PROVENANCE OF LOWER CRETACEOUS DEPOSITS OF THE WESTERN PART OF THE SILESIAN NAPPE IN POLAND (OUTER CARPATHIANS): EVIDENCE FROM GEOCHEMISTRY

摘要

The turbiditic to hemipelagic, fine-grained deposits of the Hradi?tě Formation (Hauterivian, 132 Ma) to the Lhoty Formation (Albian–Cenomanian, 99 Ma) in the western part of the Silesian Nappe (Polish Outer Car- pathians) were studied mineralogically and geochemically to determine if the main factors controlling the che- mistry of the sedimentary material can be attributed to provenance, or to post-depositional processes. A high degree of weathering of the source rocks is indicated by the chemical index of alteration (CIA) that varies from 75.98 to 89.86, and Th/U ratios (~4 with outliers at 1.85 and 6). The co-occurrence of rounded and unabraded grains of zircon and rutile, the enrichment in Zr and Hf, as well as the high Zr/Sc ratios suggest that the Hradi?tě and Ve?ovice Formations contain recycled material. Plots of La/Th versus Hf and Th against Sc show that samples occur in the field of felsic and mixed felsic/basic sources. On a ternary La–Th–Sc diagram, all of the sediments studied are referable to the continental island-arc field. The European Plate, as an alimentary area, has a mosaic structure consisting of Cadomian and Variscan elements. The Proto-Silesian Ridge was detached from the conti- nent, because of rifting. Therefore, it could have corresponded to a continental island arc. The concentrations of Fe and trace metals (e.g., Mo, Au, Cu) in the Ve?ovice Formation and silica and potassium additions to the Ve?ovice and Lhoty Formations, as well as the fractionation of REE, and Nb, Ta, Zr, Hf, and Y can be explained by the action of basinal brines. The fluids were of hydrothermal origin and/or were released, owing to the dewatering of clay minerals. Diagenetic processes could have exerted a greater influence on sedimentary rock chemistry than the provenance and sedimentary processes. A distinction between primary, terrigenous elements and those changed diagenetically is necessary for the reliable determination of provenance.