Development and subsequent demise of an oceanic spreading center : the Troodos ophiolite, Cyprus

Ash, Christopher Henry (1990) Development and subsequent demise of an oceanic spreading center : the Troodos ophiolite, Cyprus. Masters thesis, Memorial University of Newfoundland.

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Abstract

The sequence of magmatic events present within the lower plutonic crustal section exposed in the Amiandos area of the Troodos ophiolite records the death of a once active oceanic spreading center. At least three and possibly four magmatic episodes are evident on the of basis field relationships. The progressive variation in geometry, scale, composition and texture of these successive magmatic events appears to have resulted from a "decaying" or lowering oceanic geothermal gradient. -- Earliest identified crustal processes record the development of a relatively large (at least 3 to 4 km) open-system magma chamber which is defined by a differentiated plutonic suite of ultramafic to mafic cumulates. -- Lowering geotherms result in a freezing of the axial magma chamber and a lowering of the brittle-ductile transition zone into the lower crust and upper mantle. The axial magma chamber which is initially affected by high temperature ductile deformation accommodates extension by brittle failure which in turn controls the geometry and distribution of subsequent magmatic episodes. These are defined by much smaller fault controlled dike-like bodies that are lithologically and texturally highly variable, but temporally gradational. -- Accompanying the structural change is a significant reduction in the supply of magma to the ridge system. Chemically these magmas become progressively more depleted (REEs and TiO₂) and increasingly refractory (enriched in Mg, Cr and Ni) through time and appear to be correlative with the Troodos lava sequence. -- A definitive explanation for the death of the ridge system at this stage is not possible, however "ridge jumping" or a locking up of the subduction system from which the Troodos crust was being generated due to collision, are possible candidates.

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