Amphibolitization of Mg- and Fe-rich gabbroic dykes within mantle-derived serpentinites from Northern Apennine ophiolites evidence for high-temperature hydration of the oceanic lithosphere

This work shows a petrological investigation on a serpentinized peridotite which includes various gabbroic dykes. The dykes range in composition from olivine-bearing gabbro to Fe/Ti-oxide diorite and are characterized by widespread hornblende development. The study has been carried out on both gabbroic dykes and enclosing peridotite, and documents a complex polyphase ocean-floor metamorphic evolution. The gabbroic dykes in places preserve evidence for high-temperature deformation in ductile shear zones, which predates the hornblende development. The high-temperature event is testified by the crystallization of neoblastic Ca-clinopyroxene and minor Ti-pargasite at the expenses of igneous clinopyroxene. Clinopyroxenes are commonly largely replaced by hornblende which, in Fe/Ti-oxide diorites, appears to coexist with ilmenite. Hornblende growth implies a significant influx of aqueous fluids in the gabbroic dykes under amphibolite-facies conditions. The development of amphibolite-facies assemblages in the gabbroic dykes was most likely coupled with hornblende formation in the enclosing peridotite, at the expenses of clinopyroxene exsolution lamellae within mantle orthopyroxene. This metamorphic transformation was possibly accompanied by the development of Al-rich lizardite (Al(2)O(3) up to 5.5 wt. %) after orthopyroxene and of Al-bearing Fe-rich spinel + chlorite after primary spinel. Many lines of evidence in both gabbroic dykes and enclosing peridotite (e.g., replacement of hornblende, ilmenite and Al-rich serpentine by actinolite, titanite and Al-depleted serpentine, respectively) indicate a subsequent retrogression, assisted by S-bearing aqueous fluids, to greenschist- and subgreenschist-facies conditions.. In the gabbroic dykes, fine-grained aggregates of chlorite+/-pumpellyite+/-albitic plagioclase+/-epidote formed in the plagioclase microtextural domain. In the peridotite, retrogression was accompanied by brittle deformation, with fractures filled with tremolite and/or chlorite in turn followed by diffuse serpentine veinlets. The final stage recorded by the gabbro-peridotite association is characterized by the development, mainly along fractures, of calcite. This provides evidence for late infiltration of CO(2)-bearing fluids, most likely during exhumation to near sea-floor levels.