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1.
Young volcanic rocks from different sections of the Aleutian Islands-Alaska Peninsula Arc have been measured for 87Sr/86Sr, 143Nd/144Nd and some trace elements. We found the 143Nd/144Nd to be highly restricted in range ( Nd=6 to 7) and low as compared to midocean ridge ba-salts (MORB). This indicates that the source of the Aleutian Arc magmas is different from MORB and remarkably isotopically homogeneous with respect to Nd. The range reported here for arc rocks is substantially smaller than found by other workers. However, the Sr isotope ratios vary considerably ( Sr=–24 to –14). Those samples from small volcanic centers north of the main arc (second arc) are characterized by low Sr. Our data in combination with previous studies suggest that there are slight geochemical differences between discrete sections of the arc. The general uniformity of Nd isotope ratios are thought to be the surface expression of an efficient mixing or homogenization process beneath the arc plate, but which still causes a wide dispersion in Sr isotopic composition.To relate the arc rocks to the broader tectonic setting and to identify possible sources of arc magmas, measurements were done on volcanic and sedimentary rocks from the North Pacific/Bering Sea area. Alkali basalts from the back-arc islands St. George, Nunivak and St. Lawrence and alkali-rich tholeiites from the fore-arc have Nd=+4 to +9 and are correlated on the Sr- Nddiagram parallel to the mantle array but shifted to lower Sr. These samples are thought to be isotopically representative of the mantle transported to that region. A tholeiitic basalt from the Kamchatka Basin ocean floor (back-arc), however, yielded typical MORB values ( Nd=10, Sr=–24). Composite sediment samples were made from DSDP cores in the Aleutian Abyssal Plain, Gulf of Alaska and the Alka Basin which represent mixtures of continentally and arc-derived materials. These composites have intermediate Nd isotopic ( Nd= –2 and +2) and high Sr isotopic values ( Sr=+9 and +37). These data show that possible source materials of the Aleutian Arc volcanics are isotopically different from and much more heterogeneous than the arc rocks themselves.On the basis of this study and of literature data, we developed a set of alternative models for volcanic arc magma generation, based on the restricted range in Nd and the wider range in Sr for arc rocks. Different isotopic and trace element characteristics found in different arcs or arc sections are explained by varying mixing proportions or concentrations in source materials. The basic observations require rather strict mixing ratios to obtain constant Nd. The preferred model is one where the melting of subducted oceanic crust is controlled by the amount of trapped sediment with the melting restricted to the upper part of the altered basaltic layer. Homogenization within the upper part of the oceanic crust is brought about by hydrothermal circulation attending dewatering of the slab during subduction and possibly some oxygen exchange of the magmas on ascent.Division Contribution Number 3849 (411)  相似文献   

2.
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   

3.
Alkalic and tholeiitic basalts were erupted in the central Arizona Transition Zone during Miocene-Pliocene time before and after regional faulting. The alkalic lava types differ from the subalkaline lavas in Sr, Nd and Pb isotopic ratios and trace element ratios and, despite close temporal and spatial relationships, the two types appear to be from discrete mantle sources. Pre-faulting lava types include: potassic trachybasalts (87Sr/86Sr = 0.7052 to 0.7055, Nd= –9.2 to –10.7); alkali olivine basalts (87Sr/ 86Sr = 0.7049 to 0.7054, Nd= –2 to 0.2); basanite and hawaiites (87Sr/86Sr = 0.7049 to 0.7053, Nd= –3.5 to –7.8); and quartz tholeiites (87Sr/86Sr = 0.7047, Nd= –1.4 to –2.6). Post-faulting lavas have lower 87Sr/86Sr (<0.7045) and Nd from –3.2 to 2.3. Pb isotopic data for both preand post-faulting lavas form coherent clusters by magma type with values higher than those associated with MORB but within the range of values found for crustal rocks and sulfide ores in Arizona and New Mexico. Pb isotopic systematics appear to be dominated by crustal contamination. Effects of assimilation and fractional crystallization are inadequate to produce the Sr isotopic variations unless very large amounts of assimilation occurred relative to fractionation. It is impossible to produce the Nd isotopic variations unless ancient very unradiogenic material exists beneath the region. Moreover the assumption that the alkalic lavas are cogenetic requires high degrees of fractionation inconsistent with major- and trace-element data. Metasomatism of the subcontinental lithosphere above a subduction zone by a slab-derived fluid enriched in Sr, Ba, P and K could have produced the isotopic and elemental patterns. The degree of metasomatism apparently decreased upward, with the alkalic lavas sampling more modified regions of the mantle than the tholeiitic lavas. Such metasomatism may have been a regional event associated with crustal formation at about 1.6 Ga. Disruption and weakening of the subcontinental lithosphere in the Transition Zone of the Colorado Plateau by volcanism probably made deformation possible.  相似文献   

