High-pressure, low-temperature metamorphism in the Sebtides nappes, northern Rif, Morocco

On the southern bank of the Gibraltar strait, the Permian-Triassic phyllites of the Sebtides-Alpujarrides nappes yielded edogite and biueschist-fades relic assemblages. The various metamorphic units are juxtaposed through retrograde contacts. The HP-LTmetamorphism indicates an Alpine subduction event.Las metapelitas Permo-Triassicas de los mantas Sebtides-Alpurarrides cerca de Ceuta tienen asociaciones minerales de alta presión, baja temperatura (talco-fengita-distena-Mg-cloritoide-Mg-carfolita en la unidad piu baja; asociaciones de menor presión en las unidades mas alta). Los diferentes mantas están superpuestos por medio de contactos retrógrados. El metamorfismo inicial de alta presión se debió producir como consecuencia de un proceso de subducción continental

Chemical zonation of the garnet from the Beni Bouzera granulites (Rif Interne): establishmetn of a anti-clockwise metamorphic path

Textural observations and variation of chemical composition on the garnet of Beni Bouzera's granulites (kinzingites for Kornprobst, 1974) allow to recognize two prograde metamorphic event Ml and M2 corresponding, respectively to a thermal and baric peak, Ml and M2. An abrupt disminution of the Ca content in the peripheric zone of the garnet has been interpreted as a retrograde event (M3) of the metamorphic P-T path, produced after a isothermal decompression, also inferred from the metamorphic reactions between minerals included in the garnet. Thermobarometric data estimated from zoned garnet allow to reconstruct a anti clock-wise P-T path to these rock

HP-LT metamorphism of the Filali-Bénzou schist rocks (Sebtides, Morocco). Chloritoid-garnet-phengite thermobarometry

The paleozoic schist rocks of Benzou outcrop in the core of the Beni Mzala's antiformal stack (Upper Sebtides units). In these schist rocks, we have found the association garnet (XFe=0.89-0.94), chloritoid (XMg=0.12-0.22), chlorite (XMg=0.40-0.45), biotite (XMg=0.37) and fengite (Si=3.23-3.44 a.p..u.) which grows in the KFMASH system with a ferrous tendency. The metamorphic peak has been estimated around 12 Kbar in pressure and 500°C in temperature According id the garnet (XFe=0.94) ± fengite (Si=3.44 a.p.u) association and the Chloritoid + Biotitè = Garnet + Fengite equilibrium. The secondary mineral association: chloritoid, biotite, chlorite and fengite (Si=3.1 7-3.23) indicates a pressure drop accompanied by cooling during the post-pèak path. The thermobarometriC data show that these Palaeozoic rocks have suffered a high-pressure low-temperature metamorphism in contrast with the classic estimations of P-T conditions of schist rocks belonging to Filali units (Palaeozoic in âgé; P=5-7 Kbàr y T=550-700°C). We propose that Benzou schist rocks are the lower part of the Beni Mzala unit and they could be transitional levels to the metapelite formations represented in the Filali units. Similar transitional metápélite rocks have been observed in the Alpujarride unit

The pressure–temperature–time–deformation history of the Beni Mzala unit (Upper Sebtides, Rif belt, Morocco): Refining the Alpine tectono-metamorphic evolution of the Alboran Domain of the western Mediterranean

