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921.
浙江诸暨陈蔡地区发育一套副变质岩层,由中深度变质的片麻岩、片岩和大理岩组成,与双溪坞群为断层接触,其地质时代多被视为与江山—绍兴断裂带南东侧的华夏地块古老变质基底相同。陈蔡岩群地处江绍断裂带北东端,变质和变形较深,前人将其划归为华夏古陆的小块体,地层时代曾与福建古元古代麻源岩群或浙江八都岩群和江西中元古代铁砂街组对比。在陈蔡岩群下吴宅组含榴夕线黑云斜长片麻岩及含榴黑云斜长片麻岩中获得大量的锆石,具有典型的核、幔、边结构,核部年龄集中在新元古代,206Pb/238U年龄加权平均值为848±10Ma和845±9Ma,锆石后期增生边部年龄多为加里东期的年龄,206Pb/238U年龄加权平均值为431.4±7.8Ma。陈蔡岩群斜长片麻岩获得的锆石核部较单一的U-Pb年龄,反映出陈蔡岩群下吴宅组夕线石所代表的泥质成分所含的锆石单一,其锆石核部年龄应代表陈蔡岩群下吴宅组的原岩年龄;同时,认为陈蔡岩群既不能与福建古元古代的麻源岩群对比,也不能与江西地区中元古代铁砂街组对比。其原岩为稍晚于双溪坞群(905~877Ma)的新元古代地层(848~845Ma),从年代上倾向于江南造山带范畴。该年龄对江山—绍兴断裂带变质地层的区域对比和构造演化有重要意义。 相似文献
922.
2003年在对新疆喀喇昆仑地区原划长城系甜水海岩群进行地质填图及实测剖面的工作中,采集到了大量叠层石化石,时代为新元古代青白口纪。该群岩石变形非常强烈,变质达绿片岩相。本次区调依据其岩石变形变质特征,结合叠层石化石资料,对其形成时代进行了重新厘定。 相似文献
923.
One of the most relevant characteristics of the Pennsylvanian shallow-water carbonates of the Amazonas Basin is its diverse and well preserved invertebrate fossiliferous assemblages. In order to better understand the origin of these fossil concentrations, taphonomic data were obtained along well exposed areas of the uppermost part of the Monte Alegre Formation and basal part of the Itaituba Formation, which, based on conodonts, fusulinids and palynomorphs is of Atokan age. The taphonomic data focused on invertebrate organisms were supported by petrographic analysis. The understanding of the stacking pattern of the strata in the studied section allowed the identification of five type taphofacies, which contributed in the development of regional palaeoecological models, expressed as block-diagrams. These characterize the distribution of the environmental parameters, the composition of the faunal associations and the distribution and amplitude of the taphonomic processes that created the taphonomic signatures of the bioclastic elements throughout the supratidal to lower intertidal/deep subtidal depositional environments pertinent to the studied depositional environment. The regional palaeoecological models here presented are related to the particularities of the depositional environments of the studied rocks and are exclusive for the characterization of this intracratonic basin set influenced by high frequency climatic variations. Lithofacies, biofacies and taphofacies associations also reflect depositional conditions pertinent to the studied regional context, differing from the elements observed in modern intracratonic contexts analogous to the one studied, from different sedimentary basins around the world. Therefore, invertebrate taphonomy, supported by the analysis of sedimentary facies, fulfills the purposes recommended in this work, demonstrating its potential as a tool for palaeoecological analysis in the Pennsylvanian outcropping section in the southern platform of the Amazonas Basin. 相似文献
924.
925.
