Gabbros recovered at Site 894 during Leg 147 provide the first constraints on fracture formation and metamorphism in the root zones of hydrothermal systems at the fast-spreading East Pacific Rise. Metamorphism in the gabbros was related to fracture formation, fracture filling to form veins, and spatially associated fluid-rock reactions. The earliest veins are microscopic amphibole veins responsible for pervasive (10% to >50%) alteration of the gabbros to amphibolite-facies mineral assemblages which record metamorphic temperatures of 600°-750°C. Metamorphism associated with these veins locally occurred to lower temperatures of the greenschist-amphibolite transition. Rare macroscopic amphibole veins everywhere crosscut the microscopic veins. They are filled by mineral assemblages consistent with formation in the amphibolite facies to the greenschist-amphibolite transition (450°-600°C). Crosscutting the amphibole veins are chlorite-calc-silicate veins and chlorite-smectite veins. Together, these vein types define prominent, regular sets which are abundant at Site 894. Chlorite-calc-silicate vein assemblage imply formation in the greenschist facies (250°-450°C), whereas the chlorite-smectite vein assemblage probably reflects lower, sub-greenschist facies conditions (<300°C). Latest zeolite-calcite veins are rare, display no alteration halos, and formed at very low temperatures.
Reorientation of core pieces to present geographical coordinates demonstrates that macroscopic amphibole veins show no preferred orientation, whereas chlorite-bearing vein types display strong preferred east-west strikes with moderate to steep dips. Combination of these observations with petrologic and theoretical constraints on the temperatures and timing of mineral formation suggests formation of amphibolite-facies mineral assemblages within 60,000 yr and several kilometers of axial emplacement at the East Pacific Rise. The east-west strikes of later chlorite-bearing veins suggest formation when the westward-propagating Cocos-Nazca Spreading Center rifted the newly formed EPR crust to form Hess Deep. This may have taken place up to tens of kilometers and several hundred thousand years after gabbro emplacement. The inferred temperatures of up to 750°C for the onset of brittle failure and fluid-rock interaction in the gabbroic portion of fast-spread oceanic crust are higher than has previously been recognized. In addition, variable metamorphic temperatures and alteration extents imply that microfracturing did not proceed along a homogeneous isothermal cracking front; rather it developed heterogeneously in both space and time. However, high temperatures and a dependence of metamorphic mineral composition on grain-scale bulk compositional variation argue for low time-integrated fluid fluxes and rapid reaction rates.
Date of initial receipt: 2 August 1994
Date of acceptance: 24 February 1995
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