Blue Amphibole Research Articles

Pre‐Tertiary blueschist terrains in the Hellenides: evidence from detrital minerals of flysch successions

ABSTRACTDetrital blue amphiboles detected in flysch sandstones of the Hellenides of mainland Greece give important information on uplift and exhumation history of blueschist terrains. The occurrence of these HP/LT minerals in the terminal flysch of the Pindos zone as well as in the Othrys flysch of the Subpelagonian zone from late Maastrichtian/ Palaeocene time onwards proves a Pre‐Tertiary (Eohellenic) stage of blueschist formation. Detrital blue amphiboles from flysch sequences of the Ionian zone indicate a further pulse of uplift of high‐pressure rocks during the Palaeocene/Eocene. Blueschist rocks of this latter time range are radiometrically well documented in the Hellenides.

Phase petrology and P‐T conditions of mafic blueschists from the Meliata unit, West Carpathians, Slovakia

Abstract Blueschists occurring as layers in calcite marbles of the Meliata unit occur along the so‐called Roznava tectonic line situated in the southern part of the Gemericum, Slovakia. Mineral assemblages and compositions from seven blueschists localities and one occurrence of amphibolite facies rocks overprinted by blueschist metamorphism were investigated. The most common minerals in the blueschists are blue amphibole, epidote and albite. Some Fe2+‐ and Al‐rich rocks also contain garnet and chloritoid, respectively. Na‐pyroxene with a maximum 50% jadeite component was also found. The blue amphiboles correspond mostly to crossite and also to glaucophane and ferroglaucophane in some samples. Almandine‐ and spessartine‐rich garnet has very low MgO content (<3 wt%). The Si content in phengite ranges between 3.3 and 3.5 pfu calculated on the basis of 11 oxygens. The zoning patterns of blue amphibole, garnet and chloritoid suggest their formation during a prograde stage of metamorphism. The P‐T conditions of metamorphism are estimated to be about 380–460° C and 10–13 kbar. Pressures of 7.5–8.5 kbar and temperatures of 350–370° C were obtained for some actinolite‐ and aegirine‐rich rocks. Apart from chlorite, other mafic minerals formed during retrograde metamorphism are biotite and occasionally also actinolite.

Low‐grade blueschist facies metamorphism of metagreywackes, Franciscan Complex, northern California

Abstract Metagreywackes in the Eastern Belt of the Franciscan Complex contain the assemblage: Qtz + Ab + Lws + Chl + Ph + Pmp + Fgl + Hem ° Cal/Arg or compatible subassemblages. Blue amphibole first appears in the westernmost part of the belt and pumpellyite is absent in the eastern part. The compositions of the coexisting minerals and the nature of the continuous reactions in these low‐grade blueschists suggest that the distribution of blue amphibole and pumpellyite in the Eastern Belt of the Franciscan Complex reflects differences of effective bulk composition rather than differences in physical conditions of metamorphism. In rocks lacking pumpellyite, white mica may be essential to the growth of blue amphibole, but carbonate plays only a limited role. The continuous reaction that limits the appearance of blue amphibole and the disappearance of coexisting pumpellyite has the general form: Pmp + Chl + Ab + Qtz + Hem + H2O + FeMg‐1= Fgl + Lws. This reaction requires significant hydration as pressure increases in order to produce blue amphibole. Most of the Eastern Belt of the Franciscan Complex formed in limited ranges of temperature and pressure, which are estimated to be 240—280° C, 6.5‐7.5 kbar. Pressures in the westernmost part of the area were about 1 kbar lower than in the east. Pressures of about 8.5‐10 kbar are estimated for tectonic blocks that contain sodic clinopyroxene.

