The metamorphic petrology working group logged 2792 veins as well as groundmass alteration assemblages on a piece-by-piece basis downcore. This was supplemented by an examination of some 243 thin sections of the Leg 176 cores. The individual observations were made by two teams of two investigators, with one team identifying, measuring, and logging veins (Alt and Kelley) and the other characterizing and logging alteration in the groundmass of the gabbros (Bach and Bideau). The same types of observations, and others, were then made independently on the thin-section suite by a fifth investigator (Robinson), who separately confirmed and expanded the observations of the macro description teams. In hand sample, plagioclase alteration was based on the extent of a milky white appearance. This did not distinguish between hydrothermally altered plagioclase and plagioclase recrystallized by the high-temperature granulite facies deformation, which was pervasive in many sections of the core. Thus the total background alteration logged downhole is significantly greater than that seen in thin section, where secondary plagioclase due to hydrothermal alteration was more readily distinguished from that produced by dynamic recrystallization. Otherwise, the thin section observations were remarkably consistent with the observations of alteration in hand specimen. The thin section observations are presented in Figure 11. Total alteration and vein abundances downhole are plotted in Figure 12. This uses additional data on vein abundances for the upper 500 m (H.J.B. Dick, pers. comm., 1997).

The gabbros recovered during Leg 176 range from fresh to 40% altered, although there are many small intervals where alteration is far more extensive. Typically, however, there is less than a few percent total hydrothermal alteration through long sections. The most intensely altered portion of the core occurs between 500 and 600 mbsf, where amphibole in addition to secondary recrystallized plagioclase is most abundant, and the primary minerals are on average 10% to 40% altered. This contrasts sharply with the amphibolite gneisses at the top of Hole 735B, where total alteration was more intense, coming close to 100% (e.g., Robinson et al., 1991; Stakes et al., 1991). Calcite veins, associated with low temperature oxidation of the rocks, are also abundant between 500 and 600 m, evidently reflecting ongoing alteration at low temperatures due to the presence of open fractures. A second zone of intense recrystallization occurs between 800 and 1030 mbsf, where many of the rocks exhibit high-temperature plastic deformation, and veins are rare. Two less altered zones are located in an interval of abundant smectite veins at 1300 1500 mbsf. Below 1030 m, the intensity of alteration is generally much less than 10%.

The Leg 176 gabbros preserve a complex record of high-temperature metamorphism, brittle failure, and hydrothermal alteration that began at near solidus temperatures and continued down to very low temperature conditions. The highest temperature metamorphic effects are transitional with magmatic processes: they most likely overlap both temporally and spatially, and in places distinguishing the effects of these two processes is difficult. This is particularly true of the felsic vein assemblages, which range from clearly magmatic to apparently exclusively hydrothermal. It was not unusual to find plagioclase and diopside veins, and combinations thereof, on splays from apparently igneous felsic veins (Fig. 13). In general it would appear that the locus of felsic veins also served as a conduit for late fluids, possibly often of magmatic origin, often leaving a heavy overprint on the igneous assemblages, which extended to partial replacement of some veins with clays.

Granulite facies metamorphic conditions (>800° 1000°C) are clearly marked by localized, narrow zones of crystal-plastic deformation that cut igneous fabrics. These intervals are characterized by anastomosing layers of olivine and pyroxene neoblasts that are bounded by plagioclase-rich bands. In some places, the high-temperature shear zones are associated with impregnations of oxide gabbros; in many cases, these zones have abundant recrystallized brown hornblende, indicating that deformation continued down to amphibolite facies metamorphic conditions. Other high-temperature effects probably resulting from late-stage magmatic activity include the formation of plagioclase+amphibole veins and diopside-rich veins, which in some intervals are progressively transposed into localized zones of high-temperature shear. Many of these rocks, veins, and shear zones reflect the effects of late magmatic hydrous fluids, but these zones also acted as pathways for later hydrothermal fluids at various temperatures.

