Rapid Variscan exhumation and the role of magma in core complex formation: southern Brittany metamorphic belt, France   总被引：6，自引：0，他引：6  

MICHAEL BROWN  R. DAVID DALLMEYER 《Journal of Metamorphic Geology》1996,14(3):361-379

ABSTRACT The high-grade migmatitic core to the southern Brittany metamorphic belt has mineralogical and textural features that suggest high-temperature decompression. The chronology of this decompression and subsequent cooling history have been constrained with ⁴⁰Ar/³⁹ Ar ages determined for multigrain concentrates of hornblende and muscovite prepared from amphibolite and late-orogenic granite sheets within the migmatitic core, and from amphibolite of the structurally overlying unit. Three hornblende concentrates yield plateau isotope correlation ages of c. 303–298 Ma. Two muscovite concentrates record well-defined plateau ages of c. 306–305 Ma. These ages are geologically significant and date the last cooling through temperatures required for intracrystalline retention of radiogenic argon. The concordancy of the hornblende and muscovite ages suggest rapid post-metamorphic cooling. Extant geochronology and the new ⁴⁰Ar/³⁹Ar data suggest a minimum time-integrated average cooling rate between c. 725 °C and c. 125 °C of c. 14 ± 4°C Ma^-1, although below 600 °C the data permit an infinitely fast rate of cooling. Mineral assemblages and reaction textures in diatexite migmatites suggest c. 4 kbar decompression at 800–750 °C. This must have pre-dated the rapid cooling. Emplacement of two-mica granites into the metamorphic belt occurred between 345 and 300 Ma. The youngest plutons were emplaced synkinematically along shallow-dipping normal faults interpreted to be reactivated Eo-Variscan thrusts. A penetrative, west-plunging stretching lineation developed in these granites suggests that extension was orogen-parallel. Extension was probably related to regional uplift and gravitational collapse of thermally weakened crust during constrictional (escape) tectonics in this narrow part of the Variscan orogen. This followed slab breakoff during the terminal stages of convergence between Gondwana and Laurasia; detachment may have been consequent upon a change in kinematics leading to dextral displacement within the orogen. Dextral ductile strike-slip displacement was concentrated in granites emplaced synkinematically along the South Armorican Shear Zone. Rapid cooling is interpreted to have resulted from tectonic unroofing with emplacement of granite along decollement surfaces. The high-grade migmatitic core of the southern Brittany metamorphic belt represents a type of metamorphic core complex formed during orogen-parallel extensional unroofing and regional-scale ductile flow.  相似文献   

A Study on the Geological—Geochemical Dynamics of Hydrothermal Ore Deposition as Exemplified by the Muping—Rushan Gold Deposit Belt,Eastern Shandong,China     

赵伦山  高太忠等 《中国地球化学学报》1996,15(4):289-295

This paper presents a method of establishing a hydrothermal ore-forming reaction system.On the basis of the study of four typical hydrothermal deposits,the following conclusions concerning geochemical dynamic controlling during hydrothermal mineralization have been sions concerning geochemical dynaamic controlling during hydrothermal mineralization have been drawn:(1)The regional tectonic activities control the concentration and dispersion of elements in the ore-forming process in terms of their effects on the thermodynamic nature and conditions of the ore-forming reaction system.(2)During hydrothermal mineralization the activites of ore-bearing faults can be divideb into two stages:the brittle splitting stage and the brittle-tough tensing stage,which would create characteristically different geodynamic conditions for the geochemical thermodynamic ore-forming system.(3)The hydrothermal ore-forming reaaction system is an open dynamic system.At the brittle splitting stage the system was so strongly supersaturated and unequilibrated as to speed up and enhance the crystallization and differentiation of ore-forming fluids.And at the brittle-tough tensing stage,the ore-forming system was in a weak supersaturated state;with decreasing temperature and pressure the crystallization of oreforming material would show down,and it can be regarded as an equilibrated state.(4)In the lates stages of hydrothermal evolution,gold would be concentrated in the residual ore-forming solution.The pulsating fracture activite in this stage led to the crush of pyrite ore and it was then filled with gold-enriched solution,forming high-grage“fissure”gold ore.This ore-forming process could be called the coupling mechanism of ore formation.  相似文献   

贵州三都早奥陶世同高组下燕高页岩段的生物标志化合物   总被引：8，自引：1，他引：8  

许玩宏  沈平 《沉积学报》1997,15(3):72-77

本文着重报道了贵州三都早奥陶世同高组下燕高页岩段生物标志化合物特征，首次在采自该层位的样品中检出丰富的正烷烃、链状类异戊二烯烷烃、萜类化合物及甾类化合物。测试结果显示:正烷烃图谱呈双驼峰形，碳数分布为ｎＣ_１５～ｎＣ_３５，以ｎＣ_１８为主峰碳，ｎＣ_２９为次主峰碳，Ｃ－_２１/Ｃ＋_２２为０.７７～１.０２，ＯＥＰ为０.９４～１.０４；植烷占优势，Ｐｒ/Ｐｈ为０.４０～０.４６；藿烷碳数为Ｃ_２７～Ｃ_３５，以Ｃ_３０占优势，Ｃ_２７＋Ｃ_２９＜Ｃ＋_３１；仅见碳数分布为Ｃ_２０～Ｃ_２９的长链三环二萜烷，以Ｃ_２８为主峰碳；甾烷丰度顺序为Ｃ_２９甾烷＞Ｃ_２７甾烷＞Ｃ_２８甾烷，同时检出了孕甾烷和４－甲基甾烷。生物标志化合物显示出还原环境特点，且可能主要来自藻类。  相似文献   

