Hans-Peter Schertl

35 Ma old ultrahigh-pressure metamorphism and evidence for very rapid exhumation in the Dora Maira Massif, Western Alps

Abstract 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.

The pyrope-coesite rocks and their country rocks at Parigi, Dora Maira Massif, Western Alps ; Detailed petrography, mineral chemistry and PT-path

Both the coarse- and fine-grained varieties of the partly coesite-bearing pyrope-quartzites, their interlayered jadeite-kyanite rocks, and the biotite-phengite gneiss country rock common to all of them were subjected to detailed petrographic and textural studies in order to determine the sequence of crystallisation of their mineral constituents, which were also studied analytically by microprobe. Prior to pyrope and coesite growth, the Mg-rich metapelites were talc-kyanite-chlorite-rutile-ellenbergerite schists which — upon continued prograde metamorphism — developed first pyrope megacrysts in silica-deficient local environments at the expense of chlorite + talc + kyanite, and subsequently the smaller pyrope crystals with coesite inclusions from reacting talc + kyanite. Based on geobarometrically useful mineral inclusions as well as on experimentally determined phase relations, a prograde PT-path — simplified for water activity = 1 — is constructed which passes through the approximate PT-conditions 16 kbar and 560° C, 29 kbar and 720° C, and finally up to 37 kbar at about 800° C, where the Mg-rich metapelite was a pyrope-coesite rock with phengite, kyanite, and talc still present. During the retrograde path, pyrope was altered metasomatically either into phlogopite + kyanite + quartz or, at a later stage, to chlorite + muscovite + quartz. Both assemblages yield PT-constraints, the latter about 7–9 kbar, 500–600° C. The country rock gneisses have also endured high-pressures of at least 15 kbar, but they provide mostly constraints on the lowest portion of the uplift conditions within the greenschist facies (about 5 kbar, 450° C). Microprobe data are presented for the following minerals: pyrope, ellenbergerite, dumortierite (unusually MgTi-rich), jadeite, vermiculite (formed after Na-phlogopite?), paragonite, and for several generations of phengite, chlorite, talc, phlogopite, dravite, and glaucophane in the high-pressure rocks, as well as for biotite, chlorite, phengites, epidote, garnet, albite, and K-feldspar in the country rock gneisses. An outstanding open problem identified in this study is the preservation of minerals as inclusions within kyanite and pyrope beyond their PT-stability limits.

Pb−Sr−Nd isotopic behavior of deeply subducted crustal rocks from the Dora Maira Massif, Western Alps, Italy-II: what is the age of the ultrahigh-pressure metamorphism?

Pb, Nd and Sr isotope data are reported from two localities on mineral separates from Mg-rich metapelites and associated rocks that have been subducted to depths of at least 100 km, for which metamorphic conditions are estimated at 28–33 kilobars pressure and 700°–800° C, and then returned to the surface. Initial isotope ratio data from the granitoid country rock are similar to those found in the metapelites. The initial ratios indicate predominantly recycled, aged granitic crustal materials for the sources of all of the samples. Five zircon samples, 4 from pyrope megacrysts and 1 from fine-grained pyrope quartzite lenses in the metapelites accurately define a chord yielding intercept ages of 304±10 and 38.0±1.4 Ma in a concordia diagram. Zircon from the country rock also plots along the chord. The zircon data, together with initial Nd and Sr data, indicate that the sedimentary sources of the rocks were derived mainly or entirely from sialic Hercynian rocks. Ellenbergerite from pyrope megacrysts and monazite from the fine-grained ground mass yield slightly younger ages of 30–34 Ma, apparently reflecting lower blocking temperatures than that of zircon. Sm−Nd data from a pyrope megacryst give an errorchron corresponding to an age of 38 Ma, in agreement with the zircon date. A major question concerns the timing of the ultrahigh-pressure metamorphism. Experimental data suggest that pyrope and quartz/coesite as well as ellenbergerite formed by various metamorphic reactions. If, as generally assumed, the ultrahigh-pressure metamorphism occurred ca. 100 Ma ago, our data require that the zircon did not experience measurable lead loss at that time, but lost major amounts of lead 38 Ma ago during late Alpine metamorphism. Estimates of diffusion rates for Nd in pyrope further suggest that the apparent Sm/Nd age of 38 Ma for the megacryst is not consistent with that model. Those problems are resolved if the ultrahigh-pressure metamorphism occurred 38–40 Ma ago, but problems remain from Ar/Ar dates of 100 Ma on phengite, an inferred 120 Ma age for zircon lead loss from another study, and possibly by the very rapid uplift required if the metamorphism is that young.

