TI/NB RATIOS OF CLASTIC TERRIGENOUS SEDIMENTS USED AS AN INDICATOR OF PROVENANCE

Geochemical data have been compiled for the clastic sedimentary rocks of late Proterozoic to Carboniferous age in the Armorican Massif (northwest France). These data show a great similarity in the distribution of Ti and Nb in most sediments due to an association of these elements during geochemical processes. The Ti/Nb ratios remain close to 300, a result which is interpreted as being an inheritance from the source areas. However, recent published works have indicated that Ti and Nb are not fractionated by either weathering or diagenetic phenomena. In contrast to all the other formations studied, the basal Carboniferous clastic rocks are different in having abnormally low Ti/Nb ratios (< 100). The origin of this anomaly is to be sought in the widespread intra-plate volcanism developed during the Devonian and Early Carboniferous. This phase of magmatic activity is characterized by low Ti/Nb ratios in acid differentiates. Compilation of geochemical data on recent volcanic suites shows that Ti/Nb ratios are a possible indicator of mantle supply in clastic rocks.