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1.
The Pliocene and Quaternary Patagonian alkali basalts of southernmost South America can be divided into two groups. The cratonic basalts erupted in areas of Cenozoic plateau volcanism and continental sedimentation and show considerable variation in 87Sr/86Sr (0.70316 to 0.70512), 143Nd/144Nd (Nd) and 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios (18.26 to 19.38, 15.53 to 15.68, and 38.30 to 39.23, respectively). These isotopic values are within the range of oceanic island basalts, as are the Ba/La, Ba/Nb, La/Nb, K/Rb, and Cs/Rb ratios of the cratonic basalts. In contrast, the transitional basalts, erupted along the western edge of the outcrop belt of the Pliocene and Quaternary plateau lavas in areas that were the locus of earlier Cenozoic Andean orogenic arc colcanism, have a much more restricted range of isotopic composition which can be approximated by 87Sr/86Sr=0.7039±0.0004, Nd, 206Pb/204Pb=18.60±0.08, 207Pb/204Pb=15.60±0.01, and 208Pb/204Pb=38.50±0.10. These isotopic values are similar to those of Andean orogenic are basalts and, compared to the cratonic basalts, are displaced to higher 87Sr/86Sr at a given 143Nd/144Nd and to higher 207Pb/204Pb at a given 208Pb/204Pb. The transitional basalts also have Ba/La, Ba/Nb, La/Nb, and Cs/Rb ratios higher than the cratonic and oceanic island basalts, although not as high as Andean orogenic are basalts. In contrast to the radiogenic isotopes, 18O values for both groups of the Patagonian alkali basalts are indistinguishable and are more restricted than the range reported for Andean orogenic are basalts. Whole rock 18O values calculated from mineral separates for both groups range from 5.3 to 6.5, while measured whole rock 18O values range from 5.1 to 7.8. The trace element and isotopic data suggest that decreasing degrees of partial melting in association with lessened significance of subducted slabderived components are fundamental factors in the west to east transition from arc to back-arc volcanism in southern South America. The cratonic basalts do not contain the slab-derived components that impart the higher Ba/La, Ba/Nb, La/Nb, Cs/Rb, 87Sr/86Sr at a given 143Nd/144Nd, 207Pb/204Pb at a given 208Pb/204Pb, and 18O to Andean orogenic arc basalts. Instead, these basalts are formed by relatively low degrees of partial melting of heterogeneous lower continental lithosphere and/or asthenosphere, probably due to thermal and mechanical pertubation of the mantle in response to subduction of oceanic lithosphere below the western margin of the continent. The transitional basalts do contain components added to their source region by either (1) active input of slab-derived components in amounts smaller than the contribution to the mantle below the arc and/or with lower Ba/La, Ba/Nb, La/Nb, and Cs/Rb ratios than below the arc due to progressive downdip dehydration of the subducted slab; or (2) subarc source region contamination processes which affected the mantle source of the transitional basalts earlier in the Cenozoic.  相似文献   

2.
Strong compositional zonation of the 34 Ma Grizzly Peak Tuff in west-central Colorado is attended by non-monotonic trends in O, Sr, Nd, and Pb isotope ratios. Fiamme from the tuff cluster in chemical compositions and petrographic characteristics, indicating the magma chamber was not continuously zoned but consisted of at least seven compositional layers. The most mafic magma erupted (57 wt% SiO2, fiamme group 7) had 18O= +8.5, initial 87Sr/86Sr=0.7099, Nd, and 206Pb/204Pb=17.80, suggesting that the magma was produced by 50% fractional crystallization of basaltic magma that assimilated 20 to 40 wt% Proterozoic crust. Isotopic compositions of more evolved parts of the chamber (up to 77 wt% SiO2, fiamme group 1) depart from the mafic base-level composition of fiamme group 7, and reflect late-stage assimilation that occurred largely after compositional layering was established. 18O values decrease by as much as 1.5 from fiamme groups 7 through 4, indicating assimilation of hydrothermally altered roof rocks. 18O values abruptly inerease by up to 1.5 between fiamme groups 4 and 3. This discontinuity is interpreted to reflect evolution in an asymmetric chamber that had a split-level roof, allowing assimilation of wall rocks that varied vertically in degree of hydrothermal alteration. This chamber geometry is also supported by collapse structures in the caldera. Late-stage assimilation of heterogeneous wall rocks is also indicated by variations in Sr, Nd, and Pb isotope ratios. Large Sr isotope disequilibrium exists between some phenocrysts and whole-rock fiamme, and initial 87Sr/86Sr ratios in phenocrysts are as high as 0.7170. values regularly increase from-13.0 in fiamme group 7 to-11.3 in fiamme group 3, and then decrease to-12.2 in fiamme group 1. 206Pb/204Pb ratios generally increase from 17.80 to 17.94 for fiamme groups 7 through 1. The rhyolitic parts of the Grizzly Peak Tuff have isotopic compositions that could be attributed to a purely crustal melt. It is unlikely, however, that the mafic parts of the tuff were generated by crustal melting, and the compositional and isotopic variations across the entire zonation of the tuff are best explained by fractional crystallization of mantle-derived magmas, accompanied by extensive assimilation of Proterozoic crust.  相似文献   

