Search

Abstract Investigating the carbonate preservation efficiency (CPE) of continental crust is crucial to understand the global carbon cycle, which requires constraints on initial carbonate abundances (ICAs) of crustal rocks. To link Mg isotopes to ICAs, we present elemental and Mg isotopic data for Himalayan carbonate‐bearing and carbonate‐free metasedimentary rocks. Given no evident melt extraction or external‐fluid infiltration, ICAs of these samples can be independently estimated by elemental data. Despite different carbonate species in the protoliths, all the samples show congruent relationship between their δ26Mg and ICAs, owing to the elevated carbonate δ26Mg and Mg/Ca in protoliths of calcite‐rich samples resulting from diagenetic processes. When collated with literature data, we suggest the observed correlation here can be applied to most carbonate‐bearing (meta‐)sedimentary rocks. Based on a steady state box‐model, we constrained the modern net carbonate accretion flux (9.50−5.56+9.50 Tmol/year) and the average time‐integrated CPE (∼80−43+20%) for continental crust.

Outgassing of CO2 during collisional orogeny can be explained by the generation of immiscible CO2-rich fluids and hydrosaline brines in metamorphosed sediments, according to thermodynamic modelling applied to the Himalaya.

Abstract The Banchetta-Rognosa tectonic unit (BRU), covering an area of 10 km2 in the upper Chisone valley, consists of two successions referred to a continental margin (Monte Banchetta succession) and a proximal oceanic domain (Punta Rognosa succession) respectively. In both successions, Mesozoic meta-sedimentary covers discordantly lie on their basement. This paper presents new data on the lithostratigraphy and the metamorphic evolution of the continental basement of the Monte Banchetta succession. It comprises two meta-sedimentary sequences with minor meta-intrusive bodies preserving their original lithostratigraphic configuration, despite the intense Alpine deformation and metamorphic re-equilibration. Phase equilibrium modeling points to a metamorphic eclogitic peak (D1 event) of 20–23 kbar and 440–500 °C, consistent among three different samples, analyzed from suitable lithologies. The exhumation P–T path is characterized by a first decompression of at least 10 kbar, leading to the development of the main regional foliation (i.e. tectono-metamorphic event D2). The subsequent exhumation stage (D3 event) is marked by a further decompression of almost 7–8 kbar associated with a significant temperature decrease (cooling down to 350–400 °C), implying a geothermal gradient compatible with a continental collision regime. These data infer for this unit higher peak P–T conditions than previously estimated with conventional thermobarometry. The comparison of our results with the peak P–T conditions registered by other neighboring tectonic units allows to interpret the BRU as one of the westernmost eclogite-facies unit in the Alps.

In the Valmala sector of the southern Dora Maira Massif (Western Alps), two different eclogite- and blueschist-facies units (i.e. the Rocca Solei and Dronero units, respectively), are separated by a shear zone (i.e. the Valmala Tectonic Unit), which peculiarly consists of mixed slices of ophiolitic and continental rocks. A detailed geological map at 1:10,000 scale allowed to point out that the tectonic slices within the Valmala Tectonic Unit consist of ‘native’ rock slices wrenched from the overlying Dronero Unit, and ‘exotic’ rocks likely sourced from other units of the Dora Maira and from a continental margin and an oceanic basin. On the contrary, rock slices sourced from the underlying Rocca Solei Unit are lacking. The overall tectonic stack results after an early subduction-related deformation phase (i.e. the D1), and the pervasive overprinting of two subsequent exhumation-related deformation phases (i.e. the D2 and D3). The Valmala Tectonic Unit is inferred to have played a role in decoupling the southern Dora Maira Massif during subduction, and/or in driving exhumation of the ultra-high pressure rocks occurring in the adjoining Brossasco-Isasca Unit.

Geological mapping, stratigraphic observations, and U/Pb dating allow reconstructing the pre-orogenic setting of the transition zone between the distal European passive margin and the Alpine Tethys in the southwestern Alps. Although convergent tectonics overprinted the syn-rift Jurassic tectonic features, our data document an articulated Jurassic physiography. From the distal European passive margin oceanward, we distinguished: the Dronero Unit (the southernmost Dora Maira massif), represents a continental margin composite basement wherein monometamorphic metasediments are interlayered with Late Permian (253.8 ± 2.7 Ma) metavolcanic rocks; the Sampeyre Unit, represents a structural high consisting of Lower Triassic Verrucano-facies siliciclastic metasediments unconformably sealed by Cretaceous calcschist bearing Globotruncana sp.; the Maira Unit, corresponds to a Middle Triassic platform succession detached from the Sampeyre Unit; the Grana Unit, corresponds to a Late Triassic–Late Jurassic platform to basin succession; the Queyras Schistes Lustrès Complex, represents the ocean basin succession. Tectonic slices of Cambrian (513.9 ± 2.7 Ma) metadiorite hosted in the Valmala Shear Zone, separating the Dronero Unit from the underlying (U)HP units of the Dora Maira massif, suggests a potential pre-Alpine activation of the shear zone.

Earthquakes rarely affect hydrothermal systems in non-magmatic context. Here the authors report outbursts of CO2 and hydrothermal disturbances triggered by the 2015 Nepal earthquake, revealing high sensitivity of Himalayan hydrothermal systems to co-, post- and possibly pre- seismic deformation.

The article discusses the study regarding the high-T eclogites and its metamorphic evolution from the North Dabie complex zone (NDZ) in China during continental subduction and exhumation. Topics discussed include the ultrahigh-pressure (UHP) and high-pressure eclogite-facies metamorphism, the effect of Cretaceous partial melting and migmatization on the identification of metamorphic stages for the eclogites, and the application of petrological and geochronological investigation of eclogites.

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. [ABSTRACT FROM AUTHOR]