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Post-Variscan Early Permian magmatism is widespread from Corsica to the Eastern Alps. After the formation of the Variscan orogenic belt between Laurussia and Gondwana, felsic and mafic bodies were emplaced during an extensional tectonic phase. This study focuses on a mafic dyke swarm that intruded in the region of Ajaccio (Corsica, France) and on a gabbroic intrusive complex outcropping in the Eastern Alps (Bressanone/Brixen, South Tyrol, Italy) both of which were emplaced during this extensional event. New U-Pb data from zircon show that both of these intrusions were emplaced at ca. 282 Ma. Most Ajaccio dykes display a calc-alkaline affinity and are characterized by enrichment of LILE (Large Ion Lithophile Elements) and LREE over HREE (Light and Heavy Rare Earth Elements) and HFSE (Nb, Ta, Hf, Zr; High Field Strength Elements). Two dykes show tholeiitic affinity and are characterized by marked LREE and Th-U depletion and by minor negative Nb-Ta anomalies on multi-element diagrams, which is typical for within-plate basalts. Bressanone/Brixen gabbros display similar geochemical features to the Ajaccio calc-alkaline samples with LREE enrichment over MREE (Medium Rare Earth Elements) and HREE, and HFSE depletion. Such characteristics are also confirmed by trace element data obtained by LA-ICP-MS on clinopyroxene and plagioclase crystals. The calc-alkaline to tholeiitic dykes are characterized by enriched to depleted Sr-Nd-Pb isotopic compositions, respectively. Some Ajaccio dykes and the Bressanone/Brixen gabbros are clearly affected by crustal contamination. However, the combination of trace element and isotopic variations suggests that mantle source heterogeneities were also involved. Isotopic compositions of the Ajaccio dykes reveal the presence of two distinct mantle source signatures: an enriched mantle (EM) component, which dominates the composition of the calc-alkaline-like suite, while a DMM (Depleted MORB Mantle) component dominates the source of the tholeiitic suite. The EM mantle source likely formed during Variscan subduction, however the composition of the tholeiitic suite requires a source composed dominantly of depleted mantle, with a minor contribution from both the subducting slab and supra-subduction mantle wedge. The mantle source of the tholeiitic magmas possibly originated from greater depths (compared to the calc-alkaline-like suite) during the post-orogenic extensional phase potentially due to break-off and/or roll-back of the Variscan slab or mantle flow from the front or margins of the subducted slab. Notably, coeval Permian mafic intrusive bodies from throughout Corsica (Bocca di Tenda, Porto, Pila Canale) and from the Australpine and Southalpine Alps intruded at a similar time and have geochemical features that are very similar to the Ajaccio dyke swarm and Brixen gabbro. This indicates that a widespread Permian magmatic province developed in a post-orogenic extensional tectonic setting at the margin of the former Variscan belt. During the Middle-Late Triassic magmatism with similar enriched geochemical signatures occurred in the Southern Alps, Dinarides, Australpoine domains and at the end of the Triassic over central Pangea (Central Atlantic Magmatic Province). This would highlight the importance that the Variscan orogeny had in the development of a more fertile mantle.

