Preservation of Garnet Growth Zoning and the Duration of Prograde Metamorphism

Chemically zoned garnet growth and coeval modification of this zoning through diffusion are calculated during prograde metamorphic heating to temperatures of up to 850°C. This permits quantification of how the preservation or elimination of zoning profiles in garnet crystals of a given size is sensitive to the specific burial and heating (P-T) path followed, and the integrated duration spent at high temperature (dT/dt). Slow major element diffusion in garnet at T 30 Myr at amphibolite-grade conditions, but small-scale (tens of micrometres) zoning features will be lost early in the prograde stage unless this is ‘rapid' (5 Myr for rocks reaching c. 600°C). Calculations indicate that preservation of unmodified growth compositions in even relatively large (up to 3 mm diameter) pelitic garnet crystals requires prograde and exhumational events to be <10 Myr for rocks reaching c. 600°C. This timescale can be 5 Myr for garnet in rocks reaching 650°C or hotter. It is likely, therefore, that most natural prograde-zoned crystals record compositions already partially re-equilibrated between the time of crystal growth and of reaching maximum temperature. However, a large T-t window exists within which crystals begin to lose their growth compositions but retain evidence of crystal-scale zoning trends that may still be useful for thermobarometry purposes. The upper limit of this window for 500 μm diameter crystals can be as much as several tens of millions of years of heating to c. 700°C. Absolute re-equilibration timescales can be significantly different for garnet growing in different rock compositions, with examples of a granodiorite and a pelite give

Anticlockwise metamorphic pressure–temperature paths and nappe stacking in the Reisa Nappe Complex in the Scandinavian Caledonides, northern Norway: evidence for weakening of lower continental crust before and during continental collision

This study investigates the tectonostratigraphy and metamorphic and tectonic evolution of the Caledonian Reisa Nappe Complex (RNC; from bottom to top: Vaddas, Kåfjord, and Nordmannvik nappes) in northern Troms, Norway. Structural data, phase equilibrium modelling, and U-Pb zircon and titanite geochronology are used to constrain the timing and pressure–temperature (P–T) conditions of deformation and metamorphism during nappe stacking that facilitated crustal thickening during continental collision. Five samples taken from different parts of the RNC reveal an anticlockwise P–T path attributed to the effects of early Silurian heating (D1) followed by thrusting (D2). At ca. 439&thinsp;Ma during D1 the Nordmannvik Nappe reached the highest metamorphic conditions at ca. 780&thinsp;∘C and ∼9–11&thinsp;kbar inducing kyanite-grade partial melting. At the same time the Kåfjord Nappe was at higher, colder, levels of the crust ca. 600&thinsp;∘C, 6–7&thinsp;kbar and the Vaddas Nappe was intruded by gabbro at &gt;&thinsp;650&thinsp;∘C and ca. 6–9&thinsp;kbar. The subsequent D2 shearing occurred at increasing pressure and decreasing temperatures ca. 700&thinsp;∘C and 9–11&thinsp;kbar in the partially molten Nordmannvik Nappe, ca. 600&thinsp;∘C and 9–10&thinsp;kbar in the Kåfjord Nappe, and ca. 640&thinsp;∘C and 12–13&thinsp;kbar in the Vaddas Nappe. Multistage titanite growth in the Nordmannvik Nappe records this evolution through D1 and D2 between ca. 440 and 427&thinsp;Ma, while titanite growth along the lower RNC boundary records D2 shearing at 432±6&thinsp;Ma. It emerges that early Silurian heating (ca. 440&thinsp;Ma) probably resulted from large-scale magma underplating and initiated partial melting that weakened the lower crust, which facilitated dismembering of the crust into individual thrust slices (nappe units). This tectonic style contrasts with subduction of mechanically strong continental crust to great depths as seen in, for example, the Western Gneiss Region further south.</p

Geometric aspects of synkinematic granite intrusion into a ductile shear zone — an example from the Yunmengshan core complex, northern China

