Serpentinite Matrix Research Articles

Boron isotopic discrimination for subduction-related serpentinites

Abstract The Guatemala Suture Zone (GSZ), Guatemala, is a region that contains two distinct suture-related serpentinite mélanges straddling the Motagua fault and an ophiolitic complex paired with the northern mélange. The serpentinite matrix of the mélanges formed by subduction-fluid hydration of peridotite from the deep mantle wedge. The occurrence of serpentinite from both exhumed subduction channel mélange and ophiolite is not uncommon in paleo–suture zones, but distinguishing them and their tectonic origin can be difficult. A new method of discrimination, based on boron isotopes in serpentine from both mélanges and ophiolite, as well as on mica and pyroxene from the metamorphic and vein-rock blocks embedded within the mélanges, has been developed. The metamorphic and vein samples have mainly negative δ11B, ranging from –15.3‰ to +4.3‰, in the same range as the serpentine from the mélanges (–14.4‰ to +9.7‰). In addition to being the most negative δ11B values ever measured in serpentinite, comparable values from vein minerals indicate that the same fluid serpentinized the overlying mantle. In contrast, serpentine samples from the ophiolite have positive δ11B, in the range 0‰ to +18.0‰, consistent with hydration by seawater-derived fluids. As the GSZ displays two mélanges whose serpentinite originated from two different deep subductions and mantle hydration, we hypothesize that the negative signature of exhumed mélange serpentine is the norm and that the B isotopic signature can be a useful tool to discriminate the tectonic origin of serpentinization in paleo–suture zones.

Sedimentary serpentinite and chaotic units of the lower Great Valley Group forearc basin deposits, California: updates on distribution and characteristics

ABSTRACTSedimentary serpentinite and related siliciclastic-matrix mélanges in the latest Jurassic to Lower Cretaceous lower Great Valley Group (GVG) forearc basin strata of the California Coast Ranges reach thicknesses of over 1 km and include high-pressure (HP) metamorphic blocks. These units crop out over an area at least 300 km long by 50 km wide. The serpentinite also contains locally abundant blocks of antigorite mylonite. Antigorite mylonite and HP metamorphic blocks were exhumed from depth prior to deposition in the unmetamorphosed GVG, but the antigorite mylonite may be mistaken for metamorphosed serpentinite matrix in localities with limited exposure. These olistostrome horizons can be distinguished from intact slabs of serpentinized peridotite associated with the Coast Range Ophiolite (CRO) or serpentinite mélanges of the Franciscan subduction complex (FC) on the basis of internal sedimentary textures (absent in CRO), mixing/interbedding with unmetamorphosed siliciclastic matrix and blocks (differs from CRO and FC), and preserved basal sedimentary contacts over volcanic rocks of the CRO or shale, sandstone, and conglomerate of the GVG (differs from CRO and FC). Even in the relatively well-characterized Palaeo trench–forearc region of the California Coast Ranges the GVG deposits are difficult to distinguish from similar units in the FC and CRO. In typical orogenic belts that exhibit greater post-subduction disruption, distinguishing forearc basin olistostrome deposits, subduction complex, and opholite mantle sections is much more difficult. Forearc basin olistostromal deposits have probably been misidentified as one of the other trench–forearc lithologic associations. Such errors may lead to erroneous interpretations of the nature of large-scale material and fluid pathways in trench–forearc systems, as well as misinterpretations of tectonic processes associated with HP metamorphism and exhumation of the resultant rocks.

Petrogenesis and 40Ar/39Ar dating of proto-forearc crust in the Early Cretaceous Caribbean arc: The La Tinta mélange (eastern Cuba) and its easterly correlation in Hispaniola

ABSTRACTThe La Tinta mélange is a small but singular ultramafic mélange sheet that crops out in eastern Cuba. It is composed of dolerite-derived amphibolite blocks embedded in a serpentinite matrix. The amphibolite blocks have mid-ocean ridge basalt (MORB)-like composition showing little if any imprint of subduction zone component, similar to most forearc and MOR basalts worldwide. Relict Cr-spinel and olivine mineral chemistry of the serpentinized ultramafic matrix suggest a forearc position for these rocks. These characteristics, together with a hornblende 40Ar/39Ar age of 123.2 ± 2.2 Ma from one of the amphibolite blocks, suggest that the protoliths of the amphibolite blocks correspond to forearc basalt (FAB)-related rocks that formed during the earlier stage of subduction initiation of the Early Cretaceous Caribbean arc. We propose that the La Tinta amphibolites correspond to fragments of sills and dikes of hypoabyssal rocks formed in the earlier stages of a subduction initiation scenario in the Pacific realm (ca. 136 Ma). The forearc dolerite-derived amphibolites formed by partial melting of upwelling fertile asthenosphere at the beginning of subduction of the Proto-Caribbean (Atlantic) slab, with no interaction with slab-derived fluids/melts. This magmatic episode probably correlates with Early Cretaceous basic rocks described in Hispaniola (Gaspar Hernandez serpentinized peridotite-tectonite). The dikes and sills cooled and metamorphosed due to hydration at low pressure (ca. 3.8 kbar) and medium to high temperature (up to 720ºC) and reached ca. 500ºC at ca. 123 Ma. At this cooling stage, serpentinite formed after hydration of the ultramafic upper mantle. This process might have been favoured by faulting during extension of the forearc, indicating an early stage of dike and sill fragmentation and serpentinite mélanges formation; however, full development of the mélange likely took place during tectonic emplacement (obduction) onto the thrust belt of eastern Cuba during the latest Cretaceous.

