1. Eclogites, metamorphism and plate tectonics
Lecture 10

2. Chapter 19: Continental Alkaline Magmatism. Kimberlites
Figure Model of an idealized kimberlite system, illustrating the hypabyssal dike-sill complex leading to a diatreme and tuff ring explosive crater. This model is not to scale, as the diatreme portion is expanded to illustrate it better. From Mitchell (1986) Kimberlites: Mineralogy, Geochemistry, and Petrology. Plenum. New York. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

3. Chapter 19: Continental Alkaline Magmatism. Kimberlites
Figure 19-20b. Hypothetical cross section of an Archean craton with an extinct ancient mobile belt (once associated with subduction) and a young rift. The low cratonal geotherm causes the graphite-diamond transition to rise in the central portion. Lithospheric diamonds therefore occur only in the peridotites and eclogites of the deep cratonal root, where they are then incorporated by rising magmas (mostly kimberlitic- “K”). Lithospheric orangeites (“O”) and some lamproites (“L”) may also scavenge diamonds. Melilitites (“M”) are generated by more extensive partial melting of the asthenosphere. Depending on the depth of segregation they may contain diamonds. Nephelinites (“N”) and associated carbonatites develop from extensive partial melting at shallow depths in rift areas. After Mitchell (1995) Kimberlites, Orangeites, and Related Rocks. Plenum. New York. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

4. Coesite in garnet, Alps, from L. Jolivet

5. Coesite in garnet, Alps, from L. Jolivet

6. Miyashiro (1961, 1973) suggested that the occurrence of coeval metamorphic belts, an outer, high-P/T belt, and an inner, lower-P/T belt ought to be a common occurrence in a number of subduction zones, either modern or ancient
Figure Some of the paired metamorphic belts in the circum-Pacific region. From Miyashiro (1994) Metamorphic Petrology. Oxford University Press.
Miyashiro (1961, 1973) noted the paired nature of the Ryoke-Sanbagawa belts, and suggested …
Miyashiro called these paired metamorphic belts
May be separated by km of less metamorphosed and deformed material (“arc-trench gap”) or closely juxtaposed (Ryoke-Sanbagawa)
In the latter cases the contact is commonly a major fault
Most of these belts are quite complex, and are not always coeval

7. Paired Metamorphic Belts of Japan
Figure The Sanbagawa and Ryoke metamorphic belts of Japan. From Turner (1981) Metamorphic Petrology: Mineralogical, Field, and Tectonic Aspects. McGraw-Hill and Miyashiro (1994) Metamorphic Petrology. Oxford University Press.
Shikoku and Honshu in Japan: a pair of parallel metamorphic belts are exposed along a NE- SW axis parallel to the active subduction zone
These belts are of the same age, suggesting that they developed together

8. Paired Metamorphic Belts of Japan
Fig suggests that the 600oC isotherm, for example, could be as deep as 100 km in the trench-subduction zone area, and as shallow as 20 km beneath the volcanic arc

9. Island Arc Petrogenesis
Figure 16-11b. A proposed model for subduction zone magmatism with particular reference to island arcs. Dehydration of slab crust causes hydration of the mantle (violet), which undergoes partial melting as amphibole (A) and phlogopite (B) dehydrate. From Tatsumi (1989), J. Geophys. Res., 94, and Tatsumi and Eggins (1995). Subduction Zone Magmatism. Blackwell. Oxford.
Altered oceanic crust begins to dehydrate at depths ~ 50 km or less, as chlorite, phengite, and other hydrous phyllosilicates decompose
Further dehydration takes place at greater depths as other hydrous phases become unstable, including amphibole at about 3 GPa.
The slab crust is successively converted to blueschist, amphibolite, and finally anhydrous eclogite as it reaches about km depth
In most (mature) arcs, the temperature in the subducted crust is below the wet solidus for basalt, so the released water cannot cause melting, and most of the water is believed to rise into the overlying mantle wedge, where it reacts with the lherzolite to form a pargasitic amphibole and probably phlogopite (yellowish area)
Slightly hydrous mantle immediately above the slab is carried downward by induced convective flow where it heats up, dehydrates, and melts at A (120 km)