Bruno Lombardo

Geology of the Higher Himalayan Crystallines in Khumbu Himal (Eastern Nepal)

Abstract In this paper we present the current geological knowledge and the results of new geological and structural investigations in the Cho Oyu-Sagarmatha-Makalu region (Eastern Nepal and Southern Tibet). The tectonic setting of the middle and upper part of the Higher Himalayan Crystallines (HHC) and Tibetan Sedimentary Sequence is characterized by the presence of pervasive compressive tectonics with south-verging folds and shear zones overprinted by extensional tectonics. In the middle and upper part of the HHC two systems of folds (F2a and F2b) have been recognized, affecting the S1 high-grade schistosity causing kilometer-scale upright antiforms and synforms. The limbs of these upright folds are affected by F3 collapse folds, top-to-SE extensional shear zones and extensional crenulation cleavages linked to extensional tectonics. The uppermost portion of the HHC and the lower part of the Tibetan Sedimentary Sequence is affected by two major extensional fault zones with a top-to NE direction of movement. The lower ductile extensional shear zone brings into contact the North Col Formation with the high grade gneisses and micaschists of the HHC. It is regarded as the main feature of the South Tibetan Detachment System. The upper low-angle fault zone is characterized by ductile/brittle deformation and thin levels of cataclasites and brings the slightly metamorphosed Ordovician limestones into contact with the North Col Formation. Extensional tectonics continued with the formation of E–W trending high angle normal faults. Three metamorphic stages of Himalayan age are recognized in the HHC of the Sagarmatha-Makalu region. The first stage (M1) is eclogitic as documented by granulitized eclogites collected at the top of the Main Central Thrust Zone in the Kharta region of Southern Tibet. The second event recorded in the Kharta eclogites (M2) was granulitic, with medium P (0.55–0.65 GPa) and high T (750–770°C), and was followed by recrystallization in the amphibolite facies of low pressure and high T (M3). The first event has also been recorded in the overlying Barun Gneiss, where M1 was followed by decompression under increasing T, the M2 event, producing the dominant mineral assemblage (garnet-sillimanite-biotite), and then by strong decompression under high T, with growth of andalusite, cordierite and green spinel. Also, changes in phase compatibilities suggest an increase in metamorphic temperature (T) coupled with a decrease in metamorphic pressure (P) in some of the thrust sheets of the MCT Zone. A telescoped metamorphic zonation ranging from the sillimanite to the staurolite and biotite zones is characteristic of the ductile extensional shear zone which is the lower part of the STDS in the Sagarmatha region. Evidence for decompression under increasing temperature, anatexis and leucogranite emplacement accompanying extension in the HHC was found throughout the whole ductile shear zone, particularly in metapelites both below and above the Makalu leucogranite and in micaschists of the staurolite zone.

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.

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 (

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 history of HP mafic granulites from the Gesso-Stura Terrain (Argentera Massif, Western Alps, Italy)

At the Frisson Lakes (FL), a small layered mafic sequence surrounded by migmatitic orthogneisses is exposed as a tectonic window of the crystalline basement of the Gesso-Stura Terrain (GST) of the Argentera Massif (Western Alps, Italy) within its Meso-Cenozoic sedimentary cover. The mafic sequence is characterised by Pl-rich and Pl-poor layers of HP granulites consisting of porphyroclasts of Omp, Grt, Amp, Pl, and rare Qtz embedded in a fine-grained mylonitic matrix of neoblastic Amp, Pl, Cpx, Grt and minor Bt and Qtz. Symplectites of Amp + Pl partly replace porphyroclastic Grt and Omp. Chemical compositions of the same minerals from the Pl-rich and Pl-poor layers are different, suggesting very limited element diffusion between layers during metamorphic evolution. All samples show the following features: (i) porphyroclasts of Grt and Cpx characterised by cores rich in exsolved rutile and albite, respectively, (ii) zoned Cpx with Jd- and CaTi-Ts-richer core, and (iii) a systematic decrease in Al(VI) of Amp from the earliest porphyroclasts to the latest mylonitic neoblasts. Geological and petrological data indicate that the FL mafic sequence derived from the mylonitic deformation of a former layered mafic intrusion, which occurred before Carboniferous migmatization. The mafic sequence records a continuous metamorphic evolution characterised by: (i) a metamorphic peak (stage A) at HP granulite-facies conditions (735 ± 15 LC and 1.38 ± 0.05 GPa) in which coarse-grained minerals developed statically; (ii) an early decompression stage (stage B), which occurred at the transition between HP granulite- and HP-HT amphibolite-facies (709 ± 2 °C and 1.10 ± 0.02 GPa); (iii) a HT–MP amphibolite-facies stage (stage C) (665 ± 15 °C and 0.85 ± 0.15 GPa), which was accompanied by pervasive mylonitic deformation; and (iv) a final non-pervasive “symplectitic” stage (stage D) (500 < T < 625 °C and P < 0.59 GPa), which developed at shallow crustal levels, most likely favoured by a moderate influx of hydrous fluids. The P–T path reconstructed for the FL layered mafic sequence, for the first time quantitatively estimated for the Argentera Massif, allows to conclude that the granulites and eclogites previously reported in the GST belong to a single metamorphic stage, which developed at HP granulite-facies conditions. This metamorphic history, which appears to be similar to that recorded in other External Crystalline Massifs, is consistent with a subduction process followed by relatively rapid exhumation.

