The basement drilled at Site 1201 is composed of a 94-m-long sequence of volcanic rocks (recovery = 32%). Evidence for pillow lava structures, such as glassy margins, hyaloclastite fragments, and interpillow sedimentary material, as well as radiating fractures and vesicles (Fig. F29) are visible throughout the sequence. Examples of preserved pillow margins are found in intervals 195-1201D-48R-4, 12-26 cm, and 52R-1, 18-44 cm (Fig. F30). Hyaloclastite fragments are observed, for example, in interval 195-1201D-46R-4, 24-29 cm (Fig. F31A), and traces of biogenic material are found in interpillow sediments containing hyaloclastite fragments (Fig. F31B). Section 195-1201D-55R-1 is massive with no evidence for pillow margins and may therefore represent a lava flow.
In Section 195-1201D-47R-3 there are intervals of pillow breccia containing fragments of lava cemented in brown sedimentary material (Fig. F32). The latter is brown (10YR 4/3) to yellowish brown (10YR 5/4) and has been found throughout the core, for instance, in Sections 195-1201D-47R-1, 48R-1, 52R-2, and 54R-1 (Fig. F33). The sedimentary material is found both as single pieces containing fragments of green volcanic glass and infilling veins and fractures in the lava.
The lavas are highly altered, especially at the contact with the overlying sediments. In general, the degree of alteration decreases slightly with depth, as indicated by a decrease in the percentage of secondary minerals (see "Petrography") and a decrease in the loss on ignition (LOI) (see "Geochemistry"). The alteration gives different colors to the rocks, from light gray (5Y 6/1) to bluish gray (5B 6/1-5B 5/1) in most of the sections and reddish gray (10R 5/1) in the lowermost cores. Veins and amygdules are widespread between Cores 195-1201D-45R and 48R, becoming less frequent in the deeper cores, where only a few thin veins are present. The veins are usually filled by one or more of the following secondary minerals, identified in thin section: zeolites, iron oxyhydroxides, carbonate, and clay minerals. They are accompanied by large halos in which the color of the rock is generally lighter, such as gray (5Y 6/1) or reddish brown. The veins are often filled with brownish sedimentary material with dark red rims and contain a late-stage carbonate precipitate (Fig. F34). The amygdules are filled by green clay minerals and/or zeolites and sometimes also by carbonate. No quartz vein fillings were observed.
In hand specimen, the basement igneous rocks are generally very fine grained (borderline aphanitic) and aphyric. Only rare brownish red pseudomorphs of olivine can be observed. The groundmass appears glassy with signs of devitrification ("rosette texture") in the upper cores and more microcrystalline in the deeper cores.
The petrography of the basement rocks has been defined on the basis of 38 thin sections, covering all representative rock types observed throughout the core. The thin section samples were selected to characterize different textures, primary and secondary mineral phases, crystal content and size, and amygdule and vein mineralogy and size. All observations are reported in the thin section tables (see "Site 1201 Thin Sections"), following the guidelines illustrated in "Thin Section Descriptions" in "Igneous Petrology" in the "Explanatory Notes" chapter. On the basis of both visual and microscopic observations, the basement rocks are classified as basalts.
In thin section, the basalts range from aphyric to sparsely porphyritic to moderately porphyritic with up to 7% phenocrysts (crystals longer than 0.5 mm). They show variable crystallinity, from almost completely glassy along pillow rims to hypohyaline and coarser grained in pillow interiors. A general trend of increasing crystallinity is observed with depth. The grain size is generally fine (average length of microliths in the groundmass = <0.2 mm) and tends to increase slightly with depth. In most cases, the glass is completely devitrified (palagonitized), but the devitrification products cannot be identified easily under the microscope. Different colored clay minerals and zeolites are tentatively identified as the most common secondary minerals after glass. Based on XRD data, mixed-layer clay minerals and zeolites such as phillipsite, natrolite, and analcite are common alteration products (see "Lithostratigraphy").
