At Site 1276, igneous rocks were recovered from diabase sills that intrude lithologic Subunit 5C (see "Site 1276 Thin Sections" and piece logs in "Site 1276 Core Descriptions"). An upper sill (Subunit 5C1) was recovered in Sections 210-1276A-87R-6, 72 cm, through 88R-7, 122 cm. A second interval comprising several sills, here termed a "sill complex," was drilled deeper in the hole from the top of Core 210-1276A-97R through Core 102R (Subunit 5C2). The individual sills in the lower complex range from a few centimeters to >4 m in thickness. The cored igneous rocks are all aphyric diabase composed of primary plagioclase, pyroxene, magnetite, olivine, apatite, biotite, and glass. Near the contacts of the sills, hydrothermal alteration of the diabase ranges from strong to total; it becomes less intense toward the center of the sills. Secondary mineral phases include various clay minerals (mainly smectite, kaolinite, and chlorite), calcite, quartz, analcime, and, in rare cases, serpentine (Table T3).
Most of the sills preserve chilled margins. The average crystal size increases from the margins toward the centers of the sills, with a crystal size as large as 1 mm in the centers of the larger sills. Crystal size variations are accompanied by changes from a predominantly intersertal texture near the sill contacts to an intergranular to subophitic texture toward the centers of the sills. Where primary contacts with the sediments are preserved, the sediments are baked over a distance of as much as 1 m and are associated with marked color changes. Recrystallization and the formation of new minerals and very high reflectance of the organic matter observed in the sediments only occur near the intrusive contacts.
The upper sill, forming Subunit 5C1, extends from Section 210-1276A-87R-6, 72 cm (top of sill at 1612.72 mbsf), to Section 88R-7, 117 cm (base of the sill at 1623.00 mbsf). The rocks are aphanitic to fine-grained aphyric diabase. Hydrothermal alteration is high to total at the margins and moderate toward the center of the sill.
The high recovery (Core 210-1276A-87R = 93.6% and Core 88R = 97.1%) and lack of drilling disturbance make it possible to document the sill/sediment relationships, as well as the lithologic and textural variations across the sill. The sediments show a baked contact at the top of the sill, which is well preserved in Section 210-1276A-87R-6 (Fig. F119). The sill is ~10 m thick and preserves chilled margins at its upper and lower contacts. Although hydrothermal alteration is high to complete at the margins and moderate in the center of the sill, the primary magmatic textures can still be recognized, allowing three zones of magmatic origin to be defined based on grain size and textural variations (Fig. F120). From the margin to the center sill, zone 1 is marked by crystal sizes <0.3 mm in diameter and a predominant intersertal texture; zone 2 is defined by crystal sizes ranging between 0.5 and 0.8 mm in diameter and shows intersertal, intergranular, and subophitic textures; in zone 3, the average grain size is 0.8 mm in diameter and the texture is predominantly subophitic. The contacts between these zones are gradual and were determined based on observations made on the core and from thin section analysis.
The upper contact of the sill is sharp and parallel to bedding of the overlying sediments. The contact is preserved in Section 210-1276A-87R-6 (Piece 3A) (Fig. F119). Sediments overlying the contact are recrystallized grainstones (i.e., marbles) and calcareous siltstones that preserve a primary sedimentary lamination (Fig. F121). In the calcareous siltstones, calcite porphyroblasts are observed (for details see Fig. F115). They decrease in size away from the contact and become gradually less prominent until they can no longer be seen >66 cm from the contact (Section 210-1276A-87R-6, 6 cm) (Fig. F121); their maximum size near the contact is ~1 cm. These porphyroblasts, together with the recrystallized metagrainstones, are interpreted to record a thermal overprint related to emplacement of the sill. The results of this thermal overprint can be observed in interval 210-1276A-87R-6, 6-72 cm (Fig. F121). In smear slides, evidence for a thermal overprint is seen in Sample 210-1276A-87R-6, 41-42 cm, where a high reflectance of organic matter is observed. In Sample 210-1276A-87R-6, 67 cm, euhedral biotite is present and it could be an alteration product (see "Site 1276 Smear Slides"), although a detrital origin cannot be excluded.
