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排序方式: 共有434条查询结果,搜索用时 31 毫秒
141.
SYLVI HALDORSEN PER JØRGENSEN MARTIN RAPPOL PETER A. RIEZEBOS 《Boreas: An International Journal of Quaternary Research》1989,18(2):89-97
The clay mineral composition of a number of Saalian till samples from The Netherlands shows a large variation in smectite and illite abundances. The samples were selected according to their source area, as inferred from coarse erratic components. Specimens containing primarily rock fragments indicative of East and East-Central Baltic areas are associated with no or low smectite percentages in their clay fractions. Those with mainly flint and rock fragments indicative of the West Baltic areas have high smectite contents in the clay. In general the smectite-rich material is found in the lower portions of the Saalian till bed, whereas the smectite-poor occurs in the upper portions. The linkage between very fine and coarse till components in the upper smectite-free parts of the till suggests that this entire till has been subjected to englacial transport over distances of at least 1000 km. 相似文献
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The phase diagram for lherzoliteCO2H2O providesa framework for interpreting the distribution of phase assemblagesin the upper mantle with various thermal structures, in differenttectonic settings. Experiments show that at depths >80 km,the near-solidus partial melts from lherzoliteCO2H2Oare dolomitic, changing through carbonatesilicate liquidswith rising temperatures to mafic liquids; vapor, if it coexists,is aqueous. Experimental data from simple systems suggest thata critical end-point (K) occurs on the mantle solidus at anundetermined depth. Isobaric (TX) phase diagrams forvolatile-bearing systems with K elucidate the contrasting phaserelationships for lherzoliteCO2H2O at depths belowand above a critical end-point, arbitrarily placed at 250 km.At levels deeper than K, lherzolite can exist with dolomiticmelt, aqueous vapor, or with critical fluids varying continuouslybetween these end-members. Analyses of fluids in microinclusionsof fibrous diamonds reveal this same range of compositions,supporting the occurrence of a critical end-point. Other evidencefrom diamonds indicates that the minimum depth for this end-pointis 125 km; maximum depth is not constrained. Constructed cross-sectionsshowing diagrammatically the phase fields intersected by upwellingmantle indicate how rising trace melts may influence trace elementconcentrations within a mantle plume. KEY WORDS: mantle solidus; critical end-point; dolomitic magma; diamond inclusions; critical fluids 相似文献
145.
Pleistocene Underplating and Metasomatism of the Lower Continental Crust: a Xenolith Study 总被引:9,自引:5,他引:4
Xenoliths from Engeln–Kempenich in the East Eifel volcanicfield (Germany) comprise gabbroic to ultramafic cumulates, andmeta-igneous and meta-sedimentary granulite- to amphibolite-facieslithologies. They provide evidence for Pleistocene heating andmetasomatism of the lower continental crust by mafic magmas.The metamorphic xenoliths were divided into three types: (1)primitive type P, which are little affected by metasomatic replacementstructures; (2) enriched type E1 defined by metasomatic replacementof primary pyroxene and garnet by pargasitic amphibole and biotite;(3) enriched type E2 defined by breakdown of hydrous phases.Type E rocks are geochemically related to type P and cumulatexenoliths by compositional trends. During modal metasomatism,type E rocks were oxidized. Type E1 rocks were typically enrichedin Rb, Th, U, Nb, K, light rare earth elements (LREE) and Zr,and E2 enriched in Rb, Th, U, Nb, K, REE, Zr, Ti and Y, relativeto type P rocks. Formation of the hydrous, chlorine-bearingphases amphibole and scapolite containing glass and fluid inclusionsin the E1 rocks provides evidence for a water and Cl-bearingfluid phase coexisting with silicate melt. Accordingly, we calculated10 mol % H2O back into the CO2-dominated fluid inclusions, inagreement with experimental data on the composition of a fluidphase coexisting with mafic alkaline melts at elevated pressure.Primary CO2-dominated fluid inclusions coexisting with glassinclusions in metamorphic corona phases and neoblasts, and incumulate xenoliths, have overlapping densities. Fluid inclusionbarometry using the corrected densities indicates that bothcumulates and metamorphic xenoliths originated from the samedepth at 22–25 km (650 ± 50 MPa). This is interpretedas being a main magma reservoir level within the upper partof the lower crust close to the Conrad discontinuity, wherethe xenoliths represent wall-rocks. The Conrad discontinuityseparates an upper-crustal layer, consisting of preferentiallyductile granodioritic and tonalitic gneisses, and more brittlelower-crustal mafic granulites. The brittle–ductile transitionappears to be a preferred level of magma stagnation. KEY WORDS: continental lower crust; fluids; magma chamber; metasomatism; xenoliths 相似文献
146.
