The Alpine ophiolites and their original sedimentary cover (Upper Jurassic radiolarite-limestone with ophiolite breccia) occur as dismembered tectonic slices (often inverted by Alpine tectonics) or olistoliths among the calc-schists (Cretaceous?). Original petrologic features of the ophiolites are largely masked by the Alpine metamorphism, but are discernible by the relic-mineral chemistry and petrographic reconstruction. The residual peridotite is generally plagioclase lherzolite resembling those from oceanic fracture zones, but spinel lherzolite is present at Davos (Switzerland) and Péas (France). The latter is characterized by higher Na2O (1.6%) of clinopyroxene and lower Cr2O3 (8%) of spinel than the former ( < 0.7% and > 30%, respectively), and resembles subcontinental lherzolites. Geologic evidence indicates that these two lherzolites were together exposed on the Jurassic ocean floor. The mafic-ultramafic cumulates are of plagioclase-type characterized by the scarcity of orthopyroxene and high TiO2 (avg. 0.8%) of clinopyroxene. The basaltic volcanics are alkalic rather than tholeiitic in respect to their relic clinopyroxene, which is as rich in TiO2 and Na7O as that in Hawaiian alkali basalt. The Alpine ophiolite association of lherzolitic residual peridotite, plagioclase-type cumulates, and alkalic basalt is reasonable as cogenetic products of low-degree partial melting in the mantle. Geologic and petrologic features of the Alpine ophiolites are consistent with a geotectonic model postulating the generation of the Alpine ophiolites in an early Mesozoic rift zone cut by many transform faults.
ASJC Scopus subject areas
- Geochemistry and Petrology
- Earth and Planetary Sciences (miscellaneous)
- Space and Planetary Science