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101.
VINCENZO GIANFRANCO DI; ANDRIESSEN PAUL A. M.; GHEZZO CLAUDIO 《Journal of Petrology》1996,37(5):1175-1206
Detailed petrographic and geochemical data and Sr and Nd isotopecompositions of enclaves and host-granite are reported for oneof the largest strongly peraluminous cordierite-bearing intrusionsof the Hercynian Sardinia-Corsica Batholith: the San BasilioGranite. Compared with other peraluminous series, the San BasilioGranite has a non-minimum melt composition andshows variations primarily owing to fractionation of early-crystallizedplagioclase, quartz and biotite. Crystallization age is constrainedat 305 Ma, by Rb-Sr whole-rock age [30523 Ma with (87Sr/86Sr)i= 0.711050.00041], and occurred during late Hercynian tectonicevents. Nd(305Ma) values range from 7.8 to 7.5.The San Basilio Granite contains both magmatic and metamorphicenclaves. Magmatic enclaves, similar to mafic microgranularenclaves common in calc-alkaline granitoids, are tonalitic incomposition and show a variation in silica content from 60.3to 67.7 wt % correlating with a variation in (87Sr/86 Sr) (305Ma)and Nd (305 Ma) from 0.7092 to 0.7109 and from 6.6 to7.4, respectively. Together with petrographic and othergeochemical data, the Sr and Nd isotopic data record differentstages in a complex homogenization process of an unrelated maficmagma with a crustal melt. A process of simple mixing may accountfor the variations of nonalkali elements and, to some extent,of Sr and Nd isotopes, whereas the distribution of alkali elementsrequires diffusioncontrolled mass transfer. Petrographic andmineralogical data on metamorphic enclaves and geochemical modellingfor trace elements in granite indicate melt generation by high-degreepartial melting involving biotite breakdown of a dominantlyquartzo-feldspathic protolith at about T>750800Cand P>6 kbar leaving a granulite facies garnet-bearing residue,followed by emplacement at 3 kbar. Nd(305Ma) values of thegranite fall within the range defined by the pre-existing metamorphicrocks but (87Sr/86Sr) (305Ma) ratios are lower, indicating involvementof at least two distinct components: a dominant crustal componentand a minor well-mixed mafic end-member. These data point toa decoupling between the Sr-Nd isotope systematics and majorand trace element compositions, suggesting that the effect ofthe mafic component was minor on granite major and trace elementconcentrations, but significant on Sr and Nd isotopes. The studyof the magmatic enclaves and the isotopic evidence demonstratethat unrelated mafic magmas, probably derived from the mantle,had a close spatial and temporal association with the productionof on-minimum melt strongly peraluminous granites,and support the proposal that heat from the mafic magma contributedto crustal melting. KEY WORDS: cordierite-bearing granite; enclaves; felsic-mafic interaction; Sardinia-Corsica Batholith; Sr and Nd isotopes
*Corresponding author. 相似文献
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103.
PAUL F. STARRS 《Geographical review》1998,88(4):492-506
ABSTRACT. Academic geography is dotted with fly-specks from the fashion conscious who have skittered from one enthusiasm to another like aggravating bugs in midroom flight. Our disciplinary history in fact abounds in dead-end roads entered at fatally high speed, theoretical turns not negotiated, crossroads oracles proved dismally inaccurate. By contrast, the formidable cultural geographer and landscape historian J. B. “Brinck” Jackson (1909–1996) conceived of a more slowly developing world, replete with enduring geographies. Although firmly Brahmin by origin, Jackson was chary of the fleeting fashions and power tropes of what he sometimes dubbed the Establishment. He was inclined instead to attend to the structures of the everyday, emphasizing community and connection over didactic fashion. With writing grounded in daily experience and a consummate ability to witness pattern, he urged geographers to think and envision. 相似文献
104.
