Petrology, mineral and isotope geochemistry of the Sondalo gabbroic complex (Central Alps, Northern Italy): implications for the origin of post-Variscan magmatism

The origin of the Sondalo gabbroic complex has been unravelled by means of a petrological study of the least evolved rocks, troctolites to norites containing up to 20% of anhedral clinopyroxene and titanian pargasite. Pyroxenes and titanian pargasite from the troctolites have higher Mg, Al and Cr, and lower Mn than those from the norites, whereas plagioclase does not show systematic compositional variations (An ca. 65 mol%). The variation trend of anorthite content of plagioclase versus the forsterite content of olivine differs from that of are-related gabbroic rocks. Plagioclase, clinopyroxene, orthopyroxene and titanian pargasite were analyzed for REE and selected trace elements by ion microprobe. Application of crystal/liquid partition coefficients to trace element mineral compositions suggests that the parental liquids of both troctolites and norites had tholeiitic affinity and were slightly LREE and LILE enriched relative to N-MORB. A troctolite and a norite give Sm-Nd mineral isochron ages of 300 +/- 12 Ma and 280 +/- 10 Ma. Plagioclase-amphibole Rb-Sr isochron ages are 266 +/- 10 Ma and 269 +/- 16 Ma for the same rock samples, and they are interpreted to represent cooling ages. The Nd-Sr-O isotopic compositions indicate that a substantial crustal contribution was involved in the petrogenesis of the norite, which has low epsilon(Nd)(290 Ma), high Sr-87/Sr-86(290 Ma) and high delta(18)O(Px) (-2.6, 0.7057 and + 7.9 parts per thousand, respectively) compared with the troctolites. We thus conclude that the troctolite/norite association formed by concomitant fractional crystallization and crustal assimilation. The somewhat elevated delta(18)O(Cpx) (+6.4 parts per thousand) and the relatively low forsterite contents in olivine suggest that the parental liquids of the troctolites had already been evolved through an AFC type process. The epsilon(Nd)(290 Ma) and Sr-87/Sr-86(290 Ma) of these rocks (ranging from + 2.8 to + 4.4 and from 0.7037 to 0.7040, respectively) probably do not reflect the epsilon(Nd) and Sr isotopic compositions of their mantle source, and it is thus unclear whether the primary melts were derived from a slightly enriched or from a depleted mantle source. The Sondalo gabbroic complex was most likely associated with the post-Variscan gabbroic complexes of the Alpine belt. These gabbroic complexes can be ascribed to the intrusion at different crustal levels of tholeiitic mantle-derived melts and were emplaced in the time span of 300-270 Ma.