High-grade Blueschist Research Articles

Pressure–temperature estimates of the lizardite/antigorite transition in high pressure serpentinites

Serpentine minerals in natural samples are dominated by lizardite and antigorite. In spite of numerous laboratory experiments, the stability fields of these species remain poorly constrained. This paper presents petrological observations and the Raman spectroscopy and XRD analyses of natural serpentinites from the Alpine paleo-accretionary wedge. Serpentine varieties were identified from a range of metamorphic pressure and temperature conditions from sub-greenschist (P<4kbar, T~200–300°C) to eclogite facies conditions (P>20kbar, T>460°C) along a subduction geothermal gradient. We use the observed mineral assemblage in natural serpentinite along with the Tmax estimated by Raman spectroscopy of the carbonaceous matter in associated metasediments to constrain the temperature of the lizardite to antigorite transition at high pressures. We show that below 300°C, lizardite and locally chrysotile are the dominant species in the mesh texture. Between 320 and 390°C, lizardite is progressively replaced by antigorite at the grain boundaries through dissolution–precipitation processes in the presence of SiO2 enriched fluids and in the cores of the lizardite mesh. Above 390°C, under high-grade blueschist to eclogite facies conditions, antigorite is the sole stable serpentine mineral until the onset of secondary olivine crystallization at 460°C.

Tectonic evolution of early Paleozoic HP metamorphic rocks in the North Qilian Mountains, NW China: New perspectives

The North Qilian Mountain range is one of the three oldest orogenic belts preceded with a cold oceanic subduction-zone so far recognized on Earth with a subduction history from 490 Ma to 440 Ma in rock record. This orogenic belt has received much attention over the past decades. A significantly improved understanding of the tectonic evolution of the orogenic belt is now emerging thanks to detailed mineralogical, petrologic, geochemical and geochronological studies as well as field observations carried out over the past few years. In this paper, we offer new perspectives on the tectonic evolution of the North Qilian orogenesis in terms of subduction-zone metamorphism by integrating existing observations and our new data. We provide fairly comprehensive documentation of various observations to support our interpretations, which will also serve as a convenient guide for the field excursion of the 8th International eclogite conference. In the North Qilian subduction-zone complex (suture zone), two sub-belts of high-pressure (HP) metamorphism have been distinguished in terms of mineral assemblages, i.e., the low-grade blueschist belt and high-grade blueschist–eclogite belt. The low-grade blueschist belt consists of lawsonite–pumpellyite–glaucophane schist, lawsonite–glaucophanite and minor lawsonite-bearing felsic blueschist with a typical mineral assemblage of Lws + Pmp + Gln + Ab + Chl ± Arg. This assemblage constrains the metamorphic conditions of T = ∼250–350 °C and P = 0.6–1.1 GPa. Protoliths of the lawsonite-bearing mafic blueschist are similar to present-day N-type MORB. The high-grade blueschist belt occurs as three slices within the island-arc volcanic complex and consists of blueschist- to eclogite-facies metamorphosed greywacke, basite, serpentinite, meta-pelite, chert and marble. Lawsonite and Mg-carpholite occur in eclogites and meta-pelites, respectively. P− T estimation yields peak metamorphic conditions of P = 2.2–2.6 GPa and T = 460−540 °C for eclogite and carpholite meta-pelite. Geochemical data suggest that protoliths of mafic blueschist and eclogite in both belts have features of present-day MORB or OIB. These observations indicate that the North Qilian HP metamorphic rocks have experienced a history of seafloor subduction in a cold subduction-zone with a geotherm of ∼6–7 °C/km in the early Paleozoic before exhumed at the Late Silurian and Devonian in response to continental collision.

The neodymium isotopic and geochemical composition of serpentinites from ophiolitic assemblages in the Qilian fold belt, northwest China

