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1.
Abstract Rb–Sr and Sm–Nd isochron ages were determined for whole rocks and mineral separates of hornblende‐gabbros and related metadiabases and quartz‐diorite from Shodoshima, Awashima and Kajishima islands in the Ryoke plutono‐metamorphic belt of the Setouchi area, Southwest Japan. The Rb–Sr and Sm–Nd whole‐rock‐mineral isochron ages for six samples range from 75 to 110 Ma and 200–220 Ma, respectively. The former ages are comparable with the Rb–Sr whole‐rock isochron ages reported from neighboring Ryoke granitic rocks and are thus due to thermal metamorphism caused by the granitic intrusions. On the contrary, the older ages suggest the time of formation of the gabbroic and related rocks. The initial 87Sr/86Sr and 143Nd/144Nd ratios of the gabbroic rocks (0.7070–0.7078 and 0.51217–0.51231 at 210 Ma, respectively) are comparable with those of neighboring late Cretaceous granites and lower crustal granulite xenoliths from Cenozoic andesites in this region. Because the gabbroic rocks are considered to be fragments of the lower crustal materials interlayered in the granulitic lower crust, their isotopic signature has been inherited from an enriched mantle source or, less likely, acquired through interaction with the lower crustal materials. The Sr and Nd isotopic and petrologic evidence leads to a plausible conclusion that the gabbroic rocks have formed as cumulates from hydrous mafic magmas of light rare earth element‐rich (Sm/Nd < 0.233) and enriched isotopic (?Sr > 0 and ?Nd < 0) signature, which possibly generated around 220–200 Ma by partial melting of an upper mantle. We further conclude that they are fragments of refractory material from the lower crust caught up as xenoblocks by granitic magmas, the latter having been generated by partial melting of granulitic lower crustal material around 100 Ma.  相似文献   

2.
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   

3.
Abstract Rb–Sr and K–Ar chronological studies were carried out on granitic and metamorphic rocks in the Ina, Awaji Island and eastern Sanuki districts, Southwest Japan to investigate the timing of intrusion of the granitoids in the Ryoke belt. Intrusions of 'younger' Ryoke granitic magmas took place in the Ina district between 120 Ma and 70 Ma, and cooling began immediately after the emplacement of the youngest granitic bodies. Igneous activity in Awaji Island was initiated at 100 Ma and continued to 75 Ma. Along-arc variations of Rb–Sr whole-rock isochron ages suggest that magmatism began everywhere in the Ryoke and San-yo belts at almost the same time ( ca 120 Ma). The last magmatism took place in the eastern part of both belts. Rb–Sr and K–Ar mineral ages for the granitoids young eastwards. The age data suggest that the Ryoke belt was uplifted just after the termination of igneous activity. Initial Sr and Nd isotopic ratios for the Ryoke granitoids indicate that most were derived from magmas produced in the lower crust and/or upper mantle with uniform Sr and Nd isotopic compositions. Several granitoids, however, exhibit evidence of assimilation of Ryoke metamorphic rocks or older Precambrian crustal rocks beneath the Ryoke belt.  相似文献   

4.
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   

5.
A comparative analysis of the concentrations of major oxides, trace elements, and the 143Nd/144Nd ratios in representative sequences of volcanic and subvolcanic rocks in the western and eastern Vitim Upland has revealed petrogenetic groups with different relationships among components from lithosphere and sublithosphere sources. It is hypothesized that the initial 16–14-Ma eruptions of picrobasalts and Mg basanites in the east of the upland resulted from high-temperature melting, hence, the melting of sublithospheric peridotite and lithospheric Mg-pyroxenite mantle material with mildly and strongly depleted isotope compositions of Nd relative to the value in the primitive mantle (0.512638). The broad range of varying lava compositions in the 14–9 Ma time span was caused by “passive” rifting in the west of the upland and by “active” rifting in the east. The “passive” rifting manifested itself in the melting of lithospheric material with some admixture of material from the underlying asthenosphere, while the “active” rifting lifted deep-lying mantle material. The structural rearrangement that has been occurring in the Baikal Rift System during the last 9 Ma resulted in stopping the rifting in the area of study. Relaxation, flattening and thinning of the lithosphere beneath the east part of the system during the 1.1–0.6 Ma time span caused magma effusion with values of 143Nd/144Nd that are typical of a moderately depleted asthenospheric source contaminated with deeper mildly depleted mantle material.  相似文献   

