ERTH 209-02 Earth Metamorphic Rocks Fall 2024 PDF

ERTH 209-02 EARTH Fall 2024 Metamorphic rocks Metamorphic rocks - formation  Metamorphic rocks are formed through the transformation of the pre-existing rocks under the action of the following factors:  pressure;  temperature;  subterranean fluids....

ERTH 209-02 EARTH Fall 2024 Metamorphic rocks Metamorphic rocks - formation  Metamorphic rocks are formed through the transformation of the pre-existing rocks under the action of the following factors:  pressure;  temperature;  subterranean fluids.  During the metamorphic process, the rocks remain in solid state. Generalities  Changes of the preexisting rocks during the metamorphic process are:  textural;  mineralogical;  both textural and mineralogical. Nature of rock change  Rock changes in response to the changes in pressure and temperature:  little compositional changes;  changes are mostly textural;  loss of volatiles (e.g., H2O and CO2).  Rock changes in response to the fluid –induced actions:  addition of elements;  subtraction of elements;  subtraction and addition of elements (metasomatism). Limits of the metamorphism  The metamorphic changes are made while the rocks remain under the solid state.  This doesn’t happen always (e.g., migmatite). Migmatites  In the case of migmatites there is a partial melting, which results in a process of segregation of the initial material of the rock. Leucosome: acidic composition; Melanosome: basic composition. Temperature-driven metamorphism  The transformation from a limestone into a marble involves rock recrystallization. Pressure-driven metamorphism  The pressure is mostly generated by the tectonically induced stress.  The differential stress causes the minerals to align approximately perpendicular to the direction of stress.  Lining up of minerals result in the formation of ‘foliation’ or ‘layering’ in many metamorphic rocks. Foliation in metamorphic rocks Folded gneiss; significant Folded schist; significant increase in pressure, increase in pressure, relatively low temperatures. relatively low temperatures. Foliation in metamorphic rocks Metamorphosed muddy sandstone (from Vernon 2004). Types of metamorphism  There are three major types of metamorphism:  Regional metamorphism (Barrovian ~);  Contact metamorphism (Buchan ~);  Burial metamorphism. Types of metamorphism Regional metamorphism (A); contact metamorphism (B); burial metamorphism (C). Metamorphic rock textures  There are three major types of textures:  Non-foliated (granoblastic);  Foliated;  Porphyroblastic. Non-foliated texture  The metamorphic rocks with non-foliated texture are known as granoblastic.  Minerals are approximately equidimensional;  Platy minerals are randomly oriented in the rock mass and/or in such small abundance, so no foliation is developed. Granoblastic texture Aggregate of equant to irregular minerals in a metamorphic rock with granoblastic texture (from Vernon 2004). Foliated texture Minerals present preferential orientation. The foliate texture is often referred to as schistose (from Vernon 2004).  Porphyroblastic texture Larger garnet porphyroblasts with finer- grained foliated matrix (ground mass). Porphyroblastic texture Large-sized porphyroblasts, which retain the euhedral shape; note the foliation that is deflected around the porphyroblasts.