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1.
Strontium isotopic analyses are reported for ophiolitic and associated rocks of Mesozoic and Tertiary age from the Drocea Mountains. The samples and their average Sr87/Sr86 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 Sr87/Sr86 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 Sr87/Sr86 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 Sr87/Sr86 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 Sr87/Sr86 ratio suggests a direct link to a mantle source with little or no crustal contamination.  相似文献   

2.
Erciyes stratovolcano, culminating at 3917 m, is located in the Cappadocian region of central Anatolia. During its evolution, this Quaternary volcano produced pyroclastic deposits and lava flows. The great majority of these products are calc-alkaline in character and they constitute Kocdag and Erciyes sequences by repeated activities. Alkaline activity is mainly observed in the first stages of Kocdag and approximately first-middle stages of Erciyes sequences. Generally, Kocdag and Erciyes stages terminate by pyroclastic activities. The composition of lavas ranges from basalt to rhyolite (48.4–70.5 wt.% SiO2). Calc-alkaline rocks are represented mostly by andesites and dacites. Some compositional differences between alkaline basaltic, basaltic and andesitic rocks were found; while the composition of dacites remain unchanged. All these volcanics are generally enriched in LIL and HFS elements relative to the orogenic values except Rb, Ba, Nb depleted alkaline basalt. 87Sr/86Sr and 143Nd/144Nd isotopic composition of the volcanics range between 0.703344–0.703964, 0.512920–0.512780 for alkaline basalts and change between 0.704322–0.705088, 0.512731–0.512630 for alkaline basaltic rocks whereas calc-alkaline rocks have relatively high Sr and Nd isotopic ratios (0.703434–0.705468, 0.512942–0.512600). Low Rb, Ba, Nb content with high Zr/Nb, low Ba/Nb, La/Yb ratio and low Sr isotopic composition suggest an depleted source component, while high Ba, Rb, Nb content with high La/Yb, Ba/Nb, low Zr/Nb and low 87Sr/86Sr ratios indicate an OIB-like mantle source for the generation of Erciyes alkaline magma. These elemental and ratio variations also indicate that the different mantle sources have undergone different degree of partial melting episodes. The depletion in Ba, Rb, Nb content may be explained by the removal of these elements from the source by slab-derived fluids which were released from pre-collisional subduction, modified the asthenospheric mantle. The chemically different mantle sources interacted with crustal materials to produce calc-alkaline magma. The Ba/Nb increase of calc-alkaline samples indicates the increasing input of crustal components to Erciyes volcanics. Sr and Nd isotopic compositions and elevated LIL and HFS element content imply that calc-alkaline magma may be derived from mixing of an OIB-like mantle melts with a subduction-modified asthenospheric mantle and involvement of crustal materials in intraplate environments.  相似文献   

3.
Cenozoic volcanism in the Great Basin is characterized by an outward migration of volcanic centers with time from a centrally located core region, a gradational decrease in the initial Sr87/Sr86 ratio with decreasing age and increasing distance from the core, and a progressive change from calc-alkalic core rocks to more alkalic basin margin rocks. Generally each volcanic center erupted copious silicic ignimbrites followed by small amounts of basalt and andesite. The Sr82/Sr86 ratio for old core rocks is about 0.709 and the ratio for young basin margin rocks is about 0.705. Spatially and temporally related silicic and mafic suites have essentially the same Sr87/Sr86 ratios. The locus of older volcanism of the core region was the intersection of a north-south trending axis of crustal extension and high heat flow with the northeast trending relic thermal ridge of the Mesozoic metamorphic hinterland of the Sevier Orogenic Belt. Derivation of the Great Basin magmas directly from mantle with modification by crustal contamination seems unlikely. Initial melting of lower crustal rocks probably occurred as a response to decrease in confining pressure related to crustal extension. Volcanism was probably also a consequence of the regional increase in the geothermal gradient that is now responsible for the high heat flow of the Basin and Range Province. High Sr isotopic ratios of the older core volcanic rocks suggests that conditions suitable for the production of silicic magmas by partial fusion of the crust reached higher levels within the crust during initial volcanism than during production of later magmas with lower isotopic ratios and more alkaline chemistry. As the Great Basin became increasingly attenuated, progressively lower portions of the crust along basin margins were exposed to conditions suitable for magma genesis. The core region became exhausted in low temperature melting components, and volcanism ceased in the core before nearby areas had completed the silicic-mafic eruption cycle leading to their own exhaustion of crustal magma sources.  相似文献   