3.
Geology must consider the physical processes involved in the genesis of inorganic matter. These processes are ultimately based on cosmogony and cosmology and the associated physics and mathematics of these origins.The approach to genesis impinges on the unknown where a feeling that something finite, and possibly real, exists. The evidence of the unknown must then be compared with the experimental and observed evidence to create a framework from which certain conclusions can be made.The thoughts on genesis and the geological implications are discussed briefly in two parts. Part one includes the basic philosophy, the mathematical and physical concepts; it outlines the philosophy of time, zero and infinity, mass, space, and the mass environments, based on this a ring hypothesis of planetary origins is developed. Part two discusses basic problems of structural geology, fundamental tectonics, the development of continents and continental drift, climatic changes, seismicity, and paleomagnetics.The conclusions drawn from the discussion are: mass ormatter can exist in five states. Continental buttresses are largely made up of rocks of secondary igneous origin. The core of the earth is made of solar (meteoric) material rather than pure nickel-iron. Continents move under differential density forces which ultimately result in a slow twist or rotation of the earth's outer layers about the core. This rotation causes climatic changes and the many paths observed in tracing polar wandering by paleomagnetics.
Zusammenfassung Dieser Aufsatz versucht, fundamentale geologische Grenzprobleme in Beziehung zu bringen mit fundamentaien physikalischen und astronomischen Problemen, die ihrerseits z. T. noch einer Lösung harren.Ausgehend von einer Betrachtung der Herkunft von Masse, Universum und Sonnensystem werden Probleme der Kontinentalgenese, Kontinentalwanderung, hydrostatischer Druck auf und in Kontinenten, Orogenese, Klimaänderungen und Paläomagnetik in geologischer Zeit besprochen.Der Aufsatz besteht aus zwei Teilen, einem physikalischen und einem geologischen, die zu einem gewissen Grade unabhängig voneinander gelesen werden können. Für ein fruchtbares Verstehen ist es aber ratsam, beide Teile zu lesen. Mathematik ist auf das notwendigste beschränkt.Am Ausgangspunkt der Betrachtung steht die Idee von einem fundamentalen (letztlichen) Masse-Medium, da dieses die Grenze der Erkenntnis bildet.
Résumé La géologie doit considérer le processus physique entraîné dans la genèse de matière inorganique. Ce processus est finalement basé sur la cosmogonie et cosmologie, ainsi que la physique associée et mathématiques de ces originesL'approche de la genèse se heurte à l'inconnu où le sentiment de quelque chose de fini et peut-être bien réel existe. L'évidence de l'inconnu doit alors être comparée à l'évidence expérimentale et observée, pour créer une structure de laquelle certaines conclusions peuvent être tirées.Les réflexions sur la genèse et les implications géologiques sont exposées brièvement en deux parties. La première partie comprend la philosophie fondamentale, les idées générales de mathématique et physique; elle esquisse la philosophie du temps, zéro et infini, masse, espace et environnements de la masse; basée là-dessus, une hypothèse en cercle des origines planétaires est développée. La seconde partie expose les problèmes fondamentaux de géologie structurale, tectoniques fondamentales, le développement des continents et le mouvement de ceux-ci, changements de climat, séismicité et paléomagnétiques.Les conclusions tirées de l'exposé sont que masse ou matière peuvent exister dans cinq états. Les contreforts continentaux sont en grande partie formés de roche pyrogène secondaire. Le noyau de la terre est fait de matière solaire (météorique) plutôt que de nickel-fer pur. Les continents se déplacent sous des forces de densité différentielle qui résultent finalement dans une torsion lente ou rotation des couches extérieures de la terre autour du noyau. Cette rotation provoque des changements climatique et les nombreuses voies observées en traçant les écarts polaires par paléomagnétique.
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4.
The Kahoolawe shield volcano produced precaldera and caldera-filling tholeiites and mildly alkalic post-caldera lavas that petrographically and compositionally resemble such lavas from other Hawaiian shield volcanoes. However, Kahoolawe tholeiites display wide ranges in incompatible trace element ratios (e.g., Nb/Th=9–24, Th/Ta=0.6–1.3), 87Sr/86Sr (0.70379–0.70440), 143Nd/144Nd (0.51273–0.51298), and 206Pb/204Pb (17.92–18.37). The isotopic variation exceeds that at any other Hawaiian shield volcano, and spans about half the range for all Hawaiian tholeiites. Quasi-cyclic temporal evolution of Kahoolawe tholeiites is consistent with combined fractional crystallization and periodic recharge by primitive magmas. Ratios of highly incompatible trace elements and Sr, Nd, and Pb isotopic ratios from coherent sub-trends that reflect recurrent interactions between variably evolved magmas and two other mantle components whose compositions are constrained by intersections between these trends. The most MgO-rich Kahoolawe tholeiites are partial melts of a high Nb/Th (23.5) ascending plume, possibly comprising ancient subducted oceanic lithosphere. Slightly evolved tholeiites experienced combined crystal fractionation and assimilation (AFC) of material derived from a distinct reservoir (Nb/Th 9) of asthenospheric derivation. The most evolved tholeiites display compositional shifts toward a third component, having mid ocean ridge basalt-like isotopic ratios but enriched OIB-like trace element ratios, representing part of the lithospheric mantle (or melts thereof). Periodic recurrence of all three magma variants suggests that eruptions may have tapped coeval reservoirs distributed over a large depth range. Kahoolawe provides new evidence concerning the nature of the Hawaiian plume, the distribution of compositional heterogeneities in the suboeanic mantle, and the processes by which Hawaiian tholeiites form and evolve.  相似文献   

5.
The magma sources for granitic intrusions related to the Mesozoic White Mountain magma series in northern New England, USA, are addressed relying principally upon Nd isotopes. Many of these anorogenic complexes lack significant volumes of exposed mafic lithologies and have been suspected of representing crustal melts. Sm–Nd and Rb–Sr isotope systematics are used to evaluate magma sources for 18 felsic plutons with ages ranging from about 120 to 230 Ma. The possibility of crustal sources is further examined with analyses of representative older crust including Paleozoic granitoids which serve as probes of the lower crust in the region. Multiple samples from two representative intrusions are used to address intrapluton initial isotopic heterogeneities and document significant yet restricted variations (<1 in Nd). Overall, Mesozoic granite plutons range in Nd [T] from +4.2 to -2.3, with most +2 to 0, and in initial 87Sr/86Sr from 0.7031 to 0.709. The isotopic variations are roughly inversely correlated but are not obviously related to geologic, geographic, or age differences. Older igneous and metamorphic crust of the region has much lower Nd isotope ratios with the most radiogenic Paleozoic granitoid at Nd [180 Ma] of -2.8. These data suggest mid-Proterozoic separation of the crust in central northern New England. Moreover, the bulk of the Mesozoic granites cannot be explained as crustal melts but must have large mantle components. The ranges of Nd and Sr isotopes are attributed to incorporation of crust by magmas derived from midly depleted mantle sources. Crustal input may reflect either magma mixing of crustal and mantle melts or crustal assimilation which is the favored interpretation. The results indicate production of anorogenic granites from mantle-derived mafic magmas.  相似文献   

