Extensional deformation structures within a convergent orogen: The Val di Lima low-angle normal fault system (Northern Apennines,Italy)     

《Journal of Structural Geology》2014

A low-angle extensional fault system affecting the non metamorphic rocks of the carbonate dominated Tuscan succession is exposed in the Lima valley (Northern Apennines, Italy). This fault system affects the right-side-up limb of a kilometric-scale recumbent isoclinal anticline and is, in turn, affected by superimposed folding and late-tectonic high-angle extensional faulting.The architecture of the low-angle fault system has been investigated through detailed structural mapping and damage zone characterization. Pressure-depth conditions and paleofluid evolution of the fault system have been studied through microstructural, mineralogical, petrographic, fluid inclusion and stable isotope analyses. Our results show that the low-angle fault system was active during exhumation of the Tuscan succession at about 180°C and 5 km depth, with the involvement of low-salinity fluids. Within this temperature - depth framework, the fault zone architecture shows important differences related to the different lithologies involved in the fault system and to the role played by the fluids during deformation. In places, footwall overpressuring influenced active deformation mechanisms and favored shear strain localization.Our observations indicate that extensional structures affected the central sector of the Northern Apennines thrust wedge during the orogenic contractional history, modifying the fluid circulation through the upper crust and influencing its mechanical behavior.  相似文献   

电子探针测年方法应用于晶质铀矿的成因类型探讨   总被引：7，自引：4，他引：3  

赵慧博  刘亚非  阳珊  叶美芳  王志海  王博  常娜 《岩矿测试》2014,33(1):102-109

电子探针Th-U-Pb测年因其高分辨率与高精度的优势,在独居石、锆石等定年矿物中得到了推广,但在Th、U、Pb含量高的晶质铀矿、沥青铀矿等矿物中则应用较少。本文在铁矿床变质岩绿泥石、阳起石黑云母蚀变岩首次发现U含量高的晶质铀矿,基于此,结合该铁矿床地区的地质背景,利用偏光显微镜与电子探针等分析测试手段,将镜下蚀变现象、年龄计算与其他相关元素分析相结合,重点对晶质铀矿的成矿年龄及成矿规律进行探讨。研究发现:通过镜下观察判断,晶质铀矿的成因类型与澳大利亚著名的变质型铀矿相似,均为古老的变质型,且周围的脉石矿物均为绿泥石,绿泥石皆由黑云母退变质而成,铀矿的赋存位置显示其与黑云母、绿泥石之间有紧密联系,其成矿年龄与黑云母、绿泥石形成年龄息息相关。继而根据电子探针数据计算成矿年龄,判断成矿期次,得出主要成矿期在(1654±17)Ma~(1805±17)Ma,为中元古代中期,且主要成矿期与热液蚀变作用黑云母化有关,后期活化富集时期在(657±17)Ma~(807±17)Ma,为新元古代南华纪时期,此阶段是热液侵入、绿泥石化广泛发育的时期;选取较大颗粒对晶质铀矿的环带年龄进行计算,从年龄分布上证实后期有强烈的流体活动的发生,且主要与绿泥石化相关。另外,对比变质型与沉积型铀矿中Y2O3与UO2含量发现,两者之间存在负相关关系,此关系对判断铀矿成因即是否为变质型或沉积型可能有指示意义,但缺乏大量的数据佐证,需进一步研究。  相似文献   

藏北商旭造山型金矿床的发现及意义   总被引：4，自引：3，他引：1  

黄瀚霄  李光明  刘波  张智林  马东  曲扎  肖万峰  刘洪 《矿床地质》2014,33(3):486-496

西藏双湖县商旭金矿床位于班公湖-怒江缝合带中段。矿体赋存于中-下侏罗统木嘎岗日群浅变质复理石中,受北西西向断裂构造控制,呈透镜体状、脉状,或者似层状产出;矿石矿物以自然金为主,矿石类型为石英脉型和蚀变岩型,围岩蚀变特征明显。该矿床主要受地层及构造因素控制,矿区的构造、围岩蚀变、矿物、地球化学异常等可作为找矿标志。野外地质调查和矿床对比研究显示,商旭金矿床在矿物共生组合、蚀变类型、构造类型等方面,与典型的造山型金矿床相似,其成因类型属于造山型金矿床。商旭金矿床的发现为班公湖-怒江成矿带金等贵金属矿产的地质找矿工作开辟了新的方向。  相似文献   

