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21.
F. Bastida C. Brime S. García-López G. N. Sarmiento 《International Journal of Earth Sciences》1999,88(1):38-48
The palaeotemperature distribution in the transition from diagenesis to metamorphism in the western nappes of the Cantabrian
Zone (Somiedo, La Sobia and Aramo Units) are analysed by conodont colour alteration index (CAI) and illite crystallinity (IC).
Structural and stratigraphic control in distribution of CAI and IC values is observed. Both CAI and IC value distributions
show that anchizonal conditions are reached in the lower part of the Somiedo Unit. A disruption of the thermal trend by basal
thrusts is evidenced by CAI and IC values. There is an apparent discrepancy between the IC and CAI values in Carboniferous
rocks of the Aramo Unit; the IC has mainly anchizonal values, whereas the CAI has diagenetic values. Discrepant IC values
are explained as a feature inherited from the source area. In the Carboniferous rocks of the La Sobia Unit, both IC and CAI
indicate diagenetic conditions. The anchimetamorphism predated completion of emplacement of the major nappes; it probably
developed previously and/or during the early stages of motion of the units. Temperature probably decreased when the metamorphosed
zones of the sheets rose along ramps and were intensely eroded. In the context of the Iberian Variscan belt, influence of
tectonic factors on the metamorphism is greater in the internal parts, where the strain and cleavage are always present, than
in the external parts (Cantabrian Zone), where brittle deformation and rock translation are dominant, with an increasing role
of the burial on the metamorphism.
Received: 11 May 1998 / Accepted: 19 January 1999 相似文献
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23.
Some conodonts described by Tea Kolar-Jurkovsek and Bogdan Jurkovsek (2007) from the Permian-Triassic boundary section in the ?iri area of western Slovenia are revised in this paper. One specimen that they assigned to Hindeodus parvus is tentatively regarded as Hindeodus cf. scalaris Wu 2005. Two specimens that they assigned to Isarcicella sp. A is changed to Hindeodus parvus. One specimen that they described as Isarcicella lobata Perri and Farabegoli is changed to Hindeodus crenatus Wu (2006). One specimen that they assigned to Isarcicella turgida (Kozur, Mostler and Rahimi-Yazd) is changed to Hindeodus cf. parvus. Since these revisions, the three conodont zones that they defined have been correspondingly revised. Their first zone may belong to Hindeodus parvus zone, but it is not certain. Their second zone is changed to the Hindeodus parvus zone, and their third one remains as is. 相似文献
24.
Some conodonts described by Tea Kolar-Jurkovsek and Bogdan Jurkovsek (2007) from the Permian-Triassic boundary section in the Ziri area of western Slovenia are revised in this paper. One specimen that they assigned to Hindeodus parvus is tentatively regarded as Hindeodus cf. scalaris Wu 2005. Two specimens that they assigned to IsarciceUa sp. A is changed to Hindeodus parvus. One specimen that they described as IsarciceUa lobata Perri and Farabegoli is changed to Hindeodus crenatus Wu (2006). One specimen that they assigned to lsarcicella turgida (Kozur, Mostler and Rahimi-Yazd) is changed to Hindeodus cf. parvus. Since these revisions, the three conodont zones that they defined have been correspondingly revised. Their first zone may belong to Hindeodus parvus zone, but it is not certain. Their second zone is changed to the Hindeodus parvus zone, and their third one remains as is. 相似文献
25.
用87Sr/86Sr研究海平面变化与全球对比问题 总被引：9，自引：0，他引：9
综述了近年来国际上在Sr同位素研究应用方面的新成果,着重讨论了如何应用海相沉积岩(物)87Sr/86Sr比值来研究海平面变化和全球对比问题。综合分析表明,海水Sr同位素组成受Sr的来源控制,海水87Sr/86Sr比值随时间变化与海平面变化有内在的联系,高频旋回(时限约2.0Ma)的87Sr/86Sr变化与三级海平面变化相对应。研究认为牙形石是分析Sr同位素组成最理想的样品,用之建立的87Sr/86Sr变化曲线最具全球对比意义。与国外相比,我国在这方面研究成果较少,但从地质条件分析,我国独有的一些地质记录,在古海平面变化和全球对比研究中占据着不可缺少或不可代替的地位,其中扬子地块西南缘二叠—三叠纪界限附近的87Sr/86Sr变化曲线,可以弥补国外一些学者建立的自显生宙以来全球海水87Sr/86Sr变化曲线在该时段存在的“断层”现象。 相似文献
26.
