High‐P metamorphism of rodingites during serpentinite dehydration (Cerro del Almirez, Southern Spain): Implications for the redox state in subduction zones

Abstract

The transition between antigorite‐serpentinite and chlorite‐harzburgite at Cerro del Almirez (Betic Cordillera, Southern Spain) exceptionally marks in the field the front of antigorite breakdown at high pressure (~16–19 kbar) and temperature (~650°C) in a paleosubducted serpentinite. These ultramafic lithologies enclose three types of metarodingite boudins of variable size surrounded by metasomatic reaction rims. Type 1 Grandite‐metarodingite (garnet+chlorite+diopside+titanite±magnetite±ilmenite) mainly crops out in the antigorite‐serpentinite domain and has three generations of garnet. Grossular‐rich Grt‐1 formed during rodingitization at the seafloor (<2 kbar, ~150–325°C, ~FMQ buffer). During subduction, the alternating growth of Grt‐2b (richer in andradite and pyralspite components than Grt‐1) and Grt‐3 (very rich in andradite component) reflects the change from internally buffered metamorphic conditions (>10 kbar, ~350–650°C, ~FMQ buffer) to influx events of oxidizing fluids (fO2 ~HM buffer) released by brucite breakdown in the host antigorite‐serpentinite. Type 2 Epidote‐metarodingite (epidote+diopside+titanite±garnet) derives from Type 1 and is the most abundant metarodingite type enclosed in dehydrated chlorite‐harzburgite. Type 2 formed by increasing μSiO2 (from −884 to −860 kJ/mol) and decreasing μCaO (from −708 to −725 kJ/mol) triggered by the flux of high amounts of oxidizing fluids during the high‐P antigorite breakdown in serpentinite. The growth of Grt‐4, with low‐grandite and high‐pyralspite components, in Type 2 metarodingite accounts for progressive reequilibration of garnet with changing intensive variables. Type 3 Pyralspite‐metarodingite (garnet+epidote+amphibole+chlorite±diopside+rutile) crops out in the chlorite‐harzburgite domain and formed at peak metamorphic conditions (16–19 kbar, 660–684°C) from Type 2 metarodingite. This transformation caused the growth of a last generation of pyralspite‐rich garnet (Grt‐5) and the recrystallization of diopside into tremolitic amphibole at decreasing fO2 and μCaO (from −726 to −735 kJ/mol) and increasing μMgO (from −630 to −626 kJ/mol) due to chemical mixing between the metarodingite and the reaction rims. The different bulk Fe3+/FeTotal ratios of antigorite‐serpentinite and chlorite‐harzburgite, and of the three metarodingite types, reflect the highly heterogeneous oxidation state of the subducting slab and likely point to the transfer of localized oxidized reservoirs, such as metarodingites, into the deep mantle.

Subduction metamorphism of serpentinite‐hosted carbonates beyond antigorite‐serpentinite dehydration (Nevado‐Filábride Complex, Spain)

“…Meta‐serpentinites from the NFC record a history of Jurassic seafloor alteration (Alt et al., ; Laborda‐López et al., ; Puga et al., ) possibly in an ocean‐continent transition setting (the S–SE Iberian palaeo‐margin) and narrow oceanic basin, similar to that proposed for the origin of their Alps and Apennines counterparts (Lagabrielle, Vitale Brovarone, & Ildefonse, ). In the middle Miocene, subduction of the Iberian margin beneath the extended Alborán continental domain caused HP metamorphism of the NFC (cf.…”

“…(a) P–T pseudosection of Almirez meta‐ophicarbonate ( SN ‐138; Table ). Calculated mineral modes (wt%; including fluid F) are given in brackets at the peak conditions (yellow star) according to the Almirez P–T path (black dotted arrow, after Laborda‐López et al., ). All fields contain fluid, with shown as blue dashed lines.…”

“…Dark grey field: maximum P-T conditions for hydrothermal ophicarbonate formation in seafloor serpentinites (Schwarzenbach et al, 2013). The P-T path of the Almirez massif (after Laborda-López et al, 2018) hosting meta-ophicarbonates beyond the antigorite stability (this study) is shown by the black dashed arrow. P-T paths at the Moho depth of the slab for hot, intermediate and cold subduction zones (HSZ, ISZ and CSZ, respectively) after van Keken, Hacker, Syracuse, and Abers (2011).…”

“…These data indicate that Almirez meta-ophicarbonate was stable in the Chl-harzburgite facies beyond the conditions of antigorite dehydration (Figure 13a). For peak temperatures in the range of 670-680°C (Figure 13a; P-T-t path after Laborda-López et al, 2018), the assemblage Chl-Ol-Cpx-Dol-Cal limits the composition of the fluid to 0.004 < X CO 2 < 0.007 (yellow star in Figure 13a,c). Calcitedolomite thermometry of matrix carbonate yields T < 550°C, which records retrograde re-equilibration conditions (Table S1), a common feature of high-grade metamorphic carbonate rocks (Ferry, 2001;Mizuochi et al, 2010).…”

Garnets from Val d’Ala Rodingites, Piedmont, Italy: An Investigation of Their Gemological, Spectroscopic and Crystal Chemical Properties

“…Mineral assemblages of metabasites and ultramafites ( [15][16][17] and references therein) indicate that these ophiolites recorded an ocean floor-related metamorphic imprint, under conditions (zeolite-, prehnite-pumpellyite-and greenschist-facies) similar to those suggested for other metaophiolites from Western Alps [10]. Their qualitative PT (Pressure-Temperature) evolution as deduced from the literature is synthesized in Figure 2d where the low-and high-pressure stability fields of different garnet-bearing assemblages inferred from polymetamorphic rodingites of Betic Cordillera, in Southern Spain [18] are reported (light-pink fields). The ultramafic rocks consist of antigorite serpentinite bodies that represent the most widespread rock-type of Val d'Ala, and contain from decametric to metric lenses and layers of rodingites.…”