Evidence for ancient fractional melting, cryptic refertilization and rapid exhumation of Tethyan mantle (Civrari Ophiolite, NW Italy)

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 Na 2 O, 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. 143 Nd/ 144 Nd 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. Electronic supplementary material The online version of this article (10.1007/s00410-019-1603-5) contains supplementary material, which is available to authorized users.

Phlogopite-pargasite coexistence in an oxygen reduced spinel-peridotite ambient

“…The Cr content in spinel, orthopyroxene and clinopyroxene is such that Cr# Sp > Cr# Cpx > Cr# Opx (Supplementary Tables 2–4). This suggests that the compositional evolution of spinel results from a Cr-Al exchange with coexisting pyroxenes during mantle melting 49 – 51 , thus promoting an increase of the Cr/Al ratio in spinel and the formation of Al-bearing coexisting pyroxene 52 .…”

Revisiting the Australian‐Antarctic Ocean‐Continent Transition Zone Using Petrological and Geophysical Characterization of Exhumed Subcontinental Mantle

“…(a) Mg# Fetot vs. TiO 2 wt%; (b) Mg# Fetot vs. Cr 2 O 3 wt%; (c) Mg# Fetot vs. Al 2 O 3 wt%; and (d) Mg# Fetot vs. Na 2 O wt%. Data for the Western Tethys from Rampone et al (1998), Borghini et al (2007), Borghini and Rampone (2007), Rampone et al (2008), Montanini et al (2008), Renna and Tribuzio (2011), Renna et al (2016), McCarthy and Müntener (2019), and Basch et al (2019). Mid‐ocean ridge data from Sanfilippo et al (2013), Dick et al (2010), Lissenberg and Dick (2008), Suhr et al (2008) and Leuthold et al (2018).…”

“…Subcontinental mantle xenoliths are from Beyer (2002), and the field of Western Tethys mantle clinopyroxene is compiled from McCarthy and Müntener (2019); (d) REE of plagioclase from pyroxenites compared to plagioclase from gabbros and pyroxenites from (ultra‐) slow spreading systems. Plagioclase and clinopyroxene compositions from gabbros and troctolites from the Western Tethys are from Borghini and Rampone (2007), Basch et al (2018), McCarthy and Müntener (2019), from (ultra‐) slow spreading ridges from Leuthold et al (2018) and Drouin et al (2009). Plagioclase from plagioclase‐peridotites are from Rampone et al (2008) and Basch et al (2018).…”

“…The black line is a fractional melting model in the spinel stability field starting with a depleted MORB mantle from Workman and Hart (2005) and showing 3% melting increments. Details of partial melting modeling including melting modes and partition coefficients can be found in McCarthy and Müntener (2015, 2019). The spinel‐lherzolite sample can be explained by ~5% melting.…”

“…This implies that the partial melting event recorded in the depleted spinel‐lherzolites likely reveal “ancient” magmatic processes unrelated to rifting and mantle exhumation (e.g., McCarthy & Müntener, 2015; Rampone et al, 1998). Depleted mantle domains along OCTs and (ultra‐) slow spreading localities are interpreted as preserving “ancient” melting events, such as (i) Proterozoic melting events (Macquarie Island, Southern Ocean, Dijkstra et al, 2010), (ii) Permian mantle‐derived magmatism (Western Tethys, e.g., McCarthy & Müntener, 2015, 2019), (iii) subarc mantle during Pan‐African Orogeny found in the Zabargad ophiolite (Red Sea, Brooker et al, 2004), and (v) subarc mantle along Newfoundland margin (ODP Site 1277; Müntener & Manatschal, 2006) and western Iberia/Morocco margin (volcanic xenoliths; Merle et al, 2012). The presence of a relic subduction zone along eastern Gondwanaland is suggested to be responsible of the formation of the Australian‐Antarctic Discordance (Gurnis & Muller, 2003).…”