Daniele Castelli

Jadeitite from the Monviso meta-ophiolite, western Alps: occurrence and genesis

A block is described, which is exposed in the antigorite serpentinite of Vallone Bule, belonging to the Basal Serpentinite Unit of the Monviso massif (Piemonte Zone of calcschists with meta-ophiolites). The block consists of a quartz-jadeite rock core and a jadeitite rim, very similar to the rocks used by prehistoric men to make stone axeheads. In spite of their different bulk-rock compositions, both core and rim show the same trace and rare earth elements patterns, suggesting the same protolith. The quartz-jadeite rock exhibits a major, trace and rare earth elements composition consistent with that of oceanic plagiogranite, most likely a dyke cutting across upper mantle peridotites, later hydrated to serpentinites. Conversely, the jadeitite, which consists mainly of zoned jadeite crystals progressively enriched in the diopside component from core to rim, is significantly depleted in Si but enriched in Mg and Ca with respect to the quartz-jadeite rock. The trace and rare earth elements similarities and the ubiquitous presence of small zircons suggest that the jadeitite and the quartz-jadeite rock both derive from a plagiogranite; however, jadeitite would have undergone a metasomatic process involving a significant desilication and Mg- and Ca-enrichment, connected to the host peridotite serpentinization. The process, responsible for the transformation of the plagiogranite into a jadeitite, should have occurred during prograde Alpine high-pressure (eclogite-facies) metamorphism, since the first Na-pyroxene formed is jadeite, corroded and partly replaced during the metasomatic process by a progressively more omphacitic pyroxene. Because similar rocks – mostly jadeitites, but even their plagiogranite protoliths – are reported from other localities from the Western and Maritime Alps, it is likely that the raw materials of most jadeitites used to make stone axeheads, which are spread all over the Western Europe, have a similar origin and derive from the western Alps as long suggested.

Thermal and baric evolution of garnet granulites from the Kharta region of S Tibet, E Himalaya

Granulite-facies garnet-bearing metapelites, metabasics and calc-silicate rocks from the lower metamorphic complex (Kharta Gneiss) of the Greater Himalayan Crystallines in the Kharta region of S Tibet, E Himalaya, preserve textural and chemical evidence for prograde equilibration at temperatures of at least 700–720°C and pressures around 8 kbar during the main event of the Himalayan metamorphism. Post-deformational reaction textures include clinopyroxene (± orthopyroxene) - plagioclase symplectites after garnet in calc-silicate rocks, and cordierite ± spinel coronas on sillimanite and garnet in metapelite granulites. These assemblages indicate a decompressional pressure-temperature path that is confirmed by the geothermobarometry of zoned and symplectite minerals as well as by calculated phase equilibria. Isothermal decompression through ca. 3 kbar occurred at temperatures of about 700°C, and was followed by further decompression to P ∼ 3 kbar, and T ∼ 710°C. At this point, decompression was replaced by quasi-isobaric cooling ending in the andalusite stability field at P ca. 2.5 kbar. The P-T path of the Kharta Gneiss appears to be similar to those inferred for the lower Greater Himalayan Crystallines exposed in the nearby Dudh Kosi and middle Arun valleys of eastern Nepal. This type of clockwise P-T path, with most of the exhumation occurring at relatively constant metamorphic temperatures, requires a high exhumation rate and suggests that extrusion tectonics of crustal-scale wedges may have been operative during post-collisional exhumation of the Greater Himalayan Crystallines.

The Gophu La and Western Lunana granites: Miocene muscovite leucogranites of the Bhutan Himalaya

Abstract Petrographical, mineralogical and geochemical data are given for the Gophu La leucogranite, the second largest leucogranite body of the Bhutan Himalaya and for the smaller Western Lunana leucogranite. Both leucogranites were emplaced in the amphibolite-facies gneisses of the High Himalaya basement nappes during the Miocene. The Gophu La leucogranite is about 300 km 2 in outcrop surface and is homogeneous, showing only moderate variations in chemical and modal composition. The Western Lunana leucogranite has a far smaller outcrop surface (about 40 km 2 ) and is less homogeneous than the Gophu La leucogranite. Both are fine- to medium-grained, hololeucocratic granites with white mica, biotite and minor tourmaline. Significant accessories are andalusite and sillimanite. The muscovites are phengitic, being notably high in Fe and Mg, and the biotites are Fe-rich. The average chemical compositions are similar to that of other Himalayan leucogranites and are typical for minimum melts. Both granites preserve geochemical features recording the heterogeneity of the parent material and were probably produced during the uplift of tectonically thickened continental crust.

