New standardized illite crystallinity data from low- to very-low grade metamorphic rocks (Northern Apennines, Italy)

Abstract

The present study is aimed at testing the effectiveness of the Warr & Rice (1994) Crystallinity Index Standard (CIS) scale as a potential indicator of metamorphic grade. The test was carried out on rock samples belonging to well-defined metamorphic facies charaeterized by known metamorphic P-T conditions. The selected rocks come from various tectonic units of the Northern Apennines and had been the object of earlier studies on both their metamorphic conditions and illite crystallinity. All the IC data collected on the rocks of these units had been previously standardized with respect to the standard rocks slab series (32, 34 and 35) provided by Kubler. Some units belong to the Internal Liguride Domain (ILU), while others come from the Tuscan Domain (TDU). The Internal Liguride Units yielded average IC values standardized to the CIS scale of 0.29 (± 0.04) °Δ2𝛉 in the epidote blueschist facies (Cravasco/Voltaggio U.), 0.34 (± 0.06) °Δ2𝛉 in the pumpellyite-actinolite facies (Mt. Figogna U.), and 0.53 (± 0.07) °Δ2𝛉 in the prehnite-pumpellyite facies (Bracco/Val Graveglia U.). In the TDU average CIS scale-standardized IC values vary from 0.27 ± 0.03 to 0.30 ± 0.03 °Δ2𝛉 in rocks belonging to the metamorphic zone defined by Qz + Py association, from 0.33 ± 0.03 to 0.37 ± 0.03 °Δ2𝛉 in rocks of the Qz + Py + Ka zone, and from 0.40 ± 0.02 to 0.42 ± 0.03 °Δ2𝛉 in the rocks of the Qz + Ka zone. Although the standardized IC values may be considered consistent with the overall metamorphic conditions estimated in both the ILU and TDU, in general, they appear too high when compared with the temperatures estimated in the TDU by the calcite-dolomite geothermometer, which yielded maximum temperatures of 370–380°C in the Qz + Py zone, 300–312°C in the Qz + Py + Ka zone, and 260–280°C in the Qz + Ka zone. IC values previously normalized to Kubler9s standards (about 0.04 °Δ2𝛉 lower) also conform to the metamorphic facies conditions in all rocks selected from both the ILU and TDU. Although IC data and temperatures are only roughly related, and much caution must therefore be exercised in correlating these parameters, in general, Kubler9s standardized IC data from TDU also seems to exhibit good overall agreement with the thermal conditions.