6.
Sr- and Pb-isotope data from the Calabozos center (87Sr/86Sr= 0.7043, 206Pb/204Pb=18.64–18.66, 207Pb/204Pb=15.59–15.60, 208Pb/204Pb=38.52–38.55) fall within the range of values reported for the southern volcanic zone (33–42° S) of the Andean arc. The range of 18O (5.0–6.3), however, includes unusually low values compared to volcanic rocks of similar bulk composition in the region. The Calabozos caldera complex lies at 35 °30 S, where the continental crust under the Andes thins southward from >45 to 30 km. Three voluminous late Pleistocene ashflow tuffs, collectively called the Loma Seca Tuff, constitute the bulk of >1,000 km3 of eruptive products at the Calabozos caldera complex and are evidence for a major, longlived andesitic-to-rhyodacitic magma reservoir at shallow crustal levels. The 18O values of the most evolved volcanic rocks from the Calabozos center are lower than predicted for rhyodacite produced by crystal fractionation from basalt typical of the region. Variation of 18O independent of bulk composition and inferred magmatic water contents indicates that the 18O depletion is a late-stage, upper-crustal phenomenon that cannot simply be attributed to magmatic interaction with meteoric water. The data are interpreted to be the result of assimilation of 5–30% of roof and wall rocks previously depleted in 18O by isotopic exchange in a meteoric hydrothermal system overlying the magma reservoir. Combined assimilation and fractional crystallization calculations applied to Sr isotope data show that the isotopic contrast between the Calabozos magmas and the assimilated rocks is very small. Hydrothermally-altered volcanic and plutonic rocks from the Tertiary Andean arc complex and Mesozoic-to-Cenozoic volcaniclastic sediments typical of the local basement provide a geologically reasonable contaminant compatible with the Sr- and O-isotope data. Pb-isotope data from the Calabozos system lend no significant insight into upper crustal contamination.  相似文献   

7.
Previous studies of alkalic lavas erupted during the waning growth stages (<0.9 Ma to present) of Haleakala volcano identified systematic temporal changes in isotopic and incompatible element abundance ratios. These geochemical trends reflect a mantle mixing process with a systematic change in the proportions of mixing components. We studied lavas from a 250-m-thick stratigraphic sequence in Honomanu Gulch that includes the oldest (1.1 Ma) subaerial basalts exposed at Haleakaka. The lower 200 m of section is intercalated tholeiitic and alkalic basalt with similar isotopic (Sr, Nd, Pb) and incompatible element abundance ratios (e.g., Nb/La, La/Ce, La/Sr, Hf/Sm, Ti/Eu). These lava compositions are consistent with derivation of alkalic and tholeiitic basalt by partial melting of a compositionally homogeneous, clinopyroxene-rich, garnet lherzolite source. The intercalated tholeiitic and alkalic Honomanu lavas may reflect a process which tapped melts generated in different portions of a rising plume, and we infer that the tholeiitic lavas reflect a melting range of 10% to 15%, while the intercalated alkalic lavas reflect a range of 6.5% to 8% melting. However, within the uppermost 50 m of section. 87Sr/86Sr decreases from 0.70371 to 0.70328 as eruption age decreased from 0.97 Ma to 0.78 Ma. We infer that as lava compositions changed from intercalated tholeiitic and alkalic lavas to only alkalic lavas at 0.93 Ma, the mixing proportions of source components changed with a MORB-related mantle component becoming increasingly important as eruption age decreased.  相似文献   

8.
The Re — Os isotopic systematics of komatiites and spatially associated basalts from Gorgona Island, Colombia, indicate that they were produced at 155±43 Ma. Subsequent episodes of volcanism produced basalts at 88.1±3.8 Ma and picritic and basaltic lavas at ca. 58 Ma. The age for the ultramafic rocks is important because it coincides with the late-Jurassic, early-Cretaceous disassembly of Pangea, when the North- and South-American plates began to pull apart. Deep-seated mantle upwelling possibly precipitated the break-up of these continental plates and caused a tear in the subducting slab west of Gorgona, providing a rare, late-Phanerozoic conduit for the komatiitic melts.Mantle sources for the komatiites were heterogeneous with respect to Os and Pb isotopic compositions, but had homogeneous Nd isotopic compositions (Nd+9±1). Initial 187Os/186Os normalized to carbonaceous chondrites at 155 Ma (Os) ranged from 0 to +22, and model-initial values ranged from 8.17 to 8.39. The excess radiogenic Os, compared with an assumed bulk-mantle evolution similar to carbonaceous chondrites, was likely produced in portions of the mantle with long-term elevated Re concentrations. The Os, Pb and Nd isotopic compositions, together with major-element constraints, suggest that the sources of the komatiites were enriched more than 1 Ga ago by low (<20%) and variable amounts of a basalt or komatiite component. This component was added as either subducted oceanic crust or melt derived from greater depths in the mantle. These results suggest that the Re — Os isotope system may be a highly sensitive indicator of the presence of ancient subducted oceanic crust in mantle-source regions.  相似文献   