Exhumation History of the UHPM Brossasco-Isasca Unit, Dora-Maira Massif, as Inferred from a Phengite-Amphibole Eclogite

A well-preserved phengite-amphibole eclogite (Br2F) from the UHP Brossasco-Isasca Unit (BIU) of the Dora-Maira Massif was studied in detail. The eclogite consists of the peak assemblage omphacite, garnet, phengite, rutile, and quartz. A porphyroblastic blue-green amphibole statically overgrows the eclogitic foliation defined by the preferred orientation of phengite flakes, and by the alignments of abundant accessory rutile grains. Both omphacite and phengite are partially replaced by fine-grained symplectites, consisting of clinopyroxene + albite and biotite + oligoclase, respectively. The metamorphic evolution of eclogite Br2F was reconstructed combining microstructural observations, conventional thermobarometry, and pseudosection analysis. A first pseudosection was calculated in the NKCFMASH system in the pressure range 5-45 kbar to model the peak and early retrogressive conditions, whereas a second pseudosection, calculated in the NCFMASH system, was used to model the albite + clinopyroxene symplectite after omphacite. Peak metamorphic conditions of P = 37.7 kbar and T = 732°C were estimated. The decompressional P-T path is associated with significant cooling from about 730°C at 38 kbar to 630°C at 14 kbar. These data, obtained combining pseudosection analysis with conventional thermobarometric methods, are in agreement with the P-T paths estimated from other lithologies by Hermann (2003), Castelli et al. (2004), and Groppo et al. (2006), and confirm that the BIU equilibrated within the diamond stability field.

Lichens on asbestos–cement roofs: Bioweathering and biocovering effects

Asbestos-cement roofs, the most widespread sources of airborne, toxic and carcinogenic asbestos fibres, are often colonized by lichens. Since these latter are physical and chemical weathering agents, they have been often considered as significant responsible of disaggregation processes increasing fibre dispersion. Consequently, official guidelines for the management of asbestos often suggest their removal. Weathering and/or covering effects of lichens on asbestos-cement, however, have never been deeply investigated and available procedures to evaluate asbestos-cement aging do not take the biological colonization into account. In this study we show that a 25% lichen cover modifies physical and chemical properties of asbestos-cement sheets containing chrysotile and crocidolite fibres. By innovatively coupling pull up tests and image analysis of linear structures, we show that fibre loss is significantly lower ( approximately 30%) where lichens develop and offer a physical barrier to the fibre detachment. Below the most covering lichens (Acarospora cervina, Candelariella ssp.), chrysotile and crocidolite undergo a partial incongruent dissolution, which in laboratory assays generally determined a reduction of their surface reactivity. Because of their biocovering and bioweathering effects, lichens on asbestos-cement play a role which differs from the current public opinion and the assumptions of some official regulations, acting as effective spontaneous bioattenuation agents.

SEM/TEM-AEM characterization of micro- and nano-scale zonation in phengite from a UHP Dora-Maira marble petrologic significance of armoured Si-rich domains

