Natural feldspar

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Naturally deformed feldspar rocks

TOP

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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 feldspar rocks

regime 1 / regime 2 weakly deformed / regime 2 moderately - strongly deformed

Introduction

In naturally deformed rocks, dislocation creep in feldspar is usually found above temperatures of approximately 500°C. Below these temperatures, deformation takes place by fracturing. However, this simple temperature-classification of deformation mechanisms in feldspar is complicated by the dependence of feldspar chemistry on ambient P,T-conditions and bulk rock composition. Compositional disequilibrium may be a driving force that leads to recrystallization of plagioclase under much lower temperatures than 500°C in deformed rocks. In such cases, recrystallization is syndeformational but not dynamic, because it is not driven by strain energy alone and is better termed neo-mineralisation. In order to determine the recrystallization mechanism in plagioclase one needs to compare the chemical composition of recrystallized and host grains; if the compositions are identical the process was dynamic recrystallization, but if they are different it was neo-mineralization.
Dynamic recrystallization by regime 1 dislocation creep, with no chemical change, is very rare in nature. Samples shown here are from the Corvatsch shear zone (Engadine, Swiss Alps). Examples of low temperature recrystallization (low to mid-greenschist facies) are shown in micrographs 23 and 24. Fracturing is the most important contribution to the deformation of plagioclase in these examples, accompanied by neo-mineralization.
With the exception of photo 47, the series of samples of regime 2 come from a shear zone between Sogndal and Kaupanger (Norway), in an anorthosite body of the Jotun nappe, which is part of the "middle allochthonous unit" of the Caledonides. The samples represent stages of progressively higher shear strain along a strain gradient from less deformed boudins into strongly deformed mylonites. Deformation took place around 700°C at pressures below 900 MPa. The chemical composition of the recrystallized plagioclase grains is the same as that of the porphyroclasts, so that the formation of new grains represents only strain-induced dynamic recrystallization. Dynamic recrystallization took place dominantly by progressive subgrain rotation with some local grain boundary bulging (largely regime 2 dislocation creep).

further reading

4		Naturally deformed feldspar rocks
4

TOP	1-	Experimentally deformed quartz aggregates	2-	Naturally deformed quartz-rich rocks
	3-	Experimentally deformed feldspar aggregates	4-	Naturally deformed feldspar rocks
	5-	Experimentally deformed quartzo-feldspathic rocks	6-	Naturally deformed quartzo-feldspathic rocks
	7-	Experimentally deformed pyroxenite and diabase	8-	Deformation and metamorphic reactions in polyphase rocks

Naturally deformed feldspar rocks		regime 1 / regime 2 weakly deformed / regime 2 moderately - strongly deformed

		Introduction In naturally deformed rocks, dislocation creep in feldspar is usually found above temperatures of approximately 500°C. Below these temperatures, deformation takes place by fracturing. However, this simple temperature-classification of deformation mechanisms in feldspar is complicated by the dependence of feldspar chemistry on ambient P,T-conditions and bulk rock composition. Compositional disequilibrium may be a driving force that leads to recrystallization of plagioclase under much lower temperatures than 500°C in deformed rocks. In such cases, recrystallization is syndeformational but not dynamic, because it is not driven by strain energy alone and is better termed neo-mineralisation. In order to determine the recrystallization mechanism in plagioclase one needs to compare the chemical composition of recrystallized and host grains; if the compositions are identical the process was dynamic recrystallization, but if they are different it was neo-mineralization. Dynamic recrystallization by regime 1 dislocation creep, with no chemical change, is very rare in nature. Samples shown here are from the Corvatsch shear zone (Engadine, Swiss Alps). Examples of low temperature recrystallization (low to mid-greenschist facies) are shown in micrographs 23 and 24. Fracturing is the most important contribution to the deformation of plagioclase in these examples, accompanied by neo-mineralization. With the exception of photo 47, the series of samples of regime 2 come from a shear zone between Sogndal and Kaupanger (Norway), in an anorthosite body of the Jotun nappe, which is part of the "middle allochthonous unit" of the Caledonides. The samples represent stages of progressively higher shear strain along a strain gradient from less deformed boudins into strongly deformed mylonites. Deformation took place around 700°C at pressures below 900 MPa. The chemical composition of the recrystallized plagioclase grains is the same as that of the porphyroclasts, so that the formation of new grains represents only strain-induced dynamic recrystallization. Dynamic recrystallization took place dominantly by progressive subgrain rotation with some local grain boundary bulging (largely regime 2 dislocation creep).

		further reading