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Petrography and mineral chemistry
Mineral analyses have been performed with a Cameca SX100 electron microprobe (Microsonde Ouest, IFREMER, Brest-Plouzané, France). Representative analyses of selected minerals are listed in Supplementary material (Table I.1.1 – After the references). Mineral XF3=Fe3+/(Fe3++Fe2+); abbreviations are: amp: amphibole, bi: biotite, chl: chlorite, cpx: clinopyroxene, coe: coesite, ep: epidote, g: garnet, ilm: ilmenite, ksp: potassium feldspar, ky: kyanite, law: lawsonite, mu: muscovite, pl: plagioclase, q: quartz, ru: rutile, ttn: titanite, sp: spinel, sul: sulphide. Mineral endmembers (expressed in mole %) and compositional variables are: XMg = Mg/(Mg+Fe); almandine, alm = Fe/(Fe+Mg+Ca+Mn), pyrope, prp = Mg/(Fe+Mg+Ca+Mn), grossular, grs = Ca/(Fe+Mg+Ca+Mn), spessartine, sps = Mn/(Fe+Mg+Ca+Mn); jadeite, jd = Na/(Na+Ca); albite, an = Ca/(Ca+Na+K), anorthite, an = Ca/(Ca+Na+K), pistacite, ps = Fe3+/( Fe3++Al-2); orthoclase, or = K/(Ca+Na+K); geikielite, gk = Mg/(Fe2++Fe3++Mg+ Mn); pfu: per formula unit. In addition, the amount of calcium on site B labelled Ca(B) and Na on site A labelled Na(A) in amphibole have been calculated according to the procedure of Leake et al. (1997).
Under the microscope, the studied sample is a strongly retrogressed eclogite composed of garnet porphyroblasts and various fine-grained (< 25µm) symplectites (Fig. I.1.2a). It also contains minor rutile, ilmenite, apatite and zircon. A foliation is marked by the ellipsoidal shape of the symplectites, whereas the crystals that compose these aggregates are randomly oriented and lack signs of deformation which is illustrated by the preservation of the cor-sp-mu-pl coronitic textures (Fig. I.1.2a).
Interpretation of the petrographic observations
These observations can be interpreted in terms of three metamorphic events. The dominant assemblage comprises relatively coarse (several mm) crystals of garnet and three minerals now replaced by symplectites. Diopside-amphibole-plagioclase-quartz symplectites are interpreted as former omphacite, corundum/spinel-plagioclase as former kyanite, and biotite-bearing symplectites as replacing a K-rich mineral (muscovite or K-feldspar). These minerals are also preserved as inclusions in garnet, associated with amphibole, epidote, muscovite, rutile and quartz. Together with the remarkable absence of plagioclase inclusions, this suggests that the dominant metamorphic stage (M1) records eclogite-facies conditions. In detail, the core and mantle of garnet contain abundant inclusions of epidote and amphibole, whereas the rim is epidote-free and contains only rare crystals of amphibole (g1, Fig I.1.3a). The progressive decrease of the proportion of hydrous phases testifies to the prograde character of M1. This is also in agreement with garnet zoning, characterized by a decrease of spessartine and increase of pyrope, typical of prograde growth zoning.
The irregular boundary between the rim and the outer rim of garnet (Fig. I.1.3a), associated with the relatively abrupt increase of grossular, as well as the irregular garnet contour, commonly with convex faces, cutting across the concentric growth zoning, suggest a period of partial resorption of garnet (g1) before a renewed growth (outer rim – g2).
The growth of garnet 2 (with higher grossular, and lower pyrope contents) is characteristic of the second metamorphic stage, M2. This event is also marked by the renewed growth of amphibole (included in garnet 2) and by the resorption of omphacite. Minor rutile and quartz, preserved as inclusions in garnet 2, were also part of the assemblage. The albite blebs (significantly larger than plagioclase crystals in the symplectites) surrounded by a diopside corona suggest that they are not in equilibrium with the matrix symplectite and could be attributed to the M2 metamorphic stage. This interpretation suggests that M2 records high-pressure granulite-facies conditions.
The M3 event is characterized by the replacement of the large M1 crystals by plagioclase-and diopside-bearing symplectites. They typically develop during decompression of high-pressure rocks, suggesting that M3 occurred at lower pressures. This agrees with the observed relations between rutile, ilmenite and titanite that suggest a sequential growth of ilmenite at the expense of rutile and then titanite at the expense of both rutile and ilmenite.
Pseudosection investigation of a magmatic origin
In order to investigate the possibility that the garnet rich layers represent an initial heterogeneity of the protolith, a P–T pseudosection has been calculated for a SEM-measured composition analysed over an area that comprises garnet, with inclusions of epidote, amphibole and kyanite with amphibole-plagioclase symplectites as well as rutile and ilmenite. This diagram has been calculated under H2O-saturated conditions and Fe3+/(Fe3++Fe2+) = 12% (i.e. the same as the host rock). In the P–T pseudosection (Fig I.2.6a), epidote is stable with garnet and amphibole below 620°C and 24 kbar. The predicted grossular and pyrope contents of a garnet in equilibrium with epidote would be comprised between 20-30% and 10-25% respectively. The pyrope content observed in garnet core (prp35) with epidote inclusions is not consistent with the modelled garnet in the epidote stability field.