9.
Pb, Sr and Nd isotope variations are correlated in diverse lavas erupted at small seamounts near the East Pacific Rise. Tholeiites are isotopically indistinguishable from MORB (206Pb/204Pb=18.1–18.5; 87Sr/86Sr=0.7023–0.7028; 143Nd/144Nd=0.51326-0.51308); associated alkali basalts always show more radiogenic Pb and Sr signatures (206Pb/204Pb=18.8–19.2; 87Sr/86Sr=0.7029–0.7031) and less radiogenic Nd (143Nd/144Nd=0.51289–0.51301). The isotopic variability covers 80% of the variability for Pacific MORB, due to the presence of small-scale heterogeneity in the underlying mantle. Isotope compositions also correlate with trace element ratios such as La/Sm. Tholeiites at these seamounts have 3He/4He between 7.8–8.7 R A(R A= atmospheric ratio), also indistinguishable from MORB. He trapped in vesicles of alkali basalts, released by crushing in vacuo, has low 3He/4He (1.2–2.6 R)Ain conjunction with low helium concentrations ([He]<5×10–8 ccSTP/g). In many cases post-eruptive radiogenic ingrowth has produced He isotope disequilibrium between vesicles and glass in the alkali basalts; subatmospheric 3He/4He ratios characterize the He dissolved in the glass which is released by melting the crushed powders. The narrow range of 3He/4He in the vesicles of the alkali basalts suggests that low 3He/4He is a source characteristic, but given their low [He] and high (U + Th), pre-eruptive radiogenic ingrowth cannot be excluded as a cause for low inherited 3He/4He ratios. Pb, Sr and Nd isotope compositions in lavas erupted at Shimada Seamount, an isolated volcano on 20 m.y. old seafloor at 17°N, are distinctly different from other seamounts in the East Pacific (206Pb/204Pb=18.8–19.0, 87Sr/ 86Sr0.7048 and 143Nd/144Nd0.51266). Relatively high 207Pb/204Pb (15.6–15.7) indicates ancient (>2 Ga) isolation of the source from the depleted upper mantle, similar to Dupal components which are more prevalent in the southern hemisphere mantle. 3He/4He at Shimada Seamount is between 3.9–4.8 R A. Because the helium concentrations range up to 1.5×10–6, the low 3He/4He can not be due to radiogenic accumulation of 4He in the magma for reasonable volcanic evolution times. The low 3He/4He may be due to the presence of enriched domains within the lithosphere with high (U + Th)/He ratios, possibly formed during its accretion near the ridge. Alternatively, the low 3He/4He may be an inherent characteristic of an enriched component in the mantle beneath the East Pacific. Collectively, the He-Pb-Sr-Nd isotope systematics at East Pacific seamounts suggest that the range of isotope compositions present in the mantle is more readily sampled by seamount and island volcanism than by axial volcanism. Beneath thicker lithosphere away from the ridge axis, smaller degrees of melting in the source regions are less efficient in averaging the chemical characteristics of small-scale heterogeneities.  相似文献   

10.
Ridge segments and fracture zones from the American-Antarctic Ridge have been systematically dredge sampled from 4° W to 18° W. Petrographic studies of the dredged basalts show that the dominant basalt variety is olivine-plagioclase basalt, although olivine-plagioclase-clinopyroxene basalt is relatively common at some localities. Selected samples have been analysed for major and trace elements, rare earth elements and Sr and Nd isotopes. These data show that the majority of samples are slightly evolved (Mg#=69-35) N-type MORB, although a small group of samples from a number of localities have enriched geochemical characteristics (T- and P-type MORB).These different types of MORB are readily distinguished in terms of their incompatible trace element and isotopic characteristics: N-type MORB have high Zr/Nb (17–78), Y/Nb (4.6–23) and 143Nd/144Nd (0.51303–0.51308) ratios, low Zr/Y (2.2–4.2) and 87Sr/86Sr (0.70263–0.70295) ratios and have (La/Sm)N<1.0; T-type MORB have lower than chondritic Zr/Nb ratios (8.8–15.5), relatively low Y/Nb (1.9–4.3) and 143Nd/144Nd (0.51296–0.51288) ratios and relatively high Zr/Y (3.1–4.7), 87Sr/86Sr (0.70307–0.70334) and (La/Sm)N (1.1–1.5) ratios; the single sample of P-type MORB has low Zr/Nb (6.3), Y/Nb (0.9) and 143Nd/144Nd (0.51287) ratios and high Zr/Y (7.1), 87Sr/86Sr (0.70351) and (La/Sm)N (2.4) ratios. The geochemical characteristics of this sample are essentially identical to those of the Bouvet Island lavas.Geochemically enriched MORB are less abundant on the American-Antarctic Ridge than on the Southwest Indian Ridge but their geochemical characteristics are identical. The compositions of T- and P-type MORB are consistent with a regional mixing model involving normal depleted mantle and Bouvet plume type magma. On a local scale the composition of T-type MORB is consistent with derivation from depleted mantle which contains 4% veins of P-type melt.We propose a model for the evolution of the American-Antarctic Ridge lavas in which N-type MORB is derived from mantle with negligible to low vein/mantle ratios, T-type MORB is derived from domains with moderate and variable vein/mantle ratios and P-type MORB from regions with very high vein/mantle ratios where vein material comprises the major portion of the melt. The sparse occurrence of enriched lavas and by implication enriched mantle beneath the American-Antarctic Ridge, some distance (500–1,200 km) from the Bouvet plume location, is interpreted to be the result of lateral dispersion of enriched mantle domains by asthenospheric flow away from the Bouvet mantle plume towards the American-Antarctic Ridge.  相似文献   

11.
Pb, Sr and Nd isotopic compositions have been analyzed in recent granites from Northern Africa, Northern Italy and Greece. Lead isotope compositions of K-feldspars are rather homogeneous, and cluster close to the modern lead of Stacey and Kramers (1975) but with slightly higher207Pb/204Pb and208Pb/204Pb ratios. The Cyclades samples, however, have higher206Pb/204Pb ratios. Addition of mantle-derived lead was probably very limited, which supports a quasi-closed system evolution of this element in the continental crust. The Sr, Nd data fall in the enriched part of the143Nd/144Nd vs.87Sr/86Sr diagram and define a smooth hyperbolic mixing curve. Over a wide area, straddling different orogens, most granites may be accounted for by a binary mixture between a recycled crustal component and a depleted mantle-like component. No correlation is observed between either Pb and Sr or Nd isotopic ratios, or any isotopic ratio and major element contents. Quantitative modelling suggests that two cases fit the Sr and Nd characteristics of these granites: they both require anatexis of the crust on a scale large enough to average the isotopic properties of heterogeneous terranes. In the first case, the mantle-derived component may be represented by differentiated Island Arc-type magmas, and the granites result from mixing these magmas with anatectic melts. In the second case, mantle-derived igneous rocks, such as obducted ophiolites, are part of the crustal source and their variable involvement in the anatectic process causes isotopic variations.CRPG Contribution n 630.  相似文献   