4.
Initial 87Sr/86Sr and 143Nd/144Nd ratios of Phanerozoic granitoids and related intrusions of the New Zealand block display a mixing-type array indicative of the involvement in their sources of old continental crustal material, most likely of Proterozoic age. Sr(T) values range from –4 to +273 (87Sr/86Sr=0.7041–0.7233), while Nd(T) ranges from +2.7 to –11.0. Preexisting metasedimentary rocks have generally higher Sr and lower Nd (ranging to present-day values of +646 and –15.0, respectively), and, particularly for the Mesozoic intrusives, are isotopically appropriate mixing end-members. The widespread, early Paleozoic Greenland Group graywackes, which are derived from Proterozoic sources, are modeled as the source of the crustal end-member mixing with mantle-derived mafic magmas to produce the intrusive rocks. Four different types of models are applied to the isotopic and trace-element (Rb, Sr, Ba, REE) data: simple mixing; mixing with a partial melt of the metasedimentary rock, with or without isotopic equilibrium; and assimilation-fractional crystallization. Based on these models, some constraints may be applied on petrogenesis (e.g., the lack of high Rb concentrations points to the presence of biotite, and HREE depletion points to the presence of garnet); however, the models fail to adequately explain all the data. The New Zealand granitoids show similarities in isotopic character not only to rocks from offshore islands on the New Zealand block, but also to similar-aged granitoids in adjacent regions of Antarctica and Australia. This points to similarities in crustal character between continental blocks formerly proximal in Gondwanaland. We note an overall increase in Nd and decrease in Sr in felsic magmas from the Paleozoic to the Mesozoic to the Cenozoic in New Zealand, indicative of a decrease over time in the level of influence of recycled continental crust in subduction-related magmatism.Division Contribution No. 4538 (582)  相似文献   

5.
Initial Nd and Sr isotopic ratios were obtained for middle Miocene igneous rocks as well as for related rocks from the Outer Zone of Southwest Japan to investigate the petrogenesis of acidic magmas and their relation to a peculiar tectonic environment bearing on the back-arc spreading of the Japan Sea. On the Nd- Sr diagram, data points for the acidic rocks fall in the – Nd, + Sr quadrant occupying different positions from those for sedimentary and old crustal rocks, and seem to define several subparallel lines which extend towards the lower-righthand sedimentary field. The S-type acidic rocks occupy an intermediate position between I-type rocks and sedimentary ones, a fact suggesting mixing of an igneous component and a sedimentary one. The linear mixing trend observed on the Nd- Sr diagram can be attained in the restricted case that the igneous component has similar Sr/Nd concentration ratios to that of the sedimentary one, which implies an intermediate to acidic composition for the igneous component. Inconsistency between the elemental and isotopic variations observed may be reconciled by considering that mixing, probably in the relatively deep part of the crust, might have occured prior to chemical differentiation processes. The episodic igneous activity and the high heat energy required to melt such materials involving sedimentary rocks may be explained by a model in which a hot mantle region probably corresponding to the rising part of the mantle convection supplied the heating energy to the Outer Zone of Southwest Japan when passing beneath Southwest Japan in the course of movement of the hot rising part from the Shikoku basin areas to the Japan Sea area.  相似文献   

6.
Neodymium, Sr and Pb isotopic compositions, along with rare earth element (REE) concentrations were determined for twelve black ores and one yellow ore from twelve localities of the Kuroko deposits, Japan. The ores were generated by submarine hydrothermal activity during the Miocene age. Neodymium isotopic compositions of the ores (Nd: –4.9 to +6.5) mostly overlap with spatially associated igneous rocks. On a Nd versus Sr isotopic correlation diagram, however, 87Sr/86Sr ratios are shifted from the associated igneous rocks towards the higher contemporaneous seawater ratio. REE patterns are highly variable, ranging from light REE enriched to depleted, and show no Ce anomalies, as would be expected if they were derived from seawater. These results suggest that the REEs contained in ores were mainly derived from the associated igneous rocks, but that the ore Sr is a mixture derived from both seawater and the igneous rocks. Most Pb isotopic compositions fall within the range defined by the associated igneous rocks (206Pb/204Pb=18.35–18.84, 207Pb/204Pb=15.59–15.97 and 208Pb/204Pb=38.53–39.90), although several samples have very radiogenic compositions that were most likely derived from basement rocks. Our new Pb isotopic results display greater variation, and have a larger range of more radiogenic compositions than has been noted previously for these ores. In addition, the black ore with the most radiogenic Pb isotopic composition also has the least radiogenic Nd isotopic composition. This suggests that at least some of the Pb contained in the ores was derived mainly from older basement rocks. The large positive Eu anomalies for some black ores are consistent with a high-temperature origin for the parental fluids, irrespective of the source rock. The single yellow ore examined, however, has a small negative Eu anomaly, which may indicate derivation from a lower temperature fluid. Previous studies suggested that the Kuroko ores were formed in the presence of organic materials in an anoxic basin. Combined Nd, Sr, Pb and Os isotopic and REE abundance data indicate that multiple sources were involved in the genesis of Kuroko ores.  相似文献   