A possibility is that the garnet-rich layer and its host rock were not characterized by the same Fe3+/(Fe3++Fe2+) ratio. Accordingly, a TX(Fe3+) pseudosection has been calculated in order to investigate the effect of varying Fe oxidation state (Fig I.2.6b). In this diagram, 67 calculated at constant pressure (i.e. 20 kbar) the stability of epidote is largely increased toward high temperature with increasing Fe3+. However, the maximum pyrope content predicted remains lower than 25%. Therefore, the results of the P–T pseudosection calculated using the bulk composition of the garnet-rich layer does not faith fully reproduces the observations.
Pseudosection investigation of a metamorphic origin
In order to investigate the possibility that the garnet-rich layers results from the partial melting of the eclogites during the isobaric heating (M1, Fig. I.2.7), a T–X (H2O) pseudosection using the bulk composition of the host rock but varying H2O content has been calculated at 22.5 kbar to investigate the effect of varying H2O content on the modelled assemblages. In this diagram, quartz, amphibole, muscovite and kyanite disappear toward increasing H2O content. The inferred peak assemblage for the host rock is observed in the field melt-cpx-g-mu-ky-q-ru from 770°C to higher than 900°C for a H2O content ranging ~1-10 mol% (light yellow field in Fig. I.2.7). The inferred assemblage for the garnet-rich layer is modelled in the field melt-cpx-g-ky-ru with higher water content (H2O ~5-10 mol; deep orange field in Fig. I.2.7) and temperature than 860°C.
Origin of the garnet rich layers
In this section, both the possible cumulate or residuum origin for the garnet-rich layers will be discussed. To do so, the discussion will focus successively on a comparison of major element composition of the garnet-rich layer with respect to its host rock, a comparison of garnet compositions from the host rock and the garnet-rich layer and the results of phase equilibrium modelling.
Garnet-rich layers in metabasic rocks were interpreted as the result of metamorphism of various types of magmatic cumulate layers. Each has specific chemical characteristics. Plagioclase accumulations are characterized by higher Al2O3 and CaO contents than their host rock (Al2O3layer/Al2O3host ~8; CaOlayer/CaOhost ~9) (Konzett et al., 2005). Spinel accumulations are characterized by a high CaO content (~20%) and a high CaO/Al2O3 ratio (~1.5) due to the extraction of Al-rich spinel (Yang et al., 2008). The studied garnet-rich layers are characterized by a depletion of the Al2O3 content compared with the host rock (Al2O3layer/Al2O3host ~0.65) and lower CaO content (CaOlayer/CaOhost = 0.8) inconsistent with a significant accumulation of plagioclase in the protolith of the studied garnet rich layer. The high Al2O3 content of the host rock, does not support a depletion of Al2O3 because of spinel accumulation in the garnet-rich layer. On the other hand, our data indicate that garnet-rich layers are characterized by lower SiO2, CaO, Na2O and K2O contents, but higher FeO, MnO, MgO, TiO2 contents. Mafic components are concentrated in garnet and Ti in rutile. A possible alternative protolith for the garnet-rich layer would be a former ilmenite cumulate, with significant proportion of pyrophanite and geikilite, that could account for the high TiO2, FeO, MnO, MgO content of the layer (this point is discussed in the light of the pseudosection hereafter). On the other hand, the granitoid composition of melt derived from metabasites partial melting is indicated by experiments with composition ranging from andesitic (e.g. Senn and Dunn, 1994) to rhyolitic (e.g. Schmidt et al., 2004). This variability may be attributed to the variable content of K-bearing minerals (mostly phengite at high-pressure) in natural protoliths. The occurrence of biotite-plagioclase±quartz±K-feldspar bearing polyinclusions or cuspate veinlets interpreted as former melt in partially molten mafic eclogite (e.g. Cao et al., 2019, Wang et al., 2014) also suggest a granitoid composition of the melt. The bulk composition of the garnet-rich layer could therefore be explained by a depletion of incompatible elements due to partial melting and melt extraction.
In addition to the bulk composition of the layer, garnet composition may also indicate a cumulate or a residuum origin for the garnet-rich layer. Garnet crystallized form a plagioclase-rich cumulate contains higher grossular contents than the host-rock (grshost3-14; grslayer10-42; Konzett et al., 2005). Garnet from initial spinel cumulates is richer in pyrope than that from the host-rock (Yang et al., 2008). On the other hand, garnet from garnet-rich layers in partially melted mafic granulite may share a similar composition with garnet of the host-rock (Palin et al., 2018). Our data indicate similar chemical composition of garnet – the core of garnet forming the layers, corresponds to the mantle of garnet in the host eclogite (Fig. I.2.4). The respective cores of garnets from the garnet ric-layer and the host-rock also display the same zoning and inclusion pattern. These observations strongly suggest that they crystallized from the same material and underwent a similar metamorphic evolution.
