10 cm in size) together with elongate hyaloclastite fragments.
Site 1277 is located atop a basement ridge (Mauzy Ridge) beneath a thin cover of sediment. The main objective at this site was to recover basement. For this reason, Hole 1277A was drilled without coring to 103.90 mbsf. It was predicted that the bit would still be ~30 m above the anticipated sediment/basement contact at that depth. While this upper section was drilled, a "wash" core barrel was in place. When retrieved, the core barrel contained 2.29 m of fractured igneous rock and associated volcaniclastic sediments (Core 210-1277A-1W) that were indistinguishable from the rock in the top of Core 2R. Drilling records indicate that hard zones were encountered at 85–89 and 97.5–100 mbsf (see "Operations"). Thus, the rock recovered in Core 210-1277A-1W most likely came from depths between 85 and 103.90 mbsf. We define the top of Unit 1 to be at the shallower depth of 85 mbsf. Two lithologic units are recognized in Hole 1277A. Because sedimentary units cored at this site are interbedded with, and derived from, rock types characteristic of basement at this site, we discuss all the sedimentary, igneous, and structural features together in this section (see piece logs in "Site 1277 Visual Core Descriptions").
Unit 1 is an igneous–sedimentary succession of alternating basalt flows with hyaloclastites (altered volcanic glass) and laminated sediments within neptunian fissures (~50%); these rocks compose ~50% of the unit. In addition, coarse matrix-supported breccias (20% of unit) contain a wide variety of clasts of gabbro and serpentinite, and variably deformed and differentiated gabbros (20%) appear to represent large clasts or fragments in matrix-supported breccias. There are also minor sandstones, locally ferruginous (<10%).
Unit 2 contrasts with Unit 1 in that it consists almost entirely of variably deformed serpentinized peridotite (~95%) cut by magmatic and calcite veins. The lack of sediments and lavas, as well as the occurrence of a steeply inclined and locally intense high-temperature foliation within the serpentinized peridotite (Fig. F1), suggest that this unit represents true basement (i.e., massive mantle and crustal rocks that are in situ).
The serpentinized peridotite drilled at Site 1277 is interpreted to be basement that represents tectonically exhumed mantle. This interpretation is compatible with the observation that the serpentinite in Unit 2 shows pervasive brittle deformation and hydrothermal alteration that decreases downhole. The downhole decrease in alteration may explain increasing core recovery with depth, from <9.28% at the top of the serpentinite (Core 210-1277A-6R) to 105.73% at the bottom of the hole (Core 9R, the last core of ODP). After exhumation, the serpentinites were buried by lavas and basement-derived sediments of Unit 1. Postcruise studies may help to confirm our inference that tectonic exhumation of Unit 2 was coincident with sedimentation and volcanism in Unit 1 and also to clarify how the clastic sediments were preserved on what is now a basement high.
Figure F2A summarizes the main lithologies of Unit 1. Details are given in the visual core descriptions for this site (see "Site 1277 Visual Core Descriptions"). Low recovery in Unit 1 (~35%, excluding Core 210-1277A-1W) means that the lithologic record is very fragmented and incomplete, so it may be biased toward certain rock types. Examples of typical occurrences of each rock type are described and briefly interpreted below.
There are three intervals of basalt flows (intervals 210-1277A-1W-1, 61 cm, through 1W-2, 75 cm; 2R-2, 68 cm, through 3R-4, 60 cm; and 5R-1, 3–135 cm) interbedded with clastic sediments (Figs. F2, F3, F4). Together, the flows form ~50% of Unit 1. The thickest example in Cores 210-1277A-2R and 3R includes one discrete flow at the base and several overlying, possibly composite flows. The lowest flow overlies sediments with a contact that was recovered intact; it has a chilled top and shows a gradual downhole increase in crystal size within the flow, from aphanitic to fine grained. The top of the flow is marked by ~1 cm of vesicular basalt grading into green, crustose, laminar hyaloclastite up to 1 cm thick. Some fragments of hyaloclastite are spalled off into an overlying thin interflow unit of clastic sediment that is several centimeters thick.
