PRINCIPAL RESULTS

A site somewhere between Site 1185 and Site 803 would be particularly useful because, unlike other sites on the main plateau, Sites 803 and (on the basis of shipboard biostratigraphic data) 1185 contain basalt that is younger than 122 Ma, and the lava flows in the upper 125 m of basement at Site 1185 are compositionally different from any seen elsewhere on the plateau. We therefore selected Site 1187 (Fig. 1), near Site 804 (which did not reach basement) on the eastern edge of the main plateau. Site 1187 is 194 km southeast of Site 803, 146 km north of Site 1185, and <3 km from the easternmost point where Ontong Java Plateau basement can be distinguished easily, on seismic records, from that of the Nauru Basin (Fig. 34). Our principal objectives at Site 1187 were to establish the composition, age, and eruptive environment of the basement volcanic rocks and to compare them with those of lavas sampled at Sites 803 and 1185. In particular, we wanted to determine whether basement in this region was emplaced during the ~122-Ma or later magmatic events, or both.

We encountered basaltic basement at 372.5 mbsf (Table 1), ~40 m shallower than estimated from the seismic record. We had begun coring at 365.5 mbsf and recovered only 1.36 m of the sedimentary succession above basement. The sediment recovered consists of dark brown ferruginous claystone that grades downward from burrow mottled to laminated and overlies an ~2-cm-thick chalk layer. Biostratigraphic analysis of the claystone indicates an age of late Aptian to Albian. The chalk layer, which immediately overlies basalt, contains a late Aptian foraminiferal assemblage, including the planktonic taxa Globigerinelloides ferreolensis, Blowiella duboisi and Blefuscuiana praetrochoidea, and a lower-slope benthic assemblage. The calcareous nannofossils Eprolithus floralis and Hayesites irregularis, present without any Albian-restricted species, are consistent with a late Aptian age for the chalk. Washed residues of the overlying claystone very rarely contain the planktonic foraminifer Globigerinelloides aptiensis, indicating an age range of late Aptian to middle Albian. A single, questionable specimen of the Albian taxon Blefuscuiana albiana was also recovered. The claystone residues are dominated by fish-bone fragments and small ferromanganese nodules, suggesting slow accumulation below the CCD.

A 70-cm-thick interval of the claystone is reversely magnetized. The late Aptian to Albian biostratigraphic age of the claystone suggests that this interval may be ISEA, a short reverse polarity subchron (~115 Ma) within the Cretaceous normal superchron. The underlying basalt flows all exhibit normal polarity. Paleoinclination data for the basalt indicate a paleolatitude of 19°S, essentially the same as at Site 1186 and within error of values for Sites 1183 and 1185.
We cored 135.8 m of basaltic basement, which we divided into 12 units (ranging in thickness from 0.7 to 41.3 m; Fig. 35) on the basis of recrystallized limestone, significant (>10 cm thick) hyaloclastite interbeds, and/or downward changes in basalt structure from massive to pillowed (e.g., the contact between Units 6 and 7). Most of the sequence consists of pillow-lava flows. The only unequivocally massive portion is the fine-grained, 9-m-thick base of Unit 6; some basalt interpreted to be from pillows >2 m thick also may be from massive flows.

The rims of the pillows contain both unaltered and altered glass. Basalt inside the rims is aphanitic, generally altered, and often contains spherulitic zones stained by Fe oxyhydroxide. In the larger pillows, the grain size coarsens gradually from aphanitic near the margins to fine grained in pillow interiors. The basalt is aphyric to moderately olivine phyric. Fresh olivine is present in some of the least-altered intervals of fine-grained basalt in pillow interiors and in the massive basalt of Unit 6. Rare, irregular miarolitic cavities (as large as 1 cm x 2 cm) are present in the interiors of pillows and in the massive portion of Unit 6. Shipboard ICP-AES analyses (Fig. 10, Fig. 11, Fig. 12, Fig. 13) show that the basalt is relatively primitive (MgO 9 wt%; Cr 485 ppm), like the upper 125 m of lava flows at Site 1185, and virtually identical to them in its unusually low concentrations of incompatible elements (e.g., Zr and Ti). The presence of these distinctive lava flows in >100-m thick piles at two sites 146 km apart implies that substantial volumes of this type of magma were erupted on the eastern flank of the high plateau after the main plateau-forming eruptions at ~122 Ma.

Seawater-derived fluids have interacted at low temperatures with the basaltic basement, resulting in the most pervasive overall alteration observed on Leg 192. This observation is consistent with the high abundance of relatively small pillows. Alteration occurred under highly oxidizing conditions with high water:rock ratios and resulted in the development of light and dark yellow-brown colors through the complete replacement of olivine and the alteration of groundmass to smectite (saponite and nontronite) and Fe oxyhydroxide near the outer zones of pillow margins. The color grades into dark brown and dark gray in the coarser-grained pillow interiors. Despite the high average level of alteration in the basalt, unaltered glass is relatively abundant at Site 1187 because of both the large number of individual pillows (i.e., more glassy margins are present per length of core) and the greater thickness of many of the glassy margins compared with those at other Leg 192 sites. Overall, the secondary mineral assemblages and visual characteristics of basalt alteration at all Leg 192 sites are remarkably similar to those seen elsewhere in nonplateau seafloor of varying ages. This similarity indicates that alteration conditions in basement on the Ontong Java Plateau were similar to those operative in typical ocean crust formed at spreading centers.

Veins throughout the basement sequence consist mainly of calcite, zeolites (probably phillipsite with analcime), smectite, Fe oxyhydroxide, and rare celadonite and pyrite. As at Sites 1185 and 1186, the physical properties of basement at Site 1187 strongly reflect the amount of veining and alteration in the basalts. P-wave velocities are high (>5300 m/s) in the dense, relatively sparsely veined basalt of Units 6 and 7 and lower (<5300 m/s) in the more abundantly veined basalt of other units. Areas of high magnetic susceptibility also correlate with the presence of dense, unveined basalt, and the mean bulk densities of sparsely veined intervals are >2.7 g/cm³ vs. <2.7 g/cm³ in abundantly veined intervals.

The major results of drilling at Site 1187 are summarized as follows:

1. A late Aptian biostratigraphic age for an ~2-cm-thick chalk layer directly above basaltic basement and a late Aptian to Albian age for a ferruginous claystone overlying the chalk suggest that basalt at this site is older than the Albian to latest Cenomanian age indicated for the upper portion of basement at Site 1185.

2. Basement at Site 1187 consists mainly of basaltic pillow lavas, with minor massive flows. Despite the apparent difference in age, the chemical composition of Site 1187 basalt closely resembles that of the upper 125 m of lava flows at Site 1185. Both groups of lavas are significantly less differentiated and poorer in incompatible elements than other Ontong Java Plateau basalts. Relatively large amounts of incompatible element-poor, rather high Mg basalt appear to have been erupted along the eastern edge of the plateau. These rocks represent high total fractions of partial melting, possibly of previously melted mantle.

3. The level of basaltic alteration at Site 1187 is generally greater than at other Leg 192 sites, although fresh glass is present in many of the pillow rims, and unaltered olivine is abundant in some pillow interiors and in the one massive flow. The basalt flows have normal magnetic polarity and yield a paleolatitude of ~19°S, which agrees within error with values obtained for basement at Sites 1183, 1185, and 1186. A 70-cm-thick reversed polarity interval in the claystone overlying basement may correspond to ISEA, a short reverse-polarity subchron (~115 Ma) within the Cretaceous normal superchron.