In the calcareous siltstones overlying the sill, a subvertical crenulated vein occurs in interval 210-1276A-87R-6, 5-41 cm (Fig. F121). This vein is filled with calcite and pyrite. Folding of the vein is most likely related to compaction, indicating that the vein formed before there was significant compaction of the sediment into which the sill intruded (see "Physical Properties"). In interval 210-1276A-87R-6, 46-55 cm, sediment laminae warp around the calcite porphyroblasts, suggesting that they also predate compaction. Assuming that both the porphyroblasts and the vein formed as a consequence of sill intrusion, it is likely that the sill was emplaced at a very shallow level in weakly consolidated sediments. Thus, the age of emplacement may be close to that of the surrounding sediments (i.e., uppermost Aptian[?]-lower Albian; see "Biostratigraphy").
The lower contact of the sill was not recovered in one piece (Fig. F122). A chilled margin marks the base of the sill, but no baked contact is preserved in the underlying sediments. It is therefore difficult to estimate the extent of thermal overprint of these sediments. The sediments are calcareous claystones that preserve sediment laminations, and no direct evidence for a thermal overprint was found in a smear slide of the sediments (Sample 210-1276A-88R-7, 119-120 cm) (see "Site 1276 Smear Slides").
The primary mineralogy and composition of the rocks forming the sill are very similar throughout Sections 210-1276A-87R-6 through 88R-7. The igneous intrusion is diabase with a groundmass consisting of plagioclase (40%-60%), clinopyroxene (10%-30%), magnetite (<5%), subordinate olivine (<5%), and glass (<20%) (see "Site 1276 Thin Sections").
Zone 1 forms the outer parts of the sill; it includes the interval from Sections 210-1276A-87R-6, 72 cm, to 87R-7, 16 cm, and the interval from Sections 88R-7, 80 cm, to 88R-7, 117 cm. The outer margins of the zone are marked by sharp chilled margins. The inner margins are gradational to zone 2. Structures and veins are rare; vesicles are observed only in interval 210-1276A-87R-6, 78-85 cm. The rocks in this zone are massive greenish gray aphyric diabase with crystal sizes <0.5 mm. Thin sections used to classify the mineralogy and textures of zone 1 were taken from intervals 210-1276A-87R-6, 86-89 cm (2i in Fig. F120), 87R-6, 108-113 cm (3i in Fig. F120), and 88R-7, 93-96 cm (10i in Fig. F120) (see "Site 1276 Thin Sections"). XRD samples were taken adjacent to the thin section locations and were used to identify the groundmass mineral composition (Table T3). Primary minerals are rarely preserved in zone 1, and they are replaced by clay minerals (mainly kaolinite and chlorite), calcite, quartz, pyrite, and, in one sample, rare serpentine. The primary mineral composition was estimated to be plagioclase (60%), clinopyroxene (10%), magnetite (<5%), subordinate olivine (<5%), and glass (<20%). The dominant magmatic textures are intersertal (Fig. F123).
Zone 2 includes the interval from Sections 210-1276A-87R-7, 16 cm, to 88R-2, 40 cm, and the interval from Sections 88R-5, 40 cm, to 88R-7, 80 cm. Contacts with zones 1 and 3 are gradational. The rock in this zone is greenish gray massive diabase with crystal sizes ranging between 0.5 and 0.8 mm. Thin sections used to classify the mineralogy and textures of zone 2 include Samples 210-1276A-87R-7, 16-19 cm (1i in Fig. F120), 88R-1, 93-95 cm (13i in Fig. F120), 88R-5, 104-109 cm (8i in Fig. F120), and 88R-6, 69-72 cm (9i in Fig. F120) (see "Site 1276 Thin Sections").
Primary minerals are locally well preserved, and their composition is estimated to be plagioclase (60%), clinopyroxene (20%), magnetite (<5%), subordinate olivine (<5%), and glass (<10%). Magmatic textures in zone 2 are more variable than in the remainder of the sill. The dominant magmatic textures are intergranular. Intersertal and subophitic textures are also observed in contiguous domains within the same thin sections (Fig. F124). Secondary minerals are smectite- and kaolinite-group clay minerals, quartz, and pyrite (see XRD results in Table T3). Apatite, analcime, and serpentine are found only in single samples and do not occur throughout zone 2.
