Pb and Hf isotope evidence for mantle enrichment processes and melt interactions in the lower crust and lithospheric mantle in Miocene orogenic volcanic rocks from Monte Arcuentu (Sardinia, Italy)

The most primitive Monte Arcuentu rocks (MgO >8.5 wt%) were sourced from a mantle wedge metasomatized by melts derived from terrigenous sediment, likely derived from Archean terranes of northern Africa. This gave rise to magmas with high Sr-87/Sr-86 (0.705-0.709) and Pb-207/Pb-204 (15.65-15.67) with moderate epsilon Hf (-1 to + 8) and epsilon Nd (-6 to + 1), but it does not account for the full range of compositions observed. More evolved rocks (MgO <8.5 wt%) have higher Sr-87/Sr-86 (up to 0.711) and Pb-207/Pb-204 (up to 15.68), with eHf and eNd as low as -8 and -9, respectively. Mixing calculations suggest that evolved rocks with low Rb/Ba and low Pb-206/Pb-204 interacted with lower crust similar compositionally to that exposed today in Calabria, Italy, which was formerly in crustal continuity with Sardinia. High Rb/Ba and high Pb-206/Pb-204 magmas interacted with lithospheric mantle similar to that sampled by Italian lamproites. Partial melting of lower crustal and upper mantle lithologies was facilitated by the rapid extension, and subsequent passive mantle upwelling, that occurred as Sardinia drifted away from the European plate during the Oligo-Miocene (ca. 32-15 Ma). Fractional crystallization under these PT conditions involved olivine + clinopyroxene with little or no plagioclase, such that differentiation proceeded without significant increase in SiO2. The Monte Arcuentu rocks provide insights into assimilation process in the lower crust and lithospheric mantle that may be obscured by upper crustal assimilation-fractional crystallization (AFC) processes in other orogenic suites.

Miocene (ca. 18 Ma) subduction-related basalts and basaltic andesites from Monte Arcuentu, southern Sardinia, Italy, show a remarkable correlation between Sr-87/Sr-86 (from similar to 0.705 to similar to 0.711) over a small range of SiO2 (similar to 51-58 wt%) that contrasts with most other orogenic volcanic suites worldwide. New high-precision Pb and Hf isotope data help to constrain the petrogenesis of these rocks.