Anorthositic rocks compose 3540% of the Middle Proterozoic(Keweenawan; 1?1 Ga) Duluth Complexa large, compositemafic body in northeastern Minnesota that was intruded beneatha comagmatic volcanic edifice during the formation of the Midcontinentrift system. Anorthositic rocks, of which six general lithologictypes occur in one area of the complex, are common in an earlyseries of intrusions. They are characterized on a local scale(meters to kilometers) by nonstratiform distribution of rocktypes, variably oriented plagioclase lamination, and compositeintrusive relationships. Variably zoned, subhedral plagioclaseof nearly constant average An (60) makes up 8298% ofthe anorthositic rocks. Other phases include granular to poikiliticolivine (Fo6638), poikilitic clinopyrox-ene (En'7337),subpoikilitic Fe-Ti oxides, and various late-stage and secondaryminerals. Whole-rock compositions of anorthositic rocks are modelled bymass balance to consist of three components: cumulus plagioclase(7095 wt.%), minor cumulus olivine (05%), anda gabbroic postcumulus assemblage (527%) representinga trapped liquid. The postcumulus assemblage has textural andcompositional characteristics which are consistent with crystallizationfrom basaltic magma ranging from moderately evolved olivinetholeiite to highly evolved tholeiite (mg=6025). Sympatheticvariations of mg in plagioclase and in mafic minerals suggestthat cumulus plagioclase, though constant in An, was in approximateequilibrium with the variety of basaltic magma compositionswhich produced the postcumulus assemblages. Standard models of mafic cumulate formation by fractional crystallizationof basaltic magmas in Duluth Complex chambers, although ableto explain the petrogenesis of younger stratiform troctoliticto gabbroic intrusions, are inadequate to account for the field,petrographic, and geochemical characteristics of the anorthositicrocks. Rather, we suggest an origin by multiple intrusions ofplagioclase crystal mushesbasaltic magmas charged withas much as 60% intratelluric plagioclase. The high concentrationsof cumulus plagioclase (7095%) estimated to compose theanorthositic rocks may reflect expulsion of some of the transportingmagma during emplacement or early postcumulus crystallizationof only plagioclase from evolved hyperfeldspathic magma. Althoughthe evolved compositions of anorthositic rocks require significantfractionation of mafic minerals, geophysical evidence indicatesthat ultramafic rocks are, as exposure implies, rare in theDuluth Complex and implies that plagioclase crystal mushes werederived from deeper staging chambers. This is consistent withinterpretations of olivine habit and plagioclase zoning. Moreover,plagioclase could have been segregated from coprecipitatingmafic phases in such lower crustal chambers because of the buoyancyof plagioclase in basaltic magmas at high pressure. The geochemicaleffects of plagioclase suspension in basaltic magmas are consistentwith observed compositions of cumulus plagioclase in the anorthositicrocks and with the geochemical characteristics of many comagmaticbasalts. The petrogenesis of the anorthositic rocks and theoverall evolution of Keweenawan magmas can be related to thedynamics of intracontinental rift formation. 相似文献
105.
MATTHEW J. AMESBURY KEITH E. BARBER PAUL D. M. HUGHES 《Boreas: An International Journal of Quaternary Research》2011,40(1):161-174
Amesbury, M. J., Barber, K. E. & Hughes, P. D. M. 2010: The methodological basis for fine‐resolution, multi‐proxy reconstructions of ombrotrophic peat bog surface wetness. Boreas, 10.1111/j.1502‐3885.2010.00152.x. ISSN 0300‐9483. The need for Holocene peat‐based palaeoclimatic records of increased temporal resolution has been widely identified in recent research. The often rapid growth rates of ombrotrophic bogs, when combined with fine‐resolution (i.e. millimetre‐scale) sampling, provide an as yet largely unexploited potential to derive sub‐decadal palaeoclimatic data from this proxy‐archive. However, multi‐proxy, fine‐resolution analyses require changes to standard methodologies, and the application of sampling techniques that are new to peat‐based palaeoclimate research. A peat sampler was custom‐built to allow precise and replicable millimetre‐scale subsampling. Subsequent methodological testing revealed that, irrespective of sample thickness (i.e. resolution), halving the standard sample volume used for plant macrofossil (from 4 cm3 to 2 cm3) and testate amoebae (from 2 cm3 to 1 cm3) analyses and the sample weight used for peat humification analysis (from 0.2 g to 0.1 g dried peat) did not affect the interpretation of the results. A contiguous 1‐mm sampling resolution for plant macrofossil analysis was also tested, but it was found that contiguous 5‐mm samples provided a more reliable background record to fine‐resolution testate amoebae and peat humification analyses. Based on these findings, a standardized and systematic methodological approach was developed, using the custom‐built peat slicer to take millimetre‐scale samples that provide enough sample material for both testate amoebae and peat humification analyses to be performed at 1‐mm resolution. This approach will facilitate the testing of the palaeoclimatic reliability of multi‐proxy, fine‐resolution peat‐based records. 相似文献
106.