4.
Abstract Whole‐rock chemical and Sr and Nd isotope data are presented for gabbroic and dioritic rocks from a Cretaceous‐Paleogene granitic terrain in Southwest Japan. Age data indicate that they were emplaced in the late Cretaceous during the early stages of a voluminous intermediate‐felsic magmatic episode in Southwest Japan. Although these gabbroic and dioritic rocks have similar major and trace element chemistry, they show regional variations in terms of initial Sr and Nd isotope ratios. Samples from the South Zone have high initial 87Sr/86Sr (0.7063–0.7076) and low initial Nd isotope ratios (?Nd, ?2.5 to ?5.3); whereas those from the North Zone have lower initial 87Sr/86Sr (usually less than 0.7060) and higher Nd isotope ratios (?Nd, ?0.8 to + 3.3). Regional variations in Sr and Nd isotope ratios are similar to those observed in granitic rocks, although gabbroic and dioritic rocks tend to have slightly lower Sr and higher Nd isotope ratios than granitic rocks in the respective zones. Limited variations in Sr and Nd isotope ratios among samples from individual zones may be attributed partly to a combination of upper crustal contamination and heterogeneity of the magma source. Contamination of magmas by upper crustal material cannot, however, explain the observed Sr and Nd isotope variations between samples from the North and South Zones. Between‐zone variations would reflect geochemical difference in magma sources. The gabbroic and dioritic rocks are enriched in large ion lithophile elements (LILE) and depleted in high field strength elements (HFSE), showing similar normal‐type mid‐ocean ridge basalt (N‐MORB) normalized patterns to arc magmas. Geochronological and isotopic data may suggest that some gabbroic and dioritic rocks are genetically related to high magnesian andesite. Alternatively, mantle‐derived mafic or intermediate rocks which were underplated beneath the crust may be also plausible sources for gabbroic and dioritic rocks. The magma sources (the mantle wedge and lower crust) were isotopically more enriched beneath the South Zone than the North Zone during the Cretaceous‐Paleogene. Sr and Nd isotope ratios of the lower crustal source of the granitic rocks was isotopically affected by mantle‐derived magmas, resulting in similar initial Sr and Nd isotope ratios for gabbroic, dioritic and granitic rocks in each zone.  相似文献   

5.
Sr and Nd isotope analyses are presented for Tertiary continental alkaline volcanics from Cantal, Massif Central, France. The volcanics belong to two main magma series, silica-saturated and silica-undersaturated (with rare nephelinites). Trace element and isotopic data indicate a common source for the basic parental magmas of both major series; the nephelinites in contrast must have been derived from a mantle source which is isotopically and chemically distinct from that which gave rise to the basalts and basanites.87Sr/86Sr initial ratios range from 0.7034 to 0.7056 in the main magma series (excluding rhyolites) and143Nd/144Nd ratios vary between 0.512927 and 0.512669; both are correlated with increasing SiO2 in the lavas. The data can be explained by a model of crustal contamination linked with fractional crystallisation. This indicates that crustal magma chambers are the sites of differentiation since only rarely do evolved magmas not show a crustal isotopic signature and conversely basic magmas have primitive isotopic ratios unless they contain obvious crustal-derived xenocrysts. Potential contaminants include lower crustal granulites or partial melts of upper crustal units. Equal amounts of contamination are required for both magma series, refuting hypotheses of selective contamination of the silica-saturated series.The isotopic characteristics of the apparently primary nephelinite lavas demonstrates widespread heterogeneity in the mantle beneath Cantal. Some rhyolites, previously thought to be extremely contaminated or to be crustally derived, are shown to have undergone post-emplacement hydrothermal alteration.  相似文献   

