Under pressure: High-pressure metamorphism in the alps

The mechanisms attending the burial of crustal material and its exhumation before and during the Alpine orogeny are controversial. New mechanical models propose local pressure perturbations deviating from lithostatic pressure as a possible mechanism for creating (ultra-)high-pressure rocks in the Alps. These models challenge the assumption that metamorphic pressure can be used as a measure of depth, in this case implying deep subduction of metamorphic rocks beneath the Alpine orogen. We summarize petrological, geochronological and structural data to assess two fundamentally distinct mechanisms of forming (ultra-)high-pressure rocks: deep subduction; or anomalous, non-lithostatic pressure variation. Furthermore, we explore mineral-inclusion barometry to assess the relationship between pressure and depth in metamorphic rocks

Locally Resolved Stress-State in Samples During Experimental Deformation: Insights Into the Effect of Stress on Mineral Reactions

Understanding conditions in the Earth's interior requires data derived from laboratory experiments. Such experiments provide important insights into the conditions under which mineral reactions take place as well as processes that control the localization of deformation in the deep Earth. We performed Griggs-type general shear experiments in combination with numerical models, based on continuum mechanics, to quantify the effect of evolving sample geometry of the experimental assembly. The investigated system is constituted by CaCO3 and the experimental conditions are near the calcite-aragonite phase transition. All experimental samples show a heterogeneous distribution of the two CaCO3 polymorphs after deformation. This distribution is interpreted to result from local stress variations. These variations are in agreement with the observed phase-transition patterns and grain-size gradients across the experimental sample. The comparison of the mechanical models with the sample provides insights into the distribution of local mechanical parameters during deformation. Our results show that, despite the use of homogeneous sample material (here calcite), stress variations develop due to the experimental geometry. The comparison of experiments and numerical models indicates that aragonite formation is primarily controlled by the spatial distribution of mechanical parameters. Furthermore, we monitor the maximum pressure and σ1 that is experienced in every part of our model domain for a given amount of time. We document that local pressure (mean stress) values are responsible for the transformation. Therefore, if the role of stress as a thermodynamic potential is investigated in similar experiments, an accurate description of the state of stress is required

Constraining the P-T conditions of melting in stromatic migmatites from Ronda (S Spain).

none4noneL. Tajcmanova; O. Bartoli; B. Cesare; A. Acosta-VigilL., Tajcmanova; Bartoli, Omar; Cesare, Bernardo; A., Acosta Vigi

Alpine burial and heterogeneous exhumation of Variscan crust in the West Carpathians: Insight from thermodynamic and argon diffusion modelling.

International audienc

Grain-scale pressure variations and chemical equilibrium in high-grade metamorphic rocks

ISSN:0263-4929ISSN:1525-131

Geochemistry of Eocene-Early Oligocene low-temperature crustal melts from Greater Himalayan Sequence (Nepal): a nanogranitoid perspective

Despite melt inclusions in migmatites and granulites provide a wealth of information on crustal anatexis in different geological scenarios, a complete compositional study (including trace elements and H2O) is yet to be made for the Himalayan rocks. In this contribution, we focus on nanogranitoids occurring in peritectic garnet of migmatites from Kali Gandaki valley in central Nepal (Greater Himalayan Sequence, GHS). The microstructural position of the nanogranitoids proves that these melts were produced at 650-720 degrees C and 1.0-1.1 GPa, during the Eohimalayan prograde metamorphism (41-36 Ma) associated with crustal thickening. Nanogranitoid compositions (mostly granodiorites, tonalities and trondhjemites) resemble those of experimental melts produced during H2O-present melting of meta-sedimentary rocks. They have variable H2O concentrations (6.5-14.4 wt%), which are similar to the expected minimum and maximum values for crustal melts produced at the inferred P-T conditions. These compositional signatures suggest that melt formation occurred in the proximity of the H2O-saturated solidus, in a rock-buffered system. The low-to-very low contents of Zr (3-8 ppm), Th (0.1-1.2 ppm) and LREE (4-11 ppm) along with the weak-to-moderate positive Eu anomalies (Eu/Eu* = 1.2-3.3), the high B concentrations (200-3400 ppm) and the high U/Th ratio (up to 21) point to the lack of equilibration between melt and accessory minerals and are consistent with melting of plagioclase at low temperature. Kali Gandaki nanogranitoids record the beginning of melting in a H2O-present system that, in other GHS localities, may have produced voluminous crustal melts. We demonstrate that compositional comparison with nanogranitoids may be useful to reconstruct the petrogenesis of Eohimalayan granitoids