6.
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   

7.
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   

8.
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   

9.
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   

10.
The Deccan flows at Mahabaleshwar are divisible into a lower and an upper group, based on Nd and Sr isotopic ratios, which define two correlated trends. This distinction is supported by incompatible element ratios and bulk compositions. The data reflect contamination in a dynamic system of magmas from an LIL-depleted,εJUV ≥ +8 mantle by two different negative εJUV endmembers, one undoubtedly continental crust, the other either continental crust or enriched mantle. The depleted mantle source, anomalously high in (87Sr/86Sr), may have been in the subcontinental lithosphere or a region of rising Indian Ocean MORB mantle.  相似文献   

11.
The chronology and isotope geochemistry of a selection of Proterozoic Scourie dykes has been investigated in order to specify both their time of emplacement within the thermal history of the Archaean crust of N.W. Scotland, and to attempt to characterise the evolution of continental lithosphere. SmNd, RbSr and UPb isotope analyses are presented. Primary, major igneous minerals separated from four well preserved dykes yield SmNd ages of 2.031 ± 0.062Ga, 2.015 ± 0.042Ga, 1.982 ± 0.044Ga and 2.101 ± 0.078Ga, which are interpreted as crystallisation ages.The initial Nd isotope compositions in the dykes at their emplacement age of 2.0 Ga, range from +3.4 to −6.8, indicating the presence of an older lithospheric component. SmNd whole-rock isotope data for fifteen dykes, if interpreted to have age significance, yield an “age” of 3.05 ± 0.27 Ga. SmNd crustal residence ages for the same dykes average 2.95 Ga, which is interpreted as the time that small melts were added to the Lewisian lithosphere. The possibility that correlated147Sm/144Nd and143Nd/144Nd ratios are an artifact of mixing between depleted mantle melts generated at 2.0 Ga, and an older enriched lithospheric component is not eliminated by the data, but the relationship between 1/Nd and143Nd/144Nd ratios rules out any simple mixing. UPb isotope data for plagioclase feldspars and whole-rock samples of dykes provide useful estimates of initial Pb-isotope composition of the dykes at the time of their emplacement. Initial206Pb/204Pb and207Pb/204Pb ratios vary considerably and range from 13.98 to 15.78, and 14.72 to 15.56 respectively, and suggest that the UPb fractionation responsible must have occurred at least 2.5 Ga ago.The Scourie dykes have inherited a trace element enriched component from the Lewisian lithosphere, which has resided there since ca. 3 Ga ago. Whether the dykes inherited this material from the crust or the mantle portions of the lithosphere or both, it seems likely that small basaltic melts derived from asthenospheric mantle were ultimately responsible for the enrichment. The simplest view is that these small melt fractions had been resident in the mantle part of the Lewisian lithosphere. In this case the Archaean trace-element enrichment and element fractionation in the Lewisian lithospheric mantle sampled by the dykes was closely associated in time with the generation of the 2.9 Ga old crustal portion of the lithosphere [36,37].  相似文献   

