Figure 23

Naturally deformed feldspar aggregates

Figure caption

Naturally deformed feldspar rocks

regime 1 / regime 2 low deformation / regime 2 moderate-high deformation

23

Plagioclase porphyroclast deformed under lower- to mid-greenschist facies P,T-conditions.

Below temperatures of about 500°C, feldspars tend to deform by fracturing. In the micrograph, new, fine-grained plagioclase is found along the fracture, which cuts across the grain from left to right above the twin. These new grains, however, are not formed by dynamic recrystallization in the strict sense. Instead, the new grains have a different chemical composition from that of the host grain, because intermediate plagioclase usually is metastable under greenschist facies P,T-conditions. This process constitutes a syndeformational recrystallization or neo-mineralisation. The chemical decomposition of the plagioclase porphyroclast is also evident from its "dusty" appearance, resulting from numerous solid inclusions of new phases that have nucleated. See other examples in photos # 93-98. Sample is from Wyangala dam, Australia.

		Naturally deformed feldspar aggregates
		Figure caption

	Naturally deformed feldspar rocks	regime 1 / regime 2 low deformation / regime 2 moderate-high deformation


23		Plagioclase porphyroclast deformed under lower- to mid-greenschist facies P,T-conditions. Below temperatures of about 500°C, feldspars tend to deform by fracturing. In the micrograph, new, fine-grained plagioclase is found along the fracture, which cuts across the grain from left to right above the twin. These new grains, however, are not formed by dynamic recrystallization in the strict sense. Instead, the new grains have a different chemical composition from that of the host grain, because intermediate plagioclase usually is metastable under greenschist facies P,T-conditions. This process constitutes a syndeformational recrystallization or neo-mineralisation. The chemical decomposition of the plagioclase porphyroclast is also evident from its "dusty" appearance, resulting from numerous solid inclusions of new phases that have nucleated. See other examples in photos # 93-98. Sample is from Wyangala dam, Australia.