Serpentinites are a common component of ophiolitic assemblages in the Qilian fold belt. Samples associated with MORB-like basalts in the northern low-grade blueschist belt of the North Qilian terrane have lherzolitic and pyroxenitic compositions, high ε Nd( T) values of +5.5 to +6.9, and likely represent residues from melting along spreading ridge segments. Despite evidence for incompatible element enrichment by hydrothermal fluids which had interacted with the ocean floor sediment pile, samples at the Ayougou locality retain original isotope Sm–Nd systematics and yield a Silurian age of 422±20 Ma with ε Nd( T)=+6.8. Serpentinites in the southern high-grade blueschist belt of the North Qilian terrane and from the Lha-Jyi terrane of the South/Central Qilian belt are associated with volcanic arc rocks, and include lherzolitic, harzburgitic and dunitic compositions characterised by low ε Nd( T) values of +3.8 to −2.2. Harzburgitic samples from these localities display U-shaped REE profiles, and weakly developed negative Nb-anomalies superimposed on incompatible element enrichments, and are interpreted as remnants of oceanic lithosphere metasomatised by slab-derived fluids in a supra-subduction zone environment. Anomalous enrichments in the mobile elements U, Sr and Ba in serpentinites throughout the Qilian belt may be attributed to post-obduction alteration by groundwater.

Eclogite-Facies Mineral Inclusions in Clinozoisite from Paleozoic Blueschist, Central Chugoku Mountains, Southwest Japan: Evidence of Regional Eclogite-Facies Metamorphism

An eclogite-facies assemblage garnet + omphacite + rutile + glaucophane + quartz occurs as inclusions in clinozoisite porphyroblasts in a high-grade blueschist block in the Paleozoic Osayama serpentinite mélange of the Chugoku Mountains, southwest Japan. Textual relations, and parageneses and compositions of minerals, indicate that the high-grade block experienced peak eclogite-facies metamorphism (M 1) and subsequent blueschist-facies overprinting (M 2). Moreover, low-P amphibolite-facies minerals (M 0) that may represent remnants of pre-subduction conditions are locally preserved in the protolith. Omphacite + adjacent garnet inclusion pairs in clinozoisite give a minimum P = 1.1-1.3 GPa at T = ~480-550°C. Many petrologic similarities between the Osayama eclogite and the Hida Mountains eclogite provide evidence for a regional eclogite-facies metamorphism in Paleozoic terranes of southwest Japan. Considering available petrotectonic information regarding eastern Asia, we suggest two alternative models: (1) Japanese Paleozoic eclogites may represent "Pacific-type" subduction prior to collision between the Sino-Korean and Yangtze blocks, which occurred on both western and eastern sides of the Qinling-Dabie-Sulu belt; or (2) Japanese Paleozoic eclogites may represent part of a Paleozoic "Pacific-type" subduction chain, which is continuous from the China-Russia border through Japan southward to North Korea.

Structure and Metamorphism of the Franciscan Complex, Mt. Hamilton Area, Northern California

Truncation of metamorphic isograds and fold axes within coherent terranes of Franciscan meta-graywacke by intervening zones of melange indicate that the melange is tectonic and formed after the subduction-related metamorphism and folding. These relations are expressed in two terranes of blueschist-facies rocks of the Franciscan Complex in the Mt. Hamilton area, northern CaliforniA&Mdash; the Jurassic Yolla Bolly terrane and the structurally underlying Cretaceous Burnt Hills terrane. Local preservation in both terranes of basal radiolarian chert and oceanic basalt beneath continent-derived metagraywacke and argillite demonstrates thrust repetition within the coherent terranes, although these relations are scarce near Mt. Hamilton. The metagraywackes range from albite-pumpellyite blueschists to those containing well-crystallized jadeitic pyroxene, and a jadeite-in isograd can be defined in parts of the area. Primary bedding defines locally coherent structural orientations and folds within the metagraywacke units. These units are crosscut by thin zones of tectonic melange containing blocks of high-grade blueschist, serpentinite, and other exotic rocks, and a broader, but otherwise identical melange zone marks the discordant boundary between the two terranes.

Frictional resistance of a fault zone with strong rotors

As a possible mechanism to explain the lack of a heat flow anomaly along the creeping section of the San Andreas Fault, we have determined the effect of placing hard rotors along a surface between two deformable media, in this case styrofoam balls between two blocks of foam rubber. The probable presence of Franciscan rocks at depth along the creeping section of the San Andreas Fault suggested this mechanism, since the Franciscan is characterized not only by basic serpentinous rocks which are weak, but also by embedded, more or less equidimensional, blocks of very strong rocks (knockers), e.g., high grade blueschists and eclogites, which might act as rotors. The results suggest that knocker rotation may be a viable mechanism for reduction of friction on the creeping section of the San Andreas fault, and thus be at least a partial explanation of the lack of any observed frictional heat flow anomaly there.

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.