Chemically zoned phengite flakes from an impure marble of the ultra-high pressure Brossasco-Isasca Unit (Dora-Maira massif, Western Alps - Italy) were investigated by scanning, transmission and analytical electron microscopies (SEM-TEM-AEM), revealing a complex nano-structure within the crystal cores. Diffraction patterns show that this phengite is a highly ordered 3 T polytype. A close inspection of the diffracted spots reveals that, for some classes of crystallographic indexes, weak satellite spots surround a central stronger spot. In bright-field TEM images, 60–120 A wide areas of mottled contrast cross, at an angle of about 40°, the host phengite \[Phe (a)] that represents the matrix. The satellite spots, the spatially ordered mottled contrast and the AEM analyses are all indicative of the presence of high-Si phengite relics [Phe (b)\] (Si = 3.61 apfu) within the ordered matrix (Si = 3.38 apfu). The higher-Si phengite has the c cell parameter [29.68 A; Phe (b)] different from that of the lower-Si matrix [29.75 A; Phe (a)]. A zoned 3 T phengite mantle (3.41-3.31 Si apfu) surrounds the phengite cores showing decreasing c values from the inner towards the outer part ( i.e. , 29.78-29.85 A). A later 2 M1 polytype with Si = 3.22 apfu has been locally detected at the edge of the phengite crystals. Both the crystallographic and crystal-chemical data of the higher-Si phengite relics, as well as petrologic information have been used to reconstruct prograde, peak and decompressional growth stages of the phengite flakes. Differences in both the octahedral and tetrahedral contents between the two types of phengite nano-domains suggest that the formation of the high-Si Phe (b) is due to an incomplete ionic reaction affecting the prograde Phe (a) at the metamorphic peak. The proposed ionic reaction is: 1 Phe (a) + 0.20 Mg2+ + 0.23 Si4+ → 1 Phe (b) + 0.44 Al3+.

Metamorphic CO2 production from calc-silicate rocks via garnet-forming reactions in the CFAS–H2O–CO2 system

The type and kinetics of metamorphic CO2-producing processes in metacarbonate rocks is of importance to understand the nature and magnitude of orogenic CO2 cycle. This paper focuses on CO2 production by garnet-forming reactions occurring in calc-silicate rocks. Phase equilibria in the CaO–FeO–Al2O3–SiO2–CO2–H2O (CFAS–CO2–H2O) system are investigated using P–T phase diagrams at fixed fluid composition, isobaric T–X(CO2) phase diagram sections and phase diagram projections in which fluid composition is unconstrained. The relevance of the CFAS–CO2–H2O garnet-bearing equilibria during metamorphic evolution of calc-silicate rocks is discussed in the light of the observed microstructures and measured mineral compositions in two representative samples of calc-silicate rocks from eastern Nepal Himalaya. The results of this study demonstrate that calc-silicate rocks may act as a significant CO2 source during prograde heating and/or early decompression. However, if the system remains closed, fluid–rock interactions may induce hydration of the calc-silicate assemblages and the in situ precipitation of graphite. The interplay between these two contrasting processes (production of CO2-rich fluids vs. carbon sequestration through graphite precipitation) must be considered when dealing with a global estimate of the role exerted by decarbonation processes on the orogenic CO2 cycle.

The Oligocene Biella pluton (western Alps, Italy): new insights on the magmatic vs. hydrothermal activity in the Valsessera roof zone

