Rare earth element distribution in Jurassic siliceous rocks from Northern Apennines (Italy)

Data on REE, Rb, Ba, Th, Hf, Ta, Cr, Sc and Co are reported on radiolarites from Jurassic chert formations of the Ligurian and Tuscan sequences in the Northern Apennines. The samples from the Tuscan sequence shov a more homogeneous trace element composition relative to Ligurian rocks which display a rather wide scatter of trace element values. The chondrite normalized REE patterns of all the analyzed samples exhibit fractionated LREE, a significant negative Eu anomaly, a light positive Sm anomaly and no Ce anomalies. The high Sm contents are better evidenced by the REE patterns normalized against the North American Shale values (NAS). The radiolarites of the Tuscan sequence are also characterized by fractionated HREE patterns with TbN/YbN values ranging between 1.5-2.6. Comparison of the data with those reported by several authors on radiolarites from different areas demonstrates comparable REE distribution patterns in all these rocks. A major difference, however exists between the Northern Apennines samples and radiolarites coming from the Pacific sea floor essentially as regards the Ce distribution. This element, in fact, display a sharp negative anomaly in the Pacific samples which has been interpreted as indicative of an «open sea» formation and depositional environment of the biogenic silicic material. The lack of negative Ce anomalies in the REE patterns of the analyzed Northern Apennines radiolarites is interpreted as suggesting deposition in a relatively small basin in which the REE composition of sea water was heavely modelled by the input of the river waters coming from the surrounding continental masses.