Mature Subduction Zones Research Articles

Experimental synthesis and phase relations of phengitic muscovite from 6.5 to 11 GPa in a calcareous metapelite from the Dabie Mountains, China

The stability and phase relations of phengitic muscovite in a metapelitic bulk composition containing a mixed H 2O+CO 2 fluid were investigated at 6.5–11 GPa, 750–1050°C in synthesis experiments performed in a multianvil apparatus. Starting material consisted of a natural calcareous metapelite from the coesite zone of the Dabie Mountains, China, ultrahigh-pressure metamorphic complex that had experienced peak metamorphic pressures greater than 3 GPa. The sample contains a total of 2.1 wt.% H 2O and 6.3 wt.% CO 2 bound in hydrous and carbonate minerals. No additional fluid was added to the starting material. Phengite is stable in this bulk composition from 6.5 to 9 GPa at 900°C and coexists with an eclogitic phase assemblage consisting of garnet, omphacite, coesite, rutile, and fluid. Phengite dehydrates to produce K-hollandite between 8 and 11 GPa, 750–900°C. Phengite melting/dissolution occurs between 900°C and 975°C at 6.5–8 GPa and is associated with the appearance of kyanite in the phase assemblage. The formation of K-hollandite is accompanied by the appearance of magnesite and topaz-OH in the phase assemblage as well as by significant increases in the grossular content of garnet (average X grs=0.52, X py=0.19) and the jadeite content of omphacite ( X jd=0.92). Mass balance indicates that the volatile content of the fluid phase changes markedly at the phengite/K-hollandite phase boundary. At P≤8 GPa, fluid coexisting with phengite appears to be relatively CO 2-rich (XCO 2/XH 2O=2.2), whereas fluid coexisting with K-hollandite and magnesite at 11 GPa is rich in H 2O (XCO 2/XH 2O=0.2). Analysis of quench material and mass balance calculations indicate that fluids at all pressures and temperatures examined contain an abundance of dissolved solutes (approximately 40 mol% at 8 GPa, 60 mol% at 11 GPa) that act to dilute the volatile content of the fluid phase. The average phengite content of muscovite is positively correlated with pressure and ranges from 3.62 Si per formula unit (pfu) at 6.5 GPa to 3.80 Si pfu at 9 GPa. The extent of the phengite substitution in muscovite in this bulk composition appears to be limited to a maximum of 3.80–3.85 Si pfu at P=9 GPa. These experiments show that phengite should be stable in metasediments in mature subduction zones to depths of up to 300 km even under conditions in which aH 2O≪1. Other high-pressure hydrous phases such as lawsonite, MgMgAl-pumpellyite, and topaz-OH that may form in subducted sediments do not occur within the phengite stability field in this system, and may require more H 2O-rich fluid compositions in order to form. The wide range of conditions under which phengite occurs and its participation in mixed volatile reactions that may buffer the composition of the fluid phase suggest that phengite may significantly influence the nature of metasomatic fluids released from deeply subducted sediments at depths of up to 300 km at convergent plate boundaries.

The stability and composition of phengitic muscovite and associated phases from 5.5 to 11 GPa: Implications for deeply subducted sediments

The stability and composition of phengitic muscovite was investigated from 5.5–11 GPa, 700–1150°C in synthesis experiments performed in a multianvil apparatus. Starting materials consisted of natural minerals with a bulk composition similar to that of a K-deficient, intermediate dioctahedral-trioctahedral mica. Phengitic muscovite was found to be stable from 5.5–11 GPa at 900°C. Phengite melting occurs between 1075–1150°C at 7–8 GPa and 1000–1050°C at 10 GPa. At 10–11 GPa, 800°C octahedral cation deficient (OCD) muscovite and K-hollandite are observed rather than phengite.The average phengite content of muscovite is positively correlated with pressure ranging from 3.65 Si pfu at 5.5 GPa to 3.81 Si pfu at 11 GPa. The maximum phengite content of stable muscovite appears to be 3.80–3.85 Si pfu. Most phengite examined exhibits at least minor solid solution towards phlogopite, averaging 2.04 ± 0.06 (2σ) octahedral cations pfu.The hydrous phases phengite, lawsonite, topaz-OH, and Mg-pumpellyite occur between 6–8 GPa, 700–900°C. With increasing pressure lawsonite and Mg-pumpellyite dehydrate to form garnet between 8–9 GPa. With increasing temperature Mg-pumpellyite, lawsonite, and topaz-OH devolatilize to form garnet and kyanite between 900–1000°C at 7–8 GPa.Phengitic muscovite and topaz-OH would be stable in hydrous sediments in cool mature subduction zones to depths exceeding 360 km while lawsonite and Mg-pumpellyite would be stable to 240–300 km allowing the transport of H2O contained in these phases deep into the upper mantle. In warmer subduction zones the dehydration of Mg-pumpellyite, lawsonite, and topaz-OH; as well as the melting of phengite would result in fluid release at depths of 180–240 km.The presence of phengite at far greater depths than the zone of melt generation beneath arcs (100–150 km) requires a mechanism such as the partitioning of K, Be, B, Ba, and Rb into migrating fluids rather than the simple dehydration of phengite beneath arcs in order to provide for the transfer of these slab signature elements from phengite into arc magmas.

