CAN REE PATTERNS AND U-TH VARIATIONS BE USED AS A TOOL TO DETERMINE THE ORIGIN OF APATITE IN ELASTIC ROCKS

Apatites of SE Germany from late Palaeozoic and Triassic red bed elastic rocks (environments: alluvial fan, fluvial, lacustrine, with volcaniclastic intercalations) were investigated for their rare earths and U-Th contents, including their chondrite-normalized REE-patterns, Eu-Ce anomalies, LREE/HREE fractionation, U-Th distribution and U-Pb ages. For provenance purposes, apatite compositions were compared with published data from granites, volcanics, phosphorites and gneisses of the NE Bavarian basement. The applicability of REE contents of apatite to stratigraphic questions is restricted to areas and rock successions within the same heavy mineral province. The strong points of this method are in the fields of provenance studies (extrabasinal, intrabasinal) and, to a lesser degree, investigations of diagenetic alteration as demonstrated by the following examples. Apatites from Permian red beds of the Weiden embayment were probably derived from late Variscan granites in the adjacent basement on the basis of U-Pb age dating. The REE patterns and U-Th ratios of apatites from upper Stephanian rocks from Stockheim and from Autunian rocks of Erbendorf contrast conspicuously with this granite-derived apatite and probably originated from late Palaeozoic volcanic activity that generated volcaniclastic rocks. Despite their different ages of formation and depositional environments, diagenetic remobilisation gave rise to similar REE patterns and U-Th ratios in both Upper Triassic apicretes (= duricrusts composed of apatites) and Silurian marine phosphorites. The organic matter that surrounds the P-bearing strata has been responsible for the reducing conditions in both of these host environments and for the striking similarities of their REE data.