Pb-Sr-Nd isotopic behavior of deeply subducted crustal rocks from the Dora Maira Massif, Western Alps, Italy

Abstract Pb, Nd and Sr isotope data are reported on mineral separates and whole rock samples of Mg-rich metapelites and associated rocks that have been subducted to depths of ca . 100 km in the mantle within the last 100 Ma and returned to the surface, and for which metamorphic conditions were estimated to be 700–800°C at > 28 kilobars pressure. Three rock types, fine- and coarse-grained pyrope quartzites and NaFe-rich jadeite-kyanite quartz layers, were collected from a single outcrop in the coesite-bearing portion of the Dora Maira massif. Samples analyzed include pyrope, pseudomorphs after pyrope, phengite, zircon, the new mineral ellenbergerite, and whole rocks. Isotope data from all three decay systems indicate minimal, if any, isotopic exchange with mantle rocks. Isotope ratios from minerals with low parent/daughter ratios cluster around 143 Nd/ 144 Nd = 0.51225: ϵ (Nd) = −7.6; 87 Sr/ 87 /Sr = 0.7 = 15.75–15.80 with 206 Pb/ 204 Pb = 19.4–20.7. These compare closely with ratios from Precambrian granitic rocks or their derived sediments. From the Pb isotope data and the Sm/Nd whole rock model ages we estimate a mean age of 2000 ± 500 Ma for the sediment protoliths. Pyrope mineral separates record approximate ages for Alpine metamorphism of 34–38 Ma by the Sm/Nd method; the U/Pb method applied to zircon and ellenbergerite yields possible additional metamorphic ages of 50–55 Ma. These data illustrate a possible mechanism for creating isotopically enriched mantle. Had these rocks yielded their isotopes to the mantle, “enriched mantle” could have resulted, the magnitude of the enrichment depending upon the total volume attained by the mixing process. Similarly the metasediments could also provide the high potassium abundances and appropriate Pb, Sr and Nd isotope ratios that are postulated to accompany mantle metasomatic processes.

Factors in the preservation of coesite: The importance of fluid infiltration

Abstract The survival of coesite in ultrahigh-pressure (UHP) rocks is most commonly attributed to rapid exhumation, continuous cooling during uplift, and inclusion in strong phases that can sustain a high internal over-pressure during decompression. Exceptions to all of these criteria exist. Perhaps less attention has been paid to the role of fluid infiltration in the preservation of coesite. We used infrared spectroscopy to measure water contents of coesite and coesite pseudomorphs in a variety of UHP rocks. In all cases, OH concentrations in coesite are below the detection limit of ~100 ppm H2O. The silica phases surrounding coesite, however, show varying amounts of H2O. This is most spectacularly observed in pyrope quartzites from the Dora-Maira massif that contain at least three phases of silica replacing coesite, also distinguished by varying color of cathodoluminescence (CL): palisade-textured quartz (<100 ppm H2O, red-violet CL); .mosaic. quartz, which is actually chalcedony (up to 0.4 wt% H2O, yellow/brown CL); and a rare, highly hydrated silica phase interpreted to be opal (~7 wt% H2O, dark blue CL). Very similar signatures are observed in a grospydite xenolith from the Roberts Victor kimberlite. The quartz replacing coesite in other UHP samples studied contains on the order of 500 ppm H2O or less, and most measurements are under the detection limit of our technique. We infer that palisade quartz forms under dry or nearly dry conditions and at high temperatures during dilation of the host phase. The formation of hydrous silica phases such as chalcedony and opal, however, must take place at much lower temperatures, after cracking of the host phase, which allows external fluids to infiltrate. Delay of fluid infiltration to low temperatures, where kinetics are slow even in the presence of water, is the most critical factor in the preservation of coesite.

Garnet–omphacite–phengite thermobarometry of eclogites from the coesite-bearing unit of the southern Dora-Maira Massif, Western Alps

Abstract Using relevant geothermobarometric methods, P – T -data were collected for the reconstruction of the metamorphic evolution of 34 eclogite samples taken from small lenses and boudins within the ultrahigh-pressure (UHP) metamorphic coesite-bearing Brossasco-Isasca Unit (BIU) of the Dora-Maira Massif. The mineral phases used (clinopyroxene, garnet, phengite), or growth zones thereof, were identified as being coexistent for different stages of metamorphism on the basis of careful petrographic studies. Of several published geothermobarometers, the garnet–clinopyroxene thermometer of Powell [Powell, R., 1985. Regression diagnostics and robust regression in geothermometer/geobarometer calibration: the garnet–clinopyroxene geothermometer revisited. J. Metamorph. Geol., 3, pp. 231–243.] combined with the garnet–clinopyroxene–phengite barometer after Waters and Martin [Waters, D., Martin, H.N., 1993. The garnet–clinopyroxene–phengite barometer. Terra Abstr., 5, pp. 410–411.] was chosen here, because it provided the most reliable results. Nevertheless, the scatter of P – T -data points for the prograde (stage I), peak metamorphic (stage II), and retrograde (stage III) development of the eclogites is still considerable. Among the many possible reasons for this inconsistency discussed, a partial lack of equilibration of some of the eclogites during their metamorphic history should be taken into account. Despite the data scatter, an average P – T -path could be estimated, which includes the following coordinates: for stage I: 15 kbar/500°C; 25 kbar/570°C; 32 kbar/650°C; for stage II: 36 kbar/720°C; and for stage III: 24 kbar/680°C and 14 kbar/620°C. This is in fair agreement with P – T -paths derived earlier for other rock types of the BIU on the basis of other geothermobarometers.

