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Whole rock lithium (Li) and Li isotopic compositions of coesite-bearing whiteschists and their country rock metagranites from the Dora-Maira Massif in the Western Alps were investigated to understand their behavior during fluid-rock interaction in mantle depths of continental subduction zones. The whiteschists reveal relatively low δ7Li values (−6.4‰ to +2.4‰, with an average value of −2.1‰) in comparison to the country rock metagranites (δ7Li = −1.2‰ to +3.6‰, with an average value of +1.4‰). The δ7Li values of whiteschists do not show any correlations with major elements (e.g. Mg, Fe), Li, or isotopes (i.e., δ26Mg and δ18O values) and cannot be explained by kinetic fractionation. Hence, the low δ7Li values of whiteschists result from interactions with metasomatic fluids containing isotopically light Li that occurred in deep portions of a continental subduction zone. Fluid-rock interaction modeling suggests that the metasomatic fluids contain low δ7Li values (down to −4‰) and moderate Li concentrations (50 to 150 ppm), which were probably derived from the dehydration of serpentinites in the subduction channel. The presence of such low δ7Li fluids in deep subduction zones could cause Li isotopic heterogeneity in the earth's interior.

The peak metamorphic conditions of subducted continental crust in the Dora-Maira massif (Western Alps) have been revised by combining experimental results in the KCMASH system with petrologic information from whiteschists. Textural observations in whiteschists suggest that the peak metamorphic assemblage garnet+phengite+kyanite+coesite±talc originates from the reaction kyanite+talc↔garnet+coesite+liquid. In the experimentally determined petrogenetic grid, this reaction occurs above 45 kbar at ∼730 °C. At lower pressures, talc reacts either to orthopyroxene and coesite or, together with phengite, to biotite, coesite and kyanite. The liberated liquid contains probably similar amounts of H2O and dissolved granitic components. The composition of the liquid in the whiteschists at peak metamorphic conditions, a major unknown in earlier studies, was probably very similar to the liquid composition produced in the experiments. Therefore, the experimentally determined petrogenetic grid represents a good model for the estimation of the peak metamorphic conditions in whiteschists. Experimentally determined Si-isopleths for phengite further constrain peak pressures to ∼43 kbar for the measured Si=3.60 of phengite in the natural whiteschists. All these data provide evidence that the whiteschists reached diamond-facies conditions. The fluid-absent equilibrium 4 kyanite+3 celadonite=4 coesite+3 muscovite+pyrope has been calibrated on the basis of garnet and phengite compositions in the experiments and serves as a geothermobarometer for ultra-high-pressure (UHP) metapelites. For graphite-bearing metapelites and kyanite–phengite eclogites, forming the country rocks of the whiteschists, peak metamorphic pressures of about 44±3 kbar were calculated from this barometer for temperatures of ∼750 °C estimated from garnet–phengite thermometry. Therefore, the whole ultra-high-pressure unit of the Dora-Maira massif most likely experienced peak metamorphic conditions in the diamond stability field. While graphite is common in the metapelites, diamond has not been found so far. The absence of metamorphic microdiamonds might be explained by the low temperature of metamorphism, the absence of a free fluid phase in the metapelites and a short residence time in diamond-facies conditions resulting in kinetic problems in the conversion of graphite to diamond.

Rare centimeter-sized superzoned garnets (SZGs) were discovered in two coesite-bearing whiteschists of the Brossasco-Isasca Unit (BIU), southern Dora-Maira massif (DMM), Western Alps. The superzoned garnet consists of a reddish-brown almandine core crowded with inclusions of staurolite, chloritoid, kyanite, chlorite and paragonite, and of a pinkish pyrope rim with sporadic inclusions of kyanite, and magnesian chlorite. The core–rim contact is relatively sharp and marks the termination of the inclusion-rich portion. The core composition of the superzoned garnet is almost identical to, or slightly richer in Mg, than that of the rim of porphyroblastic garnet in metapelites from the same unit. In the rim of the superzoned garnet, Mg–Fe ratio increases abruptly towards the outermost rim, whose composition is identical to that of the common pyrope in the whiteschist. At the core–rim boundary, there is no chemical gap. Chloritoid and staurolite are common inclusions in the core of the superzoned garnet in the whiteschist and in the porphyroblastic garnet in the metapelite. The staurolite composition (Si=2.00 and total R 2+ Grt>Cld>Chl. The resulting petrogenetic grid suggests that the core of the superzoned garnet contains incompatible assemblages, such as St–Cld–Chl vs. Cld–Chl–Ky. New and literature data and results of experiments in the KFASH system suggest that: (1) the superzoned garnet was formed under a single prograde high-pressure/ultra high-pressure (HP/UHP) Alpine metamorphism, (2) the almandine inclusion-rich core of the superzoned garnet crystallized at disequilibrium in a pelitic composition system at around 600°C and less than 16 kbar, probably from a former metapelite xenolith included in a Variscan granitoid, and (3) the chemical environment of the host rock suddenly changed from the normal pelite to the whiteschist composition by a metasomatic process during the rim growth, i.e., at a stage close to the UHP climax.

