ABSTRACT

The Triassic to Lowermost Jurassic mudrocks from continental redbeds of the Longi-Taormina Unit (Calabria-Peloritani Arc; southern Italy) have been mineralogically, chemically and petrographically analyzed, in order to reveal their complex history, which record an important phase of the geological evolution of the Mediterranean area.
The Late Triassic corresponds with a low first-order sea level stand and a time of high continental emergence whereas the Early Jurassic was the time of the initiation of the first-order sea level rise in the mid-Mesozoic, generally marked by a transgressive trend.
The mineralogical assemblage, dominated by the occurrence of illite and illite/smectite mixed layers, coupled to the CIA index and to the A-CN-K plot, indicates post-depositional K-enrichments.
Palaeoweathering indices (CIW and PIA ratios) suggest that the source experienced intense weathering and that they likely record a recycling effect from their metasedimentary basement rocks. A recycling effect is also suggested by the distribution of Al₂O₃, TiO₂, and Zr. In the Zr/Sc vs. the Th/Sc plot the redbeds are not clustered along the primary compositional trend but fall along a trend involving zircon addition and thus sediment recycling. Recycling could significantly affect the weathering indices which likely monitor a cumulative effect including a first cycle of weathering at the source rocks. Weathering occurred under hot, episodically humid climate with a prolonged dry season. Wet-humid conditions favored the formation of stream channels that eroded the soil profiles, whereas the dry season promoted the sedimentation. The climate alternation in the Early Jurassic favored recycling.
An additional provenance terrane occurring in the basement, including metavolcanic rocks having mafic composition cannot be excluded, since the Eu anomaly is slightly higher than the PAAS value. Although the effects of recycling on REE distribution are uncertain, the Eu/Eu^* should increase, as more feldspar is destroyed during weathering and diagenesis. This involves that the Eu/Eu^* ratio could monitor a supply of low Eu/Eu^* mafic detritus which compensate for the recycling effect by reducing Eu/Eu^*. Eu released during the dissolution of feldspar could be retained by clay minerals contributing to minimize the recycling induced increase of the Eu-anomaly. This may involve that the recycling effect on the Eu-anomaly was minor and that the low Eu/Eu^* mafic detritus supply was also minor. The subordinate importance of a mafic supply is confirmed also by other provenance proxies including the La-Th-Sc plot and the Cr/V and Y/Ni ratios.
The proportions of illitic layers in I/S mixed layers coupled with the illite crystallinity values, expressed as Kübler Index (KI), suggesting an estimated temperature experienced by the Longi Units in the range of 100-150°C. Starting from this range the diagenetic/tectonic evolution should correspond to a lithostatic/tectonic loading of about 4-5 km.