Aljustrel Mining Research Articles

Sr isotopic homogenization through whole-rock systems under low-greenschist facies metamorphism in Carboniferous pyroclastics at Aljustrel (southern Portugal)

The Megacryst Tuff Formation at the Aljustrel mining centre in southern Portugal belongs to the Volcanic-Siliceous Complex, the pyritite-bearing sequence of low-grade meta-morphic volcanic tuffs and sediments in the Iberian Pyrite Belt of the southwestern Iberian peninsula. On fossil evidence the complex is accurately placed as post-Famennian and pre-Late Viséan. Ten whole-rock samples from the Megacryst Tuff Formation yield a RbSr isochron of 315 ± 7 Ma with initial 87 Sr 86 Sr = 0.7135 ± 0.0007 ( λ 87Rb = 1.39 ·10 −11 a −1; errors at 95% confidence level). The isotopic age is too young with respect to the biostratigraphic age (estimated at around 350 Ma), so it must reflect a time of Sr isotopic homogenization through a sequence of volcanic tuffs. This resetting of the wholerock RbSr isochron took place under conditions of metamorphism as low as the low-greenschist facies. It is discussed that such a resetting is a common phenomenon in waterlaid pyroclastic deposits.