PORPHYROBLASTIC MICROSTRUCTURES AND METAMORPHIC GRADE: CONTROLS ON SYSTEMATIC TRENDS FROM A HIMALAYAN INVERTED BARROVIAN SEQUENCE
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: Principal metamorphic rocks. Origin of the Metamorphic strata. Fossiliferous strata rendered metamorphic toy intrusive masses of Granite. Hydrothermal action, or the Influence of Steam and Gases in producing; Metamorpnism. Objections to tbe metamorphic theory considered.
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Metamorphic rocks form a substantial proportion of the material that makes up the Earth's crust, and metamorphic processes have been almost continually occurring throughout geological time since the origin of that crust. Rocks undergo metamorphic and metasomatic changes as they are subjected to different pressure and temperature conditions, or are infiltrated by chemically reactive fluids. Rocks that are metamorphosed in subduction/collision zones undergo metamorphic changes over broad zones, and can record evidence of passage from one metamorphic grade to another as they journey through different depths. These form the most common types of metamorphic rocks, termed the Regional Metamorphic Rocks. Where rocks are metamorphosed due to contact with hot igneous bodies they are referred to as Contact Metamorphic Rocks. Finally, where alteration and metamorphism occur due to fluids, the rocks are called Hydrothermally Altered Metamorphic Rocks.
Metasomatism
Metamorphic core complex
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Protolith
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Monometamorphism is resulted from one metamorphic event,whereas polymetamorphism resulted from two or more metamorphic events.If a metamorphic event involves only one phase of heating or pressing and then cooling or depressing,it is called monophase metamorphism.If a metamorphic process involves two or more climaxes in the changes of temperature and/or pressure,it is called a polyphase metamorphism.Although criteria are quite rare,it is very important to determine if a metamorphism belongs to a monometamorphism or a polymetamorphism.However,metamorphic processes preserved in one geologic entity with different geodynamic backgrounds are definitely of different metamorphic events.To accurately retrieve metamorphic p-T-t paths,metamorphic events should be discriminated by inspecting metamorphic reaction textures,p-T conditions should be determined by valid geothermometers and geobarometers,and finally,the geologic time should be determined by in-situ geochronology.Since uncertainties exist in all of these three aspects,refining and developing the fundamental theory and technology are much needed in the near future.
Polyphase system
Geochronology
Foliation (geology)
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