12.
Abstract Meatiq and Hafafit core complexes are large swells in the Eastern Desert of Egypt, comprising two major tectono‐stratigraphic units or tiers. The lower (infrastructure) unit is composed of variably cataclased gneissose granites and high‐grade gneisses and schists. It is structurally overlain by Pan–African ophiolitic mélange nappes (the higher unit). The two units are separated by a low‐angle sole thrust, along which mylonites are developed. Major and trace element data indicate formation of the gneissose granites in both volcanic arc and within‐plate settings. Nevertheless, all analyzed gneissose granites and other infrastructural rocks, exhibit low initial ratios (Sri) (<0.7027), positive εNd(t) (+4.9 to +10.3) and Neoproterozoic Nd model age (TDM) (592–831 Ma for the gneissose granite samples). Although these values are compatible with other parts of the Arabian– Nubian Shield considered to be juvenile, the εNd(t) values and several incompatible element ratios of the gneissose granites are too low to be derived from a mantle source without contribution from an older continental crust. Our geological, Sr–Nd isotopic and chemical data combined with the published zircon ages indicate the existence of a pre‐Neoproterozoic continent in the Eastern Desert that started to break up at ca 800 Ma. Rifting and subsequent events caused the formation of oceanic crust and emplacement within‐plate alkali basalts in the hinterland domains of the old continent. The emplacement of basaltic magma might have triggered melting of lower crust in the old continent and resulted in emplacement of the within‐plate granite masses between 700 Ma and 626 Ma. The granite masses and other rocks in the old continent have been subjected to deformation during the over‐thrusting of Pan–African nappes, probably because of the oblique convergence between East and West Gondwanaland. Rb–Sr isotopes of the gneissose granites in both Meatiq and Hafafit core complexes defines an isochron age of 619 ± 25 Ma with Sri of 0.7009 ± 0.0017 and mean squares of weighted deviates = 2.0. We interpret this age as the date of thrusting of the Pan–African nappes in the Eastern Desert. Continued oblique convergence between East and West Gondwanaland could have resulted in the formation northwest–southeast‐trending Meatiq and Hafafit anticlinoriums.  相似文献   

13.
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   

14.
The Abitibi Volcanic Belt in eastern Superior Province of the Canadian Shield is the largest continuous greenstone belt in the world and is a key example of late Archean crust. This belt has, in general, suffered a low intensity of metamorphism and deformation, and, as a result, the stratigraphy and geology are well established. Tholeiitic and calc-alkaline series of igneous rocks are present in this belt in about equal proportions. However, the undersaturated potassic and leucitic volcanics of the Timiskaming Group are a unique feature of this belt.SmNd systematics were determined for twelve Timiskaming volcanic rocks. These rocks show nepheline, diopside and/or olivine plus leucite in the norm and a highly fractionated REE pattern. Sm and Nd concentrations range from 25 to 160 and 45 to 300 times the chondritic abundance, respectively. The Sm and Nd isotopic data yield an isochron age of 2702±105Ma for these volcanic rocks with an initial εNd of +1.9±1.6. This age establishes the Timiskaming alkalic rock to be one of the oldest of their kind. From stratigraphic relations, 2705 Ma is an upper limit for the age and the εNd values of +1.8 to +2.2 at this age for the twelve rocks are also upper limits. Further, this small but positive εNd value for the isochron, when compared to other mantle-derived Archean rocks in the Superior Province, indicates that the Archean mantle was heterogeneous beneath the Canadian Shield and that the Timiskaming alkalic lavas were derived from a depleted mantle.  相似文献   