The main primary minerals observed in the basalts include plagioclase, olivine, clinopyroxene, and opaque minerals. Clinopyroxene is the only well-preserved primary mineral. Its morphology is commonly anhedral or subhedral. In glass-rich rocks, the clinopyroxene microliths show anomalous undulatory extinction, due to fast cooling. Based on its colorless or pale green color, the clinopyroxene should be Mg rich. Olivine is always completely replaced by secondary minerals and its original composition cannot be determined, but its relics are easily recognized by their common euhedral morphology. Olivine alters to clay minerals, iron oxyhydroxides (including hematite), and carbonate (Fig. F35). The plagioclase exhibits elongate skeletal morphology, often swallow-tailed, due to fast cooling (Fig. F36). Plagioclase appears highly altered in many thin sections. It is most commonly replaced by zeolites and alkali feldspar and less commonly by either clay minerals or carbonate (Fig. F37). In most cases, Na-rich rims are preserved, reflecting original chemical zoning (Fig. F38). In several thin sections, the primary plagioclase core has also been preserved, and its composition, determined using the Michel-Levy method, ranges from An75 to An49, with little or no difference between phenocrysts and small microliths in the groundmass. Opaque minerals are present as small euhedral or skeletal grains in the groundmass; the composition is presumably titanium-rich magnetite. Opaque grains often observed in the groundmass of glass-rich basalts outline branching or spherulitic (clay mineral?) patches of devitrified glass. The secondary minerals observed in thin section and those identified by XRD analysis suggest that alteration of the basalts in Hole 1201D occurred within the zeolite metamorphic facies (Table T3).
Devitrification of glass often leads to a spherulitic texture, corresponding to the "rosette" texture visible in hand specimen or by lens. Spherulites made up of radiating clinopyroxene and plagioclase microliths are common (Fig. F39). Other types of textures exhibited by glass-rich basalts are hyalopilitic and branching textures. In hyalopilitic textures, devitrified glass, vesicles, and clinopyroxene and plagioclase microliths show subparallel alignment along flow directions (Fig. F35), whereas in branching textures, the microliths and/or the devitrified glass are bent and show featherlike shapes, suggesting fast cooling (Fig. F40). In a few cases, the rocks display two or more of the above textures, sometimes exhibiting a gradual transition from spherulitic at the pillow margins to branching textures further away.
Basalts with a higher degree of crystallinity show a textural variation from felty to intersertal to intergranular to subophitic, with progressively decreasing glass and increasing amounts of crystals. In felty and intersertal textures, euhedral plagioclase microliths are distributed randomly, with variable amounts of devitrified glass occupying the angular spaces between them. In intergranular texture, the spaces between plagioclase microliths are occupied by olivine, clinopyroxene, and opaque grains, whereas in subophitic rocks, euhedral plagioclase microliths are partially enclosed by anhedral clinopyroxene or olivine grains (Fig. F41).
The textural relationships exhibited by the basalts and the morphologies displayed by the different minerals have allowed us to infer an order of crystallization of the original minerals. Plagioclase crystallized first, often together with olivine, followed by clinopyroxene, and finally, opaque minerals. Evidence for plagioclase being crystallized before or along with olivine suggests a shallow depth of crystallization as well as high PH2O conditions (Johannes, 1978).
A total of 30 samples from Hole 1201D were analyzed using inductively coupled plasma-atomic emission spectroscopy for major and trace elements. The selected samples are either plagioclase ± clinopyroxene ± olivine phyric to aphyric basalts. A few hyaloclastites and glassy pillow basalt margins were also analyzed to study the effects of seawater alteration on the bulk rock geochemistry. The basement samples are altered to various degrees (see "Petrography"), so LOI values vary considerably (0.85-11.85 wt%); moreover, many of the rocks have gained alkalis, as reflected by the appearance of nepheline in their CIPW norms (see Table T4). The upper 20 m of the pillow basalts are highly altered, with LOI values up to 12 wt% (average = 6.4 wt%). Below this depth, LOI drops to relatively constant lower values (average = 1.84 wt%), reflecting less extensive seawater alteration and less veining and fracturing of the basalts (see also Fig. F42). These deeper basalts are also hypersthene rather than nepheline normative (Table T3). Some of the major and trace element abundance vs. depth patterns correlate well with the LOI values (Fig. F43). In fact, by examining the behavior of elements easily mobilized during seafloor alteration (K, Na, Sr, Ca, and Fe) in samples that are visually altered, it has been possible to assess the effect of alteration on the bulk chemistry. The content of Si, Al, and Ti remains relatively unchanged with depth, whereas P is highly variable throughout the entire sequence (Table T4) and shows no correlation with depth. The remaining major elements show different behavior patterns above and below ~525 mbsf. Compared with the deeper basalts, the shallow basalts exhibit lower average CaO (9.7 vs. 13.3 wt%), higher Na2O (3.8 vs. 2.1 wt%), and highly variable K2O (0.9 to 2.8 wt%) and Sr (41 to 223 ppm) contents. This suggests significant seawater alteration. In the basalts below 525 mbsf, all of the above-mentioned elements except K2O show only minor variations. As expected, the immobile trace elements Zr, Y, V, and Cr show no significant variations with depth, their average values being 48.3, 24.4, 261, and 390 ppm, respectively. The effect of seawater alteration is also demonstrated by comparing the more altered light-colored basalt in interval 195-1201D-46R-1, 134-136 cm, with the adjacent darker, presumably fresher, basalt in interval 46R-1, 138-140 cm (Table T4). The two samples, respectively, show decreasing values of Na2O (from 5.5 to 4.0 wt%), Fe2O3 (from 9.8 to 8.8 wt%), and LOI (from 6.5 to 5 wt%), as well as increasing CaO (from 7.35 to 9.7 wt%), Sr (from 26 to 64 ppm), and P2O5 (from 0.13 to 0.20 wt%) contents.
