Illite ‘crystallinity’, maturation of organic matter and microstructural development associated with lowest-grade metamorphism of Neoproterozoic sediments in the Teplá -Barrandian unit, Czech Republic
Illite ‘crystallinity’, maturation of organic matter and microstructural development associated with lowest-grade metamorphism of Neoproterozoic sediments in the Teplá -Barrandian unit, Czech Republic
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伊利石“结晶度”、有机质的成熟和与捷克共和国 Teplá-Barrandian 单元新元古代沉积物最低级变质作用相关的微观结构发育
作者：
V. Suchý;I. Sýkorová;K. Melka;J. Filip;V. Machovič
摘要研究了捷克特普勒<e:1> -巴兰地亚单元东部变质品位、古温史以及构造应变对粘土矿物和有机质转化的影响。泥质沉积的变质等级从下地幔带（IC ~0.30-0.36Δº2θ）到上地幔带（IC ~0.24-0.26Δº2θ）不等。变质品位的增加与下部断裂带的缝状解理和透镜状石英晶粒的发育相一致，下部断裂带为板状解理和动态再结晶的带状石英晶粒。白色云母在高应变岩石中通常具有较高的IC值，而绿泥石在变形和劈裂样品中显示出较低的IC值。分散在沉积物中的有机质代表了一个复杂的组合，包括来历不明的高度成熟的颗粒、焦沥青和“过渡物质”。有机碎屑的反射率一般在Rmax的3.1% ~ 7.7%之间变化，这表明在更高品位的样品中，无烟煤到超无烟煤等级向半石矿等级过渡，尽管Rmax和IC值之间的总体联系很弱，如果存在的话。新形成的剪切诱导石墨在安次带-次带边界附近突然出现，与寄主沉积物中石英颗粒塑性变形和动态再结晶的发生有关。新元古代层序中最大变质温度为250 ~ 350℃，发生在卡多米期（泛非）造山运动540 ~ 550 Ma。磷灰石裂变径迹分析显示，在340-350 Ma期间，岩石温度随后下降，并持续整个晚古生代。最近一次加速降温发生在20 ~ 40 Ma之间，与阿尔卑斯造山运动引起的波希米亚地块的区域性隆起相对应。
Abstract Metamorphic grade, palaeothermal history and the influence of tectonic strain on clay minerals and organic matter transformation were studied in the eastern part of the Teplá - Barrandian unit in the Czech Republic. The metamorphic grade of pelitic sediments ranges from the lower anchizone (IC ~0.30-0.36Δº2θ) to the lowermost epizone (IC ~0.24-0.26Δº2θ). Increase in metamorphic grade is paralleled by the development of anastomosing cleavage and lenticular quartz grains in the anchizone which give way to slaty cleavage and dynamically recrystallized ribbon quartz grains in the lower epizone. White mica in highly strained rocks generally has greater IC values whereas chlorite displays reduced values in deformed and cleaved samples. The organic matter dispersed in the sediments represents a complex assemblage of highly matured particles of uncertain origin, pyrobitumen and ‘transitional matter’. The reflectance of organic fragments generally varies from 3.1% to 7.7% of Rmax which suggests anthracite to meta-anthracite rank progrades to semigraphite in higher-grade samples, although the overall link between Rmax and IC values is weak, if present at all. Newly formed shear-induced graphite appears abruptly near the anchizone-epizone boundary and correlates with the onset of plastic deformation and dynamic recrystallization of quartz grains in the host sediments. Maximum metamorphic temperatures within the Neoproterozoic sequence in the range of 250-350ºC were attained during the Cadomian (Pan-African) orogeny at 540-550 Ma. Apatite fission-track analysis reveals a subsequent decrease in rock temperature over the period 340-350 Ma that persisted throughout the late Palaeozoic. The most recent episode of accelerated cooling occurred between 20 and 40 Ma, corresponding with the regional uplift of the Bohemian Massif due to the Alpine orogeny.