Early Neoproterozoic multiple arc–back-arc system formation during subduction–accretion processes between the Yangtze and Cathaysia blocks: New constraints from the supra-subduction zone NE Jiangxi ophiolite (South China)

The NE Jiangxi ophiolite in the eastern Jiangnan Orogen is a tectonic mélange that mainly consists of individual tectonic blocks comprising pyroxenite, gabbro, basalt, diorite, granite and chert in a matrix of serpentinite or tuffaceous greywacke. A combined geochemical and geochronological study of the NE Jiangxi ophiolite was undertaken to constrain the timing and tectonic setting of its formation. The basalts were geochemically subdivided into three groups with different FeOt and TiO2 contents. Group 1 basalts have the lowest FeOt (12.17–13.07 wt.%) and TiO2 (1.48–1.62 wt.%) contents and the lowest Nb/Yb (0.80–0.88) and Th/Nb (0.02–0.03) ratios. Furthermore, they have normal mid-ocean ridge basalt (N-MORB)-like trace element patterns, suggesting derivation from an N-MORB-type mantle source without subduction input. Group 2 Fe–Ti basalts have the highest FeOt (15.52–16.30 wt.%) and TiO2 (3.06–3.23 wt.%) contents, Nb/Yb and Th/Nb ratios from 1.75 to 1.89 and from 0.11 to 0.15, respectively, and trace element patterns similar to those of back-arc basin basalts. The geochemical characteristics suggest that Group 2 basalts were derived from a slightly enriched MORB-type mantle source with a minor subduction contribution. In contrast, Group 3 Fe–Ti basalts have moderate FeOt (12.98–13.40 wt.%) and TiO2 (2.37–2.71 wt.%) contents, and Nb/Yb and Th/Nb ratios from 1.28 to 1.45 and from 0.27 to 0.30, respectively. These basalts further display markedly negative Nb–Ta anomalies and show a geochemical affinity to island-arc basalts (IAB), indicating a slightly enriched MORB-type mantle source that was significantly influenced by subduction-derived fluids and/or melts. SIMS zircon U–Pb dating on gabbros gave ages of 995±22Ma and 993±12Ma, which are interpreted as the formation age of the NE Jiangxi ophiolite. Positive zircon εHf(t) (+8.8 to +13.8) values for the gabbros and whole-rock εNd(t) (+5.5 to +6.6) values for the basalts indicate that the NE Jiangxi ophiolite originated from an isotopically homogeneous depleted mantle source. The diversity of MORB- to IAB-like basalts and the presence of Fe–Ti basalts favor a formation of the NE Jiangxi ophiolite during the initial rifting phase of an intra-oceanic back-arc basin between an oceanic arc (Huaiyu Terrane) and the continental margin of the Yangtze Block (Jiuling Terrane) at ca. 990Ma. Both the present and previous studies imply that multiple arc–back-arc systems formed during long-lasting subduction–accretion processes between the Yangtze and Cathaysia blocks during the early Neoproterozoic.

Compositional diversity in peridotites as result of a multi-process history: The Pacific-derived Santa Elena ophiolite, northwest Costa Rica

The Santa Elena ophiolite (SEO) is an ultramafic nappe of more than 270km2 overlying a tectonic serpentinite–matrix mélange in northwest Costa Rica. It is mainly composed of Cpx-rich and Cpx-poor harzburgites (~2.5km-thick), with minor lherzolite, dunite and chromitite, as well as intrusive mafic sills and subvertical dikes, which coalesce into an upper Isla Negritos gabbroic sill complex. Minerals and whole-rock features of the Cpx-rich and Cpx-poor harzburgites share features of the abyssal and supra-subduction zone (SSZ) peridotites, respectively. To explain these characteristics two-stages of melting and refertilization processes are required. By means of trace element modeling, the composition of Cpx-rich harzburgites may be reproduced by up to ~5–10% melting of a primitive mantle source, and the composition of Cpx-poor harzburgites and dunites by ~15–18% melting of an already depleted mantle. Therefore, the Cpx-rich harzburgites can be interpreted as product of first-stage melting and low-degrees of melt–rock interaction in a mid-ocean ridge environment, and the Cpx-poor harzburgites and dunites as the product of second-stage melting and refertilization in a SSZ setting. The mafic sills and the Isla Negrito gabbros are genetically related and can be explained as crystallization from the liquids that were extracted from the lower SSZ mantle levels and emplaced at shallow conditions. The Murciélagos Island basalts are not directly related to the ultramafic and mafic rocks of the SEO. Their E-MORB-like composition is similar to most of the CLIP mafic lavas and suggests a common Caribbean plume-related source. The SEO represents a fragment of Pacific-derived, SSZ oceanic lithosphere emplaced onto the southern North America margin during the late Cretaceous. Because of the predominance of rollback-induced extension during its history, only a limited amount of crustal rocks were formed and preserved in the SEO.

