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71.
ABSTRACT Microstructural and petrological data from the Jumping Brook metamorphic suite, western Cape Breton Highlands, suggest that a single episode of syntectonic prograde metamorphism, followed by uplift, cooling and associated retrogression, affected these rocks during mid-Palaeozoic times. Microstructures indicative of progressive crenulation foliation development can be traced from low-grade (chlorite zone) through high-grade (kyanite zone) rocks, allowing a clear sequence of porphyroblast growth to be established. Metamorphic reactions and P-T calculations suggest metamorphic conditions of 700-750°C at 8-10 kbar were achieved in kyanite zone rocks. Although a complete P-T-t path was not defined, combined petrological and geochronological data can be used to constrain computed P-T-t models. These models suggest that a component of post-metamorphic tectonic exhumation is required to explain the observed times of cooling and uplift. The microstructural and petrological data to not support the interpretation that the high-grade rocks represent pre-existing crystalline basement. Indeed, the metamorphic history, geochronology and computed tectonic models all point to a single, short-lived episode of Silurian-Devonian volcanism, intrusion, convergence, regional metamorphism and uplift, probably resulting from collision tectonics at an irregular continental margin. 相似文献
72.
The microstructure of halite from the subhorizontal, bedded Main Röt Evaporite Member at Hengelo, The Netherlands (AKZO well 382, depth interval of 420–460 m), was studied by transmitted and reflected light microscopy of gamma-irradiation decorated samples. Primary microstructures compare favourably with those found in recent ephemeral salt pans. Large, blocky, fluid-inclusion-poor halite grains and elongated chevrons are interpreted to have formed in the saline lake stage, while void-filling clear halite is interpreted to have formed during the desiccation stage of the salt pan. In addition, in all layers the grains are rich in deformation-related substructures such as slip bands and subgrains indicating strains of a few percent. The study of gamma-irradiation decorated thin sections shows that the main recrystallization mechanism is grain boundary migration. Grain boundary migration removes primary fluid inclusions and produces clear, strain-free new grains. Differential stresses as determined by subgrain size piezometry were 0.45–0.97 MPa. The deformation of the salt layers is probably related to Cretaceous inversion in the area. 相似文献
73.
Large maficultramafic layered intrusions may containlayers enriched in platinum-group elements (PGE). In many cases,the PGE are hosted by disseminated sulphides. We have investigatedthe distribution of the sulphides in three dimensions in twooriented samples of the Merensky Reef and the J-M Reef. Theaim of the study was to test the hypothesis that the sulphidescrystallized from a base metal sulphide liquid that percolatedthrough the cumulate pile during compaction. The distributionof sulphides was quantified using: (1) X-ray computed tomography;(2) microstructural analysis of polished thin sections orientedparallel to the paleovertical; (3) measurement of dihedral anglesbetween sulphides and silicates or oxides. In the Merensky Reefand the J-M Reef, sulphides are connected in three dimensionsand fill paleovertical dilatancies formed during compaction,which facilitated the downward migration of sulphide liquidin the cumulate. In the melanorite of the Merensky Reef, thesulphide content increases from top to bottom, reaching a maximumvalue above the underlying chromitite layer. In the chromititelayers sulphide melt connectivity is negligible. Thus, the chromititemay have acted as a filter, preventing extensive migration ofsulphide melt downwards into the footwall. This could partiallyexplain the enrichment in PGE of the chromitite layer and theobserved paucity of sulphide in the footwall. KEY WORDS: X-ray computed tomography; microstructures; sulphides; Merensky Reef; J-M Reef 相似文献
74.
美国页岩气大规模商业性开发促进了中国页岩气地质调查评价和研究工作的开展.经过近10年的探索实践,认识到中国页岩气地质条件复杂,地质理论和开发技术有待突破.从中国富有机质页岩特征及分布规律、生气机理与富集规律、页岩气储层物性及微结构、保存条件、资源评价与选区评价等方面对目前中国页岩气地质理论研究进展进行了综述分析,讨论了中国在页岩气地质理论研究中存在的问题,并对今后中国页岩气地质研究工作进行了展望.认为沉积、构造对页岩层段分布规律的控制作用研究需加强,页岩气吸附机理的理论认识需深入,微结构定性定量测试分析技术需提高,评价现今页岩气保存条件的指标体系需建立,选区参数体系和标准需进一步完善. 相似文献
75.
