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doi:10.2204/iodp.proc.336.106.2012

Basement petrology

Hole U1382B

Basement was encountered at 90 mbsf in Hole U1382B, and 58 cm from ~8.8 m of cored basement was recovered (6.6% recovery). Only one basalt piece was recovered in Section 336-U1382B-11X-CC. The rest of the basement rocks in Core 336-U1382B-12X are pebbles of completely altered plutonic and mantle rocks composed of talc-tremolite schist and serpentinite (Fig. F2). The ultramafic rocks occurring at the basement/​sediment interface are interpreted as being part of sedimentary breccia overlying the massive basalt of lithologic Unit 1 in Hole U1382A, described in the “Site U1382” chapter (Expedition 336 Scientists, 2012b).

Unexpectedly, serpentinized ultramafic rocks and one piece of troctolite were recovered from within the sediment sequence between 49.8 and 53.1 mbsf in Sections 336-U1382A-6H-7 through 7H-1 (Fig. F3). The occurrence of plutonic and ultramafic rocks of various sizes (from sand to pebble size, with clasts up to 8 cm in diameter) within a soft-sediment matrix (clayey foraminifer-rich ooze) suggests that this sequence represents a gravity flow that has traveled down the slopes of nearby rift mountains with outcrops of highly deformed plutonic and mantle rocks (i.e., oceanic core complex). This interpretation is consistent with the occurrence of similar plutonic and mantle rocks at ~60 meters subbasement (msb) in DSDP Holes 395 and 395A (Shipboard Scientific Party, 1979) and Hole U1382A (see “Petrology, hard rock and sediment geochemistry, and structural geology” in the “Site U1382” chapter [Expedition 336 Scientists, 2012b]).

Mafic and ultramafic rocks from APC core

The troctolite piece recovered in Section 336-U1382B-7H-1 has been intensively serpentinized (up to 80%) and consists of plagioclase (20%) and olivine (80%) as primary minerals. The troctolite has a granular texture of euhedral olivine and subhedral plagioclase and lacks obvious foliation and lineation texture (Fig. F3C). The serpentinite pieces retrieved from Sections 336-U1382B-6H-7 and 6H-8 are black, dark green, and greenish white on the cut surface (Fig. F3A, F3B). In some parts, the serpentinites are dominated by a tangerine color, indicating a higher degree of weathering (up to 100%). Serpentinites are composed of serpentine that has a typical mesh texture with minor Fe hydroxide–rich clays. Coarse- to medium-sized grains (0.5–1 cm) of bastite (serpentine pseudomorph of orthopyroxene) in the olivine matrix give the rock a porphyroclastic texture. Black spinel grains occur in several serpentinites, but some spinel may have been replaced by magnetite. Most of the serpentinites lack macroscopic deformation fabric, although several pieces have developed foliation defined by elongated and rounded pyroxene grains. These two types of serpentinites were grouped as “undeformed” and “deformed,” respectively.

Ultramafic rocks from XCB core

Ultramafic rocks recovered in Core 336-U1382B-12X are small pebbles with variable angular to subrounded shapes (Fig. F2). There are also several basalt and chalk fragments in Section 336-U1382B-12X-1 (Piece 4) (Fig. F2D). Three talc-amphibole rocks (Sections 336-U1382B-12X-1 [Piece 1] and 12X-1 [Piece 3]) were retrieved, and these range in color from green to greenish white. The constituent minerals are talc and amphibole (most probably tremolite). Two of the talc-amphibole rocks exhibit strong schistosity, which led to their classification as “talc-amphibole schist,” whereas rocks in Section 336-U1382B-12X-1 (Piece 1) lack schistosity. Talc-amphibole rocks may represent products of metasomatism in detachment faults (e.g., Boschi et al., 2006).

Serpentinites retrieved in Core 336-U1382B-12X are black, dark green, or greenish white, whereas several serpentinites are mainly tangerine. The serpentinite is composed of serpentine (with mesh texture) and talc with minor Fe oxide–rich clay. Coarse- to medium-sized grains (~0.2 to ~1.8 cm) are probably bastite or talc occurring as pseudomorphs of pyroxene, preserving the original texture.

Structural geology and deformation

The serpentinites obtained from both APC and XCB cores contain mesh textures and bastite, indicating static replacement and serpentinization of essentially undeformed peridotites (e.g., Mével, 2003). On the basis of macroscopic observation, most serpentinites lack deformation such as schistosity, indicating the structures are probably relics from peridotites that predate the serpentinization and Si-metasomatism (e.g., Bach et al., 2004; Boschi et al., 2006; Mével, 2003; Hirauchi and Yamaguchi, 2007; Hirauchi et al., 2010). Hence, most serpentinites from Site U1382 could derive from weakly deformed lithospheric mantle. Other serpentinites and the talc-amphibole schists, with their well-developed porphyroclastic texture, could be related to localized ductile shear zones in the lithospheric mantle (e.g., Michibayashi et al., 2009; Harigane et al., 2011). Thus, the occurrence of deformed ultramafic rocks in Hole U1382B could be related to the formation of a nearby oceanic core complex.