12.
A comprehensive Sr–Nd–Pb–O isotopic study is reported for rhyolites from the Maroa Volcanic Centre in the Taupo Volcanic Zone (TVZ) of New Zealand. The Sr–Nd isotopic compositions of the rhyolites (87Sr/86Sr=0.705236 to 0.705660 and Nd = 2.0 to 0.2) are intermediate between those of primitive basalts (87Sr/86Sr=0.70387 and Nd = 5.3) and the Torlesse basement (87Sr/86Sr=0.709 and Nd = -4.5). The relatively low mantle-like oxygen isotopic compositions of 18 O = 7 ± 0.5 are consistent with the Nd-Sr isotopic constraints in that they can be accounted for by 15% to 25% crustal contamination of a basaltic parent by relatively 18 O-rich Torlesse metasediment. High precision Pb isotopic analyses of plagioclase separates from the Maroa rhyolites show that they have essentially the same compositions as the Torlesse metasedimentary terrane which is itself distinctive from the Western or Waipapa metasediments. Due to the high concentration of Pb in the Torlesse metasediments (>20 ppm) compared to the basalts (<2 ppm), the Pb isotopic composition of the volcanics may be controlled by relatively small amounts (>10%) of crustal contamination. All these results are shown to be consistent with derivation of the rhyolites by 15% to 25% contamination of relatively primitive basaltic magmas with Torlesse metasedimentary crust, followed by extensive, essentially closed system fractionation of the basalt to a magma of rhyolite composition. It is argued that the processes of assimilation and fractionation are separated in both space and time. The voluminous high silica rhyolites, which make up >97% of the exposed volcanism in the continental margin back-are basin environment of the TVZ, therefore appear to be a product of predominantly new additions to the crust with assimilation-recycling of pre-existing crust being of secondary importance.  相似文献   

13.
The 50 km-long West Valley segment of the northern Juan de Fuca Ridge is a young, extension-dominated spreading centre, with volcanic activity concentrated in its southern half. A suite of basalts dredged from the West Valley floor, the adjacent Heck Seamount chain, and a small near-axis cone here named Southwest Seamount, includes a spectrum of geochemical compositions ranging from highly depleted normal (N-) MORB to enriched (E-) MORB. Heck Seamount lavas have chondrite-normalized La/Smcn0.3, 87Sr/86Sr=0.70235–0.70242, and 206Pb/204Pb=18.22–18.44, requiring a source which is highly depleted in trace elements both at the time of melt generation and over geologic time. The E-MORB from Southwest Seamount have La/Smcn1.8, 87Sr/86Sr=0.70245–0.70260, and 206Pb/204Pb=18.73–19.15, indicating a more enriched source. Basalts from the West Valley floor have chemical compositions intermediate between these two end-members. As a group, West Valley basalts from a two-component mixing array in element-element and element-isotope plots which is best explained by magma mixing. Evidence for crustal-level magma mixing in some basalts includes mineral-melt chemical and isotopic disequilibrium, but mixing of melts at depth (within the mantle) may also occur. The mantle beneath the northern Juan de Fuca Ridge is modelled as a plum-pudding, with plums of enriched, amphibole-bearing peridotite floating in a depleted matrix (DM). Low degrees of melting preferentially melt the plums, initially removing only the amphibole component and producing alkaline to transitional E-MORB. Higher degrees of melting tap both the plums and the depleted matrix to yield N-MORB. The subtly different isotopic compositions of the E-MORBs compared to the N-MORBs require that any enriched component in the upper mantle was derived from a depleted source. If the enriched component crystallized from fluids with a DM source, the plums could evolve to their more evolved isotopic composition after a period of 1.5–2.0 Ga. Alternatively, the enriched component could have formed recently from fluids with a less-depleted source than DM, such as subducted oceanic crust. A third possibility is that enriched material might be dispersed as plums throughout the upper mantle, transported from depth by mantle plumes.  相似文献   

14.
The early miocene Tecuya volcanic center in the southern San Joaquin basin of California consists of flows and tuffs of basalt and rhyolite that erupted, closely spaced in time, in both submarine and subaerial conditions. The rhyolites are overlain by the basalts and constitute approximately 45% of a total of at least 180 km3 of the Tecuya volcanic rocks. The basalts have Nd(t) values of +2 to +6 and (87Sr/86Sr)i values between 0.7035 and 0.7052. These rocks show LREE enrichment [(La/Yb)N =2.4–5.5; La=28–150 times chondrite] and higher Th/U, Th/Ta, Rb/Ta, Ba/Ta, Cs/Rb but lower K/Rb ratios than MORB. Combined major- and trace-element, and Sr–Nd isotopic data suggest the involvement of subcontinental lithosphere, depleted upper mantle source (MORB), and local continental crust in the basalt petrogenesis. Nd(t) values in rhyolites vary from +1.5 to +3.7 while (87Sr/86Sr)i ratios range from 0.7051 to 0.7064. The rhyolites display LREE enrichment [(La/Yb)N=10; La=100 times chondrite] along with a distinct negative Eu anomaly (Eu/Eu*=0.75) and depletion of Ti and P. Mixing relations in (87/86Sr)i – Nd(t) space among basalts, rhyolites, and local continental crust indicate that the Tecuya rhyolites were produced by assimilation of variable amounts of continental crust by MORB-related magmas and subcontinental lithosphere-derived melts. This conclusion is supported by the synchroneity of Tecuya volcanism at 22 Ma with interaction of a segment of the East Pacific Rise along the southern California margin. The Tecuya volcanic rocks thus provide an example for the generation of rhyolitic melts owing to crustal assimilation by basaltic melts during mid-oceanic ridge-induced magmatism along a continental margin.  相似文献   