6.
The Shabogamo Intrusive Suite comprises numerous bodies of variably metamorphosed gabbro which intrude Archean and Proterozoic sequences at the junction of the Superior, Churchill, and Grenville structural provinces in western Labrador. Combined Sm-Nd and Rb-Sr systematics in two bodies, ranging from unmetamorphosed to lightly metamorphosed, document a crystallization age of about 1375 m.y., and suggest that both bodies crystallized from magmas with similar Nd and Sr isotopic compositions. This age is in accordance with the existence of a regional magmatic event in the Churchill Province at approximately 1400 m.y.Rb-Sr systematics in two bodies of amphibolite-grade gabbro suggest a regional metamorphic event at about 950 m.y., corresponding to the waning stages of Grenville activity. Sm-Nd systematics in these high-grade bodies are affected to a much lesser degree than Rb-Sr.Initial ratios for143Nd/144Nd and87Sr/86Sr are lower and higher, respectively, than bulk earth values at 1375 m.y. Both these displacements are in the direction of older crustal material at 1375 m.y., and a model is proposed to produce the Shabogamo magma by mixing a mantle-derived magma with a partial melt of crustal rocks (approximately 4: 1 by volume). Young volcanic rocks with anomalous Nd and Sr isotopic ratios, which have previously been taken as evidence for “enriched” mantle, may be interpreted similarly.  相似文献   

7.
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   

8.
The Cenozoic volcanic rocks of eastern China are subalkalic to alkalic basalts erupted in an early Tertiary back-arc rift environment and from scattered late Tertiary and Quaternary volcanic centers in a continental area crossed by active faults, driven by subduction of the Pacific plate and the collision of India and Eurasia. Immobile trace elements and major elements conform very well to each other in classification of the 59 rocks for which complete data are reported and they correctly identify the tectonic setting. LIL-element enrichments of the basalts lie between those of P-MORB and ocean island alkalic basalts, and show a secular increase.87Sr/86Sr ratios of basalts vary from 0.7029 to 0.7048. Alkalic basalts are systematically less radiogenic than geographically coextensive and contemporaneous tholeiitic basalts. Increase of radiogenic Sr with increasing crustal thickness and crustal age and with silica enrichment of the magmas suggests crustal contamination but this is inadequate to explain the LIL-element enrichment patterns and variable LIL-element enrichments. The preferred hypothesis is that the alkalic magmas come from a deeper source, with long-term LIL-element depletion and low Rb/Sr ratio but relatively recent LIL-element enrichment. Conversely the tholeiitic magmas are melts of subcontinental mantle lithosphere that is more LIL-element depleted than the alkalic source, at the time of magma genesis, but has had an elevated Rb/Sr ratio for much of its post-consolidation history.  相似文献   

9.
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.  相似文献   

10.
Carbonatites are rarely igneous rocks distributed on the earth. The rocks usually form ring complexes with alkalic rocks, occurring in the environments of continental rift, collisional oro-genic zone and oceanic island[1, 2]. Numerous facts and experiment…  相似文献   

