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1.
Résumé Au Lias inférieur (au moins au Sinémurien), la région du Todrha (feuille de Tinghir au 1/100.000) est caractérisée par une large plate-forme carbonatée peu profonde. Toutefois un sillon étroit et relativement profond s'est creusé parallèlement à la chaîne haut-atlasique et a contrôlé la sédimentation, la paléomorphologie et la paléoécologie.Dans cette région, on constate du sud au nord les faciès latéraux suivants: calcaires dolomitiques, calcaires massifs plus ou moins récifaux, calcaires lités biodétritiques et calcaires turbiditiques, calcaires dolomitiques massifs et enfin dolomies litées à tipis et calcaires oolithiques.
During the Lower Lias (Sinemurian), the area of Todrha has been characterised by a carbonate tidal flat platform, however a small trough with deeper water environments was situated parallel to today's central High Atlas mountains.In the Todrha region the following facies changes from south to north are developed: dolomitic limestones, reefal massive limestones, biodetritic bedded limestones and turbiditic limestones, massive dolomitic limestones and thinly bedded dolomites with teepees and oolitic limestones.
Zusammenfassung Während des Unterlias, zumindest im Sinemurium, bestand im Gebiet der Todrha eine Karbonat-Plattform im Gezeitenbereich. Sie wurde von einem schmalen Trog tieferen Wassers durchzogen, welcher parallel zum Streichen des zentralen Hohen Atlas verlief.Folgende Fazies sind im Todrha-Gebiet entwikkelt (von S nach N): Dolomitische Kalke, massige Riffkalke, biodetritische Bankkalke und turbidische Kalke, massige dolomitische Kalke sowie dünnbankige Dolomite mit Teepee-Strukturen und oolithische Kalke.
, , - , , / Todrha . , . Todrha : , , , , Teepee' .
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2.
Major and trace element concentrations and initial 87Sr/86Sr ratios of lavas from the Aleutian volcanic centers of Adak and Atka have been used to study the evolution of their respective lithospheric plumbing systems. The centers are within 150 km of one another and show similar overall silica ranges (47–67%), but Adak (40 km3) is smaller than Atka (200 km3). Adak's lavas are chemically and isotopically heterogeneous (87Sr/86Sr:0.70285–0.70330) and two units contain lithospheric xenoliths. The lavas of the much larger Atka, on the other hand, have much less variability in major and trace elements as well as 87Sr/86Sr (0.70320–0.70345). We suggest that these characteristics are a measure of the relative maturity and cleanliness of the lithospheric plumbing systems that supply magma to these centers. Because Aleutian volcanic centers often remain fixed for relatively long periods of time (5 m.y.), once established, magmatic passageways are repeatedly used. Young plumbing systems are relatively cool and contain large amounts of wallrock contaminant, and ascending magmas undergo contamination as well as concurrent crystallization and fractionation. With time, however, heat and mass transfer between ascending magmas and wallrock produce thermal and chemical boundary layers that insulate subsequent magmas. In effect, the plumbing system matures. The chemical heterogeneity displayed by young, dirty systems (like Adak) reflects not only the magma source but also the wallrock encountered during ascent and possibly the effects of extensive crystal fractionation. Thus, it is the petrologic data of mature, clean systems, like Atka, that yield the most direct and unambiguous information on the ultimate origin of the lavas and their near surface evolution.  相似文献   