Zone 3 forms the center of the sill and includes the interval from Sections 210-1276A-88R-2, 40 cm, to 88R-5, 40 cm. The rocks in this zone are greenish gray diabase that exhibits a characteristic "patchy" pattern. Average crystal size is homogeneous and is on the order of 0.8 mm throughout the zone (Figs. F125, F126). Thin sections used to characterize the mineralogy and textures of zone 3 include Samples 210-1276A-88R-2, 93-96 cm (4i in Fig. F120), 88R-3, 56-59 cm (5i in Fig. F120), 88R-3, 121-125 cm (6i in Fig. F120), and 88R-4, 111-115 cm (7i in Fig. F120) (see "Site 1276 Thin Sections" and Table T3). The primary mineral assemblage is locally well preserved; its composition is estimated to be plagioclase (60%), clinopyroxene (30%), magnetite (<5%), subordinate olivine (<5%), and glass (<5%). The magmatic texture in zone 3 is predominantly subophitic. Intergranular and intersertal textures occur in the same thin sections but are less abundant (Fig. F127). In all the samples analyzed by XRD from zone 3, smectite- and kaolinite-group clay minerals, quartz, pyrite, and analcime are present as secondary minerals. Apatite, analcime, and serpentine are found only in occasional samples.
Segregation bands occur throughout the sill (Fig. F120) but form <5% of the total recovered interval. The bands are typically as thick as 5 cm, light colored, and subhorizontal with distinct (but not sharp) contacts with the host rock. An example of a segregation band is observed in interval 210-1276A-88R-2, 91-94 cm (Fig. F125). In thin section, this lithology shows texture, composition, and crystal shape that are different from the host rock. The major mineral phase observed in these bands is zoned plagioclase crystals as much as 1 mm in diameter. Clinopyroxene is rare or absent. Some of the bands are strongly altered. Large calcite crystals (as large as 4 mm in diameter) are observed in the bands. Pyrite shows characteristic acicular crystals that are as large as 5 mm.
The intensity of alteration is high to complete at the margins of the sill, but alteration decreases to moderate levels toward the center of the sill. It is not clear to what extent alteration is controlled by magmatic textures and crystal size, both of which define the different zones in the sill. It appears that the alteration was not sufficiently strong to mask the internal magmatic zonation of the sill. The observation that calcite and chlorite occur only at the contacts of the sill, whereas analcime is found only in the center, suggests that the sills experienced a polyphase alteration history, which merits postcruise investigation.
Twelve samples from Hole 1276A were analyzed by ICP-AES for major and trace element abundance. The results are given in Table T6. Sample preparation and analytical methods are given in "Geochemistry" in the "Explanatory Notes" chapter. At least two representative samples were taken from each of the zones discussed above. The ICP-AES samples were taken close to the thin section and XRD samples, which enables us to link compositional variations with mineralogical and textural observations.
Major and trace element variation diagrams are shown in Figure F128. Apart from two samples from the altered margins of zone 1 (Samples 210-1276A-87R-6, 86-89 cm, and 87R-6, 107-108 cm), the major and trace element concentration plots might conceivably be interpreted as primary magmatic trends (e.g., K2O, Na2O, and CaO vs. SiO2) (Fig. F128). However, the presence of analcime and quartz (both determined by XRD) indicates that some mobilization of major element oxides has taken place. One sample from zone 3 (center of the sill) shows higher values of SiO2 and Zr and is interpreted as possibly a more differentiated magma (Sample 210-1276A-88R-4, 110-111 cm). The high Zr values in this one sample deviate from any inferred magmatic trend, possibly because of the presence of the mineral zircon in this sample. In thin section, this sample contains zoned feldspar phenocrysts typical of a segregation band, suggesting that it is not representative of the whole-rock composition and, again, alteration effects are likely.
Plots of TiO2 vs. Zr and Y vs. Zr (bottom of Fig. F128), elements that are considered to be immobile under low-temperature alteration processes (subgreenschist facies in this case), may distinguish magmatic trends from alteration effects. In these plots, all of the strongly altered samples of zone 1 plot into a very narrow field that generally is indistinguishable from data points for the less altered samples of zones 2 and 3 (except for the more differentiated Sample 210-1276A-88R-4, 110-111 cm). The absence of any clear chemical distinction between samples from these three zones of the sill on these plots suggests that all three zones are magmatically related.
