Origin of the Gabbro-Peridotite Association from the Northern Apennine Ophiolites (Italy)

Abstract

The Northern Apennine ophiolites are remnants of the Middle Jurassic---Early Cretaceous lithosphere from the Ligurian Tethys. New trace element and Nd---Sr isotope investigations were performed on: (1) the rare gabbros associated with the subcontinental mantle rocks from the External Liguride ophiolites; (2) the gabbro--peridotite association from the poorly known ophiolitic bodies from Cecina valley (Southern Tuscany). Clinopyroxenes from the External Liguride and Cecina valley gabbros have similar trace element compositions, which are consistent with formation from normal mid-ocean ridge basalt (N-MORB) magmas. Sm---Nd mineral isochron ages are 179 9Ma for an External Liguride gabbro and 170 13Ma and 173 5 4 8Ma for two different gabbroic bodies from the Cecina valley ophiolites. These ages are interpreted to date the igneous crystallization of the gabbros and are slightly older than the oldest pelagic sediments of the Ligurian Tethys. Initial eNd (þ8 5 toþ8 9) and Sr/Sr of clinopyroxene are consistent with the interpretation that the studied gabbros were derived from N-MORB magmas. The least serpentinized mantle rocks from the Cecina valley ophiolites are porphyroclastic spinel lherzolites displaying a residual geochemical signature. They are similar to the least depleted residual peridotites from modern oceans. Nd and Sr isotopic ratios for separated mantle clinopyroxene are respectively higher (e.g. eNd 1⁄4 þ11) and lower than those of clinopyroxene from associated gabbros at the time of the gabbro intrusion. The gabbro---peridotite associations from the Northern Apennine ophiolites record the progression of the rifting process that led to opening of the Ligurian Tethys.