Most of the flows observed in the three intervals are cut by irregular fractures or fissures as wide as 1 cm that are filled with calcite-spar cement. Some incompletely filled fractures are lined with dogtooth calcite spar (Fig. F4). Larger fissures (up to 4 cm) are also present and are filled with sediment composed of poorly sorted silt- to granule-size clastic sediments derived from the basalts (Fig. F3). In several fissures, internal sediment has planar lamination and there are also occasional elliptical to elongate fragments of concentrically laminated hyaloclastite. In one interval, the basalt clasts and matrix are stained a reddish color, especially at the margins of the clasts, which resulted from alteration at or near the seafloor.
Chilled margins at the tops of all flows and gradual downhole increases in crystal size in the flows indicate that the basalts are relatively intact and are not large fragments within a sedimentary breccia. Hyaloclastites between the flows probably originated from the chilled tops of the flows. Hyaloclastite and basaltic sand filtered down into cracks in the flows; these sediment-filled fissures are interpreted as neptunian dikes. They are filled with carbonate, reddish iron oxide, and sediments, and they have a jigsawlike appearance. There are two generations of clastic-sediment fill. The first is soft, brown, unstructured, fine-grained ferruginous or ferromanganese-rich sediment (Fig. F5). The second generation, which cuts the first generation, is pale gray, altered silt- and granule-size clastic sediment that shows a vague subhorizontal lamination defined by coarser sand grains.
There are several intervals of disorganized breccia that contain subangular to subrounded clasts of gabbro and serpentinized peridotite (e.g., Sections 210-1277A-4R-1 through 4R-2 and 5R-1, 125 cm, through 5R-2, 29 cm) (Fig. F6). The breccias form ~20% of the recovery in Unit 1, and the clasts constitute ~30% of the breccia. The serpentinized peridotite clasts preserve a great variety of structures (Fig. F6), ranging from high-temperature peridotite mylonite (e.g., intervals 210-1277A-4R-1, 8–25 cm) to undeformed massive serpentinite.
A few gabbro clasts preserve a mylonitic structure, although most gabbro preserves primary magmatic textures. Most of the gabbro is magmatically differentiated and contains plagioclase, pyroxene, brown amphibole, and illmenite; no olivine was observed. Secondary mineral phases include chlorite, tremolite, and calcite. Most gabbro clasts are less altered than equivalent small intrusive gabbro veins in the underlying serpentinized peridotite of Unit 2.
The breccia matrix (~70% of the rock) is composed of finer-grained subangular to subrounded clasts of serpentinite and gabbro that form a reddish brown silty sandstone (Fig. F6). This is cemented by calcite spar, which may be a later hydrothermal precipitate. The breccias are unsorted, and there is a wide range of clast sizes. Like the lava flows, the breccias are cut by calcite veins, some as wide as 2 cm. Many larger clasts have rims of calcite spar up to several millimeters thick. The breccias are matrix supported and have a polymict, disorganized fabric; these features suggest emplacement by mass-flow processes.
There are two intervals of gabbroic rocks that are not associated with a sedimentary matrix; these are interpreted as large clasts or fragments within mass-flow deposits. The upper of the two intervals (Sections 210-1277A-2R-1, 13 cm, through 2R-2, 40 cm) is >177 cm thick and is composed of strongly altered and cataclastically deformed gabbro (Fig. F7). The overall gabbroic texture is well preserved, but locally it is disrupted and shows plagioclase and pyroxene porphyroclasts within a foliated chlorite-rich matrix. Individual porphyroclasts are as large as several centimeters, are angular to well rounded, and have altered margins. A weak foliation is defined by a preferred orientation of the fabric within the chloritic matrix and by a crude alignment of mainly the smaller (<2 cm) elongate porphyroclasts.