When the compositions of the diabase samples of zones 2 and 3 are plotted on the Na2O + K2O vs. SiO2 classification diagram of Le Bas et al. (1986), most plot outside the diagram because of anomalously low SiO2 contents (Fig. F129). At face value, the plots are suggestive of fractionation of an undersaturated magma giving rise to highly alkaline rocks such as basanite. However, the apparent absence of nepheline, a major phase in basanites, causes us to question whether the magma was really so alkaline. Silica mobility (see above) could have displaced compositions from an originally more basaltic character. For comparison, data from volcanic rocks recovered from the New England seamounts located off the eastern United States (Pe-Piper et al., 1990) plot on the Na2O + K2O vs. SiO2 classification diagram in the same field as the Site 1276 samples (i.e., alkaline). Volcanic rocks from the Newfoundland seamounts 180 km to the south are considerably more differentiated, and they also show an alkaline trend. Our provisional interpretation, based largely on the immobile major and trace elements chemistry, is that the diabase sills are likely to be of alkaline composition, but the extent of magmatic fractionation and chemical alteration require postcruise chemical data to elucidate.
The lower sill complex (Subunit 5C2) extends from Sections 210-1276A-97R-3 to 102R-1 at the base of the hole. Igneous rocks are observed in five intervals (210-1276A-97R-3, 141-150 cm; 97R-4, 0-3 cm; 98R-1, 112-137 cm; 98R-2, 1-31 cm; and from 98R-CC, 10 cm, to the last recovery in 102R-1, 0-2 cm) (Fig. F130). In the first four intervals, the sills range in thickness from decimeters to a few centimeters; the thinner sills may represent small apophysies (magmatic fingers) related to the larger sills. From Core 210-1276A-98R to the base of the hole, low recovery and strong drilling disturbance make it difficult to determine if this complex is a single thick sill or several thinner sills.
The rocks forming the sill complex are aphanitic to medium-grained aphyric to seriate diabase. The primary phases in most of the rocks are plagioclase, pyroxene, olivine, magnetite, and glass. In contrast to the upper sill, the lithologies forming the lower sill complex are more heterogeneous. Hydrothermal alteration of minerals is strong to complete in the smaller sills and moderate in the diabase cored in Core 210-1276A-99R. Secondary minerals are clay minerals (mainly smectite, kaolinite, and chlorite), quartz, pyrite, and calcite.
In the lower sill complex, most of the sediment/sill contacts are strongly affected by drilling disturbance and only one was recovered (interval 210-1276A-97R-3, 139-150 cm) (Fig. F131A). In this interval, the sill margin is chilled; the adjacent sediments are only slightly altered and still preserve sedimentary structures. A thin section from the upper margin (Fig. F131B), a few millimeters thick, consists of a glassy matrix separating sediments from aphanitic diabase. Evidence for recrystallization is not observed in the sediments. In the diabase, vesicles of irregular to elongate shape are seen and are filled with aggregates of small calcite crystals. Curved chilled contacts, some of which overlie a complex zone composed of sparry calcite, contain clasts of diabase and show a geopetal fabric (Fig. F131A). These structures, together with the vesicles, suggest that the emplacement of the sills occurred at very shallow levels in the sediments.
Thermal overprint in the sediments is manifested by color changes in Core 210-1276A-98R (see "Images" in "Site 1276 Core Descriptions"). In this core, color changes occur over a 188-cm interval from Sections 210-1276B-98R-2, 89 cm, to 98R-CC, 10 cm. In this interval, the sediments preserve their primary characteristics (see "Hydrothermally Altered Sediments" in "Unit 5" in "Lithostratigraphy") and, aside from the color change, no further macroscopic evidence for a thermal overprint is observed. However, smear slides from this interval (see "Site 1276 Smear Slides") show very high reflectance of organic matter as well as recrystallization of calcite and clay minerals that record a thermal overprint. In this context, it is also important to mention that smectites appear in the sedimentary rocks separating the upper and lower sills. This indicates that the thermal overprint of the sills was strongly localized to the immediate vicinity of the contact zones; otherwise, smectites probably would not have been preserved.
In the lower sill complex individual sills vary substantially in thickness, and they consequently preserve different textures and crystal sizes (Figs. F130, F131). The very thin sills in Cores 210-1276A-97R and 98R are aphanitic diabase that is dominated by intersertal textures. Crystal size and textural variations that are similar to those observed in the upper sill can be observed in the deeper sill cored in Core 210-1276A-99R. However, there are some differences. For example, ophitic and seriate textures appear in Core 210-1276A-99R; this mainly reflects the larger crystal sizes of augites that are as large as 3 mm in diameter (Fig. F132).
No complete geochemical data set is available for the lower sill complex because there was insufficient time to analyze samples on the ship. A few analyses that were entered in the Janus database should be treated with caution. Some of the silica and loss on ignition values appear to be unrealistic. Postcruise chemical data are needed to resolve this issue.