing slab of oceanic lithosphere or the overlying mantle. These values are distinctly lower than those from corresponding rocks of the central Andes.The calc-alkaline rocks of the central Andes exhibit higher Sr isotopic values (0.705–0.713) and variable Rb/Sr ratios. Different explanations are possible for this behaviour as well as for the positive correlation between 87Sr/ 86Sr and Rb/Sr expressed in an apparent isochron of 380 ± 50 m.y. It is postulated that these magmas result from a mixing process between a primary magma with basaltic affinities and crustal material of relatively young age.A model is proposed for the generation of the “andesitic” magmas of the central Andes by which crustal rocks of the upper part of the crust are added to the base of the crust by an accretionary process at the margin of the continent. Melts from these upper crustal rocks act as contaminants in “andesitic” magmas.The role of crustal material is still more significant in the generation of the ignimbritic magmas; they are considered to result from a two-stage melting process by which igneous rocks, belonging to a former stage of development of the Andes, are engulfed in the subduction zone, where they melt. 相似文献
17.
The distribution of U in volcanic rocks from two transects across the Central Andes (latitudes 27°–28° S and 16°–18° S, respectively) differs from that of K. For a given SiO 2, content of the rocks, K systematically increases with the distance from the trench, while the highest U abundances are found in the rocks overlying the thickest segments of the continental crust, which are situated in the middle parts of the transects. It is suggested that this variation of U reflects crustal contamination. 相似文献
18.
Calc-alkalic chemical trends characteristic of arc volcanic rocks mainly result from three mechanisms which act additively: (1) fractional crystallization involving separation of titanomagnetite; (2) selective concentration of plagioclase phenocrysts and selective depletion of titanomagnetite phenocryst compared with the actually fractionated proportion; and (3) mixing of magmas on continuous fractionation trends. The association of calc-alkalic and tholeiitic trends in a single composite volcano may not represent different fractional crystallization processes or different chemistries of primary magmas, but the calc-alkalic chemical trend can be considered as a mixing trend resulting from mixing of various magmas on associated tholeiitic chemical trends. Chemical variations of most arc volcanic rocks, including calc-alkalic ones, can accordingly be essentially accounted for by the low-pressure fractional crystallization of phenocrystic phases from primary basaltic magmas.Crystallization sequences of arc magmas which are strongly dependent on water content in magmas are deduced from the phenocryst assemblages. The crystallization sequence changes laterally across-arc, suggesting increasing water contents in magmas toward the back-arc side, as is also seen for other incompatible elements such as K and Rb. Systematic differences in the characteristic crystallization sequence are also observed among arcs, roughly correlating with the crustal thickness. Water content in magma, like other incompatible elements, tends to increase with increasing crustal thickness. The variation of incompatible elements including water roughly represents that of the degree of partial melting of the upper mantle, which is broadly controlled by the crustal thickness.The variation of water content indicates that arc magmas are not saturated with water during differentiation to late differentiates such as dacite or rhyolite. This strongly constrains the maximum water contents in primary basaltic magma, at most 2.5 wt.%. This value suggests that magma generation beneath arcs is dependent on dry solidus of peridotite. Diapiric uprise of the hot deeper mantle and associated adiabatic decompression would be necessary for mantle peridotite to attain the temperature as high as dry solidus. Diapirs that begin to rise from the subduction zone may stop at or near the crust-mantle boundary because of the surrounding density change, and their degree of partial melting is roughly controlled by their stopped depth assuming their similar temperature. Across-arc variation is also explained by the stopped depth of diapirs, but is not controlled by crustal thickness. 相似文献
19.
A detailed petrological study of the Banda Arc volcanism, documented by extensive microprobe whole-rock and mineral analysis, shows that the apparent geographical eastwards continuity of the Sunda Arc conceals a major geochemical discontinuity adjacent to the southern end of the Weber Deep beneath Serua. The alkali contents and Sr isotope ratios suggest that Nila, Teun and Damar form one volcanic group distinct from Banda and Manuk, and Serua is in the border discontinuity, reflected in its widely fluctuating Fe/Mg partitioning between mineral phases and its variable Sr isotope ratios. With the exception of basalt from S. W. Ambon, all lavas are quartz normative and typical of an ensimatic oceanic island arc. They range from tholeiitic basalt and dacite on S. W. Ambon and Banda, through low-K calc-alkaline andesites on Manuk and Serua, to high-K calc-alkaline andesites on Nila, Teun, Damar, Gunung Api north of Wetar, and Romang which also contains dacite. The higher potassium contents are normally contained in biotite and hornblende, and occasionally in the groundmass glass. Increasing potassium from Banda to Manuk may be related to increasing Benioff Zone depth between the 100 to 200 km contours, but the lavas of Gunung Api north of Wetar have insufficient potassium to be derived from the underlying 400–500 km Benioff Zone. Older cordierite dacites (ambonites) on North Ambon must be derived from a underlying continental crust, but the younger tholeiitic lavas of S.W. Ambon and Banda may be related to a shallow subduction zone dipping southwards from Seram. 相似文献
|