Phase equilibrium constraints on a deep crustal metamorphic field gradient: metapelitic rocks from the Ivrea Zone (NW Italy)

The metamorphic rocks of the Ivrea Zone in NW Italy preserve a deep crustal metamorphic field gradient. Application of quantitative phase equilibria methods to metapelitic rocks provides new constraints on the P–T conditions recorded in Val Strona di Omegna, Val Sesia and Val Strona di Postua. In Val Strona di Omegna, the metapelitic rocks show a structural and mineralogical change from mica‐schists with the common assemblage bi–mu–sill–pl–q–ilm ± liq at the lowest grades, through metatexitic migmatites (g–sill–bi–ksp–pl–q–ilm–liq) at intermediate grades, to complex diatexitic migmatites (g–sill–ru–bi–ksp–pl–q–ilm–liq) at the highest grades. Partial melting in the metapelitic rocks is consistent with melting via the breakdown of first muscovite then biotite. The metamorphic field gradient in Val Strona di Omegna is constrained to range from conditions of ～3.5–6.5 kbar at ≈650 °C to ～10–12 kbar at >900 °C. The peak P–T estimates, particularly for granulite facies conditions, are significantly higher than those of most earlier studies. In Val Sesia and Val Strona di Postua, cordierite‐bearing rocks record the effects of contact metamorphism associated with the intrusion of a large mafic body (the Mafic Complex). The contact metamorphism occurred at lower pressures than the regional metamorphic peak and overprints the regional metamorphic assemblages. These relationships are consistent with the intrusion of the Mafic Complex having post dated the regional metamorphism and are inconsistent with a model of magmatic underplating as the cause of granulite facies metamorphism in the region.     

Melt segregation in the continental crust: distribution and movement of melt in anatectic rocks

The grain‐ and outcrop‐scale distribution of melt has been mapped in anatectic rocks from regional and contact metamorphic environments and used to infer melt movement paths. At the grain scale, anatectic melt is pervasively distributed in the grain boundaries and in small pools; consequently, most melt is located parallel to the principal fabric in the rock, typically a foliation. Short, branched arrays of linked, melt‐bearing grain boundaries connect melt‐depleted parts of the matrix to diffuse zones of melt accumulation (protoleucosomes), where magmatic flow and alignment of euhedral crystals grown from the melt developed. The distribution of melt (leucosome) and residual rocks (normally melanocratic) in outcrop provides different, but complementary, information. The residual rocks show where the melt came from, and the leucosomes preserve some of the channels through which the melt moved, or sites where it pooled. Different stages of the melt segregation process are recorded in the leucosome–melanosome arrays. Regions where melting and segregation had just begun when crystallization occurred are characterized by short arrays of thin, branching leucosomes with little melanosome. A more advanced stage of melting and segregation is marked by the development of residual rocks around extensive, branched leucosome arrays, generally oriented along the foliation or melting layer. Places where melting had stopped, or slowed down, before crystallization began are marked by a high ratio of melanosome to leucosome; because most of the melt has drained away, very few leucosomes remain to mark the melt escape path — this is common in melt‐depleted granulite terranes. Many migmatites contain abundant leucosomes oriented parallel to the foliation; mostly, these represent places where foliation planes dilated and melt drained from the matrix via the branched grain boundary and larger branched melt channel (leucosome) arrays collected. Melt collected in the foliation planes was partially, or fully, expelled later, when discordant leucosomes formed. Leucosomes (or veins) oriented at high angles to the foliation/layering formed last and commonly lack melanocratic borders; hence they were not involved in draining the matrix of the melting layer. Discordant leucosomes represent the channels through which melt flowed out of the melting layer.     

Phase equilibrium modelling of the amphibolite to granulite facies transition in metabasic rocks (Ivrea Zone,NW Italy)