— The composite Biella pluton is part of an Oligocene volcano-plutonic complex whose origin is connected to the Alpine subduction-collision processes and that emplaced at shallow crustal levels within the eclogite-facies rocks of the Austroalpine Sesia-Lanzo Zone. In the roof zone of the pluton, small satellite igneous bodies are set within the SesiaLanzo country rocks, close to the main Biella pluton, and range in composition from quartz alkali feldspar syenite, quartz monzodiorite and monzogabbro, to quartz diorite and gabbronorite. Their geochemical features, including the REE patterns, are coherent with the calc-alkaline to shoshonitic affinity recognized in the whole volcano-plutonic complex. Field and petrographic data suggest that these bodies represent earlier crystallization/differentiation products of the Biella primary magma(s), which underwent contact metamorphic recrystallization during the multistage emplacement of the main pluton. Tourmalinebearing hydrothermal breccias and different types of hydrothermal veins (including quartz-plagioclase-, quartz-tourmalineand ankerite-quartz-sulphidesbearing veins) occur within both the intrusive rocks (satellite bodies + the main pluton) and their SesiaLanzo Zone country rocks. Both field relationships and vein assemblages suggest a close connection between the late-magmatic evolution of the Biella pluton and the multistage, boron-rich hydrothermal activity. riassunto. — Il plutone di Biella fa parte di un complesso vulcano-plutonico, legato all’evoluzione tardo-collisionale delle Alpi Occidentali, che si e messo in posto a livelli crostali relativamente superficiali all’interno delle rocce in facies eclogitica della Zona Sesia-Lanzo, appartenente al Dominio Austroalpino. Nelle rocce originariamente a tetto del plutone, sono presenti piccole intrusioni satelliti che presentano un ampio spettro composizionale, variabile da quarzo sieniti alcalifeldspatiche, quarzo monzodioriti e monzogabbri a quarzo dioriti e gabbronoriti. I loro caratteri chimici, inclusi i pattern delle Terre Rare, sono consistenti con l’affinita da calcalcalina a shoshonitica del complesso vulcanoplutonico. Le osservazioni di terreno e petrografiche suggeriscono che tali corpi rappresentano dei prodotti di cristallizazione/differenziazione precoci a partire dai magmi primari del corpo intrusivo principale e che hanno subito una ricristallizzazione metamorfica di contatto connessa con la messa in posto dell’intrusione di Biella. All’interno delle rocce intrusive (sia dei corpi satelliti sia del plutone principale) e della Zona Sesia-Lanzo incassante sono presenti brecce idrotermali ricche di tormalina e diversi tipi di sistemi di vene idrotermali (tra cui * Corresponding author, E-mail: piergiorgio.rossetti@unito.it 224 P. rossetti, a. agangi, d. Castelli, M. Padoan and r. ruFFini vene a quarzo e plagioclasio, a quarzo e tormalina, ad ankerite, quarzo e solfuri). Le relazioni di terreno e le paragenesi di vena e di alterazione suggeriscono uno stretto legame tra l’evoluzione tardo-magmatica del plutone di Biella e almeno gli stadi precoci dell’attivita idrotermale multifasica, caratterizzati dalla circolazione di fluidi con elevato contenuto di

Fragments of the Western Alpine Chain as Historic Ornamental Stones in Turin (Italy): Enhancement of Urban Geological Heritage through Geotourism

In Piemonte, stone has always been the most widely used raw material for buildings, characterizing the architectural identity of the city of Turin. All kinds of rocks, metamorphic, igneous, and sedimentary, are represented, including gneisses, marbles, granitoids, and, less commonly, limestones. The great variety of ornamental stones is clearly due to the highly composite geological nature of the Piemonte region related to the presence of the orogenic Alpine chain and the sedimentary Tertiary Piemonte Basin. This paper provides a representative list of the most historic ornamental stones of Piemonte, which have been used over the centuries in buildings and architecture. The main stones occurring in Turin have been identified and described from a petrographic and mineralogical point of view in order to find out the corresponding geological units and quarry sites, from which they were exploited. This allows the associated cultural and scientific interest of stones to be emphasized in the architecture of a town which lies between a mountain chain and a hilly region.

Relict granulites in the Ross orogen of northern Victoria Land (Antarctica), I. Field occurrence, petrography and metamorphic evolution

Abstract Granulite-facies rocks occur as mappable relics in the Wilson Terrane (northern Victoria Land), the lithotectonic unit of the Cambro—Ordovician Ross orogen closest to the East Antarctic craton. Despite the widespread amphibolite-facies overprinting of Ross age, large-scale low-strain tectonic lozenges preserve a layered unit of felsic and quartz-poor garnet + orthopyroxene ± cordierite metasedimentary granulite, with minor, metre-thick, layers of mafic two-pyroxene granulite and rare lenses of marble. Large bodies of massive enderbite are also present and locally show discordant, intrusive contacts with respect to the layered metasedimentary sequence. Mineral assemblages, reaction textures and geothermobarometric estimates in granulite rocks point to a pre-Ross decompressional evolution from higher-pressure (P = 7.7 ± 0.7 kbar at T = 820 ± 100°C) to lower-pressure (6.3 ± 0.4 kbar and 830 ± 50°C) granulite-facies conditions. Geological and petrological data suggest that the granulite-facies rocks of the Wilson Terrane form a distinct tectonometamorphic unit very similar to other Neoproterozoic granulite-facies terrains of the East Antarctic craton. In this aspect, the occurrence of granulite-rocks within the Transantarctic Mountains strongly suggests the reactivation of the palaeo-Pacific margin of East Antarctica during the Ross orogeny.