3.
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   

4.
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   

5.
New K-Ar determinations for granitoid rocks and ores from the northern Bolivian segment (ca. 15°-18° S.) of the Central Andean Cordillera Oriental (Eastern Cordillera) confirm that this region displays a remarkable repetition of magmatic activity and associated lithophile metal mineralization. Intrusion of monzogranites, in part peraluminous, and granodiorites took place in the Mid—to—Late Triassic (apparent age range: 225-202 Ma) and Late Oligocene-Early Miocene (28.4-ca. 19.2 Ma); the two plutonic domains are juxtaposed, with little or no areal overlap, along an essentially linear belt defining the inner boundary of the Central Andean magmatic arc. There is no conclusive evidence of granitoid intrusion during the intervening period, although at least two centres of Upper Cretaceous basic-intermediate hypabyssal-volcanic rocks are known. Whereas the Triassic episode simultaneously affected ca. 200 km of the belt, the Oligocene-Miocene domain experienced a longitudinal (southeastward) migration of activity which, in its later stages (22-19 Ma), coincided with the initiation of widespread volcanism and intrusion on the altiplano, to the west of the granitoid belt.Both intrusive domains may be regarded as the innermost expressions of relatively brief episodes of radical broadening of the Central Andean magmatic arc, possibly generated through abrupt, but minor, decrease in the inclination of eastward subduction at the western plate margin, and attended by a degree of crustal anatexis unattained in the outer parts of the arc. Our radiometric data permit the delimitation of two metallogenetic (W-Sn) sub-provinces in the northern part of the Bolivian tin belt. The tungsten-tin vein deposits associated with the Triassic and Tertiary plutons are very similar, although tungsten appears to be enriched relative to tin in the older domain. There is no radio-metric or petrographic evidence of significant reactivation of older hydrothermal systems in the Tertiary. The late-Early Miocene age (16.3 Ma) determined for the important Oruro Sn-Ag epithermal centre (17° 56' S.) represents an anomaly in the broad southward migration of volcanic and hydrothermal activity defined byGrant et al. (1979) in the central and southern, sub-volcanic, segments of the Bolivian tin-polymetallic belt. Minor Pb-Zn-Ag vein mineralization was probably associated with the Upper Cretaceous basic-intermediate magmatism.
Zusammenfassung Neue K-Ar Bestimmungen für die granitoiden Gesteine und Erze aus dem nord-bolivischen Segment (ca. 15°-18° S) der mittleren andeanischen Cordillera Oriental (östlichen Cordillera) bestätigen, daß dieses Gebiet eine beachtenswerte Wiederholung der magmatischen Aktivität und der damit verbundenen Erzmineralisierung aufweist. Granodiorite und teilweise Monzogranite mit Aluminium-Überschuß sind in der Mittel- und Spät-Trias (225-202 Ma) und im Spät-Oligozän bis Früh-Miozän (28.4-ca. 19.2 Ma) eingedrungen; die beiden plutonischen Bereiche liegen mit nur geringer oder fehlender Überlappung benachbart längs einem im wesentlichen linearen Gürtel, der den inneren Rand des mittelandinischen magmatischen Bogen abgrenzt. Es gibt keine entscheidende Anzeichen für granitische Intrusionen in der dazwischen liegenden Zeit, obwohl wenigstens zwei Zentren der oberkretazischen basischen bis intermediären, hypabyssalen bis vulkanischen Gesteinen bekannt sind. Während die triassische Episode gleichzeitig ungefähr 200 km des Gebiets beeinflußte, erfuhr der oligozän-miozäne Bereich jedoch eine longitudinale (südöstliche) Wanderung der Aktivität, die, in den jüngeren Stufen, mit der Einleitung des ausgedehnten Vulkanismus und der Intrusion auf dem Altiplano, westlich des granitoiden Gebietes zusammenfiel.Die beiden intrusiven Gebiete könnte man als die innersten Vorgänge verhältnismägig kurzer Episoden einer drastischen Verbreitung des mittleren andinischen magmatischen Bogens betrachten. Diese könnten vielleicht durch die plötzliche Verringerung des Winkels der östlichen Subduktion an der westlichen Plattengrenze entstanden sein, mit einem Grad der Anatexis, der in den äußeren Teilen des Bogens nicht erreicht wurde. Unsere radiometrischen Daten ermöglichen die Abgrenzung zweier metallogenetischen (W-Sn) Unterprovinzen im nördlichen Teil des bolivischen Zinn-Gebiets. Obwohl die mit den triassischen und tertiären Plutonen verbundenen Wolfram-Zinn-Erzgänge einander sehr ähneln, scheint das Wolfram im älteren Bereich dem Zinn gegenüber etwas angereichert zu sein. Es liegen keine radiometrischen oder petrographischen Zeugnisse für eine erhebliche Reaktivierung von älteren hydrothermalen Systemen im Tertiär vor. Ein spätes früh-miozänes Alter (16.3 Ma) wurde für das wichtige Sn-Ag epithermale Zentrum von Oruro bestimmt. Dieses weicht von der allgemeinen nach S gerichteten Wanderung der vulkanischen und hydrothermalen Aktivität in den zentralen und südlichen subvulkanischen Segmenten der bolivischen Zinn-polymetallischen Gürtel ab, die vonGrant et al. (1979) beschrieben wurde. Geringere Pb-Zn-Ag-Erzgangbildung stand wahrscheinlich mit dem oberkretazischen basisch-intermediären Magmatismus in Verbindung.