ANNA I. HAAPANIEMI JAMES D. SCOURSE VICTORIA L. PECK HILARY KENNEDY PAUL KENNEDY SIDNEY R. HEMMING MARK F. A. FURZE ANNA J. PIEŃKOWSKI WILLIAM E. N. AUSTIN JOHN WALDEN EMILIE WADSWORTH IAN R. HALL 《Boreas: An International Journal of Quaternary Research》2010,39(3):576-591
Haapaniemi, A.I., Scourse, J.D., Peck, V.L., Kennedy, H., Kennedy, P., Hemming, S.R., Furze, M.F.A., Pieńkowski, A.J., Austin, W.E.N., Walden, J., Wadsworth, E. & Hall, I.R. 2010: Source, timing, frequency and flux of ice‐rafted detritus to the Northeast Atlantic margin, 30–12 ka: testing the Heinrich precursor hypothesis. Boreas, Vol. 39, pp. 576–591. 10.1111/j.1502‐3885.2010.00141.x. ISSN 0300‐9483. Increased fluxes of ice‐rafted detritus (IRD) from European ice sheets have been documented some 1000–1500 years before the arrival of Laurentide Ice Sheet (LIS)‐sourced IRD during Heinrich (H) events. These early fluxes have become known as ‘precursor events’, and it has been suggested that they have mechanistic significance in the propagation of H events. Here we present a re‐analysis of one of the main cores used to generate the precursor concept, OMEX‐2K from the Goban Spur covering the last 30 ka, in order to identify whether the British–Irish Ice Sheet (BIIS) IRD fluxes occur only as precursors before H layers. IRD characterization and planktonic foraminiferal δ18O measurements constrained by a new age model have enabled the generation of a continuous record of IRD sources, timing, frequency and flux, and of local contemporary hydrographic conditions. The evidence indicates that BIIS IRD precursors are not uniquely, or mechanistically, linked to H events, but are part of the pervasive millennial‐scale cyclicity. Our results support an LIS source for the IRD comprising H layers, but the ambient glacial sections are dominated by assemblages typical of the Irish Sea Ice Stream. Light isotope excursions associated with H events are interpreted as resulting from the melting of the BIIS, with ice‐sheet destabilization attributed to eustatic jumps generated by LIS discharge during H events. This positive‐feedback mechanism probably caused similar responses in all circum‐Atlantic ice‐sheet margins, and the resulting gross freshwater flux contributed to the perturbation of the Atlantic Meridional Overturning Circulation during H events. 相似文献
107.
108.
ADITI MANDAL ARIJIT RAY MAYUKHEE DEBNATH SANKAR PRASAD PAUL 《Journal of Earth System Science》2012,121(3):793-812
109.
Testate amoebae as a proxy for reconstructing Holocene water table dynamics in southern Patagonian peat bogs 下载免费PDF全文
SIMON VAN BELLEN DMITRI MAUQUOY RICHARD J. PAYNE THOMAS P. ROLAND TIM J. DALEY PAUL D. M. HUGHES NEIL J. LOADER F. ALAYNE STREET‐PERROTT EMMA M. RICE VERÓNICA A. PANCOTTO 《第四纪科学杂志》2014,29(5):463-474
Testate amoebae are abundant and diverse in Sphagnum peat bogs and have been used extensively as indicators of past water table depths. Although these unicellular protists are widely dispersed with globally similar hydrological preferences, regional variations in communities demand region‐specific transfer functions. Here we present the first transfer function for southern Patagonian bogs, based on 154 surface samples obtained from transects in five bogs sampled in 2012 and 2013. Significant variance was explained by pH, electrical conductivity and, in particular, water table depth. Transfer functions for water table were constructed using weighted averaging and evaluated by cross‐validation and independent test sets. The optimal transfer function has predictive ability, but relatively high prediction errors given the wide range in sampled water tables. The use of independent test sets, as well as cross‐validation, allows a more rigorous assessment of model performance than most previous studies. For a subset of locations we compare surface and subsurface samples to demonstrate significant differences in community composition, possibly due to vertical zonation. Our results provide the first quantification of hydrological optima and tolerances for several rare species, which may include Southern Hemisphere endemics and pave the way for palaeohydrological reconstructions in southern Patagonian bogs. 相似文献
110.