Contact metamorphism of Silurian black shales by a basalt sill: geological evidence and thermal modeling in the Barrandian Basin


Authors: Suchý V, Šafanda J, Sýkorová I, Stejskal M, Machovič V, Melka K

Published in: Bulletin of Geosciences, volume 79, issue 3; pages: 133 - 145; Received 3 December 2003; Accepted in revised form 30 April 2004;

Keywords: contact metamorphism, basalt sill, black shale, graptolite reflectance, illite and chlorite crystallinity, thermal modeling, Barrandian, Silurian,

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Organic-rich shales of the Liteň Formation (Silurian) were intruded by a series of several-meter thick doleritic basalt sills soon after their deposition. The effect of rapid thermal stress on organic and mineral diagenesis of shales around a representative 4mthick sill has been studied using optical microscopy, Fourier-Transform infrared (FT-IR) and micro-Raman spectroscopy of dispersed graptolite particles, gas chromatography-mass spectrometry (GC-MS) of organic extracts, and X-ray diffraction (XRD) of the clay fraction.
Our data suggest that the sills have only had a local effect on the thermal maturity of the adjacent sediments. Graptolite reflectance values (3.0-3.6% Rmax) and bireflectance (up to 3.1%) higher than the regional diagenetic background (~1.8 Rmax) were found to be restricted to the narrow zone immediately adjacent to the igneous contacts, with detectable alteration starting to take effect at about 70 to 80% of sill thickness. Based on recent empirical correlations, these values may indicate contact-metamorphic temperatures between 320-420 °C. The optical properties of graptolite fragments within the contact aureole correlate closely with the chemical and structural transformation as expressed by FT-IR and Raman spectroscopy data. The graptolite periderm undergoes systematic depletion of aliphatic-containing groups toward the igneous contacts, and transforms into a condensed aromatic residuum of an increased crystalline ordering, similar to high-rank coal or kerogen. Extractable organic matter within the immediate contact zone is strongly depleted, but appreciably higher concentrations were obtained at a distance of 1.2mbelow the intrusion. This suggests that the organic matter along the contact was gasified during the igneous event, and the expelled volatiles condensed in 'micro-reservoirs' a certain distance from the sill.
In contrast to the organic matter, clay minerals from the contact aureole reveal a lower degree of thermal metamorphism. The values of illite crystallinity (IC; up to 0.44° Δ2θ) and chlorite crystallinity (ChC; 0.30-0.34° Δ2θ) recorded in a narrow contact zone imply only minor elevation above the regional diagenetic background (IC ~ 0.60-0.70° Δ θ), and broadly evidence paleotemperatures in the range of 170-300 °C. The apparent discrepancy between the degree of thermal transformation of the organic matter and the clay minerals can be ascribed to the greater sensitivity of organic materials to geologically short metamorphic heating.
The results from computer thermal modeling of the sill do not match the empirical geological thermometers, as the model predicts substantially higher temperatures and wider contact zones. A reasonable fit between the two requires water-saturated magma that has cooled to about 600-700 °C. Given these preconditions, the modeling would predict maximum temperatures in the range 380-440 °C and contact aureoles exceeding 1-2 times the thickness of the sill. The thermal perturbation caused by the intrusion was short-lived and largely decayed into the diagenetic background within weeks to months after the emplacement.