We reached basement at 968.6 mbsf and cored a 65.4-m-thick sequence of basalt flows, with an average recovery of 59%. We divided the flows into four basement units composed of pillow basalt (Unit 1) or pillow basalt with a massive base (Units 2-4; Fig. F30). The basement units range in thickness from ~10 to >29 m. The age of sediment directly overlying basement in Unit 1 is early Aptian (see "Biostratigraphy"). Igneous Units 1 and 2 are separated by a thin layer of conglomerate (Subunit 2A). Unit 3 begins with a sequence of pillows beneath the massive base of Unit 2. Unit 4 is defined on the basis of downhole wireline logging that suggested the presence of an unrecovered pillow sequence between two flows of massive basalt (see "Downhole Measurements").
The basement units are typically nonvesicular, aphyric to moderately phyric, aphanitic to fine-grained basalts that are slightly to moderately altered (see "Alteration"). Olivine is the dominant phenocryst phase, with subordinate amounts of plagioclase (±clinopyroxene). Glassy rims are present throughout the recovered pillowed sequences (Fig. F30), and some of the glass is unaltered. Immediately adjacent to the glassy rims are aphanitic zones, 1-2 cm wide; rare vesicles elongated perpendicular to the pillow rims are present immediately adjacent to the aphanitic zone. Grain size increases to fine grained toward the interior of the pillows and also toward the interior of the massive flows. In the massive portions of Units 2-4, grain-size changes are irregular, and little other evidence, such as glassy rims or vesicles, is present to indicate a pillowed structure. On the basis of these observations, we conclude that the recovered sequence represents a series of pillow basalt units or pillow units interspersed with massive flows.
Grain-size variation is a distinctive feature of Unit 4, and we define three broad textural types: aphanitic, aphanitic with fine-grained patches, and fine grained. Figure F31 illustrates the distribution of these textural types. The average grain size of the fine-grained patches is approximately the same as that of the fine-grained basalt (0.3-0.5 mm). Aphanitic basalt forms an ~20-cm layer at the top of Core 192-1186A-37R-1. The middle portion of the cored sequence, between 1015 and 1026 mbsf, contains varying proportions of aphanitic and fine-grained patches that give the core a mottled appearance. The lowermost part of Unit 4 is nearly all fine grained. Fine-grained zones in the flow interior contain larger crystals of magnetic minerals such as titanomagnetite. Variation of the magnetic mean destructive field (see "Paleomagnetism") exhibits a general correlation with variation in average grain size.
Subround plagioclase-rich xenoliths, ranging in size from 0.5 to 1.5 cm, are most common in Unit 3 and rare in Units 1, 2, and 4 (Fig. F30).
The upper three basalt units are predominantly sparsely olivine phyric with subordinate amounts of plagioclase (Fig. F32) and, possibly, clinopyroxene phenocrysts. The distribution of phenocryst phases is shown in Figure F33. Phenocryst phases are most easily identified in the aphanitic pillow rims and are commonly difficult to identify in the pillow interiors where the groundmass is more coarsely crystalline. Fine-grained patches of olivine, plagioclase, and clinopyroxene, which are common in Unit 4 and typically constitute ~50% of the rock (Fig. F31), make identification of early liquidus-phase phenocrysts difficult (Fig. F34).
The groundmass in pillow margins ranges from totally to partly glassy; most glass is devitrified. Spherulites are present and plagioclase microlites are aligned around phenocrysts in a subtrachytic texture (Fig. F35). Rare round vesicles are also found in the pillow margins. Pillow and massive flow interiors are characterized by fine-grained plagioclase and clinopyroxene. Groundmass plagioclase forms either a variolitic texture (Fig. F36) or an intergranular texture with clinopyroxene (Fig. F37). Skeletal titanomagnetite crystals form a minor constituent (1%-3%) in the groundmass of the fine-grained regions and, along with sulfide, occur as inclusions in plagioclase (Fig. F38). The sulfide phase(s) are too small (<0.01 mm) for accurate petrographic identification.
Alteration ranges from slight (away from veins) to moderate (near veins) (see "Alteration"). Olivine phenocrysts are completely altered to smectite (Fig. F39), or celadonite (Fig. F32) with subordinate sulfide or calcite. Devitrified glass is partially replaced by smectite. Miarolitic cavities and rare vesicles are filled with calcite and smectite.
We selected seven basalt samples for whole-rock analysis by ICP-AES. Weight loss on ignition (LOI) ranges from 0.38 to 2.11 wt% (Table T8), consistent with petrographic evidence of slight to moderate levels of alteration in these samples. All the basalts analyzed are tholeiitic (Fig. F40) and slightly olivine normative to slightly quartz normative. In terms of the immobile elements Zr, Ti, and Cr, they are similar to basalts at Site 1183, to the lower group of units (Units 10-12) at Site 1185, and to Units C-G at Site 807. Their compositions plot within the Kwaimbaita Formation field defined by lava flows on Malaita (Tejada et al., in press) (Figs. F41, F42). Site 1186 basalts contain 7.0-8.4 wt% MgO and have Mg# ranging from 0.58 to 0.67, values similar to those in basalts at Site 1183 and the lower group of basalts at Site 1185 (Table T8; Fig. F43).
The basaltic sequence recovered at Site 1186 is similar to those at Sites 1183, 807 (Units C-G), and 1185 (lower group Units 10-12), as well as to lava flows of the Kwaimbaita Formation on Malaita. The Site 807 and Malaita sequences are composed of fine-grained and sparsely olivine-plagioclase-clinopyroxene-phyric basalts (Kroenke, Berger, Janecek et al., 1991; Tejada et al., in press). Site 1183 is predominantly olivine (±plagioclase) phyric (see "Igneous Petrology" in the "Site 1183" chapter). The presence of Kwaimbaita-like basalt at Site 1186 further emphasizes the widespread occurrence of this magma type on the Ontong Java Plateau.