Mantle-melt and mantle-fluid interactions in suprasubduction zones : evidence from the Troodos Massif, Cyprus
The Troodos Massif exposes an intact section of harzburgitic mantle from its contact with crustal
lithologies to a depth of approximately 3 km, where it is faulted out against a mass of heavily
fractured and serpentinised peridotites: the serpentinite diapir. The harzburgites are host to
several generations of pyroxenitic and dunitic intrusives, many of which have features suggestive
of a reaction relationship with the enclosing harzburgites such as resorbed harzburgite xenoliths
and marginal dunites.
Mineral chemistry and whole-rock data suggest that the harzburgites in the Troodos sequence are
residues from up to 30% fractional partial melting in the spinel stability field. The serpentinite
diapir exposes iherzolitic lithologies which can be modelled by 10 to 15% fractional partial
melting in the spinel stability field. In both cases, the starting composition for the melt modelling
was a fertile MORB mantle source.
Deviations from the compositions expected to result from simple fractional partial melting are
found in several situations in the mantle section and suggest that melts/fluids interacted with the
mantle during and after the partial melting event. Three main situations are identified: i)
enrichments in mineral chemistry and whole-rock parameters in specific parts of the background
harzburgite section; ii) mineral chemistry enrichments around pyroxenites and iii) the
clinopyroxene crystals in the Troodos harzburgites which have LREE/HREE ratios higher than
those that could be produced by simple fractional melting models.
In the background harzburgites, mineral chemistries were enriched at the top of the sequence
(Anomaly 1) and in a layer towards the base of the sequence (Anomaly 2). Pyroxenites also
enriched their wallrocks and two trends were identified on the basis of spinel compositions. The
Type I trend is of Cr-Fe-Ti enrichment and is similar to the mineral chemistry variations in the
Anomaly 1 harzburgites. The melt involved is inferred to be tholeiitic. The Type II trend is of
Mg-Al enrichment and is similar to the mineral chemistry variations in the Anomaly 2
harzburgites. The melt involved is inferred to be boninitic. The fact that the lower pillow lavas
(LPL) have tholeiitic chemistries and the upper pillow lavas (UPL) boninitic chemistries suggests
a link between the melt which crystallised the pyroxenites and the pillow lava sequence.
The clinopyroxene trace element patterns from the background harzburgites suggest that the
LREE, Nd, Sr and Zr are enriched in these minerals compared to the expected values for
fractional melting. The enriched component was modelled from the clinopyroxene data and is
similar in trace element pattern to the enriched component in the UPL. This suggests that the
addition of the subduction component that has been proposed to explain the UPL chemistries was
probably added to the mantle both during and after the melting event which produced the UPL.