Incipient metamorphism of Cambro-Silurian clastic rocks from the Jåmtland Supergroup, Central Scandinavian Caledonides, Western Sweden: illite crystallinity and 'vitrinite' reflectance

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Abstract
The degree of incipient metamorphism in the Cambro-Silurian of the Jåmtland Supergroup (Eastern Complex) of the Jämtland Caledonides, from the Caledonian front autochthon westwards through the parautochthon towards the thrust front of the metamorphic main ('Western Complex') nappes, has been studied using: ( a ) determination of the illite crystallinity by X-ray powder diffraction on the K- and Mg-saturated –2 μm fractions; the low-grade limits of the anchizone and the 'opizone' of Kubler correspond to peak widths of c. 0.38° and 0.21°Δ2 (illite 001 diffraction peak) under the experimental conditions used; ( b ) determination of the rank of coaly particles in the clastic rocks by measurement of the 'vitrinite' reflectance using a reflected-light microscope with photomultipliers. In the Indalsälven segment S of Iat. 63°30' N between Brunflo in the E and Undersåker in the W, and further N to Rönnöfors-Juvuln, 4 zones of illite crystallinity may be distinguished: (A) The Östersund-Krokom zone: mean 'saturated' 10 Å peak widths of ∅0.32°Δ2θ, the highest peak widths occurring in the easternmost part of the zone ('diagenetic' zone of Kubler). (B) The Norderö-Näldsjön zone: mean peak widths between 0.27° and 0.35° Δ2θ middle-anchizone of Kubler. (C) The Mattmar–Alsensjön zone: predominance of peak widths between 0.19° and 0.25° 2θ. (D) The Mörsil–Jårpen zone; predominance of peak widths of 0.21° Δ2θ characteristic of the 'epizone' of Kubler. This zone extends N to Rönnöfors–Juvuln, W of the Offerdal nappe. 'Vitrinite' reflectance values were measured on 35 samples. In illite-crystallinity zone A the predominant mean maximum reflectance ranges from 1.4-2.9% R max oil , corresponding to low-volatile bituminous coal to low-rank anthracite. In zones B, C and D the ranges of R max oil are similar, from 3.7 to 4.3%, corresponding to high-rank anthracite. Similarly high reflectances (4.2-4.5% R max oil ) were found in 3 samples from the northern continuation of zone D. The comparative uniformity of the reflectances from zones B–D contrasts with the increase in illite crystallinities, particularly from zone B to C. Anatase was found in demineralization residues from zones A and B (together with rutile); in the residues from zones C and D rutile is the only titanium oxide. The results are consistent with the hypothesis that the incipient metamorphism in the Jåmtland Supergroup was at least in part caused by the overthrust metamorphic allochthon, particularly the mainly medium- and locally medium to high-grade metamorphic Seve nappe. The association of the onset of the anchizone with semi-anthracitic to low-rank anthracitic coal ranks (c. 2.5–2.8% R max oil ) is in agreement with the relationships found in several other areas of incipient regional metamorphism.