The development of thermodynamic models for tonalitic melt and the updated clinopyroxene and amphibole models now allow the use of phase equilibrium modelling to estimate P–T conditions and melt production for anatectic mafic and intermediate rock types at high‐T conditions. The Permian mid‐lower crustal section of the Ivrea Zone preserves a metamorphic field gradient from mid amphibolite facies to granulite facies, and thus records the onset of partial melting in metabasic rocks. Interlayered metabasic and metapelitic rocks allows the direct comparison of P–T estimates and partial melting between both rock types with the same metamorphic evolution. Pseudosections for metabasic compositions calculated in the Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–O (NCKFMASHTO) system are presented and compared with those of metapelitic rocks calculated with consistent end‐member data and a–x models. The results presented in this study show that P–T conditions obtained by phase equilibria modelling of both metabasic and metapelitic rocks give consistent results within uncertainties, allowing integration of results obtained for both rock types. In combination, the calculations for both metabasic and metapelitic rocks allows an updated and more precisely constrained metamorphic field gradient for Val Strona di Omegna to be defined. The new field gradient has a slightly lower dP/dT which is in better agreement with the onset of crustal thinning of the Adriatic margin during the Permian inferred in recent studies.     

Local disequilibrium of plagioclase in high-temperature shear zones of the Ivrea Zone, Italy

Abstract Microstructural and chemical analysis of plagioclase in 20 superficially similar amphibolite facies ductile shear zones in metagabbors and amphibolites of the Ivrea Zone in Italy reveals significant differences in An and Ba contents. Plagioclase, which was deformed at P-T conditions lower than those of the wall rocks, occurs in the following four different microstructural situations with different chemical compositions: (i) relatively undeformed porphyroclasts, (ii) dynamically recrystallized grains and subgrains rimming the porphyroclasts, (iii) infill of microcracks cross-cutting the porphyroclasts and (iv) fine-grained recrystallized grains in the matrix of the shear zones. The differences in the An and Ba contents are caused by partial chemical equilibration of plagioclase in the shear zones during and partly after deformation. Changes in An and Ba contents were caused by fluid-assisted grain-boundary migration recrystallization, as well as by solid-state diffusion, while fluid activity was high. The relation between the composition and microstructures of the plagioclase in the shear zones indicates that in the different shear zones, fluids ceased to be active during different stages in the late shear zone deformation history.
The interpretation of the variations in composition and microstructures reveals that only grains that developed by grain-boundary migration recrystallization and that are not adjacent to porphyroclasts reflect P-T conditions during the dominant shear-zone deformation.     

Production of metaluminous melt during fluid‐present anatexis: an example from the Maghrebian basement,La Galite Archipelago,central Mediterranean

Garnet brought to the surface by late Miocene granitoids at La Galite Archipelago (Central Mediterranean, Tunisia) contains abundant primary melt and fluid inclusions. Microstructural observations and mineral chemistry define the host garnet as a peritectic phase produced by biotite incongruent melting at ~800 °C and 0.5 GPa, under fluid‐present conditions. The trapped melt is leucogranitic with an unexpected metaluminous and almost peralkaline character. Fluid inclusions are one phase at room temperature, and contain a CO₂‐dominated fluid, with minor H₂O, N₂ and CH₄. Siderite and an OH‐bearing phase were identified by Raman and IR spectroscopy within every analysed inclusion, and are interpreted as products of a post‐entrapment carbonation/hydration reaction between the fluid and the host during cooling. The fluid present during anatexis is therefore inferred to have been originally richer in both H₂O and CO₂. The production of anatectic melt with a metaluminous signature can be explained as the result of partial melting of relatively Al‐poor protoliths assisted by CO₂‐rich fluids.     

Origin of CO2-rich fluid inclusions in leucosomes from the Skagit migmatites, North Cascades, Washington, USA

CO₂–CH₄ fluid inclusions are present in anatectic layer-parallel leucosomes from graphite-bearing metasedimentary rocks in the Skagit migmatite complex, North Cascades, Washington. Petrological evidence and additional fluid inclusion observations indicate, however, that the Skagit Gneiss was infiltrated by a water-rich fluid during high-temperature metamorphism and migmatization. CO₂-rich fluid inclusions have not been observed in Skagit metasedimentary mesosomes or melanosomes, meta-igneous migmatites, or unmigmatized rocks, and are absent from subsolidus leucosomes in metasedimentary migmatites. The observation that CO₂-rich inclusions are present only in leucosomes interpreted to be anatectic based on independent mineralogical and chemical criteria suggests that their formation is related to migmatization by partial melting. Although some post-entrapment modification of fluid inclusion composition may have occurred during decompression and deformation, the generation of the CO₂-rich fluid is attributed to water-saturated partial melting of graphitic metasedimentary rocks by a reaction such as biotite + plagioclase + quartz + graphite ± Al₂SiO₅+ water-rich fluid = garnet + melt + CO₂–CH₄. The presence of CO₂-rich fluid inclusions in leucosomes may therefore be an indication that these leucosomes formed by anatexis. Based on the inferences that (1) an influx of fluid triggered partial melting, and (2) some episodes of fluid inclusion trapping are related to migmatization by anatexis, it is concluded that a free fluid was present at some time during high-temperature metamorphism. The infiltrating fluid was a water-rich fluid that may have been derived from nearby crystallizing plutons. Because partial melting took place at pressures of at least 5 kbar, abundant free fluid may have been present in the crust during orogenesis at depths of at least 15 km.     

