Week 4. Geochemistry of Metamorphic Rocks

Geochemistry of Metamorphic Rocks

Metamorphic rocks are derived from other rocks of igneous, sedimentary or metamorphic origin. The chemical composition of this primary material ( = protolith) determines the chemical and mineralogical composition of metamorphic rocks to a large degree. The structure of metamorphic rocks is often inherited from the precursor material. In low-grade metasedimentary rocks, for example, the sedimentary bedding and typical structures of sedimentary rocks such as cross-bedding and graded bedding may be preserved. Ophitic structure, characteristic of basaltic lava, may be found in mafic metamorphic rocks. Very coarse-grained structures of igneous origin can occasionally be found even in high-grade metamorphic rocks. Most metamorphic rocks, however, exhibit structures that are of distinct metamorphic origin. The structures typically result from a combination of deformation and recrystallization. Deformation normally accompanies large-scale tectonic processes causing metamorphism.Large-scale tectono-thermal processes move rocks along unique paths in pressure-temperature-time (P-T -t) space. Rocks may undergo continuous recrystallization and new minerals may replace old ones in a complex succession. Earlier-formed minerals and groups of minerals often experience metastable survival because of unfavorable reaction kinetics. This happens particularly if an aqueous fluid phase is absent. Metamorphism may proceed episodically. The study of metamorphic rocks aims at the correct identification of the group of minerals that may have coexisted in chemical equilibrium at one stage during the evolutionary history of the metamorphic rock. This group of minerals is the mineral assemblage. The total succession of mineral assemblages preserved in a metastable state in the structure of a metamorphic rock is designated mineral paragenesis. Discussion and analysis of phase relationships in metamorphic rocks is greatly facilitated by the use of composition phase diagrams. mineral assemblages.

Metamorphic rocks are product of transformation or solid-state recrystallization of existing (protolith) igneous, sedimentary, and metamorphic rocks, due to change in physical and chemical conditions, principally temperature, pressure and introduction of chemically active fluids and gases. Metamorphism alters the mineral composition including formation of new minerals (garnet, zoisite, kyanite, chlorite, biotite, sericite, staurolite, sillimanite, talc and andalusite). Sources of temperature are geothermic gradient, effect of magmatic body and friction in rock masses of tectonic movements following prograde or retrograde mechanism. Pressure is caused by weight of sediments or crust. Common textures are crystalline, granular, xenoblastic, idioblasts, granoblastic, and porphyroblastic. Structures include gneissic, schistose and slaty.

Types of metamorphism are dynamic/kinetic, contact, regional and plutonic. Dynamic metamorphism due to mechanical deformations or dynamic stress during tectonic movement’s forms mylonites, flazer and augen gneisses. Contact metamorphism due to thermal effect of magma/lava generates skarn deposits. Regional metamorphism, caused by general increase/decrease in temperatures over large areas of continental crust, creates low-/high-grade metamorphic rocks like slate, phyllite, amphibolites, varieties of schists, para- and orthogneisses, quartzite, and marble. Plutonic metamorphism occurs at high temperatures and strong pressure in deeper parts of lithosphere producing granulites, eclogite and migmatites.Metamorphic rocks are exceptionally appreciated as decorative and building stone due to crystalline texture, layering, brilliant colors and excellent polishing capabilities.

Primary Material of Metamorphic Rocks

All metamorphic rock-forming processes make rocks from other rocks. The precursor rock or protolith determines many attributes of the new metamorphic rock. Metamorphism results from the addition (or removal) of heat and material to discrete volumes of the crust or mantle by tectonic or igneous processes. Metamorphism, therefore, may affect all possible types of rock present in the earth's crust or mantle. Protoliths of metamorphic rocks comprise rocks of all possible chemical compositions and include the entire range of sedimentary, igneous and metamorphic rocks. Metamorphic processes tend to change the original composition of the protolith. Addition of heat to rocks typically results in the release of volatiles (H20, COz, etc.) that are stored in hydrates (e. g., clay, micas, amphiboles), carbonates and other minerals containing volatile components. Therefore, many metamorphic rocks are typically depleted in volatiles relative to their protoliths. Metamorphism that releases only volatiles from the protolith is, somewhat illogically, termed isochemical. On a volatile-free basis, the chemical composition of protolith and product rock is identical in isochemicalmetamorphism. In truly isochemical metamorphism, protolith and productrocks are of identical composition including the volatile content. Isochemical metamorphism in such a strict sense is extremely rare. Many, if not most, metamorphic processes also change the cationic composition of the protolith. This type of metamorphism is termed allochemical metamorphism or metasomatism. The aqueous fluid released by dehydration reactions during metamorphism may contain dissolved cations. These are then carried away with the fluid and lost by the rock system. It has been found, for example, that many granulite facies gneisses are systematically depleted in alkalis (Na and K) relative to their amphibolite facies precursor rocks. This can be explained by loss of alkalis during dehydration. Silica saturation is a general feature of almost all metamorphic fluids. Pervasive or channeled regional-scale flow of silica-saturated dehydration fluids may strongly alter silica-deficient rocks (ultramafic rocks, dolomite marbles) that come into contact with this fluid. Unique metamorphic rock compositions may result from metasomatism on a local or regional scale. Efficient diffusion and infiltration metasomatism requires the presence of a fluid phase. Metasomatism is fluid-rock interaction at elevated temperature and pressure. Fluid-rock interaction is also important in sedimentary and other near-surface environments. Interaction of rocks with externally derived fluids is referred to as allochemical metamorphism. The volatile composition of the fluid may not be in equilibrium with the rock's mineralogy and, consequently, the rock may be altered. Some examples: flushing of rocks with pure H20 under high P-T conditions may initiate partial melting; it may form mica and amphibole in pyroxene-bearing rocks; it may induce wollastonite or periclase formation inmarbles.

Metamorphic rocks arise from the transformation of existing rock types, in a process called metamorphism, which means "change in form". The original rock (protolith) is subjected to heat (temperatures greater than 150 to 200 °C) and pressure (100 megapascals (1,000 bar) or more), causing profound physical or chemical change. The protolith may be a sedimentary, igneous, or existing metamorphic rock.


For Detail Lession see attached pdf file (Geochemistry of Metamorphic Rocks)