7.
The isotopic compositions of Nd and Sr and concentrations of major and trace elements were measured in flows and tuffs of the Woods Mountains volcanic center of eastern California to assess the relative roles of mantle versus crustal magma sources and of fractional crystallization in the evolution of silicic magmatic systems. This site was chosen because the contrast in isotopic composition between Precambrian-to-Mesozoic country rocks and the underlying mantle make the isotope ratios sensitive indicators of the proportions of crustal- and mantle-derived magma. The major eruptive unit is the Wild Horse Mesa tuff (15.8 m.y. old), a compositionally zoned rhyolite ignimbrite. Trachyte pumice fragments in the ash-flow deposits provide information on intermediate composition magma types. Crustal xenoliths and younger flows of basalt and andesite (10 m.y. old) provide opportunities to confirm the isotopic compositions of potential mantle and crustal magma sources inferred from regional patterns. The trachyte and rhyolite have Nd values of -6.2 to -7.5 and initial 87Sr/86Sr ratios mostly between 0.7086 and 0.7113. These magmas cannot have been melted directly from the continental basement because the Nd values are too high. They also cannot have formed by closed system fractional crystallization of basalt because the 87Sr/86Sr ratios are higher than likely values for parental basalt. Both major and trace element variations indicate that crystal fractionation was an important process. These results require that the silicic magmas are end products of the evolution of mantle-derived basalt that underwent extensive fractional crystallization accompanied by assimilation of crustal rock. The mass fraction of crustal components in the trachyte and rhyolite is estimated to be between 10% and 40%, with the lower end of the range considered more likely. The generation of magmas with SiO2 contents greater than 60% appears to be dominated by crystal fractionation with minimal assimilation of upper crustal rocks.  相似文献   

8.
The Closepet batholith in South India is generally considered as a typical crustal granite emplaced 2.5 Ga ago and derived through partial melting of the surrounding Peninsular Gneisses (3.3 to 3.0 Ga). In the field, it appears as a composite batholith made up of at least two groups of intrusions. (a) An early SiO2-poor group (clinopyroxene quartz-monzonite and porphyritic phyritic monzogranite) is located in the central part of the batholith. These rocks display a narrow range in both initial 87Sr/86Sr (0.7017–0.7035) and Nd(–0.9to –4.1). (b) A later SiO2-rich group (equigranular grey and pink granites) is located along the interface between the SiO2-poor group and the Peninsular Gneisses. They progressively grade into migmatised Peninsular Gneisses, thus indicating their anatectic derivation. Their isotopic characteristics vary over a wide range (87Sr/86Sr ratios=0.7028–0.7336 and Nd values from-2.7 to-8.3, at 2.52 Ga). Field and geochronological evidence shows that the two groups are broadly contemporaneous (2.518–2.513 Ga) and mechanically mixed. This observation is supported by the chemical data that display well defined mixing trends in the Sr vs Nd and elemental variation diagrams. The continuous chemical variation of the two magmatic bodies is interpreted in terms of interaction and mixing of two unrelated end-members derived from different source regions (enriched peridotitic mantle and Peninsular Gneisses). It is proposed that the intrusion of mantle-derived magmas into mid-crustal levels occurred along a transcurrent shear zone; these magmas supplied additional heat and fluids that initiated anatexis of the surrounding crust. During this event, large-scale mixing occurred between mantle and crustal melts, thus generating the composite Closepet batholith. The mantle-derived magmatism is clearly associated with granulite facies metamorphism 2.51±0.01 Ga ago. Both are interpreted as resulting from a major crustal accretion event, possibly related to mantle plume activity.  相似文献   

9.
Rb-Sr isotopic data for anorthosites, charnockites, ferrodioritic to quartz monzonitic plutons, and high-grade gneisses of the Blue Ridge of central Virginia show evidence of post-emplacement metamorphism, but in some cases retain Grenville ages. The Pedlar River Charnockite Suite yields an isochron age of 1021 +/-36 Ma, (initial 87Sr/86Sr ratio of 0.7047 +/-6), which agrees with published U-Pb zircon ages. Five samples of that unit which contain Paleozoic mylonitic fabrics define a regression line of 683 Ma, interpreted as a mixing line with no age significance. Samples of the Roseland Anorthosite Complex show excessive scatter on a Rb-Sr evolution diagram probably due to Paleozoic (475 m.y.) metamorphism. Data from the ferrodioritic to quartz monzonitic plutons of the area yield an age of 1009 +/-26 Ma (inital ratio=0.7058 +/-4), which is in the range of the U-Pb zircon ages of 1000–1100 Ma. The Stage Road Layered Gneiss yields an age of 1147 +/-34 Ma (initial ratio of 0.7047 +/- 5).Sm-Nd data for the Pedlar River Charnockite Suite reflect a pre-Grenville age of 1489 +/-118 Ma ( Nd=+6.7 +/-1.2). Data for the Roseland Anorthosite Complex and the ferrodioritic to quartz monzonitic plutons yield Grenville isochron ages of 1045 +/44 Ma ( Nd=+1.0 +/-0.3) and 1027 +/-101 Ma ( Nd=+1.4 +/-1.0), respectively. Two Roseland Anorthosite samples plot far above the isochron, demonstrating the effects of post-emplacement disturbance of Sm-Nd systematics, while mylonitized Pedlar River Charnockite Suite samples show no evidence of Sm-Nd redistribution.The disparity of the Sm-Nd age and other isotopic ages for the Pedlar River Charnockite Suite probably reflects a Sm-Nd source age, suggesting the presence of an older crust within this portion of the ca. 1 Ga old basement.  相似文献   

