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261.
The Occurrence of Garnet in the Granulite-Facies Terrane of the Adirondack Highlands 总被引:1,自引:0,他引:1
Based on the presence or absence of almandine in charnockiticand metabasic rocks, Buddington (1963) recognized two garnetisograds in the northwestern Adirondacks which were inferredto represent a general increase in grade of regional metamorphismfrom northwest to southeast and east. ACF plots of analyzed Adirondack rocks demonstrate that thepresence or absence of garnet is primarily controlled by thebulk chemistry of the rocks. In metabasites, garnet forms partof the type assemblage of the almandine-amphibolite facies inwhich it developed. The assemblage becomes unstable in the granulitefacies where quartz is present. Garnet does not occur in themore calcic metabasites. In charnockites, garnet develops concomitantlywith orthopyroxene in the aluminous varieties of this rock typeand not in the calcic varieties. The two may occur intimatelyinterlayered. The present definition of the granulite facies, however, doesnot cover clinopyroxene-almandine-bearing assemblages reportedin rocks of diverse composition from the proximity of anorthositemassifs. Reaction rims prove that within the granulite faciesa reaction occurs in rocks with excess silica by which the typomorphicorthopyroxene-plagioclase association is replaced by the higherdensity clinopyroxene-almandine-quartz association. To accommodate this assemblage a fourfold subdvision of thegranulite facies is proposed. Four subfacies are obtained bytwo intersecting boundaries: (1) PH2O-T controlled dehydrationreactions separating pyroxene-granulite from hornblende-granulitesubfacies, and (2) the anhydrous, Pload-T controlled reactionwhich separates subfacies with orthopyroxene-plagioclase assemblagesfrom those with clinopyroxene-almandine assemblages. 相似文献
262.
A graphical interpretation method is presented by which separately the depth and the product of conductivity and thickness of a vertical plate-shaped conducting orebody can be determined. The method is based on the result of model experiments. 相似文献
263.
JUAN PEDRO RODRÍGUEZ‐LÓPEZ NIEVES MELÉNDEZ POPPE L. DE BOER ANA ROSA SORIA 《Sedimentology》2008,55(5):1253-1292
The existence of a mid‐Cretaceous erg system along the western Tethyan margin (Iberian Basin, Spain) was recently demonstrated based on the occurrence of wind‐blown desert sands in coeval shallow marine deposits. Here, the first direct evidence of this mid‐Cretaceous erg in Europe is presented and the palaeoclimate and palaeoceanographic implications are discussed. The aeolian sand sea extended over an area of 4600 km2. Compound crescentic dunes, linear draa and complex aeolian dunes, sand sheets, wet, dry and evaporitic interdunes, sabkha deposits and coeval extradune lagoonal deposits form the main architectural elements of this desert system that was located in a sub‐tropical arid belt along the western Tethyan margin. Sub‐critically climbing translatent strata, grain flow and grain fall deposits, pin‐stripe lamination, lee side dune wind ripples, soft‐sediment deformations, vertebrate tracks, biogenic traces, tubes and wood fragments are some of the small‐scale structures and components observed in the aeolian dune sandstones. At the boundary between the aeolian sand sea and the marine realm, intertonguing of aeolian deposits and marine facies occurs. Massive sandstone units were laid down by mass flow events that reworked aeolian dune sands during flooding events. The cyclic occurrence of soft sediment deformation is ascribed to intermittent (marine) flooding of aeolian dunes and associated rise in the water table. The aeolian erg system developed in an active extensional tectonic setting that favoured its preservation. Because of the close proximity of the marine realm, the water table was high and contributed to the preservation of the aeolian facies. A sand‐drift surface marks the onset of aeolian dune construction and accumulation, whereby aeolian deposits cover an earlier succession of coastal