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931.
932.
933.
Sven O. Egenhoff Arndt Peterhänsel Thilo Bechstädt Rainer Zühlke & Jürgen Grötsch 《Sedimentology》1999,46(5):893-912
The 720-m-thick succession of the Middle Triassic Latemàr Massif (Dolomites, Italy) was used to reconstruct the lagoonal facies architecture of a small atoll-like carbonate platform. Facies analysis of the lagoonal sediments yields a bathymetric interpretation of the lateral facies variations, which reflect a syndepositional palaeorelief. Based on tracing of lagoonal flooding surfaces, the metre-scale shallowing-upward cycles are interpreted to be of allocyclic origin. Short-term sea-level changes led to subaerial exposure of wide parts of the marginal zone, resulting in the development of a tepee belt of varying width. Occasional emergence of the entire lagoon produced lagoon-wide decimetre-thick red exposure horizons. The supratidal tepee belt in the backreef area represented the zone of maximum elevation, which circumscribed the sub- to peritidal lagoonal interior during most of the platform's development. This tepee rim, the subtidal reef and a sub- to peritidal transition zone in between stabilized the platform margin. The asymmetric width of facies belts within individual metre-scale cycles was caused by redistribution processes that reflect palaeowinds and storm paths from the present-day south and west. The overall succession shows stratigraphic changes on a scale of tens of metres from a basal subtidal unit, overlain by three tepee-rich intervals, separated by tepee-poor units composed of subtidal to peritidal facies. This stacking pattern reflects two third-order sequences during the late Anisian to early middle Ladinian. 相似文献
934.
The lower valley of Changjiang, from Wuhan of the Hubei Province in the west to Zhenjiang of the Jiangsu Province in the east, contains more than 200 polymetallic (Cu–Fe–Au, Mo, Zn, Pb, Ag) deposits and is one of the most important metallogenic belts in China. This metallogenic belt, situated at the northern margin of the Yangzi craton and bordered by the Dabieshan ultrahigh pressure metamorphic belt to the north, consists mainly of Cambrian–Triassic marine clastic sedimentary rocks and carbonate and evaporite rocks, which overlay a Precambrian basement and are intruded by Yanshanian (205 to 64 Ma) granitoid intrusions and subvolcanic complexes. Repeated tectonism from Late Proterozoic to Triassic resulted in extensively developed networks of faults and folds involving the Cambrian–Triassic sedimentary strata and the Precambrian basement. The Yanshanian granitoid intrusions and subvolcanic complexes in the Lower Changjiang metallogenic belt are characterized by whole-rock δ18O of +8‰ to +10‰, initial 87Sr/86Sr of 0.704 to 0.708, and εNdt from −10 to −17 and have been interpreted to have originated from mixing between juvenile mantle and old crustal materials. Also, the Yanshanian granitoids exhibit eastward younging and increase in alkalinity (i.e., from older calc–alkaline in the west to younger subalkaline–alkaline in the east), which are related to oblique collision between the Yangzi and Sino-Korean cratons and tectonic evolution from early compressional to late extensional or rifting regimes. Most polymetallic deposits in the Lower Changjiang metallogenic belt are clustered in seven districts where the Yanshanian magmatism is particularly extensive: from west to east, Edong, Jiurui, Anqing–Guichi, Luzhong, Tongling, Ningwu and Ningzhen. Mineralization is characterized by the occurrence of three distinct types of orebodies in individual deposits: orebodies in Yanshanian granitoid intrusions, skarn orebodies at the contact zones between the Yanshanian intrusions and Late Paleozoic–Early Mesozoic sedimentary rocks, and stratabound massive sulfide orebodies in the Late Paleozoic–Early Mesozoic sedimentary strata. The most important host sedimentary strata are the Middle Carboniferous Huanglong Formation, Lower Permian and Lower–Middle Triassic carbonate and evaporite rocks. The intrusion-hosted and skarn orebodies exhibit well-developed zonation in alteration assemblages, metal contents, and isotopic compositions within individual deposits, and apparently formed from hydrothermal activities related to the Yanshanian magmatism. The stratabound massive sulfide orebodies in the Late Paleozoic–Early Mesozoic sedimentary strata have long been suggested to have formed from sedimentary or volcano-sedimentary exhalative processes in shallow marine environments. However, extensive research over the last 40 years failed to produce unequivocal evidence for syngenetic mineralization. On the basis of geological relationships and isotope geochemical characteristics, we propose a carbonate-hosted replacement deposit model for the genesis of these stratabound massive sulfide orebodies and associated skarn orebodies. This model suggests that epigenetic mineralization resulted from interactions between magmatic fluids evolved from the Yanshanian intrusions with carbonate and evaporite wall rocks. Mineralization was an integral but distal part of the larger hydrothermal systems that formed the proximal skarn orebodies at the contact zones and the intrusion-hosted orebodies. The stratabound massive sulfide deposits of the Lower Changjiang metallogenic belt share many features with the well-studied, high-temperature, carbonate-hosted replacement deposits of northern Mexico and western United States, particularly with respect to association with small, shallow granitoid complexes, structural and stratigraphic controls on mineralization, alteration assemblages, geometry of orebodies, metal association, metal zonation and isotopic systematics. 相似文献
935.
In this paper an attempt has been made to search a new parameter for the prediction of the Indian summer monsoon rainfall.
For this purpose the relationship of the global surface-air temperature of four standard seasons viz., Winter (December-January-February),
Spring (March-April-May), Summer (June-July-August), Autumn (September-October-November) with the Indian summer monsoon rainfall
has been carried out. The same analysis is also carried out with surface-air temperature anomalies within the tropical belt
(30°S to 30°N) and Indian summer monsoon rainfall. For the present study data for 30 years period from 1958 to 1988 have been
used. The analysis reveals that there is a strong inverse relationship between the monsoon activity and the tropical belt
temperature. 相似文献
936.
937.
938.
完达山造山带原型盆地及可能的造山机制 总被引：5，自引：1，他引：4
完达山造山带是东亚环太平洋构造带的重要组成部分,是中国北方唯一的中生代深水活动类型沉积建造出露区。三叠纪—侏罗纪硅质岩－碎屑岩系构成了这个造山带的主体,其沉积序列总体表现为一向上变浅、碎屑颗粒变粗的沉积旋回,其中的硅质岩、泥岩稀土元素以Ｃｅ的正异常为特征;砂岩构架颗粒成分与典型的弧前盆地相同。这些特征表明,完达山造山带三叠纪—侏罗纪的原型盆地是东亚陆缘区弧前盆地的组成部分。构造解析结果进一步揭示,造山带只发育一期透入性的变形构造,与增生杂岩的变形序列明显不同。基于上述结论并结合区域构造分析结果提出,完达山造山带的隆升主要是中生代东亚陆缘型岩浆弧东移、本区卷入科迪勒拉型造山带所致。 相似文献
939.
大别造山带北部的中生代火山岩 总被引：17，自引：0，他引：17
大别造山带北部的北淮阳中生代火山喷发岩带形成于后造山阶段的晚侏罗世—早白垩世。火山岩可以划分为２个独立的火山旋回,分别对应于高钾钙碱性系列（ＨＫＣＡ）和钾玄岩系列（ＳＨＯ）,从岩石构造组合和岩石地球化学数据提供的约束条件分析,前者形成于晚侏罗世的陆内挤压环境,造山带是有“山根”的增厚陆壳,而早白垩世钾玄岩系列岩石的出现表征着造山带已发生“去根”作用,北淮阳处于陆壳减薄的拉张环境 相似文献
940.