10.
Sm-Nd and Rb-Sr isotopic analyses are reported for granulite facies orthogneisses from Fiordland southwest New Zealand. Whole-rock samples define a Rb-Sr isochron age of 120±15 Ma and an initial 87Sr/86Sr ratio of 0.70391±4. Nd values (at 120 Ma) show a relatively wide range of from –0.4 to 2.7 indicating decoupling of Sr-Nd isotope systems. Associated ultramafic rocks have initial 87Sr/86Sr ratios of from 0.70380 to 0.70430 and Nd values of from 0.1 to 3.0. The different initial ratios suggest that the various intrusions, although contemporaneous, were not derived through fractionation of a single parent magma. A metasedimentary enclave incorporated during emplacement of the granulitic rocks preserves a Proterozoic isotopic signature with a measured Nd(0) value of –10.2, 87Sr/86Sr ratio of 0.73679 and a T Nd provenance age of 1490 Ma. The Rb-Sr whole rock age of the granulites is the same as obtained from recent U-Pb zircon dating (Mattinson et al. 1986) and is interpreted as the time of magmatic emplacement and essentially contemporaneous granulite facies metamorphism. Rb-Sr and Sm-Nd analyses of mineral systems indicate that the terrain had cooled below 300° C by 100 Ma providing further evidence that high grade metamorphism was of exceptionally short duration.Unmetamorphosed leucogabbros from the Early Cretaceous Darran Complex of eastern Fiordland have significantly higher Nd values (3.9 to 4.6) and slightly lower 87Sr/ 86Sr (0.70373 to 0.70386) than the western Fiordland granulites. This indicates that the western and eastern Fiordland complexes are not correlative although both have geochemical similarities to Phanerozoic calc-alkaline island-arc suites. The Fiordland granulites are LREE enriched (LaN/ YbN=12 to 40) and have trace element characteristics (e.g. high K/Rb and low Rb/Sr ratios) typical of many Rb-depleted Precambrian granulite terrains. The Fiordland trace element trends, however are attributed to magmatic, not metamorphic processes, reflecting the character of the Early Cretaceous magma sources. The range of Nd values, but uniform initial 87Sr/86Sr of the western Fiordland granulites is consistent with derivation of the parent Early Cretaceous magmas at least in part from a LREE enriched, low Rb/Sr protoliths of mid-to late-Paleozoic age. Partial melting of this protolith occurred during or immediately preceding a period of great crustal thickening culminating in rapid thickening of existing crust by 20 km following emplacement of the granulitic rocks. The rapid crustal thickening was probably a consequence of a collisional event in which an Early Cretaceous magmatic arc was over-ridden by one or more thrust sheets.  相似文献   

11.
Sm-Nd and Rb-Sr isotopic analyses of charnockitic migmatite, augen gneiss, anorthosite-leuconorite and two acid plutons from the Rogaland and Vest-Agder districts of southwest Norway constrain their crustal residence ages, origin and evolution. The charnockitic migmatites, which are a major component of the metamorphic basement complex, represent the oldest and largest episode of accretion, in which new crust was derived 1.5–1.9 Ga ago from a mantle source of depleted Nd isotopic composition. The basement complex was intruded by a number of large anorthositic to granitic plutons during and after the Sveconorwegian orogenic period. Samples from the ca. 1050 Ma old, synorogenic Håland anorthosite-leuconorite massif exhibit substantial variation of initial Nd of +2.1 to +4.4 at an anorthosite locality and –0.5 to +2.3 at a leuconorite locality, but display significant variation of initial 87Sr/86Sr ratio only between the localities (anorthosite mean=0.70369, leuconorite mean=0.70560). A model is proposed whereby the anorthosite and leuconorite were derived by major crustal contamination of, and fractional crystallization from, a picritic magma derived from isotopically-depleted mantle. Two younger acid intrusions, the 950 Ma old Lyngdal granodiorite and the 930 Ma old Farsund charnockite, both have initial Sr and Nd isotope ratios consistent with massive contamination of depleted-mantle-derived magma by old continental crustal material.  相似文献   

12.
The Ivrea zone represents a tilted cross section through deep continental crust. Sm-Nd isotopic data for peridotites from Baldissero and Balmuccia and for a suite of gabbros from the mafic formation adjacent to the Balmuccia peridotite provide evidence for an event of partial melting 607±19 Ma ago in an extended mantle source with 607 Nd =+0.4±0.3. The peridotites are interpreted as the corresponding melt residue, the lower part of the mafic formation as the complementary melts which underwent further differentiation immediately after extraction. The Finero body represents a complex with layers of phlogopite peridotite, hornblende peridotite, and amphibole-rich gabbro. The isotopic signatures fall into two groups: (1) highly radiogenic Nd and low-radiogenic Sr characterize the phlogopite-free, amphibole-rich rocks, whereas (2) low-radiogenic Nd and highly radiogenic Sr is found in ultramafics affected by phlogopite metasomatism. Phlogopite metasomatism in the Ivrea zone is dated by a Rb-Sr whole rock isochron yielding 293±13 Ma. It was fed by K-rich fluids which were probably derived from metasediments. The high initial 293 Nd value of about +7.5 for phlogopite-free samples indicates a high time-integrated Sm/Nd ratio in the Finero protolith 293 Ma ago. Sm-Nd analyses of metapelites from the paragneiss series yield Proterozoic crustal residence ages of 1.2 to 1.8 Ga. Internal Sm-Nd isochrons for three garnetiferous rocks show that closure of garnet at temperatures around 600° C or even lower occurred about 250 Ma ago.  相似文献   