M. M. Joachimski R. van Geldern S. Breisig W. Buggisch J. Day 《International Journal of Earth Sciences》2004,93(4):542-553
Oxygen isotope ratios of well-preserved brachiopod calcite and conodont apatite were used to reconstruct the palaeotemperature history of the Middle and Late Devonian. By assuming an oxygen isotopic composition of –1 V-SMOW for Devonian seawater, the oxygen isotope values of Eifelian and early Givetian brachiopods and conodonts give average palaeotemperatures ranging from 22 to 25 °C. Late Givetian and Frasnian palaeotemperatures calculated from 18O values of conodont apatite are close to 25 °C in the early Frasnian and increase to 32 °C in the latest Frasnian and early Famennian. Oxygen isotope ratios of late Givetian and Frasnian brachiopods are significantly lower than equilibrium values calculated from conodont apatite 18O values and give unrealistically warm temperatures ranging from 30 to 40 °C. Diagenetic recrystallization of shell calcite, different habitats of conodonts and brachiopods, as well as non-equilibrium fractionation processes during the precipitation of brachiopod calcite cannot explain the 18O depletion of brachiopod calcite. Moreover, the 18O depletion of brachiopod calcite with respect to equilibrium 18O values calculated from conodont apatite is too large to be explained by a change in seawater pH that might have influenced the oxygen isotopic composition of brachiopod calcite. The realistic palaeotemperatures derived from 18O apatite may suggest that biogenic apatite records the oxygen isotopic composition and palaeotemperature of Palaeozoic oceans more faithfully than brachiopod calcite, and do not support the hypothesis that the 18O/16O ratio of Devonian seawater was significantly different from that of the modern ocean. 相似文献
27.
Given the lacking of effective geothermometer， it is difficult to reconstruct the complex thermal evolution history of marine carbonate rocks. It is considered that conodonts widely distributed in Paleozoic strata are suitable for the reconstruction of thermal history of carbonate strata. In this study， we mainly discussed the basic principle and experimental test method of conodont （U-Th）/He thermochronology， and the influencing factors of conodont （U-Th）/He ages. The thermal diffusion experiment reveals that the closure temperature of conodont （U-Th）/He is 60°~70°. Conodont samples have strong inverse relationship between effective uranium concentration and （U-Th）/He data. The main reasons for the open-system behavior in conodont （U-Th）/He are possible U loss during dissolution procedure using formic acid， the fluidity of U and Th elements during diagenesis， excessive initial He， and the non-uniform parent isotope distributions， etc. The higher CAI values the conodonts have， the more brittle the conodonts will become； the possibility of parent isotope loss will increase， and consistently they have older （U-Th）/He dates and substantially more scattered individual dates. Conodont tissue type also affects the parent isotope distribution and element migration of conodonts， and the change of its microstructure leads to the dispersion of conodont （U-Th）/He dates. Finally， we explored the existing problems and put forward the future hot research and development direction of conodont （U-Th）/He thermal dating system， so as to promote the deep development of relative theory and technology of conodont （U-Th）/He thermochronology. 相似文献
28.
The record of conodonts related to the Mississippian/Pennsylvanian boundary interval was investigated in four sections in Central Iran from two different structural units. Two sections from the Sanandaj–Sirjan trend zone (Asad-abad, and Darchaleh sections) and two from the East-Central Iran Microplate (Shesh-angosht and Kale-Sardar sections) exhibit a nearly complete record previously described across the Mississippian/Pennsylvanian boundary in Iran. The investigated sections can be subdivided in three formations (Ghaleh-, Absheni-, and Zaluda Formation) which belong to the Sardar Group. The mid-Carboniferous boundary was defined by the occurrence of Declinognathus noduliferus s.l.. Bio-event characteristics of the Carboniferous conodont fauna (Mississippian genera Gnathodus and Lochriea have been replaced by Pennsylvanian genera Declinognathus and Idiognathodus) as well as sedimentological changes within overall shallow water deposits were located approximately 33° S of the paleoequator and suggest sea-level changes within the framework of the Late Paleozoic Ice Age (LPIA). Furthermore, a widespread crinoid marker horizon previously described from two localities in Iran can be subdivided into three units of different ages. 相似文献