Emplacement of serpentinites in the Chohar Gonbad-Gugher-Baft ophiolitic mélange, southeast Iran: examination of the mineral–chemical, petrologic, and structural features

The Chohar Gonbad-Gugher-Baft ophiolite melange, located along the major Baft and Shahr-e-Babak fault zones, southeast Iran, represents remnants of Neo-Tethyan oceanic lithosphere. This melange contains blocks of harzburgite, dunite, lherzolite, basalt, and other ophiolite-related lithologies tectonically mixed with and embedded in a serpentinite matrix. Field, petrographic, and geochemical data show that peridotites in this melange belong to the upper mantle. They seem to have undergone up to ~20 % partial melting in a supra-subduction zone setting, based on their spinel Cr# values (0.21–0.53). Chemical compositions and textures in the serpentinites indicate that they were partially hydrated during emplacement and further mobilized diapirically to the surface. The different deformation stages occurred in an accretionary wedge environment. Petrographic evidence shows that the first serpentinization event produced mesh-textured serpentinites formed under static conditions in an ocean floor environment (Nain-Baft ocean crust), where the initial lizardite, bastite, and chrysotile veins formed. Plastic deformation occurred due to the subduction of Nain-Baft oceanic lithospheric beneath the central Iranian microcontinent, with antigorite-bearing flare-textured serpentinites produced. During progressive exhumation of the Nain-Baft ophiolite melange, the serpentinites were affected by ductile, ductile–brittle, and brittle deformation, respectively. Accretion and resultant diapirism are the most important processes in the emplacement of serpentinite, which is a consequence of hydration of the ocean crust. In this example, late-stage emplacement via thrusting occurred along the northern extent of the southern Sanandaj–Sirjan zone (S–SZ).

Metamorphic history of riebeckite- and aegirine-augite-bearing high-pressure–low-temperature blocks within the Siuna Serpentinite Mélange, northeastern Nicaragua

The Siuna Serpentinite Mélange (SSM) is a subduction-zone-related complex that contains diverse blocks of igneous and sedimentary origin, overprinted by various metamorphic conditions. The SSM is located at the southern border of the Chortís block and marks the boundary between continental and oceanic crusts in the western margin of the Caribbean Plate. The serpentinite matrix mainly consists of lizardite/chrysotile, Cr-rich spinel, and relict orthopyroxene that suggest a harzburgitic protolith and an upper mantle supra-subduction zone origin. Blocks within the southern and central regions range from Jurassic pelagic sediments to mafic/intermediate igneous rocks that are metamorphosed to various degrees, ranging from prehnite-pumpellyite/greenschist to likely blueschist facies (e.g. riebeckite-bearing metashale) conditions. In contrast, the northern section encloses almost exclusively epidote-amphibolite facies metabasite blocks, and minor mica- and chlorite-rich rocks of metasomatic origin, respectively. Some of the epidote-amphibolite blocks contain relic garnet-rich zones embedded in an amphibole-rich matrix. The garnets appear to record two generations of growth and contain mineral inclusions such as amphibole, apatite, titanite, aegirine-augite, and quartz. Thermobarometric estimates for the garnet-rich zones and epidote-amphibolite-rich matrix suggest a prograde blueschist facies at ~1.2 GPa and 400–450°C, an eclogite facies metamorphic peak at 1.5–1.7 GPa and 565–614°C, and a post-peak epidote-amphibolite facies metamorphism. These pressure and temperature estimates indicate a classical clockwise PT path that has been observed in many palaeo-subduction zone environments worldwide. Phengite Ar–Ar dating of mica-rich rock yields 140 Ma and suggests an Early Cretaceous exhumation along the southern edge of the continental Chortís block.

Structural evolution and deformation kinematics of a subduction-related serpentinite-matrix mélange, Santa Elena peninsula, northwest Costa Rica