International audienceThe structural and thermal relaxation overprint associated with the Neogene Alboran rifting have obscured the early Alpine tectono‐metamorphic evolution of the Alboran Domain, representing the metamorphic core of the Betic–Rif orogen of the western Mediterranean region. This study focuses on the Beni Mzala unit, forming the lower and deeper structural level of the Alpine metamorphic nappe stack (Upper Sebtides) in the Moroccan Rif. Meso‐ and micro‐scale structural investigations are carried out on high‐P aluminum silicate (Ky‐bearing)‐quartz segregations that occur as boudins within the main retrogressive syn‐greenschist foliation (S2/D2) and assumed to preserve the early M1 HP metamorphism associated with the Alpine orogenic construction in the Alboran Domain. These boudins host an early crenulated high‐P foliation (S1, D1/M1) made of quartz–kyanite–white mica–rutile. A large spread in white mica composition is documented, with the highest Si content per formula unit (up to 3.18 apfu) preserved along the S1 foliation and the lower Si content observed in the white mica marking the S2 retrogressive foliation (D2/M2) and the rim of S1 mica. Microtextural evidence documents post‐tectonic andalusite growth and static recrystallization of the quartz microlithons. Inverse (Zr‐in‐Rt thermometry) and forward modelling thermobarometry are integrated with Ar–Ar white mica geochronology to define the peak and exhumation pressure–temperature–time (P–T–t) path of the Beni Mzala unit. Minimum thermo‐baric estimates for the M1 event are ~1.4 GPa and 600°C, corresponding to a metamorphic gradient of ~11°/km, consistent with subduction zone metamorphism. Exhumation is constrained by re‐equilibration of the white mica composition (from high to low celadonite) between c. 29 and 22 Ma, during a nearly isothermal retrogressive path, with final equilibration at high‐T/low‐P conditions within the andalusite stability field (~0.2–0.3 GPa and 500°C). A minimum late Oligocene age is proposed for the Alpine D1 tectono‐metamorphic stage in the Rif, suggesting as feasible the previously proposed Eocene timing for the subduction‐zone metamorphism of the Alboran Domain. Conclusive evidence is provided to link the early Miocene tectono‐metamorphic event to a late thermal perturbation that affected the Alboran Domain at shallow crustal conditions, post‐dating the almost complete exhumation of the deep roots of the Alpine belt in the western Mediterranean

Prograde and retrograde history of eclogites from the Voltri Massif (Ligurian Alps, Italy): structural and petrological constraints.

The ophiolitic rocks of the Voltri Group (Ligurian Alps) represent exhumed portions of the Liguro-Piemontaise oceanic domain, which were subducted during the formation of the Alpine belt in Tertiary times. Eclogites occur as interlayered lenses within ultramafic sequences of the Beigua Unit. They are primarily constituted by garnet (rim-core: Alm78Grs11Pyr10Sps1-Alm46Grs23Pyr1Sps30), clinopyroxene (Jd19-50), and rutile. These eclogitic lenses retain evidence of the prograde metamorphic path by inclusions in garnet cores. The inclusions consist of quartz, Ca-amphibole, rutile, ilmenite, epidote (XCa=1.97) and paragonite. The retrograde evolution is attested by Na-amphibole overgrowth on early matrix garnet and clinopyroxene. All of that is overprinted by a texturally late amphibolite/greenschist facies assemblage. Quantitative thermobarometry was evaluated using THERMOCALC3.21 and TWQ2.02. Thermobarometry related to inclusion sets in different garnets record different conditions, but all are representative of the amphibolite facies (0.6-0.9 GPa and 400°-550°C). The Fe2+/Mg exchange reaction between clinopyroxene and garnet rims yields temperatures of approximately 500°C for the eclogitic stage. A minimum pressure of 1.3-1.5 GPa at 500°C was estimated by applying the jadeite-in-omphacite geobarometer. The occurrence of glaucophane as a first retrogressive product indicates retrogression through the blueschist facies. Jadeitic mole fraction (Jd36) in clinopyroxene in association with Na-amphibole provides pressure conditions of about 1.25 GPa. The blueschist retrogression is associated to the development of a non-coaxial fabric, with dominant top-to-the-N sense of shear. Zoning of Na-amphiboles, together with Na-Ca-amphibole in association with chlorite and albite, suggests a decompressional path with a final re-equilibration within the amphibolite/greenschist facies. This re-equilibration is associated with a top-to-the-W sense of shear, as deduced from the enveloping mafic bodies. The combined P-T data show a general counter-clockwise path running from the amphibolite facies through the eclogite and blueschist facies to the greenschist facies. This path provides a complete P-T-deformation history for the Voltri eclogites recording events during and after subduction processes. The rocks also imply modalities of exhumation from the deep-seated roots of the Alpine orogen

The pressure–temperature–time–deformation history of the Beni Mzala unit (Upper Sebtides, Rif belt, Morocco): Refining the Alpine tectono-metamorphic evolution of the Alboran Domain of the western Mediterranean