11.
The isotopic compositions of Sr, Nd and Pb together with the abundances of Rb, Sr, U and Pb have been determined for mafic and felsic potassic alkaline rocks from the young Virunga volcanic field in the western branch of the East African rift system.87Sr/86Sr varies from 0.7055 to 0.7082 in the mafic rocks and from 0.7073 to 0.7103 in the felsic rocks. The latter all come from one volcano, Sabinyo. Sabinyo rocks have negative εNdvalues ofεNd = ?10. Nd and Sr isotopic variations in the basic potassic rocks are correlated and plot between Sabinyo and previously reported [1] compositions (εNd = +2.5;87Sr/86Sr≈ 0.7047) for Nyiragongo nephelinites. The Pb isotopic compositions for Sabinyo rocks are nearly uniform and average206Pb/204Pb≈ 19.4,207Pb/204Pb= 15.79–15.84,208Pb/204Pb≈ 41.2. The basic potassic rocks have similar206Pb/204Pb values but range in207Pb/204Pb and208Pb/204Pb from the Sabinyo values to less radiogenic compositions.Excellent correlations of87Sr/86Sr with Rb/Sr, 1/Sr and207Pb/206Pb for Sabinyo rocks suggest these to be members of a hybrid magma series. However, the nearly uniform Pb compositions for this series points to radiogenic growth of87Sr in the magma source region following an event which homogenized the isotopic compositions but not Rb/Sr. The Rb-Sr age derived from the erupted Sabinyo isochron-mixing line is consistent with the ～500 Myr Pb-Pb age from Nyiragongo [1], which suggests that this event affected all Virunga magma sources. The event can again be traced in the Pb-Pb, Pb-Sr and Nd-Sr isotopic correlations for all Virunga rocks, including Nyiragongo, when allowances are made for radiogenic growth subsequent to this mixing or incomplete homogenization event. Inferred parent/daughter element fractionations point to a metasomatic event during which a mantle fluid invaded two lithospheric reservoirs: a +εNd reservoir sampled by the Nyiragongo nephelinites and suggested to be the subcontinental mantle and a ?εNd reservoir sampled by the mafic and felsic potasssic volcanism. Whether this ?εNd reservoir is the crust, continental crustal material in the mantle or anomalous mantle cannot be decided from the data. The simplest answer, that this reservoir is the continental crust, seems to be at variance with experimental evidence suggesting a subcrustal origin for basic potassic magmas. Partial melting of the ancient metasomatised lithospheric domains and ensuing volcanism seems to be entirely a response to decompression and rising geotherms during rifting and thinning of the lithosphere.  相似文献   

12.
Purico-Chascon is an acid igneous complex less than 1.5 Ma old rising to 5800 m in the North Chilean Andes, and consisting of andesite-dacite cones and dacite domes on an ignimbrite shield. The rocks are subdivided into two groups: those from Chascon appear to exhibit evidence for magma mixing with more basic material now preserved as xenoliths, whereas among those at Purico no xenoliths have been found.87Sr/86Sr=0.7095?0.7081 at Purico, 0.7079?0.7069 at Chascon, and 0.7061-0.7057 in the xenoliths from the Chascon lavas:143Nd/144Nd=0.51222?0.51236 overall. The Purico lavas are characterised by higher SiO2, Rb/Sr,87Sr/86Sr, and REE abundances, and lower Sr/Nd, Sr/Ba and143Nd/144Nd than most Andean igneous suites. There is no indication ofselective crustal contamination of Sr, or any systematic change in isotope ratios during differentiation. Nonetheless the trend of, for example, high Sr/Nd and Sr contents in rocks with low87Sr/86Sr (0.704, Ecuador) to low Sr/Nd and Sr and high SiO2 in rocks with87Sr/86Sr=0.7081?0.7095 at Purico is interpreted as a shift from subduction zone related magmatism to one with greater crustal affinity. The formation of the least evolved Purico lavas (～60%SiO2) is discussed in terms of bulk assimilation of crustal material, mixing between crustal- and mantle-derived magmas, and partial melting of pre-existing crust. Although such models are still extremely primitive, the simplest explanation of the observed chemical variations is that the Purico rocks evolved from parental magmas derived by crustal anatexies. Thermal considerations suggest that such late-stage crustal anatexis is a predictable response to crustal thickening which in the Andes is thought to have taken place during the Cenozoic.  相似文献   

13.
Neodymium isotope and REE analyses of recent volcanic rocks and spinel lherzolite nodules from the Afar area are reported. The143Nd/144Nd ratios of the volcanic rocks range from 0.51286 to 0.51304, similar to the range recorded from Iceland. However, the87Sr/86Sr ratios display a distinctly greater range (0.70328–0.70410) than those reported from the primitive rocks of Iceland. Whole rock samples and mineral separates from the spinel lherzolite nodules exhibit uniform143Nd/144Nd ratios (ca. 0.5129) but varied87Sr/86Sr ratios in the range 0.70427–0.70528.The SrNd isotope variations suggest that the volcanic rocks may have been produced by mixing between two reservoirs with distinct isotopic compositions. Two possible magma reservoirs in this area are the source which produced the “MORB-type” volcanics in the Red Sea and Gulf of Aden and the anomalous source represented by the nodule suite. The isotopic composition of the volcanics is compatible with mixing between these two reservoirs.It is shown that the anomalous source with a high87Sr/86Sr ratio cannot have been produced by simple processes of partial melting and mixing within normal mantle. Instead the high87Sr/86Sr is equated with a fluid phase. A primitive cognate fluid, subducted seawater or altered oceanic lithosphere may have been responsible for the generation of the source with a high87Sr/86Sr ratio.  相似文献   