哀牢山-红河剪切带中段多阶段新生代花岗岩脉:同位素年代学及对于剪切应变型式转变时间的约束 总被引:1,自引:1,他引:0
哀牢山-红河剪切带是新生代印度板块与欧亚板块碰撞过程中发育的大规模走滑型剪切带,其发育对于碰撞过程中印支地块的南东向逃逸以及藏东南地区构造格局的形成具有重要的贡献。与剪切带演化相关,伴随发育多阶段花岗岩脉就位,它们为限定剪切变形时限、阐明剪切作用属性提供了重要证据。本文在野外观察基础上,应用显微构造和EBSD石英c-轴组构分析查明花岗岩脉的构造特点与应变型式,同时采用锆石LA-ICP-MS测年方法获得岩脉侵位与结晶年龄。年龄分析结果表明,岩脉年龄分别为27.09±0.48Ma、25.17±0.23Ma和25.16±0.50Ma,其中年龄为27.09±0.48Ma的花岗岩脉具有糜棱岩化现象,其变形特征体现为中温变形后叠加低温变形,且剪切变形形式由一般剪切转换为简单剪切;年龄为25.17±0.23Ma的花岗岩脉表现出同剪切晚期构造特征,且具有较低温度简单剪切变形特点;25.16±0.50Ma的切穿糜棱叶理,矿物未见变形,可能代表剪切期后岩脉。结合区域构造,推测剪切方式由纯剪为主的剪切向由单剪为主的剪切转换发生在27Ma和25Ma之间,哀牢山-红河剪切带中段在约25Ma走滑运动结束。 相似文献
76.
《International Geology Review》2012,54(3):348-367
The Diancang Shan metamorphic massif, the northwestern extension of the Ailao Shan Massif, is a typical metamorphic complex situated along the NW–SE-trending Ailao Shan–Red River shear zone. Diancang Shan granitic and amphibolitic mylonites collected from sheared high-grade metamorphic rocks were studied using petrographic and electron-backscatter diffraction techniques. Sensitive high-resolution ion microprobe U–Pb dating of zircon grains from the granitic mylonites constrains the timing of shearing. Macro- and microstructural and textural analysis reveals intense plastic deformation of feldspar, quartz, and amphibole under amphibolite-facies conditions, all consistently document left-lateral shearing. Porphyritic monzogranitic mylonite within the shear zone possesses evidence supporting a sequential, progressive process from crystallization during magma emplacement, through submagmatic flow to solid-state plastic deformation. We suggest that the early-kinematic pluton subsequently underwent strong left-lateral strike–slip shearing. The development of complex textures of quartz, feldspar, and amphibole from the granitic and amphibolitic mylonites apparently records successive variation of conditions attending coherent, solid-state high-temperature ductile deformation during regional left-lateral shearing. All magmatic zircons from the mylonitized porphyritic monzogranite give U–Pb ages of 30.95 ± 0.61 million years for the crystallization of the granite. This age provides the timing of onset of left-lateral shearing along the Ailao Shan–Red River shear zone in the Diancang Shan high-grade metamorphic massif. 相似文献
77.
中国东部早第三纪玄武岩中幔源包体的变形特征及其上地幔流变学意义 总被引:5,自引:0,他引:5
中国东部含幔源包体的早第三纪玄武岩已发现的有吉林双辽、河北阳原和山西繁峙。本文详细研究了这三个地点幔源包体的变形特征,结果表明,这些地点的幔源包体以尖晶石二辉橄榄岩为主,其结构主要为残斑结构。包体中橄榄石显示明显的优选方位,发育有反映高温位错蠕变机制的位错亚构造。包体的平衡温度在800一1100℃之间,压力为0.7一1.7CPa。来源深度为28一52km。采用显微构造应力计计算的差异应力在17一24MPa之间,由橄榄岩高温流变律计算的应变速率为10 ̄(-15)-10 ̄(-18)s ̄(-1)等效粘滞度为10 ̄(21)一10 ̄(25)spa.S之间。中国东部早第三纪与晚第三纪以后的幔源包体具有基本相同的特征,表明晚第三纪以后的上地幔继承和发展了早第三纪时的特征。中国东部早第三纪以来的上地幔符合大陆拉张带和裂谷上地幔的流变特征。 相似文献