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.

Geological map of the upper Pellice Valley (Italian Western Alps)

Abstract Please click here to download the map associated with this article. The 1:25,000 scale geological map of the upper Pellice Valley (Italian Western Alps) encompasses an area of roughly 60 km2 where a composite stack of both oceanic and continental Alpine units crops out. The four units distinguished in the map correspond, from bottom to top, to the Dora Maira Unit, a slice of the Paleozoic European crust, the Giulian-Sea Bianca Unit, a succession of Triassic to Lower Jurassic carbonate metasediments, the Monviso Unit, an ophiolitic remnant of the Mesozoic Tethyan Ocean, and the Bucie-Seilliere Unit, a carbonate metasedimentary succession of Upper Jurassic to Upper Cretaceous age. Each unit is bounded by syn-metamorphic faults (i.e. tectonic contacts) and is displaced by a postmetamorphic fault network, then discontinuously covered by heterogeneous alluvial, gravitative, glacial and periglacial Quaternary deposits. The map gives new and updated information about the structural and geological setting of the Pellice Valley through a detailed representation of the main lithological, structural and morphological features.

Tectonostratigraphy of the northern Monviso Meta-ophiolite Complex (Western Alps)

The Monviso Meta-Ophiolite Complex is a remnant of the Piedmont-Ligurian oceanic lithosphere stacked in the Western Alps, andconsisting of dismembered HP meta-ophiolite sequences. In thiswork, focused on the northern sector of the Complex, we differen -tiate six tectonic units which structural, petrographic and stratigraphiccharacteristics are described in detail and discussed in thelight of a comparison with the overall geology of the MonvisoMeta-Ophiolite Complex. The structural evolution has been referredto i) an early syn-eclogitic deformation phase (D1), ii) a main deformationphase (D2) occurred in the blueschist- to greenschist-faciestransition and characterized by the development of a regional foliation(S2) that is parallel to the tectonic contacts and to the axialplane of map-scale W-verging folds, and iii) a late-metamorphicdeformation phase (D3) characterized by westward extensionaltectonic. The northern Monviso Meta-Ophiolite Complex is characterizedby a poor preservation of HP paragenesis and a widespreadoverprint of the blueschist- to greenschist-facies metamorphism,but the occurrence of garnet-, omphacite-, talc- and lawsoniteassemblagein a Fe-Ti metagabbro indicates P-T eclogitic conditions(2.5-2.7 GPa for 550-570 �C) very similar to those calculated in thesouthern sector of the Complex. The stratigraphic characteristics ofthe meta-ophiolite sequences point out that, differently from thesouthern sector of the Complex where basalt-poor and basalt-richoceanic units have been distinguished, in the northern MonvisoMeta-Ophiolite Complex the different types of metasedimentsmay be the key to restore the oceanic tectonostratigraphy, markedby gabbro and mantle peridotite exposition on a puzzle-like oceanfloor where basalt effusion and different sedimentation processestook place.