Large-scale silicate liquid immiscibility: a possible example from northeastern Brazil

Ultrapotassic, peralkalic silica-saturated plutons (580 Ma) are widespread in the Cachoeirinha-Salgueiro foldbelt, northeastern Brazil. They consist of alkali-feldspar syenites with pyroxenite as co-magmatic inclusions and syn-plutonic or late-stage dikes. Pyroxenite and syenite have the same mineral phases (aegirine-augite, microcline, sphene, apatite, blue amphibole, magnetite), but only in different proportions. Rare inclusions of a “mixed” rock (about 60% syenite+40% pyroxenite in an emulsion-like texture) are also present. Pyroxenes in the three units are all only slightly zoned, silica-saturated and extremely low in Al 2O 3 (0.2–1.4%). Amphiboles are mostly K-rich richterite, characterized by high SiO 2, low Al 2O 3 and TiO 2 contents and low Mg#. The three rock types have similar REE chondrite-normalized patterns, with negative slopes and lack of Eu anomaly, with the total REE in the pyroxenite greater than that of the syenite. Trace element patterns for the mixed rock are intermediate between those for the pyroxenite and syenite. Major element partitioning between pyroxenite and syenite has the same sense as that one observed between immiscible liquids in volcanic lavas and trace element partitioning is similar to the experimentally determined partition of immiscible liquid pairs. The rocks have similar high δ 18O values (avg. w.r.+ 8‰ SMOW, corrected from pyroxene), high initial 87 Sr 86 Sr ratios (about 0.710), and low 143 Nd 144 Nd (avg. 0.51104). Field and geochemical characteristics indicate chemical equilibrium among the three rock types and suggest liquid immiscibility between syenite and pyroxenite, the mixed rock representing the original magma composition.

Blueschist metamorphism in an active subduction zone

THE high-pressure, low-temperature metamorphic rocks known as blueschists have long been considered to form in subduction zones, where the descent of a relatively cold slab leads to the occurrence of unusually low temperatures at mantle pressures. Until now, however, the link between blueschist-facies rocks and subduction zones has been indirect, relying on a spatial association of blueschists with old subduction complexes, and estimates of the geothermal gradients likely to exist in subduction zones. Here we strengthen this link, by reporting the discovery of blueschist-facies minerals (lawsonite, aragonite, sodic pyroxene and blue amphibole) in clasts from a serpentinite seamount in the forearc of the active Mariana subduction zone. The metamorphic conditions estimated from the mineral compositions are 150–250 °C and 5–6 kbar (16–20 km depth). The rocks must have been entrained in rising serpentine mud diapirs, and extruded from mud volcanoes onto the sea floor. Further study of these rocks may provide new insight into the tectonics of trench-forearc systems, and in particular, the processes by which blueschist-facies clasts come to be associated with forearc sediments in ancient subduction complexes.

Pyroxene and amphibole chemistry of peralkalic ultra-potassic syenite and associated pyroxenite, northeastern Brazil: an approach to test liquid immiscibility process

Liquid immiscibility has been a process little explored as a mechanism for generation of magmas of contrasting composition, although its importance has been recognized by some petrologists. In this work, the chemistry of mafic phases in ultrapotassic syenite and pyroxenite, which seem to have coexisted as liquids for a certain period of time, in the Late Precambrian Triunfo pluton, Pernambuco, is discussed. The two rock types have the same mineral phases (clinopyroxene, microcline, apatite, sphene, blue amphibole) only at different amounts. The chemistry of pyroxene (aegirine-augite), which is only slightly zoned, and amphibole (K-rich richterite) of both rock types, show that the mineral phases in the pyroxenite are similar to their corresponding ones in the syenite, compositionally overlapping. These features strongly suggest chemical equilibrium between syenite and pyroxenite, which is one of the requirements for two liquids constitute an immiscible pair. This hypothesis is supported by the presence of mixed-rock inclusions that consist of cmsized, near-rounded, syenitic and pyroxenitic drops mutually interstitial, in an emulsion-like texture, which record magma splitting.

