Subduction-zone initiation (SZI) is the onset of downward plate motion forming a new slab, which later evolves into a self-sustaining subduction zone. - Crameri et al. (2020)

Ocean-plate tectonics (OPT) is a mode of mantle convection characterised by the autonomous relative movement of multiple discrete, mostly rigid, portions of oceanic plates at the surface, driven and maintained principally by subducted parts of these same plates that are sinking gravitationally back into Earth’s interior and deforming the mantle interior in the process. - Crameri et al. (2019)

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Crameri, F., C.P. Conrad, L. Montési, and C.R. Lithgow-Bertelloni (2019), The life of an oceanic plate, Tectonophysics, 760, 107-135, doi:10.1016/j.tecto.2018.03.016

Early basalts (a.k.a. Fore-arc basalts (FAB) or Lower lavas) are MORB-like volcanic rocks that are likely products of the very first lavas that erupt during SZI. They underlie Boninites and arc-like volcanic rocks. These basalts differ from typical MORBs due to low Ti/V ratios (high V at similar Ti contents). They do not exhibit signatures of slab fluids and may be precursors to Boninites/arc-like volcanic rocks. Even though this type of basalt was initially observed in the fore-arc setting of the Izu-Bonin Mariana system (hence the term Fore-arc basalt - Reagan et al. 2010), it is not clear where it is actually emplaced during SZI. The clearer term Early basalts avoids misconceptions about its emplacement.

Reagan, M. K., Ishizuka, O., Stern, R. J., Kelley, K. A., Ohara, Y., Blichert‐Toft, J., ... & Hickey‐Vargas, R. (2010). Fore‐arc basalts and subduction initiation in the Izu‐Bonin‐Mariana system. Geochemistry, Geophysics, Geosystems, 11(3).

Boninites are primitive andesitic extrusive rocks with a chemical composition of >54 wt% SiO₂, <0.5 wt% TiO₂ and >8 wt% MgO (Mg# >0.6) (Crawford et al. 1989). High MgO contents and the presence of clinopyroxene suggest high temperatures and water contents in the mantle wedge with a highly depleted harzburgite residue. Boninites can be subdivided into low-Si and high-Si suites (Pearce and Reagan 2019). Whereas low-Si Boninites are not only associated to SZI, the high-Si ones appear to be uniquely associated with SZI. If associated to SZI, Boninites are the first melt products in which the influence of the slab is geochemically observed. 

Crawford, A. J., Falloon, T. J., & Green, D. H. (1989). Classification, petrogenesis and tectonic setting of boninites.

Pearce, J. A. & Reagan, M. K. (2019). Identification, classification, and interpretation of boninites from Anthropocene to Eoarchean using Si-Mg-Ti systematics. Geosphere 15, 1008-1037.

SZI ophiolites are remnants of oceanic crust that formed via spreading of the overriding plate during early stages of subduction. As ophiolites, these pieces of oceanic crust and mantle have been tectonically emplaced above sea level (e.g., Dilek and Furnes 2014), however, SZI ophiolites are distinct from back-arc or mid-ocean ridge ophiolites, which have a different geochemical signature. SZI ophiolites are also referred to as Supra-subduction-zone ophiolites in the literature.

Dilek, Y., & Furnes, H. (2014). Ophiolites and their origins. Elements, 10(2), 93-100.

Metamorphic soles are thin (<500 m thick), fault-bounded sheets of highly deformed meta-volcanic and meta-sedimentary rocks showing an inverted pressure- and temperature gradient that structurally underlie many SZI-ophiolite complexes. Metamorphic soles are interpreted to be derived from the top of a nascent oceanic slab that accreted to the base of the still hot overriding plate during the incipient stages of intra-oceanic thrusting (Casey and Dewey, 1984; Hacker, 1990; Wakabayashi and Dilek, 2000; Dewey and Casey, 2011).

Casey, J. F., & Dewey, J. F. (1984). Initiation of subduction zones along transform and accreting plate boundaries, triple-junction evolution, and forearc spreading centres—implications for ophiolitic geology and obduction. Geological Society, London, Special Publications, 13(1), 269-290.

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Dewey, J. F., & Casey, J. F. (2011). The origin of obducted large-slab ophiolite complexes. In Arc-Continent Collision (pp. 431-444). Springer, Berlin, Heidelberg.

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Hacker, B. R. (1990). Simulation of the metamorphic and deformational history of the metamorphic sole of the Oman ophiolite. Journal of Geophysical Research: Solid Earth, 95(B4), 4895-4907.

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Wakabayashi, J., & Dilek, Y. (2000). Spatial and temporal relationships between ophiolites and their metamorphic soles: a test of models of forearc ophiolite genesis. SPECIAL PAPERS-GEOLOGICAL SOCIETY OF AMERICA, 53-64.