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Kastens K A Mascle J et al 1990 Proceedings of the Ocean Drilling Program Scientific Results Vol 107 5 PETROLOGY AND GEOCHEMISTRY OF BASALTS FROM THE VAVILOV BASIN TYRRHENIAN SEA OCEAN DRILLING PROGRAM LEG 107 HOLES 651A AND 655B1 H Bertrand 2 P Boivin 3 and C Robin3 ABSTRACT During ODP Leg 107 two holes were drilled in the basement of Vavilov Basin a central oceanic area of the Tyrrhe nian sea Hole 655B is located near the Gortani ridge in off axis position at the western rim of the basin Hole 651A is located on a basement swell at the axis of the basin This paper deals with mineral chemistry major and trace element geochemistry and petrogenesis of the basalts recovered in the two holes The mineralogy of the basalts is broadly homogeneous but all of them have suffered important seawater alteration Their major element compositions are similar to both normal mid ocean ridge basalts N MORB and back arc basalts BAB except for Na 2 0 contents BAB like and K 2 0 which is somewhat enriched in upper unit of Hole 651 A Their affinity with N MORB and BAB is confirmed by using immobile trace elements such as Zr Y and Nb However basalts from the two sites present contrasting geochemical characteristics on spidergrams using incompatible elements Hole 655B basalts are homogeneous enriched tholeiites similar to those from DSDP Hole 373 located on the opposite side of the basin near the eastern rim and show affinities with enriched MORB E MORB At Hole 651 A the two basalt units are chemically distinct One sample recovered in lower unit is rather similar to those from Hole 655B but basalts from upper unit display calc alkaline characteristic evidenced by the increase of light ion lithophile element LILE high field strength element HFSE ratio and appearance of a negative Nb anomaly making them comparable with orogenic lavas from the adjacent Eolian arc The observed chemical compositions of the basalts are consistent with a derivation of the magmas from a N MORB type source

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