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Title: Mantle-melt and mantle-fluid interactions in suprasubduction zones : evidence from the Troodos Massif, Cyprus
Author: Freeman, Jonathan
Awarding Body: Durham University
Current Institution: Durham University
Date of Award: 1996
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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.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Geochemistry Geochemistry