Two High-Pressure Metamorphic Events in NE Oman: Evidence from40Ar/39Ar Dating and Petrological Data

The Saih Hatat tectonic window in NE Oman exposes basement and shelf units that structurally underlie the Semail ophiolite. These units were metamorphosed under high-pressure conditions, as evidenced by the occurrence of lawsonite schists, carpholite-bearing metasediments, blueschists, and eclogites. Conventional K-Ar and dating of whole rocks and white mica separates from the structurally highest Region I of Saih Hatat and the lowest-grade blueschists of the northern part of the structurally lowest Saih Hatat Region III yield ages of 72-80 Ma. White micas from the high-grade blueschists and eclogites of Region III, which formed at T >340°C, yielded variably discordant age spectra with weighted mean "plateau" ages of 106-111 Ma. The age data, combined with structural and petrological criteria, suggest that many units exposed in Saih Hatat experienced two high-pressure, low temperature (high P/T) metamorphic events. The first, in the Early Cretaceous, was possibly a result of partial subduction of the continental margin beneath a microcontinental fragment of Gondwanaland. The second high P/T event was a result of the Late Cretaceous emplacement of the Semail ophiolite onto the Oman continental margin and is characterized by lower temperatures.

Sediment-derived fluids in subduction zones: Isotopic evidence from veins in blueschist and eclogite of the Franciscan Complex, California

Isotopic analyses of minerals from veins that cut high-grade blueschist and eclogite blocks in the central belt of the Franciscan Complex provide constraints on the chronology of metamorphic events and on the origin and movement of fluids within the subduction zone. A Rb-Sr age of 153 {plus minus}1 Ma obtained for minerals from veins and open cavities that formed contemporaneously with retrograde blueschist facies metamorphism is a minimum age for the prograde metamorphism. The veining precedes the last episode of sedimentary-matrix melange formation by a minimum 15 to 20 Ma, during which time the blocks must have been stored within the subduction complex at low temperatures and without undergoing penetrative deformation. Initial Nd-isotope compositions ({epsilon}{sub Nd}) of the vein minerals range from +10.8 to {minus}2.4, indicating that some fluids were derived predominantly from dehydration of subducted mid-ocean ridge basalt, but that other fluids had a component derived from subducted sediment. The provenance of the subducted sediment was within old continental crust, thus associating the Franciscan paleo-subduction complex with a continental craton by the time of vein formation.

Eclogites associated with high-grade blueschists in the Cyclades archipelago, Greece: A review

Eclogites associated with high-grade blueschists in the Cyclades archipelago, Greece: A review

Preliminary fluid inclusion studies in a high-grade blueschist terrain, Syros, Greece

AbstractPreliminary fluid inclusion measurements have been made on quartz (whole rocks and segregations) and garnet from a blueschist terrain. Although further measurements are required, the fluids apparently associated with the blueschist event are aqueous with no thermometrically detectable CO2, a feature which is consistent with mineral-fluid equilibria studies. The salinity of the fluid inclusions is highly variable, from almost pure H2O to halite saturation, and a mechanism involving hydration reactions, such as proposed by Crawford et al. (1979), is suggested. Fluid inclusions associated with the greenschist overprint, which has affected the terrain, are also aqueous in nature.

Eclogites and Blueschists from Northeastern Oman: Petrology and P-T Evolution

Eclogites and blueschists occur in the basement and continental shelf deposits in the Saih Hatat tectonic window of northeastern Oman, where they structurally underlie the Semail ophiolite. These eclogites and blueschists constitute part of a coherent high-P/T metamorphic terrain comprised of interlayered metabasites, metapelites, and quartzofeldspathic and calcareous mica schists. The metabasites record a continuous regional trend of increasing P and T from west to east, where crossite-epidote schists in central and western Saih Hatat grade into blueschists and eclogites at As-Sifah, allowing for the subdivision of this terrain into three metamorphic zones. P-T conditions range from 4·5 to 5·5 kb atT <340°C for zone A to >10·2 kb at 500–580°C for zone C. Zoning patterns of garnets, pyroxenes, and phengite, and the compositions of pyroxene and amphibole inclusions in garnet, indicate that the eclogites and enclosing high-grade blueschists of zone C followed a ‘clockwise’ P-T rpath of increasing P and T followed by increasing T and decreasing P and a final stage of retrogression. Garnets and pyroxenes crystallized at different stages of P-T evolution of the terrain in the various eclogite pods and boudins. This conclusion and the contrasting mineralogies of the eclogites and the enclosing blueschists are attributed to differences in bulk-rock chemistry, fluid availability, or fluid compositions within the terrain. The blueschists and eclogites of this terrain formed as a result of A-type subduction and crustal thickening of the Oman continental margin along an east-northeast-dipping thrust or ‘subduction zone’ which was initiated by the change in plate motion between Africa and Eurasia at 131 Ma. High P/T metamorphism continued until the final stages of ophiolite emplacement in the late Cretaceous, as indicated by the metamorphism of the thrust sheets overlying the coherent terrain.