Large-scale hydration of the lithosphere above subducting slabs

During subduction, oceanic lithosphere, capped by variably hydrated uppermost mantle, oceanic crust, and sediments, undergoes progressive metamorphism and devolatilization. The petrologic evolution of subducting oceanic lithosphere and the overriding continental/oceanic lithosphere can be predicted by integrating thermal models with petrologic phase equilibria. In mature subduction zones that have achieved thermal steady state, the subducting slab lies at subsolidus conditions for depths < 100–150 km. Most subducting oceanic crust passes through the blueschist to eclogite facies transition where large amounts of H 2O are released by the breakdown of Na-amphibole, lawsonite (or clinozoisite), and chlorite. The large amounts of H 2O released by continuous blueschist → eclogite reactions can cause large-scale hydration of the overriding lithosphere and can trigger partial melting of the mantle wedge (lowermost lithosphere and asthenosphere). Upward migration of slab-derived fluids add H 2O, CO 2, SiO 2, and incompatible elements to the overlying lithospheric plate that result in the formation of new phases (e.g., serpentine, amphibole, and mica) and chemical modification of existing phases (e.g., trace element enrichment of clinopyroxene). Evidence for large-scale lithospheric hydration above subducting slabs includes serpentinite diapirs observed in the Marianas forearc, low P-wave velocities observed in the hanging wall of the Alaskan and Japanese subduction zones, and studies of high-pressure metamorphic terrains.

Fluid-melt-rock interaction in mafic eclogites and coesite-bearing metasediments: Constraints on volatile recycling during subduction

The question as to whether the metasomatizing-slab agent needed to form island-arc magmas is an aqueous fluid or a hydrous melt requires a knowledge of fluid-melt-rock interaction during high-pressure low-temperature metamorphism. In the Western Alps, typical “anhydrous” mafic eclogites (Monviso Massif) and coesite-bearing metasediments (Dora-Maira Massif) have experienced a prograde P- T path characteristic of a mature (cold) subduction zone. Microstructural, petrologic, geochemical and fluid inclusion studies show that fluid flow was limited during eclogite and high-grade blueschist facies metamorphism. Most of the fluids were driven off the rocks at pressures lower than 1 and 1.6 GPa, respectively. With increasing pressure and temperature, the remaining fluid phase was released during crystal plastic flow processes or devolatilisation reactions but retain the host rocks to form veins, partial melts or dense hydrous silicates. Tectonic erosion at modern convergent margins delivers large amounts of terrigenous sediments (KMASH system) in subduction zones. In contrast to the basalt and peridotite systems, the KMASH system is characterized by a wide range of pressure-sensitive reactions which allow the reconstruction of dehydration/hydration depths in the subducted slab. Independently of the thermal structure of subduction zones, terrigenous sediments will begin to melt at pressures < 2 GPa. The melt phase can absorb tremendeous quantities of water and represent major sinks for devolatilisation fluids. Dense hydrous silicates will form in any subducted sediments at all depths. These silicates can store significant amount of water in their structure. In the subducted oceanic crust and upper mantle, heterogeneous distribution of fluids inherited from hydrothermal alteration at mid-ocean ridges will cause contemporaneous dehydration and hydration reactions on a local scale rather than large-scale fluid infiltration in the overlying mantle wedge. The situation where a cold, competent and fluid-rich subducted slab is bounded above by a hot, relatively weak and anhydrous material is likely to act against buoyant rise of devolatilisation fluids by forcing the fluids to flow downwards. At sub-arc depths (50 ± 10 to 80 150 km ) and for any geothermal gradient, continuous internal buffering of volatile activities will retain the fluid phase in the slab. This is in contrast with shallow, sub-forearc depths (20 to 50 ± 10 km), where tectonic vein arrays, localized thrust faults, serpentine diapirs, mud volcanoes and boninite magmas attest to the release of large volume of fluids into the overriding mantle wedge.

