High-pressure Terrains Research Articles

Extraneous argon in high-pressure metamorphic rocks: Distribution, origin and transport in the Cycladic Blueschist Unit (Greece)

The 40Ar/39Ar geochronological technique has significantly contributed to the study of crustal deformation. However, the frequent occurrence of extraneous argon in high-pressure terrains can make the interpretation of 40Ar/39Ar ages problematic. This contribution attempts to: i) characterize the distribution and origin of extraneous argon in the Cycladic Blueschist Unit (CBU, Greece) by applying the 40Ar/39Ar dating technique on three Cycladic islands, Syros, Tinos and Ios, which span a complete transect of the CBU and ii) discuss regional implications for the tectonometamorphic evolution of the CBU. Our results, combined with ages available in the literature, show that periods of peak-burial conditions (55–49Ma) and syn- (45–38Ma) to post-orogenic exhumation (37–21Ma) are well constrained in the CBU by the 40Ar/39Ar method. Moreover, Variscan apparent ages are preserved in the Cycladic basement of Ios, indicating that the 40Ar/39Ar system was not completely reset during the Eocene metamorphic episode. Additionally, our 40Ar/39Ar data show that the occurrence of extraneous argon is heterogeneously distributed in the CBU. While high-pressure rocks of Syros and Tinos are only locally affected, extraneous argon is clearly evidenced in the CBU of Ios, yielding geologically meaningless ages up to 900Ma. On each island, the most affected ages were obtained in microstructures where fluid circulation is enhanced such as shear bands or strain shadows around garnets. We suggest that the Cycladic Basement, a polymetamorphic continental unit, behaved as an argon reservoir that has been mobilized by fluid circulation at the Eocene. These argon-enriched fluids have then heterogeneously contaminated metamorphic rocks when percolating through the CBU. Finally, we claim that a minimum closure temperature of 550°C must be accepted for the phengites of the CBU and conclude that volume diffusion is not the main process resetting the argon isotopic system in high-pressure rocks compare to recrystallization enhanced by fluid circulation and deformation.

True metamorphic isograds or tectonically sliced metamorphic sequence? New high-spatial resolution petrological data for the New Caledonia case study

The metamorphic belt of northern New Caledonia represents one of the best-exposed example of fossil Pacific-type subduction zone and a worldwide reference for high-pressure (HP) metamorphism and metamorphic mineral isograds, marked by a gradual evolution from very low-grade lawsonite-bearing to high-grade epidote-bearing eclogite assemblages. Despite the general agreement on the progressive increase in metamorphism, open debates hinge on the tectonic meaning of such mineral isograds, either interpreted as fossilized continuous metamorphic gradients or as major tectonic discontinuities. We present a new and extensive metamorphic dataset acquired by means of Raman spectroscopy of carbonaceous material and pseudosection modeling. Additional PT estimates were also obtained via traditional thermobarometry. Our dataset indicates the occurrence of two tectonometamorphic domains characterized by distinct patterns and significantly modifies the past tectonic interpretation of the HP terranes. The first domain, rich in metasediments, shows a continuous metamorphic gradient starting at ~300 °C and ~0.8 GPa and reaching the blueschist-eclogite transition at 500–520 °C and ~1.8 GPa and is only locally cut by minor tectonic breaks. The second one, rich in metaophiolites, shows a rather constant metamorphism at 520–550 °C and ~2.4 GPa. The two terranes are separated by a conspicuous pressure gap (0.6 GPa, or ~20 km), but no temperature gap exists. We therefore interpret the metamorphic mineral isograds in the blueschist, metasediment-rich unit as a continuous prograde metamorphic gradient corresponding to ~35 km of accreted material (in a cold subduction zone favoring lawsonite stability) later affected by a factor of two decompressional thinning. Only the epidote isograd, which localized strain as a result of fluid release during decompression, reflects regional reequilibrations. Importantly, no significant tectonic break affects the regional distribution of the classical mineral isograds, and the most significant metamorphic break is best depicted by a lithological contrast (metasediment-rich vs. metamafic/ultramafic-rich domains). Comparable patterns are observed in Tethyan-type orogens like the Western Alps or Corsica. This contribution provides useful insights into the mechanisms of exhumation and stacking of HP terrains and mountain building of both Pacific and Tethyan orogenic belts.

