STRUCTURE AND PETROLOGY OF HYDROTHERMAL VEINS IN GABBROIC ROCKS FROM SITES 921 TO 924, MARK AREA (LEG 153): ALTERATION HISTORY OF SLOW-SPREAD LOWER OCEANIC CRUST

Yildirim Dilek, Pamela D. Kempton, Peter Thy, Stephen D. Hurst, Donna Whitney, and Deborah S. Kelley

ABSTRACT

Gabbros recovered during Ocean Drilling Program (ODP) Leg 153 from Sites 921 to 924 in the Mid-Atlantic Ridge at the Kane Transform (MARK) area display metamorphic and structural features that record a complex alteration and deformational history of lower crustal rocks formed in a slow-spreading ridge environment. Exposures of gabbroic rocks on the western wall of the Mid-Atlantic Ridge, nearly 10 km west of the spreading axis and about 6 km south of the Kane Fracture Zone, indicate their rapid unroofing within 500,000 to 750,000 yr via faulting and block uplifting near the ridge-transform intersection. Hydrothermal vein assemblages in the gabbroic rocks reflect an alteration chronology that has accompanied crystal-plastic and brittle deformational episodes, as the lower crust was emplaced on the median valley wall. Hydrothermal alteration began preferentially within and along locally developed extensional discrete shear zones, which acted as high-permeability pathways for Na-rich fluids that caused albite enrichment in the plagioclase and Fe enrichment in the clinopyroxene. The concentration of Fe-Ti oxides along some of these discrete shear zones suggests that the shear zones also controlled late magmatic (evolved) and subsolidus fluid flow in the gabbroic rocks. Crack networks within and adjacent to the discrete shear zones facilitated amphibole veining with progressive cooling of the rocks below 500C. Amphibole compositions in veins changed from pargasitic and edenitic hornblende to actinolitic hornblende and actinolite, in parallel with the disappearance of secondary clinopyroxene, accompanying the gradual lowering of temperatures from amphibolite facies to greenschist/amphibolite transitional facies metamorphic conditions. Changes in the compositions of vein amphibole and vein chlorite resulted from combined effects of progressive cooling and changing water/rock ratios and water/rock reactions. Development of amphibole-chlorite and chlorite veins was facilitated by penetration of hydrothermal fluids into the lower oceanic crust along distributed microfractures, cataclastic zones, and shear zones with further cooling that attended continued tectonic extension and crustal stretching. This stage of veining overprinted the previously developed deformation fabrics and hydrothermal veins and resulted in retrograde metamorphism under greenschist metamorphic conditions in and around the discrete shear zones. Chlorite compositions indicate temperatures of chlorite equilibration of 150-200C based on an empirical chlorite solid-solution geothermometer. Rare subvertical composite veins that are composed of epidote plagioclase quartz prehnite oxide and clay minerals are probably related to cracking front in the exhumed lower oceanic crust, and may represent sealed cracks developed during emplacement of the gabbroic rocks on the seafloor. Cataclastic zones in the uppermost sections of some of the holes and in certain intervals at depth show renewed hydrothermal alteration via moderate fluid/rock ratios and represent extensional faults associated with the emplacement of the gabbroic rocks in the ridge-transform intersection massif. The vein chronology and deformation fabrics in gabbroic rocks from Sites 921 to 924 are similar to those documented from ODP Hole 735B on the Southwest Indian Ridge, and indicate that the circulation of hydrothermal fluids in gabbroic rocks at MARK occurred at progressively lower temperatures through time as the slow-spread oceanic lower crust was emplaced on the seafloor. These characteristic features of the hydrothermal alteration and spatial and temporal relations between deformation fabrics and hydrothermal alteration in slow-spread oceanic crust differ from those observed in fast-spreading oceanic crust and in the Semail ophiolite, in which hydration occurred uniformly along microfractures and crack networks that developed as a result of downward propagation of cracking front.

Date of initial receipt: 15 August 1995
Date of acceptance: 29 April 1996


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