Direct measurement of 3D elastic anisotropy on rocks from the Ivrea zone (Southern Alps, NW Italy)

Lower crustal and upper mantle rocks exposed at the earth's surface present direct possibility to measure their physical properties that must be, in other cases, interpreted using indirect methods. The results of these direct measurements can be then used for the corrections of models based on the indirect data. Elastic properties are among the most important parameters studied in geophysics and employed in many fields of earth sciences. In laboratory, dynamic elastic properties are commonly tested in three mutually perpendicular directions. The spatial distribution of P- and S-wave velocities are then computed using textural data, modal composition, density and elastic constants. During such computation, it is virtually impossible to involve all microfabric parameters like different types of microcracking, micropores, mineral alteration or quality of grain boundaries.

In this study, complete 3D ultrasonic transmission of spherical samples in 132 independent directions at several levels of confining pressure up to 400 MPa has been employed for study of selected mafic and ultrabasic rocks sampled in and nearby Balmuccia ultrabasic massif (Ivrea zone, Southern Alps, NW Italy). This method revealed large directional variance of maximum P-wave velocity and different symmetries (orthorhombic vs. transversal isotropic) of elastic waves 3D distribution that has not been recorded on these rocks before. Moreover, one dunite sample exhibits P-wave velocity approaching to that of olivine single crystal being interpreted as influence of CPO.     

山东昌乐刚玉巨晶中的流体和熔融包裹体及其流体组分特征

山东昌乐第三纪玄武岩中产有刚玉巨晶,内含丰富的原生和假次生流体包裹体和熔融包裹体。流体包裹体可分为CO2单相包裹体、H2O-CO2两相和三相包裹体。熔融包裹体类型复杂,其中富流体相包裹体可分为含CO2收缩气泡两相熔融包裹体和气-液-固多相熔融包裹体。诸类包裹体主要赋含在刚玉晶核外的“主体”部分,以CO2单相流体包裹体和两相熔融包裹体最为发育,并且不同类型包裹体常密切伴生,表明它们形成时流体发生了不混溶作用:出现熔浆相(富含挥发分)、气相(CO2为主)和富水相(H2O-CO2为主)等多相体系。激光拉曼分析结果显示,各类包裹体中的气体组分主要是CO2,另有不等量的N2和H2S,据此划分为纯CO2、CO2-N2、CO2-H2S和CO2-N2-H2S等气体组合类型,没有发现O2、CH4和H2等组分。此外,拉曼分析也证实了流体包裹体和熔融包裹体中存在H2O。上述资料表明,昌乐地区深部流体以CO2为主,同时包含H2O、N2和H2S在内的多种组分,这些流体组分也是刚玉母浆系统的重要成分。     

Crystal chemical variations in Ba-rich biotites from gabbroic rocks of lower crust (Ivrea Zone,NW Italy)

