MICROFABRICS AND PATTERNS OF DYNAMIC RECRYSTALLIZATION IN GABBROIC ROCKS FROM THE MARK AREA, MID-ATLANTIC RIDGE

John Fletcher, Georges Ceuleneer, and Mathilde Cannat

ABSTRACT

Gabbroic rocks sampled during Leg 153 record a continuous spectrum of microfabrics that range from magmatic to crystal plastic to cataclastic. Magmatic fabrics are defined to have less than 20% recrystallized plagioclase and they typically contain either aligned or randomly oriented euhedral plagioclase and ophitic to subophitic olivine and pyroxene. Crystal-plastic fabrics show a wide range of degrees of recrystallization. Plagioclase neoblasts are dominantly 100-200 Ám in size, clinopyroxene neoblasts are dominantly <100 Ám, and olivine neoblasts show no dominant size. The crystal-plastic fabric contains a strong stretching lineation oriented parallel to the maximum-dip direction of a more weakly developed foliation. Kinematic indicators consistently show normal sense of shear across the foliation. On the basis of individual microstructures and patterns of dynamic recrystallization, clinopyroxene is significantly more competent in this ductile regime than plagioclase, and olivine has a relative competence intermediate between that of clinopyroxene and plagioclase.

The degree of plagioclase recrystallization increases with decreasing modal abundance of olivine and increasing abundance of orthopyroxene such that troctolitic gabbro is relatively less recrystallized than are gabbronorite and ferrogabbro. We interpret this strong partitioning of crystal-plastic deformation into more evolved gabbros to reflect, most likely, differences in hydrolytic weakening of the gabbroic rocks. Semibrittle and cataclastic fabrics also show normal-sense shear and are likely to have developed in response to the same tectonism that produced the higher grade fabrics.

Crosscutting relationships show that neoblast size progressively diminished and deformational style evolved from ductile to brittle. Microfabrics, compositional zoning of minerals, and synkinematic mineral assemblages demonstrate that deformation developed under progressively decreasing temperature and increasing pore-water pressure.

Date of initial receipt: 3 August 1995
Date of acceptance: 4 March 1996


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