Dario Visonà

Two-mica and tourmaline leucogranites from the Everest–Makalu region (Nepal–Tibet). Himalayan leucogranite genesis by isobaric heating?

In the Higher Himalaya of the region from Cho Oyu to the Arun valley northeast of Makalu, the Miocene leucogranites are not hosted only in the upper High Himalayan Crystallines (HHC); a network of dykes also cuts the lower HHC and the Lesser Himalayan Crystallines (LHC). The plutons and dykes are mainly composed of two-mica (muscovite+biotite±tourmaline±cordierite±andalusite±sillimanite) leucogranite, with tourmaline≤2.6% and biotite>1.5% modal, and tourmaline (muscovite+tourmaline±biotite±sillimanite ±garnet±kyanite±andalusite±spinel±corundum) leucogranite, with tourmaline>2.2% and biotite<1.5% modal. Both leucogranite types were produced by partial melting in the andalusite–sillimanite facies series, under LP/HT conditions constrained by the occurrence of peritectic andalusite and cordierite. The geochemical features of the leucogranites suggest that tourmaline leucogranite was produced by muscovite dehydration melting in muscovite-rich metapelites at P∼350 MPa and T≥640°C, whereas two-mica leucogranite was produced by biotite dehydration melting in biotite-rich metapelites at P∼300 MPa and T≥660–710 °C. Melting in fertile muscovite-rich metapelites of the top of both the HHC and LHC produced magmas which were emplaced at the same structural level in which they had been generated. Melting in the biotite-rich gneiss of both the HHC and LHC produced hotter magmas which were transported upwards by dyking and eventually coalesced in the plutons of the upper HHC. A similar process also produced a network of two-mica granite at the top of the LHC in the Ama Drime–Nyonno Ri Range northeast of Makalu. The prograde character of leucogranite melt-producing reactions in the Everest–Makalu area suggests that, here, the generation of Miocene leucogranites took place in a regime of nearly isobaric heating following nearly adiabatic decompression.

Interpretation and processing of ASTER data for geological mapping and granitoids detection in the Saghro massif (eastern Anti-Atlas, Morocco)

Satellite remote sensing analysis is extensively used for geological mapping in arid regions. However, it is not considered readily applicable to the mapping of metamorphic and igneous terrains, where lithological contacts are less predictable. In this work, ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) data were used to clarify the geological framework of the Precambrian basement in the Saghro massif (eastern Anti-Atlas, Morocco). The Saghro basement is composed of low-grade metasedimentary sequences of the Saghro Group (Cryogenian), intruded by calc-alkaline plutons of late Cryogenian age. These rocks are unconformably covered by volcanic to volcaniclastic series of Ediacaran age that are broadly coeval with granitoid plutons. All of these units are cut by a complex network of faults associated with hydrothermal fluid flows, which developed during and shortly after the emplacement of the volcanic rocks. The geological mapping of the Precambrian units was challenging in particular for the Edicaran granitoid bodies, because they are characterized by very similar compositions and a widespread desert varnish coating. For this reason, a two-stage approach has been adopted. In the first step, false color composites, band ratios, and principal components analyses on visible and near infrared (VNIR) and shortwave infrared (SWIR) bands were chosen and interpreted on the basis of the field and petrographic knowledge of the lithologies in order to detect major lithological contacts and mineralized faults. In the second step, a major effort was dedicated to the detection of granitoid plutons using both thermal infrared (TIR) and VNIR/SWIR data. The ASTER TIR bands were used to evaluate Reststrahlen and Christiansen effects in the granitoid rocks spectra, whereas VNIR/SWIR false color composite and ratio images were chosen directly on the basis of the granitoid spectra (derived from both spectrophotometric analyses of samples and selected sites in the ASTER image). Finally, spectral angle mapper (SAM) and supervised maximum-likelihood classifications (MLL) were carried out on VNIR/SWIR data, mainly to evaluate their potential for discriminating granitoid rocks. The results have further demonstrated the value of ASTER data for geological mapping of basement units, particularly if the processing has been based on a detailed knowledge of the rock mineral assemblages. In addition, the analytical comparison of ASTER TIR and VNIR/SWIR data has demonstrated that the latter are very effective in the distinction of granitoids with very similar silica content, because they can be recognized by secondary effects related to their hydrothermal and surface alterations (K-feldspar kaolinitization, plagioclase saussiritization, substitution of mafic minerals with oxides, inhomogeneous desert varnish coating, and clay/oxide proportions).