Résumé De nouvelles determinations K-Ar sur des roches granitiques et sur du minerais provenant du segment bolivien septentrional (ca. 15°-18° S) de la Cordillère orientale des Andes centrales (Cordillère est) confirme que cette région fut affectée par une activité magmatique répétée auquelle s'associa une activité metalifère riche en métaux lithophiles. Des intrusions de monzogranites, en partie peralumineux et de granodiorites se mirent en place durant le Triassique moyen et supérieur (intervalle d'âge apparent: 225-202 Ma) et durant l'Oligocène moyen et le début du Miocène (24.4-ca. 19.2 Ma). Ces deux domaines plutoniques se juxtaposent avec un recouvrement spatial faible ou nul, le long d'une ceinture essentiellement linéaire définissant la bordure interne de l'arc magmatique des Andes centrales. Bien qu'aucune indication concluante nesitue la mise en place d'intrusions granitiques durant la période intermédiaire, au moins deux centres volcaniques hypabyssaux de composition basique à intermédiaire se sont formés au Crétacé supérieur. L'épisode magmatique du Triassique a affecté simultanément ca. 200 km de la ceinture. Le domaine Oligocène-Miocène, pour sa part, a subi une migration longitudinale (sudest) de l'activité magmatique dont les étapes tardives (22-19 Ma) coîcident avec le déput d'une longue période de volcanisme et d'intrusions sur l'altiplano, à l'ouest de la ceinture granitique.celle des aggregats de grande taille, par des mécanismes de dislocation. La transition est Les deux domaines intrusifs peuvent être causés par un fort élargissement répété et bref de l'arc magmatique des Andes centrales. neut-être dû à une diminution abrupte de l'inclinaison de la zone de subduction orientale à la bordure ouest de la plaque, accompagnée par un degré d'anatexie crustale qui n'atteignit pas la partie externe de l'arc. Les résultats radiométriques obtenus permettent de délimiter deux sous-provinces métallogéniques (W-Sn) dans la partie septentrionale de la ceinture bolivienne stannifère. Les gisements filoniens d'étain et de tungsténe associés aux plutons triassiques et tertiaires sont très semblables; cependant le tungstène semble être plus abondant rapport à l'étain dans le plus vieux domaine. Il n'y a aucune évidence radiométrique ou pétrographique d'une réactivation signicative du système hydrothermal tertiaire. L'age Miocène (16.3 Ma) obtenu pour l'important centre épithermal Sn-Ag d'Oruro (17° 56' S.) représente une anomalie dans la vaste migration sud-est de l'activité volcanique et hydrothermale définie parGrant et al. (1979) dans les segments «sub-volcaniques» central et sud-est de la ceinture polymétallique et stannifére bolivienne. Une faible minéralisation filonienne Pb-Zn-Ag était probalement associée avec le magmatisme intermédiaire à basique du Crétacé supérieur.
- ( 15–18° ) . , , . (225-202 ) (28,4-. 19,2 ) . , , . , , no- , : ( ) . , 200 , - , , . , . , , . . - , , , , . , , . - (16,3 ). - , Grant . (1979). , , , .
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6.
The Ascutney Mountain complex of eastern Vermont, USA, is a composite epizonal pluton of genetically related gabbro to granite intrusives. Nd isotopic data are reported for mafic rocks, granites, and nearby country rock. The parental mafic magma producing the complex 122 m.y. ago had 87Sr/86Sr=0.7039, 143Nd/144Nd=0.512678 ( Nd=+3.8) and 18O=6.1, indicating a mantle source with time-integrated lithophile element depletion. Uniform initial radiogenic isotope ratios for granites, which are undistinguishable from those for the most primitive gabbro, suggest that the granite magma evolved from the mafic magma without crustal contamination and that the increase in 18O, to about 7.8, is the result of fractional crystallization. Mafic rocks show a large range in initial 143Nd/144Nd ratio, from about 0.51267 to 0.51236 ( Nd= +3.7 to –2.5), which is correlated with elevated 87Sr/86Sr ratios and 18O. These data substantiate the production of mafic lithologies by fractional crystallization of the parental magma accompanied by assimilation of up to about 50% crust. The local country rocks include gneiss and schist and assimilation involved representatives of both rock types. The isotopic and chemical relationships preclude derivation from a single batch of magma undergoing contamination and indicate that a large magma body at depth evolved largely by fractionation with batches of melt issued from this chamber being variably contaminated at higher levels or at the level of emplacement.The Precambrian gneisses of the Chester dome and overlying lower Paleozoic schists have essentially identical Nd isotope systematics which suggest a crustal formation age of about 1.6. b.y. The parental sediments for the schists were apparently derived from a protolith similar to the gneissic basement without appreciable Sm/Nd fractionation.  相似文献   