西秦岭岷-礼成矿带岩浆活动与金矿成矿模式探讨     

肖力 《地质与勘探》2014,50(Z1):1291-1298

岷-礼成矿带发现较多大型金矿,同时在岩体内发现斑岩型钼矿。金矿床矿石中除有低温矿物外,还有黄铜矿、白钨矿等中-高温矿物,白钨矿在寨上金矿中是载金矿物之一;钼矿中也有黄铜矿、白钨矿矿物;金、钼、钨矿化分带有一定空间关系;金矿床地球化学、稳定同位素、放射性同位素和流体包裹体的研究表明成矿物质,成矿流体有深源特征;中川岩体群的岩石学、年代学、岩石化学、微量元素、稀土元素特征表明岩体为复式岩体,具有壳幔混源特征;据此建立成矿带内受印支-燕山期岩浆活动控制的金矿成矿模式。  相似文献   

Emplacement of the La Peña alkaline igneous complex,Mendoza, Argentina (33° S): Implications for the early Miocene tectonic regime in the retroarc of the Andes     

《Journal of South American Earth Sciences》2014

The La Peña alkaline complex (LPC) of Miocene age (18–19 Ma) lies on the eastern front of the Precordillera (32°41ʹ34ʺS, 68°59ʹ48″W, 1400–2900 m a.s.l.), 30 km northwest of Mendoza city, Argentina. It is a subcircular massif of 19 km² and 5 km in diameter, intruded in the metasedimentary sequence of the Villavicencio Formation of Silurian-Devonian age. It is the result of integration of multiple pulses derived from one or more deep magma chambers, which form a suite of silicate rocks grouped into: a clinopyroxenite body, a central syenite facies with a large breccia zone at the contact with the clinopyroxenite, bodies of malignite, trachyte and syenite porphyry necks, and a system of radial and annular dikes of different compositions. Its subcircular geometry and dike system distribution are frequent features of intraplate plutons or plutons emplaced in post-orogenic settings. These morphostructural features characterize numerous alkaline complexes worldwide and denote the importance of magmatic pressures that cause doming with radial and annular fracturing, in a brittle country rock. However, in the LPC, the attitude of the internal fabric of plutonic and subvolcanic units and the preferential layout of dikes match the NW–SE extensional fractures widely distributed in the host rock. This feature indicates a strong tectonic control linked to the structure that facilitate space for emplacement, corresponding to the brittle shear zone parallel to the N–S stratigraphy of the country rock. Shearing produced a system of discontinuities, with a K fractal fracture pattern, given by the combination of Riedel (R), anti-Riedel (R′), (P) and extensional (T) fracture systems, responsible for the control of melt migration by the opening of various fracture branches, but particularly through the NW–SE (T) fractures. Five different pulses would have ascent, (1) an initial one from which cumulate clinopyroxenite was formed, (2) a phase of mafic composition represented by dikes cross-cutting the clinopyroxenite, (3) a malignite facies that causes a small breccia in the clinopyroxenite, (4) a central syenite facies that develops breccias at the contact with the clinopyroxenite and, finally, (5) porphyry necks and a system of radial dikes intruding all units. At the moment of the emplacement different mechanisms would have acted, they summarized in: 1) opening of discontinuities synchronous to the magma circulation as the principal mechanism for formation of dikes and conduits; 2) stoping processes, that play an important role in the development of the breccia zone and enabling an efficient transference of material during the emplacement of the syenitic magma and 3) shear-related deformation (regional stress), affected the internal fabric of the facies, causing intracrystalline deformation and submagmatic flow, which is very evident in the central syenite intrusive. The kinematic analysis of shear planes allows proposing that emplacement of the LPC took place in a transtensive regime, which would have occurred in the back-arc of the Andes orogen, during a long period spanning from Miocene to the present, of the compressive deformation responsible, westward and at the same latitude, for the development of the Aconcagua fold and thrust belt.  相似文献   

