Cycle anatomy and variability in the storm-dominated type cincinnatian (Upper Ordovician): coming to grips with cycle delineation and genesis

Although parasequence and sequence are scale-independent terms, they are frequently applied only to specific scales of cycles. For example, meter-scale cycles are commonly assumed to be parasequences or PACs. In the Upper Ordovician Kope and Fairview Formations of northern Kentucky, we examined a succession of 50 meter-scale cycles that have been variously interpreted as deepening-upward, shallowing-upward, or showing no relationship with water depth. Our analysis shows that these cycles, characterized by shifts in storm-bed proximality, are highly variable in their thickness and internal construction. Most cycles are best considered high-frequency sequences, because deepening-upward intervals are common, and many cycles contain evidence of abrupt basinward shifts in facies as expected at sequence boundaries. A minority fit the parasequence model of shallowing-upward cycles bounded by flooding surfaces. Larger, 20 m scale cycles are defined by systematic thickening and thinning trends of meter-scale cycles. However, meter-scale cycles do not display any systematic trends in cycle anatomy as a function of position within the 20 m cycles or position within the Kope and Fairview Formations. The high cycle variability and the lack of systematic stratigraphic organization with respect to longer-term cyclicity reflect either the irregularity of relative sea-level changes, the poor recording of sea-level changes in this deep-water setting, or the generation of these cycles by climate-induced cyclicity in storm intensity. These three mechanisms would generate similar patterns at the outcrop scale, so it is not possible at the present to distinguish between them.     

壳幔混染作用——下扬子区早中生代玄武岩同位素地质特征

下扬子区中生代（231Ma）玄武质火山岩全岩εNd（t）值-8.62～1.92,87Sr/86Sr 值0.7074～0.7179，206Pb/204Pb值为17.786～18.323。岩石中含有前寒武纪捕虏晶锆石。研究表明玄武质岩石可能是由亏损地幔与元古宙地壳岩石混染作用的结果。     

大别山超高压石榴多硅白云母片岩中钻石U－Pb年龄

大别山超高压石榴多硅白云母片岩中钻石Ｕ－Ｐｂ年龄刘晓春（中国地质科学院地质力学研究所，北京１０００８１）李惠民，左义成（地质矿产部天津地质矿产研究所，天津３００１７０）关键词石榴多硅白云母片岩，Ｕ－Ｐｈ年龄，早元古代表壳岩，印支期超高压变质，大别山高...     

苏北榴辉岩Sm-Nd等时线同位素年龄及地质意义

采用Sm-Nd同位素定年方法，测得江苏北部新沂地区踢球山榴辉岩岩体时代为221.6±8.4 Ma,Sm-Nd年龄被看作为踢球山超高压变质之后板块折返过程中的高压榴辉岩相重结晶阶段年龄。中朝板块与扬子板块碰撞时代主要发生在晚三叠世。     

华北古风化壳型稀有稀土矿床的发现及意义

1995年山西国土资源局陈平和山西地质研究所柴东浩对山西中部、西部、东部约 2 0个铝土矿床进行了观察 ,取样和研究[1] 。所采铝土矿样品经作者在南京大学现代分析中心用同位素质谱稀释法发现钐 (Ｓｍ)和钕 (Ｎｄ)含量异常。后又通过多种现代分析手段在山西沁源大峪铝土矿及平陆曹川黑灰色粘土岩和铝土岩中发现锂 (Ｌｉ)、铌 (Ｎｂ)及稀土元素异常 ,其含量达到工业指标。类似样品又经国土资源部权威检测中心进行验证 ,与上述分析结果完全一致。这些稀有稀土矿床与华北Ｇ层铝 (粘 )土矿为同一层位 ,后者已被确认是“堆积沉积在长期风化古陆上…     

扩大地质市场、开展多种经营经验交流会在杭州召开

地质矿产部物探化探科技情报网于1990年6月5日至8日在杭州召开了“扩大地质市场、开展多种经营经验交流会”。来自地矿、冶金、煤炭、石油、核工业等系统的50个地勘单位近百名代表出席了会议,浙江省地矿厅领导亲临会议指导。会议期间口头和出面交流的材料有40多篇,展示了物探化探在水文工程勘察(尤     

长城系团山子组火山岩颗粒锆石U－Pb年龄及其地质意义

蓟县中－上元古界典型剖面之邻区平谷县境内，长城系团山子组连续发育两层厚度各为１ｍ左右的富钾粗面安山岩，发育气孔构造和绳状构造。其中存在三种不同成因类型锆石，应用颗粒锆石Ｕ－Ｐｂ测定年龄技术，得到：（１）自生锆石，其Ｕ－Ｐｂ不一致上交点年龄为１６８３±６７Ｍａ，代表该火山岩形成年龄；（２）捕获锆石，多为原碎屑锆石，较年轻的一种捕获锆石的２０７Ｐｂ／２０６Ｐｂ表面年龄为１８２３±６８Ｍａ，这限定了长城系底界年龄应新于１８２３Ｍａ；（３）后生热液锆石，其Ｕ－Ｐｂ年龄为４６０Ｍａ左右，反映后期热液事件发生的时代。     

白云鄂博矿田碳酸岩墙年代学再研究

白云鄂博地区发育侵入于白云鄂博群的碳酸岩墙。碳酸岩墙(00BYC19)中锆石的SHRIMPU-Pb下交点年龄为1984±180Ma,其ID-TIMS上交点年龄为2085±330Ma。碳酸岩墙(02BY80)中锆石SHRIMP的上交点年龄为2035±51Ma;02BY81中锆石的ID-TIMS上交点年龄为1934±64Ma。主矿北碳酸岩墙(00BYC21)的全岩Pb-Pb等时线年龄为1236±300Ma。本文及前人对白云鄂博地区的碳酸岩墙的定年结果表明该地区碳酸岩墙至少有两个阶段的侵入事件,第一阶段在2.0Ga左右,第二阶段在1.2Ga左右。前者是白云鄂博地区以至阴山地块北缘的一次主要的构造—岩浆事件,标志白云鄂博裂谷和白云鄂博群地层的形成。第二阶段的碳酸岩墙,代表白云鄂博裂谷在中元古代的活化,且与成矿有密切关系。     

氟碳铈矿U-Pb定年技术研究

本文以稀土矿床中的常见副矿物氟碳铈矿为研究对象,通过优化实验条件和仪器参数,分别采用激光剥蚀多接收电感耦合等离子体质谱法(LA-MC-ICP-MS)和同位素稀释热电离质谱法(ID-TIMS)对氟碳铈矿样品SAM进行U-Pb定年技术研究,并将此两种方法得到的结果进行对比。LA-MC-ICP-MS得到的U-Pb年龄为(409±18)Ma(N=27,MSWD=4.5),ID-TIMS得到的206Pb/238U U-Pb年龄为(407.8±3.3)Ma(N=3,MSWD=0.029),两种方法得到的U-Pb年龄结果在误差范围内一致。通过比较两种方法的实验流程和结果,总结了各自的优缺点,为氟碳铈矿U-Pb定年方法选择提供了参考依据。