The structural and thermal relaxation overprint associated with the Neogene Alboran rifting have obscured the early Alpine tectono-metamorphic evolution of the Alboran Domain, representing the metamorphic core of the Betic–Rif orogen of the western Mediterranean region. This study focuses on the Beni Mzala unit, forming the lower and deeper structural level of the Alpine metamorphic nappe stack (Upper Sebtides) in the Moroccan Rif. Meso- and micro-scale structural investigations are carried out on high-P aluminum silicate (Ky-bearing)-quartz segregations that occur as boudins within the main retrogressive syn-greenschist foliation (S2/D2) and assumed to preserve the early M1 HP metamorphism associated with the Alpine orogenic construction in the Alboran Domain. These boudins host an early crenulated high-P foliation (S1, D1/M1) made of quartz–kyanite–white mica–rutile. A large spread in white mica composition is documented, with the highest Si content per formula unit (up to 3.18 apfu) preserved along the S1 foliation and the lower Si content observed in the white mica marking the S2 retrogressive foliation (D2/M2) and the rim of S1 mica. Microtextural evidence documents post-tectonic andalusite growth and static recrystallization of the quartz microlithons. Inverse (Zr-in-Rt thermometry) and forward modelling thermobarometry are integrated with Ar–Ar white mica geochronology to define the peak and exhumation pressure–temperature–time (P–T–t) path of the Beni Mzala unit. Minimum thermo-baric estimates for the M1 event are ~1.4 GPa and 600°C, corresponding to a metamorphic gradient of ~11°/km, consistent with subduction zone metamorphism. Exhumation is constrained by re-equilibration of the white mica composition (from high to low celadonite) between c. 29 and 22 Ma, during a nearly isothermal retrogressive path, with final equilibration at high-T/low-P conditions within the andalusite stability field (~0.2–0.3 GPa and 500°C). A minimum late Oligocene age is proposed for the Alpine D1 tectono-metamorphic stage in the Rif, suggesting as feasible the previously proposed Eocene timing for the subduction-zone metamorphism of the Alboran Domain. Conclusive evidence is provided to link the early Miocene tectono-metamorphic event to a late thermal perturbation that affected the Alboran Domain at shallow crustal conditions, post-dating the almost complete exhumation of the deep roots of the Alpine belt in the western Mediterranean

A counter-clockwise P-T path for the Voltri Massif eclogites (Ligurian Alps, Italy)

Integrated petrological and structural investigations of eclogites from the eclogite zone of the Voltri Massif (Ligurian Alps) have been used to reconstruct a complete Alpine P-T deformation path from burial by subduction to subsequent exhumation. The early metamorphic evolution of the eclogites has been unravelled by correlating garnet zonation trends with the chemical variations in inclusions found in the different garnet domains. Garnet in massive eclogites displays typical growth zoning, whereas garnet in foliated eclogites shows rim-ward resorption, likely related to re-equilibration during retrogressive evolution. Garnet inclusions are distinctly different from core to rim, consisting primarily of Ca-, Na/Ca-amphibole, epidote, paragonite and talc in garnet cores and of clinopyroxene ± talc in the outer garnet domains. Quantitative thermobarometry on the inclusion assemblages in the garnet cores defines an initial greenschist-to-amphibolite facies metamorphic stage (M1 stage) at c. 450-500 °C and 5-8 kbar. Coexistence of omphacite + talc + katophorite inclusion assemblage in the outer garnet domains indicate c. 550 °C and 20 kbar, conditions which were considered as minimum P-T estimates for the M2 eclogitic stage. The early phase of retrograde reactions is polyphase and equilibrated under epidote-blueschist facies (M3 stage), characterized by the development of composite reaction textures (garnet necklaces and fluid-assisted Na-amphibole-bearing symplectites) produced at the expense of the primary M2 garnet-clinopyroxene assemblage. The blueschist retrogression is contemporaneous with the development of a penetrative deformation (D3) that resulted in a non-coaxial fabric, with dominant top-to-the-N sense of shear during rock exhumation. All of that is overprinted by a texturally teamphibolite/greenschist facies assemblages (M4 & M5 stages), which are not associated with a penetrative structural fabric. The combined P-T deformation data are consistent with an overall counter-clockwise path, from the greenschist/ amphibolite, through the eclogite, the blueschist to the greenschist facies. These new results provide insights into the dynamic evolution of the Tertiary oceanic subduction processes leading to the building up of the Alpine orogen and the mechanisms involved in the exhumation of its high-pressure roots. © 2005 Blackwell Publishing Ltd

