Pb-Sr-Nd isotopic behavior of deeply subducted crustal rocks from the Dora Maira Massif, Western Alps, Italy

Abstract

Pb, Nd and Sr isotope data are reported on mineral separates and whole rock samples of Mg-rich metapelites and associated rocks that have been subducted to depths of ca. 100 km in the mantle within the last 100 Ma and returned to the surface, and for which metamorphic conditions were estimated to be 700–800°C at > 28 kilobars pressure. Three rock types, fine- and coarse-grained pyrope quartzites and NaFe-rich jadeite-kyanite quartz layers, were collected from a single outcrop in the coesite-bearing portion of the Dora Maira massif. Samples analyzed include pyrope, pseudomorphs after pyrope, phengite, zircon, the new mineral ellenbergerite, and whole rocks. Isotope data from all three decay systems indicate minimal, if any, isotopic exchange with mantle rocks. Isotope ratios from minerals with low parent/daughter ratios cluster around 143Nd/ 144Nd = 0.51225: ϵ(Nd) = −7.6; 87Sr/ 87/Sr = 0.7 = 15.75–15.80 with 206Pb/ 204Pb = 19.4–20.7. These compare closely with ratios from Precambrian granitic rocks or their derived sediments. From the Pb isotope data and the Sm/Nd whole rock model ages we estimate a mean age of 2000 ± 500 Ma for the sediment protoliths. Pyrope mineral separates record approximate ages for Alpine metamorphism of 34–38 Ma by the Sm/Nd method; the U/Pb method applied to zircon and ellenbergerite yields possible additional metamorphic ages of 50–55 Ma. These data illustrate a possible mechanism for creating isotopically enriched mantle. Had these rocks yielded their isotopes to the mantle, “enriched mantle” could have resulted, the magnitude of the enrichment depending upon the total volume attained by the mixing process. Similarly the metasediments could also provide the high potassium abundances and appropriate Pb, Sr and Nd isotope ratios that are postulated to accompany mantle metasomatic processes.