Detailed structural analysis of the Isla Pelada serpentinite-matrix mélange on Santa Elena Peninsula, Pacific coast of northwest Costa Rica, elucidates for the spatial distribution and temporal progression of a décollement-related deformation. This décollement zone constitutes the fault rock of a plate boundary in a subduction to collision setting. The décollement zone is defined by a thick tectonic mélange that includes blocks and belts of polymictic breccia, turbiditic greywacke, argillite, radiolarite, basalt, gabbro and serpentinized peridotite in a variably foliated serpentinite matrix. The chemical-mineral data of the peridotite blocks in the mélange indicate that they are fragments of abyssal mantle, different of the very depleted forearc mantle peridotites of the overlying Santa Elena ultramafic nappe.The structural evolution of the mélange can be divided into four main events. Early D1 deformation took place during underthrusting in the footwall of the décollement and is characterized by subhorizontal layer-parallel extension and subsequent by heterogeneous shearing of trench-fill deposits, oceanic volcanic rocks and variably serpentinized peridotites, resulting asymmetric boudinage, mesoscopic porphyroclasts and pervasive (S1) S–C fabrics. D1 is followed by layer-parallel contraction with early D2 folding and local development of an S2 pressure-solution cleavage, and late-D2 thrusting and large-scale duplexing. Therefore, D2 deformation evolves from pervasive coaxial subhorizontal contraction to a more localized non-coaxial strain (along relatively narrow thrust surfaces, in the mélange). D2 deformation mainly occurred in the hanging-wall of the décollement and was related to the downward migration of the subduction thrust. The result is the incorporation of slices of the subducted sequence by underplating into the overriding plate, typical of accretionary prisms.Kinematic indicators in the mélange are consistent with a general eastward-directed shearing during D1 and D2 deformations. This kinematics can be correlated with the direction of emplacement of the overlying Santa Elena ultramafic nappe during the early Upper Cretaceous. This also suggests the western-directed subduction of the oceanic lower plate. After the mélange was incorporated into the accretionary prism, the whole tectonic edifice was deformed in pre-late Campanian time by WNW–ESE trending D3 cylindrical upright folding and S-directed D4 narrow brittle thrusting. The recorded evolution of deformation in rocks already lithified, combined with the exotic origin of the oceanic lithologies and the serpentinite nature of the matrix points to disruption and mélange formation mainly during tectonic burial and underthrusting. In this context, the serpentinite matrix of the mélange is the result of the low-T hydration, metamorphism and deformation of mantle peridotites in a relatively shallow subduction setting.

Fluid-metapelite interaction in an ultramafic mélange: implications for mass transfer along the slab-mantle interface in subduction zones

The slab-mantle interface in subduction zones is a site of tectonic mixing of crustal and mantle rocks. It is the interface for fluid flow of slab-derived components into the mantle wedge. To assess the fluid-rock interaction along the slab-mantle interface, we studied the bleaching of pelitic schist in an ultramafic mélange. The Nishisonogi metamorphic rocks in Kyushu, Japan, comprise ultramafic mélanges intercalated with epidote-blueschist facies schists. The ultramafic mélange consists of tectonic blocks of various lithologies and a matrix of chlorite-actinolite schist and serpentinite. Along the contact with the mélange matrix, pelitic schist blocks are bleached mainly due to the modal increase of albite and the consumption of carbonaceous material. The bleaching is probably attributed to infiltration of Na-rich external fluid from the mélange matrix. Mass balance analysis indicates losses of C, Rb, K2O, Ba, Pb, and SiO2 from the bleached pelitic schist, although Al2O3, TiO2, Sc, Y, Zr, Nb, La, Ce, and Nd remain immobile. This suggests fractionation of large-ion lithophile elements (LILE) and Pb from the high-field-strength elements and rare earth elements during the bleaching. If this ultramafic mélange is analogous to the slab-mantle interface, similar infiltration metasomatism will promote liberation of C, Si, LILE, and Pb from subducting metapelites and enhance metasomatism of the mantle wedge.

Seismic visibility of a deep subduction channel – insights from numerical simulation of high-frequency seismic waves emitted from intermediate depth earthquakes

Abstract. Return flow in a deep subduction channel (DSC) has been proposed to explain rapid exhumation of high pressure–low temperature metamorphic rocks, entirely based on the fossil rock record. Supported by thermo-mechanical models, the DSC is envisioned as a thin layer on top of the subducted plate reaching down to minimum depths of about 150 km. We perform numerical simulations of high-frequency seismic wave propagation (1–5 Hz) to explore potential seismological evidence for the in situ existence of a DSC. Motivated by field observations, for modeling purposes we assume a simple block-in-matrix (BIM) structure with eclogitic blocks floating in a serpentinite matrix. Homogenization calculations for BIM structures demonstrate that effective seismic velocities in such composites are lower than in the surrounding oceanic crust and mantle, with nearly constant values along the entire length of the DSC. Synthetic seismograms for receivers at the surface computed for intermediate depth earthquakes in the subducted oceanic crust for models with and without DSC turn out to be markedly influenced by its presence or absence. While for both models P and S waveforms are dominated by delayed high-amplitude guided waves, models with DSC exhibit a very different pattern of seismic arrivals compared to models without DSC. The main reason for the difference is the greater length and width of the low-velocity channel when a DSC is present. Seismic velocity heterogeneity within the DSC or oceanic crust is of minor importance. The characteristic patterns allow for definition of typical signatures by which models with and without DSC may be discriminated. The signatures stably recur in slightly modified form for earthquakes at different depths inside subducted oceanic crust. Available seismological data from intermediate depth earthquakes recorded in the forearc of the Hellenic subduction zone exhibit similar multi-arrival waveforms as observed in the synthetic seismograms for models with DSC. According to our results, observation of intermediate depth earthquakes along a profile across the forearc may allow to test the hypothesis of a DSC and to identify situations where such processes could be active today.