The Cretaceous Yungmengshan core complex in northern China contains a large syntectonic granodiorite batholith that intrudes a slightly older diorite intrusion. A major gently dipping ductile décollement shear zone is developed along the contact of the diorite and granodiorite. The shear zone is invaded by a large volume of granitic and pegmatite veins associated with the main granodiorite batholith during activity of the shear zone under high-grade metamorphic conditions. Progressively older veins are more strongly deformed into tight cylindrical fold structures rotated into parallelism with the lineation and foliation in the shear zone. Parallelism of veins to the foliation is partly due to this rotation, but also to foliation-parallel injection of younger syntectonic pegmatite veins. Several small-scale structures have been recognized that allow distinction of solid-state deformation of veins. Granite veins do not extend much above the ductile shear zone that seems to act as a lid and an effective depository to intruding granite veins from the underlying batholith. There was considerable volume increase in the footwall and lower part of the shear zone by vein intrusion

Preservation of Garnet Growth Zoning and the Duration of Prograde Metamorphism

Chemically zoned garnet growth and coeval modification of this zoning through diffusion are calculated during prograde metamorphic heating to temperatures of up to 850°C. This permits quantification of how the preservation or elimination of zoning profiles in garnet crystals of a given size is sensitive to the specific burial and heating (P-T) path followed, and the integrated duration spent at high temperature (dT/dt). Slow major element diffusion in garnet at T 30 Myr at amphibolite-grade conditions, but small-scale (tens of micrometres) zoning features will be lost early in the prograde stage unless this is ‘rapid' (5 Myr for rocks reaching c. 600°C). Calculations indicate that preservation of unmodified growth compositions in even relatively large (up to 3 mm diameter) pelitic garnet crystals requires prograde and exhumational events to be <10 Myr for rocks reaching c. 600°C. This timescale can be 5 Myr for garnet in rocks reaching 650°C or hotter. It is likely, therefore, that most natural prograde-zoned crystals record compositions already partially re-equilibrated between the time of crystal growth and of reaching maximum temperature. However, a large T-t window exists within which crystals begin to lose their growth compositions but retain evidence of crystal-scale zoning trends that may still be useful for thermobarometry purposes. The upper limit of this window for 500 μm diameter crystals can be as much as several tens of millions of years of heating to c. 700°C. Absolute re-equilibration timescales can be significantly different for garnet growing in different rock compositions, with examples of a granodiorite and a pelite give

Transformation weakening: Diffusion creep in eclogites as a result of interaction of mineral reactions and deformation

International audienceThe deformation of eclogites and the driving forces for their fabric development are an important topic, potentially allowing to determine deformation rates and stresses in subduction zones, where the greatest number of large earthquakes occurs. Here, fabric studies of grain size and shape, texture, and chemical composition from two locations of Variscan and Alpine eclogites are presented. All samples show a well-developed crystallographic preferred orientation (CPO) of omphacite with a strong maximum of [001] in the lineation direction and a weaker maximum of poles to (010) normal to foliation. Garnet shows no systematic CPO. Anisotropic chemical zoning developed in omphacite and garnet during growth together with elongated grain shapes and can be related to a prograde (in terms of pressure change) P,T-path. The individual chemically zoned and elongated grains orientated in the stretching direction are single crystals without major internal mis-orientations. Chemical, microstructural, and CPO data indicate that the deformation microstructure and texture were produced by preferential crystal growth of garnet and omphacite grains in the extension direction. Dislocation creep can be excluded as a possible fabric formation process by the systematic and oriented chemical zonation of single crystals and absence of dynamic recrystallization microstructures. The dominant deformation is inferred to be diffusion creep, where dissolution of material took place in reacting mafic phases (plagioclase, pyroxene) and precipitation took place in the form of new eclogite facies minerals (omphacite, garnet, zoisite). This type of diffusion creep deformation represents a transformation process involving both, deformation and metamorphic reactions. It is emphasized that the weakening is directly connected to the transformation and therefore transient. The weakening facilitates diffusion creep deformation of otherwise strong minerals (pyroxene, garnet, zoisite) at far lower stresses than dislocation creep. The results imply low stresses during the deformation of eclogite blocks in subduction zones. These results can be applied to other rock types, too