Holes U1383D and U1383E

Basement was encountered at 43.3 mbsf in Hole U1383D and at 43.21 mbsf in Hole U1383E. Less than 110 cm of basement was recovered from both sites (76 cm in Hole U1383D and 30 cm in Hole U1383E). All basement rocks were assigned to the same basement lithologic unit, although several sedimentary breccia intervals are also present. The basalts, mainly aphyric and sparsely vesicular, are slightly to moderately altered and grayish brown in color, and they display brown alteration halos along veins and exposed fractures. Interestingly, the basement rocks are different from those encountered at 69.5 mbsf in Unit 1 of Hole U1383C (sparsely plagioclase-olivine-phyric basalts), which suggests the existence of at least one volcanic unit between 43.2 and 69.5 mbsf. Note also that the interflow sediments recovered in Hole U1383D are less consolidated than those recovered in Hole U1383C, indicating that the occurrence of limestone deeper in the section may result from baking and carbonate recrystallization during emplacement of successive basalt flows.

Hole U1383D

The volcanic section recovered in Core 336-U1383D-7X is composed of aphyric to sparsely porphyritic glassy to microcrystalline basalts (e.g., Fig. F4). Half of the recovered pieces have glassy chilled margins. The basalts are avesicular to sparsely vesicular, and small amounts (up to 1%) of elongated tabular plagioclase phenocrysts were observed in sparsely porphyritic specimens. Core 336-U1383D-7X also features common sedimentary breccia, either as individual pieces or still in contact with basaltic rocks (Fig. F5). The sedimentary breccias are composed of 60%–40% of poorly sorted palagonitized glass shards in a matrix of light brown clayey chalk. Recrystallized carbonate cement occasionally occurs around glass clasts and at the contact between the breccias and basalt (Fig. F5B).

Groundmass alteration is patchy to pervasive, with an overall alteration intensity ranging from fresh (<1% alteration) to moderate (up to 30% alteration). Chilled margins display variably palagonitized glass with common reddish-brown alteration. Vesicle abundance ranges from 1% to 3%, with <30% filling with mainly carbonate (possibly associated with zeolite) and minor smectite and Fe oxyhydroxide. Average vein density is ~25 veins/m, which compares well with vein density in Unit 1 of Hole U1383C (see the “Site U1383” chapter [Expedition 336 Scientists, 2012c]). Vein width is generally <0.4 mm, and veins are composed of mixed smectite and Fe oxyhydroxide (± iddingsite). A single vein (as wide as 3 mm) was found in Section 336-U1383D-7X-CC (Piece 8) and is composed of mixed carbonate and clayey sediment infill. Dark brown to grayish-brown halos are well developed along veins (Fig. F4).

Hole U1383E

Only seven pieces were recovered in Core 336-U1383E-7X, comprising mainly aphyric avesicular to sparsely vesicular microcrystalline basalts (Sections 336-U1383E-7X-1 [Piece 2] through 7X-1 [Piece 7]) and including two samples with chilled margins. As observed in Hole U1383D, small amounts of sediment and sedimentary breccia were recovered in contact with exposed fractures of basalt. Section 336-U1383E-7X-1 (Piece 1) is a fragment of highly plagioclase-clinopyroxene-phyric holocrystalline dolerite, which differs markedly from other basalts recovered at Site U1383. The basalts are sparsely vesicular to nonvesicular (<2% vesicles), and vesicles generally lack secondary mineral filling, except in Section 336-U1383E-7X-1 (Piece 1), where vesicles are filled with dark green smectitic clay.

Alteration of the groundmass is patchy and generally moderate (<6%), except in Section 336-U1383E-7X-1 (Piece 2), which is pervasively altered (<40%). The dolerite, Section 336-U1383E-7X-1 (Piece 1), is also moderately to highly altered (<50%).

Hole U1384A

Basement was encountered at 94.71 mbsf in Hole U1384A, and 58 cm of basement was recovered. All volcanic rocks, interflow sediments, and sedimentary breccias were assigned to the same basement lithologic unit. The basalts are aphyric and sparsely vesicular with a glassy to microcrystalline groundmass. This unit is composed of pillow lava with common variolitic texture that lacks glassy chilled margins. The basalts are slightly to moderately altered (3%–10%) and grayish brown in color, and display brown alteration halos along veins and exposed fractures. Variolitic basalts display blotchy alteration textures, similar to the pillow lava units encountered in Holes U1382A and U1383C.

Vesicle abundance ranges from <0.5% to 3%, with very limited filling (<10%). Only Section 336-U1384A-12X-1 (Piece 13) has >80% vesicle filling, composed of dark green smectite and Fe oxyhydroxide. Only two veinlets (0.1 mm width) were logged in this sequence; they are composed of typical smectite-iddingsite assemblages found deeper in the basement in Holes U1382A and U1383C (see the “Site U1382” and “Site U1383” chapters [Expedition 336 Scientists, 2012b, 2012c]). Dark brown to brown halos are well developed along veins and the exposed surface of the rocks (Fig. F6).

Section 336-U1384A-12X-1 features common sedimentary breccia, either as individual pieces of interflow sediments or still in contact with basaltic rocks (Fig. F6). The sedimentary breccia (Sections 336-U1384A-12X-1 [Piece 1], 12X-1 [Piece 2], 12X-1 [Piece 3], 12X-1 [Piece 4], and 12X-2 [Piece 11]) is composed of poorly sorted altered basalt and clayey sediment clasts (defined as slumps) within a matrix composed of light brown sediment (clayey chalk). Note also that the interflow sediment recovered in Hole U1384A is less consolidated than that recovered in Hole U1383C and generally similar to that recovered at the sediment/​basement interface in Hole U1383D.

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