Strain localization and fluid infiltration in the mantle wedge during subduction initiation: Evidence from the base of the New Caledonia ophiolite

Abstract

International audienceDespite decades of petrological and geochemical studies, the nature and setting of obducted ophiolites remain controversial: the influence of supra-subduction zone environments on pre-existing oceanic lithosphere is yet to assess, and the processes leading to subduction/obduction initiation are still poorly constrained. Our study documents successive influx of slab-derived fluids and progressive strain localization within the upper mantle in a supra-subduction environment during the first few My of the subduction history. We focus on strongly sheared mafic amphibolites intruding peridotites near the mantle–crust transition of the New Caledonia obducted ophiolite and ~ 50 to 100 m above the basal thrust contact of the ophiolite. These m- to hm-long and several m-thick shear bands are interpreted as inherited small-scale intrusions of mafic melts, probably dikes or sills, which were derived from a moderately refractory mantle source refertilized by supra-subduction zone fluids. 40Ar/39Ar age constraints on pargasite at ca. 90 Ma suggest that they could be inherited from the former Pacific west-dipping subduction.Secondary deformation of these mafic intrusions is intimately associated to three major stages of fluid infiltration: (1) the first stage of deformation and metasomatism is marked by syn-kinematic growth of Ca-amphibole (at 700–800 °C and 3–5 kbar) with a distinctive supra-subduction zone signature, and controlled later channelization of aqueous fluids. 40Ar/39Ar dating on magnesio-hornblende indicates that this deformation episode occurred at ca. 55 Ma, coincident with east-dipping subduction initiation; (2) the main metasomatic stage, characterized by the development of a phlogopite-rich matrix wrapping peridotites and amphibolite boudins, points to the percolation of alkali-rich aqueous fluids at still high temperature (650–750 °C); (3) the last, low temperature (< 600 °C) metasomatic stage results in the formation of deformed veinlets containing talc, chlorite and serpentine

Deformation mechanisms in mafic amphibolites and granulites: record from the Semail metamorphic sole during subduction infancy

“…This unit results from the juxtaposition of several slices of oceanic crust ( Fig. 1c-d; Soret et al, 2017, and references therein) that were buried, strained and metamorphosed against the hot overlying mantle wedge, from granulite-facies (for the uppermost slice) to upper-greenschist-facies conditions (for the lowermost one). In practice, it is convenient to distinguish in the field a high-temperature (HT) sole, found directly below the mylonitic basal peridotite, from a low-temperature (LT) sole onto which the HT sole is thrust.…”

“…M. Soret et al: Deformation mechanisms in mafic amphibolites and granulites cretion at characteristic pressure-temperature-time (P -Tt) conditions Dilek, 2000, 2003;Plunder et al, 2016;Soret et al, 2017): two major accretion events have been identified in the mafic, high-temperature amphibolite to granulite facies portions of the Semail ophiolite sole (Oman and United Arab Emirates, UAE) at 850 ± 50 • C and 0.9±0.2 GPa and 750±50 • C and 0.7±0.2 GPa (Soret et al, 2017). The striking similarity of P -T conditions across the whole ophiolite width (∼ 150 km) also indicates that these slivers experienced shear strains of at least 4-5 gamma during accretion/exhumation (Soret et al, 2017), coeval with large ductile deformation in the banded peridotites above (Boudier et al, 1988;Prigent et al, 2018a, c).…”

“…In contrast to plagioclase and clinopyroxene (e.g., Bascou et al, 2002;Rybacki and Dresen, 2004;Hier-Majumder et al, 2005;Dimanov et al, 2007), the physical properties and mechanical behavior of amphibole and amphibolebearing polymineralic aggregates, commonly found over a large range of P -T and water activity conditions, have been far less studied and remain puzzling (see Getsinger and Hirth, 2014). Understanding their deformation mechanisms is therefore expected to shed light on the plate interface rheology during subduction infancy (Soret et al, 2016(Soret et al, , 2017Prigent et al, 2018b) or beneath warm subduction zones (where similar temperatures, lithologies and mechanical coupling are expected at ∼ 60 km depth; Abers et al, 2017), as well as on the rheological behavior of amphibole-bearing rocks and strain localization in mid-crustal environments (e.g., continental crust: Imon et al, 2004;Tatham et al, 2008;Getsinger et al, 2013;Giuntoli et al, 2018; oceanic detachment faults or transform faults: Boschi et al, 2006;Escartín et al, 2003).…”

Petrological evidence for stepwise accretion of metamorphic soles during subduction infancy (Semail ophiolite, Oman and UAE)

“…Large uncertainties remain, however, on the detailed structure and P–T conditions of metamorphic soles (Gnos, ; Hacker, ; Hacker & Mosenfelder, ; Searle, ), or on the amount of associated melting and genetic link with localized magmatic intrusions in the ophiolite (Boudier et al., ; Hacker & Gnos, ; Ishikawa, Fujisawa, Nagaishi, & Masuda, ; Searle & Cox, ; Soret et al., ), due to the fact that analytical and modelling methods with contrasting accuracy have been used over the last 30 years. Previous studies have shown that metamorphic soles show a temperature decrease structurally downward yet with large uncertainties, while pressure variations remain poorly constrained (see Agard et al., and references therein).…”

Pervasive Eclogitization Due to Brittle Deformation and Rehydration of Subducted Basement: Effects on Continental Recycling?

“…Fluids are major players in subduction processes, but their game remains poorly understood, especially in continental slabs. Thermal breakdown of hydrous minerals will liberate fluids, and these can trigger processes as diverse as seismic failure (Abers et al, 2013;Barcheck et al, 2012), mantle infiltration (Soret et al, 2016), or hydrous melting (Labrousse et al, 2015). But continental slabs comprise mostly pretty infertile lithotypes, with sparse hydrates, and little is known about how fluids in the subduction channel interact, especially with very dry rocks (Angiboust et al, 2017) before these reach their solidus.…”