Biotites from mafic rocks occurring at different stratigraphic levels of the Ivrea-Verbano Mafic Complex are studied. The rocks are gabbros and diorites. All the biotites are intermediate between phlogopite and annite [0.282 (up to 7.14 and 9.32 wt%, respectively) with respect to those of the diorites (up to 1.26 and 6.26 wt%, respectively). Systematic compositional variations support the substitution model 2 ^IV Si+( ^IV R²⁺)2 ^IV Al+ ^VI Ti (R²⁺=Fe+Mg+Mn) in gabbros and ^IV Si+ ^VI Al ^IV Al+ ^VI Ti in diorites. A predominance of disordered stacking sequences, coexisting with 1M, 2M ₁ and 3T polytypes was observed in all biotites. It was possible to carry out structural refinements only on three biotites-2M ₁ from diorites (R-values between 2.68 and 3.77) and one biotite-1M from gabbros (R-value=3.09). It was shown that: (1) the reduced thickness of the tetrahedral sheet in Ba-rich biotites supports the coupled substitution ^IV Si+ ^XII K ^IV Al+ ^XII Ba; (2) the interlayer site geometry is affected by the whole layer chemistry and does not reflect only local chemical variations; (3) in two samples of the 2M ₁ polytype, the M(1) octahedral site is larger and more distorted than the M(2) sites because of the preferential ordering of Fe²⁺ in the M(1) site, whereas one sample shows complete cation disorder in the octahedral sites. Biotite-1M shows that Fe²⁺ can also be located in the M(2) site. Some of the differences between the biotites of gabbros and diorites (e.g. Ba concentration and exchange vectors) may be linked to the host rock composition and to its crystallization process. Biotite occurs in trace amounts in gabbros and its crystallization is related to the interstitial melt which contributed to the adcumulus growth of the main rock forming phases and became highly enriched in K, Ba and Ti. Diorites are the result of equilibrium crystallization of a residual melt rich in incompatible elements, where biotite is a major constituent.     

Silica-rich Melts in Quartz Xenoliths from Vulcano Island and their Bearing on Processes of Crustal Anatexis and Crust-Magma Interaction beneath the Aeolian Arc, Southern Italy

Quartz-rich xenoliths in lavas and pyroclastic rocks from VulcanoIsland, part of the Aeolian arc, Italy, contain silicic meltinclusions with high SiO₂ (73–80 wt %) and K₂O (3–6wt %) contents. Two types of inclusions can be distinguishedbased on their time of entrapment and incompatible trace element(ITE) concentrations. One type (late, ITE-enriched inclusions)has trace element characteristics that resemble those of themetamorphic rocks of the Calabro-Peloritano basement of theadjacent mainland. Other inclusions (early, ITE-depleted) havevariable Ba, Rb, Sr and Cs, and low Nb, Zr and rare earth element(REE) contents. Their REE patterns are unfractionated, witha marked positive Eu anomaly. Geochemical modelling suggeststhat the ITE-depleted inclusions cannot be derived from equilibriummelting of Calabro-Peloritano metamorphic rocks. ITE-enrichedinclusions can be modelled by large degrees (>80%) of meltingof basement gneisses and schists, leaving a quartz-rich residuerepresented by the quartz-rich xenoliths. Glass inclusions inquartz-rich xenoliths represent potential contaminants of Aeolianarc magmas. Interaction between calc-alkaline magmas and crustalanatectic melts with a composition similar to the analysed inclusionsmay generate significant enrichment in potassium in the magmas.However, ITE contents of the melt inclusions are comparablewith or lower than those of Vulcano calc-alkaline and potassicrocks. This precludes the possibility that potassic magmas inthe Aeolian arc may originate from calc-alkaline parents throughdifferent degrees of incorporation of crustal melts. KEY WORDS: melt inclusions; crustal anatexis; magma assimilation; xenoliths; Vulcano Island     

Internal morphology, habit and U-Th-Pb microanalysis of amphibolite-to-granulite facies zircons: geochronology of the Ivrea Zone (Southern Alps)

Several types of growth morphologies and alteration mechanisms of zircon crystals in the high-grade metamorphic Ivrea Zone (IZ) are distinguished and attributed to magmatic, metamorphic and fluid-related events. Anatexis of pelitic metasediments in the IZ produced prograde zircon overgrowths on detrital cores in the restites and new crystallization of magmatic zircons in the associated leucosomes. The primary morphology and Th-U chemistry of the zircon overgrowth in the restites show a systematic variation apparently corresponding to the metamorphic grade: prismatic (prism-blocked) low-Th/U types in the upper amphibolite facies, stubby (fir-tree zoned) medium-Th/U types in the transitional facies and isometric (roundly zoned) high-Th/U types in the granulite facies. The primary crystallization ages of prograde zircons in the restites and magmatic zircons in the leucosomes cannot be resolved from each other, indicating that anatexis in large parts of the IZ was a single and short lived event at 299 ± 5 Ma (95% c. l.). Identical U/Pb ages of magmatic zircons from a metagabbro (293 ± 6 Ma) and a metaperidotite (300 ± 6 Ma) from the Mafic Formation confirm the genetic context of magmatic underplating and granulite facies anatexis in the IZ. The U-Pb age of 299 ± 5 Ma from prograde zircon overgrowths in the metasediments also shows that high-grade metamorphic (anatectic) conditions in the IZ did not start earlier than 20 Ma after the Variscan amphibolite facies metamorphism in the adjacent Strona–Ceneri Zone (SCZ). This makes it clear that the SCZ cannot represent the middle to upper crustal continuation of the IZ. Most parts of zircon crystals that have grown during the granulite facies metamorphism became affected by alteration and Pb-loss. Two types of alteration and Pb-loss mechanisms can be distinguished by cathodoluminescence imaging: zoning-controlled alteration (ZCA) and surface-controlled alteration (SCA). The ZCA is attributed to thermal and/or decompression pulses during extensional unroofing in the Permian, at or earlier than 249 ± 7 Ma. The SCA is attributed to the ingression of fluids at 210 ± 12 Ma, related to hydrothermal activity during the breakup of the Pangaea supercontinent in the Upper Triassic/Lower Jurassic. Received: 7 July 1998 / Accepted: 4 November 1998     

