Transition from eclogite to amphibolite-facies metamorphism in the Austroalpine Ulten Zone     

C. A. Hauzenberger  W. Höller  G. Hoinkes 《Mineralogy and Petrology》1996,58(3-4):111-130

Summary The Ulten Zone of the Austroalpine crystalline basement south-west of Meran (Italy) contains metapelitic schists and granoblastic paragneisses, leucocratic orthogneisses, migmatites (in both gneiss-lithologies), metabasites and ultramafic lenses. Metamorphic textures of the metapelitic schists and granoblastic paragneisses indicate two different metamorphic events, characterized by two mineral assemblages, which differ in mineral chemistry: (1) an eclogite facies mineral assemblage (M1) comprising Grt-Ky I-Bt. Ms-Kfs-PI-Qtz-Rt, and (2) an amphibolite facies mineral assemblage (M2) comprising Grt-KyII-Bt-Ms-PI-Qtz-Ilm±St. For the M1 event, pressures of at least 15kbar and temperatures of about 700°±50°C can be estimated. The later amphibolite facies overprint occurred at pressures of 6 to 8kbar and about 600°±50°C. The M1 and M2 assemblages belong to a continuous clockwise metamorphic evolution during the Variscan orogeny. Evidence for Alpine metamorphism can only be detected by sericite rims around kyanite and reset biotite ages. The migmatites, which contribute about 15–30vol.% of all rocks in the investigated area, were formed on the prograde path during the M1 event. Dissolution of H₂O in the melted part of the migmatites resulted in a CO₂dominated fluid, which was trapped in primary kyanite (M1) fluid inclusions. Secondary H₂O-rich fluid inclusions are found in quartz grains and may represent the fluid which enabled a pervasive equilibration during M2.

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Übergang von eklogit-zu amphibolitfazieller Matamorphose in der austroalpinen Ultenzone

Zusammenfassung Die Ulten Zone, ein Teil des ostalpinen kristallinen basements, südwestlich von Meran, wird aus Metapeliten and granoblastischen Paragneisen, leukokraten Orthogneisen, Migmatiten (in beiden Lithologien), Metabasiten and ultramafischen Linsen aufgebaut. Metamorphe Texturen der Metapelite und granoblastischen Paragneise lassen auf zwei verschiedene metamorphe Ereignisse schließen, die durch unterschiedliche Mineral-chemismen und Paragenesen charakterisiert sind: (1) eine eklogitfazielle Paragenese (M1), bestehend aus Grt-KyI-Bt-Ms-Kfs-P1-Qtz-Rt und (2) eine amphibolitfazielle Paragenese (M2), bestehend aus Grt-KyII-Bt-Ms-P1-Qtz-Ilm±St. Für M1 konnten Minimaldrucke von 15kbar und Temperaturen von 700°±50°C abgeleitet werden. Die spätere amphibolitfazielle Überprägung fand bei 6 bis 8kbar und 600°±50°C statt. M1 und M2 gehören einer kontinuierlichen Metamorphoseentwicklung während der variszischen Orogenese an.Die Migmatite, ungefähr 15–30vol.% der Gesteine im untersuchten Gebiet, wurden am prograden Pfad während des M1 Ereignisses gebildet. Aufgrund der höheren Löslichkeit von H₂0 in der Schmelze, blieb ein CO₂, reiches Fluid zurück, das im primären Kyanit (M1) eingeschlossen wurde. Wässrige Flüssigkeitseinschlüsse können in Quarzkörnern gefunden werden. Dieses Fluid ist wahrscheinlich für die Reequilibrierung zu amphibolitfaziellen Bedingungen verantwortlich.

