The Pressure–Temperature–time–deformation evolution of the high- to ultra-high-pressure units of the Western Alps has been progressively refined in the last 40
years, leading to several paradigm ...shifts in the understanding of orogenesis. This set of information, combined with Cretaceous–Tertiary plate kinematic reconstructions and Mesozoic palaeogeography, indicates that the Western Alps represent the amalgamation of a Cretaceous and an Eocene orogen, which developed at the expense of the Adriatic and European rifted margins, respectively. In the Cretaceous, NNE-directed drift of the Adriatic plate, parallel to the inherited Jurassic structural trends, led to the development of a highly oblique subduction boundary. In this setting, the Canavese Zone and parts of the Sesia Zone, derived from the hyper-extended Adriatic margin, underwent deformation and metamorphism at conditions ranging from sub-greenschist to eclogite facies. A Lower Eocene switch to NNW-directed motion, perpendicular to the rift-related Jurassic structural trends, culminated in the collision between the proto-Alps and the Briançonnais block at ca. 44
Ma. As a result, the Briançonnais Domain and parts of its hyper-extended margin, preserved in the (U)H
P Piemonte Units, were accreted to the orogen. Continued convergence was subsequently accommodated by subduction of the more external Valaisan basin and of the thinned European margin. The arrival of thick European continental crust at the subduction zone at ca. 35–30
Ma marked the onset of the final continental collision.
To a first order, the orogen grew through the progressive episodic accretion of units located towards north-westerly positions. Accretion of subducting units at the front of the orogen was coeval with kinematic reworking of tectonic contacts in the hangingwall units, locally resulting in renewed deformation/metamorphism.
The Main Central Thrust Zone (MCTZ) is a key tectonic feature in the architecture of the Himalayan chain. In the Arun valley of the eastern Nepal Himalaya, the MCTZ is a strongly deformed package of ...amphibolite- to granulite-facies metapelitic schist and granitic orthogneiss. This package is tectonically interposed between the underlying, low-grade, Lesser Himalaya sequences and the overlying, high-grade and locally anatectic, Higher Himalayan Crystallines (HHC). The MCTZ is characterized by a well documented inverted metamorphism from the Grt–Bt zone, across the Ky-in, St-in and -out, Kfs-in, Ms-out and Sil-in isograds. Partial melting with local occurrence of migmatitic segregations has been rarely reported from the highest structural levels of the MCTZ.
While it is widely accepted that thrusting along the MCT occurred during the Miocene, geochronological data constraining the timing of crustal anatexis in the upper portion of the MCTZ are still lacking. In order to understand the link between partial melting in the MCTZ and the Miocene activation of the MCT, we present the
P–
T–time evolution of a kyanite-bearing anatectic gneiss occurring at the highest structural levels of the MCTZ, along the Arun–Makalu transect (eastern Nepal).
Microstructural observations combined with
P–
T pseudosection analysis show that dehydration partial melting occurred in the kyanite-field. After reaching peak conditions at about 820
°C, 13
kbar, the studied sample experienced decompression accompanied by cooling down to 805
°C, 10
kbar, which caused
in situ melt crystallization. SHRIMP monazite and zircon geochronology provides evidence that the anatexis affecting the upper portion of the MCTZ occurred during Early Oligocene (∼
31
Ma). These results demonstrate that in the upper MCTZ, at least in the eastern Himalaya, crustal anatexis was earlier than, and not a consequence of, decompression linked to exhumation along the MCT.
Different lithologies (impure marble, eclogite and granitic orthogneiss) sampled from a restricted area of the coesite-bearing Brossasco-Isasca Unit (Dora Maira Massif) have been investigated to ...examine the behaviour of 40Ar-39Ar and Rb-Sr systems in phengites developed under ultrahigh-pressure (UHP) metamorphism. Mineralogical and petrological data indicate that zoned phengites record distinct segments of the P-T path: prograde, peak to early retrograde in the marble, peak to early retrograde in the eclogite, and late retrograde in the orthogneiss. Besides major element zoning, ion microprobe analysis of phengite in the marble also reveals a pronounced zoning of trace elements (including Rb and Sr). 40Ar-39Ar apparent ages (∼35-62 Ma, marble; ∼89-170 Ma, eclogite; ∼35-52 Ma, orthogneiss), determined through Ar laserprobe data on phengites (step-heating and in situ techniques), show wide intra-sample and inter-sample variations closely linked to within-sample microchemical variations: apparent ages decrease with decreasing celadonite contents. These data confirm previous reports on excess Ar and, more significantly, highlight that phengite acted as a closed system in the different lithologies and that chemical exchange, not volume diffusion, was the main factor controlling the rate of Ar transport. Conversely, a Rb-Sr internal isochron from the same eclogite yields an age of ∼36 Ma, overlapping with the time of the UHP metamorphic peak determined through U-Pb data and thereby corroborating the previous conclusion that UHP metamorphism and early retrogression occurred in close succession. Different phengite fractions of the marble yield calcite-phengite isochron ages of ∼36 to ∼60 Ma. Although this time interval matches Ar ages from the same sample, Rb-Sr data from phengite are not entirely consistent with the whole dataset. According to trace element variations in phengite, only Rb-Sr data from two wet-ground phengite separates, yielding ages of ∼36 and ∼41 Ma, are internally consistent. The oldest age obtained from a millimetre-sized grain fraction enriched in prograde-peak phengites may represent a minimum age estimate for the prograde phengite relics. Results highlight the potential of the in situ 40Ar-39Ar laser technique in resolving discrete P-T stages experienced by eclogite-facies rocks (provided that excess Ar is demonstrably a negligible factor), and confirm the potential of Rb-Sr internal mineral isochrons in providing precise crystallization ages for eclogite-facies mineral assemblages.