13.
Nd and Sr isotopic compositions and Rb, Sr, Sm and Nd concentrations are reported for madupites, wyomingites and orendites from the Pleistocene volcanic field of the Leucite Hills, Wyoming. All Leucite Hills rocks have negative εNd signatures, indicating derivation or contribution from an old light rare earth element (LREE) enriched source. In this respect they are similar to all occurrences of high potassium magmas so far investigated. But Sr isotopic variations are comparatively small and 87Sr/ 86Sr ratios are unusually low for high-K magmas (0.7053–0.7061, one sample excluded). These values suggest that the light REE enrichment of the source was not accompanied by a strong increase in Rb/Sr. Wyomingites and orendites are isotopically indistinguishable which is consistent with chemical and petrographic evidence for their derivation from a common magma series depending on emplacement conditions. Basic to ultrabasic madupites and more silicic wyomingites/orendites are distinct in their Nd isotopic variations (madupites: εNd= ?10.5 to ?12.3; wyomingites/orendites: εNd= ?13.7 to ?17.0) despite similar Sm/Nd ratios and complete overlap in 87Sr/86Sr. Selective or bulk assimilation of crustal material is unlikely to have significantly affected the Nd and Sr isotopic compositions of the magmas. The measured isotopic ratios are considered to reflect source values. The distinct isotopic characteristics of madupite and wyomingite/orendite magmas preclude their derivation by fractional crystallization, from a common primary magma, by liquid immiscibility or by partial melting of a homogeneous source. Two isotopically distinct, LREE enriched and slightly heterogeneous sources are required. Heterogeneities were most pronounced between magma sources from each volcanic centre (butte or mesa). The relationship between the madupite and wyomingite/orendite sources and their evolution is discussed on the basis of two simple alternative sets of models:
  1. a two-stage evolution model with an old enrichment event (a metasomatic event?) perhaps taking place during the stabilization of the Wyoming Craton 3.2 to 2.5 Gyr ago but not later than 1.2 Gyr ago or
  2. a mixing model involving mixing between one endmember with εNd near zero and another end-member with a strong negative εNd signature.
  相似文献   

14.
Hornblende- and clinopyroxene-phyric lamprophyre dykes exposed in the Roaring River Complex, Superior Province are alkaline, nepheline-normative, basaltic compositions (>50 wt% SiO2), that range from primitive to fractionated [Mg/(Mg + total Fe)=0.66–0.40; Ni=200–35 ppm], and which have high abundances of light rare earth elements (REE) [(Ce/Yb)n=16–26, Cen=60–300; n = chondrite normalized], Sr (870–1,800 ppm), P2O5 (0.4–1.3 wt%), and Ba (150–900 ppm). Crystal fractionation of the lamprophyres produced coeval gabbro and clinopyroxenite cumulate bodies. A whole-rock Sm–Nd isochron for lamprophyres and gabbro-pyroxenite yields a crystallization age of 2,667±51 Ma Ma (I=0.50929±0.0004; Nd = + 2.3 0.7). Whole-rock Sr isotope data are scattered, but suggest an initial 87Sr/86Sr ratio of 0.7012, similar to bulk Earth. The elevated levels of light REEs and Sr in the lamprophyres were not due to crustal contamination or mixing with contemporaneous monzodioritic magmas, but a result of partial melting of a mantle source which was enriched in these and other large-ion-lithophile elements (LILEs) shortly before melting. The lamprophyres were contemporaneous with mantle-derived, high-Mg, LILE-enriched monzodiorite to granodiorite of the Archean sanukitoid suite. Both suites have concave-downward light REE profiles, suggesting that depleted mantle was common to their source regions, but the higher light REE abundances, higher Ba/La ratios, and lower Nd values (+1.3±0.3) of the parental monzodiorites suggest a more enriched source. The lamprophyres and high-Mg monzodiorites were derived from a mineralogically and compositionally heterogeneous, LILE-enriched mantle lithosphere that may have been part of a mantle wedge above a subducting plate in an arc environment.  相似文献   