New occurrences of jadeitite, jadeite quartzite and jadeite-lawsonite quartzite in the Dominican Republic, Hispaniola: petrological and geochronological overview

New occurrences of jadeitite and jadeite-rich rocks have been discovered in the Rio San Juan Complex (RSJC) of the northern Dominican Republic in serpentinite melanges associated with a former intra-oceanic subduction zone. Allochthonous blocks in lag deposits developed on the melange outcrops or boulders in river beds are common. A very unusual feature for the RSJC is the occurrence of concordant layers and discordant veins of cm to dm thickness in blocks of jadeite±lawsonite- or omphacite-garnet-bearing blueschist of the melange. Two suites of jadeite-rich rocks can be recognized. The first is represented by quartz-free jadeitite s.str . (>90 vol% jadeite) found so far only as blocks and boulders. The second suite comprises quartz-bearing jadeitite s.str . grading into jadeitite quartzite (JQ), jadeite-lawsonite quartzite (JLQ) and jadeite-free lawsonite quartzite (LQ). The second suite is found both as blocks and boulders as well as layers and veins in blueschist blocks. One single occurrence of a cross-cutting omphacitite vein in blueschist has also been observed. Additional important phases so far found in both suites are omphacite, phengite, glaucophane, epidote, albite, calcite, titanite and zircon. Apatite and pumpellyite have only been identified in quartz-free jadeitite s.str .; almandine-rich garnet has so far been observed only in JLQ. The two suites of jadeite-bearing rocks occur in various shades of green, are fine- to coarse-grained, and usually equigranular. Mineral distribution is commonly homogeneous, but may be patchy in JLQ, giving this rock type a distinctly mottled appearance. Cathodoluminescence (CL) images show oscillatory zoning patterns in jadeite, zircon, apatite and calcite; this is evidence for crystallization from an aqueous fluid under open-system conditions. Zircons separated from a sample of quartz-free jadeitite s.str . contain primary inclusions of high-pressure matrix minerals such as jadeite and omphacite, indicating coeval zircon growth. The cores of the zircons yield ages of 114.9 ± 2.9 Ma, thus defining a crystallization age close to the initiation of subduction in the Rio San Juan Complex, when “warm” geotherms of ≈15°/km prevailed. These ages are in contrast with the crystallization ages of the blueschists hosting the second, quartz-bearing suite of jadeite-rich rocks. These range from 80 to 62 Ma, towards the end of subduction-zone activity at ≈55 Ma and “cool” geotherms of 8–9 °C/km. For the younger quartz-bearing suite, the combination of phengite compositions with the available P-T-t paths of the host blueschists suggests crystallization temperatures of ≈ 350 to ≈ 500 °C at minimum pressures of 15–16 kbar. The P-T conditions for the older quartz-free suite are more difficult to constrain, but the combination of phengite compositions with the prevailing geotherms in the young and warm subduction zone suggest minimum conditions of at least 500 °C and 11 kbar. However, temperatures and pressures as high as 600 °C and 15 kbar, as documented for jadeitites of similar age in the same subduction zone exposed in neighbouring eastern Cuba, are possible. Jadeitites and jadeite-rich rocks of the RSJC are thus interpreted to have crystallized over a time-span of ≥ 60 Myr at initial temperatures of at least 500 °C, later evolving down to 350 °C in a single, thermally self-organizing, cooling subduction zone. The P-T conditions suggested for the younger quartz-bearing suite correlate well with those of jadeitite formation in Guatemala south of the Motagua Fault Zone, the only other occurrence world-wide where jadeitite with both lawsonite and quartz appears to be common. Further evidence is needed to corroborate that the older quartz-free suite represents another example of rare high-temperature jadeitite as documented in Cuba.

