Evolution of gabbroic rocks of the Northern Apennine ophiolites (Italy): Comparison with the lower oceanic crust from modern slow-spreading ridges

Abstract

Field and petrographic relationships together with major and trace element mineral chemistry have allowed us to determine the igneous to high-temperature metamorphic evolution of the gabbroic rocks of the Northern Apennine ophiolites. Gabbroic rocks formed by the intrusion of liquids of normal mid-oceanic-ridge (N-MORB) type in a heterogeneous mantle section under low-pressure conditions. These liquids underwent an igneous-differentiation process controlled by fractional crystallization, most likely associated with late percolation in the gabbroic cumulate pile of a volatile-bearing igneous agent, possibly a trondhjemite-type liquid. Such an igneous-differentiation process produced highly evolved liquids that gave rise to Fe-rich rocks (mostly Fe-Ti oxide-bearing diorites). The gabbroic rocks underwent high-temperature (T ഠ 900 ؇C) recrystallization in ductile shear zones in the absence of seawater-derived fluids. The gabbroic rocks of the Northern Apennine ophiolites bear striking similarities to those recovered from modern slow-spreading ridges, such as the Southwest Indian Ridge, the Mid-Atlantic Ridge (at its intersection with the Kane Fracture Zone, i.e., the MARK area), and the Mid-Cayman Rise. The Northern Apennine ophiolites are interpreted to represent a fossil analogue of a magmapoor slow-spreading center that formed as a result of continental lithospheric extension.

The architecture of an incipient oceanic basin: a tentative reconstruction of the Jurassic Liguria-Piemonte basin along the Northern Apennines–Alpine Corsica transect

“…The geochemistry of the mantle peridotites is similar to that of residual abyssal peridotites; however, significant differences exist (Rampone et al 1996;Piccardo et al 2004). By contrast, the gabbros and basaltic rocks display a clear N-MORB Nd-isotopic signature (Rampone et al 1996;Tribuzio et al 2000;Tribuzio et al 2004). Geochemically, the basalts and gabbros from Internal Ligurian ophiolites sequence are representative of a more evolved stage among the today-exposed remnants of the Liguria-Piemonte basin.…”

“…The mantle peridotites, characterized by pyroxenite layers and minor dunitic lenses, are intruded by a shallowlevel gabbroic complex and cut by rare clinopyroxenite and widespread gabbro dikes (Beccaluva et al 1984;Rampone et al 1998;Piccardo et al 2004). The gabbroic complex consists of large, up to several kilometer-wide bodies, volumetrically dominated by isotropic olivine-bearing gabbros showing small lenticular bodies of layered melatroctolites and troctolites (Tribuzio et al 2000 and references therein). Chromitite layers or pockets also occur in the largest layered gabbros bodies.…”

“…Rare gabbroic slide-blocks are represented by troctolite to olivine-bearing gabbro, showing an MOR geochemistry Tribuzio et al 2000). Sm/Nd isochrons on plagioclase-clinopyroxenewhole rock from one sample of gabbro yielded an age of 179 ± 9 Ma interpreted to date igneous crystallization ).…”

Geochemical constraints on the origin of the late Jurassic proto-Caribbean oceanic crust in Hispaniola

“…Al 2 O 3 and CaO increase slightly to reach a maximum at about 7-8 wt.% MgO, then decrease in the evolved basalts. These trends are tholeiitic and can be attributed to the low-pressure fractionation and/or accumulation of olivine plus Cr-spinel, plagioclase, and clinopyroxene (Tribuzio et al 2000), which is compatible with the igneous mineralogy. However, the TiO 2 -MgO variation shows at least two distinct trends of relative low-and high-Ti samples, related not only to mineral fractionation/accumulation, but also to distinct, source related, geochemical features of the magmas (see below).…”

The microstructural record of porphyroclasts and matrix of partly serpentinized peridotite mylonites – from brittle and crystal-plastic deformation to dissolution–precipitation creep

“…The ophiolite complex from the Voltri Massif is related to the Late Jurassic Piemont-Ligurian Ocean, the Alpine-Apennine part of the Mesozoic Tethys (Bodinier et al, 1986;Scambelluri et al, 1997Scambelluri et al, , 2004Hermann et al, 2000;Piccardo and Vissers, 2007). The Jurassic Piemont-Ligurian Tethys has been proposed to represent an analogue of a slow-ultraslowspreading ocean basin (Tribuzio et al, 2000;Piccardo, 2008Piccardo, , 2013, which is characterized by extended continental margins and ocean continent transition zones, whereby lithospheric subcontinental mantle can be preserved in the rift system (Manatschal et al, 2007;Péron-Pinvidic and Manatschal, 2009;Piccardo, 2013). The Voltri Massif ophiolite complex comprises units that are derived from oceanic lithosphere (Beigua-type serpentinites), associated with their volcanic-sedimentary cover (Voltri-Rossiglione-type calcschists and meta-volcanics), and units derived from the subcontinental mantle (Erro-Tobbio-type peridotites; T < 1000 − 1100 • C and spinel-facies conditions; e.g.…”