Blue amphiboles and their significance for the metamorphic history of the Pan‐African Gariep belt, Namibia

ABSTRACTThe metamorphic history of mafic exotic blocks from a tectonic melange zone within an allochthonous ophiolitic terrane (Marmora Terrane) of the Pan‐African Gariep orogenic belt in south‐western Namibia was studied, based on mineral parageneses and amphibole composition. Glaucophane described previously from these rocks could not be verified. Instead, two types of blue amphiboles were distinguished: (i) rims of (ferro‐) edenitic to pargasitic to barroisitic hornblende composition around brownish amphibole phenocrysts replacing magmatic clinopyroxene, and (ii) deep blue porphyroblasts of magnesio‐riebeckite with little ferro‐glaucophane component in a highly metasomatized albite‐rich rock. Textural and mineralogical evidence, particularly the existence of up to three different amphibole generations in metagabbro samples, supports a multiphase metamorphic history experienced by these exotic blocks. The first metamorphic event, M1, is interpreted as very low‐P hydrothermal oceanic metamorphism that affected the igneous protoliths at up to amphibolite facies temperatures. Subsequent M2 metamorphism was syntectonic and is characterized by temperatures similar to those attained during M1 but higher pressures indicating burial to 15–20 km. This event is related to a subduction process. The third metamorphic event, M3, was low grade and of regional nature. It is the only one recorded in the sedimentary envelope of the exotic blocks. The formation of magnesio‐riebeckite is considered a retrograde reaction at greenschist facies during M2. The results indicate that in the Gariep belt subduction and subsequent obduction have occurred, although blueschist facies metamorphism has not been reached.

Chemical-microstructural changes in eclogite-facies shear zones (Monviso, Western Alps, north Italy) as indicators of strain history and the mechanism and scale of mass transfer

Variations of eclogite-facies mineral chemistry are related to the textural characteristics of shear zones in FeTi metagabbros from the Monviso meta-ophiolitic complex in the Italian Western Alps. Three different kinds of microstructural domains are described: (1) small lenses of low-strain rocks with preserved igneous textures; (2) larger high-strain domains where deformation caused pervasive mylonitic foliation (S 1) and a stretching mineral lineation (L 1); and (3) two types of eclogite-facies veins. The latter can be subdivided into veins composed of only Na-pyroxenes filling small tension gashes and dilatation fractures and veins composed of a Na-pyroxene-garnet-rutile-apatite assemblage occupying large fractures that randomly cross-cut the mylonites. In low-strain domains and mylonites the Na-pyroxenes are omphacites and the garnets are almandine rich. In some mylonite layers containing phengite and blue amphibole crosscutting S 1, chloromelanite (iron-rich omphacites) exsolved from omphacite. In type-1 veins the pyroxene is either omphacite or concentric and sector-zoned omphacite-chloromelanite. In type-2 veins pyroxenes are omphacite and/or concentric and sector-zoned omphacite-chloromelanite and/or zoned kosmochlor (chromium-rich omphacite)-omphacites. Associated garnets are richer in pyrope component than those from low-and high-strain domains. Possible origins of both sector and concentric zoning and exsolution are discussed. It is emphasized that a miscibility gap between the omphacite and acmite end-members partly controlled the omphacite-chloromelanite distribution. We define a chemical vector coat constrains the strain history in eclogite-facies rocks and give indications of the scale and mechanism of mass transfer involved during foliation development and fracturing. P- T conditions are calculated for each microstructural domain. The garnet-pyroxene thermometer yields scattered temperature values. These values relate to the dependence of temperature estimates on the amount of Fe 3+ in the pyroxene and possibly on non-ideal substitution in omphacite-acmite solid solution.

Geochronology of Blueschists from West-Central Baja California and the Timing of Uplift in Subduction Complexes

A geochronologic study of blueschists from west-central Baja California has provided constraints on the timing of high-pressure, low-temperature metamorphism and the subsequent uplift of this subduction complex. Mineral separates from coherent blueschists and blueschist blocks enclosed in serpentinite melange were analyzed by {sup 40}Ar/{sup 39}Ar, fission track, and Rb/Sr methods. {sup 40}Ar/{sup 39}Ar age spectra for white mica separated from four blueschist blocks and an overprinted eclogite block in serpentinite-matrix melange show slow cooling gradients between 115-95 Ma. Rb/Sr apparent ages for one block are concordant with these results. {sup 40}Ar/{sup 39}Ar ages of blue amphibole are primarily the result of the degassing of white mica intergrowths as confirmed by microprobe analyses. Apatite fission track ages indicate a blueschist block and an eclogite block cooled to a temperature below {approximately}100 C at 22 Ma and 32 Ma, respectively. Subduction-related metamorphism of the coherent blueschists occurred in late Early Cretaceous time, based on a {sup 40}Ar/{sup 39}Ar age or 109 Ma for metamorphic white mica from a metasandstone. {sup 40}Ar/{sup 39}Ar analysis of partially albitized K-feldspar from a plutonic clast in a metaconglomerate indicates this sample cooled to a temperature of {approximately} 145 C at approximately 20 Ma. Geochronologic and petrologicmore » data for coherent blueschists indicate an average uplift rate of 0.1 mm/yr for one portion of the subduction complex. The relatively slow uplift rate and lack of any higher temperature overprinting assemblages in the coherent blueschists suggest that synsubduction uplift was gradual and proceeded through a dynamic accretionary wedge characterized by low geothermal gradients. An increase in uplift rate (to 1.0 mm/yr) during post-Miocene time coincides with a change from a convergent to a transform plate boundary.« less

