H
HIGH-PRESSURE, LOW-TEMPERATURE
METAMORPHISM
Hayato Ueda
Department of Geology, Niigata University, Niigata,
Japan
Synonyms
High-pressure/temperature (high-P/T) metamorphism;
High-pressure metamorphism
Definition
Metamorphism: Changes of mineral textures and phases in
rocks at depths without significant change of bulk-rock
chemical composition except for vapor components.
High-pressure/low-temperature metamorphism: Metamorphism that undergoes at high pressure with
low-temperature conditions than usual, typically with
thermal gradients as low as 10
C/km.
High-pressure/temperature metamorphism: Metamorphism under conditions of high pressure versus temperature ratios, i.e., of low thermal gradients.
Introduction
In Earth’s interior, temperature generally increases as the
depth increases as a result of heat conduction. Because
rocks as well as melts and fluids sometimes migrate fast
enough for evident heat advection, geothermal fields are
modified by regional or local geodynamics. In subduction
and collision zones, lower temperatures than normal
geotherms are maintained with thermal gradients as low
as 10
C/km, because the slabs cooled on the surface are
constantly supplied to the depths. High-pressure/lowtemperature (HPLT) metamorphism is believed almost
exclusively to occur at such cool conditions inside the
convergent plate margins. Although only a few cases of
seafloor exposures of HPLT metamorphic rocks are so
far known, HPLT metamorphism might be ongoing inside
every subduction zone corresponding to deep-sea
trenches. HPLT rocks occasionally record physical and
chemical conditions, dynamics and element transport,
and their historical changes inside deep subduction or collision zones as mineral phases and textures. HPLT metamorphic rocks commonly contain significant amounts of
hydrous and/or carbonate minerals. When these rocks
are further subducted down to deeper portions with
increasing temperature and pressure, they release volatile
fluids such as water and carbon dioxide as well as other
elements as solutes via dehydration or decarbonation.
The released volatiles affect chemistry, rheology, and
melting conditions of the upper mantle. Therefore, HPLT
metamorphic rocks act as containers which deliver volatiles from the surface to the mantle.
Geodynamic settings and source rocks
Source rocks (protoliths) of HPLT metamorphic rocks differ between two contrasting convergent settings of collision and subduction zones (Maruyama et al., 1996). In
collision zones, materials being subducted are leading
edges of passive continental margins consisting of granitic basements, clastic and calcareous sediments of continental shelves and slopes, and rift-related basaltic rocks.
Aluminous and/or iron-rich metapelites originated from
mud and soil affected by subaerial weathering are also
occasionally accompanied. Materials input into subduction zones consist of oceanic crust and overlying pelagic
sediments, as well as trench-fill deep-sea turbidites and
mudstone, and seamounts. This subduction zone rock
assemblage is comparable to rocks of accretionary complexes. Basaltic rocks have chemical compositions correlative to mid-ocean ridge and oceanic island basalts.
Metachert characteristically occur, and some of them are
occasionally manganese. HPLT metamorphic rocks are
J. Harff et al. (eds.), Encyclopedia of Marine Geosciences, DOI 10.1007/978-94-007-6238-1,
© Springer Science+Business Media Dordrecht 2016
HIGH-PRESSURE, LOW-TEMPERATURE
METAMORPHISM
Hayato Ueda
Department of Geology, Niigata University, Niigata,
Japan
Synonyms
High-pressure/temperature (high-P/T) metamorphism;
High-pressure metamorphism
Definition
Metamorphism: Changes of mineral textures and phases in
rocks at depths without significant change of bulk-rock
chemical composition except for vapor components.
High-pressure/low-temperature metamorphism: Metamorphism that undergoes at high pressure with
low-temperature conditions than usual, typically with
thermal gradients as low as 10
C/km.
High-pressure/temperature metamorphism: Metamorphism under conditions of high pressure versus temperature ratios, i.e., of low thermal gradients.
Introduction
In Earth’s interior, temperature generally increases as the
depth increases as a result of heat conduction. Because
rocks as well as melts and fluids sometimes migrate fast
enough for evident heat advection, geothermal fields are
modified by regional or local geodynamics. In subduction
and collision zones, lower temperatures than normal
geotherms are maintained with thermal gradients as low
as 10
C/km, because the slabs cooled on the surface are
constantly supplied to the depths. High-pressure/lowtemperature (HPLT) metamorphism is believed almost
exclusively to occur at such cool conditions inside the
convergent plate margins. Although only a few cases of
seafloor exposures of HPLT metamorphic rocks are so
far known, HPLT metamorphism might be ongoing inside
every subduction zone corresponding to deep-sea
trenches. HPLT rocks occasionally record physical and
chemical conditions, dynamics and element transport,
and their historical changes inside deep subduction or collision zones as mineral phases and textures. HPLT metamorphic rocks commonly contain significant amounts of
hydrous and/or carbonate minerals. When these rocks
are further subducted down to deeper portions with
increasing temperature and pressure, they release volatile
fluids such as water and carbon dioxide as well as other
elements as solutes via dehydration or decarbonation.
The released volatiles affect chemistry, rheology, and
melting conditions of the upper mantle. Therefore, HPLT
metamorphic rocks act as containers which deliver volatiles from the surface to the mantle.
Geodynamic settings and source rocks
Source rocks (protoliths) of HPLT metamorphic rocks differ between two contrasting convergent settings of collision and subduction zones (Maruyama et al., 1996). In
collision zones, materials being subducted are leading
edges of passive continental margins consisting of granitic basements, clastic and calcareous sediments of continental shelves and slopes, and rift-related basaltic rocks.
Aluminous and/or iron-rich metapelites originated from
mud and soil affected by subaerial weathering are also
occasionally accompanied. Materials input into subduction zones consist of oceanic crust and overlying pelagic
sediments, as well as trench-fill deep-sea turbidites and
mudstone, and seamounts. This subduction zone rock
assemblage is comparable to rocks of accretionary complexes. Basaltic rocks have chemical compositions correlative to mid-ocean ridge and oceanic island basalts.
Metachert characteristically occur, and some of them are
occasionally manganese. HPLT metamorphic rocks are
J. Harff et al. (eds.), Encyclopedia of Marine Geosciences, DOI 10.1007/978-94-007-6238-1,
© Springer Science+Business Media Dordrecht 2016