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 Na20 contents (BAB-like), and K20 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 progressively contaminated by LILE-enriched fluids released from dehydration of the bordering subducted plate. Implications for evolution of the Tyrrhenian basin are tentatively proposed taking into consideration geochemical and chronological relationships between basalts from Leg 107 Holes 655B and 651 A, together with data from Leg 42 Site 373 and Vavilov Seamount. These data illustrate back-arc spreading in ensialic basin closely associated with the maturation of the adjacent subduction, followed by the growth of late off-axis central volcano, whereas the active sub-duction retreats southeastward. INTRODUCTION Geological Setting Developed in a general collisional context, the Tyrrhenian Sea is a small triangular sea surrounded by the Sardinian conti-nental margin, the Eolian volcanic arc, and the Italian Penin-sula (Fig. 1). Its central basin is considered to be back-arc basin (Boccaletti and Guazzone, 1972; Barberi et al., 1973, 1978) that has opened behind a still active subduction zone. Subduction is evidenced by northwestward deepening of seismic foci along the Benioff plane (Gasparini et al., 1982) and by the orogenic vol-canism of Eolian islands (Barberi et al., 1974; Keller, 1974; Bec-caluva et al., 1985). Evolution of the back-arc spreading is pre-sumably linked to the southeastward migration of the Cala-brian-Sicilian arc system, probably in response to complexities in subduction of the Ionian lithosphere (Moussat, 1983; Mous-sat et al., 1985; Kastens et al., 1986; Rehault et al., 1987). The main rifting phase started during the Tortonian and led, around the early Pliocene, to the creation of a narrow central oceanic area subdivided into two basins, the Vavilov Basin and the Mar-sili Basin (Rehault et al., 1987). These two basins are thought to be diachronous, possibly as an effect of the retreat toward the southeast of the subduction zone (Kastens et al., 1986). Geo-1 Kastens, K. A., Mascle, J., et al., 1990. Proc. ODP, Sci. Results, 107: Col-lege Station, TX (Ocean Drilling Program). 2 Laboratoire de Petrologic, Unite Associee no. 726 C.N.R.S.—E.N.S. Lyon, 46 allee d'Italie, 69364 Lyon Cedex, France. 3 Unite Associee no. 10 C.N.R.S.—Universite, et Centre de Recherches Volcanologiques (CRV), Universite Clermont II, 5, rue Kessler, 63038 Clermont-Ferrand Cedex, France. physical studies indicate that the basement of the basins is oce-anic-type lithosphere (Steinmetz et al., 1983; Recq et al., 1984; Delia Vedova et al., 1984; Hutchison, 1983). These data are cor-roborated by petrological and geochemical investigations made on basalts recovered at DSDP Site 373, which show an oceanic tholeiite affinity (Dietrich et al., 1977; Barberi et al., 1978), with some "transitional" characteristics (Hamelin et al., 1979). Moreover, in each basin, the abyssal plain is dominated by a vo-luminous central volcano elongated parallel to the axis of the basin. They are Vavilov and Marsili seamounts. Lavas from Vavilov have been described as tholeiitic/transitional basalts (Selli et al., 1977; Beccaluva et al., 1982), to mildly alkaline ba-salts independent of the surrounding oceanic crust (Robin et al., 1987). During Leg 107, two holes were drilled in the basement of Vavilov Basin. Hole 651A is located on a ridge in the axis of the basin, in a water depth of 3578 m. This ridge extends on strike of Vavilov Seamount, and is considered as an embryonic center of accretion (Kastens et al., 1986). Hole 655B is located on the north-south Gortani ridge (water depth 3290 m) which lies in an off-axis position to the Sardinian margin, near the transition between oceanic and continental lithosphere. This site is nearly symmetrical to Hole 373 of Leg 42 with respect to the axis of the basin. Objectives The first aim of this study is to characterize and compare the petrology and geochemistry of lavas recovered in Holes 651A and 655B, and to compare them with other lavas from Hole 373 and Vavilov Seamount. These combined data are used to exam-ine the problem of space-time relationships of magma genesis within Vavilov Basin. 75H. BERTRAND, P. BOIVIN, C. ROBIN 43°N-Figure 1. Location map of drill sites in basement of the Vavilov Basin: ODP Leg 107 (Holes 651A and 655B); DSDP Leg 42 (Site 373). Stippled area corresponds to central oceanic type basins. The second goal is to compare the composition of these rocks with those erupted in rather similar tectonic environ-ments, i.e., well