15.
Geochemical analyses of dikes, sills, and volcanic rocks of the Mesozoic Appalachian Tholeiite (MAT) Province of the easternmost United States provide evidence that continental tholeiites are derived from continental lithospheric mantle sources that are genetically and geochronologically related to the overlying continental crust. Nineteen olivine tholeiites and sixteen quartz tholeiites from the length of this province, associated in space and time with the last opening of the Atlantic, display significant isotopic heterogeneity: initial εNd = +3.8 to −5.7; initial 87Sr/86Sr= 0.7044−0.7072; 206Pb/204Pb= 17.49−19.14; 207Pb/204Pb= 15.55−15.65; 208Pb/204Pb= 37.24−39.11. In PbPb space, the MAT define a linear array displaced above the field for MORB and thus resemble oceanic basalts with DUPAL Pb isotopic traits. A regression of this array yields a secondary PbPb isochron age of ≈ 1000 Ma (μ1 = 8.26), similar to Sm/Nd isochrons from the southern half of the province and to the radiometric age of the Grenville crust underlying easternmost North America. The MAT exhibit significant trace element ratio heterogeneity (e.g., Sm/Nd= 0.226−0.327) and have trace element traits similar to convergent margin magmas [e.g., depletions of Nb and Ti relative to the rare earth elements on normalized trace element incompatibility diagrams, Ba/Nb ratios (19–75) that are significantly greater than those of MORB, and low TiO2 (0.39–0.69%)].Geochemical and geological considerations very strongly suggest that the MAT were not significantly contaminated during ascent through the continental crust. Further, isotope and trace element variations are not consistent with the involvement of contemporaneous MORB or OIB components. Rather, the materials that control the MAT incompatible element chemistry were derived from subcontinental lithospheric mantle. Thus: (1) the MAT/arc magma trace element similarities; (2) the PbPb and Sm/Nd isochron ages; and (3) the need for a method of introducing an ancient (> 2−3 Ga) Pb component into subcontinental mantle that cannot be much older than 1 Ga leads to a model whereby the MAT were generated by the melting of sediment-contaminated arc mantle that was incorporated into the continental lithosphere during arc activity preceding the Grenville Orogeny (≈ 1000 Ma).  相似文献   

16.
Abstract The Hakkoda‐Towada caldera cluster (HTCC) is a typical Late Cenozoic caldera cluster located in the northern part of the Northeast Japan Arc. The HTCC consists of five caldera volcanoes, active between 3.5 Ma and present time. The felsic magmas can be classified into high‐K (HK‐) type and medium‐ to low‐K (MLK‐) type based on their whole‐rock chemistry. The HK‐type magmas are characterized by higher K2O and Rb contents and higher 87Sr/86Sr ratios than MLK‐type magmas. Both magmas cannot be derived from fractional crystallization of any basaltic magma in the HTCC. Assimilation‐fractional crystallization model calculations show that crustal assimilation is necessary for producing the felsic magmas, and HK‐type magmas are produced by higher degree of crustal assimilation with fractional crystallization than MLK‐type magmas. Although MLK‐type magmas were erupted throughout HTCC activity, HK‐type magmas were erupted only during the initial stage. The temporal variations of magma types suggest the large contribution of crustal components in the initial stage. A major volcanic hiatus of 3 my before the HTCC activity suggests a relatively cold crust in the initial stage. The cold crust probably promoted crustal assimilation and fractional crystallization, and caused the initial generation of HK‐type magmas. Subsequently, the repeated supply of mantle‐derived magmas raised temperature in the crust and formed relatively stable magma pathways. Such a later system produced MLK‐type magmas with lesser crustal components. The MLK‐type magmas are common and HK‐type magmas are exceptional during the Pliocene–Quaternary volcanism in the Northeast Japan Arc. This fact suggests that exceptional conditions are necessary for the production of HK‐type magmas. A relatively cold crust caused by a long volcanic hiatus (several million years) is considered as one of the probable conditions. Intensive crustal assimilation and fractional crystallization promoted by the cold crust may be necessary for the generation of highly evolved HK‐type felsic magmas.  相似文献   

17.
The occurrence of ultrahigh pressure (UHP) minerals, such as coesite and diamond in crustal rocks in orogenic belts suggests that a huge amount of continental crust can be subducted to man-tle depth during the continental-continental collision[1—6]. This…  相似文献   