Jurassic–Paleogene intraoceanic magmatic evolution of the Ankara Mélange, north-central Anatolia, Turkey

Abstract. Oceanic rocks in the Ankara Mélange along the Izmir–Ankara–Erzincan suture zone (IAESZ) in north-central Anatolia include locally coherent ophiolite complexes (~ 179 Ma and ~ 80 Ma), seamount or oceanic plateau volcanic units with pelagic and reefal limestones (96.6 ± 1.8 Ma), metamorphic rocks with ages of 256.9 ± 8.0 Ma, 187.4 ± 3.7 Ma, 158.4 ± 4.2 Ma, and 83.5 ± 1.2 Ma indicating northern Tethys during the late Paleozoic through Cretaceous, and subalkaline to alkaline volcanic and plutonic rocks of an island arc origin (~ 67–63 Ma). All but the arc rocks occur in a shale–graywacke and/or serpentinite matrix, and are deformed by south-vergent thrust faults and folds that developed in the middle to late Eocene due to continental collisions in the region. Ophiolitic volcanic rocks have mid-ocean ridge (MORB) and island arc tholeiite (IAT) affinities showing moderate to significant large ion lithophile elements (LILE) enrichment and depletion in Nb, Hf, Ti, Y and Yb, which indicate the influence of subduction-derived fluids in their melt evolution. Seamount/oceanic plateau basalts show ocean island basalt (OIB) affinities. The arc-related volcanic rocks, lamprophyric dikes and syenodioritic plutons exhibit high-K shoshonitic to medium- to high-K calc-alkaline compositions with strong enrichment in LILE, rare earth elements (REE) and Pb, and initial εNd values between +1.3 and +1.7. Subalkaline arc volcanic units occur in the northern part of the mélange, whereas the younger alkaline volcanic rocks and intrusions (lamprophyre dikes and syenodioritic plutons) in the southern part. The late Permian, Early to Late Jurassic, and Late Cretaceous amphibole-epidote schist, epidote-actinolite, epidote-chlorite and epidote-glaucophane schists represent the metamorphic units formed in a subduction channel in the northern Neotethys. The Middle to Upper Triassic neritic limestones spatially associated with the seamount volcanic rocks indicate that the northern Neotethys was an open ocean with its MORB-type oceanic lithosphere by the early Triassic (or earlier). The latest Cretaceous–early Paleocene island arc volcanic, dike and plutonic rocks with subalkaline to alkaline geochemical affinities represent intraoceanic magmatism that developed on and across the subduction–accretion complex above a N-dipping, southward-rolling subducted lithospheric slab within the northern Neotethys. The Ankara Mélange thus exhibits the record of ~ 120–130 million years of oceanic magmatism in geological history of the northern Neotethys.

The microstructural record of porphyroclasts and matrix of partly serpentinized peridotite mylonites – from brittle and crystal-plastic deformation to dissolution–precipitation creep

Abstract. We present microfabrics in high-pressure, metamorphic, partly serpentinized peridotite mylonites from the Voltri Massif, in which porphyroclasts and matrix record independent deformation events. The microfabrics are analysed using polarization microscopy and electron microscopy (SEM/EBSD, EMP). The mylonites contain diopside and olivine porphyroclasts originating from the mantle protolith embedded in a fine-grained matrix consisting mainly of antigorite and minor olivine and pyroxene. The porphyroclasts record brittle and crystal-plastic deformation of the peridotite at upper-mantle conditions and differential stresses of a few hundred MPa. After the peridotites became serpentinized, deformation occurred mainly by dissolution–precipitation creep resulting in a pronounced foliation of the antigorite matrix, crenulation cleavages and newly precipitated olivine and pyroxene from the pore fluid at sites of dilation, i.e. in strain shadows next to porphyroclasts and folded fine-grained antigorite layers. Antigorite reveals a pronounced associated shape preferred orientation (SPO) and crystallographic preferred orientation (CPO) with the basal (001) cleavage plane oriented in the foliation plane. In monomineralic antigorite aggregates at sites of stress concentration around porphyroclasts, a characteristically reduced grain size and deflecting CPO as well as sutured grain boundaries indicate also some contribution of crystal-plastic deformation and grain-boundary migration of antigorite. In contrast, the absence of any intragranular deformation features in newly precipitated olivine in strain shadows reveals that stresses were not sufficiently high to allow for significant dislocation creep of olivine at conditions at which antigorite is stable. The porphyroclast microstructures are not associated with the microstructures of the mylonitic matrix, but are inherited from an independent earlier deformation. The porphyroclasts record a high-stress deformation of the peridotite with dislocation creep of olivine in the upper mantle probably related to rifting processes, whereas the serpentinite matrix records dominantly dissolution–precipitation creep and low stresses during subduction and exhumation.

