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Igneous petrology and volcanology

In Hole U1374A, 474.9 m of igneous rock (excluding sedimentary interbeds) was penetrated after the igneous basement was reached at 16.7 mbsf. The igneous sequence was divided into 148 lithologic units, which were grouped into 15 stratigraphic units (Units III–VIII, X, and XII–XIX; Fig. F18). Two sedimentary units (Units IX and XI; 21.03 and 6.59 m thick, respectively) and three thinner sedimentary layers with a combined thickness of 2.81 m are included in the basement section. The uppermost igneous basement begins with a 1.3 m thick flow of olivine-augite-plagioclase-phyric basalt, followed downhole by a series of aphyric basalt lava flows and volcanic breccias that compose Units III–VII. Below this, the hole is dominated by poorly sorted volcanic breccia. The breccia contains numerous basalt cooling units that could be lava flow lobes or larger clasts. Units XVI–XIX are cut by a series of steeply inclined to vertical intrusive sheets of aphyric basalt. To help achieve the paleomagnetic objectives of this expedition, each of the more massive igneous lithologic units was assigned an in situ confidence index (ISCI) following the procedures described in “Igneous petrology and volcanology” in the “Methods” chapter (Expedition 330 Scientists, 2012a). The lithologic units and their ISCI values are summarized in Table T7. In this section we briefly describe the types of igneous clasts found in the sedimentary units, followed by a description of each of the igneous units and our interpretation of the entire igneous succession.

Basaltic clasts in sedimentary Units II, IX, and XI

The larger basaltic clasts within the conglomerate and breccia of Units II, IX, and XI were divided into 12 types on the basis of their appearance in hand specimen. Each clast type is described below, together with the sedimentary subunit(s) in which it was found (see “Sedimentology”):

  • Type 1 (Subunits IID, IIE, IXA, IXC, and XIB): highly olivine-pyroxene-phyric basalt that is medium gray and mottled orange and green, with 10% olivine phenocrysts (completely altered to iddingsite; maximum size = 10 mm, modal size = 4 mm) and 3% pyroxene phenocrysts (unaltered; maximum size = 8 mm, modal size = 3 mm). The groundmass is fine grained, with 1% vesicles (moderate sphericity, subrounded).

  • Type 2 (Subunits IIE and IXC): highly olivine-phyric basalt that is medium gray and mottled orange, with 10% olivine phenocrysts (completely altered to iddingsite; maximum size = 7 mm, modal size = 3.5 mm). The groundmass is fine grained, with 0.1% vesicles (high sphericity, rounded).

  • Type 3 (Subunits IID, IIE, IXA, and IXC): aphyric basalt that is medium gray, with 1% olivine microphenocrysts (altered; maximum size = 0.5 mm, modal size = 0.2 mm). The groundmass is fine grained, with 2% vesicles (low sphericity, subangular).

  • Type 4 (Subunits IXA, IXB, and IXC): sparsely olivine-phyric basalt that is medium gray to medium reddish gray, with 1% olivine phenocrysts (altered). The groundmass is fine grained, with 10%–20% vesicles.

  • Type 5 (Subunits IXA and IXC): aphyric basalt that is medium gray, with 0.2% olivine (altered). The groundmass is fine grained, with 15% vesicles.

  • Type 6 (Subunits IXA, IXB, and IXC): basaltic froth in small (modal size = 5 mm) angular clasts that are pale yellow, with 1% olivine (altered) and 0.1% hornblende. The clasts have an aphanitic texture and a vesicularity of 60%.

  • Type 7 (Subunits IXA and IXC): very highly olivine-phyric basalt that is mottled orange-gray, with 30% olivine phenocrysts (altered; maximum size = 8 mm, modal size = 4 mm) and 1% pyroxene phenocrysts (unaltered; maximum size = 5 mm, modal size = 3 mm). The groundmass is fine grained, with 5%–10% vesicles (high sphericity, rounded).

  • Type 8 (Subunits IXA, IXB, and IXC): sparsely olivine-phyric basalt that is medium gray, with 1% olivine phenocrysts (altered; maximum size = 4 mm, modal size = 1.5 mm), small feldspar laths, and 1% vesicles (high sphericity, rounded).

  • Type 9 (Subunits IXA, IXC, XIA, and XIB): aphyric basalt that is medium gray and fine grained, with 0.5% vesicles (elongate, rounded).

  • Type 10 (Subunits IXB and IXC): moderately olivine-phyric basalt that is medium gray, with 3% olivine phenocrysts (altered; maximum size = 6 mm, modal size = 2 mm) and 0.5% pyroxene phenocrysts (unaltered; maximum size = 5 mm, modal size = 3 mm). The groundmass is fine grained, with 3% vesicles (elongate, rounded).