Timing of HP metamorphism in the Schistes Lustrés of Alpine Corsica: New Lu–Hf garnet and lawsonite ages

Alpine Corsica is crucial in understanding Western Mediterranean geodynamics. Available paleotectonic models attribute the high-pressure terrains of Corsica to the Western Alps (east dipping subduction), or to the Apennines (west-dipping subduction) or to a flip from one to the other subduction zone. Constraints on the timing of high-pressure metamorphism in Alpine Corsica are sparse and ages range from Late Cretaceous to Late Eocene. This represents a major point of debate. Here we present high-precision garnet Lu–Hf ages for three samples from the highest grade, Lawsonite-eclogite unit yielding ages of 34.23±1.6Ma; 34.11±0.89Ma, 34.65±0.55Ma. In addition to the well-established garnet dating, we employed lawsonite Lu–Hf analyses on a lawsonite-bearing (garnet-free) metagabbro from the neighboring lawsonite-blueschist unit that yielded an age of 37.5±1.3Ma. Because metamorphic temperatures of the eclogite-facies samples (ca. 500–550°C) are below the Lu–Hf closure temperature in garnet and since the eclogitic garnet displays a core-to-rim increase in Mn concentrations typical of prograde zoning, we attribute the above Lu–Hf ages to prograde subduction metamorphism. Similarly, the age of lawsonite in the blueschist-facies unit can be safely referred to prograde-peak conditions based on the available clockwise metamorphic path. The slightly older age of the lawsonite-blueschist sample suggests a diachronous timing in the achievement of high-pressure conditions in the eclogite and in the blueschist facies units. Since both units are thought to derive from a former ocean–continent transition zone, our geochronological data provide precise time constraints to the subduction history of Alpine Corsica in the frame of the Western Mediterranean geodynamics. Our data provide Late Eocene ages for the subduction event in Corsica, which are significantly younger than previously published Cretaceous ages. This calls for partial revision of the current evolutionary models on the Corsican segment of the Alpine belt.

Highly siderophile element behaviour accompanying subduction of oceanic crust: Whole rock and mineral-scale insights from a high-pressure terrain

Highly siderophile element concentrations (HSE: Re and platinum-group elements (PGE)) are presented for gabbros, gabbroic eclogites and basaltic eclogites from the high-pressure Zermatt–Saas ophiolite terrain, Switzerland. Rhenium and PGE (Os, Ir, Ru, Rh, Pt, Pd) abundances in gabbro- and eclogite-hosted sulphides, and Re–Os isotopes and elemental concentrations in silicate phases are also reported. This work, therefore, provides whole rock and mineral-scale insights into the PGE budget of gabbroic oceanic crust and the effects of subduction metamorphism on gabbroic and basaltic crust. Chondrite-normalised PGE patterns for the gabbros are similar to published mid-ocean ridge basalts (MORB), but show less inter-element fractionation. Mean Pt and Pd contents of 360 and 530 pg/g, respectively, are broadly comparable to MORB, but gabbros have somewhat higher abundances of Os, Ir and Ru (mean: 64, 57 and 108 pg/g). Transformation to eclogite has not significantly changed the concentrations of the PGE, except Pd which is severely depleted in gabbroic eclogites relative to gabbros (∼75% loss). In contrast, basaltic eclogites display significant depletion of Pt (⩾60%), Pd (>85%) and Re (50–60%) compared with published MORB, while Os, Ir and Ru abundances are broadly comparable. Thus, these data suggest that only Pt, Pd and Re, and not Os, Ir and Ru, may be significantly fluxed into the mantle wedge from mafic oceanic crust. Re–Os model ages for gabbroic and gabbroic eclogite minerals are close to age estimates for igneous crystallisation and high-pressure metamorphism, respectively, hence the HSE budgets can be related to both igneous and metamorphic behaviour. The gabbroic budget of Os, Ir, Ru and Pd (but not Pt) is dominated by sulphide, which typically hosts >90% of the Os, whereas silicates account for most of the Re (with up to 75% in plagioclase alone). Sulphides in gabbroic eclogites tend to host a smaller proportion of the total Os (10–90%) while silicates are important hosts, probably reflecting Os inheritance from precursor phases. Garnet contains very high Re concentrations and may account for >50% of Re in some samples. The depletion of Pd in gabbroic eclogites appears linked, at least in part, to the loss of Ni-rich sulphide. Both basaltic and gabbroic oceanic crust have elevated Pt/Os ratios, but Pt/Re ratios are not sufficiently high to generate the coupled 186Os– 187Os enrichments observed in some mantle melts, even without Pt loss from basaltic crust. However, the apparent mobility of Pt and Re in slab fluids provides an alternative mechanism for the generation of Pt- and Re-rich mantle material, recently proposed as a potential source of 187Os– 186Os enrichment.