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.

Eocene partial melting recorded in peritectic garnets from kyanite-gneiss, Greater Himalayan Sequence, central Nepal *

Abstract Anatectic melt inclusions (nanogranites and nanotonalites) have been found in garnet of kyanite-gneiss at the bottom of the Greater Himalayan Sequence (GHS) along the Kali Gandaki valley, central Nepal, c. 1 km structurally above the Main Central Thrust (MCT). In situ U–Th–Pb dating of monazite included in garnets, in the same structural positions as melt inclusions, allowed us to constrain partial melting starting at c. 41–36 Ma. Eocene partial melting occurred during prograde metamorphism in the kyanite stability field (Eo-Himalayan event). Sillimanite-bearing mylonitic foliation wraps around garnets showing a top-to-the-SW sense of shear linked to the MCT ductile activity and to the exhumation of the GHS. These findings highlight the occurrence of an older melting event in the GHS during prograde metamorphism in the kyanite stability field before the more diffuse Miocene melting event. The growth of prograde garnet and kyanite at 41–6 Ma in the MCT zone, affecting the bottom of the GHS, suggests that inverted metamorphism in the MCT zone and folded isograds in the GHS should be carefully proved with the aid of geochronology, because not all Barrovian minerals grew during the same time span and they grew in different tectonic settings.

Interpretation and processing of Aster data for geological mapping of the Precambrian Basement in the Saghro Massif (Eastern Anti-Atlas, Morocco)

Satellite remote sensing analysis is extensively used for geological mapping in arid regions. However, it is not considered readily applicable to the mapping of metamorphic and igneous terrains, where lithological contacts are less predictable. In this work, ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) data were used to clarify the geological framework of the Precambrian basement in the Saghro massif (eastern Anti-Atlas, Morocco). The Saghro basement is composed of low-grade metasedimentary sequences of the Saghro Group (Cryogenian), intruded by calc-alkaline plutons of late Cryogenian age. These rocks are unconformably covered by volcanic to volcaniclastic series of Ediacaran age that are broadly coeval with granitoid plutons. All of these units are cut by a complex network of faults associated with hydrothermal fluid flows, which developed during and shortly after the emplacement of the volcanic rocks. The geological mapping of the Precambrian units was challenging in particular for the Edicaran granitoid bodies, because they are characterized by very similar compositions and a widespread desert varnish coating. For this reason, a two-stage approach has been adopted. In the first step, false color composites, band ratios, and principal components analyses on visible and near infrared (VNIR) and shortwave infrared (SWIR) bands were chosen and interpreted on the basis of the field and petrographic knowledge of the lithologies in order to detect major lithological contacts and mineralized faults. In the second step, a major effort was dedicated to the detection of granitoid plutons using both thermal infrared (TIR) and VNIR/SWIR data. The ASTER TIR bands were used to evaluate Reststrahlen and Christiansen effects in the granitoid rocks spectra, whereas VNIR/SWIR false color composite and ratio images were chosen directly on the basis of the granitoid spectra (derived from both spectrophotometric analyses of samples and selected sites in the ASTER image). Finally, spectral angle mapper (SAM) and supervised maximum-likelihood classifications (MLL) were carried out on VNIR/SWIR data, mainly to evaluate their potential for discriminating granitoid rocks. The results have further demonstrated the value of ASTER data for geological mapping of basement units, particularly if the processing has been based on a detailed knowledge of the rock mineral assemblages. In addition, the analytical comparison of ASTER TIR and VNIR/SWIR data has demonstrated that the latter are very effective in the distinction of granitoids with very similar silica content, because they can be recognized by secondary effects related to their hydrothermal and surface alterations (K-feldspar kaolinitization, plagioclase saussiritization, substitution of mafic minerals with oxides, inhomogeneous desert varnish coating, and clay/oxide proportions).

