A Stable Isotope Study of Anorogenic Magmatism in East Central Asia   总被引：8，自引：0，他引：8  

WICKHAM  STEPHEN M.; ALBERTS  AMY D.; ZANVILEVICH  ADA N.; LITVINOVSKY  BORIS A.; BINDEMAN  ILYA N.; SCHAUBLE  EDWIN A. 《Journal of Petrology》1996,37(5):1063-1095

A stable isotope study of 168 plutonic igneous rock and fivewater samples from Transbaikalia, East Asia, has been made,including 318 whole-rock and mineral ¹⁸O/¹⁶O analyses and 14D/H analyses. This represents the first detailed isotopic studyof the enormous Phanerozoic K-rich (mostly anorogenic) granitoidbelts of this region, which are thousands of kilometers in lengthand span an age range of >250Ma. Of the five main intrusivesuites, emplacement of the youngest (Permo-Triassic) was accompaniedby intense meteoric-hydrothermal activity, locally causing extreme¹⁸O depletio (¹⁸O _feldspar<–12). This reflects thevery low ¹⁸O of the water involved in these systems, which probablyhad ¹⁸O < – 20 and D < – 150, consistent withthe high paleolatitude of Transbaikalia in the early Mesozoic(80N). Despite local post-emplacement, hydrothermal ¹⁸O-depletioneffects near Permo-Triassic and younger plutons, the variationof magmatic isotopic composition in the five intrusive suites,in space and time, can be clearly discerned using the ¹⁸O/¹⁶Ovariation in phases such as quartz and sphene that are resistantto sub-solidus exchange. A procedure for analyzing ¹⁸O/¹⁶O insphene using a laser fluorination technique is described: analysisof samples as small as 0.4 mg (including single crystals ofsphene from granitoids) is possible and provides an effectiveway to estimate the magmatic ¹⁸O value of plutonic igneous rocks.Most sphene and quartz ¹⁸O values vary by 1.0–2.0% withineach of the five main intrusive suites in Transbaikalia (rangingin age from mid-Paleozoic to Mesozoic), and are uniform bothwithin individual plutons and among plutons of the same suiteseparated by tens or hundreds of kilometers. However, each suitehas a unique range in ¹⁸O/¹⁶O, indicating that, on a regionalscale, the magmatic ¹⁸O values of these granitoids decreasedprogressively in 1% decrements from +10 in the earliest groupto +6 in the youngest. This progression was accompanied by increasesin the concentration of elements such as K and Zr, and decreasesin the concentration of elements such as Sr and Ba. These systematicsrequire large scale deep crustal melting and mixing processesto generate the compositional uniformity of individual plutonsand groups over such wide areas, and also a progressive hybridizationof the crust with alkalic, mantle-derived magmas to generatesyenites and granites with progressively lower ¹⁸O values. Thisprocess may be a hallmark of anorogenic granitoid petrogenesisand the intracontinental cratonization process in general, andalso represents an important (though largely cryptic) crustalgrowth mechanism. KEY WORDS: anorogenic granitoids; crustal growth; hybridization; hydrothermal systems; stable isotopes ¹Present address: Galson Sciences Limited, 5 Grosvenor House, Melton Road, Oakham LE15 6AX, UK.  相似文献   

The application of high-resolution sequence stratigraphy to fluvial systems: a case study from the Upper Carboniferous Breathitt Group, eastern Kentucky, USA   总被引：13，自引：0，他引：13  

JOHN F. AITKEN  STEPHEN S. FLINT 《Sedimentology》1995,42(1):3-30

The Pennsylvanian Pikeville, Hyden and Four Corners formations of the Breathitt Group in eastern Kentucky, USA, contain six major facies associations along with a number of subassociations. These facies associations are offshore siltstone, rhythmically bedded mouthbar heteroliths, predominantly fine-grained floodplain deposits, minor channel fills, major distributary channels and major, stacked fluvial bodies. The stacked fluvial bodies are incised into a variety of open marine and delta plain deposits, have widths of several kilometres and exhibit a range of sandy fill types. These fluvial complexes are interpreted as incised valley fills. Parasequences and parasequence sets are not identifiable. Nonetheless, it is possible to identify systems tracts on the basis of sequential position, facies associations and systematic changes in architectural style and sediment body geometries. The studied portion of the Breathitt Group comprises stacked 4th-order sequences, which occur in lowstand, transgressive and highstand sequence sets related to the development of a lower frequency base level cycle. In the lowstand sequence set, incision associated with successive 4th-order sequence boundaries has commonly removed all the HST and TST of the underlying sequences, such that succeeding 4th-order incised valley fills are amalgamated. Within the transgressive sequence set, incision is at a minimum and incised valley fills tend to stack discretely with the maximum amount of fine-grained TST and HST between them. The highstand sequence set is transitional between the lowstand and transgressive sequence sets in terms of the amount of transgressive and highstand deposits preserved. Incised valley fills tend to stack discretely.  相似文献   

