Reconstructing chemical weathering during the Lower Mesozoic in the Western-Central Mediterranean area: A review of geochemical proxies

The Triassic-Jurassic rift-valley stage of Tethyan rifting in the Western-Central Mediterranean area is characterized by a development of a puzzle of plates and microplates with the deposition of continental redbeds (in the internal domains of the Gibraltar Arc and Calabria-Peloritani Arc) that can be considered a regional lithosome. This paper aims to reconstruct the chemical weathering conditions of the Triassic-Jurassic boundary in the Western-Central Mediterranean area using the geochemical and mineralogical composition of continental redbed mudrocks of Mesozoic age. The mudrocks from the Calabria-Peloritani Arc show higher values of weathering (mobility) indices (α Mg=(Al/Mg)sed/(Al/Mg)UCC; α K=(Th/K)sed/(Th/K)UCC; α Ba=(Th/Ba)sed/(Th/Ba)UCC) than the Gibraltar Arc samples. Furthermore, the CIA (Chemical Index of Alteration) and MIA (Mineralogical Index of Alteration) values and the 'Rb-type indices' (e.g. Rb/Sr and Rb/K ratios) are higher for the Calabria-Peloritani Arc mudrocks than the Gibraltar Arc samples. All these geochemical proxies closely resemble each other and show similar variations suggesting climatic changes towards humid conditions through the Uppermost Triassic to Lowermost Jurassic that favoured chemical weathering conditions. This period is probably characterized by seasonal climate alternations corresponding to an increase in palaeoclimatic humidity. The mineralogical compositions of the Mesozoic mudrocks further confirm these indications as shown by a higher abundance of kaolinite, related to warm-humid conditions, in the Calabria-Peloritani Arc mudrocks than in those of the Gibraltar Arc.