The Himalayan belt has been long studied to deduce large-scale tectonics of shear zones regarding both their kinematics and the age of their timing, especially in the frame of exhumation of the Greater Himalayan Sequence (GHS), the metamorphic core of the orogenic belt. The GHS is delimited at its bottom by an orogenscale shear zone, the Main Central Thrust zone (MCTz), a top-to-the-SW km-thick zone of intensively sheared rocks. As quantitative vorticity analyses in deformed rocks and isotope dating are fundamental in the study of the kinematics of flow and time of activity in shear zones, four samples have been investigated to infer these parameters along the two tectonic boundaries of the MCTz in the Bhagirathi valley: the Munsiari (lower) and the Vaikrita Thrust (upper). To constrain the timing of deformations recorded within the Munsiari and Vaikrita Thrust rocks we undertook 40Ar/39Ar dating. We prepared biotite and muscovite separates with different degrees of purity in order to quantify the bias given by fine-grained impurity phases, which make 100% mica purity unattainable. Combining Argon Differential Release Plots (DRP) with EPMA and Ca-Cl-K signatures of each mica populations we identified the step ages dating deformation. Microstructures of the Munsiari Thrust show the occurrence of a main disjunctive foliation defined by biotite and minor muscovite. 40Ar/39Ar stepheating coupled with correlation diagrams constrains biotite growth on the main foliation at c. 5 Ma. The microstructures of mylonitic micaschists from the Vaikrita Thrust include three different textural generation of micas: a relict foliation, a main mylonitic foliation, and a late generation of static muscovite and chlorite overprinting the relict foliation. 40Ar/39Ar dating constrains muscovite growth on the main foliation around 10 Ma and the growth of large, static muscovite at c. 8 Ma. In the study area, Bhagirathi valley (NW India), our results support an in-sequence shearing from Vaikrita to Munsiari Thrust from c. 10 to c. 5 Ma. Adding information about the kinematic of flow to the age of the bounding shear zones of the GHS is a key purpose for its exhumation models. The use of stable porphyroclasts analysis used for vorticity estimates can suffer from severe limitations, because of the reduction to two dimensions of motion of rigid clasts that is a complex 3D problem. We propose an alternative method to acquire data based on the use of X-ray micro computed tomography that allows to considerably decrease the limitation of the method. We selected the mylonitic orthogneisses from the Munsiari Thrust containing K-feldspar porphyroclasts in order to estimate the kinematic vorticity, which ranges between 0.49 and 0.57. We stress here the importance of a multidisciplinary approach based on detailed meso and microstructural, chemical and geochronological investigations of pervasively sheared rocks from shear zones having multiple generations of fine-grained foliations.

In the Alaknanda-Dhauli Ganga valleys (Garhwal Himalaya, NW India) a nearly complete crustal- structural section of the Himalaya crops out (Jain et al., 2014). In this contribution we present field observations, as well as, microstructural, petrological and geochronological data from the Lesser Himalayan Sequences up to the Tethyan Himalayan Sequence from this area. The pressure-temperature-deformation-time history (i.e. the P-T-d-t path) of selected samples from the Main Central Thrust zone (MCTz) and the up to ductile-sheared portion of the South Tibetan Detachment System (STDS) has been reconstructed with the aid of equilibrium assemblage diagrams, coupled with multiequilibrium and trace-element based thermobarometry after detailed electron microprobe analytical work and microstructural analysis have been performed. U-Th-Pb in situ monazite geochronology from selected samples of key-structural positions (MCTZ up to STDS) allowed us to put an absolute temporal constraint both on the prograde metamorphic history and on the exhumation-related overprint. These data, joined with the geological literature (e.g. Thakur et al., 2015; Hunter et al., 2018), shed light on the tectono-metamorphic evolution of the Himalayan metamorphic core in this portion of the belt. Moreover, these new P-T-d-t paths could be quite well compared with data from other portions of the belt located in Central and Central-Western Nepal (e.g. Iaccarino et al., 2017), since the same approach was followed. Hunter N.J.R., Weinberg R.F., Wilson CJ.L., Luzin W. & Misra S. (2018) - Microscopic anatomy of a “hot-on-cold” shear zone: Insights from quartzites of the Main Central Thrust in the Alaknanda region (Garhwal Himalaya). GSA Bulletin, 130, 1519-1539. Iaccarino S., Montomoli C., Carosi R., Massonne H.-J., Visonà D. (2017) - Geology and tectono-metamorphic evolution of the Himalayan metamorphic core: insights from the Mugu Karnali transect, Western Nepal (Central Himalaya). J. Metam. Geol., 35, 301-325. Jain A.K., Shreshtha M., Seth P., Kanyal L., Carosi R., Montomoli C., Iaccarino S. & Mukherjee P.K. (2014) - The Higher Himalayan Crystallines, Alaknanda – Dhauli Ganga Valleys, Garhwal Himalaya, India. J. Virtual Explorer, 47, Paper 8. Thakur S.S., Patel S.C. & Singh A.K. (2015) - A P-T pseudosection modelling approach to understand metamorphic evolution of the Main Central Thrust Zone in the Alaknanda valley, NW Himalaya. Contrib. Mineral. Petrol., 170, 1-26.