Timing and modes of granite magmatism in the core of the Alboran Domain, Rif chain, northern Morocco: Implications for the Alpine evolution of the western Mediterranean

The Betic-Rif orogen forms the western termination of the Alpine orogenic system in the Mediterranean region. The precise timing, structural evolution, and distribution of high-grade metamorphic units (Alpine versus pre-Alpine) in the inner zones of the orogen (Alboran Domain) remain controversial issues. In this paper we report occurrence of distinct generations of peraluminous granitic bodies intruded within Beni Bousera peridotites and their amphibolite-to-granulite facies envelope, in the core of the Alboran Domain of the Rif chain (northern Morocco). These granitic bodies are central to the reconstruction of the high-grade evolution of the Alboran Domain because they provide first-order structural markers to assess the P-T-t-deformation history of the high-grade terranes. Here we document the petrography and structural relationships with the host rocks and constrain the timing of granite emplacement using laser ablation.inductively coupled plasma.mass spectrometry U-Pb zircon and/or monazite dating, complemented by 40Ar/39Ar dating. The results indicate that granite emplacement occurred in two major episodes of anatectic magmatism, during the Hercynian (circa 300 Ma) and Alpine (circa 22 Ma) periods, respectively. These data (1) provide conclusive evidence for an important phase of Hercynian magmatism and high-grade metamorphism in the Alboran Domain and (2) permit a revaluation of the significance of the high-grade early Miocene event documented in the Alboran Domain in terms of a late stage, thermal pulse that reworked a polymetamorphic (Hercynian and Alpine) nappe pile. These results provide new constraints for construction of a feasible tectonometamorphic model for the Alpine evolution of the western Mediterranean

A counter-clockwise P-T path for the Voltri Massif eclogites (Ligurian Alps, Italy)

Integrated petrological and structural investigations of eclogites from the eclogite zone of the Voltri Massif (Ligurian Alps) have been used to reconstruct a complete Alpine P–T deformation path from burial by subduction to subsequent exhumation. The early metamorphic evolution of the eclogites has been unravelled by correlating garnet zonation trends with the chemical variations in inclusions found in the different garnet domains. Garnet in massive eclogites displays typical growth zoning, whereas garnet in foliated eclogites shows rim-ward resorption, likely related to re-equilibration during retrogressive evolution. Garnet inclusions are distinctly different from core to rim, consisting primarily of Ca-, Na/Ca-amphibole, epidote, paragonite and talc in garnet cores and of clinopyroxene ± talc in the outer garnet domains. Quantitative thermobarometry on the inclusion assemblages in the garnet cores defines an initial greenschist-to-amphibolite facies metamorphic stage (M1 stage) at c. 450–500 C and 5–8 kbar. Coexistence of omphacite + talc + katophorite inclusion assemblage in the outer garnet domains indicate c. 550 C and 20 kbar, conditions which were considered as minimum P–T estimates for the M2 eclogitic stage. The early phase of retrograde reactions is polyphase and equilibrated under epidote–blueschist facies (M3 stage), characterized by the development of composite reaction textures (garnet necklaces and fluid-assisted Na-amphibole-bearing symplectites) produced at the expense of the primary M2 garnet-clinopyroxene assemblage. The blueschist retrogression is contemporaneous with the development of a penetrative deformation (D3) that resulted in a non-coaxial fabric, with dominant topto- the-N sense of shear during rock exhumation. All of that is overprinted by a texturally late amphibolite/greenschist facies assemblages (M4 & M5 stages), which are not associated with a penetrative structural fabric. The combined P–T deformation data are consistent with an overall counter-clockwise path, from the greenschist/amphibolite, through the eclogite, the blueschist to the greenschist facies. These new results provide insights into the dynamic evolution of the Tertiary oceanic subduction processes leading to the building up of the Alpine orogen and the mechanisms involved in the exhumation of its high-pressure roots