18.
Post-glacial tholeiitic basalts from the western Reykjanes Peninsula range from picrite basalts (oldest) to olivine tholeiites to tholeiites (youngest). In this sequence there are large systematic variations in rare earth element (REE) abundances (La/Sm normalized to chondrites ranges from 0.33 in the picrite basalts to 1.25 in the fissure tholeiites) and corresponding variations in 143Nd/144Nd (0.51317 in the picrite basalts to 0.51299 in the fissure tholeiites). The large viaration in 143Nd/144Nd, more than one-third the total range observed in most ocean islands and mid-ocean ridge basalts (MORB), is accompanied by only a small variation in 87Sr/86Sr (0.7031–0.7032). These 87Sr/86Sr ratios are within the range of other Icelandic tholeiites, and distinct from those of MORB.We conclude that the mantle beneath the Reykjanes Peninsula is heterogeneous with respect to relative REE abundances and 143Nd/144Nd ratios. On a time-averaged basis all parts of this mantle show evidence of relative depletion in light REE. Though parts of this mantle have REE abundances and Nd isotope ratios similar to the mantle source of “normal” MORB, 87Sr/86Sr is distinctly higher. Unlike previous studies we find no evidence for chondritic relative REE abundances in the mantle beneath the Reykjanes Peninsula; in fact, the data require significant chemical heterogeneity in the hypothesized mantle plume beneath Iceland, as well as lateral mantle heterogeneity from the Reykjanes Ridge to the Reykjanes Peninsula. The compositional range of the Reykjanes Peninsula basalts is consistent with mixing of magmas produced by different degrees of melting in different parts of the heterogeneous mantle source beneath the Reykjanes Peninsula.  相似文献   

19.
Equations describing trace element and isotopic evolution in a magma chamber affected simultaneously by fractional crystallization and wallrock assimilation are presented for a model where the mass assimilation rate(?a) is an arbitrary fraction(r) of the fractional crystallization rate(?c). The equations also apply to recharge of a crystallizing magma. Relatively simple analytical expressions are obtained for both radiogenic isotope variations (Nd, Sr, Pb) and stable isotopes (O, H) including the effects of mass-dependent fractionation. Forr = 1 a modified zone refining equation is obtained for trace element concentrations, but forr < 1 behavior is a combination of zone refining and fractional crystallization. Asr → ∞, simple binary mixing is approached. The isotopic and trace element “mixing” trends generated can be much different from binary mixing, especially forr < 1. The model provides the basis for a more general approach to the problem of wallrock assimilation, and shows that binary mixing models are insufficient to rule out crustal assimilation as a cause of some of the isotopic variations observed in igneous rocks, including cases where clustering of isotopic values occurs partway between presumed endmember values. The coupled assimilation-fractional crystallization model provides an explanation for certain trace element and isotopic properties of continental margin orogenic magmas (e.g. Sr concentration versus87Sr/86Sr) which had previously been interpreted as evidence against assimilation. So-called “pseudoisochrons” can be understood as artifacts of contamination using this model. A significant correlation exists between country rock age and low143Nd/144Nd ratios in continental igneous rocks, clearly suggestive that crustal contamination is generally important.  相似文献   

20.
Published data showing a linear correlation between initial Nd and Sr isotope compositions in young basalts indicate the existence of a spectrum of isotopically distinct reservoirs in the mantle which represent either (1) mixtures of two homogeneous endmember reservoirs, one of which may be undifferentiated material or (2) fractionated reservoirs all derived from a homogeneous initial reservoir with the same ratio of enrichment factors for Sm/Nd and Rb/Sr. The slope of the correlation, which can be described approximately by (87Sr/86Sr) = ?3.74114 (143Nd/144Nd) + 2.61935orεNd = ?2.7 εSr, places constraints on the origin of these reservoirs and hence on the chemical evolution of the crust-mantle system. The reservoirs could be residual regions of the mantle left after ancient partial melting events. If so, the requirement of constant relative fractionation of Sm/Nd and Rb/Sr in refractory residues is a strong constraint on partial melting models. Calculations suggest that batch melting models are more compatible with this constraint than are fractional melting models, but models incorporating currently accepted distribution coefficients and residual phase assemblages cannot reproduce the observed isotope effects except under highly specific conditions. The slope of the correlation is not consistent with the hypotheses that chemical structure in the mantle is due to accretional heterogeneity or variable loss of elements to the core. If the mantle reservoirs are complementary in composition to the continental crust, and if the crust + mantle has εNd = 0andεSr = 0 and chondritic Sr/Nd, then Rb/Sr in the crust is calculated to be less than 0.10, suggesting that the crust may be more mafic in composition and contain a smaller proportion of the earth's Rb and heat-producing elements than previously estimated.  相似文献   

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