The effects of sub-blocking temperature metamorphism on the K/Ar systematics of hornblendes: 40Ar/39Ar dating of polymetamorphic garnet amphibolite from the Franciscan Complex, California

The effects of sub-blocking temperature metamorphism on the K/Ar system in hornblende, as revealed by 40Ar/39Ar release spectra, have been studied in a polymetamorphic “knocker” from the Franciscan Complex (FC), California. A primary amphibolite assemblage of horn-blende +rutile+epidote+apatite±garnet±sphene is variably overprinted by a blueschist facies assemblage of blue amphibole +lawsonite+chlorite+white mica+pumpellyite±sphene. The secondary assemblage formed at a temperature of ≤370° C, below that at which rapid Ar diffusion is expected in hornblende. Hornblendes from three, variably-altered samples of garnet amphibolite yield total gas ages of 147 to 161 Ma, but the corresponding plateau ages of 163.0±2.8, 160.6±2.2, and 161.8±2.2 Ma are identical within error. Hornblende separates with lower total gas ages come from more highly overprinted rocks, have “excess” K compared to that expected on the basis of electron probe analyses, and exhibit anomalously high K/Ca ratios in the low-temperature fractions of their incremental heating spectra. The reduced total gas ages result from the presence of thin (∼2 μm) sheets of younger white mica in hornblendes from the moderately and highly altered amphibolites. The secondary micas are difficult to detect because of their small size and low abundance (≤2%), but because their K content is 50 to 100 times that of the host hornblende, they contribute significantly to the K and Ar budgets of the sample. The mica intergrowths are not removed by normal sample preparation, but because the mica inclusions degas at lower temperatures than hornblende during vacuum extraction, incremental heating analyses can provide precise cooling ages for the hornblendes as well as useful estimates of the age of the mica inclusions. The hornblende separate from the most altered sample contained 20±10% younger blue amphibole replacing hornblende, but its plateau age was not significantly affected. This is consistent with a replacement process in which K and Ar loss from the hornblende are coupled, leaving the K/Ar system undisturbed in relict primary grains. The K and Ar budgets of the sample are not strongly affected by the blue amphibole because of its very low K content. Because partial replacement of primary amphibole by high-K phyllosilicates occurs in many geological environments, effects like those described here could be widespread.

KAr and [formula omitted]Ar study of metamorphic rocks associated with the Oman ophiolite: Tectonic implications