Aqueous fluids and hydrous melts in high-pressure and ultra-high pressure rocks: Implications for element transfer in subduction zones

High-pressure (HP) and ultra-high pressure (UHP) terranes are excellent natural laboratories to study subduction-zone processes. In this paper we give a brief theoretical background and we review experimental data and observations in natural rocks that constrain the nature and composition of the fluid phase present in HP and UHP rocks. We argue that a fluid buffered by a solid residue is compositionally well defined and is either an aqueous fluid (total amount of dissolved solids < 30 wt.%) or a hydrous melt (H 2O < 35 wt.%). There is only a small temperature range of approximately 50–100 °C, where transitional solute-rich fluids exist. A review of available experimental data suggest that in felsic rocks the second critical endpoint is situated at 25–35 kbar and ∼ 700 °C and hence must be considered in the study of UHP rocks. Despite this, the nature of the fluid phase can be constrained by relating the peak metamorphic conditions of rocks to the position of the wet solidus even if the peak pressure exceeds the pressure where the wet solidus terminates at the second critical endpoint. Transitional solute-rich fluids are expected in UHP terrains ( P > 30 kbar) with peak temperatures of about 700 ± 50 °C. At higher temperatures, hydrous granitic melts occur whereas at lower temperatures aqueous fluids coexists with eclogite-facies minerals. This argument is complemented by evidence on the nature of the fluid phase from high-pressure terrains. We show that in the diamond-bearing, high-temperature UHP rocks from the Kokchetav Massif there are not only hydrous felsic melts, but probably also carbonate and sulfide melts present. Hydrous quartzo-feldspathic melts are mainly produced in high temperature UHP rocks and their composition is relatively well constrained from experiments and natural rocks. In contrast, constraining the composition of aqueous fluids is more problematic. The combined evidence from experiments and natural rocks indicates that aqueous fluids liberated at the blueschist to eclogite facies transition are dilute. They contain only moderate amounts of LILE, Sr and Pb and do not transport significant amounts of key trace elements such as LREE, U and Th. This indicates that there is a decoupling of water and trace element release in subducted oceanic crust and that aqueous fluids are unable to enrich the mantle wedge significantly. Instead we propose that fluid-present melting in the sediments on top of the slab is required to transfer significant amounts of trace elements from the slab to the mantle wedge. For such a process to be efficient, top slab temperature must be at least 700–750 °C at sub-arc depth. Slab melting is likely to be triggered by fluids that derive from dehydration of mafic and ultramafic rocks in colder (deeper) portions of the slab.