15.
Geochemical and Nd-Sr-Pb-O isotope data for a suite of syn-collisional (ca. 520 Ma) syenites associated with a major shear zone in the Proterozoic Damara orogen (Namibia) constrain their sources and petrogenesis. Major rock types from within and outside the shear zone range from highly potassic nepheline syenites to quartz syenites and were primarily generated by fractional crystallization from a mantle-derived alkaline magma. Even the most primitive samples show pronounced depletion in Nb, Ti, Sr and P on a primitive mantle-normalized diagram, indicating the involvement of a recycled crustal component in the source. Extrapolation of the Sr-Nd-Pb-O isotope composition of the syenites from within the shear zone back to a hypothetical parental melt with 10 wt% MgO suggests derivation from a moderately enriched lithospheric upper mantle (87Sr/86Sr: 0.705, Nd: –2, 18O: 6, 206Pb/204Pb: 19.40, 207Pb/204Pb: 15.82). More evolved quartz syenites show increasing 87Sr/86Sr ratios, increasing 18O values but less radiogenic Nd values and Pb isotopes with decreasing MgO, indicating assimilation of ca. 10% Archaean to Proterozoic local lower crust with unradiogenic Nd, high 87Sr/86Sr and low U/Pb. For samples from outside the shear zone a hypothetical parental melt with 10 wt% MgO has distinctly more radiogenic Sr but less radiogenic Nd isotopic composition (87Sr/86Sr: 0.712, Nd: –13), with strongly unradiogenic Pb isotope ratios (206Pb/204Pb: 17.40, 207Pb/204Pb: 15.50), suggesting another strongly enriched lithospheric mantle source for these rocks. Differentiated syenites from outside the shear zone show decreasing 87Sr/86Sr, increasing 18O values, more radiogenic Nd values and Pb isotope ratios with decreasing MgO indicating interaction with a lithospheric component with low Rb/Sr but high Sm/Nd and U/Pb.  相似文献   

16.
Over 200 H, O, Sr, Nd, and Pb isotope analyses, in addition to geologic and petrologic constraints, document the magmatic evolution of the 28.5–19 Ma Latir volcanic field and associated intrusive rocks, which includes multiple stages of crustal assimilation, magma mixing, protracted crystallization, and open- and closed-system evolution in the upper crust. In contrast to data from younger volcanic centers in northern New Mexico, relatively low and restricted primary 18O values (+6.4 to +7.4) rule out assimilation of supracrustal rocks enriched in 18O. Initial 87Sr/86Sr ratios (0.705 to 0.708), 18O values (-2 to-7), and 206Pb/204Pb ratios (17.5 to 18.4) of metaluminous precaldera volcanic rocks and postcaldera plutonic rocks suggest that most Latir rocks were generated by fractional crystallization of substantial volumes of mantle-derived basaltic magma that had near-chondritic Nd isotope ratios, accompanied by assimilation of crustal material in two main stages: 1) assimilation of non-radiogenic lower crust, followed by 2) assimilation of middle and upper crust by inter-mediate-composition magmas that had been contaminated during the first stage. Magmatic evolution in the upper crust peaked with eruption of the peralkaline Amalia Tuff (26 Ma), which evolved from metaluminous parental magmas. A third stage of late, roofward assimilation of Proterozoic rocks in the Amalia Tuff magma is indicated by trends in initial 87Sr/86Sr and 206Pb/204Pb ratios from 0.7057 to 0.7098 and 19.5 to 18.8, respectively, toward the top of the pre-eruptive magma chamber. Highly evolved postcaldera plutons are generally fine grained and are zoned in initial 87Sr/86Sr and 206Pb/204Pb ratios, varying from 0.705 to 0.709 and 17.8 to 18.6, respectively. In contrast, the coarser-grained Cabresto Lake (25 Ma) and Rio Hondo (21 Ma) plutons have relatively homogeneous initial 87Sr/86Sr and 206Pb/204Pb ratios of approximately 0.7053 and 17.94 and 17.55, respectively. 18O values for all the postcaldera plutons overlap those of the precaldera rocks and Amalia Tuff, except for those for two late-stage rhyolite dikes associated with the Rio Hondo pluton that have 18O values of-8.6 and-9.5; these dikes are the only Latir rocks which may be largely crustal melts.Chemical and isotopic data from the Latir field suggest that large fluxes of mantle-derived basaltic magma are necessary for developing and sustaining large-volume volcanic centers. Development of a detailed model suggests that 6–15 km of new crust may have been added beneath the volcanic center; such an addition may result in significant changes in the chemical and Sr and Nd isotopic compositions of the crust, although Pb isotope ratios will remain relatively unchanged. If accompanied by assimilation, crystallization of pooled basaltic magma near the MOHO may produce substantial cumulates beneath the MOHO that generate large changes in the isotopic composition of the upper mantle. The Latir field may be similar to other large-volume, long-lived intracratonal volcanic fields that fundamentally owe their origins to extensive injection of basaltic magma into the lower parts of their magmatic systems. Such fields may overlie areas of significant crustal growth and hybridization.  相似文献   

