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

I. Martínez Segura and M. J. Román Alpiste are thanked for their kind assistance during sample preparation and SEM operation, and M. T. Gómez‐Pugnaire and A. Jabaloy for early work on Almirez ophicarbonates. We are grateful to the Sierra Nevada National Park for providing permits for fieldwork and sampling at the Almirez massif. We further acknowledge the editorial handling by D. Whitney and D. Robinson and the reviews of M. Galvez and T. Pettke, whose comments and constructive criticism helped to improve the manuscript. We acknowledge funding from the European Union FP7 Marie‐Curie Initial Training Network ABYSS under REA Grant Agreement no. 608001 in the framework of M.D.M.'s PhD project, the Spanish ‘Agencia Estatal de Investigación’ (AEI) grants no. CGL2016‐75224‐R to V.L.S.‐V and CGL2016‐81085‐R to C.J.G and C.M and grant no. PCIN‐2015‐053 to C.J.G. The ‘Junta de Andalucía’ is also thanked for funding under grants no. RNM‐131, RNM‐374 and P12‐RNM‐3141. C.M. thanks MINECO for financing a Ramón y Cajal fellowship no. RYC‐2012‐11314 and K.H. for a Juan de la Cierva Fellowship no. FPDI‐2013‐16253 and a research contract under grant no. CGL2016‐81085‐R. This work and the research infrastructure at the IACT have received (co)funding from the European Social Fund and the European Regional Development Fund.At sub‐arc depths, the release of carbon from subducting slab lithologies is mostly controlled by fluid released by devolatilization reactions such as dehydration of antigorite (Atg‐) serpentinite to prograde peridotite. Here we investigate carbonate–silicate rocks hosted in Atg‐serpentinite and prograde chlorite (Chl‐) harzburgite in the Milagrosa and Almirez ultramafic massifs of the palaeo‐subducted Nevado‐Filábride Complex (NFC, Betic Cordillera, S. Spain). These massifs provide a unique opportunity to study the stability of carbonate during subduction metamorphism at P–T conditions before and after the dehydration of Atg‐serpentinite in a warm subduction setting. In the Milagrosa massif, carbonate–silicate rocks occur as lenses of Ti‐clinohumite–diopside–calcite marbles, diopside–dolomite marbles and antigorite–diopside–dolomite rocks hosted in clinopyroxene‐bearing Atg‐serpentinite. In Almirez, carbonate–silicate rocks are hosted in Chl‐harzburgite and show a high‐grade assemblage composed of olivine, Ti‐clinohumite, diopside, chlorite, dolomite, calcite, Cr‐ bearing magnetite, pentlandite and rare aragonite inclusions. These NFC carbonate–silicate rocks have variable CaO and CO2 contents at nearly constant Mg/ Si ratio and high Ni and Cr contents, indicating that their protoliths were variable mixtures of serpentine and Ca‐carbonate (i.e., ophicarbonates). Thermodynamic modelling shows that the carbonate–silicate rocks attained peak metamorphic conditions similar to those of their host serpentinite (Milagrosa massif; 550–600°C and 1.0–1.4 GPa) and Chl‐harzburgite (Almirez massif; 1.7–1.9 GPa and 680°C). Microstructures, mineral chemistry and phase relations indicate that the hybrid carbonate–silicate bulk rock compositions formed before prograde metamorphism, likely during seawater hydrothermal alteration, and subsequently underwent subduction metamorphism. In the CaO–MgO–SiO2 ternary, these processes resulted in a compositional variability of NFC serpentinite‐hosted carbonate–silicate rocks along the serpentine‐calcite mixing trend, similar to that observed in serpentinite‐hosted carbonate‐rocks in other palaeo‐subducted metamorphic terranes. Thermodynamic modelling using classical models of binary H2O–CO2 fluids shows that the compositional variability along this binary determines the temperature of the main devolatilization reactions, the fluid composition and the mineral assemblages of reaction products during prograde subduction metamorphism. Thermodynamic modelling considering electrolytic fluids reveals that H2O and molecular CO2 are the main fluid species and charged carbon‐bearing species occur only in minor amounts in equilibrium with carbonate–silicate rocks in warm subduction settings. Consequently, accounting for electrolytic fluids at these conditions slightly increases the solubility of carbon in the fluids compared with predictions by classical binary H2O–CO2 fluids, but does not affect the topology of phase relations in serpentinite‐hosted carbonate‐ rocks. Phase relations, mineral composition and assemblages of Milagrosa and Almirez (meta)‐serpentinite‐hosted carbonate–silicate rocks are consistent with local equilibrium between an infiltrating fluid and the bulk rock composition and indicate a limited role of infiltration‐driven decarbonation. Our study shows natural evidence for the preservation of carbonates in serpentinite‐hosted carbonate–silicate rocks beyond the Atg‐serpentinite breakdown at sub‐arc depths, demonstrating that carbon can be recycled into the deep mantle.Funding from the European Union FP7 Marie‐Curie Initial Training Network ABYSS under REA Grant Agreement no. 608001Spanish ‘Agencia Estatal de Investigación’ (AEI) grants no. CGL2016‐75224‐R to V.L.S.‐V and CGL2016‐81085‐R to C.J.G and C.M and grant no. PCIN‐2015‐053 to C.J.GJunta de Andalucía Funding under grants no. RNM‐131, RNM‐374 and P12‐RNM‐3141MINECO for financing a Ramón y Cajal fellowship no. RYC‐2012‐11314 and K.H. for a Juan de la Cierva Fellowship no. FPDI‐2013‐16253 and a research contract under grant no. CGL2016‐81085‐

