Subduction of the lithosphere and metamorphism of the oceanic crust
This sequence aims to better understand the characteristics of metamorphism in the oceanic lithosphere, and more specifically in the oceanic crust, during subduction. A subduction zone is characterized by the plunge of oceanic lithosphere beneath continental or oceanic lithosphere. Under the influence of convergent forces, the oceanic lithosphere plunging into the underlying mantle is subjected to an increase in pressure and temperature, leading to metamorphic transformations. The oceanic lithosphere constituting the subducting plate is a geological unit consisting of oceanic crust at the surface and the upper lithospheric mantle at greater depths. Here, we will focus primarily on the metamorphism of the oceanic crust. The oceanic crust is composed of basalts at the surface and gabbros at greater depths. As it moves away from the mid-ocean ridge, the cooling of the oceanic crust, combined with water circulation within the crust, triggers hydroxylation reactions of the minerals that make up the rocks. Basalts and gabbros will metamorphose into greenschist-type metabasalts and metagabbros containing new minerals such as hornblende, chlorite, and actinolite, which are hydroxylated minerals indicative of hydrothermal metamorphism. It is important to understand that this metamorphism is not at all related to subduction. During subduction, the oceanic lithosphere—that is, the oceanic crust—plunges to great depths. Greenschist-type metagabbros are therefore subjected to an increase in pressure and temperature, which will lead to their transformation into blueschist-type metagabbros, characterized in particular by the appearance of a new metamorphic mineral such as glaucophane. When the oceanic lithosphere plunges even deeper during subduction, blueschists transform into eclogites, metamorphic rocks containing jadeite and garnet minerals—two new minerals indicative of high-pressure metamorphism. The subduction of the oceanic lithosphere thus led to metamorphism of the oceanic crust due to the increasing pressure and temperature conditions to which the rocks were progressively subjected. Finally, it's worth noting that the oceanic lithosphere also contains a portion of the upper lithospheric mantle composed of peridotite. Under the influence of water circulation within the lithosphere, the uppermost part of the mantle also undergoes hydroxylation, and the peridotites become serpentinized. These are called serpentinized peridotites or serpentinites. This metamorphism of the upper part of the mantle is not related to subduction.