New tectono-metamorphic data along the Alaknanda-Dhauli Ganga valleys in the Garhwal Himalaya (NW India) are presented focusing on sheared rocks of the Main Central Thrust zone (MCTz), one of the first order regional structures in the Himalaya. In regard of large hot orogens (LHOs), the MCTz corresponds to the base of the low-viscosity infrastructure. Our new findings support that the MCTz is a c. 4 km thick mylonitic zone, affecting different lithologies and encompassing a possible (previously defined) protolith boundary. Four selected mylonitic samples, at different structural position along the MCTz, were studied with a multi-disciplinary approach in detail, supporting that an inverted metamorphic field gradient, with temperature (T) and pressure (P) ranging from c. 500 up to c. 700 °C and c. 0.50–0.60 GPa up to >1.0 GPa, was present. This inverted P-T array is also discernable by the distribution of Al-rich minerals (e.g. chloritoid, staurolite, kyanite) in the intercalation of sheared quartzite. In situ U-(Th)-Pb monazite geochronology indicates that the high-T shearing in the upper part of the MCTz mylonites developed from c. 20 Ma on after a stage of prograde metamorphism that lasted until c. 23 Ma. The integration of our data with available structural, metamorphic and geochronological information along the MCTz profile, in the study area, sheds light on the development of the associated inverted metamorphic sequence (IMS). These data carry also empirical finite patterns to be tested against theoretical models investigating the possible roles of viscous heating and “tectonic overpressure” in the development of the IMS in the Himalaya, and possibly in LHOs in general. Both phenomena, if have been active at all, are not obvious in regard of the current state of geothermobarometric methods. On the contrary, a model for the tectonic assembly of the IMS is favored. The anomalous apparent P and T inverted field gradients are interpreted to be the result of the differential and diachronous transport along a shear zone experiencing heterogeneous vertical thinning linked to the non-coaxial flow.

A peculiar feature of the Himalaya is the occurrence of a system of low-angle normal faults and shear zones, the South Tibetan Detachment System (STDS), at the mountain crests. The STDS was active during synconvergent tectonics. We describe the STDS-related sheared rocks along the Dhauli Ganga valley, in the Garhwal Himalaya (NW India), where the Malari granite, reported as an undeformed igneous body crosscutting the STDS, occurs. A detailed multidisciplinary study, integrating field-based, microstructural, petrographic, and geochronological analyses, was carried out on rocks along this valley. We demonstrate how the noncoaxial ductile portion of the STDS affected the upper part of the Greater Himalaya Sequence migmatite, which experienced peak pressure (P)-temperature (T) conditions of 0.9–1.1 GPa and ≥750°C at ≥24 Ma. This migmatite has been reworked structurally upward leading to the formation of high-T sillimanite-bearing mylonites. Further upward, medium-T shearing deformed the Malari granite and leucogranite dykes, forming medium-T mylonites. Ductile shearing was temporally constrained, based on new in situ monazite datings and previously published Ar-Ar geochronology, between ~20 and 15 Ma. We demonstrate that a preserved ductile to brittle spatial and temporal transition of the STDS deformation exists, with the brittle features overprinting ductile ones. Our data shed new light on the geological evolution of the STDS in the NW Himalaya with implications for the relationship and relative timing of partial melting, granite emplacement, and deformation along low-angle normal faults.

A peculiar feature of the Himalaya is the occurrence of a system of low-angle normal faults and shear zones, the South Tibetan Detachment System (STDS), at the mountain crests. The STDS was active during synconvergent tectonics. We describe the STDS-related sheared rocks along the Dhauli Ganga valley, in the Garhwal Himalaya (NW India), where the Malari granite, reported as an undeformed igneous body crosscutting the STDS, occurs. A detailed multidisciplinary study, integrating field-based, microstructural, petrographic, and geochronological analyses, was carried out on rocks along this valley. We demonstrate how the noncoaxial ductile portion of the STDS affected the upper part of the Greater Himalaya Sequence migmatite, which experienced peak pressure (P)-temperature (T) conditions of 0.9-1.1GPa and 750 degrees C at 24Ma. This migmatite has been reworked structurally upward leading to the formation of high-T sillimanite-bearing mylonites. Further upward, medium-T shearing deformed the Malari granite and leucogranite dykes, forming medium-T mylonites. Ductile shearing was temporally constrained, based on new in situ monazite datings and previously published Ar-Ar geochronology, between similar to 20 and 15Ma. We demonstrate that a preserved ductile to brittle spatial and temporal transition of the STDS deformation exists, with the brittle features overprinting ductile ones. Our data shed new light on the geological evolution of the STDS in the NW Himalaya with implications for the relationship and relative timing of partial melting, granite emplacement, and deformation along low-angle normal faults.