15.
Subduction-related Late Permian shoshonites of the Sydney Basin, Australia   总被引：2，自引：0，他引：2  
Summary The Late Permian shoshonitic province of the southern Sydney Basin consists of lavas and intrusions confined to a 140 km long coastal belt, but geophysical and sedimentological data indicate that the province once had a length of at least 340 km. Both petrographic and geochemical data indicate shoshonitic affinities for these Late Permian rocks and, although the compositional range is from 48.4 to 60.6% SiO2, all units except an andesite lava have <55% SiO2. Subduction-related attributes of the province include enrichment in Al2O3 and LILE (Rb, K and LREE), depletion in HFSE (Nb, Ta, Zr, Hf and Ti), low Nb/U, and Sr/Nd of 30–35. Initial (at 250 Ma)87Sr/86Sr ranges from 0.70294 to 0.70440 whereas Nd values range from +5.11 to +2.14 and plot almost exclusively in the mantle array. Low MgO and mg-numbers (maximum 7.37% and 64.5, respectively) demonstrate that none of the shoshonites represent primary magmas in equilibrium with mantle peridotite. Isotopic data and elemental contents are not supportive of a model in which crustal contamination is the dominant process in magma petrogenesis, but do not exclude contamination with Sr-rich material having a low87Sr/86Sr and high Nd values.A temporal correlation between igneous rocks from the southern Sydney Basin, Dampier Ridge, New England and Tasmania indicates a widespread magmatic event which was, at least in part, shoshonitic in character and which developed in response to subduction along the east coast of Gondwanaland. The geochemical similarity between the Dampier Ridge and Sydney Basin samples does not support a model for spatial compositional variation across a subduction-related magmatic belt but, coupled with the variations in isotopic ratios, implies heterogeneity in the source.
Spät-permische Schoschonite des Sidney Beckens in Australien
Zusammenfassung Die spät-permische Schoschonit Provinz des südlichen Sidney Beckens besteht aus Laven und Intrusionen, die in einem 140 km langen Gürtel längs der Küste auftreten. Geophysikalische und sedimentologische Daten weisen darauf hin, daß diese Provinz ursprünglich eine Länge von mindestens 340 km hatte. Sowohl petrographische wie geochemische Daten weisen auf schoschonitische Affinität dieser spät-permischen Gesteine hin. Obwohl die SiO2 Gehalte von 48.4 bis 60.6% schwanken, haben alle Einheiten mit Ausnahme einer andesitischen Lava < 55%. Auf Subduktion weisen u.a. Anreicherungen an Al2O3 und LILE (Rb, K and LREE), die Verarmung an HFSE (Nb, Ta, Zr, Hf and Ti) niedrige Nb/U und Sr/Nd von ungefähr 30–35 hin. Die87Sr/86Sr Initiale, berechnet für 250 Ma, liegen zwischen 0.70294 und 0.70440 und die Nd-Werte zwischen +5,11 und +2,14 und fallen fast ausschließlich in den Mantel-Bereich. Niedrige MgO und mg-Zahlen (Maximal 7,37% bzw. 64,5) zeigen, daß die Schoschonite nicht primäre Magmen, die mit Mantelperidotiten im Gleichgewicht sind, darstellen. Isotopendaten und Elementgehalte weisen nicht auf ein Modell hin, bei dem Krusten-Kontamination bei der Magmen-Genese eine wichtige Rolle spielte, aber eine Nichtkontamination mit Material, das niedrige87Sr/86Sr Werte und hohe Nd Werte hat, ist nicht auszuschließen. Eine zeitliche Beziehung zwischen den magmatischen Gesteinen des südlichen Sydney-Beckens, dem Dampier Ridge, Neuengland und Tasmanien weist auf ein weit verbreitetetes magmatisches Ereignis hin, das zumindest teilweise schoschonitischen Charakter hatte und das auf Subduktion längs der Ostküste von Gondwanaland zurückführbar ist. Die geochemische Ähnlichkeit von Proben des Dampier Ridge und des Sydney Beckens weist nicht auf räumliche Variationen in der Zusammensetzung über einen subduktionsbezogenen magmatischen Gürtel hin, sondern läßt eher, zusammen mit den Variationen der Isotopendaten, heterogene Quellen für diese Gesteine vermuten.

With 10 Figures  相似文献   

16.
Ijolites from the type locality at Iivaara, Finland, form a continuous series of magmatic rocks ranging from urtites to melteigites. Both Ni and Cr, but also the large ion lithophile light-rare-earth elements, Zr, Hf, Nb, Rb, Sr and Ba are low in concentration. The Nd contents equal those of the neighboring fenites, Sr is distinctly less abundant, and there is no significant Eu anomaly. The 143Nd/144Nd and 87Sr/86Sr of the ijolites demonstrate a systematic covariation between the data of carbonaties from the Kola Alkaline Province (Sr – 13.8, Nd + 5.6) and those of the fenites at Iivaara (Sr + 132.9, Nd – 24.7) with Sr varying from +0.3 to +23.9 and Nd varying from-9.2 to-19.3. The trace element abundances and the isotopic data give evidence for a crystallization of the rocks from a liquid generated by melting (rheomorphism) of high-grade fenitized country rocks rather than from a primary mantle-derived magma which was contaminated at crustal levels. The fenitization of wall rocks preceding the ijolite magma formation was clement selective. Mixing of elements during the fenitization process between the designated components carbonatite (or derivative fenitizing fluid) and wall rock should have been dynamical depending on the stability of the wall rock mineral assemblages in contact with the fenitizing fluids, the migration velocity of these fluids, and their capacity of the respective elements. Such dynamical mixing explains best the variation of the isotope ratios withont systematic covariation of the respective element concentrations.  相似文献   