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With 5 Figures  相似文献   

Evolutionary status of the pre-protostellar core L1498     

Kuiper TB  Langer WD  Velusamy T 《The Astrophysical journal》1996,468(2):761-773

L1498 is a classic example of a dense cold pre-protostellar core. To study the evolutionary status, the structure, dynamics, and chemical properties of this core we have obtained high spatial and high spectral resolution observations of molecules tracing densities of 10(3)-10(5) cm-3. We observed CCS, NH3, C3H2, and HC7N with NASA's DSN 70 m antennas. We also present large-scale maps of C18O and 13CO observed with the AT&T 7 m antenna. For the high spatial resolution maps of selected regions within the core we used the VLA for CCS at 22 GHz, and the Owens Valley Radio Observatory (OVRO) MMA for CCS at 94 GHz and CS (2-1). The 22 GHz CCS emission marks a high-density [n(H2) > 10(4) cm -3] core, which is elongated with a major axis along the SE-NW direction. NH3 and C3H2 emissions are located inside the boundary of the CCS emission. C18O emission traces a lower density gas extending beyond the CCS boundary. Along the major axis of the dense core, CCS, NH3 and C3H2 emission show evidence of limb brightening. The observations are consistent with a chemically differentiated onion-shell structure for the L1498 core, with NH3 in the inner and CCS in the outer parts of the core. The high angular resolution (9"-12") spectral line maps obtained by combining NASA Goldstone 70 m and VLA data resolve the CCS 22 GHz emission in the southeast and northwest boundaries into arclike enhancements, supporting the picture that CCS emission originates in a shell outside the NH3 emitting region. Interferometric maps of CCS at 94 GHz and CS at 98 GHz show that their emitting regions contain several small-scale dense condensations. We suggest that the differences between the CCS, CS, C3H2, and NH3 emission are caused by a time-dependent effect as the core evolves slowly. We interpret the chemical and physical properties of L1498 in terms of a quasi-static (or slowly contracting) dense core in which the outer envelope is still growing. The growth rate of the core is determined by the density increase in the CCS shell resulting from the accretion of the outer low-density gas traced by C18O. We conclude that L1498 could become unstable to rapid collapse to form a protostar in less than 5 x 10(6) yr.  相似文献   

Detection of a new interstellar molecular ion, H2COH+ (protonated formaldehyde)     

Ohishi M  Ishikawa SI  Amano T  Oka H  Irvine WM  Dickens JE  Ziurys LM  Apponi AJ 《The Astrophysical journal》1996,471(1):L61-L64

A new interstellar molecular ion, H2COH+ (protonated formaldehyde), has been detected toward Sgr B2, Orion KL, W51, and possibly in NGC 7538 and DR21(OH). Six transitions were detected in Sgr B2(M). The 1(1,0)-1(0,1) transition was detected in all sources listed above. Searches were also made toward the cold, dark clouds TMC-1 and L134N, Orion (3N, 1E), and a red giant, IRC + 10216, without success. The excitation temperatures of H2COH+ are calculated to be 60-110 K, and the column densities are on the order of 10(12)-10(14) cm-2 in Sgr B2, Orion KL, and W51. The fractional abundance of H2COH+ is on the order of 10(-11) to 10-(9), and the ratio of H2COH+ to H2CO is in the range 0.001-0.5 in these objects. The values in Orion KL seem to be consistent with the "early time" values of recent model calculations by Lee, Bettens, & Herbst, but they appear to be higher than the model values in Sgr B2 and W51 even if we take the large uncertainties of column densities of H2CO into account. We suggest production routes starting from CH3OH may play an important role in the formation of H2COH+.  相似文献   

Evidence for Multiple Mechanisms of Crustal Contamination of Magma from Compositionally Zoned Plutons and Associated Ultramafic Intrusions of the Alaska Range   总被引：3，自引：3，他引：0  

REINERS  PETER W.; NELSON  BRUCE K.; NELSON  STEVEN W. 《Journal of Petrology》1996,37(2):261-292