14.
Major and trace element along with representative Sr, Nd and Pb isotope data are presented for drill core samples which intersect an 800 m lava pile in eastern Uruguay. The lavas form part of the Paraná flood basalt province, are low-Ti in composition but distinct from the low-Ti Gramado magma type, and have been termed the Treinte Y Trés magma type. The lava pile overlies a large positive gravity anomaly inferred to reflect an east–west trending, mid-crustal mafic intrusive body with a calculated volume of 35,000 km3. Smooth up-section compositional variations in the basalts are interpreted to record magma evolution within this mid-crustal magma chamber. 87Sr/86Sr and 206Pb/204Pb increase throughout the sequence yet Mg remains relatively constant in the lower 200 m of the sequence, suggesting a role for magma chamber recharge. Above this the lavas show a regular, up-section decrease in Mg coupled with increasing 87Sr/86Sr and 206Pb/204Pb and this is interpreted to reflect crystal fractionation combined with crustal contamination. The data provide further evidence that contamination of flood basalt magmas in crustal magma chambers is a common phenomenon and calculations suggest that the amount of crustal addition may be as high as 60–70%. Nevertheless, the effects of this crustal contamination do not appear able to account for the discrepancy between key incompatible trace element ratios and isotope ratios of the lavas and those of any putative mantle plume. In fact, La/Ta decreases with decreasing Mg and increasing 87Sr/86Sr indicating that the effects of crustal contamination were actually to reduce La/Ta and implying that the parental magmas had very high La/Ta (90). These constraints are clearly inconsistent with an asthenospheric origin for the parental magmas and so, consistent with mass balance calculations, it is inferred that they were derived from the lithospheric mantle.  相似文献   

15.
The Bishop Tuff represents a single eruption of chemically zoned rhyolitic magma. Six whole rock samples spanning the compositional and temperature range yield initial87Sr/86Sr of 0.7060–0.7092 andδ18O of 5.9–10.3‰. Six constituent sanidines yield smaller ranges of initial87Sr/86Sr of 0.7061–0.7069 andδ18O of 6.7–7.9. In contrast143Nd/144Nd ratios for the six whole rocks and two constituent magnetites exhibit negligible variation with a mean of0.51258 ± 1. These data are used to show that the phenocrysts were precipitated from an already chemically zoned liquid, that the zoning process involved negligible assimilation of, or exchange with, country rocks and that the extreme Sr and O isotopic disequilibria are probably the result of post-eruptive interaction with meteoric water. The parent magma had?Nd = ?0.9, ?Sr = +23 andδ18O = 7‰ and was formed from mantle-derived magmas and/or melts of lower crustal rocks isotopically similar to parts of the Sierra Nevada Batholith.  相似文献   