The Austridic eclogites, metabasites and metaultrabasites from the Pohorje area (Eastern Alps, Yugoslavia): 2. The metabasites and metaultrabasites, and concluding considerations

Petrography, mineral chemistry, metamorphic evolution and geothermo-barometric estimations concerning the Pohorje metabasites and metaultrabasites are presented and discussed. Consistently with the evolution detected in the associated eclogites, and disregarding serpentinization, these rocks record a three-stage metamorphic evolution: (i) an older, dry, high-pressure/high-temperature recrystallization;(it) a later, lower-pressure/lower temperature granulitic stage;(iii) a younger, epidote-amphibolite overprint. The latter water-controlled recrystallization is believed to be Alpine, while the main part of the metamorphic history is to be considered pre-Alpine. Such chronological classification is consistent with the prevailing interpretation concerning similar rocks in the Eastern Alps, and is supported by the occurrence of foliated leucocratic veins which crosscut the metaultrabasites and the eclogites.RiassuntoLe metabasiti e le metaultrabasiti affioranti nell’area di Pohorje vengono studiate dai punti di vista petrografico, petrologico, del chimismo delle fasi mineralogiche e dell’evoluzione metamorfica. Prescindendo dalla serpentinizzazione. talc evoluzione risulta congruente con quella nconosciuta nelle eclogiti associate e precisamente articolata in tre stadi: i) una antica ricristallizzazione sotto alti valon di T e P in condizioni anidre;ii) un successivo stadio granulitico svoltosi sotto valori di TeP piu bassi; iii@#@) una più recente rielaborazione in facies delle antiboliti ad epidoto, in presenza di H2O. Quest’ultimo stadio viene considerato di età Alpina. Invecc, sulla base di considerazioni geologiche, i precedenti stadi sono ritenuti pre-Alpini, congruentemente con le opinioni prevalenti nella letteratura regionale su analoghe rocce austroalpine. Tale interpretazione e awalorata dalla presenza di vene leucocratiche foliate che attraversano le metabasiti e le eclogiti studiate.

Is there any detachment in the Lower Dolpo (western Nepal)

A structural transect in the Lower Dolpo highlights that the deformation and metamorphism of the Tibetan Zone (TZ) increase toward the bottom of the sequence. The contact with the underlying HHC is marked by a metamorphic jump from amphibolite facies in the carbonatic rocks of the upper part of the HHC to greenschist facies marbles in the TZ. Moreover, the HHC and the TZ show different metamorphic histories. The contact zone shows a strain increase accompanied by asymmetric folds with a top-to-the-northeast vergence, connected to a down-to-the-northeast tectonic transport. The contact is interpreted as an extensional shear zone, connected to the South Tibetan Detachment System. To cite this article: R. Carosi et al., C. R. Geoscience 334 (2002) 933–940.

Middle to late Eocene exhumation of the Greater Himalayan Sequence in the Central Himalayas: Progressive accretion from the Indian plate

We investigated a contractional shear zone located in central Nepal, known as Kalopani shear zone. This high-temperature shear zone triggered the early exhumation of the metamorphic core in the Himalayan belt and deeply affected the tectono-metamorphic history of the crystalline rocks soon after the collisional stage. Pseudosection modeling and inverse geothermobarometry reveal that rocks involved in the Kalopani shear zone experienced pressure-temperature conditions between 0.60 and 0.85 GPa and 600 and 660 °C. U-Th-Pb in situ laser ablation−inductively coupled plasma−mass spectrometry and sensitive high-resolution ion microprobe dating on monazite points to retrograde metamorphism related to the Kalopani shear zone starting from ca. 41 to 30 Ma. The kinematics of the Kalopani shear zone and associated erosion and/or tectonics caused the middle-late Eocene exhumation of the Greater Himalayan Sequence in the hanging wall of the Kalopani shear zone at least 9 m.y. before the activities of the middle tectonic-metamorphic discontinuity in the Greater Himalayan Sequence (High Himalayan discontinuity), the Main Central thrust, and the South Tibetan detachment. Structural data, metamorphic conditions, and geochronology from the Kalopani shear zone, compared to those of other major tectonic discontinuities active within the Greater Himalayan Sequence in the Kali Gandaki valley, indicate that shear deformation and exhumation were not synchronous all over the Greater Himalayan Sequence but migrated downward and southward at different lower levels. These processes caused the exhumation of the hanging wall rocks of the activated shear zones. The main consequence is that exhumation has been driven since the middle-late Eocene by an in-sequence shearing mechanism progressively involving new slices of the Indian crust, starting from the metamorphic core of the orogen and later involving the outer portions of the belt. This challenges the common view of exhumation of the Greater Himalayan Sequence mainly driven by the coupled activity of Main Central thrust and South Tibetan detachment between ca. 23 and 17 Ma.