The South Tasman Rise (STR) consists of the west- and east-South Tasman Rise, which are submerged continental fragments that were part of the East Gondwana Continent. The STR formed following the extension between Antarctica and Australia that generated the Tasmanian Gateway. It is located between Tasmania and Antarctica and represents a vital, but often overlooked, link to understand the geological evolution of both North Victoria Land and Tasmania. Reconstruction of Gondwana shows that STR was contiguous to several continental fragments that are now thousands of km apart. Because of its central location in the plate boundary framework that developed within East Gondwana, the STR underwent all the major tectonic events that led to the dispersal of Gondwana. The STR is therefore a centrepiece in understanding the complex tectonic history of this region. Extensive sampling of STR rocks took place during AGSO cruise 147. Investigation on the metamorphic rocks of the STR pointed out interesting correlations with rocks from the Wilson Terrane and the Lanterman Metamorphics (Antarctica) and from Tasmania (Berry et al., 1977). Based on this evidence, and aiming at clarifying the correlation between mainland Australia, Tasmania and Antarctica, we undertook a detailed petrologic and geochronologic study on magmatic rocks dredged at 10 different sites from South Tasman Rise. Seven of these rocks yield Cambrian ages in the range 487-507 Ma, consistent with the Ross-Delamerian orogen; one sample is a granite gneiss of late Proterozoic crystallization age (ca. 740 Ma), which corresponds to the Wickham event (Tasmania) or Beardmore orogen (Antarctica); one sample has Grenville age (Fioretti et al., 2005); and one sample is Late Devonian (ca. 363 Ma), an age that is well represented in Australia, Tasmania and Antarctica. Petrographic observation, geochemical investigation and zircon typologic study are presented here to characterize these rocks, and to test their possible correlation with coeval rocks in Antarctica and Australia.

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 tectonicmetamorphic 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 core of the Greater Himalayan Sequence in the Mugu-Karnali area (Western Nepal) is affected by a thick shear zone with development of nearly 4 km of mylonites (Mangri shear zone). It is a contractional shear zone showing a top-to-the-SW and WSW sense of shear. The shear zone developed during the decompression, in the sillimanite stability field, of rocks that previously underwent relatively high-pressure metamorphism deformed under the kyanite stability field. P–T conditions indicate that the footwall experienced higher pressure (1.0–0.9 GPa) than the hanging wall (0.7 GPa) and similar temperatures (675°–700 °C). U–Pb in-situ dating of monazites indicate a continuous activity of the shear zone between 25 and 18 Ma. Samples from the lower part of the Greater Himalayan Sequence underwent similar ductile shearing at ~ 17–13 Ma. These ages and the associated P–T–t paths revealed that peak metamorphic conditions were reached ~ 5–7 Ma later in the footwall of the shear zone with respect to the hanging-wall pointing to a diachroneity in the metamorphism triggered by the shear zone itself. Mangri Shear Zone, with the other recently documented tectonic and metamorphic discontinuities within the Greater Himalayan Sequence, point to the occurrence of a regional tectonic feature, the High Himalayan Discontinuity, running for more than 500 km along the strike of the Central Himalayas. It was responsible of the exhumation of the upper part of the Greater Himalayan Sequence starting from 28 Ma, well before the activation of the Main Central Thrust and the South Tibetan Detachment. Our data point out that exhumation of the Greater Himalayan Sequence was partitioned in space and time and different slices were exhumed in different times, starting from the older in the upper part to the younger in the lower one.

Note illustrative Foglio geologico scala 1:50.000