Static high-temperature alteration is commonly associated with vein formation, and it is patchy throughout the section. Extensive intervals (>300 m) are marked by less than 10% total background alteration. This alteration is generally manifested by coronitic alteration halos around olivine grains, and the common replacement of clinopyroxene by variable amounts of brown amphibole. In more evolved rocks, magnesium-amphibole±talc typically replaces orthopyroxene. The secondary minerals most likely formed under low water-to-rock ratios over a range of temperature, from >600° 700°C down to much lower temperatures.

Ingress of high to moderate temperature fluids (400° 550°C) was facilitated by subvertical amphibole veins that are probably related to cooling and cracking of the rocks in the subaxial environment (e.g., Fig. 14). However, the abundance of amphibole veins decreases markedly with depth, as does the alteration, and below 600 mbsf amphibole veins are rare. In the Leg 118 section of Hole 735B amphibole veins and groundmass alteration were associated with zones of deformation at the top and bottom of the hole, with a sharp drop in abundance in undeformed intervals. The greatest alteration, often with complete replacement of mafic phases by amphibole and the highest vein abundances, was situated in the upper 100 m in a zone of intense deformation (Dick et al., 1991a; Stakes et al., 1991). A second more irregular zone of deformation and alteration was present in the bottom 100 m, where generally replacement of mafic phases by amphibole was only partial. Although this lower zone of deformation and associated amphibole alteration continued into the upper 100 m of the Leg 176 section, this is not observed in deeper intervals of the core, where foliated gabbros largely underwent crystal-plastic deformation at high temperatures. Microcracks filled with talc, magnetite, amphibole, sodic plagioclase, chlorite, and epidote are sporadically present throughout the core and represent smaller scale fracturing and fluid penetration under greenschist facies metamorphic conditions.

Cessation of hydrothermal fluid flow is marked by abundant late smectite, carbonate, and zeolite ± prehnite veins and iron oxyhydroxide minerals that are associated with intense alteration at 500 600 m. These minerals reflect low-temperature alteration by circulating seawater solutions, and they are most likely related to the presence of a fault at 560 mbsf. Smectite veins (Fig. 15), unlike the higher temperature vein assemblages, are often associated with alteration haloes where olivine and even pyroxene are extensively replaced by smectite. Below this interval, veins of smectite ± pyrite ± calcite, together with associated smectitic alteration of surrounding wallrock, reflect low temperature hydrothermal reactions under more reducing conditions. These effects occur throughout much of the core, but the abundant smectite veins at 600 800 m are most likely related to a second fault at 690 mbsf. This lower temperature set of veins formed in tensional fractures and is most likely related to cooling of the block during uplift of the massif to form the transverse ridge.

An anomalous feature of the Hole 735B cores is that high-temperature alteration assemblages are most abundant at the top of the section and low-temperature assemblages near the base. Greenschist assemblages, representing intermediate conditions, are relatively minor. This inversion of the normal order of things, as for example has been found for the in situ section of sheeted dikes and pillow lavas at Hole 504B (e.g., Alt, Kinoshita, Stokking, et al., 1993), is reasonably attributed to the unusual cooling history of the section. Apparently, at an early stage, conditions for the percolation of water to great depth did not exist beneath the rift valley, and alteration was largely limited to the upper portions of the gabbroic crust. Before a normal cooling profile could be established, however, the massif was unroofed and rapidly emplaced to the seafloor. The rapid cooling and relatively static conditions in the interior of the uplifted block inhibited extensive greenschist facies alteration, with only a single large epidote vein found in the entire lower two-thirds of the hole. In contrast, lower temperature alteration phases are abundant in the lower two-thirds of the core. This reflects alteration of the massive gabbro as it cooled during uplift. Further alteration occurred during subsequent circulation of seawater as the gabbros were transported away from the rift axis through a series of relatively restricted zones of fracture associated with block uplift.

To 176 Geochemistry

To 176 Table of Contents

ODP Publications

ODP Home