Franco Rolfo

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.

Two contrasting eclogite types in the Himalayas: implications for the Himalayan orogeny

The metamorphic evolution of granulitized eclogites recently discovered in the Eastern Himalaya compared to that of the eclogites of the Northwestern Himalaya (upper Kaghan Nappe and Tso Morari Dome) suggests the possibility of a Himalaya-wide eclogitic metamorphism of Early Tertiary age. Eclogites from the Northwestern Himalaya record peak metamorphic temperatures of 580–600°C at metamorphic pressures in excess of 23–24 kbar. They have glaucophane as a retrograde phase and followed a nearly isothermal decompression path into the field of epidote amphibolite facies. In contrast, the Eastern Himalaya eclogites have a strong granulite-facies overprint at metamorphic temperatures of about 750°C and pressures of 7–10 kbars, and followed a clockwise decompression path strongly convex towards high metamorphic temperatures. The main difference between the crystalline nappes of the Northwestern Himalaya and those of the East Himalaya appears to lie in the different P–T path they followed during exhumation. In particular the Northwestern Himalaya crystalline nappes lack the Miocene high temperature and low pressure overprinting which is characteristic of the Eastern Himalaya, where thermal relaxation of the thickened continental crust erased almost completely the mineralogical record of the early stages of continental collision.

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.

Tectonometamorphic evolution of the central Karakorum (Baltistan, northern Pakistan)

Abstract New structural and metamorphic data from the Chogo Lungma glacier area and Basha valley enlighten the polyphased tectonometamorphic evolution of the Metamorphic Complex of Karakorum. A pre- to early D1 event is defined by a metamorphic cleavage in the core of some zoned garnet and staurolite porphyroblasts. The main tectonometamorphic event (D1) is characterized by N110°E south-vergent isoclinal folds associated with a metamorphic axial-plane cleavage formed in a predominantly flattening regime of deformation. It corresponds to a Grt-Bt-Mu-Ky peak metamorphism (M1: 620–730°C and 7.5–11 kbar). The D2 tectonometamorphic event corresponds to the development of several conical domes elongated striking N110°E to N140°E. The S2 cleavage is seldom individualized as it usually corresponds to a reworking of S1. In some areas, late evolution of D2 doming deformation is marked by a crenulation. Petrologically, D2 is characterized by shearing and boudinage of M1 minerals and by development of sillimanite. It corresponds to a decompressional evolution (7.5 down to 4 kbar) with only slight changes in temperature. A late event corresponds to large undulations and open folds which affect the D2 crenulation. We propose here a model of vertical plus dextral extrusion of the middle crust to explain the dome structures in Karakorum. The steep pattern of the metamorphic P-T path implies rapid exhumation rates and suggests that the different phases could be part of a continuum of deformation. Preliminary results by 40 Ar 39 Ar method give young cooling ages (

Glaucophane and barroisite eclogites from the Upper Kaghan Nappe: implications for the metamorphic History of the NW Himalaya.