17.
Granitoids within the Precambrian basement of north-eastern and southern Somalia are subdivided on the basis of geology, geochronology and petrology into three different assemblages. The post-kinematic assemblage in north-eastern Somalia ( 630 Ma) comprises granodiorites and granites which belong to a medium-K calc-alkaline suite. Average initial Sr, Nd and Pb isotopic ratios [Sri = 0.7048, Nd = –1.8,206Pb/204Pb(i) = 17.704 and207Pb/204Pb(i) = 15.611] indicate that these melts were derived from a mantle or juvenile crustal source with only slight involvement of pre-existing crust as a contaminant. Two different assemblages are found in southern Somalia. The older assemblage is composed of crustal anatectic, synkinematic, parautochthonous granites ( 600 Ma) related to amphibolite facies retrogression of an intensively reworked pre-Pan-African crust [Sri = 0.7100, Nd = –8.4,206Pb/204Pb(i) = 15.403 and207Pb/204Pb(i) = 15.259]. These monzo- and syenogranites are moderately potassic and peraluminous. The younger assemblage ( 470 Ma) consists of post-kinematic monzonites to syenogranites with A-type affinities. Initial Sr, Nd and Pb isotopic data for this metaluminous assemblage [Sri = 0.7114, Nd = –13.1,207Pb/204Pb(i) = 16.913 and207Pb/204Pb(i) = 15.512] indicate a significant lower crustal component but, however, also a mantle signature. The late Proterozoic to early Palaeozoic granitoids in Somalia thus express contrasting regimes, characterized by strong juvenile input in the north, close to the Arabian-Nubian Shield, whereas intense crustal reworking with little addition of juvenile material prevailed in the south. Somalia was definitively not a cratonic area during the Pan-African, but a zone of high crustal mobility.  相似文献   

18.
Nd, Sr and O isotopic data were obtained from silicic ash-flow tuffs and lavas at the Tertiary age (16–9 Ma) Timber (Mountain/Oasis Valley volcanic center (TMOV) in southern Nevada, to assess models for the origin and evolution of the large-volume silicic magma bodies generated in this region. The large-volume (>900 km3), chemically-zoned, Topopah Spring (TS) and Tiva Canyon (TC) members of the Paintbrush Tuff, and the Rainier Mesa (RM) and Ammonia Tanks (AT) members of the younger Timber Mountain Tuff all have internal Nd and Sr isotopic zonations. In each tuff, high-silica rhyolites have lower initial Nd values (1 Nd unit), higher87Sr/86Sr, and lower Nd and Sr contents, than cocrupted trachytes. The TS, TC, and RM members have similar Nd values for high-silica rhyolites (-11.7 to -11.2) and trachytes (-10.5 to -10.7), but the younger AT member has a higher Nd for both compositional types (-10.3 and -9.4). Oxygen isotope data confirm that the TC and AT members were derived from low Nd magmas. The internal Sr and Nd isotopic variations in each tuff are interpreted to be the result of the incorporation of 20–40% (by mass) wall-rock into magmas that were injected into the upper crust. The low Nd magmas most likely formed via the incorporation of low 18O, hydrothermally-altered, wall-rock. Small-volume rhyolite lavas and ash-flow tuffs have similar isotopic characteristics to the large-volume ash-flow tuffs, but lavas erupted from extracaldera vents may have interacted with higher 18O crustal rocks peripheral to the main magma chamber(s). Andesitic lavas from the 13–14 Ma Wahmonie/Salyer volcanic center southeast of the TMOV have low Nd (-13.2 to -13.8) and are considered on the basis of textural evidence to be mixtures of basaltic composition magmas and large proportions (70–80%) of anatectic crustal melts. A similar process may have occurred early in the magmatic history of the TMOV. The large-volume rhyolites may represent a mature stage of magmatism after repeated injection of basaltic magmas, crustal melting, and volcanism cleared sufficient space in the upper crust for large magma bodies to accumulate and differentiate. The TMOV rhyolites and 0–10 Ma old basalts that erupted in southern Nevada all have similar Nd and Sr isotopic compositions, which suggests that silicic and mafic magmatism at the TMOV were genetically related. The distinctive isotopic compositions of the AT member may reflect temporal changes in the isotopic compositions of basaltic magmas entering the upper crust, possibly as a result of increasing basification of a lower crustal magma source by repeated injection of mantle-derived mafic magmas.  相似文献   

