GEOCHEMICAL AND RADIOGENIC ISOTOPE CHARACTERISTICS OF POST-COLLISIONAL K-RICH MAGMATISM OF THE WESTERN MEDITERRANEAN BASINS: THE CASE STUDIES OF TUSCANY, MURCIA-ALMERIA, CORSICA, AND WESTERN ALPS

Lamproitic rocks associated to shoshonitic and calc-alkalic ones are found in the Western Mediterranean. The oldest ones were intruded as dykes into the metamorphic rocks of the Western Alps during the Oligocene. The youngest ones were intruded and poured out in Southern Tuscany from Pliocene to Pleistocene. Shoshonitic and calc-alkalic rocks are invariably timely and spacely associated to lamproites. These lamproites are ultrapotassic rocks extremely enriched in MgO, but with low Al2O3, CaO, and Na2O. They are plagioclase-free, but K-feldspar is abundant beside other K-bearing phases. Lamproite-like rocks are also characterised by a strong enrichment in incompatible elements, which sometimes prevent further enrichment due to crustal contamination during ascent to surface. The sample studied are characterised by strong fractionation of HFSE with respect to LILE. Extreme enrichment in Th, U, and Pb with respect to Ba, Nb, Ta, Sr, P, and Ti is a common characteristic observed. Sr isotopic data of lamproite are extremely enriched in radiogenic compositions with initial 87Sr/86Sr in the ranges between 0.716350 and 0.717652 for Western Alps lamproites, between 0.716415 and 0.723595 for Spanish lamproites and between 0.71579 and 0.71672 for Tuscany lamproites. The Corsica lamproite shows the lowest values of initial 87Sr/86Sr (0.71229). Conversely Nd isotopes are extremely low with initial values in the ranges between 0.511990 and 0.512009 for Western Alps lamproites, between 0.511190 and 0.512140 for Spanish lamproites, and between 0.512086 and 0.512123 for Tuscany lamproites. Also in this case Corsica lamproite distinguishes clearly from others Western Mediterranean lamproites showing the highest value of initial 143Nd/144Nd (0.512148). No large differences are observed among the 207Pb/204Pb values of lamproites (15.629-15.715) from the different provinces, whereas significant differences are observed in the 206Pb/204Pb and 208Pb/204Pb values, with Tuscan Lamproites showing the lowest values (206Pb/204Pb = 18.653-18.677, 208Pb/204Pb = 38.855-39.026) and Western Alps the highest values (206Pb/204Pb = 18.684-18.904, 208Pb/204Pb = 39.029-39.485).