Fluid evolution during metamorphism of the Otago Schist, New Zealand: (I) Evidence from fluid inclusions

Fluid inclusion salinities from quartz veins in the Otago Schist, New Zealand, range from 1.0 to 7.3 wt% NaCl eq. in the Torlesse terrane, and from 0.4 to 3.1 wt% NaCl eq. in the Caples terrane. Homogenization temperatures from these inclusions range from 124 to 350  °C, with modal values for individual samples ranging from 163 to 229  °C, but coexisting, low-salinity inclusions exhibiting metastable ice melting show a narrower range of T  _h from 86 to 170  °C with modes from 116 to 141  °C. These data have been used in conjunction with chlorite chemistry to suggest trapping conditions of ≈350–400  °C and 4.1–6.0  kbar for inclusions showing metastable melting from lower greenschist facies rocks, with the densities of many other inclusions reset at lower pressures during exhumation of the schist. The fluid inclusion salinities and Br/Cl ratios from veins from the Torlesse terrane are comparable to those of modern sea-water, and this suggests direct derivation of the vein fluid from the original sedimentary pore fluid. Some modification of the fluid may have taken place as a result of interaction with halogen-bearing minerals and dehydration and hydration reactions. The salinity of fluids in the Caples terrane is uniformly lower than that of modern sea-water, and this is interpreted as a result of the dilution of the pore fluid by dehydration of clays and zeolites. The contrast between the two terranes may be a result of the original sedimentary provenance, as the Torlesse terrane consists mainly of quartzofeldspathic sediments, whilst the Caples terrane consists of andesitic volcanogenic sediments and metabasites which are more prone to hydration during diagenesis, and hence may provide more fluid via dehydration at higher grades.     

Genesis and evolution of subduction-zone andesites: evidence from melt inclusions

Andesite magmatism plays a major role in continental crustal growth, but its subduction-zone origin and evolution is still a hotly debated topic. Compared with whole-rock analyses, melt inclusions (MIs) can provide important direct information on the processes of magma evolution. In this article, we synthesize data for melt inclusions hosted by phenocrysts in andesites, extracted from the GEOROC global compilation. These data show that melt inclusions entrapped by different phenocrysts have distinct compositions: olivine-hosted melt inclusions have basalt and basaltic andesite compositions, whereas melt inclusions in clinopyroxene and othopyroxene are mainly dacitic to rhyolitic. Hornblende-hosted melt inclusions have rhyolite composition. The compositions of melt inclusions entrapped by plagioclase are scattered, spanning from andesite to rhyolite. On the basis of the compositional data, we propose a mixing model for the genesis of the andesite, and a two-chamber mechanism to account for the evolution of the andesite. First, andesite melt is generated in the lower chamber by mixing of a basaltic melt derived from the mantle and emplaced in the lower crust with a felsic melt resulting from partial melting of crustal rocks. Olivine and minor plagioclase likely crystallize in the lower magma chamber. Secondly, the andesite melt ascends into the upper chamber where other phenocrysts crystallize. According to SiO₂-MgO diagrams of the MIs, evolution of the andesite in the upper chamber can be subdivided into two distinct stages. The early stage (I) is characterized by a phenocrystal assemblage of clinopyroxene + othopyroxene + plagioclase, whereas the late stage (II) is dominated by crystallization of plagioclase + hornblende.     