Models of continental crustal magmagenesis commonly invoke theinteraction of mafic mantle-derived magma and continental crustto explain geochemical and petrologic characteristics of crustalvolcanic and plutonic rocks. This interaction and the specificmechanisms of crustal contamination associated with it are poorlyunderstood. An excellent opportunity to study the progressiveeffects of crustal contamination is offered by the compositeplutons of the Alaska Range, a series of nine early Tertiary,multiply intruded, compositionally zoned (Peridotite to granite)plutons. Large initial Sr and Nd isotopic contrasts betweenthe crustal country rock and likely parental magmas allow evaluationof the mechanisms and extents of crustal contamination thataccompanied the crystallization of these ultra-mafic throughgranitic rocks. Three contamination processes are distinguishedin these plutons. The most obvious of these is assimilationof crustal country rock concurrent with magmatic fractionalcrystallization (AFC), as indicated by a general trend towardcrustal-like isotopic signatures with increasing differentiation.Second, many ultramafic and mafic rocks have late-stage phenocrystreaction and orthocumulate textures that suggest interactionwith felsic melt. These rocks also have variable and enrichedisotopic compositions that suggest that this felsic melt wasisotopically enriched and probably derived from crustal countryrock. Partial melt from the flysch country rock may have reactedwith and contaminated these partly crystalline magmas followingthe precipitation and accumulation of the cumulus phenocrystsbut before complete solidification of the magma. This suggeststhat in magmatic mush (especially of ultramafic composition)crystallizing in continental crust, a second distinct processof crustal contamination may be super imposed on AFC or magmamixing involving the main magma body. Finally, nearly all rocks,including mafic and ultramafic rocks, have (⁸⁷Sr/⁸⁶Sr)_i thatare too high, and (T) _Nd that are too low, to represent theexpected isotopic composition of typical depleted mantle. However,gabbro xenoliths with typical depicted-mantle isotopic compositionsare found in the plutons. This situation requires either anadditional enriched mantle component to provide the parentalmagma for these plutons, or some mechanism of crustal contaminationof the parent magma that did not cause significant crystallizationand differentiation of the magma to more felsic compositions.Thermodynamic modeling indicates that assimilation of alkali-andwater-rich partial melt of the metapelite country rock by fractionating,near-liquidus basaltic magma could cause significant contaminationwhile suppressing significant crystallization and differentiation. KEY WORDS: crustal contamination; Alaska Range; isotope geochemistry; zoned plutons; assimilation ^*Corresponding author. e-mail: preiners{at}u.washington.edu; fax: (206) 543-3836.  相似文献   

Springs of Great Britain     

J. B. W. Day 《Environmental Geology》1996,27(2):77-78

Predictably, in a country such as Britain, with its preponderance of consolidated, sedimentary, mainly fissure-flow aquifers, there is a very large number of springs, many of which are, or have been, used for public supply. Migratory springs are a feature of the British (Ur. Cretaceous) Chalk, the most important British aquifer. The Chalk's low specific yield and high capillary moisture retention together give rise to very considerable fluctuations (more than 33 m in some areas) of the unconfined water table. Along the gentle dip slopes of the Chalk (North and South Downs of southern and southeastern England) springs may migrate laterally for several miles, giving rise to seasonal streams locally known as bournes or lavants. However, springs such as at Duncton, West Sussex, at the base of the much steeper scarp slopes of the Chalk, form point sources, the flows from which tend to be relatively steady; such springs commonly supply and are the original reason for the existence of many of the small towns and villages which nestle along the bases of the chalk scarps of Sussex and Kent.Where the Chalk forms coastal cliffs, a number of springs break out at the base of the cliff between high and low tide levels; there are major chalk coastal springs, for instance, at St. Margaret's Bay (Kent) and at Arish Mells, east of Lulworth Cove, Dorset. Such springs are not used for direct supply (their salinity is usually too high) but are indicators of the presence of local reserves of groundwater for possible future development.  相似文献   

Integrated chronostratigraphy of Proterozoic-Cambrian boundary beds in the western Anabar region, northern Siberia   总被引：2，自引：0，他引：2  

Kaufman AJ  Knoll AH  Semikhatov MA  Grotzinger JP  Jacobsen SB  Adams W 《Geological magazine》1996,133(5):509-533