泌阳凹陷核桃园组微量元素演化特征及其古气候意义   总被引：31，自引：6，他引：31  

王随继  黄杏珍 《沉积学报》1997,15(1):65-70

泌阳凹陷下第三系核桃园组总体上表现为一个大的湖退体系，而其中的微量元素含量及有关元素比值却呈现出细微而明显的旋回变化特征。本文依据泥岩、页岩和碳酸盐岩中Ｔｉ、Ｓｒ、Ｂａ等十六种微量元素含量及Ｓｒ/Ｂａ、Ｆｅ/Ｍｎ、Ｓｒ/Ｃｕ等七种比值的变化特征，结合古盐度及岩相资料，对核桃园组沉积时的古气候作了系统的研究，基本上分出了温暖潮湿、干湿交替、炎热干旱、温热半干旱四类较为特征的古气候类型，以期为油气勘探提供依据。  相似文献   

鹤岗盆地晚侏罗世石头河子组沉积环境及幕式聚煤作用   总被引：3，自引：0，他引：3  

杨子荣  姜剑虹 《沉积学报》1997,15(3):55-61

鹤岗盆地是一中-新生代内陆盆地，主要含煤地层上侏罗统石头河子组是一套非海相含煤地层，充填于盆地底部，自下而上可划分为三个岩性段，五个亚段和２６个含煤旋回。底部第一亚段沉积期以冲积扇、辫状河环境为主；第二亚段以河流环境为主；第三亚段以河流-三角洲-湖泊环境为主；第四亚段演化为河流环境；第五亚段则以河流、冲积扇环境为主。垂向上五个亚段对应于盆地的填平补齐-扩张-稳定沉降-差异沉降-收缩五个演化阶段。富煤带位于同沉积中心的侧翼，一般含砂率６０%～７０%的区域含煤性最好。含煤旋回受控于盆地周期性地快-慢沉降，聚煤作用发育于每期幕式沉降的缓慢稳定沉降阶段。厚-特厚煤层都与湖泊三角洲冲积平原大面积沼泽化有关。  相似文献   

四川成都盆地某深层富钾卤水的地球化学特征及成因   总被引：7，自引：0，他引：7  

黄思静  曾允孚 《沉积学报》1997,15(3):67-71

本文报道了四川成都盆地某深层富钾层卤水的化学组成，氧、氢、硫和硼同位素组成以及２５℃等温蒸发析盐过程中，从卤水中获得的固相物质组成。在此基础上讨论了卤水的成因，认为该卤水是蒸发浓缩的海相卤水、非海相卤水以及埋藏成岩过程中封存卤水对地层中海相蒸发盐溶滤作用形成的混合水。  相似文献   

东风山前寒武纪含铁建造金矿床中的稀土元素          下载免费PDF全文

魏菊英  曾强 《地质科学》1997,32(4):506-514

东风山含铁建造金矿床中稀土元素含量普遍较低。东风山群是含矿层,其不同层位岩石和矿物的稀土含量有所差别。大理岩和含铁层中铁闪石的稀土含量最低,甚至低于稀土地壳丰度;上部角岩、下部角岩和石榴子石中稀土含量近似其他壳丰度;而板岩和花岗岩中稀土含量远高于其他壳丰度。稀土元素分布的特点反映了东风山金矿床形成于一个半封闭和较还原的海盆地,金矿化是在弱碱性条件下产生。黄铁矿中出现独居石类矿物,这可能与陆壳重熔改造形成花岗岩时产生的局部富轻稀土热液沿黄铁矿裂隙渗入有关。  相似文献   

关于侵入岩体中金矿床成因研究若干问题的讨论   总被引：9，自引：1，他引：9  

莫测辉  王秀璋 《地质论评》1997,43(2):139-147

各时代、不同类型侵入岩中的金矿床是金矿的主要类型。关于金矿床与岩体的成因关系目前尚存在不同的看法，但传统的岩浆热液成因观点仍居统治地位，其原因往往在于不少研究者把岩液热液成因当成了固定模式应用于具体矿床的研究，而不是建立在综合研究的基础之上。本文结合近年来金矿的研究进展，强调侵入岩中金矿床的研究的研究要综合考虑成岩成矿的时代与时差，成矿流体和成矿物质的来源等各方面因素。金矿床定年的进展研究表明，矿  相似文献   

不同岩溶动力系统的碳稳定同位素和地球化学特征及其意义——以我国几个典型岩溶地区为例   总被引：7，自引：0，他引：7       下载免费PDF全文

刘再华  袁道先  何师意 《地质学报》1997,71(3)

结合水化学的野外观测及室内计算,作者对桂林岩溶试验场、四川黄龙风景区和贵州乌江渡坝区3个岩溶动力系统的碳稳定同位素特征进行了分析,进而对系统的性质、系统中CO_2的来源、碳酸盐沉积过程中的碳同位素动力分馏、水化学和钙华的成因及热水钙华的~(14)C测年等地球化学问题作了探讨。结果表明,桂林岩溶试验场属于表层岩溶作用动力系统,其中的侵蚀动力主要来源于大气降水溶解土壤中的CO_2;四川黄龙风景区属于深部岩溶动力系统,侵蚀动力来自大气降水溶解幔源的CO_2;贵州乌江渡坝区岩溶系统,虽然属于表层岩溶动力系统,但其中一部分的同位素和水化学特征已受到人类活动的重大影响。  相似文献   

用地质学的观点探讨洞庭湖的治理   总被引：11，自引：0，他引：11  

董和金 《湖南地质》1997,16(3):141-146

洞庭湖是一断陷盆地，至今仍在继续下沉，因围湖筑垸使水面减小，泥砂堆积速度大于湖区下沉速度使湖面进一步缩小，致使洪水季节常泛滥成灾。按地质规律科学治理方案，一是退垸还湖，二是垸湖置换，才能从根本上消除洞庭湖的水灾。  相似文献