The crystalline units of the High Himalayas in the Lahul-Zanskar region (northwest India): metamorphic-tectonic history and geochronology of the collided and imbricated Indian plate

In the High Himalayan belt of northwest India, crustal thickening linked to Palaeogene collision between India and Eurasia has led to the formation of two main crystalline tectonic units separated by the syn-metamorphic Miyar Thrust: the High Himalayan Crystallines sensu stricto (HHC) at the bottom, and the Kade Unit at the top. These units are structurally interposed between the underlying Lesser Himalaya and the very low-grade sediments of the Tibetan nappes. They consist of paragneisses, orthogneisses, minor metabasics and, chiefly in the HHC, leucogranites. The HHC registers: a polyphase metamorphism with two main stages designated as M1 and M2; a metamorphic zonation with high-temperature recrystallization and migmatization at middle structural levels and medium-temperature assemblages at upper and lower levels. In contrast, the Kade Unit underwent a low-temperature metamorphism. Rb-Sr and U-Th-Pb isotope data point to derivation of the orthogneisses from early Palaeozoic granitoids, while the leucogranites formed by anatexis of the HHC rocks and were probably emplaced during Miocene time. Most of the complicated metamorphic setting is related to polyphase tectonic stacking of the HHC with the ‘cooler' Kade Unit and Lesser Himalaya during the Himalayan history. However, a few inconsistencies exist for a purely Himalayan age of some Ml assemblages of the HHC. As regards the crustal-derived leucogranites, the formation of a first generation mixed with quartzo-feldspathic leucosomes was possibly linked to melt-lubricated shear zones which favoured rapid crustal displacements; at upper levels they intruded during stage M2 and the latest movements along the syn-metamorphic Miyar Thrust, but before juxtaposition of the Tibetan nappes along the late- metamorphic Zanskar Faul

Permian high-temperature metamorphism in the Western Alps (NW Italy)

During the late Palaeozoic, lithospheric thinning in part of the Alpine realm caused high-temperature low-to-medium pressure metamorphism and partial melting in the lower crust. Permian metamorphism and magmatism has extensively been recorded and dated in the Central, Eastern, and Southern Alps. However, Permian metamorphic ages in the Western Alps so far are constrained by very few and sparsely distributed data. The present study fills this gap. We present U/Pb ages of metamorphic zircon from several Adria-derived continental units now situated in the Western Alps, defining a range between 286 and 266 Ma. Trace element thermometry yields temperatures of 580-890°C from Ti-in-zircon and 630-850°C from Zr-in-rutile for Permian metamorphic rims. These temperature estimates, together with preserved mineral assemblages (garnet-prismatic sillimanite-biotite-plagioclase-quartz-K-feldspar-rutile), define pervasive upper-amphibolite to granulite facies conditions for Permian metamorphism. U/Pb ages from this study are similar to Permian ages reported for the Ivrea Zone in the Southern Alps and Austroalpine units in the Central and Eastern Alps. Regional comparison across the former Adriatic and European margin reveals a complex pattern of ages reported from late Palaeozoic magmatic and metamorphic rocks (and relics thereof): two late Variscan age groups (~330 and ~300 Ma) are followed seamlessly by a broad range of Permian ages (300-250 Ma). The former are associated with late-orogenic collapse; in samples from this study these are weakly represented. Clearly, dominant is the Permian group, which is related to crustal thinning, hinting to a possible initiation of continental rifting along a passive margin

A geological transect from Kun Lun to Karakorum (Sinkiang, China): the western termination of the Tibetan Plateau. Preliminary note

A geological-geophysical expedition (Ev-K2-CNR 1988) visited the area from West Kun Lun to Karakorum (K2-Gasherbrum). Seven tectonic units including sedimentary, magmatic and metamorphic rocks were distinguished in this area; the northernmost are suggested to belong to the Kun Lun and Qiangtang Microplates. The sedimentary sequence of Shaksgam is proved to extend from the Permian to the Jurassic, with Carboniferous and Cretaceous ages more doubtful. This sequence shows intermediate affinities between the Karakorum and the Qiangtang. The two southernmost units belong to the Karakorum Microplate. The Karakorum Fault Zone comprises a complex pattern of faults and thrusts, with brittle deformation and uplifting of granitoid bodies

Incomplete blueschist re-crystallization in high grade metamorphics from the Sesia-Lanzo unit (Vasario-Sparone subunit, Western alps): a case history of metastability

The Vasario-Sparone (VS) subunit is part of the Sesia-Lanzo unit (Western Alps) and represents a thin and discontinuous slice of pre-Alpine deep continental crust. It is tectonically interposed between the "eclogitic mica schists" and the "gneiss minuti" subunits. Metastable relics and a sequence of incomplete transformations controlled by a local equilibrium reaction mechanism are preserved in the VS subunit. The VS rocks record a pre-Alpine high grade of metamorphism (T=780±50°C, P≈6 kbar) which is comparable to the metamorphism dated around 500 Ma in the Ivrea Zone. During the Alpine evolution the VS rocks sustained an early blueschist stage (T=300-400°C, P≥8-10 kbar) of probable Cretaceous age and a later low-T, low-P stage of Tertiary age. The blueschist transformations are governed by slow reaction kinetics, by the heterogeneous distribution of pervasive deformations and by syn-metamorphic fluid phase. In particular, transient kyanite formed in the early stages of the blueschist re-crystallization in response to a process controlled by fluid-deficient conditions and by internal buffering of aH2O at low value