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Reaction softening by dissolution–precipitation creep in a retrograde greenschist facies ductile shear zone,New Hampshire,USA 
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下载免费PDF全文 R. J. McAleer D. L. Bish M. J. Kunk K. R. Sicard P. M. Valley G. J. Walsh B. A. Wathen R. P. Wintsch 《Journal of Metamorphic Geology》2017,35(1):95-119
We describe strain localization by a mixed process of reaction and microstructural softening in a lower greenschist facies ductile fault zone that transposes and replaces middle to upper amphibolite facies fabrics and mineral assemblages in the host schist of the Littleton Formation near Claremont, New Hampshire. Here, Na‐poor muscovite and chlorite progressively replace first staurolite, then garnet, and finally biotite porphyroblasts as the core of the fault zone is approached. Across the transect, higher grade fabric‐forming Na‐rich muscovite is also progressively replaced by fabric‐forming Na‐poor muscovite. The mineralogy of the new phyllonitic fault‐rock produced is dominated by Na‐poor muscovite and chlorite together with late albite porphyroblasts. The replacement of the amphibolite facies porphyroblasts by muscovite and chlorite is pseudomorphic in some samples and shows that the chemical metastability of the porphyroblasts is sufficient to drive replacement. In contrast, element mapping shows that fabric‐forming Na‐rich muscovite is selectively replaced at high‐strain microstructural sites, indicating that strain energy played an important role in activating the dissolution of the compositionally metastable muscovite. The replacement of strong, high‐grade porphyroblasts by weaker Na‐poor muscovite and chlorite constitutes reaction softening. The crystallization of parallel and contiguous mica in the retrograde foliation at the expense of the earlier and locally crenulated Na‐rich muscovite‐defined foliation destroys not only the metastable high‐grade mineralogy, but also its stronger geometry. This process constitutes both reaction and microstructural softening. The deformation mechanism here was thus one of dissolution–precipitation creep, activated at considerably lower stresses than might be predicted in quartzofeldspathic rocks at the same lower greenschist facies conditions. 相似文献
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东昆仑及相邻地区中生代-新生代早期构造过程的热年代学记录 总被引：2，自引：0，他引：2
针对东昆仑及相邻地区研究较薄弱的中生代—新生代早期时段的构造过程提供了系列新的热年代学资料.不同热年代学方法综合揭示了东昆仑及相邻地区在中生代—新生代早期至少存在3次明显的热事件记录.第一次大约启动于200Ma的晚三叠世晚期,并可能一直延续到早中侏罗世之交,是一次具有广泛影响并奠定造山带区域构造格架的构造热事件.区域动力背景可能和南部羌塘地块与昆仑地块的碰撞、松潘—甘孜—巴颜喀拉浊积盆地闭合相关.第二次发生在大约130~150Ma的早白垩世,并可能延续到早白垩世末,主要表现为系列区域性NWW-SEE向的挤压性断裂活动,可对应于白垩纪时期拉萨地块沿班公湖—怒江缝合带与欧亚大陆的增生拼贴事件.第三次为大约56~45Ma的古新世,表现为一期伸展抬升.热年代学记录与零星保存的地质记录具有良好的匹配性,并对构造过程提供了更确切的时间限定. 相似文献
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Jung Hoo Lee Donald R. Peacor Daniel D. Lewis Robert P. Wintsch 《Contributions to Mineralogy and Petrology》1984,88(4):372-385
Phyllosilicates in rocks which are transitional from mudstone to slate from Lehigh Gap, Pa., have been studied by a variety of techniques, including high resolution Transmission Electron Microscopy and Analytical Electron Microscopy. The principal minerals are white mica which is transitional from illite in mudstone to ordered twolayer mica in slate, and chlorite. 7Å berthierine occurs more rarely. Dioctahedral and trioctahedral layers are shown to be interleaved in individual crystals at all scales between the following two end members: (1) both random and regular 11 interlayering at the scale of individual layers, as shown, in part, by lattice fringe images. (2) packets of trioctahedral and dioctahedral layers up to a few thousand Ångstroms or microns in thickness, detectable with ordinary optical techniques. The complete range of intermediate structures is represented in samples which are in transition to slate. Bulk analytical (EMPA), X-ray diffraction or other measurements are shown to result in averages over both kinds of layers when TEM techniques are not used.Contribution No. 400 from the Mineralogical Laboratory, Department of Geological Sciences, The University of Michigan 相似文献
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Guo-Can Wang Robert P. Wintsch John I. Garver Mary Roden-Tice She-Fa Chen Ke-Xin Zhang Qi-Xiang Lin Yun-Hai Zhu Shu-Yuan Xiang De-Wei Li 《Island Arc》2009,18(3):444-466
