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Naturally deformed quartz-rich rocks

 

 

  

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Experimentally deformed quartz aggregates

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Naturally deformed quartz-rich rocks

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Experimentally deformed feldspar aggregates

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Naturally deformed feldspar rocks

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Experimentally deformed quartzo-feldspathic rocks

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Naturally deformed quartzo-feldspathic rocks

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Experimentally deformed pyroxenite and diabase

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Deformation and metamorphic reactions in polyphase rocks

Naturally deformed quartz-rich rocks

 

undeformed / semi-brittle & bulging recrystallization / subgrain rotation recrystallization / grain boundary migration recrystallization

 


Introduction

In naturally deformed quartz-rich rocks, the microstructures show systematic differences with increasing temperatures of deformation, similar to the different dislocation creep regimes in experimentally deformed examples. The strain rate and stress conditions of the naturally deformed rocks usually are unknown or poorly constrained, so that a direct correlation with dislocation creep regimes could be difficult. However, the dominant recrystallization mechanisms can be determined from the microstructures and can be compared with the experimentally established dislocation creep regimes. Three microstructural regimes corresponding to three main mechanisms of dynamic recrystallization can be distinguished:

1. Bulging recrystallization which is dominated by local grain boundary migration (slow migration) and occurs at the lowest temperatures of deformation. The grain boundary lobes are very small. Favorite sites for bulging are triple junctions, and - if present - fractures.

2. Progressive subgrain rotation which is dominated by polygonization of old grains and formation of newly recrystallized grains. This recrystallization occurs at intermediate temperatures.

3. Grain boundary migration recrystallization which is dominated by fast grain boundary migration and occurs at high temperatures. During this recrystallization, whole grains may be swept. Progressive subgrain rotation is only important for the initial formation of new grains.

Generally, the recrystallization mechanisms listed above as 1,2,3 correspond approximately to the dominant recrystallization processes identified in the respective experimental dislocation creep regimes 1,2,3.

The Heavitree quartzite is from the Ruby Gap duplex (RGD) which forms a part of the internal ductile zones of the Alice Springs orogen in central Australia. The RGD consists of 5 thrust sheets of Heavitree quartzite deformed under greenschist-facies conditions (less than 400°C). Sheets 1, 2 and 3 form an imbricate system. Finite strain and temperature of deformation generally increase upward through the duplex. The microstructures are typical of the full spectrum of dislocation creep regimes.

The chert samples are from the Warrawoona syncline which is part of an Archean greenstone belt accumulated between 3450 Ma and 3320 Ma. The syncline is a tight keel structure developed between two granitic domes. In the axis of the syncline, a chert series was deformed and recrystallized under greenschist-facies conditions. The tectonites are very strongly lineated in the central part of the syncline, and shape and crystallographic fabric analyses indicate a deformation in constriction. During deformation-recrystallization, the chert underwent substantial grain growth.

The quartz veins are from the Tonale Line, a major strike slip fault in the Alps. At the eastern end, the synkinematic Adamello intrusion has imposed a thermal gradient across the fault. The quartz samples shown here come from the Edolo shists which were deformed under greenschist-facies conditions.

 

 

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