Metamorphic history of glaucophane‐paragonite‐zoisite eclogites from the Shanderman area, northern Iran

AbstractThe Shanderman eclogites and related metamorphosed oceanic rocks mark the site of closure of the Palaeotethys ocean in northern Iran. The protolith of the eclogites was an oceanic tholeiitic basalt with MORB composition. Eclogite occurs within a serpentinite matrix, accompanied by mafic rocks resembling a dismembered ophiolite. The eclogitic mafic rocks record different stages of metamorphism during subduction and exhumation. Minerals formed during the prograde stages are preserved as inclusions in peak metamorphic garnet and omphacite. The rocks experienced blueschist facies metamorphism on their prograde path and were metamorphosed in eclogite facies at the peak of metamorphism. The peak metamorphic mineral paragenesis of the rocks is omphacite, garnet (pyrope‐rich), glaucophane, paragonite, zoisite and rutile. Based on textural relations, post‐peak stages can be divided into amphibolite and greenschist facies. Pressure and temperature estimates for eclogite facies minerals (peak of metamorphism) indicate 15–20 kbar at ~600 °C. The pre‐peak blueschist facies assemblage yields <11 kbar and 400–460 °C. The average pressure and temperature of the post‐peak amphibolite stage was 5–6 kbar, ~470 °C. The Shanderman eclogites were formed by subduction of Palaeotethys oceanic crust to a depth of no more than 75 km. Subduction was followed by collision between the Central Iran and Turan blocks, and then exhumation of the high pressure rocks in northern Iran.

Jadeitite formed during subduction: In situ zircon geochronology constraints from two different tectonic events within the Guatemala Suture Zone

Jadeitite is a rare rock type associated with high-pressure–low-temperature blocks within serpentinite matrix mélanges. Models of formation involve precipitation from subduction-zone aqueous fluids veining the overlying mantle wedge (P-type), or metasomatism of igneous and/or sedimentary protoliths previously emplaced into the mélange (R-type). Age determinations of mélange lithologies provide constraints on the timing of “peak metamorphism” and subsequent exhumation. The timing of jadeitite formation, particularly in the rich source of the Guatemala Suture Zone (GSZ), is a controversial subject needing further attention.Over 80 in situ zircon crystals from three jadeitites and two mica–albite rocks from the North Motagua Mélange and one phengite jadeitite from the South Motagua Mélange of the GSZ were studied for age and trace-element determination. Most of these zircons are characterized by low Th/U ratios, depleted chondrite-normalized REE patterns relative to zircons from oceanic gabbros, and contain fluid and mineral inclusions that reflect the primary mineralogy (i.e., jadeite) and context (i.e., crystallization from an aqueous fluid) of the host rock, and thus formed during jadeitite crystallization. The SHRIMP-RG and LAM-ICP-MS U–Pb dates from zircon indicate that jadeitites and mica–albite rocks from the GSZ were formed through vein precipitation at ~98−80 and ~154–158Ma, respectively. These data show (a) older ages that indicate jadeitite crystallization occurred ~10–30Ma before the preserved subduction-zone peak metamorphism (e.g., exhumed eclogite), and (b) a second group of ages slightly younger than, or similar to, exhumation ages given by Ar–Ar dates from micas. Similar relationships occur at other jadeitite occurrences, such as the Syum-Keu ultramafic complex in the Polar Urals (Russia) and the serpentinite mélanges of the Río San Juan complex (Dominican Republic). The data argue for formation of jadeitite within the mantle wedge during active subduction. Thus, jadeitite provides a record of fluid introduction into the mantle wedge during subduction rather than during exhumation.

Subduction initiation along transform faults: The proto-Franciscan subduction zone

The initiation of subduction is a process that cannot be observed directly but must be inferred from the rock record after subduction is well established. There are many approaches possible to infer the origin of subduction zones that are still active, but paleosubduction zones present special challenges, since their geodynamic setting can no longer be directly observed. In this study, we examine evidence for subduction initiation of the proto-Franciscan subduction zone along a transform fault, based on a subduction initiation origin for the Coast Range ophiolite, and on the Tehama-Colusa serpentinite melange, which underlies the ophiolite and separates it from high-pressure/temperature metamorphic rocks of the Franciscan complex. The Coast Range ophiolite consists of volcanic, plutonic, and mantle components, each of which contains elements that reflect subduction initiation or hydrous melting within a subduction-zone setting. The volcanic assemblage includes forearc basalts and boninites, as well as more evolved calc-alkaline rocks; the plutonic complex contains intrusive suites that reflect this same range of parent magmas. Peridotites of the mantle section include both abyssal-like and refractory peridotites formed by hydrous decompression melting. The Tehama-Colusa serpentinite melange consists of blocks of basalt, chert, sedimentary rocks, and peridotite (harzburgite and lherzolite) in a sheared serpentinite matrix. The melange matrix represents hydrated refractory peridotites with forearc affinities, and blocks within the melange consist largely of upper-plate lithologies (harzburgite, arc volcanics, and arc-derived sediments). Lower-plate blocks within the melange include oceanic basalts and chert with rare blueschist and amphibolite. The abyssal peridotites have low pyroxene equilibration temperatures that are consistent with formation in a fracture-zone setting. However, the current melange reflects largely upper-plate lithologies in both its matrix and its constituent blocks. We propose that the proto-Franciscan subduction zone nucleated on a large offset transform fault or fracture zone that evolved into a subduction-zone melange complex. The nucleation of subduction zones along former transform boundaries has long been proposed for both modern arc systems and for the Franciscan–Coast Range ophiolite system. Our data support this interpretation and document more fully how this mechanism is expressed by mixing within the evolving serpentinite melange.