16.
Andesites from the Peruvian Andes and the Banda arc of Indonesia are characterized by unusually high and variable 87Sr/86Sr ratios. The Banda arc samples, including two cordierite-bearing lavas from Ambon, show a clear positive correlation between 87Sr/86Sr and δ18O. The andesitic rocks have δ18O values that range from 5.6 to 9.2‰. Over that range in δ18O, 87Sr/86Sr increases from 0.7044 to 0.7095. The cordierite-bearing lavas have δ18O values of approximately 15‰ and 87Sr/86Sr ratios of approximately 0.717. The similarity between δ18O values and 87Sr/86Sr ratios in total rocks and separated plagioclase phenocrysts of the Banda arc samples indicates that the measured isotope ratios are primary and have not been affected by secondary, low-temperature post-eruptive alteration. The observed variation between O and Sr isotopic ratios can be modeled by two-component mixing in which one component is of mantle isotopic composition. As the crust beneath the Banda arc is probably oceanic, contamination of the manle component may have resulted from the subduction of either continentally-derived sediments or continental crust. Mixing calculations indicate that the contaminant could have an isotopic composition similar to that observed in the cordierite-bearing lavas.The Andean samples, despite petrographic evidence of freshness, exhibit whole-rock δ18O values significantly higher than those of corresponding plagioclase phenocryst separates, indicating extensive low-temperature post-eruptive alteration. The plagioclase mineral separates show a range of δ18O values between 6.9 and 7.9‰. The 87Sr/86Sr ratios of these same samples are, in most instances, not significantly different from those measured for the whole rock, thus signifying that the phenocrysts and groundmass were in isotopic equilibrium at the time of eruption. Unlike the lavas of the Banda arc, the Andean lavas show no strong positive correlation between 87Sr/86Sr ratios and δ18O values, but instead lower 87Sr/86Sr ratios appear to be associated with higher δ18O values. The δ18O and 87Sr/86Sr values of the Peruvian samples are both slightly higher than those of “normal” island arc volcanics.The small proportions of contaminant implied by the O isotope results seem to preclude continental crustal contamination as a primary cause of high 87Sr/86Sr ratios. The most plausible process that can explain both O and Sr isotope results is one in which sediments of continental origin are partially melted in the subduction zone. These melts rise into overlying mantle material and subsequently participate in the formation of calc-alkaline magmas.If the involvement of a sialic component in the genesis of andesitic magma occurs in the subduction zone, melting of that sialic material signifies temperatures of at least 750–800°C at the top of the subducted lithospheric slab at depths of approximately 150 km. The fact that contamination has apparently occurred in the Banda arc samples without producing any simple widespread correlations between Sr and O isotopic compositions on the one hand and major or trace element abundances on the other, shows that isotopic correlations, possibly including pseudo-isochrons, can be produced by mixing without producing trace element mixing correlations. Because O versus Sr isotope correlations are little affected by processes of partial melting of differentiation, they provide a direct means of testing whether Sr isotopic variations in volcanic rocks are of mantle origin or are due instead to mixing with sialic material.  相似文献   

17.
New Pb, Sr and O isotopic analyses of rocks from the Skaergard intrusion indicate the following: (1) initial87Sr/86Sr of the gabbroic magma was less than or equal to 0.7041; (2) limited contamination of magma with crustal Sr and Pb may have occurred in a deep reservoir below the presently exposed parts of the intrusion; (3) marked crustal contamination occurred at high level in marginal border group rocks, but these rocks effectively shielded the main magma body from further interaction with country rock gneisses; (4) subsolidus interaction between Skaergard gabbros and hydrothermal fluids modified δ18O values but had little effect on Sr and perhaps Pb isotopic ratios; (5) late-stage melanogranophyres may be comagmatic with the Skaergard magma, but silicic granophyres are not; (6) silicic granophyres contain large and varied proportions of crustal Sr and Pb; some may be largely anatectic melts derived from the deep crust whereas others may represent mixing of such anatectic melts with late-stage differentiated liquids of the Skaergard intrusion (e.g. Sydtoppen sill).  相似文献   

