Zircon ages of high-grade gneisses in the Eastern Erzgebirge (Central European Variscides)—constraints on origin of the rocks and Precambrian to Ordovician magmatic events in the Variscan foldbelt     

M. Tichomirowa  H.-J. Berger  E.A. Koch  B.V. Belyatski  J. Gtze  U. Kempe  L. Nasdala  U. Schaltegger 《Lithos》2001,56(4):303-332

This study is an attempt to unravel the tectono-metamorphic history of high-grade metamorphic rocks in the Eastern Erzgebirge region. Metamorphism has strongly disturbed the primary petrological genetic characteristics of the rocks. We compare geological, geochemical, and petrological data, and zircon populations as well as isotope and geochronological data for the major gneiss units of the Eastern Erzgebirge; (1) coarse- to medium-grained “Inner Grey Gneiss”, (2) fine-grained “Outer Grey Gneiss”, and (3) “Red Gneiss”. The Inner and Outer Grey Gneiss units (MP–MT overprinted) have very similar geochemical and mineralogical compositions, but they contain different zircon populations. The Inner Grey Gneiss is found to be of primary igneous origin as documented by the presence of long-prismatic, oscillatory zoned zircons (540 Ma) and relics of granitic textures. Geochemical and isotope data classify the igneous precursor as a S-type granite. In contrast, Outer Grey Gneiss samples are free of long-prismatic zircons and contain zircons with signs of mechanical rounding through sedimentary transport. Geochemical data indicate greywackes as main previous precursor. The most euhedral zircons are zoned and document Neoproterozoic (ca. 575 Ma) source rocks eroded to form these greywackes. U–Pb-SHRIMP measurements revealed three further ancient sources, which zircons survived in both the Inner and Outer Grey Gneiss: Neoproterozoic (600–700 Ma), Paleoproterozoic (2100–2200 Ma), and Archaean (2700–2800 Ma). These results point to absence of Grenvillian type sources and derivation of the crust from the West African Craton. The granite magma of the Inner Grey Gneiss was probably derived through in situ melting of the Outer Grey Gneiss sedimentary protolith as indicated by geological relationships, similar geochemical composition, similar Nd model ages, and inherited zircon ages. Red Gneiss occurs as separate bodies within fine- and medium-grained grey gneisses of the gneiss–eclogite zone (HP–HT overprinted). In comparison to Grey Gneisses, the Red Gneiss clearly differs in geochemical composition by lower contents of refractory elements. Rocks contain long-prismatic zircons (480–500 Ma) with oscillatory zonation indicating an igneous precursor for Red Gneiss protoliths. Geochemical data display obvious characteristics of S-type granites derived through partial melting from deeper crustal source rocks. The obtained time marks of magmatic activity (ca. 575 Ma, ca. 540 Ma, ca. 500–480 Ma) of the Eastern Erzgebirge are compared with adjacent units of the Saxothuringian zone. In all these units, similar time marks and geochemical pattern of igneous rocks prove a similar tectono-metamorphic evolution during Neoproterozoic–Ordovician time.  相似文献   

Fossil Rotifers and the Early Colonization of an Antarctic Lake     

Kerrie M. Swadling  Herbert J. G. Dartnall  John A. E. Gibson  milie Saulnier-Talbot  Warwick F. Vincent 《Quaternary Research》2001,55(3):380

Early Holocene sediments from a continental Antarctic lake (Ace Lake, Vestfold Hills, East Antarctica) contained abundant fossil rotifers of the genus Notholca. The fossil is similar to specimens of Notholca sp. present in modern-day Ace Lake and other fresh and brackish lakes of the Vestfold Hills. Cyanobacteria and protists (chrysophyte cysts, dinoflagellate cysts, and rhizopod tests) were also recovered from the core samples. These sediments were deposited early in the freshwater phase of Ace Lake, soon after deglaciation of the area. The occurrence of this trophically diverse assemblage of organisms at an early stage in the evolution of the lake suggests either that they were part of an endemic Antarctic flora and fauna which pre-dated the last glacial maximum and survived in glacial refugia or that efficient intercontinental dispersal had occurred.  相似文献   