7.
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   

8.
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   

9.
Obduction of the late Ordovician Solund-Stavfjord Ophiolite Complex (443±3 Ma), west Norwegian Caledonides, involved generation and high-level emplacement of granitic and granodioritic dikes and plutons. Initial 87Sr/86Sr ratios in the granites are low (0.7042–0.7059), suggesting either a mantle component or a Rb-poor crustal source. Initial Nd (Nd(t)) ranges from-0.8 to-8.8, indicating that the granites represent recycling of old crustal rocks, which is supported by Precambrian inheritance in zircons from two of the studied granites. I argue that the Rb-Sr and the Sm-Nd isotope systems are decoupled in the sense that the Sr-and the Nd-isotopes derive their dominant signals from two different sources, a mantle source and a crustal source respectively. The granites are metaluminous to peraluminous and typically have high Sr, Ba and Na2O/K2O ratios. SiO2 contents range from 66 to 74 wt%. REE abundances are highly variable; the La contents range from 80 to 200 times chondrite, and are inversely correlated with the contents of SiO2. The concentration of Nd in the granites decreases asymptotically with decreasing Nd(t) suggesting fractional crystallization of accessory phases and assimilation of continental crust. This argument is supported by the presence of partly dismembered xenoliths in the granites with Nd(t)-values that are significantly lower than Nd(t)-values in the host granite. The following models are suggested for the granites. When the ophiolite complex obducted, an outboard subduction zone approached the continental margin, and subduction-related magmas accumulated beneath the continental margin, and probably intruded the overlying eugeosynclinal deposits. The mantle-derived magmas most likely evolved to granitoid composition by assimilation of these eugeosynclinal sediments and by fractional crystallization of amphibole, feldspar, sphene, and allanite. Alternatively, but less likely, the heat content of the mantle-derived magmas caused extensive melting of immature graywackes and calc-alkaline volcaniclastic rocks in the deepest portions of the eugeosyncline. Either way, during ascent, the compositions of the granitic melts were modified by fractional crystallization of LREE-rich phases and by assimilation of continental metasediments.  相似文献   

10.
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   

11.
Meta-igneous granulite (MIG) xenoliths in lavas from Mount Ruapehu, Taupo Volcanic Zone, New Zealand, have variable but relatively high 18O (+8.2 to +11.7) and 87Sr/86Sr (0.70506 to 0.70872), and Nd ranging from +1.5 to +6.2. They show a strong positive correlation between 87Sr/86Sr and 18O, both of which are also broadly correlated with Mg number, but lack any correlation between 87Sr/86Sr and Nd. The xenoliths have been mineralogically re-equilibrated at lower-crustal temperatures (800–930°C) and pressures (7–10 kbar). Geochemical and isotopic evidence suggests they are fragments of oceanic crust that have been altered previously in an ocean floor hydrothermal system. Alternatively, they may be igneous rocks of unknown origin hydrothermally altered in the lower crust. Irrespective of this uncertainty, the xenoliths provide rare samples of the lower crustal basement beneath Taupo Volcanic Zone and represent a potential source rock for the voluminous rhyolites and ignimbrites that dominate the zone.  相似文献   

