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Abstract:
The Alpine-Apennine ophiolites are considered analogs of the oceanic lithosphere formed at ocean-continent transition zones and very slow oceanic spreading centers. They are lithospheric remnants of the Jurassic Piedmont-Ligurian ocean, a branch of the Mesozoic Tethys separating the European and Adriatic continental margins. Previous geochronological studies on gabbroic rocks of the Alpine Tethys indicated a rather large time span of crust formation. In this paper, we present Sm-Nd geochronological data on well-preserved olivine gabbros intruded in mantle peridotites from the Erro-Tobbio ophiolitic unit (Ligurian Alps, Italy). Borghini et al. (2007) documented that these gabbros crystallized at low-P conditions (<5 kb) from primitive N-MORE melts, similar to many gabbroic rocks from Alpine-Apennine ophiolites. Here four plagioclase-dinopyroxene internal Sm-Nd isochrons are presented, yielding equivalent ages and initial epsilon(Nd) values. The ages are 177 +/- 7 Ma, 179 +/- 7 Ma, 178 +/- 21 Ma and 182 +/- 19 Ma, reflecting a weighted mean age of 178 5 Ma with initial epsilon(Nd) of 9.2 +/- 0.4 (2SD). Similar ages are only recorded in gabbroic rocks from the External Liguride Units (Northern Apennines) and represent the oldest ages available for the gabbroic crust of the Alpine Tethys. These Mg-rich gabbros can therefore be considered as representative of early (syn-rift) melt intrusions in thinned lithospheric mantle exhumed at ocean-continent transition domains, likely close to Adria's continental margin, similar to the hyper-extended Western Iberian Margin. Our new results together with previous ages of the Tethys oceanic crust allow for the reconstruction of the spatial distribution of oceanic gabbros over time, and evaluation of the spreading and propagation rates of this paleo-ocean. The northward propagation rifting velocity, estimated at similar to 5 cm/year, is presumably higher than the lateral spreading rate of similar to 2 cm/year. Our rate estimates suggest that the Red Sea is a present-day analog for the evolution of the Piedmont-Ligurian ocean. (C) 2014 Elsevier B.V. All rights reserved.