17.
Summary Ultramafic xenoliths from the basaltic tuff of Kapfenstein (Graz Basin) are mainly protogranular and protogranular- to- porphyroclastic spinel lherzolites. In addition to these common rocks, a coarse-grained spinel harzburgite and a heterogeneous spinel peridotite consisting of a clinopyroxene-poor harzburgite and a clinopyroxene-rich lherzolite have been found. Interstitial amphiboles (pargasite and kaersutite) are present in most xenoliths. The magmatic activity which brought these xenoliths to the surface is part of the Plio-Pleistocene extension-related alkali basalt volcanism which took place in the Carpatho-Pannonian Region, post-dating the subduction-related calc-alkaline volcanism which formed the Inner Carpathian arc.Mineralogical and bulk geochemical data for Kapfenstein xenoliths are similar to those for other sub-continental shallow mantle xenolith suites. Correlation diagrams for major and trace elements indicate that their chemical variations are related to different degrees of partial melting due to the extraction of mafic melts. Rare earth element (REE) patterns of separated clinopyroxenes from the xenoliths are generally light rare earth element (LREE)-depleted, although LREE-enriched and more complex patterns have also been found.87Sr/86Sr and143Nd/.144Nd isotopic ratios of separated clinopyroxenes vary between 0.70193 and 0.70330 (Sr, = – 36.4 to –17.0) and 0.512824 and 0.513589 (NdNd = 4.2 to 18.l), respectively. The highest87Sr/86Sr and lowest1443Nd/144Nd.Nd ratios occur in those samples which have the most refractory character and also have the most unusual REE patterns. REE determined by ion microprobe on separated interstitial amphibole grains indicate equilibrium with the host peridotite in most of the xenoliths but disequilibirum conditions have been observed, including some extreme LREE enrichments.Striking textural and some geochemical and isotope geochemical similarities have been recognised between Kapfenstein xenoliths and those from the Eastern Transylvanian Basin, both of which are situated on the edges of the diapiric upwelling located in the central part of the Pannonian Basin. The deformation, depletion and enrichment processes which occurred in these two areas are thought to be similar. The LREE-enriched interstitial amphiboles and the presence of cross-cutting amphibole veins suggest that the shallow depleted lithospheric mantle beneath the Graz Basin has been affected by metasomatising events related to the mafic alkaline host magmas rather than by subduction-related fluids or melts.
Spinel-peridotit Xenolithe von Kapfenstein (Grazer Becken, Ostösterreich): Eine geochemische und petrologische Studie
Zusammenfassung Die ultramafischen Xenolithe aus dem Basaltuff von Kapfenstein (Grazer Becken) bestehen großteils aus protogranularem und porphyroklastischem Spinell-Lherzolith. Abgesehen von diesen weitverbreiteten Gesteinen wurden auch ein grobköringer Spinell-Harzburgit und ein heterogener Spinell-Peridotit, der aus einem Klinopyroxen-armen Harzburgit und einem Klinopyroxen-reichen Lherzolith besteht, gefunden. Interstitielle Amphibole (Pargasit und Kaersutit) sind in den meisten Xenolithen vorhanden. Die magmatischen Prozesse, die diese Xenolithe an die Erdoberfläche befördeten, stehen mit dem mit Extension assoziierten Alkali-Basalt-Vulkanismus, der während des Pliozän/ Pleistozäns dem durch Subduktion ausgelösten Kalk-alkalischen Vulkanismus des inneren Karpathenbogens folgte, in Zusammenhang.Die mineralogischen und geochemischen Vollanalysen der Kapfensteiner Xenolithe sind mit den Daten von anderen Gruppen von subkontinentalen Xenolithen des oberen Mantels vergleichbar. Korrelationsdiagramme für Haupt- und Spurenelemente zeigen, daß die chemische Variation auf verschiedene Grade von Teilschmelzbildung zurück-zuführen ist. Die Verteilungsmuster der Seltenen Erden in Klinopyroxenen der Xenolithe sind generall LREE-verarmt, aber LREE angereichte und kompliziertere Verteilungsmuster kommen vor. Die87Sr/86Sr und143Nd/144Nd Isotopenverhältnisse der Klinopyroxene schwanken zwischen 0.70193 und 0.70330 (Sr = von – 36.4 bis –17.0), beziechungsweise zwischen 0.512854 und 0.513589 (Nd=von –4.2 bis 18.l). Die höchsten87Sr/86Sr und niedrigsten143Nd/144Nd Isotopenverhältnisse treten in den Proben mit dem stärksten refraktären Charakter und den ungewöhnlichsten REE-Verteilungsmustern auf. Die mittels Zonensonde ermittelten Gehalte an REE der interstitiellen Amphibole deuten ein Gleichgewicht mit dem Wirt-Peridotit an, obwohl auch Anzeichen von gestörtem Gleichgewicht, unter anderem extreme Anreicherungen an LREE, beobachtet wurden.Ein Vergleich der Kapfenstein-Xenolithe mit denen aus dem östlichen Transsylvanischen Becken zeigt bemerkenswerte strukturelle und einige geochemische und isotopengeochemische Ähnlichkeiten. Beide Gegenden befinden sich am Rande des Erdmantel diapirs, im Zentrum des Pannonischen Beckens. Die Deformationen und Verarmungsund Anreicherungsprozesse, die in beiden Gebieten stattfanden, sind vergleichbar. Die an LREE angereicherten interstitiellen Amphibole und das Auftreten von diskordanten Amphibolgängen legen nahe, daß der obere verarmte lithosphärische Mantel unter dem Grazer Becken von, mit den mafischen alkalischen Grund-Magmen verwandten, metasomatischen Prozessen erfaßt worden ist, und nicht von mit Subduktion assoziierten Flüssigkeiten oder Schmelzen.