K-Ar analyses on extracted minerals are reported for a variety of metamorphic rocks associated with the Sumail ophiolite. Amphibolites lying at the sole of the ophiolite yield ages of 95–100 Ma, which are viewed as reflecting times of crystallization. High-pressure metamorphics of the Saih Hatat reveal complex results: white micas range in age from 80 to 131 Ma whereas blue amphiboles indicate ages that are systematically lower than those of coexisting white micas. Investigation of a few white micas by the 40Ar 39Ar step heating method yields rather intricate age spectra, featuring low apparent ages in the first and the last stages of gas release and high apparent ages in between. Two explanations can be equally envisaged for these convex-upward age spectra. The first is the mixing of two generations of micas, corresponding to two main metamorphisms. The first one ( M 1) is a low- to medium-temperature, high-pressure event which conceivably occurred between 130 and 114 m.y. ago. The second ( M 2) overprints M 1 and has produced rocks typical of the greenschist facies. It took place 80 m.y. ago and also affected the sole of the ophiolites. The second is the presence of excess argon in mica mixtures with complex degassing properties. Thus, the two metamorphic phases identified by microscopic inspection are not significantly different in age. They occurred in the 70–80 Ma interval. Moreover, K-Ar dates on amphibole from gabbroic dikes intersecting the peridotites suggest that they are genetically linked to the mafic part of the ophiolites. Assuming that metamorphism is a tracer of tectonic events, we view the infraophiolitic amphibolites as the result of an intraoceanic thrusting which took place near a spreading center. Nevertheless, the uncertainty as to the age of the blueschist metamorphism precludes the possibility of indicating a timetable, based on metamorphic ages, for the motion shift of Africa relative to Eurasia during the Late Cretaceous. A tentative model of the obduction of the Sumail ophiolite is proposed assuming an age of about 130 Ma for M 1. We stress differences with respect to earlier reconstructions which advocated an 80 m.y. old blueschist metamorphism.

A case of obduction-related high-pressure, low-temperature metamorphism in upper crustal nappes, Arabian continental margin, Oman: P-T paths and kinematic interpretation

In the southeastern Oman Mountains, the Saih Hatat tectonic window reveals Arabian continental material beneath the oceanic (sensu. lato) nappes i.e., the Hawasina sedimentary units and the huge Sumail ophiolite. The northeastern (internal) half of the window suffered an intense deformation associated with a high-pressure, low-temperature ( HP- LT) metamorphism, approximately dated at 80-75 Ma according to the available stratigraphie data. After the deformation, due to the absence of collision, the obduction-inherited structural and metamorphic pattern was virtually unmodified. A metamorphic map is established based on mineral assemblages in metapelites and metabasites. Low-grade assemblages occur in the highest units (Muscat nappes, Quryat unit). Characteristic minerals are Fe Mg carpholite (± lawsonite) associated with kaolinite or pyrophyllite and locally sudoite in the metapelites while lawsonite and blue amphiboles are found in the metabasites. High-grade rocks outcrop in the deepest and easternmost units (As Sifah antiform) and are typified by an epidote-glaucophane-bearing eclogite assemblage. Intermediate units (Hulw) show assemblages of intermediate grade (chloritoid schists and glaucophane-bearing metabasites). Higher grade assemblages such as talc-chloritoid were not found. Inverted gradients are observed locally in the Muscat nappes (pyrophyllitebearing units above kaolinite-quartz-bearing units) and these are ascribed to late metamorphic thrust displacements which are demonstrated by microstructures. A complex P- T evolution is fairly well documented by the mineral assemblages. Prograde evolution in the subophiolitic (continental rocks) began with a rather “hot” gradient of about 30°–35°C/km (with chloritoid growth in the higher units). Thereafter pressure increased with the temperature remaining constant or even decreasing (chloritoid replaced by kaolinite + oxides and carpholite growth) in synkinematic conditions. The peak P- T conditions are close to 8 kbar, 270°C for the Muscat nappes and 11 kbar, 400°C for the As Sifah unit. The corresponding surface-related gradient is low (10°–15°C/km). During the retrograde evolution, pressure and temperature generally decreased concomitantly, but in some of the upper units temperature remained constant or increased slightly for some time, at which point sudoite growth from carpholite took place. The metamorphic pattern and mineral evolution are interpreted in the framework of the Late Cretaceous obduction. The already cold oceanic lithosphere was thrust upon a relatively hot thinned passive margin (with coeval basaltic volcanism). Pressure progressively increased in the passive margin due to the wedge shape of the advancing ophiolite, the maximum thickness of which was about 25 km in the As Sifah area. In this area, the overburden was also attributable to the accumulation of continental material (up to 5 km thick) by means of a complex set of synmetamorphic, often conjugated folds and thrusts. The basal peridotites of the obducted slab were probably not hotter than 300° C during its thrusting upon the continental margin. The temperature of the tectonic pile below the ophiolite remained low because it was thrust above more external and still colder continental areas. Late thrusting in the same southerly direction controlled the retrograde evolution, together with erosional and gravity-driven tectonic unloading.

