Granulite facies
Introduction
Granulite facies is a metamorphic facies characterized by high temperatures and moderate to high pressure conditions. It is named after the coarse-grained metamorphic rock, Granulite, which is the most common rock type formed under these conditions. The granulite facies is significant in understanding the metamorphic history of the earth's crust, as it provides insights into the tectonic processes that lead to the formation of the deep continental crust.
Formation
The formation of granulite facies involves the metamorphism of pre-existing rocks under high temperature and pressure conditions. This process, known as metamorphism, involves the physical and chemical alteration of rocks in the solid state. The specific conditions under which granulite facies form are typically temperatures above 700°C and pressures of 0.6 to 1.2 GPa. These conditions are usually found at depths of 15 to 45 km in the earth's crust.
Mineral Assemblages
The mineral assemblages in granulite facies are typically characterized by the presence of orthopyroxene, clinopyroxene, plagioclase, and garnet. Other minerals that may be present include quartz, olivine, cordierite, biotite, and aluminosilicates. The specific mineral assemblage present in a granulite facies rock depends on the composition of the original rock that was metamorphosed and the specific pressure and temperature conditions during metamorphism.
P-T Conditions
The pressure-temperature (P-T) conditions of the granulite facies are typically high temperature and moderate to high pressure. The granulite facies is usually associated with the highest grades of metamorphism, which occur at temperatures above 700°C and pressures of 0.6 to 1.2 GPa. These conditions are typically found at depths of 15 to 45 km in the earth's crust. The P-T conditions of the granulite facies can be determined using various geothermobarometry techniques, which involve the use of mineral assemblages and their compositions to estimate the pressure and temperature conditions of metamorphism.
Distribution
Granulite facies rocks are found in various parts of the world, including the Canadian Shield, the Scandinavian Shield, and the Indian subcontinent. They are also found in the deep roots of ancient mountain ranges, such as the Appalachian Mountains and the Himalayas. The distribution of granulite facies rocks provides important clues about the tectonic history of these regions.
Significance
The study of granulite facies is significant in understanding the metamorphic history of the earth's crust. The high temperature and pressure conditions under which these rocks form provide insights into the tectonic processes that lead to the formation of the deep continental crust. In addition, the mineral assemblages and textures of granulite facies rocks can provide valuable information about the conditions of metamorphism and the composition of the original rock that was metamorphosed.