Erosion by turbidity currents changes the morphology of the sea floor. The relief of the scoured surface may affect the dynamics of the flow and thereby the pattern of deposition; this could, in turn, affect flow and deposition patterns in subsequent events. This study investigates shallow, centimetre to decimetre scale erosion beneath turbidite sheet sandstones of the Oligocene Macigno Formation of North‐west Italy, where erosion and deposition are variably coupled at the bed scale in a net‐aggradational setting. The research focus was on: (i) the recognition of scour edges and erosive surfaces; (ii) quantification of spatial differences in the amount of erosion; and (iii) an investigation of how this differential erosion can be compensated by the deposits directly overlying the erosional surfaces. Where they can be observed, scour edges commonly have sills of the overlying sandstone intruding beneath blocks and wings of the substrate that is being eroded. A consequence of this de‐laminating scouring style is that erosional surfaces are bedding parallel when followed away from the scour edges, giving the appearance of normal conformable bed bases. Despite their cryptic nature, such bedding‐parallel scour surfaces can be recognized by comparing serial detailed sedimentary logs (here, 16 bed‐parallel scour surfaces were identified in a succession comprising 95 beds). Different styles of compensation by the overlying turbidite beds are defined based on differential sedimentation inside and outside of the scour relief. It is found that differential erosion is on average under‐compensated by differential sedimentation. In some cases, the overlying deposits anti‐compensate, being thinner at the location where more erosion has occurred. Unequal spatial distribution of differential erosion in the study area combines with sedimentary under‐compensation to result in a trend of accumulating section thickness differences over multiple beds. In one ca 25 m thick package, the maximum cumulative change in lateral gradient during some 20 events reached 0·17°, before being reset by a single event. This process can be interpreted either as a lobe compensation effect, or as a scour enhancement effect, depending on the orientation of the palaeohorizontal datum. If allowed to proceed, the latter process could force the system past a channellization threshold, prompting a change from sheet to channelled architecture. This type of shallow substrate scouring and differential deposition is likely to be an important process in the build‐up of sheet turbidite sandstone units and could play a major role in autocyclic adjustment of local sea‐floor gradients. 相似文献
147.
RUSSELL J. WHITE GLENN A. SPINELLI PETER S. MOZLEY NELIA W. DUNBAR 《Sedimentology》2011,58(5):1138-1154
A minor amount (ca 1 wt%) of amorphous silica cement sourced from volcanic glass inhibits consolidation of hemipelagic sediment approaching the Nankai Trough subduction zone throughout the Shikoku Basin. The distribution and nature of the cement were examined via secondary and backscattered electron imaging. The amorphous silica occurs as altered material in contact with volcanic glass, coating grains (including grain contacts) and filling pores. Based on chemical and petrographic evidence, the cement is probably sourced from volcanic glass; this is in contrast to a previous suggestion that this silica cement is sourced dominantly from biogenic silica. Amorphous silica sourced from disseminated volcanic glass shards has the ability to form a thin coating on clay‐dominated sediment throughout the Shikoku Basin. Measured amorphous silica content in hemipelagic sediments suggests that the cementing process is active throughout the Shikoku Basin (at sites separated by >500 km). The cementation process may occur in other locations where sediment containing hydrated disseminated volcanic glass is buried sufficiently for heat to facilitate alteration (i.e. Central America, Cascadia and the Gulf of Alaska). 相似文献
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149.
JULIAN B. MURTON COLIN A. WHITEMAN PETER ALLEN 《Boreas: An International Journal of Quaternary Research》1995,24(3):269-280
Involutions in the early Anglian Barham Soil at Newney Green, Essex, and Badwell Ash, Suffolk, in eastern England, are attributed to soft-sediment deformation during an episode of regional thermokarst development. The involutions show a striking resemblance in morphology and size to thermokarst involutions within a palaeo-thaw layer at Crumbling Point, western arctic Canada. By analogy with the thermokarst involutions, the involutions in the Barham Soil are reinterpreted to have formed by loading during the melting of an ice-rich layer at the top of Anglian permafrost. This period of thermokarst development may have coincided with an episode of intra-Anglian climatic amelioration. Reinterpretation of the Barham Soil involutions implies that many other Pleistocene involutions in Britain may have formed during periods of thermokarst development rather than by active-layer cryoturbation. 相似文献
150.