The occurrence of deerite in highly oxidizing conditions within the ‘schistes lustrés’of eastern Corsica

Abstract A deerite‐bearing rock occurs at the boundary between quartzite and metabasites within the ‘schistes lustrés’of eastern Corsica. It contains the typomorphic assemblage pyroxene, blue amphibole, hematite and magnetite. Pyroxene shows homogeneous composition close to the aegirine end‐member and blue amphibole is zoned from crossite core to riebeckite rim. The bulk chemical analysis of the rock is remarkable by its very high iron content and the presence of an unusually large amount of Zn which is concentrated in both deerite and amphibole. Electron microprobe analyses of the Corsican deerite are compared with those published in the literature; as shown by deerite from the Fransciscan iron formation, the principal substitution for Fe2+ is Mn whereas the amount of substitution for Fe3+ is low. In the system SiO2‐FeO‐Fe2O3‐Al2O3‐Na2O‐MgO‐H2O the typomorphic paragenesis can be described by an univariant reaction interpreted as the result of a pressure decrease. P‐T conditions of metamorphism, previously estimated to be 8 kbar and 300°C, are in good agreement with present knowledge of the deerite stability field. The occurrence of hematite and magnetite in equilibrium permits an estimation of the oxygen fugacity (log fo2= ‐29.41 bar). Oxidation conditions are higher than those previously mentioned in the literature for similar assemblages.

Source Rock Potential of Franciscan Complex: The Galloping Gourmet and the Hydrocarbon Kitchen: ABSTRACT

Previous workers suggested that western North America was characterized by coastal upwelling during much of the Mesozoic and Cenozoic Eras. Areas of coastal upwelling are commonly characterized by excellent source rocks - because high-quality organic matter is preserved in oxygen-minimum zones - and by high sedimentation rates. The Franciscan complex, of Jurassic to Miocene age, records part of the interaction between Pacific oceanic plates and the North American continental margin. The Franciscan complex is an amalgam of terranes, most of which represent deposition in deep to intermediate-depth waters on oceanic lithosphere and trench-related continental margin facies. The authors analyzed the source rock potential of Franciscan complex rocks using Rock-Eval pyrolysis techniques. Turbiditic mudstones of the eastern belt Franciscan complex contain moderate amounts of total organic carbon (TOC about 1%), but have low volatile contents as indicated by hydrogen and oxygen indexes. TOC is 1-3% for organic matter in argillaceous melange matrix of the coastal belt of northern California, the Permanente terrane, and San Simeon; however, volatile content is low. The Calera limestone of the Permanente terrane was deposited under locally anoxic conditions, probably on a mid-Pacific seamount, and later accreted to North America. It contains from 0 to 7% TOC,more » and volatile content is low. Significant volatile content is found only in turbiditic mudstones of the coastal belt Franciscan, northern California, where TOC averages about 1% in Type III kerogens. Slates locally bearing blue amphiboles, lawsonite, jadeite, and limestones tectonically interleaved with slates in melange on Cedros Island, Baja California, contain up to several percent TOC, but are extremely depleted in volatiles.« less

Pre-Neogene nappe structure and metamorphism of the North Sporades and the southern Pelion peninsula

Summary On the North Sporades and on the southern Pelion Peninsula, four tectonic units can be distinguished. The lowermost is the Pelagonian Nappe which is a sequence of Permo-Triassic metaclastics and platform carbonates of Middle Triassic to Upper Jurassic age. It is overlain by the Eohellenic Nappe, which consists of a melange with ophiolite bodies and crystalline slices, followed by a series of calc-schists, platy marbles and spilites, and, finally, a flyschoid sequence. Subsequent to the overthrusting of the Eohellenic Nappe in the Early Cretaceous, both nappes were partly eroded and then covered by the meso-autochthonous sediments of Middle Cretaceous to Lower Tertiary age. In the Eohellenic Nappe, as well as in the higher units, blue amphiboles often appear, i.e., magnesio-riebeckite in quartzitic and phyllitic layers and crossite or glaucophane in metabasites. The pelitic layers contain phlogopite and stilpnomelane as well as chlorite and phengite. In meta-radiolarites, spessartine and piemontite have also been found. This association, indicating high pressure during very low-grade metamorphism, contrasts greatly with the albite-biotite-almandine assemblage in the underlying Pelagonian Nappe, which is typical of low-grade metamorphism at more moderate pressures. A geodynamic explanation is proposed.

