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Illite ‘crystallinity’, maturation of organic matter and microstructural development associated with lowest-grade metamorphism of Neoproterozoic sediments in the Teplá-Barrandian unit, Czech Republic

V. SUCH¹, I. SYKOROVÁ^2,*, K. MELKA³, J. FILIP³ and V. MACHOVI^2,4

¹ Jiránkova 1136/4, 163 00 Prague 6 - epy, Czech Republic,² Institute of Rock Structure and Mechanics, Academy of Sciences of the Czech Republic, V Holeovikách 182 09 Prague 8, Czech Republic,³ Institute of Geology, Academy of Sciences of the Czech Republic, Rozvojová 135, 165 00 Prague 6 – Suchdol, Czech Republic, and⁴ Institute of Chemical Technology, Technická 5, 166 28 Prague, Czech Republic

^* E-mail: sykorova{at}irsm.cas.cz

(Received 22 January 2007; revised 28 August 2007)

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 ~0.30–0.36°2 ) to the lowermost epizone (IC ~0.24–0.26°2 ). 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 R_max which suggests anthracite to meta-anthracite rank progrades to semigraphite in higher-grade samples, although the overall link between R_max 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 250–350°C 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.

KEYWORDS: illite ‘crystallinity’, chlorite ‘crystallinity’, Kübler index, organic matter maturation, graphitization, apatite fission-track analysis, cleavage, very low-grade metamorphism, Proterozoic, Teplá-Barrandian