Carbonate-rich sedimentary rocks of the western Anabar region, northern Siberia, preserve an exceptional record of evolutionary and biogeochemical events near the Proterozoic/Cambrian boundary. Sedimentologically, the boundary succession can be divided into three sequences representing successive episodes of late transgressive to early highstand deposition; four parasequences are recognized in the sequence corresponding lithostratigraphically to the Manykal Formation. Small shelly fossils are abundant and include many taxa that also occur in standard sections of southeastern Siberia. Despite this coincidence of faunal elements, biostratigraphic correlations between the two regions have been controversial because numerous species that first appear at or immediately above the basal Tommotian boundary in southeastern sections have first appearances scattered through more than thirty metres of section in the western Anabar. Carbon- and Sr-isotopic data on petrographically and geochemically screened samples collected at one- to two-metre intervals in a section along the Kotuikan River, favour correlation of the Staraya Reckha Formation and most of the overlying Manykai Formation with sub-Tommotian carbonates in southeastern Siberia. In contrast, isotopic data suggest that the uppermost Manykai Formation and the basal 26 m of the unconformably overlying Medvezhya Formation may have no equivalent in the southeast; they appear to provide a sedimentary and palaeontological record of an evolutionarily significant time interval represented in southeastern Siberia only by the sub-Tommotian unconformity. Correlations with radiometrically dated horizons in the Olenek and Kharaulakh regions of northern Siberia suggest that this interval lasted approximately three to six million years, during which essentially all 'basal Tommotian' small shelly fossils evolved.  相似文献   

Derivation of A-type Granites from a Dehydrated Charnockitic Lower Crust: Evidence from the Chaelundi Complex, Eastern Australia   总被引：28，自引：3，他引：25  

LANDENBERGER  B.; COLLINS  W. J. 《Journal of Petrology》1996,37(1):145-170

Triassic I- and A-type granites of the Chaelundi Complex, NewEngland Fold Belt, eastern Australia, were generated in a subduction-relatedtectonic setting. Although isotopic ages of the suites are indistinguishable,field relations indicate that the A-type is younger. The mostmafic granitoids from each suite have similar silica contents(66–68% SiO₂), slightly LREE enriched patterns withoutEu anomalies, low Rb/Sr and K/Ba ratios, and high K/Rb ratios,suggesting that both represent parental magmas. The A-type isdistinguished mineralogically by abundant orthoclase and sodicplagioclase (total >60%), ferro-hornblende, annite and allanite.In contrast, the I-type has more hornblende and biotite, whichare more magnesian in composition, and less feldspar. The parentalmagmas of both suites have many similar geochemical characteristics,although the A-type has slightly higher alkalis, Zr, Hf, Znand LREE, and lower CaO, MgO, Sr, V, Cr, Ni and Fe³⁺/Fe. Thegeochemical properties characteristic of leucocratic A-typegranites, such as high Ga/Al, Nb, , HREE and F contents, areonly manifest in the more felsic members of the A-type suite.These features were produced by 70% fractional crystallizationof feldspar, hornblende, quartz and biotite. Both granite suites were generated by water-undersaturated partialmelting of a similar source, but the A-type parent magma resultedfrom lower aH₂O conditions during partial melting. Generationand rapid ascent of the earlier 1-type magma during disequilibriumpartial melting produced a relatively anhydrous, but not refractory,charnockitic lower crust. Continued thermal input from mantle-derivedmagmas, during continuing subduction, partially melted the ‘charnockitized’lower crust at temperatures in excess of 900C, to produce A-typemagmas. Charnockitic magmas (C-type) form in a similar way toA-type magmas, although their different composition reflectsvariations in the anhydrous lower-crustal mineral assemblagesthat remain after the previous (1-type) granite-forming event. The New England Fold Belt was a subduction—accretion complexuntil the late Carboniferous, when the deeper parts underwentpartial melting to produce S-type granites. As the I-and A-typegranites intruded penecontemporaneously, a tonalitic sourcemodel for genesis of the Chaelundi A-type is untenable. KEY WORDS: A-type; charnockitization; eastern Australia ^*Corresponding author.  相似文献   

Geochemical framework for water quality studies in sub-Saharan Africa     

W.M. Edmunds 《Journal of African Earth Sciences》1996,22(4):385-389

The scope for using hydrogeochemical techniques in water quality studies in Africa is reviewed as a background to a set of thematic papers. Water quality problems are emerging as a key issue in Africa either:

• 1. 

  i) in view of the pressures of man-made pollution on finite resources; or

• 2. 

  ii) the existence of regions with naturally induced geological problems, for example fluoride endemic areas.