Numerical simulation of metamorphic pressure‐temperature‐time paths and fluid production in subducting slabs

In order to investigate metamorphic reactions and fluid production in subduction zones, a 2‐dimensional finite difference model has been constructed. The model simulates heat transfer in a subduction zone with a 27° dip. A spatial resolution of 1 km is used in the region of the subduction thrust in order to carefully monitor metamorphic reactions; elsewhere the spatial resolution is 5 km. In these calculations, the oceanic crust is assumed to contain 2 wt % bound volatiles, distributed homogeneously within the 7.5‐km‐thick crust. Fluids are released by continuous and discontinuous end‐members of three different dehydration models: pressure‐sensitive, temperature‐sensitive, and amphibole (negative dP/dT) dehydration. Subduction zone pressure‐temperature‐time (P‐T‐t) paths predicted by the model intersect the wet basaltic solidus only at the initiation of subduction. In mature subduction zones, and in the absence of significant frictional heating and induced mantle convection, P‐T‐t paths encounter subsolidus conditions at depths of 100–150 km beneath magmatic arcs, suggesting that slab dehydration reactions play an important role in arc magma genesis. Metamorphic dehydration reactions that consume 50 kJ/kg result in P‐T‐t paths that are only 5 K cooler than models that neglect such reactions. In both the temperature and amphibole dehydration models, the fluid production region moves to greater depths over time as the subduction zone cools. Within the oceanic crust, fluid production starts at the top of the crust and migrates downward in the temperature and amphibole dehydration models. This phenomenon, coupled with the weakening effect of devolatilization reactions, may result in the accretion of oceanic crust to the hanging wall of the subduction zone. For a convergence rate of 3 cm/yr, average fluid fluxes out of the subducting slab range from ∼0.1 kg fluid/(m² yr) for the continuous reaction models to &gt;1 kg fluid/(m² yr) for the discontinuous pressure and amphibole dehydration models. Vertical fluid fluxes on the order of 1 kg fluid/(m² yr) can substantially perturb the thermal structure of overlying rocks and can cause large‐scale hydration and metasomatism of the overlying mantle wedge. In contrast, the thermal effect of fluid flow parallel to the subduction thrust zone is insignificant compared to the thermal effect of subducting oceanic lithosphere.

The petrology of the Harkerbreen Group, Ny Friesland, Svalbard: protoliths and tectonic significance

AbstractThe late Precambrian–early Palaeozoic rocks of Ny Friesland, which have been subjected to Caledonian deformation and metamorphism, constitute part of the Eastern Province or Terrane of Svalbard. The Harkerbreen group and other divisions of the Stubendorffbreen supergroup form a high-grade and intensely deformed core complex to this terrane which is bounded to the west by the Billefjorden Fault Zone and to the east by a major north–south shear zone. The Stubendorffbreen rocks exhibit two gneissic foliations, one axial planar to a large scale, F1fold nappe closing to the east and the other axial planar to kilometre-scale upright F2folds subsidiary to the Atomfjella Arch. Metamorphism in the mid-amphibolite facies range coincided with generation of these folds, and F3.crenulation folding was accompanied by waningP–Tconditions. A significant proportion of the gneisses within the Harkerbreen group display silica–major element covariation patterns consistent with their position in the granodiorite field on the AFM plot. Incompatible, immobile element ratios Zr/Ti v. Nb/Y indicate affinities with rhyolites to rhyodacites which is also suggested by their REE profiles. Normalized multi-element plots of the gneisses are similar to those of granites from attenuated within-plate settings such as Mull and Skaergaard. The amphibolites which were intruded in the D1–D2interval appear to be derivatives of fractionated basalts. They plot across the calk-alkaline tholeiite boundary on the AFM diagram, and the calc-alkaline character of some of the amphibolites is further suggested by their Yb-normalized Ce-Ta abundances. Zr-Ti-Y and REE abundances would support their derivation from a related suite of fractionated basalt liquids. On the Zr/Y v. Zr discrimination diagram the amphibolites appear to have compositions transitional between Mid Ocean Ridge and Within-Plate basalts whilst the Zr-Nb-Ta plot indicates Volcanic Arc Basalt affinities. Th-Hf-Ta and multi-element plots, however, indicate a marginal to back-arc basin setting possibly above a mature subduction zone. The late Caledonian Chydenius granite is an adamellite with mixed within-plate and syn-orogenic characteristics typical of post-collisional granites.

