Evolution of the Sardinia Channel (Western Mediterranean): new constraintsfrom a diving survey on Cornacya seamount off SE Sardinia

Sarcya 1 dive explored a previously unknown 12 My old submerged volcano, la belled Cornacya. A well developed fracturation is characterised by the foll owing directions: N 170 to N-S, N 20 to N 40, N 90 to N 120, N 50 to N 70, which corresponds to the fracturation pattern of the Sardinian margin. The sampled lavas exhibit features of shoshonitic suites of intermediate co mposition and include amphibole- and mica-bearing lamprophyric xenoliths wh ich are geochemically similar to Ti-poor lamproites. Mica compositions refl ect chemical exchanges between the lamprophyre and its shoshonitic host roc k suggesting their simultaneous emplacement. Nd compositions of the Cornacy a K-rich suite indicate that continental crust was largely involved in the genesis of these rocks. The spatial association of the lamprophyre with the shoshonitic rocks is geochemically similar to K-rich and TiO2-poor igneous suites, emplaced in post-collisional settings. Among shoshonitic rocks, sample SAR 1-01 has been dated at 12.6 +/- 0.3 My using the Ar-40/Ar-39 method with a laser microprobe on single grains. The age of the Cornacya shoshonitic suite is similar to that of the Sisco lampr ophyre from Corsica, which similarly is located on the western margin of th e Tyrrhenian Sea. Thus, the Cornacya shoshonitic rocks and their lamprophyr ic xenolith and the Sisco lamprophyre could represent post-collisional suit es emplaced during the lithospheric extension of the Corsica-Sardinia block , just after its rotation and before the Tyrrhenian sea opening. Drilling on the Sardinia margin (ODP Leg 107) shows that the upper levels o f the present day margin (Hole 654) suffered tectonic subsidence before the lower part (Hole 652). The structure of this lower part is interpreted as the result of an eastward migration of the extension during Late Miocene an d Early Pliocene times. Data of Cornacya volcano are in good agreement with this model and provide good chronological constraints for the beginning of the phenomenon. (C) 2001 Elsevier Science B.V. All rights reserved.