The Austridic eclogites, metabasites and metaultrabasites from the Pohorje area (Eastern Alps, Yugoslavia): 1. The eclogries and related rocks

Petrography, petrology, mineral chemistry, metamorphic evolution and geothermo-barometric estimations concerning the Pohorje eclogites are discussed. These rocks record a three-stage metamorphic evolution: (0 an older, dry, high-pressure/high-temperature recrystallization (eclogite stage);(ii) a later, lower-pressure/lower temperature symplectite stage, partly developed under granulite faciès conditions;(iii) a younger, epidote-amphibolite overprint. The latter water-controlled recrystallization is believed to be Alpine. However, the main part of the metamorphic history is to be considered pre-Alpine: such chronological classification is consistent with the prevailing interpretation concerning similar Austridic rocks in the Eastern Alps.RiassuntoLe eclogiti dell’area di Pohorje vengono studiate dal punto di vista petrografico, petrologico, della composizione delle fasi mineralogiche e dell’evoluzione metamorfica. Tale evoluzione risulta articolata in tre stadi: (i) una antica ricristallizzazione in ambiemte anidro, sotto alte T eP (stadio eclogitico);(ii) un successivo stadio simplettitico, a più basseT e P, svoltosi nella sua ultima parte in condizioni granulitiche;(Hi) una sovrimpronta recente in facies delle anfiboliti ad epidoto. Quest’ultima ricristallizzazione in presenza di H2O viene ritenuta Alpina, mentre la parte principale della storia metamorfica di queste eclogiti viene ritenuta pre-Alpina. Tale classificazione cronologica è in accordo con le prevalenti interpretazioni riguardanti simili rocce del basamento Austridico delle Alpi Orientali.

Geology and tectono-metamorphic evolution of the Himalayan metamorphic core: Insights from the Mugu Karnali transect, Western Nepal (Central Himalaya)

New structural and tectono-metamorphic data are presented from a geological transect along the Mugu Karnali valley, in Western Nepal (Central Himalaya), where an almost continuous cross section from the Lesser Himalaya Sequence to the Everest Series through the medium-high-grade Greater Himalayan Sequence (GHS) is exposed. Detailed meso- and micro-structural analyses were carried out along the transect. Pressure (P) -temperature (T) conditions and P-T-deformation paths for samples from different structural units were derived by calculating pseudosections in the MnNKCFMASHT system. Systematic increase of P-T conditions, from ~ 0.75 GPa–560 °C up to ≥1.0 GPa–750°C have been detected starting from the garnet zone up to the K-feldspar + aluminosilicate zone. Our investigation reveals how these units are characterized by different P-T evolutions and well-developed tectonic boundaries. Integrating our meso- and micro-structural data with those of metamorphism and geochronology, a diachronism in deformation and metamorphism can be highlighted along the transect, where different crustal slices were underthrust, metamorphosed and exhumed at different times. The GHS is not a single tectonic unit, but it is composed of (at least) three different crustal slices, in agreement with a model of in-sequence shearing by accretion of material from the Indian plate, where coeval activity of basal thrusting at the bottom with normal shearing at the top of the GHS is not strictly required for its exhumation. This article is protected by copyright. All rights reserved.