  • Type 11 (Subunit XIB): moderately plagioclase-olivine-phyric basalt that is greenish medium gray, with 3% plagioclase phenocrysts, 2% pyroxene phenocrysts, and 0.8% olivine phenocrysts (altered). The groundmass is fine grained, with 1% vesicles.

  • Type 12 (Subunit XIB): moderately pyroxene-olivine-phyric basalt that is greenish medium gray, with 3% pyroxene phenocrysts, 2% olivine phenocrysts (altered), and 0.5% plagioclase phenocrysts. The groundmass is fine grained, with 1% vesicles.

Lithologic and stratigraphic igneous units

Unit III

  • Interval: Sections 330-U1374A-3R-2, 89 cm, to 3R-3, 76 cm

  • Depth: 16.72–18.04 mbsf

  • Lithology: highly olivine-augite-plagioclase-phyric basalt

  • Lithologic unit: 1

The conglomerate of Subunit IIE sits on the eroded but apparently unweathered surface of a 1.32 m basalt body, interpreted as a lava flow, that forms Unit III. The original flow top is not preserved at the contact, but fine-grained matrix sand from the overlying conglomerate has infiltrated into cracks (Fig. F19). The lava flow is conspicuously porphyritic, with large olivine and titanaugite phenocrysts and smaller phenocrysts of plagioclase (Fig. F20). One of the titanaugite phenocrysts examined in thin section contains an inclusion of hornblende (Fig. F20D), one of only two occurrences of amphibole recorded at Site U1374.

Unit IV

  • Interval: Sections 330-U1374A-4R-1, 0 cm, to 5R-4, 99 cm

  • Depth: 20.40–29.69 mbsf

  • Lithology: aphyric basalt

  • Lithologic unit: 2

A 2 m thick interval without core recovery separates Units III and IV. The latter unit consists of a single 9.29 m thick aphyric basalt lava flow that in thin section is seen to be composed largely of tiny flow-aligned plagioclase laths, titanaugite crystals, and some altered groundmass olivine (e.g., Sample 330-U1374A-5R-2, 137–139 cm [Thin Section 120]). There are no phenocrysts. Core recovery in this unit was virtually 100%.

Unit V

  • Interval: Sections 330-U1374A-5R-4, 99 cm, to 6R-4, 34 cm

  • Depth: 29.69–33.98 mbsf

  • Lithology: aphyric basalt breccia

  • Lithologic unit: 3

Unit V is a 4.29 m thick layer of basalt breccia, the upper 62 cm of which consists of aphyric basalt clasts mingled with bioclast-bearing micrite (see “Sedimentology”). This sediment-lava mingling may be peperitic (Fig. F21A). Some of the aphyric basalt clasts below the peperitic layer have altered glassy margins, radial cracks at their edges, rare pipe vesicles, and occasional vesicular patches in their centers. Because Unit V is composed of aphyric basalt clasts similar to those forming the more massive units above and below, it is difficult to determine whether Unit V is a separate volcaniclastic entity or is derived from the mingling of Unit IV and/or VI lava flows with wet sediment.

Unit VI

  • Interval: Sections 330-U1374A-7R-1, 0 cm, to 7R-3, 72 cm

  • Depth: 34.20–37.60 mbsf

  • Lithology: aphyric basalt and breccia

  • Lithologic units: 4–5

Unit VI consists of a 3.25 m thick aphyric basalt lava flow and a lower 15 cm of breccia that is probably the base of the lava flow. Its upper contact with the volcanic breccia of Unit V was not recovered. The breccia at its base has a micritic-sandy matrix.

Unit VII

  • Interval: Sections 330-U1374A-7R-3, 72 cm, to 8R-3, 52 cm

  • Depth: 37.60–41.84 mbsf

  • Lithology: aphyric lithic-vitric sandstone and breccia

  • Lithologic units: 6–7b

The brecciated base of Unit VI sits on, and mixes with, the top of a 4.24 m thick interval of palagonitized volcaniclastic sandstone composed of aphyric basalt fragments and occasional bioclasts. The sandstone in lithologic Unit 6 has well-defined bedding inclined at angles of up to 25° to horizontal (see “Structural geology”). An interval of coarser (up to 5 cm) clasts occurs in interval 330-U1374A-8R-1, 84–94 cm. The base of Unit VII (lithologic Subunit 7b) consists of a sparsely olivine-rich basalt breccia with a vitric-lithic volcanic sand matrix that includes bioclasts. Although the phenocryst assemblage in the clasts is similar to that in the unit below, this lithologic unit is grouped with Unit VII on the basis of its sedimentologic features. Further details regarding Unit VII are given in “Sedimentology.”


