The preservation of mineral assemblages that were¯uid-present during their prograde history is primarily related to the consumption of the¯uid by growth of more hydrous minerals as the retrograde history begins. The range of behaviour relating to the preservation of mineral assemblages is examined using calculated phase diagrams for¯uid-saturated conditions, contoured for the H 2 O content of the mineral assemblage. At equilibrium, as a mineral assemblage crosses contours of decreasing H 2 O content along a pressure±temperature path, it dehydrates, the¯uid being lost from the rock. If the assemblage crosses contours of increasing H 2 O content, the mineral assemblage starts to rehydrate using any¯uid on its grain boundaries. When the rock has consumed its¯uid, the resulting mineral assemblage is that preserved in the rock. Conditions relating to the preservation of mineral assemblages are discussed, and examples of the consequences of different pressure±temperature paths on preservation in a metapelitic and a metabasic rock composition are considered on phase diagrams calculated with THERMOCALC.

Petrological investigations supported by multi-scale structural analysis of eclogitized serpentinite in the Zermatt-Saas Zone of the Western Alps allows for the determination of mineral assemblages related to successive fabrics, upon which the P-T-d-t path of these hydrated mantle rocks can be inferred. Serpentinites of the upper Valtournanche, with lenses and dykes of metagabbro and meta-rodingite, display an Alpine polyphase metamorphic evolution from eclogite to epidote-amphibolite facies conditions associated with three successive foliations having different parageneses in these rocks. Serpentinite mainly consists of serpentine with minor magnetite; however, where S1 and S2 foliations are pervasive, metamorphic olivine, together with Ti-clinohumite and clinopyroxene, are also found. The mineral assemblage associated with D1 includes serpentine1, clinopyroxene1, opaque minerals, titanite ± olivine1, Ti-clinohumite1 and ilmenite; the D2 assemblage is the same (±chlorite) but minerals have different compositions. The assemblage associated with D3 comprises serpentine3, opaque minerals, ±chlorite3, ilmenite and amphibole3. Ti-clinohumite is associated with veins that are older than D2 and pre-date D3. Veins that post-date D3 are characterized by amphibole + chlorite or by serpentine. P-T conditions for S2 parageneses evaluated using two pseudosections for different bulk compositions suggest that these rocks experienced pressures >2.5 ± 0.3 GPa at temperatures slightly higher than 600°C. The late epidote-amphibolite facies re-equilibration associated with D3 and D4 developed during late syn-exhumation deformation related to folding and testifies to a small temperature decrease. These results, which were integrated in the regional framework, suggest that different portions of the Zermatt-Saas Zone registered different P-T peak conditions and underwent different exhumation paths. In addition, the inferred P-T-d-t path suggests that the Valtournanche serpentinites re-equilibrated close to the UHP conditions registered by the Cignana meta-cherts. These results imply that tectonic slices exhumed after UHP metamorphism might be wider than previously reported or that small-size UHP units, tectonically sampled during the Alpine convergence, are more abundant than those that have been detected to date.

Lower temperature eclogite (with T ¼ 600°C) represents a significant part of the occurrences of eclogite in orogenic belts. ÔTrueÕ eclogite, with, for example, garnet + omphacite >70%, is well represented in such an occurrence. Calculated phase equilibria in Na 2 O-CaO-K 2 O-FeO-MgO-Al 2 O 3 -SiO 2 -H 2 O-TiO 2 -O (NCKFMASHTO), for just one rock composition -that of a representative midocean ridge basalt, MORB MORB -are used to see under what circumstances ÔtrueÕ eclogite is predicted to occur. The variables considered are not only pressure (P) and temperature (T) but also water content and oxidation state. The latter two variables are known to exert a significant control on mineral assemblage but are difficult to establish retrospectively from the observed rocks themselves. It is found that whereas oxidation state does have a strong effect on mineral assemblage, the key control on developing ÔtrueÕ eclogite is shown to be temperature and water content. If temperature is established to be <600°C, water content has to be low (less or much less than that for H 2 O saturation) in order for ÔtrueÕ eclogite to form. Moreover, unless pressure is at the high end in the range considered, lawsonite eclogite and ÔtrueÕ eclogite will tend to be mutually exclusive, with the former requiring high water content at the lower temperature where it occurs, but the latter requiring low water content.

Cr‐rich magnesiochloritoid in the eclogitized ophiolites of the Monviso massif occurs in the least differentiated rocks of the gabbroic sequence (troctolites to melatroctolites). Chloritoid (XMg=0.63–0.85; Cr≤0.55, atoms) co‐exists with omphacite, talc and garnet. Minor, syn‐eclogitic minerals are chromite, rutile and sometimes magnesite and Cr–Ti oxides. Coronitic textures, indicative of a static recrystallization, characterize the analysed samples. Layers of variable mineral composition develop among igneous plagioclase, olivine, clinopyroxene and spinel. The minerals in the coronitic layers display sharp compositional zonings. The igneous minerals are commonly not preserved; their presence in the original assemblage is inferred from the mineralogical composition of the pseudomorphs. Syn‐eclogitic volatile components are indicated by the development of OH‐bearing minerals (e.g. chloritoid & talc) and carbonates (e.g. magnesite), and supported by the presence of coarse‐grained and fibrous mineral growths. The complex pseudomorphic replacements of igneous minerals suggest that these rocks changed their mineralogical composition prior to the eclogite facies recrystallization, most likely during ocean‐floor metamorphism. It is suggested that syn‐eclogitic fluids formed by breakdown reactions of pre‐eclogitic volatile‐bearing minerals. Geothermobarometry indicates that the investigated rocks recrystallized at a depth corresponding to 2.4 GPa and temperatures of 620±50 °C. The attainment of high‐pressure conditions is supported by the presence of magnesiochloritoid, magnesite and garnet with high pyrope content (up to 58 mol%). P–T  estimates point to a very low thermal gradient (about 9 °C km−1), comparable to that deduced in the adjacent Dora‐Maira ultra‐high pressure unit.

Records of Variscan structural and metamorphic imprints in the Alps indicate that before Pangaea fragmentation, the continental lithosphere was thermally and mechanically perturbed during Variscan subduction and collision. A diffuse igneous activity associated with high-temperature (HT) metamorphism, accounting for a Permian–Triassic high thermal regime, is peculiar to the Alpine area and has been interpreted as induced either by late-orogenic collapse or by lithospheric extension and thinning leading to continental rifting. Intra-continental basins hosting Permian volcanic products have been interpreted as developed either in a late-collisional strike-slip or in a continental rifting setting. Two-dimensional finite element models have been used to shed light on the transition between the late Variscan orogenic evolution and lithospheric thinning that, since Permian–Triassic time, announced the opening of Tethys. Comparison of model predictions with a broad set of natural metamorphic, structural, sedimentary and igneous data suggests that the late collisional gravitational evolution does not provide a thermo-mechanical outline able to justify mantle partial melting, evidenced by emplacement of huge gabbro bodies and regional-scale high-temperature metamorphism during Permian–Triassic time. An active extension is required to obtain model predictions comparable with natural data inferred from the volumes of the Alpine basement that were poorly reactivated during Mesozoic–Tertiary convergence.