Subduction zones metamorphism

Bebout, G.E., Ryan, J.G., Leeman, W.P. and Bebout, A.E. (1999) Fractionation of trace elements by subduction-zone metamorphism-effect of convergent-margin thermal evolution. Earth and Planetary Science Letters, 171(1), 63-81. 

Bebout G. E. (1995) The impact of subduction-zone metamorphism on mantle-ocean chemical cychng. Chem. Geol. 126, 191-218. 

A- -Vi = B- -V2 (where A, B are volatile free phases and Vi, V2 are hydrous phases or carbonates), involve hydrates and/or carbonates and change the mineralogy of a rock volume according to the stability fields of the minerals, but do not liberate a fluid. Prograde subduction zone metamorphism (as is true for any type of prograde metamorphism) generally reduces the amount of H2O that can be stored in hydrous minerals with depth. Thus, almost any part of the oceanic crust sooner or later becomes fluid saturated. In an equilibrium situation, the volatile content bound in hydrous phases and carbonates remains constant until fluid saturation occurs. Either continuous or discontinuous reactions may lead to fluid saturation in a rock. The point at which this occurs depends on initial water content, and pressure and temperature, and somewhat counter-intuitively, initial low water contents do not cause early complete dehydration, but delay the onset of fluid production to high pressures. 

Ultra-high-pressure metamorphic rocks. Diamonds occur sporadically in crystals of garnet or zircon in various ultra-high-pressure metamorphic rocks formed in deep subduction zones. Crystals are of micrometer size, and the morphology is mostly spherulitic or cuboid, but octahedral is also... 

The arsenic concentrations of metamorphic rocks generally decrease with increased metamorphism (Ryan et al., 1996), 265. As temperatures increase, more arsenic volatilizes out of the rocks. Specifically, metamorphic rocks that formed at <45 km in an ancient subduction zone in California lost about 80-85 % of their arsenic as metamorphic temperatures and pressures increased from about 275 °C and. 5 kilobars (kb) to approximately 750 °C and 12 kb (Bebout et al., 1999). As discussed in Section 3.6.2, the volatilization of arsenic during metamorphism may be important in transferring the element from subduction zones back into the crust (Figure 3.2). 

Cowan, D.S. 1974. Deformation and metamorphism of Fransiscan subduction zones complex northwest of Pacheco Pass, California. Geol. Soc. Am. Bull. 85 1623-1634. 

Figure 27 Occurrences of isoclinally folded and boudinaged mafic layers in the Beni Bousera otogenic peridotite, northern Morocco, after Allegre and Turcotte, 1986 reproduced by permission of Nature Publishing Group from Nature, 1986, 323, 123-127. In their marhle cake model, Allegre and Turcotte interpret these mafic layers as subducted oceanic lithosphere—modified by partial melting and metamorphism—that was stirred, stretched, and thinned by mantle convection (see, also, Kornprobst et al., 1990 Blichert-Toft et al., 1999). For Pearson et al. (1993), these pyroxenites would rather derive from crystallization products of partial melts from subducted crust. They would have been emplaced as dikes in the hanging wall of a subduction zone and deformed during the exhumation of the...

The insights gained by these specialized studies of the central Transantarctic Mountains must be examined critically to determine whether the master hypothesis concerning the formation of the Ross orogen presented by Stump (1995) is still valid and still explains satisfactorily the deposition of detrital sediment along a passive rift margin which later evolved into an active subduction zone that caused the deformation and regional metamorphism of the rocks that now form the basement complex of the central Transantarctic Mountains. 

Fig. 8.23 The continental collision model considers that the structure of the Shackleton Range is the result of compression caused by the collision of the East Antarctic craton with the African continent. The resulting closure of the Mozambique ocean is recorded by the Mozambique fold belt. The Ross Orogen, which underlies the Transantarctic Mountains formed by compression of sedimentary and volcanic rocks in an active subduction zone followed by intrusion of the anatectic granitoids of the Granite Harbor Intrusives. Both tectonic processes affected the deposition and subsequent deformation and metamorphism of the basement rocks of the Shackleton Range (Adapted from Tessensohn et al. 1999a)...

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