Triassic turbidites dominate the Songpan–Ganzi–Bayan Har (SGBH) terrane of the northern Tibetan Plateau. U‐Pb dating on single detrital zircon grains from the Triassic Bayan Har Group turbidites yield peaks at 400–500 m.y., 900–1000 m.y., 1800–1900 m.y., and 2400–2500 m.y., These results are consistent with recently published U‐Pb zircon ages of pre‐Triassic bedrock in the East Kunlun, Altyn, Qaidam, Qilian and Alaxa areas to the north, suggesting that provenance of the Bayan Har Group may include these rocks. The similarities in the compositions of the lithic arkosic sandstones of the Bayan Har Group with the sandstones of the Lower‐Middle Triassic formations in the East Kunlun terrane to the north also suggests a common northern provenance for both. A well exposed angular unconformity between the Carboniferous–Middle Permian mélange sequences and the overlying Upper Permian or Triassic strata indicates that regional deformation occurred between the Middle and Late Permian. This deformation may have been the result of a soft collision between the Qiangtang terrane and the North China Plate and the closure of the Paleo‐Tethyan oceanic basin. The Bayan Har Group turbidites were then deposited in a re‐opened marine basin on a shelf environment. Fission‐track dating of detrital zircons from the Bayan Har Group sandstones revealed pre‐ and post‐depositional age components, suggesting that the temperatures did not reach the temperatures necessary to anneal retentive zircon fission tracks (250–300°C). A 282–292 m.y. peak age defined by low U concentration, retentive zircons likely reflects a northern granitic source. Euhedral zircons from two lithic arkoses with abundant volcanic fragments in the southern area yielded a ～237 m.y. zircon fission track (ZFT) peak age, likely recording the maximum age of deposition. A dominant post‐depositional 170–185 m.y. ZFT peak age suggests peak temperatures were reached in the Early Jurassic. Some samples appear to record a younger thermal event at ～140 m.y., a short lived event that apparently affected only the least retentive zircons. 相似文献
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R. P. Wintsch 《Contributions to Mineralogy and Petrology》1980,73(4):421-428
A pE-pH diagram for supercritical aqueous fluids at 500° C and 2 kb total pressure is calculated from thermodynamic data assuming ideal mixing of gas species. The experimental data on the stability of annite-phlogopite solid-solutions (Wones and Eugster 1965) and on the stability of aluminous biotite (Rutherford 1968) have been used to calculate the stability of these biotite solid solutions in sanidine, muscovite and Al2SiO5 bearing assemblages. The resulting pE-pH diagrams show that Al increases the stability field of annite more per atom than does Mg. However, the addition of Al to biotite does not increase the stability of annite in very acid and alkaline solutions. Environments sufficiently acid to render aluminous biotite metastable are probably not found in nature, but the mildly alkaline environments are attainable. At constant f
O
2, f
H
2 and
the Al content of biotite is at a minimum in muscovite + sanidine bearing assemblages, and increases relatively rapidly with increases in pH in the sanidine field, and increases more slowly with decreases in pH through the muscovite and Al2SiO5 fields.These diagrams show that the composition of biotite solid-solutions containing more aluminum than ideal annite-phlogopite and coexisting with sanidine and magnetite cannot be used to infer intensive parameters (T
f
O
2,
) prevailing during the crystallization of the biotite, and that in no case can any biotite composition in muscovite or Al2SiO5 bearing assemblages be used to extract this information. 相似文献
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Robert P. Wintsch Anne F. O'Connell Barbara L. Ransom Mark J. Wiechmann 《Contributions to Mineralogy and Petrology》1981,77(3):207-213
Disseminated carbon has been extracted from 19 samples of arkosic rocks from the chlorite, biotite and garnet zones in the
Narragansett basin, and analysed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). XRD of these samples yields
only the (002) peak which may be as much as four times wider than the silicon internal standard peak and is always skewed
to low angles 2θ. All (002) peaks were digitized and the four moments calculated. The results suggest that width at half-height
(W1/2) provides a poorer estimate of peak shape than coefficients of skewness and kurtosis. The positive correlation of these coefficients
with grain size determined by SEM in the range of 0.1–100 μm suggest a direct relationship between peak shape and grain size.
No variable measured in this study correlates with metamorphic grade. However, W1/2, skewness and kurtosis of the (002) diffraction peak of the carbonaceous material in the biotite and garnet zones correlates
inversely with modal carbon. This suggests that the volatilization reaction: C+2H2=CH4 may be an important step in the mechanism of graphitization. Variable fugacities of H2 and CH4 and/or variable permeabilities may be responsible for the non-uniform development of graphite crystallinity in the greenschist
facies rocks in Rhode Island. 相似文献
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东昆仑东段哈拉郭勒-哈图一带中生代的岩石隆升剥露--锆石和磷灰石裂变径迹年代学证据 总被引：4，自引：2，他引：4
根据对东昆仑地区东段哈拉郭勒—哈图一带不同高度基岩的系列锆石裂变径迹年龄分析，结合磷灰石裂变径迹年龄分析和中酸性侵入岩角闪石压力计分析揭示了东昆仑东段中生代的岩石隆升剥露冷却历史．巴隆哈图一带中酸性侵入岩角闪石压力计分析结果反映晚海西—印支期以来的总体剥露幅度约8～9km，早二叠世至晚三叠世初剥蚀作用极为缓慢，大约为20～40m／Ma．不同高程样品的锆石裂变径迹年龄分析结果揭示了东昆仑地区东段在中晚侏罗世处于缓慢的岩石隆升剥露阶段，其中中侏罗世相对较快，抬升速率77～88m／Ma，晚侏罗世相对较慢，抬升速率小于37m／Ma，且呈减慢趋势，这种减慢趋势反映了早中侏罗世之交强构造抬升期后的逐渐衰退．锆石裂变径迹—磷灰石裂变径迹年龄分析结果反映了中侏罗世以来的剥蚀速率一般不超过55m／Ma，岩石的剥蚀速率与岩石的抬升速率基本为同一量级，中侏罗世—白垩纪剥蚀作用与岩石抬升作用基本处于平衡状态。 相似文献
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