The lower interval (Sections 210-1277A-5R-2, 68 cm, through 6R-1, 52 cm) is a foliated gabbro cataclasite. It is considered to have a tectonic rather than sedimentary origin, and it is described as part of Unit 2 (Fig. F2B) (see "Foliated Cataclasites and Gouges" in "Unit 2").
Several intervals of locally ferruginous clastic sediment appear in Unit 1, forming less than ~10% of the overall recovery. These beds are all less than ~50 cm thick and have variable sedimentary textures and structures. Several examples are described below.
In intervals 210-1277A-1W-2, 90–104 cm (Fig. F8), 2R-2, 41–68 cm, and 3R-4, 61–108 cm, there is calcite-cemented, polymict breccia conglomerate with angular to rounded clasts as large as 2 cm. The highest of these intervals is normally graded, ranging from pebbles to coarse sand, with a few scattered larger clasts toward the top. The clasts are dominantly mafic (~60%) and ultramafic (~40%). The lower two intervals consist of granule- to pebble-size grains of gabbro and serpentinite set in a poorly sorted matrix of siltstone to sandstone consisting of the same lithologies. The clasts are subangular to subrounded. There is postdepositional calcite replacement of the matrix in these breccias.
A 40-cm-thick sedimentary breccia in interval 210-1277A-1W-1, 10–61 cm (Figs. F5, F9), is composed of basalt fragments randomly distributed in a matrix of greenish, unsorted, clastic sediment cemented by calcite spar. The angular clasts are mainly unaltered aphyric basalt (up to 10 cm in size) together with elongate hyaloclastite fragments.
In interval 210-1277A-5R-2, 33–42 cm (Fig. F10), there is a single piece of finely laminated, reddish to purple and gray, fine- to medium-grained, graded sandstone that may be either a sedimentary bed or the sedimentary fill of a large neptunian dike.
Interval 210-1277A-1W-1 (Piece 2A, 5–9 cm) contains a single piece of well-indurated, brown, clastic sediment impregnated with ferromanganese oxide (Fig. F5). Coarse sand grains in this piece consist of variably altered basalt, feldspar, and bioclasts that include strongly altered, agglutinated benthic foraminifers. These grains are enveloped by a microlaminated ferromanganese crust showing centimeter-scale, laterally linked, irregular, and wrinkled domes. We interpret this distinctive ferruginous sediment to be part of a thin authigenic cap on the basement high.
The top of Unit 2 is defined by the downward disappearance of sedimentary rocks and basalt flows. It consists of a thin 50-cm-thick interval of strongly altered and tectonized gabbro that overlies serpentinized peridotite. The degree of serpentinization is >95% throughout Unit 2. Downhole, the intensity of brittle deformation decreases and a transition to serpentinized peridotite mylonite is observed. The peridotite shows a complex history of hydrothermal alteration and polyphase veining. This includes intrusion of highly differentiated plutonic rocks. At the time of the intrusions, the peridotite was already serpentinized, as indicated by complex reaction rims of talc, amphibole, and calcite at the interface between the magmatic veins and the altered ultrabasic rock. Extensive calcite veining is also observed throughout the succession.
Figure F2B summarizes Unit 2. Details are given in the core descriptions for this site (see "Site 1277 Visual Core Descriptions"). Examples of typical occurrences of each rock type are described below.
The top of Unit 2 (interval 210-1277A-6R-1, 0–52 cm) (Fig. F2B) consists of greenish gray heterogeneous foliated cataclasites that comprise ~5% of Unit 2. The rocks contain angular to subrounded gabbro clasts in a fine groundmass formed from abraded minerals (formerly plagioclase and pyroxene) altered to albite, chlorite, and calcite. Fragmented spinel grains derived from mantle peridotite were also observed. Elongated clasts and minerals within these rocks define a vague foliation, and the rocks are interpreted to be tectonic rather than sedimentary. The occurrence of foliation and the absence of carbonate cement, such as that found in Unit 1 sediments, support this observation. The presence of spinel within the gabbroic rocks suggests that ultramafic rocks were involved in the formation of the foliated cataclasites. We tentatively suggest that both mafic and ultramafic rocks were deformed in a shear zone as they were exhumed.
