Why does a subduction zone produce a serpentinization diapir rather than volcanism? The classic Troodos Ophiolite in Cyprus has been uplifted by a 'serpentinization event'. Upper mantle (peridotite) has been serpentinized creating a buoyant diapir. This has uplifted the ocean crust sufficiently for it to be thrust over continental crust (underlying continental crust has been inferred from gravitational surveys).

The last sentence of the abstract of "Tertiary uplift history of the Troodos massif, Cyprus", AHF Robertson says:


  The dominant driving force may have involved the liberation of water
  from a subduction zone dipping northward beneath Cyprus.


This makes sense - serpentinization is the alteration process of olivine -> serpentine in the presence of water. And, as we know, subduction slabs release fluids (primarily water) into the overlying mantle wedge.

These fluids usually promote partial melting leading to andesite arc volcanism. However, Cyprus has this big serpentinite diapir instead of volcanism. Why?

What causes the fluids from the subducting slab to occasionally result in serpentinization of the mantle wedge instead of partial melting?
After some brief research, serpentinite diapirism occurs due to deep penetration of seawater at temperature around 100-200$^o$C [1]. To the south of the rift that was forming the ophiolite sequence there was an oceanic plate subducting as African plate moved north. This subducting oceanic plate provided the seawater that serpentinized the harzburgite in the ophiolite sequence. From my understanding this removed the water from the system before it reached depths to form partial melt. The subducting plate also assisted in pushing the diapir to the surface through the detachment faults present from the rifting.[1]:http://earth.huji.ac.il/data/pics/Nuriel%20et%20al%202009.pdf [image]:http://www.moa.gov.cy/moa/gsd/gsd.nsf/dmlTectonic_en/dmlTectonic_en?OpenDocument

tectonic evolution http://www.moa.gov.cy/moa/gsd/gsd.nsf/f6b12283222a96a3c225734000315725/a9e0eeb1c9dc2e5ec225734000329c12/MainText/0.8A?OpenElement&amp;FieldElemFormat=jpg