Geochemistry of mudrock units from the Meso-Cenozoic Algarve Basin, Portugal

摘要

Geochemistry of mudrock units deposited in the sedimentary basin of Algarve (South Portugal) during Mesozoic and Cenozoic depositional cycles were studied, aiming a better knowledge of the evolution of the Basin. Two types of mudrock units were found: (i) silicic and Ca-poor (Cretaceous and Cenozoic), and (ii) calcareous (Middle-Upper Jurassic Ca-rich mudrock units and Upper Triassic to Lower Jurassic Mg-rich redbeds). LILE appear to be controlled by a K-bearing phyllosilicate, which is consistent with the abundant illite in the redbeds. Positive trends of: (i) Sc, Cr, Th, LREE with Al2O3 suggests their incorporation in clay minerals, and (ii) HREE with TiO2, Zr and Hf point to their presence in heavy minerals. The source area of the Algarve mudrock units was dominated by felsic rocks; features typical of a mafic source occur in some Upper Triassic to Lower Jurassic samples. Carboniferous metasediments of the basement were probably the first order source for the Algarve sedimentary basin. A more severe weathering in the Cenozoic and Cretaceous mudrock units occurred, whereas diagenetic enrichment in potassium is observed in the Upper Triassic to Lower Jurassic continental redbeds.