The UHP Unit in the Dora-Maira Massif, Western Alps

New results acquired in the coesite-bearing terrane of the Dora-Maira massif, Italian Western Alps, indicate that the UHP unit extends over less than 45 km2 and is bounded by two lower-grade continental units. The contact to the footwall unit is a late, lowangle, top-to-the-SW extensional one, which probably cuts across the original thrust contact. The protolith of the pyrope “whiteschist” of the UHP unit is shown to be the metasomatic transformation product of granitic rock. Nearend member magnesiochloritoid and magnesiostaurolite occur in pyrope megablasts. Ordering in mag-nesiostaurolite leads to cross-hatched twinning and superstructures with either a doubled a or doubled c parameter. The Si3.5 phengite-3T from the same whiteschist contains nano- to microscale quartz platelets and talc layers parallel to (001) of the host mica, interpreted as in situ decompression products of the mica. Some of the pyrope megablasts contain late, crack-related granulite-facies mineral assemblages, which reflect very lo...

UHP-metamorphic rocks from Dora Maira/Western Alps and Kokchetav/Kazakhstan New insights using cathodoluminescence petrography

Thin sections of ultrahigh pressure (UHP) metamorphic rocks from the Dora Maira Massif (Italy) and the Kokchetav Massif (Kazakhstan) were investigated using the hot cathode cathodoluminescence (CL) technique. Coloured images of important, but otherwise invisible growth features could be easily identified with this tool within seconds. These features are in excellent correlation with chemical variations of minerals revealed by electron microprobe (EMP). Generally, CL is induced by activator-elements (e.g. Mn and REE) and lattice defects whereas so-called quencher-elements like Fe may reduce or even extinct luminescence. Since X-ray-intensity mapping images (MAPS) of minerals can take up to 50 hours, the CL-method represents an ideal and rapid approach prior to chemical characterization. In addition to typical carbonates such as calcite, Mg-bearing calcite and dolomite, a number of rock forming and accessory minerals including Mg- and Mg-Ca-garnets, diopsidic and jadeitic pyroxenes, kyanite, K-feldspar, quartz, coesite, diamond, zircon, apatite, and bearthite were examined. Features observed in garnets include small-scale oscillatory zoning patterns, changes in morphology during growth as well as different crack generations which were partly annealed. SiO2 phases (coesite, quartz, chalcedony) as well as exsolution textures of dolomite and Mg-bearing calcite are easy to distinguish due to their different CL-colours. Pyroxene displays complex zonation patterns and -to some extent- exsolution-textures of K-feldspar. Kyanite reveals distinct growth zones; in combination with mineral inclusion studies it is possible to discriminate between different kyanite-forming reactions. The different crystallographical orientation of twinned kyanite crystals leads to various luminescence colours, thus, the suture of the twin plane is well defined. Prior to SHRIMP analyses, knowledge of the internal structures of zircon is indispensable. Even very tiny coesite crystals are easy to distinguish from quartz or chalcedony by their disparate luminescence colours. Accessory luminescent minerals like diamond, apatite, bearthite are easy to identify in thin section even if they occur in very small abundance within the matrix or as inclusions. The CL method presented here for UHP-metamorphic rocks is recommended as a pathfinder for the discovery of internal structures of minerals prior to their chemical characterization using EMP.

Reconnaissance isotopic investigations on rocks of an undeformed granite contact within the coesite-bearing unit of the Dora Maira Massif

Abstract Recent research on ultrahigh-pressure rocks in Val Gilba and the Brossasco area has revealed the complete textural preservation of large volumes of a premetamorphic granite, together with its former contact aureole, that was subducted to depths of > 100 km, as indicated by coesite pseudomorphs. Petrographic studies indicate that, although most minerals have been affected by metamorphism, biotite and potassium feldspars in the rock are largely preserved except for thin peripheral rimming of biotite by phengite and garnet. RbSr analyses on seven biotite samples fail to produce a valid isochron, indicating either incomplete resetting, or inheritance of radiogenic Sr, during metamorphism. Ar/Ar data from two biotites likewise yield very irregular patterns that are consistent with an episode of severe Ar loss from a pre-Alpine parent rock, followed later by addition of inherited radiogenic Ar. The biotite RbSr and KAr systems have therefore been extensively disturbed during Alpine metamorphism in spite of the largely unaltered appearance of the crystals in thin sections. Attempts to measure UPb ages with apatite also failed. Although the geochronological studies yielded no reliable ages, Pb and Sr ratios from four potassium feldspar samples are within the ranges found in known Hercynian rocks, providing new evidence that Hercynian rocks played a prominent role as source materials for the Dora Maira rocks. Sr ratios from two apatite samples agree closely with the potassium feldspar values. The ratios are also substantially lower than those found in the whiteschists in completely recrystallized rocks at Parigi, ca. 3 km north of the present rocks. These results indicate that the undeformed rocks in Val Gilba-Brossasco area are less disturbed by Alpine ultrahigh-pressure metamorphism, thereby preserving the record of the protoliths somewhat better than do the completely recrystallized rocks at Parigi to the north. The Pb and Sr isotope data further show that the minerals of the undeformed rocks failed to reach isotopic equilibrium despite subduction to > 100 km and heating to > 700°C.