HIGH-TEMPERATURE DUCTILE DEFORMATION OF SITE 920 PERIDOTITES

Georges Ceuleneer and Mathilde Cannat

ABSTRACT

The high-temperature textures of the serpentinized harzburgites drilled during Ocean Drilling Program Leg 153 range from coarse-grained equigranular to mylonitic. Coarse-grained equigranular texture is fully preserved in about 5% of core, and becomes increasingly abundant downhole. It consists of a mosaic of equant olivines and equant to slightly elongate pyroxenes, ranging in size from a few mm to more than 1 cm. Olivine presents a moderately developed lattice fabric resulting from crystal-plastic flow under conditions of very high temperature (>1200C) and low stress (< 1 MPa). The flow plane associated with this fabric is subhorizontal (average dip of 10). The flow line is preferentially oriented west-northwest-east-southeast, at a low angle to the spreading direction. In most samples, the coarse olivine crystals are partly (70% of the studied samples) to totally (25% of the samples) replaced by a medium-grained (around 1 mm) mosaic of olivine crystals displaying well-developed euhedral shapes. Grain-size reduction of pyroxene crystals is locally well developed. We interpret this texture in terms of static recovery and recrystallization that postdated the formation of the lattice fabric and that is synchronous with, or slightly postdates, a melt percolation event evidenced by trails of strain-free poikilitic clinopyroxenes enclosing medium-grained olivine crystals. In about 30% of the samples, virtually restricted to the upper half of the hole, the coarse- and medium-grained textures are slightly overprinted by a higher-stress (10-100 MPa) and lower-temperature (around 1000C) deformation texture, as shown by the development of a fine-grained (0.1-0.3 mm) matrix and of closely spaced subgrain boundaries in olivine and pyroxene (porphyroclastic textures). Low total strain is associated with this lower-temperature deformation, as evidenced by the limited development of the fine-grained matrix (typically 10%-20% matrix only). Mylonites (>90% matrix) are scarce (<1% of the core recovery), and are restricted to millimeter- to centimeter-thick shear zones that, in all cases, are spatially related to gabbroic veins.

Date of initial receipt: 7 August 1995
Date of acceptance: 25 April 1996


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