Such natural problems are the focus of this paper and the following topics were emphasised during a workshop in Sodere, Ethiopia: the need to determine natural baselines as a means of recognizing anthropogenic pollution; the need for high quality samples and field data, but relatively basic analytical data; the use of chloride to assist in recharge estimation and water-balance studies; an understanding of depth stratification of water quality as part of the design of well/borehole drilling programmes; the use of hydrogeochemistry in geothermal studies; the recognition of health and/or acceptability problems, especially for F, Fe, Mn, As, I and Al. Water quality standards for African countries need to be considered in the context of local geochemical environments and some of the WHO limits, especially for major ions, may be unattainable owing to naturally high total mineralization.  相似文献   

Stability of Yoderite in the Absence and in the Presence of Quartz: an Experimental Study in the System MgO-Al2O3-Fe2O3-SiO2-H2O     

FOCKENBERG  T.; SCHREYER  W. 《Journal of Petrology》1994,35(5):1341-1375

The water-pressure temperature stability field of yoderite,ideally Mg₂Al_5.6Fe^{3 +} _0.4Si₄O₁₈(OH)₂, was determined at highoxygen fugacities by high-pressure bracketing runs on eightpossible breakdown reactions involving the phases chlorite,kyanite, talc, staurolite, pyrope, enstatite, boron-free kornerupine,cordierite, quartz, and invariably an excess of hematite. Yoderitewas found to be stable over the surprisingly large PT rangefrom 6 to 25 kbar water pressure and 590 to 795 C. It is thusa high-pressure mineral covering the upper amphibolite and portionsof the eclogite facies. In the presence of quartz its upperpressure stability is reduced to some 15 kbar, and its uppertemperature stability to 715 C. Two of the yoderite-producingreactions are anomalous as they show dehydration in the directiontowards lower temperatures. Importantly, this is also true forthe reaction kyanite + talc + hematite+H₂O=yoderite+quartz whichis responsible for the only yoderite occurrence in nature atMautia Hill, Tanzania. Preliminary thermodynamic calculationsindicate that—owing to this unusual dehydration behavior—thestability field for the assemblage yoderite+quartz disappearsfor water activities lower than 0.5. The rarity of yoderitein natural rocks, which is in contrast to its large PT stabilityfield, must be explained on chemical rather than on physicalgrounds. Yoderite can only occur in whiteschist-type bulk compositionsrich in MgO, Al₂O₃, SiO₂, and containing some iron, but poorin alkalis and CaO. Oxygen fugacities must be unusually highto keep Fe trivalent, and—at least for rocks with excessquartz—the water activity must be high as well. In anenvironment of this kind, yoderite formation in the Mautia Hillwhiteschist may have occurred even at constant total pressureand temperature simply by an influx of hydrous fluid duringthe late stages of metamorphism under amphibolite facies conditions.  相似文献   

Possible consequences of absence of “jupiters” in planetary systems     

George W. Wetherill 《Astrophysics and Space Science》1994,212(1-2):23-32

The formation of the gas giant planets Jupiter and Saturn probably required the growth of massive 15 Earth-mass cores on a time scale shorter than the 10⁷ time scale for removal of nebular gas. Relatively minor variations in nebular parameters could preclude the growth of full-size gas giants even in systems in which the terrestrial planet region is similar to our own. Systems containing failed Jupiters, resembling Uranus and Neptune in their failure to capture much nebular gas, would be expected to contain more densely populated cometary source regions. They will also eject a smaller number of comets into interstellar space. If systems of this kind were the norm, observation of hyperbolic comets would be unexpected. Monte Carlo calculations of the orbital evolution of region of such systems (the Kuiper belt) indicate that throughout Earth history the cometary impact flux in their terrestrial planet regions would be 1000 times greater than in our Solar System. It may be speculated that this could frustrate the evolution of organisms that observe and seek to understand their planetary system. For this reason our observation of these planets in our Solar System may tell us nothing about the probability of similar gas giants occurring in other planetary systems. This situation can be corrected by observation of an unbiased sample of planetary systems.Paper presented at the Conference onPlanetary Systems: Formation, Evolution, and Detection held 7–10 December, 1992 at CalTech, Pasadena, California, U.S.A.  相似文献