Exhumation of alpine high-pressure rocks: insights from petrology of eclogite clasts in the Tertiary Piedmontese basin (Ligurian Alps, Italy)

The study of late and postorogenic sedimentary basins is a powerful tool to understand uplift, exhumation and erosion of an orogen. In the Ligurian Alps, high-pressure ophiolitic rocks are directly overlain by the sediments of the Tertiary Piedmontese basin (TPB); the conglomerates at the bottom of the TPB succession contain clasts of metaophiolites and metasediments which display deformation structures acquired at peak metamorphic conditions ranging from eclogite to blueschist facies. The deformation events prevalently recorded by the high-pressure rocks of the Ligurian Alps are either related to subduction and peak metamorphism, or to the late stage greenschist collision. The main structures which accomplish the early exhumation of such high-pressure terrains are only locally recorded and poorly explored. Further information on these early tectonic events can be gathered through the study of the clasts in the conglomerates. In this paper, we present a textural and petrologic outline and some thermobarometric estimates of the main types of high-pressure clasts sampled and their comparison with the high-pressure equivalents presently exposed in the Ligurian Alps. In particular, some clasts displaying a Na-amphibole+white mica+epidote+sphene blueschist foliation superimposed to an eclogitic garnet+Na-clinopyroxene+rutile tectonitic assemblage have been studied in detail. This kind of rock records a pressure–temperature ( P– T) path implying cooling during exhumation, whereas the high-pressure bedrocks are characterized by either isothermal decompression, or initial heating and subsequent cooling. We conclude that the blueschist foliation of these clasts most likely developed along shear zones and/or contacts among different slices, formed during tectonic coupling of warm, uprising eclogite units, with cooler blueschist slices during the early stages of exhumation.

EXHUMATION OF ALPINE HIGH PRESSURE ROCKS: INSIGHTS FROM PETROLOGY OF ECLOGITE CLASTS IN THE TERTIARY PIEDMONTESE BASIN (LIGURIAN ALPS, ITALY)

The study of late and post-orogenic sedimentary basins is a powerful tool to understand uplift, exhumation and erosion of an orogen. In the Ligurian Alps, high-pressure ophiolitic rocks are directly overlain by the sediments of the Tertiary Piedmontese Basin (TPB); the conglomerates at the bottom of the TPB succession contain clasts of metaophiolites and metasediments which display deformation structures acquired at peak metamorphic conditions ranging from eclogite- to blueschist- facies. The deformation events prevalently recorded by the highpressure rocks of the Ligurian Alps are either related to subduction and peak metamorphism, or to the late-stage greenschist collision. The main structures which accomplish the early exhumation of such high-pressure terrains are only locally recorded and poorly explored. Further information on these early tectonic events can be gathered through the study of the clasts in the conglomerates. In this paper we present a textural and petrologic outline and some thermobarometric estimates of the main types of high-pressure clasts sampled and their comparison with the high pressure equivalents presently exposed in the Ligurian Alps. In particular, some clasts displaying a Na-amphibole + white mica + epidote + sphene blueschist foliation superimposed to an eclogitic garnet + Na-clinopyroxene + rutile tectonitic assemblage have been studied in detail. These kind of rocks record a pressure-temperature path implying cooling during exhumation, whereas the high-pressure bedrocks are characterized by either isothermal decompression, or initial heating and subsequent cooling. We conclude that the blueschist foliation of these clasts most likely developed along shear zones and/or contacts among different slices, formed during tectonic coupling of warm, uprising eclogite units, with cooler blueschist slices during the early stages of exhumation.

