<p><span><span>The granulite occurrences from the Moldanubian zone were extensively studied in the last three decades and their metamorphic overprint at high pressures and at UHT conditions are well constrained. However, there are still some discrepancies regarding the prograde PT-path evolution, the genesis of the granulites and the tectonic processes required to produce the proposed PT-paths. Here we present a comprehensive petrological study where we have investigated more than 300 granulite samples from one of the largest occurrences, the Poechlarn-Wieselburg area - Dunkelsteinerwald. C</span><span>onventional geothermobarometry, garnet zoning pattern, thermodynamic modelling and Zr-in-rutile thermometry on rutile grains enclosed in garnets in felsic and mafic granulites allowed to constrain the prograde as well as the retrograde segments of the PT path. Polycrystalline melt inclusions and high-Ti biotite relics as well as a uniform temperature of approximately 800&#176;C obtained from rutile inclusions (Zr-in-rutile thermometry) in garnet cores disagree with a continuous prograde garnet growth but favour a metastable overstepping of the garnet-in reaction and growth by the peritectic biotite breakdown reaction to garnet and melt within a very narrow PT interval. Subsequent heating to T>1000&#176;C initiated a second stage of garnet growth with a very distinct chemical composition. The preservation of the zoning pattern at these metamorphic conditions clearly document a very short lived process. Diffusion models predict a time span of <5 Ma and cooling rates of 50-60&#176;C/my.</span><span> Zircon U-Pb ages usually cluster around 340 Ma representing the metamorphic peak. However, in mafic granulites zircon ages from approximately 410 Ma to 340 Ma are obtained indicating either an older formation age for the precursor rock of the mafic granulites or just documenting the occurrence of xenocrysts. We applied a series of coupled petrological&#8211;thermomechanical tectono-magmatic numerical model to reproduce our deduced PTt-path that evolved from exhumation of subducted lower crust followed by intense heating at the crust-mantle boundary.</span></span></p>
During the last decades, the intrinsic association of ultrahigh-pressure crustal rocks with ultrabasic rocks has been recognised in modern and ancient collisional orogens worldwide but their tectonic origins remain intriguing and controversial. In this study, we performed a series of 2D petrological–thermomechanical numerical experiments of continent-continent collision in order to: (i) characterize the variability of orogenic mantle rocks appearing in the exhumed crust during continental collision, (ii) investigate their relation to the host rocks, (iii) distinguish indicative P–T–t paths of peridotites for different crustal exhumation mechanisms. Based on the modeling results, we have identified 18 genetically different types of peridotites which may occur in the resulting crustal orogens. Peridotites from the mantle wedge are most commonly emplaced, but additionally we found peridotites of crustal origin, asthenospheric peridotites, and also peridotites from lithospheric mantle of the subducting continental plate. While some of these peridotites were initially located close to the subduction zone, others moved over hundreds of kilometres before incorporation into the subducted crust. Our results show that vertical extrusion and trans-lithospheric diapirism are the key exhumation mechanisms by which the largest variety of different peridotite types are emplaced in a collisional orogen. The crustal material associated with the peridotites did not always record the same P–T–t evolution but showed in some cases even higher peak metamorphic pressures. The range of P–T conditions predicted by the peridotites in the experiments cover all fields of P–T conditions recorded by peridotites from natural collisional orogens. Comparing the results from this study with individual P–T–t paths from natural occurrences may allow a better understanding on their origin.
Abstract A series of 2D petrological–thermomechanical numerical experiments was conducted to: (i) characterize the variability of exhumation mechanisms of ultrahigh pressure metamorphic (UHPM) rocks during collision of spontaneously moving plates and (ii) study the possible geodynamic effects of melting at ultrahigh pressure conditions for the exhumation of high‐temperature–ultrahigh pressure metamorphic (HT–UHPM) rocks. To this end, the models include fluid‐ and melt‐induced weakening of rocks. Five distinct modes of exhumation of (U)HPM rocks associated with changes in several parameters in the models of plate collision and continent subduction are identified as follows: vertical crustal extrusion, large‐scale crustal stacking, shallow crustal delamination, trans‐lithospheric diapirism, and channel flow. The variation in exhumation mechanisms for (U)HPM rocks in numerical models of collision driven by spontaneously moving plates contrasts with the domination of the channel flow mode of exhumation in a majority of the published results from numerical models of collision that used a prescribed plate convergence velocity and/or did not include fluid‐ and melt‐induced weakening of rocks. This difference in the range of exhumation mechanisms suggests that the prescribed convergence velocity condition and the neglect of fluid‐ and melt‐related weakening effects in the earlier models may inhibit development of several important collisional processes found in our experiments, such as slab breakoff, vertical crustal extrusion, large‐scale stacking, shallow crustal delamination and relamination, and eduction of the continental plate. Consequently, the significance of channel flow for the exhumation of UHPM rocks may have been overstated based on the results of the earlier numerical experiments. In addition, the results from this study extend over a larger proportion of the high‐temperature range of P–T conditions documented from UHPM rocks, including those retrieved from HT–UHPM rocks, than the results of experiments from previous numerical models. In particular, the highest peak metamorphic temperatures (up to 1000 °C) are recorded in the case of the vertical crustal extrusion model in which subducted continental crust is subjected to a period of prolonged heating by asthenospheric mantle abutting the continental side of the vertically hanging slab. Nonetheless, some extreme temperature conditions which have been suggested for the Kokchetav and Bohemian massifs, perhaps up to 1100–1200 °C, are still to be achieved in experiments using numerical models.
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