Quaternary coastal lithofacies,sequence development and stratigraphy in a passive margin setting,North Carolina and Virginia,USA   总被引：3，自引：0，他引：3  

PETER R. PARHAM  STANLEY R. RIGGS  STEPHEN J. CULVER  DAVID J. MALLINSON  W. JACK RINK  KEVIN BURDETTE 《Sedimentology》2013,60(2):503-547

Lithofacies analysis is fundamental to unravelling the succession of depositional environments associated with sea‐level fluctuations. These successions and their timing are often poorly understood. This report defines lithofacies encountered within the north‐eastern North Carolina and south‐eastern Virginia Quaternary section, interprets their depositional environments, presents a model for coastal depositional sequence development in a passive margin setting and uses this understanding to develop the stratigraphy and Quaternary evolutionary history of the region. Data were obtained from numerous drill cores and outcrops. Chronology was based on age estimates acquired using optically stimulated luminescence, amino acid racemization, Uranium series and radiocarbon dating techniques. Geomorphic patterns were identified and interpreted using light detection and ranging imagery. Since lithofacies occurrence, distribution and stratigraphic patterns are different on interfluves than in palaeo‐valleys, this study focused on interfluves to obtain a record of highstand sea‐level cycles with minimal alteration by fluvial processes during subsequent lowstands. Nine primary lithofacies and four diagenetic facies were identified in outcrops and cores. The uppermost depositional sequence on interfluves exhibits an upward succession from shelly marine lithofacies to tidal estuarine lithofacies and is bounded below by a marine ravinement surface and above by the modern land surface. Older depositional sequences in the subsurface are typically bounded above and below by marine ravinement surfaces. Portions of seven depositional sequences were recognized and interpreted to represent deposition from late middle Pleistocene to present. Erosional processes associated with each successive depositional sequence removed portions of older depositional sequences. The stratigraphic record of the most recent sea‐level highstands (Marine Isotope Stage 5a and Marine Isotope Stage 3) is best preserved. Glacio‐isostatic adjustment has influenced depositional patterns so that deposits associated with late Quaternary sea‐level highstands (Marine Isotope Stages 5c, 5a and 3), which did not reach as high as present sea‐level according to equatorial eustatic records, are uplifted and emergent within the study area.  相似文献   

Geological excursion guide 9: Jamaica     

STEPHEN K. DONOVAN 《Geology Today》1993,9(1):30-34

Apart from its obvious attractions for the visiting tourist, Jamaica is geologically significant, with a rock record dating from the Cretaceous and occupying a position at the north–east end of the Nicaraguan Rise, adjacent to the North Caribbean Plate Boundary Zone. During the Cretaceous, Jamaica was part of an island arc, but since the Eocene the rock record has been dominated by limestones.  相似文献   

Sediment magnetic properties of glacial till deposited since the Little Ice Age maximum for selected glaciers at Svartisen and Okstindan, northern Norway     

STEPHEN D. GURNEY  KEVIN WHITE 《Boreas: An International Journal of Quaternary Research》2005,34(1):75-83

Samples of glacial till deposited since the Little Ice Age (LIA) maximum by two glaciers, North Bogbre at Svartisen and Corneliussen-breen at Okstindan, northern Norway, were obtained from transects running from the current glacier snout to the LIA (c. AD 1750) limit. The samples were analysed to determine their sediment magnetic properties, which display considerable variability. Significant trends in some magnetic parameters are evident with distance from the glacier margin and hence length of subaerial exposure. Magnetic susceptibility (χ) decreases away from the contemporary snout, perhaps due to the weathering of ferrimagnetic minerals into antiferromagnetic forms, although this trend is generally not statistically significant. Trends in the ratios of soft IRM/hard IRM which are statistically significant support this hypothesis, suggesting that antiferromagnetic minerals are increasing relative to ferrimagnetic minerals towards the LIA maximum. Backfield ratios (IRM -100 mT/SIRM) also display a significant and strong trend towards magnetically harder behaviour with proximity to the LIA maximum. Thus, by employing a chronosequence approach, it may be possible to use sediment magnetics data as a tool for reconstructing glacier retreat in areas where more traditional techniques, such as lichenometry, are not applicable.  相似文献   