18.
Abstract We present chemical and Sr–Nd–Pb isotopic compositions of three Triassic (226–241 Ma) calc‐alkaline granitoids (the Yeongdeok granite, Yeonghae diorite and Cheongsong granodiorite) and basement rocks in the northern Gyeongsang basin, south‐eastern Korea. These plutons exhibit typical geochemical characteristics of I‐type granitoids generated in a continental magmatic arc. The Yeongdeok and Yeonghae plutons have similar initial Sr, Nd and Pb isotope ratios (87Sr/86Srinitial = 0.7041 ~ 0.7050, ?Nd(t) = 2.3 ~ 4.0, 206Pb/204Pbfeldspar = 18.22 ~ 18.34), but distinct rare earth element patterns, suggesting that the two plutons formed from partial melting of a similar source material at different depths. The Cheongsong pluton has slightly more enriched Sr–Nd–Pb isotopic compositions (87Sr/86Srinitial = 0.7047 ~ 0.7065, ?Nd(t) = 3.9 ~ 2.8, 206Pb/204Pbfeldspar = 18.24 ~ 18.37) than the other two plutons. The Nd model ages of the basement rocks (1.1 ~ 1.4 Ga) are slightly older than those of the plutons (0.6 ~ 1.0 Ga). The initial Sr and Nd isotopic ratios of the plutons can be modeled by the mixing between the mid‐oceanic ridge basalt‐like depleted mantle component and the crustal component represented by basement rocks, which is also supported by Pb isotope data. The Sr and Nd isotope data from granitoids and basement rocks suggest that the Gyeongsang basin, the Hida belt and the inner zone of south‐western Japan share relatively young basement histories (middle Proterozoic), compared with those (early Proterozoic to Archean) of the Gyeonggi and Yeongnam massifs and the Okcheon belt. The Nd isotope data of basement rocks suggest that the Hida belt might be better correlated with the basement of the Gyeongsang basin than the Gyeonggi massif, the Okcheon belt or the Yeongnam massif, although it may represent an older continental margin of East Asia than the Gyeongsang basin considering its slightly older Nd model ages.  相似文献   

19.
Isotopic and major and trace element analysis of nine samples of eruptive products spanning the history of the Mt. St. Helens volcano suggest three different episodes; (1) 40,000–2500 years ago: eruptions of dacite with εNd = +5, εSr = ?10, variable δ18O,206Pb/204Pb ～ 18.76, Ca/Sr ～ 60, Rb/Ba ～ 0.1, La/Yb ～ 18, (2) 2500-1000 years ago: eruptions of basalt, andesite and dacite with εNd = +4 to +8, εSr = ?7 to ?22, variable δ18O (thought to represent melting of differing mantle-crust reservoirs), 206Pb/204Pb= 18.81?18.87, variable Ca/Sr, Rb/Ba, La/Yb and high Zr, (3) 1000 years ago to present day: eruptions of andesite and dacite with εNd = +6, εSr = ?13, δ18O～6‰, variable206Pb/204Pb, Ca/Sr ～ 77, Rb/Ba= 0.1, La/Yb ～ 11. None of the products exhibit Eu anomalies and all are LREE enriched. There is a strong correlation between87Sr/86Sr and differentiation indices. These data are interpreted in terms of a mantle heat source melting young crust bearing zircon and garnet, but not feldspar, followed by intrusion of this crustal reservoir by mantle-derived magma which caused further crustal melting and contaminated the crustal magma system with mafic components. Since 1000 years ago all the eruptions have been from the same reservoir which has displayed a much more gradual re-equilibration of Pb isotopic compositions than other components suggesting that Pb is being transported via a fluid phase. The Nd and Sr isotopic compositions lie along the mantle array and suggest that the mantle underneath Mt. St. Helens is not as depleted as MORB sources. There is no indication of seawater involvement in the source region.  相似文献   

20.
New Sr and Pb isotope data are presented for a selection of lavas and associated coarse-grained blocks from Ascension Island. K-Ar dates for the lavas range up to1.5±0.2Ma. Initial87Sr/86Sr ratios are consistent with earlier measurements and for most rocks are ca. 0.7029, but range up to 0.7135 in the case of the most evolved lavas and blocks. Pb isotope data are also consistent with earlier measurements, but the Pb in two gabbroic blocks is less radiogenic than Pb in the other rocks. It is suggested that these gabbroic blocks crystallized from a magma of tholeiitic composition whose source was similar to that of mid-oceanic ridge basalt whereas the lavas and other blocks crystallized from mildly alkaline magmas derived from a source further from the crest of the Mid-Atlantic Ridge. The high87Sr/86Sr ratios result from contamination of the most silicic magma by radiogenic Sr from pelagic sediments. These data and their interpretation are consistent with the petrological and geochemical observations that the granite blocks are the coarse-grained equivalents of the volcanic suite [11] and not fragments of relict continental material [2,3].  相似文献   

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