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131.
辽宁省兴城市夹山地区中元古界长城系白云岩中首次发现叠层石,其产出层位为前人在这一地区划分的"常州沟组"顶部,自下而上为Stratifera sp.层叠层石(未定种),Cryptozoon sp.卷心菜叠层石(未定种),Eucapsiphora sp.叠球藻叠层石(未定种)。该组合以及邻区葫芦岛等地团山子组中的叠层石类型均属于燕山、太行地区团山子组叠层石组合带分子。上述叠层石的发现为夹山地区划分出团山子组提供了重要的生物地层学证据。根据岩石组合特征、叠层石形态垂向分布规律及其与古环境演化的关系分析,认为兴城地区团山子组沉积环境演化为海退沉积序列(浅滩到潮间、潮上带)到海侵沉积序列(潮间、潮上带到潮间、潮下带)。 相似文献
132.
The Ukwortung area of the Obudu Plateau exposes high-grade metamorphic rocks of upper amphibolite to granulite facies. Zircon grains from three locations of Southwest Obudu Plateau were dated by the Pb-Pb dating method. This single zircon radiometric dating method confirms the occurrence of Mesoproterozoic crust in the southeastern Nigeria basement complex. The pyroxene gneiss (granulite facies) sample from Biereberi near Ukpe yielded a mean age of 2061.4±0.4 Ma, suggesting the presence of Paleoproterozoic crust. The combined 207Pb/206Pb ratios from the garnet-biotite gneiss in the area gave a mean age of 1794.4±0.4 Ma. This further confirms the pres-ence of Paleoproterozoic crustal components within the Obudu Plateau. The Mesoproterozoic (1548.8±0.5 Ma) and Neoproterozoic (619.8±0.9 Ma) ages were obtained from two populations of zircon grains from the leucogranite in Okordem, Southwest Obudu. These "signatures" collaborate the occurrence of Mesoproterozoic ages in the Afar region and Cameroon basements along with the southeastern Nigerian basement complex, constituting the major portion of the West Central African mobile belt. The zircon age of 1794.4±0.4 Ma may probably be the maximum depositional age of the meta-sediment (garnet-biotite gneiss) and the age of 1548.8±0.5 Ma may be interpreted as the time of emplacement of the granitoid. The Neoproterozoic age (619.8±0.9 Ma) probably records the metamorphic event that was prevalent during the Pan-African period and thus affected the area. 相似文献
133.
西昆仑北缘库斯拉甫一带侵入前寒武纪结晶基底(Ch)的中元古代两期岩体,早期以喀特列克岩体为代表,年龄为1 567 Ma,晚期以阿孜巴勒迪尔岩体为代表,年龄为1 423 Ma。早期岩体(δoPt)具有贫硅、高钙、中碱和准铝质等特征; 岩性主要为钙碱性石英闪长岩,岩石稀土总量为168.4×10-6～374.38×10-6、轻重稀土比值为9.3～18.23,中等负铕异常(δEu=0.53～0.82)。晚期岩体(ηγPt)具有富硅、高碱、富钾、强过铝质和全铁含量高等特征; 岩性主要为碱性二长花岗岩,岩石具高的104*Ga/Al比值(3.01～3.5),很高的REE含量(375.68×10-6～1 075.46×10-6),高LREE/HREE(7.07～16.87)和高(LaN/YbN)(8.32～25.2)及强烈负Eu异常(δEu=0.25～0.31)。通过对两期岩体(δoPt和ηγPt)同位素年龄、矿物学、岩石化学、微量元素及稀土元素等特征系统地研究,认为西昆仑北缘在中元古代发生两次明显的岩浆-构造事件,早期事件(δoPt的侵位)与兴地运动(Ⅰ幕)有关; 晚期事件(ηγPt的侵位)在时间及区域上与兴地运动(Ⅱ幕)比较吻合; 早期岩浆(δoPt)可能存在部分幔源物质的加入,显示Ⅰ型花岗岩的特征,可能属于造山期后花岗岩类; 晚期岩浆(ηγPt)可能经历过下地壳的部分熔融或较高程度的分异演化过程,属于A2型花岗岩的范畴。两期岩浆活动可能为古塔里木板块的固结-裂解提供了新的证据及裂解模式的补充。 相似文献
134.
星星峡石英闪长质片麻岩的锆石年代学:对天山造山带构造演化及基底归属的意义 总被引：5，自引：2，他引：3
天山造山带是中亚造山带(CAOB)的主要组成部分,对于其前寒武纪古老基底的起源、古生代构造单元划分和造山作用过程的认识还存在很大分歧。本文对分布在星星峡镇西的石英闪长质片麻岩开展了系统地岩相学观察和锆石U-Pb年龄、Hf同位素及全岩地球化学分析。根据矿物组成推测它们的原岩为石英闪长岩,两个样品中的锆石具有基本一致的内部结构特征,均发育黑色、均一的边部和具震荡环带的核部,部分保留有更早的继承锆石核。分析结果表明,它们的原岩形成于～425Ma,变质作用年龄为约320～360Ma,继承锆石的年龄为1381～1743Ma。原岩结晶锆石具有正的且变化较大的εHf(t)值(0.9～17.8),继承锆石的tDM2模式年龄变化于1.54～2.44Ga。在全岩地球化学组成上,石英闪长质片麻岩具有明显富集Rb、Ba、Th、K等大离子亲石元素和Pb、U元素,亏损Nb、Ta、Ti等高场强元素的特点。结合区域上已有的前寒武纪基底、高级变质岩、蛇绿混杂岩、岩浆岩的研究资料,获得以下认识:中天山地块的前寒武纪基底的起源与塔里木板块没有明显的相关性,可能是中元古代时期,由东欧板块边缘的弧增生造山作用形成;中天山地块东部在早古生代为大陆边缘弧的构造环境,可能形成于南天山洋向中天山板块的俯冲作用;南天山洋在天山造山带的东部和西部可能具有一致的闭合时间。 相似文献
135.