coal deposits formed in a more humid period. A prominent aeolian super‐surface forms an angular unconformity that divides the aeolian succession into two erg sequences. This super‐surface formed in response to a major tectonic reactivation in the basin, and also marks the change in style of aeolian sedimentation from compound climbing crescentic dunes to aeolian draas. The location of the mid‐Cretaceous palaeoerg fits well to both the global distribution of other known Cretaceous erg systems and with current palaeoclimate data that suggest a global cooling period and a sea‐level lowstand during early mid‐Cretaceous times. The occurrence of a sub‐tropical coastal erg in the mid‐Cretaceous of Spain correlates with the exposure of carbonate platforms on the Arabian platform during much of the Late Aptian to Middle Albian, and is related to this eustatic sea‐level lowstand. 相似文献
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高原夏季风指数的定义及其特征分析 总被引:6,自引:0,他引:6
基于1958~2002年ECMWF再分析资料,我国160个台站降水和气温资料,从夏季高原季风环流系统特点出发,定义了能较好表征高原夏季风环流变化的特征指数,分析了高原夏季风年际、年代际变化特征,并揭示了高原夏季风强弱异常时的环流特征及其与中国夏季降水和气温的关系,主要结论为:(1)用6~8月600hPa (27.5~30°N,80~100°E)范围内平均的西风分量距平与(35~37.5°N,80~100°E)范围内平均的东风分量距平差定义了高原夏季风指数(PM I)。该指数计算简单,意义清楚,代表性好。(2)1958~2002年高原夏季风整体呈增强趋势,在20世纪60年代中期之前是高原夏季风的强盛期,之后是高原夏季风弱期,在80年代以后又转为季风强期。(3)高原夏季风与中国夏季降水和气温相关很好。将该指数与之前汤懋苍定义的指数进行性能综合比较后,发现该指数对川渝地区的夏季降水及气温有更好的指示意义。 相似文献
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270.
O. VIDAL V. DE ANDRADE E. LEWIN M. MUNOZ T. PARRA S. PASCARELLI 《Journal of Metamorphic Geology》2006,24(7):669-683
Quantitative X‐ray maps of composition from a chlorite, K‐white mica, albite, quartz and garnet bearing thin section from a Sambagawa blueschist facies metapelite were combined with a multi‐equilibrium calculation method to calculate a P–T‐Fe3+/Fe2+‐deformation map at the millimetre scale. The studied sample was chosen because elongated chlorite crystallization tails (pressure shadows) rimmed by phengite are present, which is an appropriate assemblage for the quantification of the P–T evolution. Chlorite temperature and Fe3+ content maps were calculated by successive iterations for each pixel analysis of Fe3+ until convergence of the four chlorite‐quartz‐H2O equilibria that can be written using the Fe‐ and Mg‐amesite, clinchlore, daphnite and sudoite chlorite end‐members. The calculated map of Fe2+/Fe3+ in chlorite is in good qualitative agreement with the in situ mapping of this ratio using XANES (X‐ray absorption near edge structure) techniques. The temperature map indicates that high temperature chlorite zones with low Fe3+ contents alternate with lower temperature zones and higher Fe3+ contents in the crystallization tail. Late fractures perpendicular to the elongation axis of the tail are filled by very low temperature chlorite (<250 °C) showing Fe3+/Fetotal up to 0.4. Groups of chlorite and mica pixels were then identified based on compositional and structural criteria, and a P–T‐deformation map was calculated using representative analyses of these groups. The calculated P–T‐deformation map suggests that in contrast to chlorite, the composition of most mica did not change significantly during exhumation. Mica reequilibrated in late EW shear bands only. EW shearing was already active at 0.1 GPa, 500 °C, which corresponds to the peak temperature (and probably pressure) conditions, at reduced redox conditions. The intensity of deformation probably decreased with decrease in temperature to ~350–400 °C. At this temperature, a second main deformation event corresponding to a further EW stretching occurred and was still active below 250 °C and more oxidizing conditions. These results indicate that the scale at which P–T data can be obtained is now close to the scale of observation of structural geologists. A close link between deformation and mineral reaction is therefore possible at the microscopic scale, which provides information about the relationship between deformation and mineral reactivity, the modalities of deformation with time and the P–T conditions at which it occurred. 相似文献