19.
The sedimentary evolution of the North Peruvian margin during upper Cretaceous can be summarized as follows:The upper Aptian transgression involved the whole margin, and was accompanied by minor, local, synsedimentary tectonic features. It could be coeval with the individualization of the Western ensialic marginal basin. The transgression reached its maximum during middle Albian, and is associated with anoxic deposits, and with the main activity of the marginal basin.A regression followed during the upper Albian. It was materialized during lower Cenomanian by intertidal deposits, by a westward deltaic progradation, and by synsedimentary tectonic features. This regressive maximum is correlatable with the tectonic closure and folding of the western marginal basin.Then, the great upper Cretaceous transgression affected the eastern parts of the margin. Major deepening stages occured during late Middle Cenomanian and at the base of the Turonian. During Turonian, a widespread, calm carbonate shelf developed. A new major transgressive pulse occured at the Turonian-Coniacian boundary. As a result of a general uplift, the whole margin emerged diachronically between Santonian and Campanian, leading to the deposition of continental Red Beds. The latter recorded a new tectonic phase of uppermost Senonian age.During upper Cretaceous times, the sedimentary evolution of the North Peruvian margin appears to be controlled both by the eustatic sea-level changes, and by the uppermost Albian to early Cenomanian, and the upper Senonian tectonic events.
Zusammenfassung Die sedimentäre Entwicklung des nordperuanischen Kontinentalrandes zeigt während der Ober-Kreide folgende Etappen: die Transgression des Oberaptes erstreckt sich über den gesamten Kontinentalrand und geht mit leichten synsedimentären tektonischen Diskonformitäten einher, die gleichzeitig mit der Entstehung des westlichen Randbeckens zu sehen sind. Die Transgression erreicht ihren Höhepunkt im mittleren Alb, sie ist verbunden mit anoxischen Ablagerungen und mit den Hauptbewegungen im randlichen Becken. Im Oberalb folgt eine Regression. Sie zeigt sich im unteren Cenoman durch intertidale Ablagerungen, durch nach Westen progradierende Deltas und durch synsedimentäre Tektonik. Der Höhepunkt der Regression ist verbunden mit der tektonischen Isolation und Faltung des westlichen Randbeckens.Die große Transgression während der Oberkreide beeinflußt vor allen Dingen die östlichen Teile des Randes. Wesentliche Eintiefungsphasen erscheinen im oberen Mittelcenoman und an der Basis des Turons. Während des Turons entwickelt sich ein weiter Karbonatschelf. Ein größerer Transgressionsvorstoß erscheint an der Grenze von Turon zu Coniac. Als Ergebnis einer generellen Heraushebung wird der gesamte Kontinentrand diachron zwischen Santon und Campan herausgehoben und es entstehen kontinentale Rotablagerungen. Diese Sedimente dokumentieren eine neue tektonische Phase im obersten Senon. Während der Oberkreide wird die sedimentäre Weiterentwicklung des nordperuanischen Kontinentalrandes durch globale Meeresspiegelschwankungen zusammen mit tektonischen Ereignissen (Oberapt, Oberalb-unteres Cenoman, Senon und oberstes Senon) kontrolliert.
Résumé L'évolution sédimentaire de la marge nord-péruvienne au Crétacé supérieur peut être résumée ainsi:La transgression de l'Aptien supérieur affecte toute la marge, et s'accompagne d'une légère instabilité tectonique localisée. Celle-ci peut être contemporaine de l'individualisation du bassin marginal occidental. La transgression atteint son maximum à l'Albien moyen. Elle est contemporaine de dépôts anoxiques et de l'activité du bassin marginal.Une régression survient au cours de l'Albien supérieur. Elle se matérialise au Cénomanien inférieur par l'installation d'une plate-forme carbonatée, puis par la progradation du delta oriental et par des mouvements tectoniques synsédimentaires. Cette régression est en relation avec la fermeture du bassin marginal occidental.La grande transgression du Crétacé supérieur affecte ensuite les zones orientales de la marge, les étapes majeures d'approfondissement se situant à la fin du Cénomanien moyen et à la base du Turonien. Au Turonien, une vaste plate-forme carbonatée s'installe, et un nouvel approfondissement survient à la limite Turonien-Coniacien. A la suite d'une surrection générale, la marge émerge de façon diachrone entre le Santonien et le Campanien, donnant lieu au dépôt des Couches Rouges continentales qui enregistrent au Sénonien terminal un nouvel événement tectonique.Au Crétacé supérieur, l'evolution sédimentaire de la marge nord-péruvienne semble donc contrôlée à la fois par les variations eustatiques, et par les événements tectoniques de l'Albien terminal-Cénomanien inférieur, et du Sénonien.
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20.
On the basis of their textures and mineral compositions spinel-peridotite xenoliths of the Cr-diopside group (group I) from Cenozoic volcanic fields of Arabia can be classified into different subtypes. Type IA is of lherzolitic to harzburgitic composition; mineral compositions are similar to those of group I mantle xenoliths from worldwide occurrences. Type IB xenoliths have lherzolitic to wehrlitic compositions; Mg/(Mg+Fe) ratios of the clinopyroxenes (0.862–0.916) and olivines (0.872–0.914) are similar too or slightly lower than those of typical IA minerals. Texturally, type IB xenoliths are distinguished from type IA rocks by the presence of intragranular spinel, intragranular relict Cr-pargasite, and subordinate intergranular Ba-phlogopite (11.1% BaO). The hydrous minerals in type IB xenoliths are interpreted to document an earlier metasomatism 1 which did not affect type IA lithospheric mantle. Subsequent recrystallization caused the partial replacement of Cr-pargasite in type IB materials and resulted in the formation of less hydrous mineral assemblages. Some of the type IA xenoliths are characterized by secondary intergranular amphibole which must have formed recently. The absence or presence of this intergranular amphibole is used to distinguish an anhydrous subtype IA1 from a hydrous subtype IA2. Type IB xenoliths may also contain secondary intergranular amphibole (similar to the one in subtype IA2) or they contain abundant formermelt patches now consisting of glass and phenocrysts of olivine, clinopyroxene, amphibole, and spinel. The secondary intergranular amphiboles and the former melt patches, both are interpreted as results of a second metasomatism (metasomatism 2). In their trace element and isotopic characteristics, type IA1 and type IA2 clinopyroxenes do not exhibit any systematic differences. Furthermore, type IA2 clinopyroxenes are in Sr isotopic disequilibrium with intergranular amphiboles. This suggests that type IA2 clinopyroxenes were not modified during the second metasomatism 2. All type IA clinopyroxenes have low Sr contents (100 ppm); most of them show Sm/Nd ratios higher than inferred for bulk earth. In their 87Sr/86Sr and 143Nd/144Nd ratios, type IA clinopyroxenes exhibit a large spread from 0.70226–0.70376 and from 0.51375–0.51251, respectively. Highly variable Sr/Nd ratios (5.0–79.3) and variable TUR and TCHUR model age relationships require different evolutions of the respective mantle portions. Nevertheless, all but two type IA clinopyroxenes form a linear array in a Sm–Nd isochron diagram which probably can not be explained by mixing. If taken as an isochron the slope of the array corresponds to an age of around 700 Ma. The mean initial Nd of 5.8±1.7 (1) is similar to values for juvenile Pan-African (i.e. 850–650 Ma old) crust of the Arabian-Nubian shield. It is suggested that type IA lithospheric mantle and the juvenile Pan-African crust are two counterparts fractionated from a common source during the earlier stages of the Pan-African. Type IB clinopyroxenes have high Sr contents (200 ppm), variable Sr/Nd ratios (9–111) and Sm/Nd ratios generally below that inferred for bulk earth, and show a small spread in their Sr and Nd isotopic compositions (0.70299–0.70318 and 0.51285–0.51278, respectively). In a Sm–Nd isochron diagram the data points form a linear, horizontal array indicating a close-to-zero age for the earlier metasomatism 1 and suggesting a close genetic relationship to mantle processes related to the formation of the Red Sea.  相似文献   

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