We reached basaltic basement in Holes 1185A and 1185B at 308.5 and 309.5 mbsf, respectively. The age of the radiolarian nannofossil chalk directly above basement is middle Eocene, but fragments of limestone interbeds recovered within the basement section gave a tentative age of latest Cenomanian to Albian for the upper 15 m of basalt and possibly Aptian for interbeds 126 m below the top of basement in Hole 1185B (see "Biostratigraphy"). We cored 16.7 m of basaltic basement in Hole 1185A (Cores 192-1185A-8R through 11R; 67% recovery) and divided it into five units. The units range in thickness from 0.2 to 9.9 m (Table T7; Fig. F16) and consist of pillow lava sequences separated by limestone and hyaloclastite interbeds (Fig. F17). In Hole 1185B, we cored 216.6 m of basaltic basement (Cores 192-1185B-2R through 28R had 42% recovery; Core 29R had 0% recovery) and divided it into twelve units ranging in thickness from 1 to 65.2 m (Table T7; Fig. F16). Units 1, 3, 4, and 6-9 are composed of pillow lava (Figs. F17, F18), whereas Units 2, 5, and 10-12 are massive lava flows (Fig. F14) with minor pillow lavas at their tops and/or bases. We defined these units using a combination of criteria (Table T7), including the presence of limestone, breccia or hyaloclastite interbeds, changes in degree and type of alteration, downward changes from massive to pillowed flow type, and, in one instance, a marked change in drilling rate (see "Igneous Petrology" in the "Explanatory Notes" chapter). In Section 192-1185B-22R-7, we recovered the only intact contact between two units, represented by the chilled base of Unit 11 in direct contact with the fine-grained blocky top of Unit 12 (Fig. F19).
Holes 1185A and 1185B are only 20 m apart, and the petrologic characteristics of basalts from Hole 1185A are similar to those of the pillowed units at the top of the basement section in Hole 1185B. Thus, we present a single description for all the Site 1185 basement rocks.
The pillows generally have glassy margins typical of submarine lava flows (e.g., Kirkpatrick, 1978). Inside the glassy margins are aphanitic zones containing spherulites (Figs. F17, F20), some apparently nucleated on olivine phenocrysts. Some of the isolated spherulites are highlighted by alteration to smectite and Fe oxyhydroxide, giving them an orange-brown color (Figs. F21, F22; see "Alteration"). Pillow interiors are fine grained with variolitic texture imparted by elongate, radiating plagioclase laths with interstitial clinopyroxene and titanomagnetite. Recovery of pillowed units (Fig. F16) was generally poor (average = 10%), in contrast to the high (average = 74%) recovery of massive units (Sections 192-1185B-4R-1 through 5R-6; Cores 192-1185B-6R and 192-1185A-17R through 22R).
We observed systematic grain-size variations from the margins to the interiors of the massive units. The interiors of the massive flows (Fig. F23) are generally coarser grained than the interiors of the pillowed units. The massive flows typically contain alternating aphanitic and fine-grained layers. This grain-size banding is particularly well developed in the lower part of Unit 11 in Section 192-1185B-22R-1 (~473.2 mbsf), where several subparallel centimeter-wide bands alternating from very fine grained to aphanitic are present in the normally fine grained variolitic interior. The aphanitic layers become wider (tens of centimeters) downcore and have subhorizontal, diffuse contacts with the fine-grained layers (Section 192-1185B-22R-5). Section 192-1185B-22R-6 is dominated by aphanitic layers, which additionally contain rare subround, elongate or irregular patches of more coarsely crystalline material as much as a few centimeters in size (Fig. F24). We interpret the grain-size banding as a product of repeated inflation of the Unit 11 lava flow, with the aphanitic layers representing more rapidly crystallized regions of the flow. The more coarsely crystalline patches within the aphanitic layers may represent earlier, more slowly cooled, nearly crystallized parts of the same flow.
One characteristic feature of samples from Hole 1185A and from Units 1-9 of Hole 1185B is their relatively high (as much as 10%; e.g., Section 192-1185B-3R-1) olivine phenocryst content (Fig. F25). This petrologic feature is accompanied by a high MgO content (9-10 wt%; see "Geochemistry"). The olivine is generally replaced by green or black clay, Fe oxyhydroxide, or calcite, except for a few places where it is unaltered (e.g., Sample 192-1185A-8R-1, 15-18 cm). Olivine is the only phenocryst phase in the glassy rims and aphanitic margins and commonly forms glomerocrysts. The olivine phenocrysts are generally euhedral and equant, although some have elongate extensions. Most of the euhedral to subhedral, equant olivine crystals in the fine-grained interiors of pillows and massive flows are approximately the same size as the groundmass plagioclase and clinopyroxene crystals. This characteristic made determination of olivine phenocryst content in the fine-grained interiors difficult in hand specimen.