  • Interval: Sections 330-U1374A-8R-3, 52 cm, to 12R-4, 100 cm

  • Depth: 41.84–63.67 mbsf

  • Lithology: moderately olivine-phyric breccia

  • Lithologic unit: 8

Unit VIII is a 21.83 m thick interval of breccia composed of moderately olivine-phyric basalt clasts as large as 22 cm that contain occasional large, sieve-textured plagioclase phenocrysts. The breccia is mostly clast supported; most of the clasts are angular, but some have altered glassy margins. Spaces between clasts are either filled with calcite and zeolite cement or are open. Open-work patches of breccia are noticeably more oxidized than the areas that are cemented. Grains <2 mm are largely absent. The upper contact of Unit VIII may be gradational with Unit VII. The lower contact was not recovered.

Unit IX

  • Interval: Sections 330-U1374A-12R-4, 100 cm, to 16R-1, 10 cm

  • Depth: 63.67–84.70 mbsf

  • Lithology: sedimentary interval

Sedimentary basalt breccia and sandstone separate the olivine-phyric basalts of Unit VIII from Unit X, which consists of aphyric basalt. This sedimentary interval does not include bioclasts. For more details, see “Sedimentology.” Basaltic clast Types 1–10 are found in the breccia and sandstone. In particular, this unit contains abundant small clasts of pale yellow, altered frothy basalt glass (Type 6).

Unit X

  • Interval: Sections 330-U1374A-16R-1, 10 cm, to 21R-3, 59 cm

  • Depth: 84.70–109.86 mbsf

  • Lithology: aphyric basalt and breccia

  • Lithologic units: 9–14

Unit X is 25.16 m thick and consists of a 2.71 m thick aphyric basalt lava flow overlying aphyric basalt breccia. Within the breccia are two intervals of massive aphyric basalt interpreted as thin flow lobes that are 1.15 and 1.10 m thick. Clasts in the uppermost 3.30 m of breccia are mingled with a basaltic sand sediment matrix, and some clasts resemble pillows (Fig. F21B). These clasts may have formed through interaction between magma and wet sediment and, if so, the breccia could be a peperitic base to the lava flow above (see “Sedimentology”). In the rest of Unit X the breccia is mostly clast supported, and most of the clasts are angular, as in Unit VIII. Spaces between clasts are either filled with calcite and zeolite cement or are open, again showing distinctive alteration differences between the open-work and cemented zones. As with Unit VIII, grains <2 mm are largely absent from the breccia matrix. Some of the basalt clasts in Unit X contain glass and unaltered groundmass olivine (Fig. F22).

Unit XI

  • Interval: Sections 330-U1374A-21R-3, 59 cm, to 22R-5, 24 cm

  • Depth: 109.86–116.45 mbsf

  • Lithology: sedimentary interval

Unit XI is another sedimentary interval consisting of two subunits: a thin upper subunit of basaltic sandstone and a 6.17 m subunit of basalt conglomerate, which includes bioclasts. Unit XI grades upward into Unit X and is defined by the dominance of sediment matrix. Thin sections show that the conglomerate is made up of glassy aphyric basalt clasts cemented by carbonate (Sample 330-U1374A-21R-3, 19–21 cm [Thin Section 150]). For a full description of this unit, see “Sedimentology.” Basaltic clast Types 11 and 12 are found only in this unit, where they occur together with clast Types 1, 5, and 9.

Unit XII

  • Interval: Sections 330-U1374A-22R-5, 24 cm, to 29R-2, 15 cm

  • Depth: 116.45–146.06 mbsf

  • Lithology: highly olivine-plagioclase-augite-phyric basalt and breccia

  • Lithologic units: 15–31

The polylithic conglomerate of Unit XI grades downhole into monolithic breccia that defines the top of Unit XII, the upper limit of which is defined by a weathered horizon. Lithologic Units 15–19 (interval 330-U1374A-22R-5, 24 cm, to 24R-4, 20 cm) are peperitic breccia with a central, incompletely recovered 2.08 m thick interval of massive basalt (lithologic Unit 16), which might be an in situ lava flow. The peperitic breccia is composed of basalt mingled with sandy sediment (Fig. F21C) containing bioclasts (see “Sedimentology”). Seven additional intervals of massive basalt, ranging in thickness from 0.17 to 0.36 m, were recovered within Unit XII, but it is uncertain whether or not they cooled in situ. Overall, the breccia of Unit XII is more poorly sorted than that in Units VIII and X, and the clasts commonly have irregular outlines (Fig. F23A), suggesting that they are primary volcanic clasts rather than broken and transported lava fragments. The breccia and massive basalt intervals that form Unit XII are highly plagioclase-olivine-augite-phyric and contain large (up to 20 mm) plagioclase phenocrysts that frequently show signs of resorption and are filled with interconnecting inclusions of dark glass (Fig. F24).


