Roberto Compagnoni

Dating of eclogite-facies zircons : The age of Alpine metamorphism in the Sesia-Lanzo Zone (Western Alps)

Abstract Zircons from eclogite-facies rocks of the Sesia–Lanzo Zone have been investigated by cathodoluminescence and dated by ion microprobe (SHRIMP). Most of the zircons show a oscillatory or sector zoned core, which is surrounded by a weakly zoned or unzoned rim, irregular in shape. The U–Th–Pb analyses revealed that, opposite to the cores, the zircon rims have low U- and Th-contents, very low Th/U ratios and that their U–Pb systems were reset or they lost lead at the time of metamorphism. This study provides evidence that the zircon rims recrystallized and changed their internal zoning as well as their chemical and isotopic composition during high-pressure (15–18 kbar) moderate temperature (550–600°C) metamorphism. In the Sesia–Lanzo Zone, eclogite-facies zircons from three different localities and four different rock types were analysed. The age of metamorphic zircon rims was obtained from an eclogite from Monte Mucrone (65±5 Ma), an eclogitic micaschist from the Aosta Valley (65±3 Ma) and two mafic rocks from Cima di Bonze (68±7 Ma). The ∼65 Ma age is further supported by the ages of the youngest zircon rims in three other samples. These results constrain the age of eclogite-facies metamorphism in the Sesia–Lanzo Zone. An age of 76±1 Ma determined on zircons from a hydrothermal vein is interpreted as dating the prograde formation of the vein at temperatures well below 550°C. The age results suggest a maximum rate of 0.7 cm/y for the subduction of the Sesia–Lanzo Zone to eclogite-facies conditions. The Cretaceous–Tertiary age for the eclogite-facies metamorphism in the Sesia–Lanzo Zone implies that this continental unit was not subducted together with either portions of the Adriatic margin or the neighbouring Mesozoic Tethys. We suggest that the Sesia–Lanzo Zone behaved as a separate continental block at the time of Alpine convergence.

New finding of micro-diamonds in eclogites from Dabie-Sulu region in central-eastern China

Micro-diamonds were only found ten years ago in eclogite associated with marble at Xindian in the Dabie Mountains. This paper reports our new finding of micro-diamonds not only in eclogites at Maobei in the Sulu region and at Xindian and Laoyoufang in the south part of the Dabie Mountains (South Dabie), but also in eclogites at Baizhangya and Huangweihe in the northern part of the Dabie Mountains (North Dabie) that has usually been considered not to experience ultrahigh pressure metamorphism. Except the micro-diamond at Huangweihe that was found from the artificial heavy sands of zircons used for isotopic dating, the micro-diamonds from other localities were identified in thin sections of the eclogites. Besides a few interstitial grains, most of the micro-diamond grains in thin sections occur as inclusion in garnet. Three crystals of microdiamond at Maobei in the Sulu region are sized in 120, 60 and 30 μm, respectively. Crystal forms look like octahedron and the composite of octahedron and hexahedron. The largest micro-diamond crystal comes from Xindian, which is measured to be 180 μm in diameter with distinct zonal structure and inclusions. The zonal structure occurs as an inclined octahedron inside rounded by an incomplete hexagonal girdle. A smaller micro-diamond inclusion occurs inside the central octahedron, and a larger graphite inclusion is within the outer zone. The Laoyoufang micro-diamond is partially retrograded to graphite. Micro-diamond from the Baizhangya eclogite in the ultramafic rock belt of North Dabie is an aggregate of 70 μm×90 μm in size. All the micro-diamonds are confirmed by the Raman spectrum analysis. The occurrence of the micro-diamonds from the eclogites in the ultramafic rock belt of North Dabie demonstrates that this region was also subjected to ultrahigh pressure metamorphism as well as the South Dabie did.