Ten well‐preserved, earthquake‐triggered liquefaction mounds and a carbonate sand volcano have been found in the Mesoproterozoic Wumishan Formation (1550–1400 Ma) in the Beijing area, North China. These features crop out in a roadcut near Zhuanghuwa Village. All ten mounds occur in the same sedimentary layer and have rounded shapes with some concentric and radial fissures arising from the centre. They range from 1.5 to 4 m in diameter and from 10 cm to 30 cm in height. The carbonate sand volcano has a diameter of 110 cm and the ‘crater’ at the top has a depth of about 30 cm. Associated with these mounds and the sand volcano are many ‘normal’ sedimentary structures and numerous soft‐sediment deformation structures. The former include ripple marks, cross‐bedding, stromatolites and desiccation cracks, indicating deposition in a stable shallow‐water peritidal platform environment. The latter include intrastratal faults and folds, seismically formed breccias and carbonate clastic dykes. The morphological features and the genesis of these liquefaction mounds are very similar to mounds formed recently by the great Wenchuan Earthquake of China (2008). Detailed thin‐section study of the mounds found no signs of any kind of biological constructional process; instead it reveals some obvious fluidification and liquefaction characteristics. Comparative studies have shown that these features are probably the products of Mesoproterozoic earthquake activity. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
136.
华北陆块北部燕山地区广泛发育侵位于下马岭组的一套辉绿岩岩床。其岩石地球化学特征表明,燕山地区辉绿岩以高TiO2(1.70%~4.29%),低Al2O3(11.33%~14.98%)和低MgO(4.17%~5.86%),以及Na2OK2O为特征,属大陆拉斑玄武岩系列。稀土元素特征显示,∑REE总量较高,LREE明显富集,轻重稀土分馏不明显;配分曲线右倾,无明显Eu异常。结合Zr-Y、Zr-Nd、Zr/Y-Zr、Ti/100-Zr-Y×3、Ta/Hf-Th/Hf及Nb/Zr-Th/Zr等图解综合分析认为:该辉绿岩岩浆源自过渡性地幔,形成于华北陆块北部中元古代晚期的板内伸展构造背景。 相似文献
137.
In this investigation, we reconstruct the latest Palaeoproterozoic to Early Mesoproterozoic orogenic events along the southern margin of the Central Indian Tectonic Zone (CITZ), using sensitive high resolution ion microprobe (SHRIMP) U‐Pb zircon dating and Lu‐Hf isotope analyses of zircon and Th‐U‐Pb chemical dating of monazite from samples of the Tirodi biotite gneiss (TBG) unit in the Sausar Mobile Belt (SMB), the latter constituting the southernmost litho‐tectonic component of the CITZ. U‐Pb zircon dating of one migmatitic gneiss sample from the type locality of the Tirodi biotite gneiss in the northern domain of the SMB has yielded an age of 1618 ± 8 Ma, which is considered to be the time of magmatic crystallization of its protolith. Combined U‐Pb zircon and monazite chemical dating of two granite gneiss samples from the southern domain of the SMB broadly constrain magmatic crystallization between 1603 ± 23 Ma and 1584 ± 17 Ma and an overprinting metamorphic recrystallization event at 1572 ± 7 Ma. Monazites from the granite gneiss samples also record a terminal metamorphic event at 1415 ± 23 Ma. Lu‐Hf isotopic analyses of zircons reveal fundamentally different source rock reservoirs for the protoliths of these magmatic rocks across the SMB. While the type TBG from the northern domain was derived from an Early Palaeoproterozoic source T(Hf) from 2093 to 2523 Ma, with a mean value at 2379 Ma) of essentially juvenile material with minor crustal components (εHf(t) from −3.3 to + 3.7), the granite from the southern domain had a mature crustal source (εHf(t) from −12.5 to −21.9) of Palaeoarchaean age T(Hf) from 3051 to 3630 Ma, with a mean value at 3218 Ma). When integrated with metamorphic information previously obtained from the 1.6 Ga ultra‐high temperature granulite facies metamorphic event in the SMB, the discrete magmatic and metamorphic events between 1.62/1.60 Ga and 1.42 Ga can be correlated with the formation of an Early Mesoproterozoic accretionary orogen in the CITZ. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
138.
叠层石被广泛认为是地球上的一些最古老的生命痕迹,是研究地球早期生命的重要窗口。作为前寒武纪地层中的唯一常见化石,可以作为地层对比的标志,特别是中、新元古代盆地间的地层对比。青海省中祁连地层分区东岔沟村花石山群克素尔组以白云质碳酸盐岩组合为主,赋存大量叠层石。依据曹瑞骥和袁训来总结的分类方法,在研究区识别出9种不同的叠层石属,分别为：Baicalia、Chihsienella、Anabaria、Tielingella、Conicodomenia、Tungussia、Colonnella、Stratifera和Cryptozoon。研究区叠层石组合与中元古界上部蓟县系铁岭组叠层石组合具有明显的可比性,与国内其他地区的中元古代晚期—新元古代早期叠层石组合也有很强的相似性,据此推测花石山群克素尔组的年代大致为中元古代晚期。青海叠层石的报道将为我国和世界范围内中、新元古代叠层石的分布提供新产地的数据,常见叠层石属、种的垂直分布延限也有新的化石证据,对提升地层对比的精度具有重要意义。 相似文献
139.
140.