柴达木盆地西部昆北断阶带基底花岗岩锆石U-Pb年龄、地球化学特征及其地质意义     

刘桂珍  张玉修  薛建勤  吴光大  陈琰 《岩石学报》2014,30(6):1615-1627

柴达木盆地西部基底分布有大量的有花岗岩类岩石。通过对柴达木盆地西部昆北断阶带钻遇的基底花岗岩样品开展详细的岩石学、锆石激光探针等离子体质谱U-Pb同位素年代学及岩石地球化学研究表明,锆石U-Pb同位素年龄为467~450Ma,显示基底花岗岩的结晶年龄为中-晚奥陶世,属于加里东期岩浆侵入旋回。详细的岩石地球化学分析表明,昆北断阶带基底花岗岩属过铝高钾钙碱性系列,其稀土元素配分模式为具有Eu负异常的轻稀土元素富集型,昆北断阶带中南部基底花岗岩属上地壳物质熔融,同碰撞环境下形成的花岗岩。综合区域上的研究成果,昆北断阶带及其以西地区存在中奥陶世-早志留世的加里东期构造-岩浆事件,这对探讨柴达木盆地西部基底花岗岩成因类型及岩浆演化具有重要的意义。  相似文献   

雅鲁藏布江蛇绿岩带西段达机翁地幔橄榄岩组成特征及其形成环境分析   总被引：6，自引：5，他引：1  

连东洋  杨经绥  熊发挥  刘飞  王云鹏  周文达  赵一珏 《岩石学报》2014,30(8):2164-2184

雅鲁藏布江蛇绿岩带自萨嘎以西分成南北两个亚带。对两个亚带蛇绿岩的各自特征及成因联系的研究,是探讨雅鲁藏布江西段的新特提斯洋构造演化的关键。北亚带蛇绿岩呈构造岩块产于冈底斯山前喀喇昆仑断裂带的南侧。其中,位于北亚带西北段的达机翁蛇绿岩,主要由地幔橄榄岩,玄武岩夹硅质岩组成,各单元间断层接触。对达机翁蛇绿岩的地幔橄榄岩开展的组成特征研究表明:(1)地幔橄榄岩主体为方辉橄榄岩,含少量的纯橄岩。方辉橄榄岩内产有豆荚状铬铁矿(呈豆状,块状以及浸染状),铬铁矿有一层纯橄岩的外壳;(2)达机翁方辉橄榄岩单斜辉石含量低,组成矿物以及全岩的地球化学特征均指示了这些样品经历了相对高的部分熔融作用;(3)方辉橄榄岩具有U型的球粒陨石标准化的稀土元素分配模式,Nb相对亏损,Ta,Zr和Hf具有弱的正异常,同时Sr和U具有强烈的正异常,这些特征可能与残余地幔和俯冲带熔/流体之间相互作用导致的轻稀土元素和部分微量元素的选择性富集有关。定量计算表明,达机翁地幔岩中的方辉橄榄岩来源于一个尖晶石相地幔源区的部分熔融,部分熔融程度大于25%,高于深海地幔橄榄岩的部分熔融程度(10%~22%)。这些橄榄岩形成时的氧逸度条件位于FMQ和FMQ+1之间,高于深海地幔橄榄岩(FMQ-1),与俯冲带环境的氧逸度条件一致。因此,我们认为达机翁蛇绿岩中的地幔橄榄岩形成于大洋中脊的环境,随后发生了洋内俯冲作用,位于俯冲带上部的地幔橄榄岩经历了俯冲带流/熔体的交代作用。  相似文献   