The P–T pseudosection calculated for the bulk composition of the garnet-rich layer predicts that epidote is stable at T lower than 620°C, below the temperature inferred for the crystallisation of the garnet core in the host rock (650°C; Fig. I.2.6). In addition, garnet in equilibrium with epidote is predicted to contain less than 25% pyrope, way below the pyrope content observed (prp35). Though garnet might have crystallized at lower P–T conditions because of a different composition, the discrepancy between the observed and modelled composition of garnet at equilibrium with epidote is significant. As a consequence, an unmodified bulk composition hypothesis for the garnet-rich layers, like an ilmenite-rich protolith, is not supported by P–T pseudosection modelling. The T–X(H2O) pseudosection (Fig. I.2.7) shows that the assemblage of both the host rock and the garnet rich layer can be modelled for various H2O-contents and similar T (870-875°C) at given P (22.5kbar). Garnet mode increases with temperature (Fig. I.2.9a) and the pressure varying mode box (Fig. I.2.9b) shows a maximum garnet mode between 22-23 kbar at 870°C consistent with the inferred P–T path (Fig. I.1.7). The maximum garnet mode modelled is around 50-60%. It reaches 70-75% in the observed assemblage, but this limited discrepancy could be explained by modelling uncertainties including uncertainties in the estimation of the effective bulk composition (e.g. the host rock displays variable kyanite content) and also imperfections in the thermodynamic data and activity–composition relations. Accordingly, partial melting and subsequent melt extraction reproduce the first order observations of the garnet-rich layer.
Consequences of partial melting on Rutile and Zircon precipitation
The garnet-rich layer contains a large amount of rutile and zircon. In the above calculations, the mode of rutile is not reproduced correctly (observed ~5-10%; modelled ~0.5%) suggesting an enrichment of Ti and also possibly of Zr during the genesis of the garnet-rich layer (i.e. during partial melting). The presence of rutile, zircon or baddeleyite in eclogitic veins and fluid inclusions in omphacite from eclogites indicates that Ti and Zr may be mobilized by an aqueous fluid at high pressure (e.g. Philippot and Selverstone, 1991; Rubatto and Hermann, 2003). Consequently, partial melting might have been triggered by the influx of an aqueous fluid enriched in Zr and Ti. H2O was incorporated in the melt, whereas Zr and Ti precipitated as zircon and rutile.
The solubility of Ti and Zr in melt vary as a function of the melt composition and tend to decrease the increasing peraluminous character of a melt (e.g. Watson, 1979; Dickinson and Hess, 1985; Gwinn and Hess, 1989). The modelled composition of the melt at 850°C and 22.5 kbar is peraluminous (A/NK = 1.45 and A/CNK = 1.27; Table I.2.2) suggesting only limited solubility of Zr and Ti. Using the example of Zr solubility in the modelled melt, the partitioning coefficient of Zr between zircon and the melt can be estimated using the zircon saturation equation of Boehenke et al. (2013). In this equation, Zr solubility in melt increases with increasing temperature and Na, Ca and K content of the melt, but decreases with increasing Al and Si content. Using the modelled composition of the melt and assuming a melting temperature of 850°C the calculated partitioning coefficient suggest a strong partioning of Zr (DZr ~ 1 x 106) in zircon rather than in the melt. Accordingly, a consequence of partial melting would be the precipitation of dissolved Zr and Ti into rutile and zircon.
Table of contents :
Introduction
Eclogite facies metamorphism in the Haut-Allier (Massif Central)
Variscan eclogite- to granulite-facies metamorphism in the Haut-Allier (French
Massif Central): geodynamic implications
1. Introduction
2. Geological setting
3. Petrography and mineral chemistry
4. Interpretation of the petrographic observations
5. Pseudosections
6. Discussion
7. Conclusion
Petrogenetic investigations of garnet-rich layers in eclogites: evidence of partial melting at high pressure?
1. Introduction
2. Petrography and mineral chemistry of the garnet-rich layer
3. Pseudosection
4. Discussion
5. Conclusions
Petrologically constrained U-Pb dating of the “La Borie” eclogites
1. Introduction
2. Analytical methods
3. Petrological constraints on zircon, rutile and apatite
4. U/Pb results
5. Discussion
Petrologic study of French Variscan metagranites
1. Mineralogical indicators of HP metamorphism in metagranite
2. Methods
3. A petrologic study of Variscan metagranites
4. Discussion and implications of the petrological study of the metagranites
Petrologic study of metapelites from the Variscan Pyrenees
Two-stage Variscan metamorphism in the Canigou massif: evidence for crustal thickening in the Pyrenees
1. Introduction
2. Geological setting
3. Structural data
4. Petrography and mineral chemistry
5. P–T estimations
6. Geochronology
7. Discussion
8. Conclusions
General discussion and conclusions
References
