Eclogites, metamorphism and plate tectonics Lecture 10

Chapter 19: Continental Alkaline Magmatism. Kimberlites Figure 19-19. 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.

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.

Coesite in garnet, Alps, from L. Jolivet

Coesite in garnet, Alps, from L. Jolivet

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 21-13. 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 100-200 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

Paired Metamorphic Belts of Japan Figure 21-12. 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

Paired Metamorphic Belts of Japan Fig. 16-15 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

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, 4697-4707 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 80-100 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)