Eclogitization
Eclogitization is the process in which the high-pressure, metamorphic rock, Eclogite is formed. Eclogitization is the proper metamorphic term describing the appearance of the eclogite facies. The eclogite facies records pressures between 11 kbar - 20 kbar and a range in temperature of 300°C - 1000°C. Eclogite can be aptly termed a Ultra-High Pressure metamorphic rock (UHP) due to its formation in extreme pressures. To reach the eclogite facies the source melt will cross through different Metamorphic facies, more often than not from the advancement of the blueschist facies.
Geologic Setting and Effect of Eclogitization
Eclogitization occurs due to the subduction of continental or oceanic material into the mantle. Eclogitization is an important metamorphic process because it provides insight into previous plate tectonic processes and records pressures that have occurred in a specific area. At these zones within the earth, high pressures are reached as well as medium to high temperatures and eclogitization commences. Metamorphic re-crystallization during burial can lead to a significant density increase (up to 10 % in the case of eclogitization)[1] or approximately 300–600 km/m3 of crustal rocks and continental lower crust and oceanic crust reach higher density than mantle.[2] This density increase is a main driver in the convection of Earth and is a legtimate answer to questions such as, why is a tectonic unit disconnected from the descending lithosphere and why does the slab undergo exhumation after?[3]
Localities
A difficult aspect of studying eclogitization is that eclogites constitute only a very minor volume of continental basement exposed today at Earth's surface.[3] The few areas that are available to study eclogitization and view eclogites include garnet peridotites in Greenland and in other ophiolite complexes. Examples are also known in Saxony, Bavaria, Carinthia, Norway and Newfoundland. A few eclogites also occur in the northwest highlands of Scotland and the Massif Central of France. Glaucophane-eclogites occur in Italy and the Pennine Alps. Occurrences exist in western North America, including the southwest[4] and the Franciscan Formation of the California Coast Ranges.[5] Transitional Granulite-Eclogite facies granitoid, felsic volcanics, mafic rocks and granulites occur in the Musgrave Block of the Petermann Orogeny, central Australia. Recently, coesite- and glaucophane-bearing eclogites have been found in the northwestern Himalaya.[6]
Field Studies
- The Western Gneiss Region and the Bergen Arc of Western Norway: Known as one of the largest eclogitized pieces of continental crust that was exhumed during the Caledonian orogeny, studies here have shown that recrystallization of the eclogite facies is also accompanied with a significant reduction in rocks strength.[7] This is shown by a localisnation of shear zones where the host granulites have been transformed to eclogites.[3] The main point of this study was to explore the kinematics of syn-eclogite deformation in the Bergen arc which suggested that eclogitization is ultimately responsible for the separation of tectonic units from the descending lithosphere. Furthermore, despite density increase, studies show that eclogitization may trigger exhumation due to the reduction in rock strength and requires that eclogitization is not complete. This is especially true in basic and intermediate lithologies that may become denser than the mantle if eclogitization in case of complete recrystallization[7] which is shown by a localisnation of shear zones where the host granulites have been transformed to eclogites.[3]. Thus the Bergen Arc provides an excellent example of eclogitization and its impact in a continental subduction region.
- Mechanical Models: Simulations with viscous (ductile) and plastic (brittle) rheologies have been used to investigate the effect of eclogitization on the dynamics of convergence. A plethora of geologic settings have been considered such as intracontinental deformation, subduction, and continental collision to determine the density and buoyancy impact of eclogitization. In most models, lithospheric bending or subduction is obtained, material from the lower continental crust and, in the case of the oceanic subduction, from the oceanic crust is entrained to great depths (more than 100 km). In all these cases, the force required for convergence at a constant velocity is significantly reduced in the case with eclogitization, compared to models without eclogitization. Although models have shown that eclogitization does not impact subduction initiation, eclogitized oceanic crust contributes to the slab negative buoyancy and could help the subduction of young oceanic lithosphere.[8]
References
- ^ Jolivet, L; et al. (June 6, 2005). "Softening Triggered by Eclogitization, the first step towards exhumation during continental subduction" (PDF). Earth and Planetary Science Letters. pp. 533–545. Retrieved October 11, 2012.
{{cite web}}: Explicit use of et al. in:|author=(help) - ^ Doin, Marie- Pierre; et al. (2001). "Subduction initiation and continental crust recycling: the roles of rheology and eclogitization". Tectonophysics. 342 (1–2): 163–191.
{{cite journal}}: Explicit use of et al. in:|author=(help); Unknown parameter|month=ignored (help) - ^ a b c d Steltonphol, Mark; et al. (September 15, 2010). "Eclogitization and exhumationof Caledonian continental basementin Lofoten North Norway". Geologic Society of America. pp. 202–218. Retrieved October 12, 2012.
{{cite web}}: Explicit use of et al. in:|author=(help) - ^ William Alexander Deer, R. A. Howie and J. Zussman (1997) Rock-forming Minerals, Geological Society, 668 pages ISBN 1-897799-85-3
- ^ C. Michael Hogan (2008) Ring Mountain, The Megalithic Portal, ed. Andy Burnham
- ^ "Eclogite". wikipedia. Retrieved October 14, 2012.
- ^ a b Austrheim, H., Griffin, W.L. (1985). "Shear deformation and eclogite formation with the granulite-facies anorthositesof the Bergen, Western Norway". Chem. Geol. 50: 267–281.
{{cite journal}}: CS1 maint: multiple names: authors list (link) - ^ Doin, Marie-Pierre; et al. (2001). "Subduction initiation and continental crust recycling: the roles of rheology and eclogitization". Tectonophysics. 342 (1–2): 163–191.
{{cite journal}}: Explicit use of et al. in:|author=(help); Unknown parameter|month=ignored (help)