A unique magnesiochloritoid-bearing, high-pressure assemblage from the Monte Rosa, Western Alps: petrologic and 40Ar-39Ar radiometric study

A phengite-talc-chloritoid-chlorite-kyanite-quartz assemblage is reported from a nearly undeformed quartz-rich metapelite found in the Monte Rosa massif (Western Alps). Chloritoid contains up to 74 mol % of the Mg end member and is the most magnesian ever reported. Textural relationships and mineral compositions suggest equilibrium and therefore a low-variance assemblage which represents the high-pressure stability limit of chlorite+quartz according to the terminal reaction $${\text{chlorite + quartz }} \rightleftarrows {\text{ talc + chloritoid + kyanite + H}}_{\text{2}} {\text{O}}{\text{.}}$$ Mineral compositions combined with new experimental data on the stability of the Mg-chloritoid end member lead, for a temperature close to 500° C, to a pressure estimate of 16 kbar and a water activity of 0.6 which is supported by fluid inclusions study. Chloritoid composition is in fact a fine metamorphic indicator which opens new ways for barometry in high-grade blueschists. It demonstrates here the existence of a high-pressure metamorphism in the Monte Rosa massif. The assemblage remained mineralogically unaffected during the subsequent lower-pressure evolution. Two size fractions of the single phengite generation were analysed by the 39Ar-40Ar incremental release method. Both spectra are identical with a plateau at 110±3 Ma representing over 96% of the 39Ar degassed. The ages of the first heating steps are discordant and increase with increasing temperature from values near 70 Ma to the plateau age. Isotope correlation diagrams show two 36Ar components, one released at high temperature and correlated with 40Ar and 39Ar, the other released at low temperature in a mixture of atmospheric argon and of a loosely held argon of 70 Ma apparent age. The 110 Ma plateau age may reflect the presence of homogeneously incorporated excess argon, the 70 Ma value might then be a true age. However we favour the alternative hypothesis that the 110 Ma plateau age is a true age, implying that the internal crystalline massifs of the Western Alps have endured high-pressure metamorphism as early as mid-Cretaceous. Whatever the interpretation chosen, the preserved high-pressure mineral assemblage remained isotopically unaffected during the low-pressure mid-Tertiary event which is recorded by the 37 Ma plateau age of phengite from a foliated, recrystallised quartzite collected in the same, westernmost part of the massif. The contrasting behaviour of the two samples shows that even at temperatures as high as 400–450° C deformation and recrystallisation are also major controlling factors of isotope mobility.

Coesite and pure pyrope in high-grade blueschists of the Western Alps: a first record and some consequences

A pyrope-quartzite originally described by Vialon (1966) from the Dora Maira massif was resampled and reinvestigated. Garnet (up to 25 cm in size), phengite, kyanite, talc and rutile are in textural equilibrium in an undeformed matrix of polygonal quartz. The garnet is a pyrope-almandine solid solution with 90 to 98 mol % Mg end-member. It contains inclusions of coesite which has partially inverted to quartz, resulting in a typical radial cracking of the host garnet around the inclusions. Several lines of evidence show that coesite crystallised under nearly static pressure conditions and that the whole matrix has once been coesite.