12.
The analysis of geophysical and geological data on the structure of the pre-Miocene substratum of the Pannonian basin has revealed a block structure. The crust was divided by deep-seated faults of mainly NE-SW orientation. The deep-seated faults separate zones of continental crust (with granitoids), which were more intensely consolidated during the Hercynian, from strips with volcanic-sedimentary rocks of eugeosynclinal character (sub-oceanic crust). In geological development of the Pannonian basin crust during the last 330 million years, subsidence predominated over uplift and denudation. The great mobility of the crust ist related to its lesser thickness compared to the thick crust in the orogenic border and median massifs, e. g. of the Balkan region. The block structure is superimposed on the original belt structure and nucleus stage of continental crust formation. In the pre-Hercynian stage, tectonic division along E-W and N-S lines predominated. The long persistence and magmatic activity of deep-seated faults and their almost vertical dip do not support proposed models of partial subduction and closing of microoceans in the region of the Pannonian basin. Strips or blocks of suboceanic crust are considered to be relicts of more weakly sialized original thin oceanic crust. The Pannonian basin is not a typical ensialic basin. Tectonic development has distinctly changed the mantle diapir, which originated in the Late Cretaceous. Along the circumference of this diapir, shear zones originated in the crust, along which seismic activity has persisted to the present. The Pannonian megablock is a type of simatic median massif, preserving long-lasting subsidence mobility. The Pannonian mantle diapir is considered to be an autonomous deep structure in the sense ofvan Bemmelen (1972). We do not relate the ascent of the mantle diapir to subduction of lithospheric plates, which must have been the cause of folding of the Carpathians according toStegena et al. (1975).
Zusammenfassung Die Analyse der geophysikalischen und geologischen Angaben über den Bau des vormiozänen Untergrundes des Pannonischen Becken enthüllte einen Blockbau. Die Erdkruste war durch Tiefenbrüche gegliedert, vorwiegend in NO-SW Richtung. Die Tiefenbrüche trennen Streifen der kontinentalen Kruste mit Granitoiden, epiherzynisch intensiver konsolidiert, von den Streifen mit vulkanisch-sedimentären Serien eugeosynklinalen Charakters (subozeanischer Krustentyp). In der geologischen Entwicklung der Kruste während der letzten 330 Mill. Jahren herrschte die Subsidenz über Hebungen und Denudationen vor. Die große Mobilität der Kruste wird mit ihrer geringeren Dicke verbunden zum Unterschied von der mächtigen Kruste im orogenen Raum und der medianen Massive, z. B. der Balkan-Region. Der Blockbau ist aufgelegt auf den ursprünglichen Zonenbau und das Nukleus-Stadium der kontinentalen Krustenbildung. In der vorherzynischen Etappe war die tektonische Gliederung nach den Linien W-O und N-S vorherrschend.Die lange Lebensdauer und magmatische Aktivität der Tiefenbrüche, ihre fast vertikale Neigung unterstützen die Modelle der Teil-Subduktionen und Schließung der Mikroozeane im Raume des Pannonischen Beckens nicht. Die Streifen-Blöcke der subozeanischen Kruste werden als Relikte der schwächer sialisierten ursprünglichen dünnen ozeanischen Kruste betrachtet. Das Pannonische Becken ist kein typisches ensialisches Becken.Die tektonische Entwicklung verändert deutlich den Manteldiapir, dessen Entstehung schon in die Oberkreide gestellt wird. Am Umfang dieses Diapirs sind in der Kruste Scherzonen entstanden, auf welchen die seismische Aktivität bis zur Gegenwart andauert. Der pannonische Megablock ist ein Typ eines simatischen medianen Massivs, welches eine langdauernde Subzidenzmobilität erhält. Der pannonische Manteldiapir wird als autonome Tiefstruktur im Sinnevan Bemmelens (1972) betrachtet.Wir setzen den Aufstieg des Manteldiapirs nicht mit der Subduktion der lithosphärischen Platten in Verbindung, welche nachStegena et al. (1975) die Ursache der Faltung der Karpaten sein sollte.
Résumé L'analyse des données géologiques et géophysiques sur la constitution du soubassement prémiocène du bassin pannonique a fait découvrir l'existence d'une structure en bloc. La croûte a été compartimentée par des failles profondes de direction NE-SW. Ces cassures profondes séparent des bandes de croûte continentale avec granitoïdes, bien consolidée par l'orogenèse epihercynienne, de bandes à séries volcano-sédimentaires à caractère eugéosynclin (type sub-océanique). Au cours du développement géologique de la croûte pendant ces dernières 330 M/A, c'est la subsidence qui a prédominé sur les soulèvements et al dénudation. La grande mobilité de la croûte est liée à sa faible épaisseur, différant en cela de la croûte épaisse du domaine orogénique et des massifs médians (p. e. la région balcanique). La structure en blocs est superposée à la structure zonaire primitive d'une part, et au stade de «nucleus» de la croûte continentale d'autre part. Dans l'étape anté-hercynienne, la subdivision prédominante se faisait conformément aux direction W-E et N-S.La longévité et l'activité magmatique des cassures profondes, leur position quasiverticale, sont en contradiction avec les modèles de subduction partielle et de fermeture de microocéans dans la région du bassin pannonique. Les bandesblocs de l'écorce subocéanique sont considérées comme des relictes faiblement sialisés de l'écorce océanique originellement peu épaisse. Le bassin pannonique n'est pas un bassin typiquement ensialique.Le développement tectonique modifia de toute évidence le diapir du manteau qui prend déjà naissance au Crétacé supérieur. Dans le pourtour de ce diapir, des zones de cisaillement sont apparues dans l'écorce, qui ont été et sont restées jusqu'à ce jour le siège d'une activité seismique. Le megabloc pannonique est un type de massif médian simatique à mobilité subsidente de longue durée. Le diapir pannonique est considéré comme une structure autonome et profonde dans le sens devan Bemmelen (1972). L'ascension du diapir du manteau n'est pas, d'après nous, liée à la subduction de plaques lithosphériques, laquelle, d'aprèsStegena et al. (1975) devrait être responsable de l'orogenèse des Carpathes.
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13.
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   