Abstract This paper presents the results of a petrographical and mineral chemical study of glaucophane- and barroisite-bearing eclogites from the Upper Kaghan nappe in the Higher Himalayan Crystallines of the Pakistan Himalaya, and discusses the implications of the P-T path recorded in such rocks for the tectonic and metamorphic history of the NW Himalaya. The eclogites described here come from a previously undescribed outcrop at Gittidas, but belong to the same suite as the garnet-omphacite-phengite-quartz-rutile eclogites previously described elsewhere in the Upper Kaghan nappe. The metamorphic peak assemblage is garnet-omphacite-rutile-quartz in glaucophane eclogite and garnet-omphacite-zoisite-rutile±kyanite±phengite±ankerite in barroisite eclogite. Most samples contain a significant amount of amphibole, white mica and quartz. White mica may be present either as part of the peak assemblage (phengite) or as a retrogressive phase after kyanite (paragonite). Amphibole is later than the metamorphic peak assemblage and is barroisite in most samples, but in relatively Fe-rich eclogite it is glaucophane with significant Na in the A site, Ca in the M4 site and tetrahedral A1. Garnet displays strong prograde zoning in the barroisite eclogites, with Mg increasing and Fe decreasing from core to rim. The iron-rich core is crowded with mineral inclusions of the peak assemblage, but inclusions of earlier paragonite, green and blue-green amphibole were also found. Peak metamorphic conditions in the barroisite eclogites have been estimated at T = 610 ± 30 °C and P = 24 ± 2 kbar from Fe/Mg partition in garnet-omphacite pairs, and from the garnet-omphacite-phengite barometer. These values are close to the equilibration conditions estimated for the eclogites of the North Himalayan Tso-Morari Dome. Strong similarities between metamorphic evolution of the Upper Kaghan nappe and metamorphic evolution of the eclogite-bearing units of the Neelum valley just to the east of the Kaghan valley indicate that the eclogite occurrences of the Kaghan-Neelum area define an eclogite-bearing terrane of regional extent, which was subjected to high pressure metamorphism in middle Eocene times. A comparison of the metamorphic evolution recorded in the eclogites of the NW Himalaya with that of the granulitized eclogites recently discovered in the E Himalaya suggests the possibility of a Himalaya-wide eclogitic metamorphism of pre-Miocene age. Therefore, the main difference between the Higher Himalayan Crystalline nappes of the NW Himalaya and those of the E Himalaya appears to lie less in the early part of their metamorphic evolution than in the different P-T paths they followed during exhumation.

CHARACTERISTICS OF UHP PELITES, GNEISSES, AND OTHER UNUSUAL ROCKS

The main felsic lithologies experiencing ultrahigh-pressure metamorphism (UHPM) are described, with a focus on rock association, nature, and composition of the peak minerals, as well as nature of the protolith. Felsic rocks usually preserve poor evidence of UHPM, being pervasively deformed and retrogressed during exhumation. Locally, however, unstrained lithologies are preserved that show UHPM minerals and even microstructures inherited from the protolith. Among the number of localities where UHPM has been reported, the following have been considered in detail— the southern Dora-Maira massif and Lago di Cignana in the Western Alps, the Dabie Shan-Sulu region in northeastern China, the Kokchetav massif in northern Kazakhstan, and the Western Gneiss Region of the Scandinavian Caledonides. At Lago di Cignana, metasediments of oceanic affinity are carbonate-bearing garnet-phengite-quartz schist and oxidized manganiferous quartz schists, all preserving rare coesite relics. In the Dora-Maira massif, the Brossas...

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.

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.

Fluid evolution from metamorphic peak to exhumation in Himalayan granulitised eclogites, Ama Drime range, southern Tibet

A fluid inclusion study has been carried out on granulitised eclogites and associated amphibolite-facies veins from the eastern Himalaya (Ama Drime range, southern Tibet) in order to better characterise the fluid evolution of continental crust involved in Cenozoic subduction and continent-continent collision processes. Six distinct events of fluid influx have been characterised on the basis of petrographic observations and microthermometric measurements: 1) a high-density - medium-salinity aqueous fluid with CO 2 , trapped at the eclogitic peak (metamorphic stage M1); 2) pure CO 2 , present at the granulitic stage M2; 3) a low-salinity – low-density aqueous fluid with minor CO 2 in equilibrium with amphibole-bearing LP-M/HT mineral assemblage (stage M4); 4) a low-density aqueo-carbonic fluid responsible of vein formation (stage M5); 5) a subsequent influx of a low-density CO 2 -rich fluid; 6) a late influx of very-low salinity aqueous fluid. Comparing these data with those obtained from other localities of the Himalayas and from other collisional orogens, an internal origin is proposed for eclogitic and granulitic fluids, whereas an origin from the underlying metasediments of the Lesser Himalaya is suggested for the subsequent types of fluid, safe for the meteoric origin proposed for the latest fluid influx.