Serpentinized peridotites were recovered from Section 210-1277A-6R-1, 52 cm, to the bottom of the hole. They comprise ~95% of Unit 2. The uppermost interval in Cores 210-1277A-6R to 8R is composed of serpentinized peridotite that generally is strongly brecciated and calcite veined, although the rocks commonly preserve a high-temperature foliation (Figs. F1, F11). Below this, primary mantle fabrics are better preserved in Core 210-1277A-9R, which contains a spectacular intact section of serpentinized peridotite mylonite (105.73% recovery) (Figs. F12, F13, F14). The original mineralogical composition of this peridotite is masked by serpentinization (locally up to 100%), and crystal size has been strongly reduced by dynamic recrystallization. Visual inspection of the core reveals 5%–10% orthopyroxene porphyroclasts with minor spinel, but this may be an underestimate of the total amount of pyroxene. Most of the spinel crystals range from needle shaped to vermicular to small aggregates intergrown with orthopyroxene (<1 mm). Postcruise investigation will be needed to define the composition and nature of these rocks more precisely.
Well-developed mylonitic foliation in Core 210-1277A-9R is defined by preferred elongation of orthopyroxene porphyroclasts (<1 cm in size) with aspect ratios of 4:1 and higher. Unfortunately, the matrix is completely serpentinized and no primary microstructures are observed. Thin section observations show that the mylonitic foliation is overgrown statically by serpentine, indicating that the deformation predates the serpentinization and is probably unrelated to final exhumation of these rocks to the seafloor. The mylonitic foliation dips at angles as much as 80° and is cut by later subhorizontal brittle shear zones, which also suggests that the foliation is an early fabric.
Colors ranging from orange green to greenish gray are observed in Core 210-1277A-9R. They are unrelated to the underlying structures or primary mineralogical composition. Thin section observations indicate that they are controlled by variable intensity of alteration.
The serpentinized peridotite in Unit 2 is intruded by magmatic veins that show complex reactions with the ultramafic host rock (e.g., Sections 210-1277A-9R-1 and 9R-2) (Fig. F14). Within these reaction rims, the growth of talc and amphibole over previously serpentinized peridotite indicates that the veins postdate serpentinization. The veins show a polyphase alteration history, ending with fracturing and precipitation of calcite that filled the veins or was deposited at their edges. In some places, calcite was precipitated as veins and as calcite cement in yellowish brown serpentinite breccia (e.g., intervals 210-1277A-6R-1, 52–68 cm, and 7R-1, 0–20 cm). These breccias have a jigsawlike fabric but no sedimentary structure, showing that they are tectonic rather than sedimentary in origin.
Although pervasive veining in Unit 2 records several stages of fracturing and mineral precipitation, there is no clearly defined succession in terms of vein structure, size, or composition. Many of the smaller veins that cut the serpentinized peridotite are composed of calcite with subordinate talc and magnetite. The earlier veins tend to be relatively gently dipping (e.g., in interval 210-1277A-8R-2, 0–100 cm). In contrast, the larger veins (reaching 1.5 cm wide) are mainly subvertical and infilled with calcite spar (e.g., intervals 210-1277A-8R-1, 1–27 cm, and 108–119 cm).
Calcite veins in both the basalt flows of Unit 1 and the underlying serpentinized peridotites of Unit 2 suggest that fracturing and precipitation of calcite occurred after both units were in place. Therefore, calcite veins are considered to be the youngest structures observed in the basement rocks at Site 1277. The calcite-veined serpentinites are comparable to certain ophicalcites known from ultramafic sections of the western Tethyan region (e.g., Alps and Liguria) (Bernoulli and Weissert, 1985).