14.
Hf isotope systematics in granitoids from the central and southern Alps   总被引：1，自引：0，他引：1  
First initial-Hf isotopic compositions for samples from the Alpine domain are presented and discussed. The results are mainly based on zircons and a few whole rocks with ages between 30 and 450 Ma. Of those so far analyzed, the present-day Hf isotopic compositions of zircons from non-metamorphic and metamorphic granitoid rocks vary between 0.2824 and 0.2829. Zircon populations with concordant U-Pb ages have much higher initial 176Hf/177Hf than inversely discordant populations which have been contaminated with older zircons containing less radiogenic Hf. Correlated Nd-Hf crustal-residence ages have been found involving model parameters of Hf/Nd=f(Lu/Hf)/f(Sm/Nd) 1.6 for the depleted mantle and f(Lu/Hf)/f(Sm/Nd) 1.2 for elemental fractionations in the crust. The model implies 176Lu/177Hf of 0.017 for the bulk crust. It is suggested that the granitoid rocks are the result of mixing of subcontinental mantle-derived magmas with 1.7 Ga old recycled and partially molten crustal material. The continental/mantle component mass-ratio values for the granitoids range between 0.3 and 2.  相似文献   

15.
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   

16.
The mean (87Sr/86Sr) and mean (87Rb/86Sr) ratios of the intrusive granites from the North and South of the Orogen's Central Zone plot on straight lines. These are interpreted as areal isochrons indicating the time of last Sr isotope homogenization 526 and 571 Ma ago in the respective source rocks.Initial (87Sr/86Sr) and mean (87Rb/86Sr) ratios of approximately coeval granites of the main magmatic pulses in the North (470 Ma) and in the South (520 Ma) line up along isochrons suggesting that in both cases 60 Ma passed after the homogenization in the protolith before large scale intrusions took place.The data require that the intrusions have preserved the Rb/Sr ratios of their source rocks permitting only very little assimilation or fractionation.The source rocks in the North and South had rather unradiogenic Sr 526 and 571 Ma ago, respectively. At Ri0.7066 all presently known Damaran metasediments and metavolcanics as well as the basement must be excluded as the protoliths. A hypothetical source with a large proportion of low (87Sr/86Sr) volcanic material is required. In the center, on the other hand, the Sr isotope ratios are more radiogenic and derivation from common Damaran metasediments is a distinct possibility.The total rock 18O values show an unusual spread from 7.1 to 15.2, the majority being very heavy. This excludes granulites and requires sediments or heavily altered volcanics as source rocks. A plot of 18O vs. initial Sr isotope ratios of the granites from the center which could have been derived from Damaran metasediments has a very clear negative slope. No trend is visible for the southern granites. Of the northern granites the older group shows a negative, the younger group a positive correlation. This is interpreted as indicating mainly altered volcanics (perhaps spilites) for the older and a mixture of volcanogenic and metasedimentary rocks as the source for the younger group. The high 18O values show that the granites are crustal remelts.  相似文献   