Long-term deformation and fluid-enhanced mass transport in a Variscan peridotite shear zone in the Ivrea Zone,northern Italy: a microtextural,petrological and geochemical study of a reactivated shear zone

The microtextural, petrological and geochemical study of a ductile shear zone in the phlogopite peridotite of Finero/Ivrea Zone (northern Italy) reveals the long-term deformation of this zone. The zone is divided into a protomylonitic and an ultramylonitic part. Both parts reflect different periods of deformation, although the orientation of the mineral lineations does not change. In the coarse-grained part (period 1) the deformation started under granulite facies conditions (about 775°C). Olivine, ortho- and clinopyroxene and phlogopite recrystallized dynamically. In the ultramylonitic part relics of the granulite facies event and evidence for a continuous or two-stage deformation history under amphibolite facies (minimum 640°C) to upper greenschist facies conditions (maximum 520°C) are preserved (period 2). Amphibolite facies conditions are indicated by olivine recrystallization, the monoclinization of orthopyroxene porphyroclasts and the recrystallization and chemical changes of clinopyroxene. The greenschist facies final stage of period 2 is characterized by decreased X _{CO 2} and the syntectonic formation of antigorite, tremolite and phlogopite at the expense of recrystallized and porphyroclastic olivine and pyroxene. Between both deformation periods a short break in deformation continuity is probable. Continuous deformation or reactivation in shear zones of the Ivrea Zone has not been described so far. During the granulite facies shearing, small amounts of channelized fluid flow led to a slight mass transfer. The shear zone shows a moderate enrichment of Na, Ba, Cu, Cs, H₂O and CO₂ and a small loss of P. The mass balance of the ultramylonite indicates a significant increase in mass transport. A mass gain can be inferred for H, Na, K, Ba, Al, Ti, P, S, Cs, Sr, Rb, C, Zn, Zr, S, Sc, light rare earth elements, Nb, Cl and Au. The zone is depleted in Ca, Cu, Co, F and Ni. Si, Mg, Cr, Mn, Y, Nb and V are constant within analytical error. Deformation and fluid infiltration led to a change in volume which increases during the granulite facies event by 5.7% and during the lower temperature phase by 3.3%. The calculated fluid to rock ratios by standard equations results in unrealistically high values. For the interpretation of highly deformed rocks with drastic grain size reduction it is therefore necessary to consider the enhanced diffusion, which is mainly controlled by the increased grain boundary surface.     

Andalusite-bearing veins at Vedrette di Ries (eastern Alps, Italy): fluid phase composition based on fluid inclusions

Abstract Andalusite-bearing veins formed during contact metamorphism in the aureole of the Vedrette di Ries tonalite. In the veins, quartz crystals that are completely armoured by andalusite or that occur in strain shadow areas contain three generations of fluid inclusions: low-salinity H₂O-CO₂-CH₄ mixtures with CH₄/(CO₂+ CH₄) ± 0.35 (type A); low-salinity aqueous fluids (type B); H₂O-free, CO₂-CH₄ fluids with the same carbonic speciation as A (type C). Carbonic types A and C typically have a dark appearance, which is attributed to graphite coatings on inclusion walls. Microstructural analysis of the host quartz and calculated densities indicate that type A inclusions were likely trapped during vein formation. These inclusions underwent strain-assisted re-equilibration during cooling that resulted in density increases without change of composition. After the rocks had cooled below about 350 ° C, type C inclusions appear to have formed from one of the immiscible fractions after unmixing of the H₂O-CO₂-CH₄ fluid mixtures. Aqueous type B inclusions, apparently trapped between 225 and 350 ° C, could represent an independent fluid, or could be the H₂O-rich fraction of unmixed type A fluids. Taking account of the uncertainties, the composition and density of the complex type A inclusion fluids are in good agreement with the properties of primary fluids calculated from the petrological data. The fluid inclusion data support the model of vein formation by hydrofracturing as a result of dehydration of graphitic metapelites. These new results also demonstrate the importance of considering strain in the interpretation of metamorphic fluid inclusions.     