High-pressure-low-temperature metamorphism and polyphase alpine deformation at Sant'Andrea di cotone (eastern corsica, france)

Abstract A quarry, located in the “Schistes lustres” of eastern Corsica, has been studied in detail for both structural geology and metamorphic petrology. The superimposed structures belong to three successive generations. For each generation, folds, foliations, lineations and behaviour of metamorphic minerals are briefly described. Attention is devoted to the connection between deformation—recrystallization processes of minerals in thin section and macroscopic contrasts in ductility. Typical HP—LT metamorphic minerals were developed during the two first generations of structures and were little transformed during the third, Clinopyroxenes, garnets, blue amphiboles, phengites, chlorites, stilpnomelane, piemontite, lawsonite, pumpellyite and deerite occur in different lithologies. P—T conditions of metamorphism are estimated to 8 kbar and 300°C. fO2 and XCO2 in metamorphic fluids are specified. Furthermore, evolution and behaviour of metamorphic fluids are described in this example of HP—LT metamorphism, with emphasis on interactions between these fluids, deformation processes and mineral assemblages.

Glaucophanic amphibole in a Monian shear zone on the mainland of N Wales

Glaucophanic amphibole occurs within recrystallized basic volcanic rocks derived from the Gwna Group at Penrhyn Nefyn in the Lleyn Peninsula. The glaucophanic rocks are restricted to the ‘Penmynydd Zone of Metamorphism’ which at Nefyn is interpreted as a narrow blastomylonite belt produced by the shearing of the Gwna Group against a body of tonalite. The blue amphibole coexists with blue-green amphibole and grades into actinolite greenschist away from the focus of shearing. Fragments of Gwna Group (dated as Lower Cambrian) and blastomylonitic ‘Penmynydd’ lithologies lie in the Cambro–Ordovician sediments of the Welsh Basin. Therefore, the metamorphism of parts of the Gwna Group during the ‘Penmynydd’ (subduction zone?) shearing is interpreted as having occurred in Cambrian times.

Detrital blue sodic amphibole in Recent sediments, southern coast, Turkey

Detrital blue sodic amphibole, sporadic lawsonite and pumpellyite occur in beach and river sediments between Gazipasa and Manavgat on the southern Turkish coast. This area lies S of the metamorphic Alanya massif, which contributes to the detritus, but blueschists have not previously been recorded in the region. The blue amphiboles have a provincial variation related to differing conditions during their formation. In the eastern region, in the vicinity of Gazipasa and Demirtas, homogeneous glaucophanes, sometimes with remnant Ca-amphiboles, are characteristic. In the western region the grains exhibit a considerable variation of optical and physical properties and are crossitic-glaucophane, crossite and occasionally magnesio-riebeckite. The efficacy of sedimentary mineralogy in reconstructing a hinterland is demonstrated.

Mesozoic ophiolite and blue amphibole on Timor and the dispersal of eastern Gondwanaland

A dismembered metamorphosed ophiolite on Timor is connected with Jurassic rifting of Australia's northwestern margin. The rift developed into an ocean basin which carried a microcontinental block northwards. During the middle to late Cretaceous the block accreted to South-east Asia and experienced a low grade metamorphism which generated crossitic amphibole.

A Caledonian blueschist from the Irish Dalradian

THE occurrence of the blue amphiboles glaucophane and crossite in metamorphosed basic rocks is characteristic of terrains metamorphosed at high pressures. Most examples are from relatively recent fold belts. We record here a Caledonian blueschist from the Dalradian Supergroup in South Achill Island, County Mayo (Fig. 1). It is the first ‘Old’ blueschist from Britain to occur in a regularly bedded and folded sequence and may imply an early high pressure metamorphic event in this part of the Dalradian, evidence for which was generally destroyed by either erosion or later metamorphism.