GEOL-1021EL Lecture Notes - Lecture 7: Geothermal Gradient, Metasomatism, High High
16 Jun 2018
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Nov 6, 2017
GEOL-1021
Metamorphic Rocks
•Definition: a rock produced by changes in mineralogy and texture. (in th solid state e.g.. no
crystallization from melting precipitation from solution)
•How?
•Recrystallization
•Formation of new minerals
•Partial or complete
•Variables
•Heat (geothermal gradient, igneous intrusions)
•Temperature increase with depth, but rate is variable (roughly at a rate of 20*-60* C
per km)
•pressure (burial, tectonic, shock)
•Pressure/stress mostly related to weight of overlaying rocks (but also compressional/
extensional forces). Rate of pressure increase is roughly constant- 0.4-0.4 kbar/km
•increase in pressure promotes compaction (decrease in available space), therefore
metamorphic minerals (created at increasing pressures) tend to be denser than initial
minerals. Increased pressure ay come from non-directed stress (confining pressure) or
directed stress (burial). directed stress will promote referred orientation of minerals.
•amount of fluids (water dominated, but also CO2)
•(water based) act as catalysts rung metamorphism aid the exchange of ions between
growing crystals
•may also add or remove minerals, significantly changing the composition
(metasomatism)
•Metamorphic Grades
•Refers to the intensity of metamorphism
•Grades:
•Low- low temperature and low pressure
•intermediate
•High- high temperature and high pressure
•TYPES
•Shock
•Used by meteoritic impact d localized concentric to the impact area.
•Changes in rocks as the result of rapid increases in pressure (at relatively low
temperature)