  • Interval: Sections 330-U1374A-29R-2, 15 cm, to 43R-6, 7.5 cm

  • Depth: 146.06–256.75 mbsf

  • Lithology: sparsely to moderately olivine-phyric basalt and breccia ± plagioclase and/or augite

  • Lithologic units: 32–87

The uppermost lithologic unit of Unit XIII is a 1.12 m thick layer of a sedimentary basalt breccia supported by a sandy matrix that does not contain bioclasts (see “Sedimentology”). The rest of Unit XIII consists of basaltic breccia, in places supported by a sandy matrix (lithologic Units 55 and 57). The breccia contains 27 massive basalt bodies that may be lava lobes or larger fragments in the breccia. These basalt bodies range in thickness from 0.13 to 2.16 m. Some of the smaller bodies in the upper part of this unit have altered glassy margins, vesicular cores, and radial fractures (e.g., lithologic Unit 36), and occasionally have irregular protruding margins. These features are consistent with in situ lobes or pillows. Some of the lava bodies are autobrecciated (lithologic Units 72–74) and capped by breccia with a sandy matrix (lithologic Units 73 and 74). Most of the breccia, however, is composed of large angular basalt fragments and is free of fine material. Some of these clasts have broken edges, broken altered margins, and sharp contacts with the groundmass, suggesting that they were transported. Spaces between clasts are either empty or filled with calcite and zeolite (Fig. F23B). The basalt bodies and clasts in the breccia are largely olivine-phyric, with most units containing 1–5 vol% olivine phenocrysts, although thin sections (Samples 330-U1374A-29R-4, 17–19 cm [Thin Section 169]; 29R-4, 107–110 cm [Thin Section 170]; and 31R-6, 20–22 cm [Thin Section 175]) show that olivine is absent from, or forms only a minor part of, the phenocryst assemblage in a few clasts. There are also intervals where plagioclase and/or augite join the phenocryst assemblage (Fig. F25A, F25B). One sample (32R-4, 2–5 cm) contains a single 2 mm aggregate of pyroxene and magnetite with the characteristic amphibole outline (Fig. F25C, F25D), suggesting it was a hornblende phenocryst that became unstable in its magmatic environment. Lithologic Unit 84, 9.81 m above the base of Unit XIII, is a 40 cm thick basalt interval that is moderately plagioclase-phyric (Fig. F18). This is the first significant interval encountered downhole in which olivine is absent from the phenocryst assemblage since Unit X, 136.68 m higher in Hole U1374A.

Unit XIV

  • Interval: Sections 330-U1374A-43R-6, 7.5 cm, to 45R-8, 10 cm

  • Depth: 256.75–278.73 mbsf

  • Lithology: aphyric to moderately plagioclase-phyric basalt breccia

  • Lithologic unit: 88

As with Unit XIII, the top of Unit XIV is defined by a thin (74 cm) sedimentary interval, in this case grain-supported, micrite-bearing, greenish-black basalt breccia with occasional bioclasts (see “Sedimentology”). Unit XIV is basalt breccia in which many of the clasts exhibit reasonably well preserved glassy margins and could therefore be fragments of pillows (Fig. F26). Occasionally, the clasts are supported by a sandy matrix, but most of the spaces between the clasts are voids filled by calcite and zeolite. Many of the clasts in this unit have sharp broken edges, suggesting transportation. The top of this unit marks a significant downhole change in magmatic character from the predominantly olivine-phyric basalt in the overlying units to plagioclase-phyric and aphyric basalts below (Fig. F18).

Unit XV

  • Interval: Sections 330-U1374A-45R-8, 10 cm, to 47R-3, 5 cm

  • Depth: 278.73–290.32 mbsf

  • Lithology: aphyric basalt and breccia

  • Lithologic units: 89–91

The top of Unit XV is defined by the disappearance of phenocrysts. Otherwise, the breccia resembles that of Unit XIV in being composed of large angular clasts that may be fragments of pillows (Fig. F26B). As with Units VIII, X, XIII, and XIV, the breccia is clast supported, lacks fine material, and is cemented with calcite and zeolite.