Flat plateau and impossible isochrons: apparent 40Ar- 39Ar geochronology in a high-pressure terrain

Until recently most ages determined by the 40Ar- 39Ar technique have been represented by the age spectrum and isochron. Early in the history of the technique a series of criteria were outlined in order to discriminate between spectra which were considered to reflect disturbed isotopic systems and spectra which were considered to reflect the accurate age of the rock or mineral. With the advancement of 40Ar- 39Ar instrumentation and the introduction of laser-ablation techniques, it is now possible to obtain an age from single grains and on a sub-grain scale. In this case there is no production of an age spectra or determination of a plateau age; however, isochrons are still readily produced and used in the discussion of Ar isotope ratios, and specifically excess argon. Here, new data illustrate the possible drawback first of the age spectrum and plateau age in the absence of an independent geochronological constraint in high-pressure terranes, and second, of the accompanying isochron, which we demonstrate can be both incorrect and geologically meaningless.

About this title

The evolution of oceanic plates is very well described by plate tectonic theory, but the study of continental tectonics is more complicated. Continental plates have a different rheology, a greater mean age, a higher heat productivity, and are more heterogeneous in composition. An understanding of continental tectonics requires, therefore, that these effects upon rigid plate behaviour be taken into account. This book brings together a series of papers which explore various aspects of the deformation of continental lithosphere, covering different tectonic settings from the Palaeozoic to the present day.These include the processes of terrane accretion and juxtaposition, the exhumation of high-pressure terrains, and mechanisms of crustal extension and rifting. The book will be of general interest to a broad audience of earth scientists concerned with global tectonics, continental growth processes, and the deformation of the continents during collision, exhumation and extension.

Singular Equilibria in Paragonite Blueschists, Amphibolites and Eclogites

Chemographic constraints from high-pressure mafic rocks of the INTRODUCTION Nevado–Filabride Complex (Betic–Rif mountain belts, SE Spain) The relevance of singular equilibria has been largely and from worldwide high-pressure terrains have been used to unravel recognized in chemical systems of interest for the Earth phase relationships between amphibole, garnet, clinopyroxene, and Sciences. After the early experiments of Konovalov on albite in the system Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O azeotropic or congruent transformations, Ricci (1951) (NCFMASH) at quartz-, paragonite-, clinozoisiteand waterexhaustively discussed the fundamentals of singularities. saturated conditions. Extensive analysis of element partitioning Two types of singularities were recognized: the first, between amphibole and clinopyroxene suggests that the Fe–Mg called type m and most relevant, is associated with azeopartition coefficient is low at low-temperature and high-pressure tropic points, and the second, called type k, is related to conditions (‘high-temperature’ blueschists–low-temperature eclogites) the intersection of solvi with compositional loops, as and a progressive increase is achieved by an increase in temperature, occurs in the alkali feldspar system at low pressures or by a decrease in pressure, or by a decrease in the sodium content (Morse, 1970). Examples and discussion of both type m of the clinopyroxene. Mass-balance analysis of paragonite eclogites and type k singular points in igneous systems have been and amphibolites suggests that at least two singular reactions are given by Morse (1980, e.g. see chapters 16 and 18). The possible in NCFMASH: clinopyroxene+ paragonite+ quartz= widespread occurrence of singularities in igneous systems amphibole + albite + clinozoisite + water (garnet-absent), and is obviously related to the extremely large compositional amphibole+ clinozoisite= garnet+ clinopyroxene+ paragonite range of silicate liquids. The well-known concept of + quartz + water (albite-absent). Albite-absent singular equithe ‘thermal barrier’ or ‘thermal divide’ implies the librium is relevant for the blueschist–eclogite transition, whereas occurrence of singular equilibria, and their importance garnet-absent equilibrium may control the eclogite to amphibolite in the petrogenesis of granites [e.g. modelled in the binary transition. A complex arrangement of divariant reactions, singular system kalsilite–silica (Lindsley, 1966)] or basalts [e.g. in reactions and conjugate non-degenerate reactions arises in topologies the binary system forsterite–silica or in the ternary system at the blueschist–eclogite–amphibolite transitions. Schreinemakers’ forsterite–silica–diopside or forsterite–silica–anorthite analysis in P–T diagrams, and mixed-variable T–XM′–XC′ and (Presnall et al., 1979)] has been widely discussed. P–XM′–XC′ diagrams suggest that complex pressure–temperature In contrast, the significance and implications of singular paths are not mandatory if a recurrent appearance and disappearance point nets at subsolidus conditions have been largely of similar phase assemblages is observed in paragonite-bearing mafic undervalued. Although Korzhinskii (1963) analysed azeorocks. The P–T location of the singular curve net is discussed in tropic transformations in subsolidus open systems, only the context of Nevado–Filabride Complex mafic rocks. recently have applications to metamorphic assemblages