陆相红层盆地1:5万区调中的遥感应用研究   总被引：3，自引：0，他引：3  

刘登忠 《国土资源遥感》1996,7(2):45-50

本文采用遥感地质方法,对陆相红层盆地的沉积岩相、沉积体系界面、盆地充填序列、岩相变化以及地层格架影像特征进行了识别研究,取得了较好效果,为遥感在红层盆地1:5万区调中的应用,开辟了新思路。  相似文献   

肃南5.7级地震过程剪切波分裂特征   总被引：3，自引：0，他引：3  

李清河  刘希强 《地震》1996,16(4):417-426

剪切波分裂应用于地震预报中的３个主要困难是：（１）如何自动定量识别剪切波快慢波到时；（２）用以认识其基本特征的震例太少；（３）因各次地震的复杂性，对所给剪切波分裂结果难以给出恰当的解释，本文研究了１９８８年１１月２２日甘肃肃南５．７级地震过程剪切波分裂的特征。所用资料为１９８８年８月到１９９０年８月中法合作张掖数字台网的记录。通过对张掖～肃南地区地壳结构模型的研究，确认了所用各次地震的真实出射角基  相似文献   

脑脓肿的CT诊断     

房桂云  赵延亮 《CT理论与应用研究》1996,5(2):15-17

作者报告了１００例脑脓肿，其中一般性脑脓肿６２例、脑内小脓肿３８例（强化灶小于１．５ｃｍ），并对发病部位，临床表现及ＣＴ表现的主要特点进行了讨论。  相似文献   

妊娠期脑积水畸形的宫内CT诊断     

李晓阳  李顺平 《CT理论与应用研究》1996,5(2):23-24

本文报告了４例在妊娠期间ＣＴ检查出来脑积水畸形病例，ＣＴ表现为幕上大片低密度区域，基底节和部分枕叶地结构完整，幕上脑室系统消失，脑组织形状结构不规则，大脑镰结构完整。  相似文献   

CT球管耙心偏移的检测与判断     

孔秋雁 《CT理论与应用研究》1996,5(2):30-31

ＣＴ球管耙偏移是较为少见的，本文以ＧＥ ｓｙｔｅｃ ２０００ｉ为例，详细描述了由此引起的扫描停止，机器伪影等现象及分析、处理过程，并通过与管衰老相比较。进一步述了两者的共同点及在图像质量，管电流稳定性，检测过程等方面的区别，最后总结出一套检测与判断耙心偏移的方法。  相似文献   

景泰地震台短周期地震仪记录的几种异常波形特征与景泰6.2级地震的关系     

姚同福  付印发  王先 《华北地震科学》1996,(1)

本文研究了景泰地震台记录到的几种异常地震波形的形态特征及随时间分布特征,采用测震学的方法和数理统计的方法,对各个阶段的异常波形出现的数量、类型和振幅进行了定量分析,发现在1990年10月20日景泰6.2级地震前,异常波形数量明显地呈平静一密集交替出现的现象;临震前各种异常波形的数量增多,振幅增大,异常持续时间增长,震后异常波形很快衰减并消失.与其他前兆对比,可以认为异常波形的出现可能与景泰地震的孕育过程有关,根据该类波形的变化特征有可能对地震作出预报.  相似文献   

天祝盆地边缘断层的全新世活动及盆地的演化与形成   总被引：3，自引：0，他引：3  

何文贵  刘百  吕太乙  袁道阳  刘小凤 《地震工程学报》1996,18(1):61-66

根据作者１／５０，０００活断层地质填图的资料，讨论了天祝盆地内断层的全新世活动及盆地的形成与演化历史。结果表明，天祝盆地内断层的全新世活动强烈，上窑洞沟深槽揭示３９８０±５０ａＢ．Ｐ．曾发生过一次古地震事件，天祝盆地是一个典型的拉分盆地，其形成及演化与断裂活动密切相关．  相似文献   

天水地震区深部地温场的有限元计算   总被引：2，自引：0，他引：2       下载免费PDF全文

董治平  郭守年  蒋梅  刘宝勤  张立光 《地震工程学报》1996,18(3):48-53

用有限元方法计算了天水地震区上地壳地温场。结果表明，影响该区浅部（层）地温场的主要因素是基底构造和断裂构造。祁连褶皱系为高热异常区，秦岭褶皱系为低热异常区，西秦岭北缘断裂带为地温梯度带。认为该带历史地震的发生与热应力差有关  相似文献