Coal rank and illite crystallinity associated with the zeolite facies of Southland and the pumpellyite-bearing facies of Otago, southern New Zealand

Coal rank (vitrinite reflectance) and clay mineralogy from the North Range Group of the Southland Syncline, and illite crystallinity in pumpellyite- and lawsonite-bearing terranes in the textural zone I of the Caples and Torlesse terranes were studied. Vitrinite reflectances range from 0.60 to 1.33% R_{m oil} (high-volatile C to medium-volatile bituminous coal ranks) in 31 out of 34 samples studied; in the lower 2 units of the North Range Group the reflectances are higher than 0.81% R_{m oil} (high-volatile A and medium-volatile bituminous ranks). This conforms to earlier extrapolations of at least low-volatile bituminous rank expected to be associated with the high-grade limit of the zeolite facies and the onset of the prehnite-pumpellyite facies. The illite 10 Å peak widths (illite crystallinity) in many of the samples can be measured only after removal of the 10-14 Å illite-smectite mixed-layer plateau through EG-saturation; the 10 Å peak widths in the North Range samples are believed to largely reflect clastic micas. Highest grade “diagenetic” illite 10 Å peak widths (B_Mg and B_K mainly 0.36-0.72° Δ2θ; B_EG mainly 0.37-0.55° Δ2θ) are found in the laumontite-bearing Benmore Dam area in the Torlesse terrane. Predominantly low grade “anchimetamorphic” illite crystallinities (B_Mg mainly 0.30-0.45° Δ2θ; B_K and B_EG mainly 0.22-0.37° Δ2θ) occur in association with the prehnite-pumpellyite facies in the Tuapeka Group (Caples-Pelorus terrane) of South Otago, whereas mainly “epizonal” illite 10 Å peak widths (<0.21° Δ2θ) seem to predominate somewhat before the appearance of unequivocal pumpellyite-actinolite facies mineral assemblages, that is, in the highgrade part of textural zone I. In 1 area in North Otago, “epizonal” peak widths appear close to the I-IIA isotect, already within the pumpellyiteactinolite facies. In the Caples Group at Routeburn, western Otago, “epizonal” illite 10 Å peak widths, accompanied by paragonite, were found in the lawsonite-bearing zone within the higher grade part of textural zone I. The results are compared with earlier correlations of lowest grade mineral facies with coal rank and illite crystallinity, particularly in the Taveyanne Greywacke in the Helvetic zone of the Swiss Alps. It is suggested that measurement of the illite 10 Å peak width may be a useful indicator of degree of incipient metamorphism in clastic rocks, at least up to the equivalent of textural zone II, that is, in a metamorphic range that includes the highest grade part of the laumontite zone, the prehnitepumpellyite facies, and possibly the low-grade part of the pumpellyite-actinolite facies.