Mesozoic Oceanic Terranes of Southern Central America-Geology, Geochesmistry and Geodynamics.

The Northwestern edge of the modem Caribbean Plate, located in central Middle America (S-Guatemala to N-Costa Rica), is characterized by a puzzle of oceanic and continental terranes that belonged originally to the Pacific facade of North America. South of the Motagua Fault Zone, the actual northern strike slip boundary of the Caribbean Plate, three continental slivers (Copan, Chortis s. str. and Patuca) are sandwiched between two complex suture zones that contain HP/LT mafic and ultramafic oceanic rocks: The Motagua Melanges to the North, extensively studied in the last ten years and the newly defined Mesquito Composite Oceanic Terrane (MCOT) to the South. No modern geological data were available for the oceanic terrane located in the southern part of the so called continental “Chortis Block”. Classically, the southern limit of this block with the Caribbean Large Igneous Province (CLIP) was placed at a hypothetical fault line connecting the main E-W fault in the Santa Elena Peninsula (N-Costa Rica) with the Hess Escarpment. However, our study in eastern Nicaragua and northwestern Costa Rica evidences an extensive assemblage of oceanic upper mantle and crustal rocks outcropping between the Chortis/Patuca continental blocks and the CLIP. They comprise collided and accreted exotic terranes of Pacific origin recording a polyphased tectonic history. We distinguish: 1The MCOT that comprises a Late Triassic to Early Cretaceous puzzle of oceanic crust and are-derived rocks set in a serpentinite matrix, and 2The Manzanillo and Nicoya Terranes that are made of Cretaceous plateau-like rocks associated with oceanic sediments older than the CLIP. This study has been focused on the rocks of the MCOT. The MCOT comprises the southern half of the former “Chortis Block” and is defined by 4 corner localities characterized by ultramafic and mafic oceanic rocks of Late Triassic, Jurassic and Early Cretaceous age: 1 The Siuna Serpentinite Melange (NE-Nicaragua), 2The El Castillo Melange (Nicaragua-Costa Rica border), 3DSDP Legs 67 and 84 (Guatemala fore-are basin), and 4The Santa Elena Peridiotite (NWCosta Rica). The Siuna Serpentinite Melange (SSM) is a HP/LT subduction zone melange set in a serpentinite matrix that contains Oceanie crust and arcrelated greenschist to blueschist/eclogite facies metamafic and metasedimentary blocks. Middle Jurassic (Rajocian-Bathonian) radiolarites are found in original sedimentary contact with arc-derived greenstones. Late Jurassic black detrital chert possibly formed in a marginal (fore-arc?) basin shortly before subduction. A phregnite 40Ar/39Ar -cooling age dates the exhumation of the high pressure rocks as 139 Ma. The El Castillo Melange Mesozoic oceANic TeRRANes oF soUTHeRN ceNTRAL AMeRicA-GeoLoGY, GeocHesMisTRY AND GeoDYNAMics

Progressive deformation partitioning and recrystallization of olivine in the lithospheric mantle

Intense serpentinization within the Machinoyama ultramafic body, eastern-most part of the Yakuno ophiolite in the Paleozoic Maizuru belt, SW Japan, resulted in the development of block-in-matrix structure in the brittle regime, possibly associated with exhumation along a fault. Various microstructures are heterogeneously distributed throughout peridotite blocks in a serpentinite matrix. The microstructures are classified into four domains according to olivine grain size: coarse (1.0–1.5mm), medium (0.5–1.0mm), small (0.2–0.5mm), and fine-grained (0.01mm). Even in a single peridotite block, the medium-, small- and fine-grained domains occur at various scales. In particular, the fine-grained domain occurs in thin zones that cut across the other domains. These observations suggest that deformation in the peridotites before the serpentinization was partitioned into anastomosing zones during progressive inhomogeneous non-coaxial shear, resulting in the simultaneous development of a range of microstructures from the coarse-grained domain to the small-grained domain. Olivine crystal-preferred orientations tend to be weaker with decreasing olivine grain size, indicating a change in deformation mechanism from dislocation creep to grain-size-sensitive creep. Moreover, the fine-grained domains contain much more second phase than the small-grained domain even in the same sample, suggesting the role of the second phases on strain localization during dynamic recrystallization of olivine.

