Major, minor, trace element, Sm-Nd and Sr isotope compositions of mafic rocks from the earliest oceanic crust of the Alpine Tethys

Recent isotopic and biochronologic dating has demonstrated that the Gets na ppe contains remnants of the oldest part of the oceanic crust of the Alpine Tethys. The ophiolites are associated with deep sea sediments, platform ca rbonates and continental crustal elements suggesting a transitional environ ment between continental and oceanic crust. Therefore, the ophiolites from the Gets nappe provide the opportunity to assess the nature of mantle sourc e and the magma evolution during the final rifting stage of the European li thosphere. Trace clement analyses of mafic rocks can he divided into two se ts: (1) P, Zr and Y contents are consistent with those of mid-ocean ridge b asalts and REE patterns have a P-MORB affinity. (2) P,Zr Ti and Y contents are compatible with within-plate basalts and are characterized by REE spect ra similar to that of T-MORB. Both have Nd isotopic compositions similar to those of synrift magma of the Red Sea and to the Rhine Graben. The model a ges are in agreement with an LREE-enriched subcontinental mantle source der ived from depleted mantle 800 to 900 Ma ago. Minor, trace element and Sm-Nd compositions suggest that these rocks are basaltic relies of an earliest s tage of oceanic spreading i.e. an embryonic ocean. Comparison between REE p atterns, Nd and Sr isotope compositions, isotopic and biochronologic ages f rom different Alpine Tethys ophiolites shows that samples with enriched LRE E are from the older ophiolitic suites and are relies of the embryonic ocea n floor. Later phases of ocean spreading are characterized by basalts that are depleted in LREE.