Antigorite polysomatism : behaviour during progressive metamorphism

Antigorite forms a polysomatic series of discrete compositions that are chemographically colinear with chrysotile/lizardite, Mg3Si2O5(OH)4 and talc, Mg3Si4O10(OH)2. The compositional variations of antigorite correspond to discrete changes in the lattice parameter, a. A complete suite of antigorites, collected from a cross-section representing increasing metamorphic grade through the Swiss and Italian Alps, has been studied by optical and transmission electron microscopy (TEM). The specimens within this suite range from those formed near the lower stability limit of antigorite (250 °C) through to those formed near its breakdown temperature (550 °C). The lower grade samples belong to the regionally metamorphosed upper Pennine Ophiolites of the Oberhalbstein-Malenco area, while higher grade antigorites were obtained from regionally metamorphosed Malenco serpentinites. The highest grade samples are also from Malenco. They underwent a later contact metamorphism within the thermal aureole of the Bregaglia Intrusive. The lattice parameter a of antigorites evolves from longer (60 Å) to shorter (35 Å) values with increasing metamorphic grade. However, individual antigorites almost invariably show a heterogeneous distribution of a periodicities with higher values close to grain boundaries or reaction fronts and lower values towards the grain centers. The crystal-chemical evolution of antigorite, expressed by reduction in a, is usually accompanied by increased crystallinity. With the TEM, this is seen as an increase in crystallite size and a decrease in the number of crystal defects (twinning, polysomatic disorder, modulation dislocations, wobbling, offset). The structural and compositional evolution of antigorite requires intracrystalline diffusion and reconstructive transformations at relatively low temperatures. Therefore, the process of evolution is sluggish. Equilibrium is frequently not attained, and relics of longer a periodicities can be observed. In addition, relics of chrysotile may be observed in high-grade metamorphic rocks of the Malenco area, in which antigorite coexists with new-formed olivine. Only at one locality is there evidence of “equilibrium”: antigorite formed at 435 °C has a=43 Å; it shows very little variation in the a periodicity, and it is characterized by a homogeneous annealing texture. A geothermometer based upon a periodicities, as proposed by Kunze (1961) has limited practical applicability.

Micro-Raman spectroscopy for a quick and reliable identification of serpentine minerals from ultramafics

Identifying serpentine minerals in rocks is generally accomplished by means of Scanning Electron Microscopy - Energy Dispersion Spectrometry (SEM-EDS) and Transmission Electron Microscopy (TEM), both of which require complicated sample preparation. In this work, we evaluate the use of micro-Raman spectroscopy, which requires no sample preparation, in identifying the different serpentine minerals contained in thin sections of serpentinized peridotites, where the various phases occur in different microstructural positions. The micro-Raman spectra were obtained from samples previously characterized by optical microscopy, SEM-EDS and TEM. Micro-Raman spectroscopy proved to be a quick, easy and reliable method for the identification of serpentine minerals.

POTENTIAL TOXICITY OF NONREGULATED ASBESTIFORM MINERALS: BALANGEROITE FROM THE WESTERN ALPS. PART 1: IDENTIFICATION AND CHARACTERIZATION

In the Italian western Alps, asbestos mineralization (both chrysotile and tremolite amphibole) takes place from serpentinites, together with other less common asbestiform minerals not regulated by the current legislation. In the context of a study on the evaluation of the asbestos risk in this area, the possible role played by the associated asbestiform minerals in the overall toxicity of the airborne fraction has been examined. The first mineral investigated was balangeroite [(Mg,Fe2+,Fe3+,Mn2+)42Si16O54(OH)36], an iron-rich asbestiform contaminant of chrysotile from the Balangero mine (Piedmont), which crystallizes as rigid and brittle fibers. In order to prepare a sample in a form appropriate for chemical and cellular tests, the fibers were separated from the rock and comminuted without damage to their crystalline structure and surface state (as confirmed by X-ray diffraction [XRD] and ultraviolet–visible [UV-Vis] spectroscopy). The first properties examined were durability in simulated body fluids (Gamble’s solution) and toxicity to epithelial cells. When compared to UICC crocidolite (the amphibole blue asbestos, regarded as the most pathogenic form), balangeroite appears even more durable than crocidolite. Balangeroite and UICC crocidolite showed a similar in vitro cytotoxic effect on a human epithelial cell line, as evidenced by leakage of intracellular lactate dehydrogenase (LDH) activity, which, observed after a 24-h incubation, was dose dependent and maximal at 12 μg/cm2 for each fiber type. Data show that chemical composition, form, durability, and cell toxicity indicate balangeroite as a potentially harmful fibrous mineral that needs to be examined by further chemical and cellular tests.