Overall, the basement rocks recovered from Site 1201 are primitive basalts, as confirmed by their overall high MgO and Cr contents (averages = 7.8 wt% and 400 ppm, respectively) and high Mg number values (average = 64.2, ranging from 55 to 68). These features are indicative of mantle-derived primary magmas (BVSP, 1981). Plotted on a total alkalis vs. silica diagram (LeBas et al., 1986), the majority of the analyzed samples fall in the basalt field with a few falling in the trachybasalt field (Fig. F43). The basalt samples have low contents of TiO2 (0.77-1.02 wt%; average = 0.93 wt%) and Zr (43-52 ppm; average = 48 ppm).
The two samples of hyaloclastite (intervals 195-1201D-46R-4, 24-27 cm, and 46R-4, 31-33 cm) are geochemically similar to the basalts except for the higher concentration of alkalis (average Na2O = 4.7 wt%; average K2O = 2.0 wt%). Sample 195-1201D-46R-4, 31-33 cm, has an LOI value of almost 12 wt% and unusually low values of Cr, V, and Y.
The chemical composition of the basalts from Site 1201 has been plotted on tectonic discrimination diagrams using major and trace elements of proven immobility to infer their tectonic affinity. The basalt suite from Site 1201 straddles the MORB and island arc tholeiite (IAT) fields on both (Ti/100)-Zr-(Y x 3) (Fig. F44) and Ti vs. V (Fig. F45) tectonic discrimination diagrams. On the other hand, the same samples plot in the IAT field on the TiO2-(MnO x 10)-(P2O5 x 10) (Fig. F46) but in the MORB field on the Y vs. Cr (Fig. F47) diagrams (Pearce, 1982; Pearce and Cann, 1973; Shervais, 1982; Mullen, 1983).
The basalts at Site 1201 lack the orthopyroxene typical of island arc basalts and display early crystallization of plagioclase, suggesting a fertile island arc mantle source (e.g., Wilson, 1989, and quoted references). Based on this and the proximity of Site 1201 to the Palau-Kuyshu Ridge remnant arc, it is likely that the mantle from which these rocks originated had geochemical features transitional between typical arc and backarc basin mantle (see DSDP Leg 59; Kroenke, Scott, et al., 1981). The basalts at Site 1201 are depleted in Zr (which is typical for island arc basalts) (Taylor et al., 1995) but are enriched in Cr and Mg (which is typical for primitive MORB and backarc basin basalt [BABB]). An arc affinity is also suggested by the low Ti content, which is uncommon for BABBs such as those from the Sumisu Rift (Gill et al., 1992) and the Lau Basin (Hawkins, 1995). Furthermore, the Site 1201 basalts have high Zr/Y ratios (average = 1.9 vs. 2.5-5.0 for typical MORBs) (Taylor et al., 1995) and low Ba/Zr ratios (average = 0.32). Both the Zr/Y and the Ba/Zr ratios are virtually identical to the Palau-Kyushu arc values of Wood et al. (1980). Taylor et al. (1995) reported rocks with arc affinities from sills penetrating the forearc basement of both the Mariana (ODP Site 781) and Izu-Bonin (Site 793) regions. Studies of the Kasuga Seamounts in the Mariana Trough by Fryer et al. (1997) have shown that arc volcanism can also occur on the backarc side of the frontal arc. The transitional arc affinity of the basalts recovered at Site 1201 is possibly a result of a similar tectono-magmatic setting.