Pseudotachylytes in the Balmuccia peridotite (Ivrea Zone) as markers of the exhumation of the southern Alpine continental crust

Two types of pseudotachylytes are observed in the Balmuccia peridotite of the Ivrea zone (Southern Alps, Italy). A-type pseudotachylytes correspond to previously studied occurrences and were formed under temperatures comprising between 550 and 900 °C and pressures comprising between 0.6 and 1.2 GPa. These conditions were met in the Ivrea crust between 350 and 270 Ma, suggesting that A-type pseudotachylytes were formed during Variscan tectonics or Permian transtensional tectonics. B-type pseudotachylytes post-date A-type pseudotachylytes. Textural characteristics of B-type veins suggest a formation in the upper continental crust, at depths of about 5–10 km or less. Petrological constraints indicate that the exhumation of the Ivrea crust at such shallow depths was achieved later than c.  70 Ma, thus providing a maximum age of 70 Ma for B-type veins. Pseudotachylytes appear as markers of the poly-orogenic evolution of the Alpine belt.     

Metasomatism of continental crust during subduction: the UHP whiteschists from the Southern Dora-Maira Massif (Italian Western Alps)

The ultrahigh‐pressure pyrope whiteschists from the Brossasco‐Isasca Unit of the Southern Dora‐Maira Massif represent metasomatic rocks originated at the expense of post‐Variscan granitoids by the influx of fluids along shear zones. In this study, geochemical, petrological and fluid‐inclusion data, correlated with different generations of pyrope‐rich garnet (from medium, to very‐coarse‐grained in size) allow constraints to be placed on the relative timing of metasomatism and sources of the metasomatic fluid. Geochemical investigations reveal that whiteschists are strongly enriched in Mg and depleted in Na, K, Ca and LILE (Cs, Pb, Rb, Sr, Ba) with respect to the metagranite. Three generations of pyrope, with different composition and mineral inclusions, have been distinguished: (i) the prograde Prp I, which constitutes the large core of megablasts and the small core of porphyroblasts; (ii) the peak Prp II, which constitutes the inner rim of megablasts and porphyroblasts and the core of small neoblasts; and (iii) the early retrograde Prp III, which locally constitutes an outer rim. Two generations of fluid inclusions have been recognized: (i) primary fluid inclusions in prograde kyanite that represent a NaCl‐MgCl₂‐rich brine (6–28 wt% NaCl_eq with Si and Al as other dissolved cations) trapped during growth of Prp I (type‐I fluid); (ii) primary multiphase‐solid inclusions in Prp II that are remnants of an alumino‐silicate aqueous solution, containing Mg, Fe, alkalies, Ca and subordinate P, Cl, S, CO₃^2‐, LILE (Pb, Cs, Sr, Rb, K, LREE, Ba), U and Th (type‐II fluid), at the peak pressure stage. We propose a model that illustrates the prograde metasomatic and metamorphic evolution of the whiteschists and that could also explain the genesis of other Mg‐rich, alkali‐poor schists of the Alps. During Alpine metamorphism, the post‐Variscan metagranite of the Brossasco‐Isasca Unit experienced a prograde metamorphism at HP conditions (stage A: ～1.6 GPa and ≤ 600 °C), as indicated by the growth of an almandine‐rich garnet in some xenoliths. At stage B (1.7–2.1 GPa and 560–590 °C), the influx of external fluids, originated from antigorite breakdown in subducting oceanic serpentinites, promoted the increase in Mg and the decrease of alkalies and Ca in the orthogneiss toward a whiteschist composition. During stage C (2.1 < P < 2.8 GPa and 590 < T < 650 °C), the metasomatic fluid influx coupled with internal dehydration reactions involving Mg‐chlorite promoted the growth of Prp I in the presence of the type‐I MgCl₂‐brine. At the metamorphic peak (stage D: 4.0–4.3 GPa and 730 °C), Prp II growth occurred in the presence of a type–II alumino‐silicate aqueous solution, mostly generated by internal dehydration reactions involving phlogopite and talc. The contribution of metasomatic external brines at the metamorphic climax appears negligible. This fluid, showing enrichment in LILE and depletion in HFSE, could represent a metasomatic agent for the supra‐subduction mantle wedge.     

