A WINDOW INTO LOWER CRUST AND UPPER MANTLE INTERACTIONS IN THE PETROGENESIS OF OROGENIC MAGMAS: THE EXAMPLE OF MONTE ARCUENTU, SOUTHERN SARDINIA, ITALY

Isotope and trace element variations in Miocene (ca. 18Ma) subduction-related basalts and basaltic andesites from Monte Arcuentu (MA), southern Sardinia, provide insights into the nature of deep crust/upper mantle interactions without the geochemical overprinting of mid- to upper-crustal AFC processes that affect most other orogenic suites. The rocks show a remarkable correlation between ⁸⁷Sr/⁸⁶Sr (up to 0.711) and SiO₂ over a small range in SiO₂ values (51-56 wt%), which has been interpreted as evidence for minimal mid- to upper crustal contamination. Metasomatism of the mantle source by melts derived from terrigenous sediment, likely sourced from Archean terranes of northern Africa, explains the compositional variation of the most primitive MA rocks (MgO >8.5 wt%), i.e. high ⁸⁷Sr/⁸⁶Sr (0.705-0.709) and ²⁰⁷Pb/²⁰⁴Pb (15.65-15.67) with moderate εHf (-1 to +8) and εNd (-6 to +1). But source contamination alone cannot explain the full spectrum of compositional variations observed: more evolved MA rocks (MgO <8.5 wt%) have higher ⁸⁷Sr/⁸⁶Sr (up to 0.711) and ²⁰⁷Pb/²⁰⁴Pb (up to 15.68), with εHf and εNd as low as -8 and -9, respectively, and require an additional component in their petrogenesis. Mixing calculations suggest evolved rocks with low Rb/Ba and low ²⁰⁶Pb/²⁰⁴Pb interacted with lower crust compositionally similar to that exposed today in Calabria, which was formerly in crustal continuity with Sardinia. High Rb/Ba and high ²⁰⁶Pb/²⁰⁴Pb magmas interacted with lithospheric mantle similar to that sampled by Italian lamproites. Partial melting of these normally refractory lower crustal and upper mantle lithologies was facilitated by the rapid extension, and subsequent mantle upwelling, that occurred as Sardinia drifted away from the European plate during the Oligo-Miocene (32-15 Ma). Fractional crystallization under these PT conditions involved ol + cpx with little or no plag, such that differentiation proceeded without significant increase in SiO₂. Thus, in contrast to most other orogenic settings, Monte Arcuentu parental magmas stalled at the base of the crust or at the top of the lithospheric mantle on their way to the surface where they underwent MASH-type processes. From there, they escaped to the surface without significant interaction with shallower crust.