Evidence for excess argon during high pressure metamorphism in the dora maira massif (Western Alps, Italy), using an ultra-violet laser ablation microprobe 40Ar-39Ar technique

Ultra-high pressure eclogite/amphibolite grade metamorphism of the Dora Maira Massif in the western Alps is a well established and intensively studied event. However, the age of peak metamorphism and early cooling remains controversial. The 40Ar-39 Ar step-heating and laser spot ages from high pressure phengites yield plateau ages as old as 110 Ma which have been interpreted as the time of early cooling after the high pressure event. Recent U/Pb and Sm/Nd results challenge this assertion, indicating a much younger age for the event, around 45 Ma, and hence a radically different timing for the tectonic evolution of the western Alps. In a new approach to the problem, samples from the undeformed Hercynian metagranite, Brossasco, were studied using an ultra-violet laser ablation microprobe technique for 40Ar-39 Ar dating. The new technique allowed selective in situ analysis, at a spatial resolution of 50 urn, of quartz, phengite, biotite and K-feldspar. The results demonstrate the frequent occurrence of excess argon with high 40Ar-39 Ar ratios (1000–10000) and a strong relationship between apparent ages and metamorphic textures. The highest excess argon ratios are always associated with high closure temperature minerals or large diffusion domains within single mineral phases. The best interpretation of this relationship seems to be that excess argon was incorporated in all phases during the high pressure event, then mixed with an atmospheric component during rapid cooling and retrogression, producing a wide range of argon concentrations and 40Ar/36Ar ratios. Step-heating analysis of minerals with this mixture would produce linear arrays on a40Ar/36Ar versus 40Ar/36 Ar correlation diagram, leading to geologically meaningless plateau ages, older than the true closure age. In the present case, some ages in the range 60–110 Ma could be explained by the presence of excess argon incorporated around 40–50 Ma ago. Similar results found in other high pressure terrains in the Alps may reconcile the argon geochronometer with other systems such as Rb/Sr, U/Pb or Sm/Nd. This study therefore calls for an increasing use of high resolution in situ sampling techniques to clarify the meaning of 40Ar/ 36 Ar ages in many high pressure terrains.

On the occurrence of metamorphic 2M1 phengites: some thoughts on polytypism and crystallization conditions of 3T phengites

Potassic white micas occur in metamorphic rocks as polytype 2M 1 and sometimes 3T. Metamorphic phengites typically crystallize under high-pressure/low-temperature conditions (occurrence type I), but they are also expected to be common in rocks of low metamorphic grade which have particular bulk compositions, specifically low in Al 2 O 3 (occurrence type II). Occurrence type I is well known, and polytype 3T is common therein; in contrast occurrence type II is only sparsely documented, and no data exist concerning polytypism therein. The question has been raised as to whether the crystallization of 3T polytype is controlled by pressure or by mica chemistry. The present paper reports the occurrence of metamorphic phengites not related to high-pressure metamorphism but clearly due to rock bulk compositional constraints (Kfs-Chl ± Bt-bearing mineral assemblage). These phengites are very similar in composition to those occurring in high pressure terrains. All of them are 2M 1 , suggesting that 3 T stacking arrangement largely depends on factors other than mica chemistry. The factors which may control the mica polytypism are discussed. A high-pressure/low-temperature environment seems to be the most necessary condition for the crystallization of 3T phengites. These 3 T micas are expected to invert to 2M 1 during the later tectonometamorphic evolution, due to the effect of the pressure release concomitant with exhumation and recovery of the isotherms. However a fully comprehensive crystallochemical understanding of how the physical conditions facilitate the formation of the 3T polytype remains elusive

