GEOCHEMISTRY AND PETROLOGY OF RECYCLED OROGEN-DERIVED SEDIMENTS - A CASE-STUDY FROM UPPER PRECAMBRIAN SILICICLASTIC ROCKS OF THE CENTRAL IBERIAN ZONE, IBERIAN MASSIF, SPAIN

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-grai ned rocks indicates the absence or scarcity of igneous clasts, the pre sence 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 QmFL t triangular diagrams, suggesting provenance from a recycled orogen. T he geochemical data on the shales and sandstones from the two areas st udied are very uniform. Both groups of rock show similar ranges of rel evant major element ratios such as Al2O3/TiO2 (17.46-21.51; 17.46-21.3 4, for shale and sandstone, respectively), Al2O3/FeOT (2.40-3.79; 2.18 -3.33) and FeOT/FeOT+Mg0 (0.71-0.77; 0.68-0.76), together with paralle l average rare earth element (REE) patterns and similar values for som e 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 unif orm in the shales group and also in the sandstones group, although min or differences become apparent when both groups are compared. These di fferences are probably related to the quartz dilution effect affecting the relative abundance of phyllosilicates in these rocks and, indirec tly, the Sc and Cr contents, given their covariations with Al2O3. 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 bet ween 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 duri ng the transport of detrital material and/or the relative importance o f other minerals, probably phyllosilicates, in the control of Yb, Y an d Th abundances. Apparently, all the groups of rocks show a remarkable geochemical homogeneity despite the distance between the sampling are as. 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. (C) 1997 Elsevier Science B.V.