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.
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.
Identifying the location and nature of the Main Central Thrust Zone (MCTZ) is a major challenge in most of the Himalayan chain. As a contribution to clarifying this conundrum, in eastern Nepal a ...number of metapelite samples were selected for petrological study along a transect on the eastern flank of the Arun Tectonic Window. Both to the west and east of the study area, the Makalu and Kangchendzonga transects show metamorphic units characterized by a well-documented inverted metamorphism, with metamorphic grade increasing northward from lower (Lesser Himalaya) to higher (High Himalayan Crystallines) structural levels across the MCTZ. A detailed petrological study of the selected metapelite sequence allowed us to recognize three superimposed tectonometamorphic units, which are separated by cryptic and transitional metamorphic discontinuities. These units are characterized by different P–T evolutions, peculiar zoning styles of garnets and contrasting T/depth ratios. Specifically: (1) the structurally lowest sample shows a prograde P–T path characterized by an increase in both P and T, up to peak metamorphic conditions of 550°C and 0·65 GPa; (2) two structurally intermediate samples preserve relics of a prograde history characterized by heating and decompression up to peak metamorphic conditions of 600–650°C and 0·85–0·95 GPa; (3) the structurally highest samples consist of mostly unzoned minerals and assemblages, interpreted as chemically equilibrated, that do not preserve relics of their prograde metamorphic history. Peak metamorphic conditions of 655°C, 0·75 GPa (still inside the white mica stability field), and a minimum temperature of 790°C at 1·05 GPa (beyond the stability limit of white mica) have been determined for these samples. These data provide new constraints on the location, tectonic setting and metamorphic evolution of the Himalayan units in a hitherto poorly known area.
The age of high-pressure metamorphism is crucial to identify a suitable tectonic model for the vast Variscan orogeny. Banded H
P granulites from the Gesso-Stura Terrain in the Argentera Massif, ...Italy, have been recently described (Ferrando et al., 2008) relicts of high-pressure metamorphism in the western part of the Variscan orogen. Bulk rock chemistry of representative lithologies reveals intermediate silica contents and calc-alkaline affinity of the various cumulate layers. Enrichment in incompatible elements denotes a significant crustal component in line with intrusion during Ordovician rifting. Magmatic zircon cores from a Pl-rich layer yield scattered ages indicating a minimum protolith age of 486
±
7
Ma. Carboniferous zircons (340.7
±
4.2 and 336.3
±
4.1
Ma) are found in a Pl-rich and a Pl-poor layer, respectively. Their zoning, chemical composition (low Th/U, flat HREE pattern and Ti-in-zircon temperature) and deformation indicate that they formed during the high-pressure event before decompression and mylonitisation. The proposed age for high-pressure metamorphism in the Argentera Massif proves that subduction preceded anatexis by less than 20
Ma. The new data allow a first-order comparison with the Bohemian Massif, which is located at the eastern termination of the Variscan orogen. Similarities in evolution at either end of the orogen support a Himalayan-type tectonic model for the entire European Variscides.
The 1:25,000 scale geological map of the Monviso massif encompasses an area of 61 km
2
, where different tectonic units of the Monviso Meta-ophiolite Complex (i.e. the Basal Serpentinite Unit, the ...Lago Superiore Shear Zone, the Viso Mozzo Unit, the Forciolline Unit and the Vallanta Unit) and different Quaternary deposits (i.e. alluvial, gravitative, glacial and periglacial deposits) have been distinguished. The Monviso Meta-ophiolite Complex is an important remnant of the Mesozoic Tethyan lithosphere stacked in the Western Alps, and consists of various sequences of serpentinized peridotites, metagabbros, metabasalts and metasediments, that are deformed by syn- to late-metamorphic folds and displaced by syn- to post-metamorphic faults. The geological map presented here provides new detailed lithological, structural and morphological data regarding (i) the tectonostratigraphy of the central sector of the Monviso Meta-ophiolite Complex and (ii) the Quaternary glacial and post-glacial evolution of the Monviso massif.
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–Nyönno 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.
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