Geochemistry and petrology of recycled orogen-derived sediments: a case study from Upper Precambrian siliciclastic rocks of the Central Iberian Zone, Iberian Massif, Spain

Abstract

Two areas of Upper Precambrian siliciclastic rocks separated from each other by 150 km were selected in the Central Iberian Zone (CIZ) for a petrological and geochemical study. The petrology of the coarser-grained rocks indicates the absence or scarcity of igneous clasts, the presence of extrabasinal and intrabasinal clasts and homogeneous features of sandstones ranging from very-fine to fine-grained. The modal data of these rocks plot close to the Qt-L and Qm-Lt sides of QtFL and QmFLt triangular diagrams, suggesting provenance from a recycled orogen. The geochemical data on the shales and sandstones from the two areas studied are very uniform. Both groups of rock show similar ranges of relevant major element ratios such as Al 2O 3 TiO 2 (17.46–21.51; 17.46–21.34, for shale and sandstone, respectively), Al 2O 3 FeO T (2.40–3.79; 2.18–3.33) and FeO T FeO T + MgO (0.71–0.77; 0.68–0.76), together with parallel average rare earth element (REE) patterns and similar values for some key trace element ratios ( La Th , Th Yb , Th U , Zr Hf , Sm Nd and Cr Sc ). Other trace element ratios ( La Sc , Th Sc , Cr Th and Cr Zr ) are uniform in the shales group and also in the sandstones group, although minor differences become apparent when both groups are compared. These differences are probably related to the quartz dilution effect affecting the relative abundance of phyllosilicates in these rocks and, indirectly, the Sc and Cr contents, given their covariations with Al 2O 3. The Zr Yb , Zr Y and Zr Th ratios of the shales are uniform; the sandstones show higher and more variable values, and there is no correlation between Zr abundance and any of these ratios in the shales or sandstones. Together with the parallel REE patterns shown by both groups of rock, these results suggest the absence of heavy mineral fractionation during the transport of detrital material and/or the relative importance of other minerals, probably phyllosilicates, in the control of Yb, Y and Th abundances. Apparently, all the groups of rocks show a remarkable geochemical homogeneity despite the distance between the sampling areas. Thus, the petrological and chemical data are consistent and point to a very extensive, homogeneous and stable source region as the most plausible contributor to the Upper Precambrian siliciclastic rocks in the CIZ. The possible relationship between this zone and N Gondwana is also discussed.