Metamorphic History of a High-Grade Blueschist Exotic Block from the Franciscan Complex, California

A high-grade blueschist tectonic block from the Franciscan Complex of the northeast Diablo Range shows evidence of three episodes of retrograde blueschist facies metamorphism ± deformation developed under progressively declining P-T conditions. The first retrograde metamorphism involved formation of an outer rind of actinolite + chlorite + rutile ± phengite, and the growth of coarse-grained chlorite + pumpellyite within the block. During the second event the rind and outer edge of the blueschist were folded, sheared and fractured, and primary glaucophane within the blueschist was replaced by albite, medium-grained chlorite, and glaucophane-crossite. The third retrograde metamorphism was marked by the pseudomorphic replacement of rind actinolite by aragonite and quartz. Aragonite also crystallized extensively within th block, accompanied by lawsonite, chlorite, jadeitic pyroxene, and crossite; this last mineral assemblage is identical to that of the surrounding Franciscan metasedimentary rocks. Features characteristic of the first and second retrograde metamorphic events are readily observed in other high-grade tectonic blocks of the Franciscan Complex and the correlative Otter Point Formation of Oregon. In contrast, evidence of a third retrograde metamorphism, matching that of presently associated Franciscan and Otter Point rocks, has been found in some but not all blocks examined so far. The high P/T conditions of prograde metamorphism and the available metamorphic age determinations suggest that the tectonic blocks originally formed in a pre-Franciscan subduction zone setting. Fragments of blueschist and eclogite from this metamorphic terrain were tectonically incorporated in a serpentinite diapir, and they developed alteration rinds through interaction with the enclosing ultramafic rock. The available data suggest that the examined exotic block and at least some others were transferred to the Franciscan as detritus from a body of serpentinite that reached the earth's surface. Such blocks were then resubducted and metamorphosed along with their presently associated sedimentary sequences.

Magnesiochloritoid, a key-mineral for the petrogenesis of high-grade pelitic blueschists

Magnesiochloritoid, a key-mineral for the petrogenesis of high-grade pelitic blueschists

Sodic pyroxenes from metabasites in the eastern mediterranean

The compositions of sodic pyroxenes from blueschist metabasites in Northwest Turkey were investigated by electronprobe microanalysis. These are compared with sodic pyroxenes from high-grade blueschist terrains and eclogites with special reference to the island of Syros in Greece. Pyroxenes from lawsonite zone metabasites in Northwest Turkey are chloromelanites, aegerine augites and aegerine jadeites with the acmite contents ranging from 65% down to 30%. Epidote-garnet bearing high-grade blueschist metabasites of Syros on the other hand contain omphacite with the acmite content less than 30%. All the analyzed sodic pyroxenes from eclogites and high-grade blueschists in the literature are also omphacites with low acmite contents. It is argued that the acmite content of the sodic pyroxene decreases with rising temperature of metamorphism. This is almost entirely related to the appearance of epidote as an additional ferric ion bearing phase with an increase in temperature.

Comments on the Distribution and Age of High-Grade Blueschists, Associated Eclogites, and Amphibolites from the Tiburon Peninsula, California

Comments on the Distribution and Age of High-Grade Blueschists, Associated Eclogites, and Amphibolites from the Tiburon Peninsula, California

Distribution and Age of High-Grade Blueschists, Associated Eclogites, and Amphibolites from Oregon and California

Isolated blocks of high-grade blueschist and amphibolite facies metamorphic rocks occur within the Jurassic and Cretaceous eugeosynclinal deposits of the Coast Ranges of southwestern Oregon and California. The blocks range in size from individual rock masses commonly 5 to 1,000 ft in diameter to a few larger masses as much as 7 mi long and 2 mi wide. The high-grade blocks are predominantly basaltic in composition and include glaucophane schists, eclogites, and gneissic rocks of the amphibolite facies. Field relationships indicate that the blocks are closely associated with serpentine, that high-grade blueschist and amphibolite blocks, lower grade blueschists, volcanic rocks, and cherts occupy disturbed zones that may be related to thrusting, and that there is no exposed in situ provenance for the high-grade blueschists, eclogites, and amphibolites. Potassium-argon mineral ages of white mica and actinolite from the blueschists and of hornblende from the amphibolites indicate that these minerals crystallized approximately 150 m.y. ago, but the ages measured on glaucophane from the blueschist blocks are commonly younger. These data suggest that the high-grade blue-schist and amphibolite blocks represent fragments of a cryptic metamorphic terrane of pre-Tithonian age that have been tectonically mixed with younger rocks of the Franciscan Formation in California and Otter Point Formation in Oregon. The younger ages for glaucophane probably reflect metamorphic episodes in which lower grade in situ blueschist facies mineral assemblages were developed in the blocks after their emplacement within the Franciscan Formation. This pre-Tithonian cryptic metamorphic terrane probably developed as a result of interaction between oceanic and continental plates. The occurrence of tectonic blocks of this terrane within melange zones in Oregon and California may be related to later plate interaction.