Carbonation of Cl-rich scapolite boudins in Skallen, East Antarctica: evidence for changing fluid condition in the continental crust

Spectacular reaction textures in poikiloblastic scapolitite boudins, within marbles in the continental crust exposed in the Lützow–Holm Complex, East Antarctica, provide insights into the changing fluid composition and movement of fluid along grain boundaries and fractures. Petrographic and geochemical features indicate scapolite formation under contrasting fluid compositions. Core composition of scapolite poikiloblasts (Scp_I) are marialitic (Cl = 0.7 apfu) whereas rims in contact with biotite or clinopyroxene are meionite rich. Fine‐grained recrystallized equigranular scapolite (Scp_II) shows prominent chemical zoning, with a marialitic core and a meionitic rim (Cl = 0.36 apfu). Scapolite poikiloblasts are traversed by Scp_III reaction zones along fractures with compositional gradients. Pure CO₂ fluid inclusions are observed in healed fractures in scapolite poikiloblasts. These negative crystal‐shaped fluid inclusions are moderately dense, and are believed to be coeval with Scp_III formation at temperatures >600 °C and a minimum pressure of c. 3.8 kbar. Grain‐scale LA‐ICPMS studies on trace and rare earth elements on different textural types of scaplolites and a traverse through scapolite reaction zone with compositional gradient suggest a multistage fluid evolution history. Scp_I developed in the presence of an internally buffered, brine‐rich fluid derived probably from an evaporite source during prograde to peak metamorphism. Recrystallization and grain size reduction occurred in the presence of an externally sourced carbonate (CaCO₃)‐bearing fluid, resulting in the leaching of Cl, K, Rb and Ba from Scp_I along fractures and grain boundaries. Movement of fluids was enhanced by micro‐fracturing during the transformation of Scp_I to Scp_III. Fractures in fluorapatite are altered to chlorapatite proving evidence for the pathways of escaping Cl‐bearing fluids released from Scp_I. The present study thus provides evidence for the usefulness of scapolite in fingerprinting changing volatile composition and trace element contents of fluids that percolate within the continental crust.     

硅酸盐-盐（硫酸盐）流体不混溶：蒙古南部Mushugai-Khuduk含火成碳酸岩杂岩体矿物中的熔体包裹体研究

Crystalline and melt inclusions were studied in garnet,diopside,potassium feldspar,and sphene from the garnet syenite porphyry of the carbonatite-bearing complex Mushugai-Khuduk,southern Mongolia.Phlogopite,clinopyroxene,albite,potassium feldspar,spheric,wollastonite,magnetite,Ca and Sr sulfates,fluorite,and apatite were identified among the crystalline inclusions. The melt inclusions were homogenized at 1010～1080℃and analyzed on an electron microprobe.Silicate,salt,and combined silicate- salt melt inclusions were found.Silicate melts show considerable variations in SiO_2 concentration(56 to 66wt% ),high Na_2O K_2O (up to 17wt% ),and elevated Zr,F,and C1 contents.In terms of bulk rock chemistry,the silicate melts are alkali syenites.During thermometric experiments,salt melt inclusions quenched into homogeneous glasses of predominantly sulfate compositions containing no more than 1.3wt% SiO_2.These melts are enriched in alkalis,Ba,Sr,P,F,and C1.The investigation of the silicate and salt melt inclusions in minerals of the garnet syenite porphyries indicate that these rocks were formed under influence of the processes of crystallization differentiation and magma separation into immiscible silicate and salt(sulfate)liquids.     

Syndeformational fluid trapping in quartz: determining the pressure-temperature conditions of deformation from fluid inclusions and the formation of pure CO2 fluid inclusions during grain-boundary migration

Abstract Observations and microthermometric data on fluid inclusions from a terrane that underwent deformation following peak metamorphic conditions show that grain-boundary migration recrystallization favours the entrapment of carbonic inclusions whereas microfracturing during brittle deformation favours the infiltration and eventual entrapment of aqueous fluids. Our results imply that pure CO₂ fluid inclusions in metamorphic rocks are likely to be the residue of deformation-recrystallization process rather than representing a primary metamorphic fluid.
Where the temperature of deformation can be deduced by other means, the densities of fluid inclusions trapped during recrystallization, which we call recrystallization-primary fluid inclusions, can be used to constrain the ambient pressure during deformation. Using these constraints, the data imply that the post-metamorphic Hercynian exhumation in Sardinia brought rocks at 300° C to within 3km of the surface. This conclusion is similar to that described for the rapidly uplifted Southern Alps in New Zealand.