Units 10-12 have a generally lower abundance of olivine pheno-crysts than do Units 1-9. Plagioclase-rich xenoliths and solitary plagioclase xenocrysts (Fig. F25), similar to those observed from Hole 1183A (see "Igneous Petrology" in the "Site 1183" chapter), are present in Units 10-12 of Hole 1185B. The xenoliths are generally small (<6 mm), irregular to subround and equant, and contain colorless, translucent plagioclase. Rare, small (<2 mm), angular and equant miarolitic cavities are present throughout Unit 9; some are interconnected. Miarolitic cavities are also present but rare in Units 10-12 (also see "Alteration").
Thin section examination confirmed our macroscopic observation that a traverse from pillow margin to flow interior shows the following sequence of textures: glassy pillow rim (e.g., Fig. F26); partly glassy to cryptocrystalline and dendritic margin near the glassy rim; and fine-grained, holocrystalline interior (e.g., Fig. F27). The degree of alteration depends highly on proximity to veins but is generally slight to moderate (see "Alteration"). All the basalts are essentially nonvesicular. For complete thin section descriptions, see the "Core Descriptions" contents list.
The basalts are generally sparsely to moderately olivine phyric or sparsely to moderately olivine-plagioclase phyric with clinopyroxene as a minor phenocryst phase. The basement sequence can be divided broadly into upper and lower groups on the basis of different olivine phenocryst abundances (2%-7% and 1%-3%, respectively), combined with the appearance in Units 10-12 of plagioclase and rare clinopyroxene phenocrysts and plagioclase-rich xenoliths (Fig. F25). The upper group consists of all the units in Hole 1185A and Units 1-9 in Hole 1185B, whereas the lower group consists of Units 10-12 in Hole 1185B (Fig. F16). The patches of more coarsely crystalline basalt in the fine-grained portions of Units 10-12 in Hole 1185B consist of olivine, plagioclase, clinopyroxene, and titanomagnetite. Some of the clinopyroxene and plagioclase phenocrysts in the finer grained portions of Units 10-12 may have been disaggregated from the more coarsely crystalline patches or may be xenocrysts similar to those found at Site 1183 (see "Igneous Petrology" in the "Site 1183" chapter). Some of the plagioclase phenocrysts exhibit oscillatory zonation (Fig. F28).
Olivine phenocrysts (0.1-0.6 mm) are euhedral to subhedral in basalts from both holes. Olivine phenocrysts in the pillow margins are generally unaltered (Fig. F29), whereas those in the fine-grained interiors of pillows and massive flows are usually replaced completely by smectite, celadonite, and calcite (Fig. F30; see "Alteration"). However, a few olivine phenocrysts in fine-grained areas are unaltered (Fig. F31). Unaltered olivine phenocrysts contain rare, minute (<0.01 mm) glass inclusions. Chrome spinel (<0.01-0.02 mm) is commonly present in the upper group of lava flows but is rare to absent in the lower group. It is euhedral to subhedral and is usually present either as inclusions in olivine phenocrysts (Fig. F32) or as discrete crystals (Figs. F33, F34).
Plagioclase phenocrysts (0.1-0.6 mm wide) in the lower basalt group of Hole 1185B are typically subhedral to euhedral and tabular (Figs. F28, F30, F35). Clinopyroxene phenocrysts (0.15-0.2 mm) are anhedral to euhedral (Figs. F30, F35); some exhibit simple twinning and oscillatory and sector zoning. In Unit 12, clinopyroxene can form radiating needles, elongate crystals, and phenocrysts, all in the same sample (Fig. F30). Equant clinopyroxene phenocrysts are commonly observed in the cryptocrystalline to microcrystalline groundmass of Units 10-12. More coarsely crystalline patches are also present in these units, suggesting that the clinopyroxene crystals could simply be remnants of disaggregated patches (e.g., Samples 192-1185B-17R-1 [Piece 4E, 80-82 cm] and 22R-7 [Piece 2, 42-43 cm]).
We examined thin sections of two unaltered, glassy pillow rinds from Holes 1185A and 1185B, respectively. These rinds contain small olivine phenocrysts and elongate vesicles (Figs. F26, F36). Adjacent to the glassy rinds are aphanitic zones containing spherulites. The spherulites consist of skeletal to euhedral olivine surrounded by radiating clusters of acicular plagioclase and elongate olivine crystals (Fig. F37). Both dendritic olivine and small, euhedral olivine crystals with elongate extensions parallel to the c crystallographic axis (Fig. F38) are present in the groundmass.