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

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 (10 kbar, ~350–650°C, ~FMQ buffer) to influx events of oxidizing fluids (fO ~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 μSiO (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 fO 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 Fe/Fe 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.“Ministerio de Economía, Industria y Competitividad” (MINECO), Grant/Award Number: CGL2012-32067, CGL201675224-R; Junta de Andalucía, Grant/ Award Number: RNM-145, P12-RNM3141; Ramón y Cajal, Grant/Award Number: RYC-2012-11314; MINECO, Grant/Award Number: CGL2016-81085-R, PCIN-2015-05

Xenolitos de metapelitas de alto grado en metabasitas: evidencia del emplazamiento en corteza continental del magmatismo básico Nevado-Filábride (Cordilleras Béticas)

Xenoliths from the continental crust have been found in the olivine-bearing dolerites in the Nevado-Filábride complex. This material is partially melted and assimilated by the igneous rocks and therefore chemical and mineralogical composition of both basaltic melts and xenoliths was modified: the xenoliths are composed of metamorphic and restite portions and in the basaltic melt the ,4/203 and SiQ content increased in the sites with a higher xenolith content The latter represent part of a metamorphic continental crust enclosed within the magma during its ascent and indicate, as do other chemical and geological features, that the metabasites were amplaced in the continental crus

Mica-chlorite intermixing and altered chlorite from the Nevado-Filabride micaschists, Southern Spain

12 páginas, 4 figuras, 2 tablasMinerals with biotite-like optical properties occur in the Nevado-Filabride micaschists. The grains are highly pleochroic, variable in colour from light yellow to deep red, and strongly recall the descriptions given for "oxychlorite". Based on X-ray powder diffraction, microprobe analysis and transmission electron microscopy, two main types are recognized in optically similar grains: 1) Those consisting of a chlorite matrix interleaved with approximately 5 % by volume of very thin biotite lamellae. 2) Those consisting of a chlorite-like, hydrated material that produces irrational 16-16.5 angstrom spacings in the X-ray diffraction pattern. These values progressively decrease upon heating. This material gives microprobe analyses close to coexisting chlorite, but shows systematically lower oxide sums, of the order of 79-85 wt.%. Wavy rather than straight lattice fringes are obtained on these grains. Lamellae of haematite, or of a precursor of haematite, a few hundred angstrom in thickness and with granular appearance, may occur interleaved with the 16 angstrom material. The 16 angstrom hydrated chlorite is a retrograde alteration product which is supposed to have been derived from the grains consisting of chlorite-mica association. Low temperature reactions occurred during the very late evolution stage of the Veleta unit, producing the hydration of chlorite and alteration of plagioclase, chloritoid and garnet. These reactions occurred during the recent, relatively rapid uplift of the Veleta rocks.Peer reviewe

U-Pb ages of detrital zircons from the Internal Betics: A key to deciphering paleogeographic provenance and tectono-stratigraphic evolution

© 2018 Elsevier B.V. Zircons from the Nevado-Filábride Complex metamorphic rocks yielded 891 concordant inherited detrital ages. The 342 concordant ages from the Aulago Fm indicate a Pennsylvanian maximum depositional age, while the age density distribution indicates that this formation is closely related to the Cantabrian Zone (north of the Iberian Massif). The dominant Ediacaran and Cryogenian populations, at ca. 596 and 788 Ma respectively, relate the Nevado-Filábride Complex to northern Gondwana terranes. A discrete Mesoproterozoic zircon population at ca. 1031 Ma (Tonian-Late Estenian population) suggests that the basement of the Nevado-Filábride Complex could have been located near of the Saharan Metacraton and west of the East African Orogen at the beginning of the Paleozoic. The 549 concordant ages from the Tahal Fm yield an Early Permian age for the protoliths. The age density distribution pattern records erosion of rocks from the Variscan belt, including Late-Variscan magmatic rocks. Zircon spectra differ from those of the Alborán Domain, supporting the hypothesis that the Nevado-Filábride Complex is part of the South Iberian Domain. Furthermore, data from the Tahal Fm are similar to those of Permo-Triassic rocks from the Iberian Ranges

The role of dehydration of subducted serpentinites in the genesis of goldrich magmatic-hydrothermal ore deposits

Goldschmidt 2019, Barcelona (Spain), 18-23 august, 201