Coarse-grained whiteschist, containing the assemblage: garnet+kyanite+phengite+talc+quartz/coesite, is an abundant constituent of the ultrahigh-pressure metamorphic (UHPM) belt in the Kulet region of the Kokchetav massif of Kazakhstan. Garnet displays prograde compositional zonation, with decreasing spessartine and increasing pyrope components, from core to rim. Cores were recrystallized at T=380°C (inner) to 580°C (outer) at P

Ion microprobe (SHRIMP) data on zircons from various rock types of an ultrahigh-pressure (UHP) metamorphic whiteschist-type pyrope quartzite lens of the Dora Maira Massif (DMM) consistently show domains giving a Late Eocene age of 35.4 ± 1.0 Ma which is taken as the age of UHP metamorphism. These domains partially replace the older oscillatory zoning pattern of the zircons formed during primary magmatic crystallization at about 275 Ma. Zircons of a metagranitic country rock next to the UHP metamorphic lens have these primary features best preserved. All zircons measured also yield intermediate ‘ages’ between 275 and 35 Ma with a statistical concentration between 260 and 210 Ma. Thus the uniformity of the initial zircon population both in the lens and the country rock evidences a common protolith, that is a granite intruded during the Late Herynian. While the intermediate ages are at least partly due to incomplete resetting of the zircons during UHP metamorphism, those in the 260–210 Ma range may be related to rifting episodes in the Permotriassic. The Mg-rich chemistry of the whiteschist lenses is due to local metasomatic alterations of the granite, perhaps by fluids derived from evaporitic sediments dating as early as the Permotriassic as well. The more pervasive resetting of zircon ages during UHP metamorphism in the pyrope quartzite lenses is explained by the ubiquity of fluids and/or melts produced during subduction by a series of dehydration reactions that occurred only in the more hydrous Mg-rich protoliths and not in their drier granitic neighborhood. Fission track ages determined partly on the same zircon samples yielding 29.9 ± 1.4 Ma mark the time when the UHPM unit had reached about 290°C at a shallow location within the crust. Thus exhumation over a vertical distance of about 120 km must have occurred within about 5–6 Ma indicating an average uplift rate of about 20–24 km/Ma and an average cooling rate of about 85–100°C/Ma. The radiometric data obtained do not lend any support to an Eo-Alpine Cretaceous subduction event so that deep subduction and immediately following exhumation all took place during Early to Mid Tertiary time. This scenario seems to apply to large portions of the Western and Central Alps as well calling for drastic geodynamic reinterpretations of these parts of the Alps.

This paper describes the second known occurrence of whiteschist from Norway. Whiteschists and orthoamphibole-cordierite rocks, which can both be described in the M (F) ASH system, are found in the Modum Complex together with albitites in contact zones between metagabbros and supracrustals. They are formed due to extensive hydrothermal activity. Bulk rock and mineral chemistry define a bimodal FeMg distribution; talc-kyanite schists and Fe-poor orthoamphibole-cordierite rocks form a leucocratic group and Fe-rich orthoamphibole-cordierite rocks a melanocratic group. The break-down of an early orthoamphibole-cordierite paragenesis to a later talc-kyanite paragenesis indicates a cooling path at isobaric conditions or under increasing pressure. The temperature-pressure regime is of the order of 600–750°C and 6–8 kb. Reaction textures with talc+albite ±kyanite forming pseudomorphs after gedrite indicate excess albite. These nonisochemical reactions indicate the presence of an active Na-bearing fluid at deep levels in the crust.