17.
The ca. 2.7–2.5 Ga Slave Province is a granitegreenstone terrane comprising deformed sedimentary and subordinate volcanic belts extensively intruded by granitoid rocks. The Nd isotopic data are reported for 58 samples of supracrustal and granitoid rocks exposed along a 400 km, east-west, transect at 65°N across the structural grain of the province. Initial Nd values reveal distinctly different crustal sources in the eastern compared to the western parts of the province, as expected from tectonic assembly of the province through accretion of juvenile crust to older continental crust. Supracrustal sequences (ca. 2.71–2.65 Ga) from the central and eastern parts of the province have positive Nd(1) values (+0.3 to +3.6), consistent with juvenile sources and formation remote from significantly older crust. Syn to late-deformation (ca. 2.63–2.60 Ga), mantle-derived diorites and related tonalites (type I) from the central and eastern parts of the province have similar initial Nd values (-0.1 to +2.7). In contrast, samples from the westernmost plutons, which intrude exposed pre-3.1 Ga crust, have much lower Nd(1) values (-1.0 to4.6) suggesting contamination of these magmas by older crust. The Nd(1) values of post-deformation granites (s.s.) (type II) also vary systematically across the province: values for granites west of longitude 110°30W range from-0.2 to -5.3; those to the east range from +0.6 to +3.7. These data suggest mixed crustal sources dominated by Mid to Early Archean material ( Nd-2.6 to- 17 at 2.6 Ga) for the western granitoid rocks and juvenile sources for the eastern granites. The Nd isotopic data are consistent with the geology of the province in that exposures of Mid to Early Archean crustal rocks, predating the principal 2.7–2.5 Ga orogenic event are restricted to the western part of the province. The asymmetric pattern defined by the Nd isotopic data indicates the presence of distinct crustal rocks beneath the Slave Province. Similar isotopic variations observed across Phanerozoic collisional orogens have been interpreted to reflect tectonic assembly of crust by accretion of juvenile crustal terranes to an older continental margin. This process may also have been an important mechanism in the cratonization of the Slave Province.  相似文献   

18.
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   

19.
The high-K Tuzgle volcanic center, (24° S, 66.5° W) along with several small shoshonitic centers, developed along extensional Quaternary faults of the El Toro lineament on the east-central Puna plateau, 275 km east of the main front of the Andean Central Volcanic Zone (CVZ). These magmas formed by complex mixing processes in the mantle and thickened crust (>50 km) above a 200 km deep scismic zone. Tuzgle magmas are differentiated from shoshonitic series magmas by their more intraplate-like Ti group element characteristics, lower incompatible element concentrations, and lower 87Sr/86Sr ratios at a given Nd. Underlying Mio-Pliocene volcanic rocks erupted in a compressional stress regime and have back-arc like calc-alkaline chemical characteristics. The Tuzgle rocks can be divided into two sequences with different mantle precursors: a) an older, more voluminous rhyodacitic (ignimbrite) to mafic andestitic (56% to 71% SiO2) sequence with La/Yb ratios <30, and b) a younger andesitic sequence with La/Yb ratios >35. La/Yb ratios are controlled by the mafic components: low ratios result from larger mantle melt percentages than high ratios. Shoshonitic series lavas (52% to 62% SiO2) contain small percentage melts of more isotopically enriched arc-like mantle sources. Some young Tuzgle lavas have a shoshonitic-like component. Variable thermal conditions and complex stress system are required to produce the Tuzgle and shoshonitic series magmas in the same vicinity. These conditions are consistent with the underlying mantle being in transition from the thick mantle lithosphere which produced rare shoshonitic flows in the Altiplano to the thinner mantle lithosphere that produced back-are calc-alkaline and intraplate-type flows in the southern Puna. Substantial upper crustal type contamination in Tuzgle lavas is indicated by decreasing Nd (-2.5 to-6.7) with increasing 87Sr/86Sr (0.7063 to 0.7099) ratios and SiO2 concentrations, and by negative Eu anomalies (Eu/Eu* <0.78) in lavas that lack plagioclase phenocrysts. Trace element arguments indicate that the bulk contaminant was more silicic than the Tuzgle ignimbrite and left a residue with a high pressure mineralogy. Crustal shortening processes transported upper crustal contaminants to depths where melting occurred. These contaminants mixed with mafic magmas that were fractionating mafic phases at high pressure. Silicic melts formed at depth by these processes accumulated at a mid to upper crustal discontinuity (decollement). The Tuzgle ignimbrite erupted from this level when melting rates were highest. Subsequent lavas are mixtures of contaminated mafic magmas and ponded silicic melts. Feldspar and quartz phenocrysts in the lavas are phenocrysts from the ponded silicic magmas.  相似文献   

20.
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   

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