Superzoned garnets in the coesite-bearing Brossasco-Isasca Unit, Dora-Maira massif, Western Alps, and the origin of the whiteschists

Abstract 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.

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.

First report of felsic whiteschist in the ultrahigh-pressure metamorphic belt of Dabie Shan, China

We report the first occurrence of a talc-kyanite assemblage typical of whiteschists in felsic ultrahigh-pressure metamorphic rocks of Dabie Shan, China. The whiteschist assemblage occurs in a leucocratic layer, less than half a meter thick, which crosscuts a coesite-bearing eclogite. Both its geologic setting and geochemistry suggest that the protolith was a felsic dyke, which suffered loss of some elements, in particular alkalies. The whiteschist consists of quartz, minor epidote, kyanite, talc and omphacite. Common accessories are rutile, apatite and zircon. Epidote is zoned, and the core includes quartz pseudomorphs after coesite and contains up to 5 wt% REE. Kyanite includes fresh omphacite with the highest jadeite content ( Jd ≤ 0.60). The host eclogite consists of omphacite (X Jd = 0.44 to 0.37 from core to rim), garnet, quartz, kyanite, epidote, phengite (Si ≤ 3.47 a.p.f.u.) and amphibole, and accessory rutile, apatite, zircon and ilmenite. Fresh coesite occurs in epidote, and its polycrystalline quartz pseudomorphs are included in both omphacite and kyanite. The whiteschist and the host eclogite share the same tectonometamorphic evolution, and show three metamorphic stages: (I) coesite-eclogite-facies stage at P ≥ 2.6–2.7 GPa and T = 710 ± 20°C; (II) early decompression stage at P ≥ 1.5–2.0 GPa and T = 650 ± 30°C; (III) late adiabatic decompression to P = 0.7 GPa and T = 670 ± 40°C.

Middle Oligocene extension in the Mediterranean Calabro‐Peloritan belt (southern Italy): Insights from the Aspromonte nappes pile

The Calabro-Peloritan belt constitutes the eastward termination of the southern segment of the Alpine Mediterranean belt. This orogenic system was built up during the convergence between the Eurasian and the African plates, roughly directed North-South since the Upper Cretaceous. It was subsequently fragmented during the opening of the Western Mediterranean basins since Oligocene times. The curved shape of the Calabro-Peloritan belt was acquired during the opening of the Tyrrhenian basin since Tortonian. The origin, kinematics and significance of the Calabro-Peloritan tectonic pile are still debated. Our data in the Aspromonte Massif of Southern Calabria reveal an Alpine history marked by two main superimposed kinematic regimes. (i) A first phase corresponds to the piling up of basement nappes with a top-to-the-SE vergence, i.e in a direction orthogonal to the belt trend and towards the Adriatic foreland. This external vergence is similar to what is observed in both Northeastern Sicily and Northern Calabria. In Sicily, the age of nappe piling is Alpine, as evidenced by pinched slices of Mesozoic sediments. In the Aspromonte Massif, thrusting age is less constrained. Our data suggest remnants of late Hercynian structuration before the Alpine stacking. (ii) A second phase corresponds to the thinning of the continental crust, dated at around 30 Ma by both geochronological and stratigraphical data. This extension is mainly localized on two low-angle detachment contacts, with top-to-the-NE displacement. The lower one corresponds to the reworking of the former main nappe contact. The upper one is a large detachment fault cutting across the pile from upper sedimentary levels down to metamorphic basement. Extension of similar Alpine age and similar internal vergence has been already recognized in other parts of the Calabro-Peloritan Arc: in the basement nappes of Northeastern Sicily and in the ophiolitic units of Northern Calabria. Coming back to the original geometry and position of the Calabro-Peloritan belt, before its bending and the opening of the Liguro-Provencal and Tyrrhenian basins, we evidence a homogeneous Oligocene NE-SW extension all along the Calabro-Peloritan segment of the Alpine Mediterranean belt. This tectonometamorphic history is best explained within the framework of the continuous Tertiary westward dipping subduction of the Tethyan oceanic domain below the European active margin and the progressive eastward retreat of the Apennine trench since Oligocene times.

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.