Sandstone matrix olistostrome deposited on intra-subduction complex serpentinite, Franciscan Complex, western Marin County, California

Detailed field mapping in the Franciscan Complex in western Marin County, California, offers insight into the origins and development of mélanges, as well as accretionary complex architecture. From structurally high to low, the study area units comprise a structurally high sandstone matrix olistostrome, serpentinite, a structurally intermediate sandstone matrix olistostrome, Early Cretaceous Nicasio Reservoir Terrane greenstone, and coherent Cretaceous greywacke, all exhibiting prehnite–pumpellyite burial metamorphism. The structurally high sandstone matrix olistostrome includes volcanic blocks up to 700m in diameter and chert blocks up to 100m in diameter, whereas the structurally intermediate olistostrome unit comprises a lithic-rich sandstone matrix enclosing volcanic blocks 1–20m in diameter. The youngest detrital zircons in the high olistostrome and the coherent greywacke have U–Pb ages of ≈20m.y. older than the inferred depositional ages of the units themselves, suggesting that the bulk of the detritus comprising these units was derived by exposure and sedimentary recycling of significantly older, previously accreted sediments. The matrix of both olistostrome units exhibits minimal strain with weak local development of pressure solution cleavage. These findings demonstrate introduction of Franciscan exotic blocks and development of Franciscan block-in-matrix fabrics through sedimentary sliding and support sedimentary interpretations for more highly strained shale and serpentinite matrix mélanges. Additionally, these data suggest that sandstone matrix olistostromes/mélange may constitute a larger part of the Franciscan and other accretionary complexes than previously believed.The structurally high olistostrome preserves a depositional contact on serpentinite, verifying an intra-subduction complex origin for the serpentinite. The serpentinite lacks block-in-matrix structure and likely represents a tectonically emplaced unit derived from the subducting plate as a coherent nappe, rather than a sedimentary serpentinite deposit. Multi-kilometer scale west-vergent overturned folds with subhorizontal fold axes deform the nappes of the field area, and a later generation of open, northeast-trending folds overprints the earlier folds.

Serpentinites and serpentinites within a fossil subduction channel: La Corea mélange, eastern Cuba

A variety of metaultramafic (serpentinite) rocks in La Corea melange, Sierra de Cristal, eastern Cuba, show differences in chemical, textural and mineralogical characteristics demonstrating a variety of protoliths. The melange originated during the Cretaceous as part of the subduction channel associated with the Caribbean island arc. This melange contains high pressure blocks in a serpentinite matrix and occurs at the base of the large tabular Mayari-Cristal ophiolite. Two principal groups of serpentinites have been identified in the melange: a) antigorite serpentinite, mainly composed of antigorite and b) antigorite-lizardite serpentinite, composed of mixtures of antigorite and lizardite and bearing distinctive porphyroblasts of diopsidic clinopyroxene. Antigorite serpentinites are closely related to tectonic blocks of amphibolite (representing subducted MORB) and constitute deep fragments of the serpentinitic subduction channel formed during hydration of the mantle wedge. The composition of the antigorite-lizardite serpentinites and the presence of clinopyroxene porphyroblasts in this type of rock suggest that abyssal lherzolite protoliths transformed into serpentinite before and during incorporation (as tectonic blocks) in the shallow part of the subduction channel. Although the studied rocks have different origin, mineralogical compositions and textures, they display similar PGE compositions, suggesting that these elements experienced no significant redistribution during metamorphism. Both types of serpentinites were exposed together in the La Corea melange during the Late Cretaceous, during obduction of the overriding Mayari-Baracoa ophiolitic belt that led to exhumation of the subduction channel (melange)

Subducted sedimentary serpentinite mélanges: Record of multiple burial–exhumation cycles and subduction erosion

Serpentinite matrix mélanges give insight into large-scale convergent plate margin processes, particularly because of the derivation of the serpentinite from oceanic mantle. Similar to shale-matrix mélanges, a field geologist may easily recognize the sedimentary origins of little-deformed serpentinite matrix mélanges, but mélanges within accretionary prisms have undergone significant deformation and recrystallization of matrix. Serpentinite mélanges of the Franciscan subduction complex of California have a seemingly intact and foliated matrix. Such exposures contrast sharply with the granular undeformed sedimentary serpentinite mélanges of the coeval Great Valley Group (GVG) forearc basin deposits that depositionally overlie Coast Range Ophiolite (that structurally overlies the Franciscan). Nonetheless, Franciscan serpentinite mélanges display evidence of sedimentary origins, including sedimentary breccia composed of exotic block material (Tolay Ridge), sedimentary serpentinite breccia (Panoche Pass Road), basal serpentinite conglomerate with exotic clasts (Sunol Regional Wilderness), and serpentinite sandstones and conglomerates, including a basal conglomerate overlying coherent metagraywacke (Tiburon Peninsula). These examples record two burial–exhumation cycles to blueschist facies depths. In addition, a mélange/breccia in the Panoche Pass area may have components that record three burial–exhumation cycles to blueschist (or greater) depth. Exhumation rates for various cycles ranged from about 1.2 to 10mm/year. The Tiburon Peninsula serpentinite mélange occupies the structurally highest horizon in the Franciscan of the San Francisco Bay area, and regional field relationships indicate deposition at ca. 100Ma. Apparently, about 65Ma of subduction erosion/non accretion followed initiation of Franciscan subduction in this region. The oldest Franciscan serpentinite mélanges are at least 35Ma younger than sedimentary serpentinites of the GVG. Subduction erosion may have facilitated reworking of forearc sedimentary serpentinite deposits into the trench. Evidence of multiple-burial exhumation cycles indicates reworking of subduction complex material, consistent with existence of clastic units that have fossils indicative of significantly older ages than detrital zircon ages.