Petrology and Geochemistry of Basalts from the Vavilov Basin (Tyrrhenian Sea), Ocean Drilling Program Leg 107, Holes 651A and 655B

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 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.

Seismic structure of the Central Tyrrhenian basin: Geophysical constraints on the nature of the main crustal domains

“…In domain 3 the velocity‐depth profiles that match best clearly correspond to that of partially serpentinized peridotite (Figures and b), with the exception of two narrow segments just below the Gortani Ridge and the Vavilov seamount, which are better explained by an oceanic crustal affinity (Figures a and b). These two seamounts have been previously described as basaltic edifices [ Robin et al ., ; Beccaluva et al ., ; Bertrand et al ., ; Savelli , ], possibly intruding a basement made of serpentinized mantle rocks.…”

“…[53] Additionally, there is some evidence of igneous rocks in the basin that include the E-MOR basalts drilled on top of the Gortani Ridge (ODP site 655) and at the basement of the Vavilov basin (ODP site 651) (Figures 1b, 4, and 6) [Beccaluva et al, 1990;Bertrand et al, 1990;Savelli, 2002]. The Gortani Ridge is intersected by the tomographic model (between OBH79 and 80 in Figures 1b and 3a) and consequently imaged in depth (at ~300 km in Figure 3a).…”

“…[59] We interpret anomaly r3 (Figure 10b) as the root of the Gortani Ridge. This ridge is composed by E-MOR basalts (ODP site 655) dated at 4.3 Ma [Beccaluva et al, 1990;Bertrand et al, 1990;Savelli, 2002], being the oldest magmatic body imaged in the central basin but younger than the opening of the Vavilov basin where it intrudes.…”

“…Ground truthing of the nature of the basement was achieved by Deep Sea Drilling Project (DSDP) Leg 42A and Ocean Drilling Program (ODP) Leg 107 (Figure 1b). Drilling found enriched mid-ocean ridge (E-MOR) basalts at the top of the Gortani Ridge (ODP site 655 in Figure 1b) and in the Vavilov basin (ODP site 651 and DSDP site 373 in Figure 1b) [Barberi et al, 1978;Dietrich et al, 1977;Beccaluva et al, 1990;Bertrand et al, 1990]. Drilling of the top of the basement under about 1 km of sediment of the Vavilov basin found few tens of meters of basalt, but deeper drilling found serpentinized peridotites (ODP site 651 in Figure 1b) [Bonatti et al, 1990;Kastens and Mascle, 1990].…”

Expedition 402 Scientific Prospectus: Tyrrhenian Continent–Ocean Transition

“…All three sites recovered basalts with major element compositions similar to MORB and an enriched trace element pattern typical of enriched MORB (E-MORB) (Beccaluva et al, 1990). MORB at Site 651 is overlain by a basaltic unit with a back-arc basin (BAB)-like signature (Bertrand et al, 1990). The upper basaltic unit at Site 651 has major calc-alkaline affinity and shows enrichment in large ion lithophile element (LILE) and negative anomalies of high field strength elements (HFSE) (particularly Nb) (Bertrand et al, 1990).…”