The groundmass of pillow interiors exhibits variolitic, intergranular, and subophitic textures (Figs. F30, F31). The groundmass consists of plagioclase and clinopyroxene, with minor titanomagnetite, chrome spinel, and sulfide. Titanomagnetite (0.01-0.1 mm) can be euhedral to subhedral but is mainly skeletal (Fig. F39); it can be unaltered or partly to completely replaced by maghemite. Chrome spinel (<0.01-0.02 mm) is commonly mantled by titanomagnetite if the olivine host crystal is altered (Fig. F40). Sulfide (<0.01 mm) is present as subround to elongate blebs in the groundmass and as inclusions in the silicate minerals.
Units 10-12 contain more coarsely crystalline patches with intergranular, subophitic, or variolitic textures (Fig. F41). The size of the more coarsely crystalline patches is typically a few millimeters, but some are as large as 3 cm across (e.g., Sample 192-1185B-22R-6 [Piece 6A, 51-54 cm]; Fig. F42). As mentioned above, we interpret these patches as portions of slowly cooled crystal mush that were entrained in a later and more rapidly cooled pulse of magma within the same flow unit.
We analyzed 16 basalt samples by inductively coupled plasma-atomic emission spectrometry (ICP-AES) to determine whole-rock composition. All are tholeiites (Fig. F43; Table T8). In terms of their silica and alkali contents, the Site 1185 basalts are indistinguishable from those at all previous DSDP and ODP sites, as well as the other Leg 192 sites. However, all samples from Hole 1185A and Units 1-9 of Hole 1185B are olivine normative, whereas those from Units 10-12 of Hole 1185B are generally quartz normative (Table T8). All data for Hole 1185A basalts plot with the data for Units 1-9 of Hole 1185B in elemental variation diagrams. Incompatible minor and trace element data confirm the distinction between the upper and lower groups of basalts at Site 1185 (Fig. F44). All samples from Hole 1185A and Units 1-9 of Hole 1185B have lower TiO2, Zr, Sc, Y, V, and Sr abundances than do Units 10-12 of Hole 1185B. The lower units of Hole 1185B are compositionally similar to basalts from Sites 1183, 1186, and 807 (Units C-G), and the Kwaimbaita Formation on Malaita. The presence of chrome spinel in Units 1-9 is consistent with the elevated Cr contents (Fig. F45); in addition, these basalts have relatively high Ni and MgO contents (Table T8). The variation in Mg# within each of the two groups of basalts at Site 1185 is not accompanied by a corresponding variation in TiO2; each group has a restricted range in TiO2 content (Fig. F46). The range in Mg#, however, may have been exaggerated by mobility of MgO during alteration. Thus, the downhole chemical variation at Site 1185 shows that a more primitive basalt sequence (all units of Hole 1185A and Units 1-9 of Hole 1185B) overlies a more evolved series of flows (Units 10-12 of Hole 1185B; Fig. F47).
Units 10-12 in Hole 1185B are petrographically and geochemically similar to the basalt sequences recovered from Sites 1183, 1186, and 807 (Units C-G) but are mainly massive rather than pillowed flows. Basalts from Site 803 and Units C-G at Site 807 were described as fine grained and sparsely phyric with plagioclase, clinopyroxene, and olivine phenocrysts (Kroenke, Berger, Janecek, et al., 1991); the basalts from Site 1183 are mostly sparsely olivine (± plagioclase) phyric (see "Igneous Petrology" in the "Site 1183" chapter). In Units 10-12 of Hole 1185B, olivine and plagioclase are the dominant phenocryst phases. Basalts from Sites 1183 and 1186 and Units 10-12 in Hole 1185B contain plagioclase-rich xenoliths. Such xenoliths are also present in basalts of the Kwaimbaita Formation on the island of Malaita, Solomon Islands, and in Units C-G at Site 807. Tejada et al. (in press) noted the geochemical similarities between basalts from Units C-G at Site 807 and the Kwaimbaita Formation on Malaita. Shipboard ICP-AES analyses show that Units 10-12 from Hole 1185B are compositionally similar to basalts from Sites 1183 and 1186 and to those of the Kwaimbaita Formation. Basalt from Hole 1185A and Units 1-9 of Hole 1185B is petrographically and geochemically similar to that from Site 1187.
In summary, basalt flows in all units of Hole 1185A and in Units 1-9 of Hole 1185B have the lowest concentrations of incompatible elements yet measured in basalts from the Ontong Java Plateau. They are also the most primitive (Mg-, Ni-, and Cr-rich and with high Mg#) basalts so far recovered from the plateau. These primitive basalt units overlie an older sequence of more evolved basalts that closely resembles those of the Kwaimbaita Formation on Malaita.