IGNEOUS PETROLOGY

We reached basement at 372.5 mbsf (366.97 mbsf curated depth) and penetrated a further 135.8 m, recovering 74.3% of the rock cored (Cores 192-1187A-2R to 16R; Fig. F10). The age of the thin chalk layer directly above basement is late Aptian (~114-112 Ma; see "Biostratigraphy"). The basaltic basement was divided into 12 units ranging in thickness from 0.7 to 41.2 m. We assigned the unit boundaries on the presence of recrystallized limestone (Fig. F11), significant hyaloclastite intervals (Fig. F12), or downcore changes in character from massive to pillowed (see "Igneous Petrology" in the "Explanatory Notes" chapter). A summary of the individual unit and unit boundary characteristics is given in Table T4. The units are composed of pillow lavas, except Units 6 and 7, which have massive bases with overlying pillows. The pillows, which we define as small cooling units with visible upper and lower chilled margins, range in thickness from several centimeters to 2.5 m (Fig. F10). Some of the cooling units interpreted as large pillows actually may be thin, massive flows, but the lack of three-dimensional information makes it impossible to distinguish between these alternatives.

The recovered basalt units are aphyric to moderately olivine phyric (7% olivine; e.g., Section 192-1187A-13R-1 [Piece 9B]). Many complete vertical sections through individual pillows were recovered (Figs. F12, F13); one section along a pillow margin shows a breakout pillow (Fig. F14). From margin to interior, the pillows have the following features: a generally altered (but locally unaltered) glassy margin, an Fe oxyhydroxide-stained spherulitic zone (Figs. F12, F13), and an aphanitic central zone without spherulites. In the larger pillows, grain size coarsens gradually from aphanitic rims toward fine-grained interiors.

The fine-grained groundmass of both the pillowed and massive basalt is composed of plagioclase and clinopyroxene ± black oxides and generally has a poorly developed variolitic texture. Some basalt from the massive part of Unit 6 and the interior of the largest pillows has a mottled appearance produced by varying proportions of coarser grained and aphanitic patches.

Two plagioclase-rich xenoliths were found in Subunit 3B; one measures 15 mm × 25 mm (e.g., Section 192-1187A-3R-5 [Piece 7]). Rare, round to elongate and irregular vesicles (1.5 mm; e.g., in Section 192-1187A-16R-2 [Piece 1A]) are present adjacent to pillow margins and are filled with calcite and green clay. Rare to sparse irregular miarolitic cavities (as large as 10 mm × 20 mm) are most abundant in the fine-grained interiors of pillows and the massive portion of Unit 6. These cavities are filled with calcite, green and brown clay, Fe oxyhydroxide, and zeolites. Basalt in Hole 1187 is generally moderately to highly altered, although the fine-grained interiors of pillows and massive basalt (away from veins) and some of the glassy margins are only slightly altered (see "Alteration"). Olivine phenocrysts are generally replaced by Fe oxyhydroxide in spherulitic zones and by black or green clay near veins in the fine-grained intervals, although several intervals of fine-grained basalt contain unaltered olivine (e.g., Section 192-1187A-10R-7 [Piece 1B]).

Olivine phenocrysts are commonly present as glomerocrysts and are generally replaced by Fe oxyhydroxide, brown clay, or calcite (Fig. F15). In the least altered areas (away from veins), the olivine crystals have unaltered interiors (Fig. F16) that occasionally contain glass inclusions (mostly devitrified). Euhedral to subhedral chrome spinel, typically associated with the olivine phenocrysts, is also present (Fig. F17).

The groundmass in the pillow margins is spherulitic and cryptocrystalline. Elongate, quenched, groundmass olivine is present, and spherulites contain radiating skeletal to feathery plagioclase (Fig. F15). A trace of skeletal to subhedral titanomagnetite is also present.

Fine-grained parts of the basalt have a variety of textures, including subophitic, variolitic, intergranular, and, less commonly, intrafasciculate (Fig. F18). In some thin sections from Unit 6, bimodal groundmass grain-size distributions are apparent (Fig. F19), giving the basalt the mottled appearance observed in hand specimen. Variolitic and intergranular textures are characteristic of the aphanitic basalt. Plagioclase in these aphanitic zones is typically skeletal to feathery and, less commonly, subhedral to euhedral. Feathery, anhedral clinopyroxene is present between the plagioclase crystals. The coarser-grained patches of Unit 6 generally display subophitic and intrafasciculate textures. Titanomagnetite is present as small but ubiquitous crystals throughout the fine-grained basalt, and ranges from skeletal to euhedral. Blebs of sulfide (0.01 mm; too small for petrographic identification) are present within the mesostasis and as inclusions within the silicate minerals.

Seven basalt samples from Hole 1187A were selected for whole-rock analysis by ICP-AES. All of the samples analyzed are olivine-normative tholeiitic basalts (Fig. F20; Table T5). Six are relatively primitive, having high Cr (464-501 ppm), Ni (171-196 ppm), and MgO (8.7-9.4 wt%) contents, Mg# of 0.65-0.66, and low TiO₂ (0.72-0.75 wt%) and Zr (36-41 ppm) contents. One sample (192-1187A-7R-6 [Piece 2, 66-70 cm]) taken from an aphanitic pillow margin has lower MgO content (but similar Cr, Ni, TiO₂, and Zr abundances) than the other samples (Table T5). Its elevated alkali contents, high weight loss on ignition, and apparent silica undersaturation (it is nepheline normative; Table T5) indicate that its composition has been affected by alteration processes; thus, it is excluded from Figures F20, F21, F22, and F23.

The basalt recovered from Hole 1187 is petrologically and chemically very similar to basalt of the upper group of units at Site 1185 (all units of Hole 1185A and Units 1-9 of Hole 1185B). Together, these basalts are the most Mg-rich yet found on the Ontong Java Plateau. The relatively primitive nature of the Site 1187 basalt and its similarity to that of the upper group at Site 1185 are clearly illustrated by immobile-element concentrations (Figs. F21, F22) and Mg# (Fig. F23). The basement units in Hole 1187 differ from the upper group at Site 1185 in having plagioclase-rich xenoliths. However, basalt flows from the two sites clearly represent a magma type hitherto unrecorded on the Ontong Java Plateau. Lavas of this type may be rather extensive in this region of the plateau's eastern flank, as they are >136 m thick in Hole 1187 and 125 m thick at Site 1185, 146 km to the south.