Anomalously Old Ar-Ar Ages in High Pressure Metamorphic Terrains

An increasing number of Ar-Ar ages for phengites in blueschist and eclogite terrains are in conflict with peak metamorphic ages determined by other geochronological techniques (Rb/Sr, Sm/Nd and U/Pb). The Ar-Ar ages are generally much older than the 'true' metamorphic age, and despite the fact that such samples yield apparently acceptable stepped-heating plateaux, the high apparent ages are most probably the result of excess argon. Recent laser microprobe Ar-Ar studies of within crystal age variations and the distribution of argon components between mineral phases confirm the widespread preservation of excess argon in minerals from high pressure eclogite/ blueschist metamorphic terrains. In the light of these uncertainties, should we abandon phengite Ar-Ar ages despite their importance in the construction of T-t profiles models for high pressure terrains?

The structure of the Bracco Ophiolite complex (Ligurian Apennines, Italy): a change from Alpine to Apennine polarity

The Bracco ophiolite complex, exposed in the area studied as the Bracco Subunit of the Melange Nappe, is tectonically intercalated between the underlying Canetolo Nappe and the overlying Lavagna Nappe. Detailed structural analysis and mapping in these units reveals a multi-phase deformation history. In the Bracco Subunit and the Lavagna Nappe, this history is characterized by a change, from SW (i.e. ‘Alpine’) directed folding of the ophiolitic basement and its sedimentary cover, to NE (i.e. ‘Apennine’) directed folds and thrusts. The Canetolo Nappe only shows Apennine vergent structures. The Alpine vergent structures are considered to be developed during the Late Cretaceous to Palaeocene in an accretionary wedge associated with an east-dipping subduction zone. The first Apennine directed structures are low-angle extensional faults and cascading folds, indicating that the change to Apennine polarity resulted from gravitational collapse of the accretionary wedge during the Palaeocene to Eocene. Gravitational instability is considered to have resulted from a drastic decrease of subduction rate during the Palaeocene, which has led to extension and uplift of the wedge. The rapid decompression under isothermal to slightly decreasing temperature conditions of the high-pressure terrains in the Voltri Massif concurs well with this tectonic scenario.

A Discussion on the evolution of the Precambrian crust - Geochemistry of granulite facies rocks and problems of their origin

Occurrences of granulite facies rocks are widespread in continental regions where they mostly are parts of stable shield areas. Granulite facies terrains are classified as low-, medium- or high-pressure terrains on the basis of critical mineral associations. Special interest is attached to the medium- and highpressure terrains, as they are representative of the deepest crustal levels available for study in any areal extent on the surface, and may give information about the composition of the lower continental crust. Granulite facies terrains are mainly composed of metamorphic and metasomatic rocks, but magmatic rocks with primary igneous textures interpreted as formed by crystallization of magmas under granulite facies conditions are frequent in some areas. Examples of such rocks are anorthosites, gabbros and mangerites. The low-pressure—high-temperature granulite facies rocks are chemically indistinguishable from the amphibolite facies gneisses with which they characteristically occur. It is therefore important to make a distinction between these and the higher pressure types. The medium- to high-pressure granulite facies terrains are characterized by a less ‘acidic’ average major element compositions, and significant depletions in Rb, Cs, Th and U compared with average surface shield compositions. Available data also indicate low initial Sr isotope ratios, even in the gneissic types. In the author’s opinion the important problem associated with granulite facies rocks is not that of their origin, but rather of their importance as constituents of the continental crust, and how they attained their present chemistry.