<p>The causes for heterogeneous deformation with strain partitioning into kilometre-scale shear zone within the partially molten crust and the spatiotemporal feedback relationships between strain localization and melt organization still remain unclear. In order to tackle these questions and unravel the strain localization in a partially molten crustal scale shear zone, we used field observations and thermodynamic modelling in the Eastern Variscan Shear Zone (EVSZ) located in the Aiguille-Rouge massif (Western Alps). The EVSZ is an orogen scale, 10 km wide and 600 km long, transpressional high strain corridor recognized in the French External Crystalline Massifs. The EVSZ affected the partially-molten late Variscan crust during late Carboniferous times (340-300 Ma). In this contribution we present a detailed field-map survey of the mid- and lower crusts focussed on the partitioning and strain pattern in the Aiguille-Rouge EVSZ. Detailed mapping revealed that high-strain deformation domains and orthogneiss occurrences are spatially related. New petrological, thermobarometrical and LA-ICP-MS dating also better constrain the P-T-t-D evolution of the partially molten crust along the EVSZ. Field observations and P-T pseudosection calculations show that among the three dominant lithologies forming the mid- and lower-crusts, i.e. metapelite, metagreywacke and orthogneiss, the latter is the most fertile if considering H2O-fluid-saturated melting. During prograde evolution at pressure between-12-15 kbar, orthogneisses reached the solidus at lower temperature and produced higher melt fraction than the metasedimentary rocks. &#160;The water-present melting in the orthogneisses may have initiate strain localization at the end of the prograde evolution. Thus, the favoured localization of the shear zone within the metagranites is explained by a higher melt fraction than in the metapelites and metagreywackes. PTDt path and thermobarometrical modelling suggest that these transpressional deformation conditions occurred under suprasolidus conditions from at least 12 kbar to 4 kbar during a near isothermal decompression. During this cooling path, while crystallization of anatectic melts might have provoked strain hardening in the orthogneisses, a strength decrease might be controlled by a higher proportion of micas in metapelites and metagreywackes as suggested by forward modelling of modal proportion of mica. This change in the nature of the weakest phase, starting with melt in metagranites and followed by micas in metasedimentary rocks, seems to control the progressive localization and broadening of the crustal scale shear zone during clockwise P-T-t path. Our results suggest that H2O-fluid-saturated melting of metagranites has a first order rheological impact on the birth and growth of the orogen scale shear zone in the lower continental crust.</p>
Abstract Where, when, and why large-scale shear zones nucleate and propagate into the continental lithosphere are critical issues that challenge the research in tectonics. The East Variscan shear zone is one of the crustal-scale strike-slip faults that shaped the Variscan orogenic crust during late Carboniferous time. Field-based structural analysis and petrological observations demonstrate that suprasolidus high-strain deformation zones and metagranite occurrences are spatially correlated. Among the three dominant lithologies forming this orogenic middle crust (metapelite, metagraywacke, and metagranite), petrological observations and phase equilibrium modeling indicate that the latter is the first lithology that melts during collision-induced heating, in response to H2O-fluid-saturated melting. Our field data and modeling suggest that the water-fluxed melting of metagranite has a primary rheological control on the localization, instigation, and growth of crustal-scale shear zones in the middle crust. Thus, the distribution and geometry of metagranite at the crustal scale could be regarded as critical parameters influencing the rheological inheritance governing the tectonic evolution and localization of bulk strain in the continental lithosphere.
Abstract A lower amphibolite Alpine shear zone from the Fibbia metagranite (Gotthard Massif, Central Alps) has been studied to better understand the parameters controlling strain localization in granitic rocks. The strain gradient on the metre‐scale shows an evolution from a weakly deformed metagranite (Qtz I –Kfs I –Ab I –Bt I ± Pl II –Zo I –Phg I –Grt) to a fine banded ultramylonite (Qtz II –Kfs II –Ab II –Pl II –Bt II –Phg II ± Grt–Zo II ). Strain localization is coeval with dynamic recrystallization of the quartzofeldspathic matrix and a modal increase in mica, at the expense of K‐feldspar. The continuous recrystallization of plagioclase during deformation into a very fine‐grained assemblage forming anastomosed ribbons is interpreted as the dominant process in the shear zone initiation and development. The shear zone initiated under closed‐system conditions with the destabilization of metastable Ab I –Zo I porphyroclasts into fine‐grained (20–50 μm sized) Ab II –Pl II aggregates, and with minor crystallization of phengite at the expense of K‐feldspar. The development of the shear zone requires a change in state of the system, which becomes open to externally derived fluids and mass transfer. Indeed, mass balance calculations and thermodynamic modelling show that the ultramylonite is characterized by gains in CaO, FeO and H 2 O. The progressive input of externally derived CaO drives the continuous metamorphic recrystallization of the fine‐grained Ab II –Pl II aggregate into a more Pl II ‐rich and finer aggregate. Input of water favours the crystallization of phengite at the expense of K‐feldspar to form an interconnected network of weak phases. Thus, recrystallization of 50% of the bulk rock volume would induce a decrease of the strength of the rock that might contribute to the development of the shear zone. This study emphasizes the major role of metamorphic reactions and more particularly plagioclase on strain localization process. Plagioclase represents at least one‐third of the bulk rock volume in granitic systems and forms a stress‐supporting framework that controls the rock rheology. Therefore, recrystallization of plagioclase due to changes in P–T conditions and/or bulk composition must be taken into account, together with quartz and K‐feldspar, in order to understand strain localization processes in granites.
Abstract We document the first occurrence of Fe‐rich olivine‐bearing migmatitic metapelite in the Khondalite Belt, North China Craton. Petrological analyses revealed two exotic assemblages of orthopyroxene+spinel+olivine and orthopyroxene+spinel+cordierite. Phase relation modelling suggests that these assemblages are diagnostic of ultra‐high temperature (UHT) metamorphism in the Fe‐rich system, with temperatures from 1,000 to 1,050°C at 0.6 GPa. U–Th–Pb SIMS analyses on zircon reveal a similar age of c . 1.92 Ga for the olivine‐bearing migmatite and an adjacent gabbronoritic intrusion that is therefore identified as the heat source for the UHT metamorphism. These results, coupled with additional analysis of the famous Tuguiwula sapphirine‐bearing granulite, lead to a re‐appraisal of the P–T path shape and heat source for the UHT metamorphism. We suggest that UHT metamorphism, dated between 1.92 and 1.88 Ga, across the whole Khondalite belt, proceeded from a clockwise P–T evolution with an initial near‐isobaric heating path at ~0.6–0.8 GPa, and a maximum temperature of 1,050°C followed by a cooling path with minor decompression to ~0.5 GPa. Considering our results and previous works, we propose that the orogenic crust underwent partial melting at temperature reaching 850°C and depth of ~20 to ~30 km during a period of c . 30 Ma, between 1.93 and 1.90 Ga. During this time span, the partially molten crust was continuously intruded by mafic magma pulses responsible for local greater heat supply and UHT metamorphism above 1,000°C. We propose that the UHT metamorphism in the Khondalite belt is not related to an extensional post‐collisional event, but is rather syn‐orogenic and associated with mafic magma supplies.