With 7 Figures  相似文献   

18.
The petrogenesis of Abu Khruq, an 89 Ma alkaline ring complex of eastern Egypt which is composed of alkali gabbros and both silica over- and undersaturated syenites, has been investigated. Major and trace element relationships and Nd and Sr isotope data are consistent with formation of the gabbros from an alkaline mafic magma that experienced extensive fractionation, and all syenites from a felsic derivative of this melt. The parental magma had an 87Sr/86Sr of 0.7030 and an 143Nd/144Nd of 0.512750 (Nd = +4.4) indicating derivation from a depeleted mantle source. The initial 143Nd/144Nd ratios are: 0.512721 to 0.512748 for the gabbros, 0.512739 to 0.512750 for the alkali syenites and trachytes, 0.512717 to 0.512755 for the nepheline syenites, and, 0.512706 to 0.512732 for the quartz syenites. In contrast, analyzed Precambrian granites from eastern Egypt have generally lower 143Nd/144Nd ratios (ranging from 0.51247 to 0.51261 or Nd = -0.8 to 1.7, for 89Ma); their Nd model ages range from 775 to 935 Ma and suggest there was no significant input of pre-Pan-African crust in their formation. Among Abu Khruq rocks, 143Nd/144Nd ratios indicate that the quartz syenites formed by open-system, crustal contamination processes whereas the nepheline syenites experienced little or no contamination. Modeling shows that contamination occurred at various stages, affecting both mafic and more evolved compositions with input of about 20% crustal Nd for the most contaminated samples. The degree of contamination is related to the silica saturation of the quartz syenites. Simplified modeling of magma evolution within Petrogeny's Residua System demonstrates the ability of AFC processes to cause a critically undersaturated magma to evolve across the feldspar join and produce oversaturated rocks. The oversaturated syenites at Abu Khruq were produced in this manner whereas the nepheline syenites formed by fractionation without similarly large degrees of contamination. The results have broad implications for the formation of subvolcanic complexes in continental settings beyond the important production of silica oversaturated compositions from crustal interaction. They underscore the importance of crustal interactions in the formation of the various lithologies. Such interactions occur at various stages in the evolution of the magmas and, as such, are not strictly coupled with fractional crystallization. While previous study of Abu Khruq has demonstrated extensive hydrothermal alteration of O and Sr isotopes, the present work shows that the Nd isotope ratios were not significantly affected and thus reflect magmatic signatures. This feature combined with relatively small corrections for initial ratios emphasizes the utility of Nd isotopes for petrogenetic studies.  相似文献   

19.
The Mariánské Lázn complex (MLC) is located in the Bohemian Massif along the north-western margin of the Teplá-Barrandian microplate and consists of metagabbro, amphibolite and eclogite, with subordinate amounts of serpentinite, felsic gneiss and calcsilicate rocks. The MLC is interpreted as a metaophiolite complex that marks the suture zone between the Saxothuringian rocks to the north-west and the Teplá-Barrandian microplate to the south-east. Sm-Nd geochronology of garnet-omphacite pairs from two eclogite samples yields ages of 377±7, and 367±4 Ma. Samples of eclogite and amphibolite do not define a whole rock Sm-Nd isochron, even though there is a large range in Sm/Nd ratio, implying that the suite of samples may not be cogenetic. Eclogites do not have correlated Nd values and initial 87Sr/86Sr ratios. Five of the eight eclogite samples have high Nd values (+10.2 to +7.1) consistent with derivation from a MORB-like source, but variable 87Sr/86Sr ratios (0.7033 to 0.7059) which probably reflect hydrothermal seawater alteration. Three other eclogite samples have lower Nd values (+ 5.4 to –0.8) and widely variable 87Sr/86Sr ratios (0.7033 to 0.7096). Such low Nd values are inconsistent with derivation from a MORB, source and may reflect a subduction or oceanic island basalt component in their source. The MLC is an important petrotectonic element in the Bohemian Massif, providing evidence for Cambro-Ordovician formation of oceanic crust and interaction with seawater, Late Devonian (Frasnian-Famennian) high- and medium-pressure metamorphism related to closure of a Saxothuringian ocean basin, Early Carboniferous (Viséan) thrusting of the Teplá terrane over Saxothuringian rocks and Late Viséan extension.  相似文献   

20.
The Sr-Nd isotopic data for selected granitoids of the Central Bohemian Pluton show a broad negative correlation with the total range of (87Sr/86Sr)330 = 0.7051–0.7129 and Nd 330 = +0.2 to –8.9. The older intrusions have more depleted Sr-Nd compositions and calc-alkaline geochemistry (Sázava suite), whereas the younger intrusions shift towards K-rich calc-alkaline (Blatná suite) and shoshonitic rocks (íany and ertovo bemeno suites) with more evolved isotopic signatures. The distribution of the data is interpreted as reflecting a diversity of sources and processes, rather than a single progressive crustal contamination trend. The Sázava suite could have originated by partial melting of metabasites, or of a mantle source with an isotopic composition close to bulk earth, or by hybridization of crustally-derived tonalitic and mantle-derived magmas. Variation within the Blatná suite is modelled by mixing between a moderately enriched [(87Sr/86Sr)330 0.708, Nd 330 –3] mantle component with either an isotopically evolved metasedimentary component, or with more evolved magmas of the suite. The íany suite was most probably produced by partial melting of peraluminous lithologies, possibly of the adjacent Moldanubian unit. The ertovo bemeno suite evolved from strongly enriched mantle-derived magmas [(87Sr/86Sr)3300.7128, Nd 330 –7], either through closed-system fractional crystallization or interaction with magma corresponding to leucogranites of the Central Bohemian Pluton.  相似文献   

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