Abstract
Western Tethyan peridotites exposed in the European Alps show limited amounts of partial melting and mostly fertile compositions. Here we investigate the Civrari Ophiolite (northwestern Italy), which is composed of depleted spinel-harzburgites and serpentinites associated with MOR-type gabbros and basalts. The ultramafic rocks are unique amongst western Tethyan peridotites, showing homogeneous residual compositions after ~ 15% near-fractional melting, lack of pervasive melt percolation and mineral compositions that indicate high-temperature equilibration ≥ 1200 °C. Clinopyroxene chemistry records some of the lowest abundances of Na2O, Ce, and Zr/Hf amongst abyssal peridotites worldwide, suggesting that most abyssal peridotites have been affected by variable degrees of melt retention upon melting or cryptic melt percolation. Locally, cryptic MORB-like melt migration in Civrari peridotites produced orthopyroxene + plagioclase intergrowth around reacted clinopyroxene. These clinopyroxene preserve micron-scale chemical zoning indicating rapid cooling after melt crystallization. 143Nd/144Nd isotopic data indicate that Civrari mantle rocks, gabbros, and basalts are not in isotopic equilibrium. Civrari spinel-peridotites represent a highly radiogenic endmember amongst Western Tethys depleted spinel-peridotites, which together form a partial melting errochron of 273 Ma ± 24 Ma. Ancient near-fractional melting and cryptic melt–rock reaction cause variations in radiogenic εNd and εHf, leading to isotopic heterogeneity of Western Tethys mantle rocks. Such inherited signatures in mantle rocks are most likely to be preserved along (ultra-)slow-spreading systems and ocean–continent transition zones.
Highlights
The formation of oceanic lithosphere at mid-ocean ridges was spearheaded by the discovery of mid-ocean ridges (e.g., Heezen et al 1964) and the characterization of ophiolites as remnant oceanic lithosphere thrust upon continental margins (e.g., Anonymous 1972; Decandia and Elter 1972)
Fe contents of non-serpentinised peridotites are consistent with other ultramafic suites ( Fe2O3 = 8.63–9.42 wt%), with the narrow compositional field of Civrari ultramafic rocks mirroring the compositional field of abyssal peridotites (Fig. 3)
Mantle clinopyroxene shows some of the most fractionated REE and lowest Na2O ever measured in abyssal peridotites and imply that these peridotites are the product of near-fractional melting starting in the garnet stability field
Summary
The formation of oceanic lithosphere at mid-ocean ridges was spearheaded by the discovery of mid-ocean ridges (e.g., Heezen et al 1964) and the characterization of ophiolites as remnant oceanic lithosphere thrust upon continental margins (e.g., Anonymous 1972; Decandia and Elter 1972). The last 30 years, the diversity of oceanic spreading systems has been underscored by the discovery of (ultra-)slow spreading systems (e.g., Dick et al 2003) and magma-poor rifted margins (e.g., Boillot et al 1980). In these cases, oceanic lithosphere is dominated by exhumed mantle, oceanic core complexes and minor basalts and gabbros, with extension being accommodated by the exhumation of mantle to the ocean floor along detachment faults (e.g., Dick et al 1981; Tucholke et al 1998). Field studies have pointed out numerous similarities between modern ultra-slow spreading systems, magma-poor rifted-margins and ophiolites in the Western Tethys (e.g., Lagabrielle and Cannat 1990; Lagabrielle et al 2015; Manatschal and Müntener 2009)
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