中天山南缘桑树园子剪切带早志留世韧性变形事件及其地质意义     

王盟  张进江  戚国伟  郑勇  刘凯 《岩石学报》2014,30(10):3051-3061

桑树园子剪切带位于中天山南缘,经历了多期韧性变形事件。对其变形样式和变形历史进行深入研究有利于加深对天山造山带构造演化的认识。本文对剪切带北部发生强烈韧性变形的岩石进行变形样式及相关年代学研究,识别出一期早志留世右旋剪切运动。该期走滑剪切事件造成部分前寒武纪斜长角闪岩发生深熔作用形成浅色脉体,使花岗闪长岩发生强烈的糜棱岩化石,并伴随同构造花岗质脉体的侵入。斜长角闪岩中浅色脉体中的锆石具弱的振荡环带或无环带,Th/U比值较低,具深熔作用中新生锆石的特点,其206Pb/238U加权平均年龄为430.1Ma。糜棱状花岗闪长岩样品锆石核部LAICPMS U-Pb年龄为496.3Ma,是中天山北缘古天山洋向南俯冲的记录。其锆石的变质增生边给出430.5Ma的年龄,记录了花岗闪长岩遭受后期糜棱岩化改造的时间。侵入花岗闪长岩中的同构造花岗质脉体也给出较为一致的432.9Ma的锆石结晶年龄。这一年龄范围与中天山北缘古天山洋盆闭合的时间一致,因此桑树园子剪切带北部在~430Ma的右旋走滑事件可能是吐哈地块与中天山碰撞事件的陆内响应。  相似文献   

酒东盆地红山地区下白垩统新民堡群灰白色蚀变带特征及其与铀成矿的关系     

郭长林  耿海军  何大鹏  李万华  陈云杰  李斌 《铀矿地质》2014,(5):283-286

研究区位于酒东盆地东北缘红山地区。通过对下白垩统新民堡群后生氧化改造作用的研究,认为灰白色弱氧化、强水解蚀变带是层间氧化带的一部分。铁和有机质是沉积岩中常见的色素,当含氧水进入目的层砂体时,砂体中的有机质、低价铁(Fe2+)等化合物被氧化,导致岩石褪色;同时氧化作用导致砂岩中长石水解,形成以高岭石为主的白色黏土矿物,使岩石增白。增白与褪色的双重作用是砂岩总体上变白的重要原因,其所形成的灰白色蚀变岩位于强氧化的紫红色-黄色蚀变亚带与灰色原生岩石带之间的一定范围内。铀矿化(体)产在弱氧化、强水解灰白色砂岩与原生灰色砂岩界面附近偏灰色岩石一侧[1-2]。灰白色蚀变带与铀成矿关系密切,可作为区域性白垩系砂岩型铀矿的找矿标志之一。  相似文献   

东昆仑东段东昆中构造带韧性剪切作用及其地质意义   总被引：2，自引：1，他引：1       下载免费PDF全文

李小兵裴先治  刘成军陈有炘李瑞保  李佐臣陈国超 魏刚锋 《中国地质》2014,41(2):419-436

东昆仑东段东昆中构造带经历了多期(次)构造事件,现今保留了复杂多样的构造样式。哈图沟、清水泉—塔妥和沟里等地区构造变形十分强烈,且发育有呈NWW-SEE展布的韧性剪切带。通过野外调研及显微构造研究,对东昆中构造带几何学、运动学及动力学等进行了分析,并通过费氏台和EBSD技术对其进行石英C轴组构分析后认为该剪切带为一压-扭性构造界面,且早期为左旋逆冲,晚期为右旋走滑,变形温度为400~550℃。对剪切带内的石英亚颗粒及动态重结晶粒度测量计算出研究区内韧性剪切带古应力值约为153.791 MPa。哈图沟牦牛山组变形砾岩(4个测点)有限应变测量结果显示,越靠近东昆中断裂带砾石Flinn指数、应变程度、罗德系数(绝对值)越大,且Flinn指数均大于1,属拉伸变形。结合区域地质资料,认为东昆仑东段东昆中韧性剪切带形成于晚海西—印支期。  相似文献