THE LANGUAGE OF THE EARTH – PART III METAMORPHIC ROCKS

Rock Cycle

Definition Metamorphism- changes in the mineral assemblage and textures of igneous, sedimentary or other metamorphic rocks due to prolonged exposure to elevated temperatures and pressures (or increased grades of metamorphism)

Types of Metamorphism 1) Burial metamorphism (also known as Diagenesis or Lithification) - Low T, low-mod confining to differential P; original sedimentary/igneous features generally preserved; forms in deeper parts of thick sedimentary sequences in non-tectonic area (e.g., on passive continental margins)

Types of Metamorphism 2) Contact (or Thermal) metamorphism - High T, low-mod. confining P; forms adjacent to igneous intrusions.

Types of Metamorphism 3) High-pressure metamorphism - High P (strongly differential), low-mod T; forms at plate boundaries convergent boundary zones. Also know as blueschist metamorphism.

Types of Metamorphism 4) Regional metamorphism -Variable T, mod-high differential P; characteristic of over-thickened plates (i.e., mountain belts) above convergent boundaries; affects large areas

Types of Metamorphism 5) Cataclasis metamorphism -Variable T, very high directed P; typically localized to narrow zones of intense mechanical deformation (shear zones).

Types of Metamorphism 6) Metasomatism -Variable T&P; distinguished from other forms of metamorphism by the loss and/or gain of material (usually transported by a fluid). Other types of metamorphism are thought to occur as nearly closed processes (except for water loss).

Controls on Rates of Metamorphic Change Fluid content (particularly water) of the rock. Water helps to catalyze the mineral transformations. Water gets “baked” out of rocks during prograde metamorphism. Temperature - chemical reactions occur faster at higher temperatures. Time - For a rock to develop a new metamorphic mineral assemblage corresponding to a particular P & T, it must exist under those condition for a sufficiently long period of time (generally tens of thousands to millions of years).

Why do we see high grade metamorphic rocks at the earth’s surface? Isostatic Rebound “Rapid” erosion of mountain belts of “dry” rocks

Mineralogic Response to Metamorphism Minerals that form depend on: T and P conditions Bulk composition of the source rock Progressive metamorphism of a graywacke (dirty sandstone)

Textural Response to Metamorphism Reflects the intensity and directionality of pressure (or stress). Increased grain size - During prograde metamorphism or at a particular grade that is maintained for a long period of time, minerals will tend to increase in size. Foliation - As new platy minerals grow, they will align themselves perpendicular to the maximum stress direction. For clay mineral and fine-grained micas, the planar fabric that results is referred to as a slaty cleavage. In higher grade rocks, coarser grained mica minerals are said to impart a schistosity to the rock. Gneissic Banding - In very high grade rocks, the dark minerals tend to segregate from the lighter colored minerals (feldspar and quartz) resulting in banded rock..

Development of Foliation in Bedded Sedimentary Rocks

Other Textural Responses Porphyroblasts Some metamorphic minerals have strong growth habits, meaning they will develop as large well-formed crystals called porphyroblasts Alkali Feldspar Staurolite Garnet

Textural Response to Thermal Metamorphism Granoblastic Texture Creates a very dense, granular, fine-grained mineral texture called granoblastic texture Metamorphic rocks displaying granoblastic texture are called hornfels basaltic hornfels sedimentary hornfels

Summary of Metamorphic Rock Types

STRUCTURAL DEFORMATION Large-scale Response to Directed Pressure Dependent on Temperature and Rate of Stress High Temperature, Low Rate of Stress → FOLDING

STRUCTURAL DEFORMATION Large-scale Response to Directed Pressure Low Temperature, Rapid Rate of Stress → FAULTING

Rock Cycle Limestone Quartz + Clay+Lithics Clay+Lithics Qtz Sandstone Basalt Graywacke Shale Rhyolite Quartzite Greenstone Slate Marble Gabbro Granite Amphibolite Schist Gneiss Migmatite Felsic Mafic