C. I. Mathison 《Australian Journal of Earth Sciences》2013,60(1):57-86
The Somerset Dam Layered Basic Intrusion is probably a sub‐volcanic magma chamber, and it consists of 20 saucer‐shaped layers composed of troctolites, olivine gabbros, ferrigabbros, and leucogabbros. The layered sequence is 1650 ft (500 m) thick, and comprises several repetitions of a standard pattern termed a zone. Each zone is generally composed of four layers, and successive mineral assemblages from the base upwards are: plagioclase‐olivine, plagioclase‐augite‐olivine, plagioclase‐augite‐magnetite‐ilmenite, and plagioclase‐uralite. Pronounced modal and textural changes define the boundaries between these layers. Within a zone, systematic variation in the proportions and compositions of minerals is thought to be the result of a slight decrease in temperature, and an increase in the partial pressures of water vapour and oxygen from the base to the top. Repetition of zones is explained by a mechanism involving periodic renewals of magma. Lack of progressive changes in mineral compositions and proportions from the base to the top of the layered sequence is also a consequence of the magma composition, the control of water vapour pressure, and the limited range of crystallisation temperatures. Gravitational settling of early minerals does not explain the variation within a zone, or the small‐scale rhythmic layering that is locally developed, and it is concluded that diffusion has been an important control. 相似文献
926.
927.
928.
Halton Arp 《Journal of Astrophysics and Astronomy》1987,8(3):241-255
Galaxies of redshiftz ≲ 1000 km s−1 are investigated. In the South Galactic Hemisphere there are two large concentrations of these galaxies. One is in the direction
of the centre of the Local Group, roughly aligned with M 31 and M 33. The other concentration is centred almost 80 degrees
away on the sky and involves the next nearest galaxies to the Local Group, NGC 55, NGC 300 and NGC 253.
The large scale and isolation of these concentrations, and the continuity of their redshifts require that they are all galaxies
at the same, relatively close distance of the brightest group members. The fainter members of the group have higher redshifts,
mimicking to some extent a Hubble relation. But if they are all at the same average distance the higher redshifts must be
due to a cause other than velocity.
The redshifts of the galaxies in the central areas of these groups all obey a quantization interval of δcz0 = 72.4 kms−1. This is the same quantization found by William Tifft, and later by others, in all physical groups and pairs which have been
tested. The quantization discovered here, however, extends over a larger interval in redshift than heretofore encountered.
The majority of redshifts used in the present analysis are accurate to ± 8 km s−1. The deviation of those redshifts from multiples of 72.4 km s-1 averages ±8.2 km s−1. The astonishing result, however, is that for those redshifts which are known more accurately, the deviation from modulo
72.4 drops to a value between 3 and 4 km s−1! The amount of relative velocity allowed these galaxies is therefore implied to be less than this extremely small value. 相似文献
929.
We discuss the distribution of radial velocities of galaxies belonging to the Local Group. Two independent samples of galaxies as well as several methods of reduction from the heliocentric to the galactocentric radial velocities are explored. We applied the power spectrum analysis using the Hann function as a weighting method, together with the jackknife error estimation. We performed a detailed analysis of this approach. The distribution of galaxy redshifts seems to be non‐random. An excess of galaxies with radial velocities of ∼24 km s–1 and ∼36 km s–1 is detected, but the effect is statistically weak. Only one peak for radial velocities of ∼24 km s–1 seems to be confirmed at the confidence level of 95%. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
930.
Daniel J. Rolfe Carole A. Haswell Joseph Patterson 《Monthly notices of the Royal Astronomical Society》2001,324(3):529-536
We present two simple dynamical models for Sagittarius based on N -body simulations of the progressive disruption of a satellite galaxy orbiting for 12.5 Gyr within a realistic Galactic potential. In both models the satellite initially has observable properties similar to those of current outlying dwarfs; in one case it is purely stellar while in the other it is embedded in an extended massive halo. The purely stellar progenitor is a King model with a total velocity dispersion of 18.9 km s−1 , a core radius of 0.44 kpc and a tidal radius of 3 kpc. The initial stellar distribution in the other case follows a King profile with the same core radius, a slightly larger total velocity dispersion and similar extent. Both these models are consistent with all published data on the current Sagittarius system, they match not only the observed properties of the main body of Sagittarius, but also those reported for unbound debris at larger distances. 相似文献