Paleoclimatic conditions and paleoweathering processes on Mesozoic continental redbeds fromWestern-Central Mediterranean Alpine Chains

摘要

Chemical and mineralogical analyses of the Triassic to lowermost Jurassic mudstones from continental redbeds outcropping in the Internal Domains of the Betic–Rifian and Calabria–Peloritani chains have been used to infer the relationships between paleoclimatic conditions and paleoweathering processes during rifting of a continental crust block that finally detached from adjoining western Tethyan realms to form an independent microplate (Mesomediterranean Microplate) from Jurassic to lower Miocene time. The studied mudstone samples come from Middle Triassic and Upper Triassic beds of the Saladilla Formation (both in the Betic Cordillera and in the Rif), whereas the Calabria–Peloritani Arc studied mudstones come from Upper Triassic to lowermost Jurassic beds (both in the Sila and Longi Taormina Units). Major and trace element concentrations, based on the massbalance calculation relative to the upper continental crust, show negative values both in Gibraltar and Calabria–Peloritani Arcs, implying that mudstone formation in the EarlyMesozoic involved moderate to intense continental paleoweathering of the crust. In particular, CaO, Na_2O, MgO, Sr, Ba, Fe_2O_3, MnO and transition metal elements (V, Cr, Co and Ni) aremore depleted in the Triassic to Upper Jurassic samples of the Calabria–Peloritani samples than in the Middle to Upper Triassic Betic–Rifian samples, and suggest an increase of continental paleoweathering in the Mediterranean region from the Triassic to the Jurassic. In addition enrichment in SiO_2, TiO_2, Al_2O_3, K_2O and incompatible elements in the Calabria–Peloritani Arc mudstones indicates sediment recycling effects that gradually increase from the Triassic to Jurassic time. The hinterland paleoweathering and sediment recycling effects have been mathematically distinguished using principal component analysis (PC1 is a measure of paleoweathering rate mainly due to humidity (positive values) against aridity (negative values), whereas PC2 corresponds to the extent of sediment recycling). The results strongly indicate that humidity had increased from the Triassic to the Jurassic and that the depositional environments in Calabria–Peloritani Arc were probably more suitable for sediment recycling. These seasonal climate alternations corresponding to an increase in paleoclimatic humidity that favored paleoweathering conditions and recycling processes. These results are also confirmed by the mineralogical data, which show a higher abundance of kaolinite, typical of warm–humid conditions, in the Calabria-Peloritani mudstones than in those of the Betic Cordillera and the Rif. Furthermore, the comparison among geochemicalweathering index values of the studied samples and of recent soils from different climatic zones likely suggests a tropical rainforest climate in the studied area during the Triassic to the lowermost Jurassic.