A first-order analysis of the potential rôle of CO₂ fertilization to affect the global carbon budget: a comparison of four terrestrial biosphere models     

DAVID W. KICKLIGHTER  MICHELE BRUNO  SILKE DÖNGES  GERD ESSER  MARTIN HEIMANN  JOHN HELFRICH  FRANK IFT  FORTUNAT JOOS  JÖRG KADUK  GUNDOLF H. KOHLMAIER  A. DAVID McGUIRE  JERRY M. MELILLO  ROBERT MEYER  BERRIEN MOORE III  REAS NADLER  I. COLIN PRENTICE  WALTER SAUF  ANNETTE L. SCHLOSS  STEPHEN SITCH  UWE WITTENBERG  GUDRUN WÜRTH 《Tellus. Series B, Chemical and physical meteorology》1999,51(2):343-366

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30 000 years of productivity and salinity variations in the late Quaternary Cariaco Basin revealed by dinoflagellate cysts     

KENNETH NEIL MERTENS  CATALINA GONZÁLEZ  IRINA DELUSINA  STEPHEN LOUWYE 《Boreas: An International Journal of Quaternary Research》2009,38(4):647-662

Dinoflagellate cysts and other palynomorphs were studied from ODP Hole 1002C in the Cariaco Basin over the past 30 000 years. The assemblage shifts between a dominance of heterotrophic dinoflagellate cysts (mainly Brigantedinium spp., Lejeunecysta spp., Selenopemphix nephroides and Stelladinium reidii ) and autotrophic dinoflagellate cysts (mainly Spiniferites ramosus , Lingulodinium machaerophorum and Operculodinium centrocarpum ). These assemblage shifts are associated with stronger upwelling during stadials and stronger river influx during interstadials. Increases in productivity caused by enhanced upwelling are coupled to improved preservation and vice versa. More stratified water is indicated by higher abundances of L. machaerophorum and succeeds Heinrich events. The average process length of L. machaerophorum can be used to track changes in salinity, since this shows a similar pattern as the δ¹⁸O_SW (paired Mg/Ca −δ¹⁸O) reconstruction. During the last glacial, conditions were more saline than during the current interglacial. On a millennial scale, changes in salinity are opposite to open ocean salinities and the hydrological proxies, which can be explained by a modulation of the signal by stratification, isolation of the Basin or advection of freshwater masses. These results highlight both generalities and particularities of the palaeoecological record of this tropical semi-enclosed basin.  相似文献   

Broody sea urchins     

STEPHEN K. DONOVAN 《Geology Today》1994,10(5):180-180

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Late-glacial ice advances in the western Italian Alps   总被引：1，自引：0，他引：1  

STEPHEN c. PORTER  GIUSEPPE OROMBELLI 《Boreas: An International Journal of Quaternary Research》1982,11(2):125-140

The extent of ice on the south side of the Mont Blanc massif during two late-glacial readvances is known primarily from the distribution of crystalline erratics on sedimentary and metasedimentary terrain, and from the distribution of end moraines and outwash remants. Similar dual moraine complexes are found elsewhere in upper Val d'Aosta and indicate that the pattern is regional in character. During the earlier readvance equilibrium-line altitudes were ca. 450 ± 50m lower than at present, whereas during the later readvance they were ca. 350 ± 50 m lower. The moraines, not yet closely dated, were deposited sometime after 14,200 ¹⁴C Years ago by which time Lago'd Alice near the margin of the würm moraine system at Ivrea had been deglaciated, and before 8400 years ago, at which time Rutor Glacier in upper Val d'Aosta was no larger than at present. The younger moraines are believed to correlate with Egesen moraines on the north side of the Alps, whereas the older set may either correlate with an early phase of the Egesen or with Daun advance.  相似文献   

Negotiating Technology — (Re)considering the Use of GIS by Indigenous Peoples     

STEPHEN T. ENGLE 《New Zealand geographer》2001,57(1):27-35

Geographic Information Systems (GIS) have been increasingly touted for their ability to help empower indigenous peoples. The opportunity to discuss the development of an iwi‐based GIS with Te Ruunanga O Raukawa provides a platform for reflecting critically on issues that impact the effective use of GIS by indigenous groups. The dialectical context of GIS is explored by (re)considering the opportunities and challenges of applying GIS as a cross‐cultural communication device. A position is negotiated which interprets GIS as conditionally empowering provided that indigenous peoples consider their symbolic representation in, and physical access to GIS before making an initial investment in the technology.  相似文献