Enrichment of trace elements in garnet amphibolites from a paleo-subduction zone: Catalina Schist, southern California

The abundance, P- T stability, solubility, and element-partitioning behavior of minerals such as rutile, garnet, sphene, apatite, zircon, zoisite, and allanite are critical variables in models for mass transfer from the slab to the mantle wedge in deep regions of subduction zones. The influence of these minerals on the composition of subduction-related magmas has been inferred (and disputed) from inverse modelling of the geochemistry of island-arc basalt, or by experiment. Although direct samples of the dehydration + partial-melting region of a mature subduction zone have not been reported from subduction complexes, garnet amphibolites from melanges of circumpacific and Caribbean blueschist terranes reflect high T (>600°C) conditions in shallower regions. Such rocks record geochemical processes that affected deep-seated, high- T portions of paleo-subduction zones. In the Catalina Schist, a subduction-zone metamorphic terrane of southern California, metasomatized and migmatitic garnet amphibolites occur as blocks in a matrix of meta-ultramafic rocks. This mafic and ultramafic complex may represent either slab-derived material accreted to the mantle wedge of a nascent subduction zone or a portion of a shear zone closely related to the slab-mantle wedge contact, or both. The trace-element geochemistry of the complex and the distribution of trace elements among the minerals of garnet amphibolites were studied by INAA, XRF, electron microprobe, and SEM. In order of increasing alteration from a probable metabasalt protolith, three common types of garnet amphibolite blocks in the Catalina Schist are: (1) non-migmatitic, clinopyroxene-bearing blocks, which are compositionally similar to MORB that has lost an albite component; (2) garnet-amphibolite blocks, which have rinds that reflect local interaction between metabasite, metaperidotite, and fluid; and (3) migmatites that are extremely enriched in Th, HFSE, LREE, and other trace elements. These trace-element enrichments are mineralogically controlled by rutile, garnet, sphene, apatite, zircon, zoisite, and allanite. Alkali and alkaline earth elements are much less enriched in the solid assemblage, and thus appear to be decoupled from the other elements in the inferred metasomatic process(es). The compositions of migmatitic garnet amphibolite blocks seem to complement that of “average” island-arc tholeiite. Trace-element metasomatism reflects fluid-solid, rather than melt-solid, interaction. The metasomatic effects indicate that H 2O-rich fluid, perhaps with a significant component of Na-Al silicate and alkalis, carried Th, U, Sr, REE, and HFSE. Fractionations of LREE in migmatites resemble those of migmatitic metasedimentary rocks underlying the mafic and ultramafic complex. “Exotic” LREE deposited in allanite in migmatites could have been derived from fluids in equilibrium with subducted sediment. If the paleo-subduction zone represented by the mafic and ultramafic complex of the Catalina Schist had continued its thermal and fluid evolution, a selvage of similarly enriched rocks might have been generated along the slab-mantle wedge contact between ~30 and 85 km depth. Rocks affected by “subduction-zone metasomatism,” although rarely recognized at the surface, could be volumetrically significant products of the initiation of subduction and may prove to be geochemical probes of convergent margins that approach the significance of xenoliths in the study of other magmatic environments.

The tectonic implications of high-pressure metamorphism in the western Alps

Five mineralogical stages are recognized in the metamorphism of Alpine ophiolites from one area in the Zermatt–Saas zone. Constraints on P-T values indicate an abrupt change at the metamorphic peak (550–600 °C, 17–20 kbar) from a cold positive slope of 25°C/kbar or less, to cooling initially without discernible change of pressure. Correspondence of the prograde path to the P-T trend of T (in °C) = 150 + 23 P (in kbar) shown by a variety of Alpine units suggests that they all have P-T histories for which an England &amp; Richardson (1977) relaxation model (also England &amp; Thompson 1984) is inappropriate. Major tectonism at the peak pressure is indicated by association between minerals (kyanite, chloritoid, lawsonite in eclogite), vein growths, fluid action and thrust surfaces. The proposed metamorphic environment involves thrust repetition, subcreting ophiolite to the over-riding slab in a thermally mature subduction zone at a depth of approximately 70 km. Preservation is favoured when ocean closure leads to subduction of sialic crust, so providing a combination of buoyant uplift with the continued refrigeration by underthrusting of cold surface materials.