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761.
Greshake A 《Geochimica et cosmochimica acta》1997,61(2):437-452
The mineralogical and chemical characteristics of the fine-grained matrix (< or = 3 micrometers) of the unique primitive carbonaceous chondrite Acfer 094 have been investigated in detail by scanning electron microscopy (SEM) and analytical transmission electron microscopy (ATEM). Generally, the fine-grained matrix represents a highly unequilibrated assemblage of an amorphous material, small forsteritic olivines (200-300 nm), low Ca-pyroxenes (300-400 nm), and Fe,Ni-sulfides (100-300 nm). The matrix is basically unaffected by secondary processes. Only minor amounts of serpentine and ferrihydrite, as products of hydrous alteration, are present. Texturally, the amorphous material acts as a groundmass to olivines, pyroxenes, and sulfides, mostly exhibiting rounded or elongated morphologies. Only very few clastic mineral grains have been found. The texture and chemical composition of the amorphous material are consistent with an origin by disequilibrium condensation in either the cooling solar nebula or a circumstellar environment. As such, the amorphous material may be considered as a possible precursor of matrix materials in other types of chondrites. The non-clastic matrix olivines (Fo98-99) and pyroxenes (En97-100) are suggested to have formed either by condensation in the solar nebula under highly oxidizing conditions or by recrystallization from the amorphous material. The formation of these grains by fragmentation of chondrule components is unlikely due to chemical and microstructural reasons. Rapid cooling caused the observed intergrowths of clino/orthoenstatite in the Mg-rich matrix pyroxenes. Although some similarities exist comparing the fine-grained matrix of Acfer 094 with the matrices of the unequilibrated CO3 chondrite ALHA77307 and the unique type 3 chondrite Kakangari, Acfer 094 remains unique. Since it contains the highest measured concentrations of circumstellar SiC and the second highest of diamond (highest is Orgueil), it seems reasonable to suggested that at least parts of the amorphous material in the fine-grained matrix may be of circumstellar origin. 相似文献
762.
HuiJun Jin XiaoLi Chang DongLiang Luo RuiXia He LanZhi L SiZhong Yang DongXin Guo XueMei Chen Stuart A. Harris 《寒旱区科学》2016,8(4):269-296
In Northeast China, permafrost advanced and retreated several times under the influences of fluctuating paleo-climates and paleo-environments since the Late Pleistocene. During the last 60 years, many new data were obtained and studies were conducted on the evolution of permafrost in Northeast China, but so far no systematic summary and review have been made. Based on sedimentary sequences, remains of past permafrost, paleo-flora and-fauna records, and dating data, permafrost evolution since the Late Pleistocene has been analyzed and reconstructed in this paper. Paleo-temperatures reconstructed from the remains of past permafrost and those from paleo-flora and-fauna are compared, and thus the southern limit of permafrost(SLP) in each climate period is inferred by the relationship of the permafrost distribution and the mean annual air/ground temperatures(MAAT/MAGT). Thus, the evolutionary history of permafrost is here divided into five stages:(1) the Late Pleistocene(Last Glaciation, or LG)(65 to 10–8.5 ka), the Last Glaciation Maximum(LGM, 21–13 ka) in particular, the coldest period in the latest history with a cooling of about 6~10 ?C, characterized by extensive occurrences of glaciation, flourishing Mammathas-Coelodonta Faunal Complex(MCFC), widespread aeolian deposits, and significant sea level lowering, and permafrost greatly expanded southwards almost to the coastal plains(37?N–41?N);(2) the Holocene Megathermal Period(HMP, 8.5–7.0 to 4.0–3.0 ka), 3~5 ?C warmer than today, permafrost retreated to about 52°N;(3) the Late Holocene Cold Period(Neoglaciation)(4.0–3.0 to 1.0–0.5 ka), a cooling of 1~3 ?C, some earlier thawed permafrost was refrozen or attached, and the SLP invaded southwards to 46?N;(4) the Little Ice Age(LIA, 500 to 100–150 a), the latest cold period with significant permafrost expansion; and(5) climate warming since the last century, during which Northeast China has undergone extensive permafrost degradation. The frequent and substantial expansions and retreats of permafrost have greatly impacted cold-region environments in Northeast China. North of the SLP during the HMP, or in the present continuous permafrost zone, the existing permafrost was largely formed during the LG and was later overlapped by the permafrost formed in the Neoglaciation. To the south, it was formed in the Neoglaciation. However, many aspects of permafrost evolution still await further investigations, such as data integration, numerical reconstruction, and merging of Chinese permafrost history with those of bordering regions as well as collaboration with related disciplines. Of these, studieson the evolution and degradation of permafrost during the past 150 years and its hydrological, ecological, and environmental impacts should be prioritized. 相似文献
763.
Dmitry P. GLADKOCHUB Tatiana V. DONSKAYA Richard E. ERNST Sergei A. PISAREVSKY Michael T.D. WINGATE Ulf S?DERLUND 《《地质学报》英文版》2016,90(Z1):6-7
正We present a summary of late Paleoproterozoic to Neoproterozoic mafic magmatism in the Siberian craton which allows us distinguish following main pulses of mafic dyke emplacement:1)1860–1850 Ma mafic dykes are localized within the 相似文献
764.
Sergei A. PISAREVSKY 《《地质学报》英文版》2016,90(Z1):40-40
正There are only two quantitative tools for Precambrian paleogeographic reconstructions–paleomagnetic data and dyke swarms geometries.Paleomagnetic data provide information about paleolatitudes and orientation of rigid 相似文献
765.
正About 300 mafic dykes are intruded Eocene volcanic and pyroclastic rocks,north east of Meshkinshahr city in the NW of Iran.Most of dyke swarms display NNE-SSE trend.Their composition varies from basalt to tephrite and 相似文献
766.
A. S. A. A. ABU SHARIB 《《地质学报》英文版》2016,90(1):88-105
Foliation inflexion/intersection axes(FIAs)preserved within porphyroblasts that grew throughout Isan orogenesis reveal significant anticlockwise changes in the direction of bulk horizontal shortening between 1670 and 1500 Ma from NE-SW,N-S,E-W to NW-SE.This implies an anticlockwise shift in relative plate motion with time during the Isan orogeny.Dating monazite grains amongst the axial planar foliations defining three of the four FIAs enabled an age for the periods of relative plate motion that produced these structures to be determined.Averaging the ages from monazite grains defining each FIA set revealed 1649±12 Ma for NE-SW shortening,1645±7 Ma for N-S shortening,and 1591±10 Ma for that directed E-W.Inclusion trail asymmetries indicate shear senses of top to the SW for NW-SE FIAs and dominantly top to the N for E-W FIAs,reflecting thrusting towards the SW and N.No evidence for tectonism related to early NE-SW bulk horizontal shortening has previously been detected in the Mount Isa Inlier.Amalgamation of the Broken Hill and possibly the Gawler provinces with the Mount Isa province may have taken place during these periods of NE-SW and N-S-directed thrusting as the ages of tectonism are similar.Overlapping dates,tectonic,metamorphic,and metallogenic similarities between eastern Australia(Mount Isa and Broken Hill terranes)and the southwest part of Laurentia imply a most probable connection between both continental masses.Putting Australia in such position with respect to North America during the Late-Paleo-to-Mesoproterozoic time is consistent with the AUSWUS model of the Rodinia supercontinent. 相似文献
767.
MIGRATION OF GEOSYNCLINES 总被引:3,自引:0,他引:3
A.W.GRABAU 《地质学报》1924,3(3):207-318
The term geosyncline was applied by J. D. Dana in 1873 to the long earth trough in which he conceived that the strata now involved in the folds of the Appalachian Mountains of eastern North America were deposited. He extended this term to include all similar earth troughs, whether simple or complex, which are iormed in regions of excessive deposition. Such earth 相似文献
768.
A.W.GRABAU 《地质学报》1932,11(3)
In the past, fossil faunas have generally been referred to the continent in which they were first studied. Thus it has been customary to speak of European Silurian or Devonian faunas, or of the North American fauna. This has led to the tacit assumption that these faunas were endemic in those regions, 相似文献
769.
C. Y. HSIEH M. A 《地质学报》1929,8(1):67-105
That the geology cf the Chinese coal fields is becoming better and better known is evidenced by the numerous publications put out from tirne to time by the different investigators, especially by the members of the National Geological Survey in Peking. But little attention seems to have been paid to the microscopical 相似文献
770.
PAR A. LACROIX 《地质学报》1929,8(1):53-57
Depuis la publication du memoire que la Societe geologique de Chine m'a fait l'honneur d'inserer dans son Bulletin,~1 j'ai complete a un point de vue general, l'etude du type lithologique auquel appartient la roche du Wei-tchang que j'ai decrite dans ce travail sous le nom de dolerite pegmatoide et que j'appelle aujour- 相似文献