Unit XVI

  • Interval: Sections 330-U1374A-47R-3, 5 cm, to 53R-5, 87 cm

  • Depth: 290.32–351.66 mbsf

  • Lithology: moderately plagioclase-augite-olivine-phyric basalt and breccia with aphyric intrusive sheets

  • Lithologic units: 92–104

In interval 330-U1374A-47R-3, 4–6.5 cm, the fines-free breccia of Unit XV rests on an inclined surface of sandy breccia that marks the upper boundary of Unit XVI (Fig. F27). This interval is the top of a 95 cm thick sedimentary interval of grain-supported, micrite-bearing, greenish-black basalt breccia with occasional bioclasts and is the deepest interval in Hole U1374A in which bioclasts were found (see “Sedimentology”). Clasts in Unit XVI are generally less angular than in overlying units and frequently have rounded lobate margins. Only three massive basalt intervals thicker than 30 cm were recovered in Unit XVI (Fig. F18). This unit is marked by increasingly green fragments making up the breccia. The first record of a greenish color in the description of igneous rocks in this hole is just below the sedimentary interval at the top of this unit. The green color appears to reflect increasing amounts of fine altered glass particles (hyaloclastite) filling the spaces between the clasts within the breccia, rather than the pore spaces being empty or filled by light-colored secondary minerals. Initially, the two types of breccia are interspersed, but from the top of lithologic Unit 100 until the bottom of the hole, the breccia is mostly supported by an altered green hyaloclastite matrix. At the top of Unit XVI and in several intervals in the rest of Hole U1374A the clasts are more strongly altered, giving the breccia an orange color. The increasing dominance of green altered hyaloclastite matrix down through Unit XVI and the rest of the hole is seen clearly in the color reflectance data (Fig. F28B). The breccia is composed of plagioclase-augite-olivine-phyric basalt. Unit XVI also contains the first of a series of aphyric basalt intrusive sheets (lithologic Unit 103) that were encountered at Site U1374. These are described separately in “Intrusive sheets.”


  • Interval: Sections 330-U1374A-53R-5, 87 cm, to 56R-1, 118.5 cm

  • Depth: 351.66–375.19 mbsf

  • Lithology: aphyric basalt breccia with aphyric intrusive sheets

  • Lithologic units: 105–107

Clasts of aphyric basalt start to appear at the bottom of Unit XVI in Section 330-U1374A-53R-3 and become more abundant throughout Sections 53R-4 and 53R-5. The disappearance of clasts of porphyritic basalt at Section 53R-5, 87 cm, defines the upper boundary of Unit XVII. The greenish color noted in the lower part of Unit XVI persists through the upper 2 m of Unit XVII and is then followed downhole by a 7.3 m thick interval in which the smaller clasts are orange in color as a result of alteration of glass to clays and iron oxyhydroxides. This orange zone coincides with an interval in which the breccia is mostly clast supported, the clasts are blockier, and the spaces between clasts are free of fine particles. Increased permeability in this interval probably facilitated circulation of seawater, which caused the observed oxidation. The breccia becomes green again at Section 54R-6, 20 cm (361.90 mbsf), and remains green or greenish gray to the bottom of the hole (Fig. F28B). A notable feature of Unit XVII is the appearance at Section 54R-6, 140 cm (363.10 mbsf), of highly vesicular and often frothy clasts of altered basaltic glass that are found throughout the lower 12.1 m of the unit. Unit XVII contains another intrusive sheet of aphyric basalt (lithologic Unit 106).


  • Interval: Sections 330-U1374A-56R-1, 119 cm, to 63R-3, 140 cm

  • Depth: 375.19–445.02 mbsf

  • Lithology: sparsely to moderately plagioclase-phyric basalt and breccia with aphyric intrusive sheets

  • Lithologic units: 108–134

The abrupt return of plagioclase phenocrysts at Section 330-U1374A-56R-1, 119 cm, defines the upper boundary of Unit XVIII, which consists mostly of basaltic breccia with eight short (0.32–0.86 m) intervals of massive basalt. Clasts in the breccia are subangular to subrounded and often have lobate margins (Fig. F23C). Frothy clasts, first seen in the lower 12.1 m of Unit XVII, are also present in the upper 3.8 m of Unit XVIII, down to Section 56R-4, 80 cm (379.03 mbsf). An angular aphyric basalt clast was noted at Section 60R-7, 95 cm (421.4 mbsf), and aphyric basalt clasts increase steadily in abundance downhole from this point. Unit XVII contains several aphyric basalt intrusive sheets: lithologic Units 116, 120, 122, 124, 125 (interpreted as the peperitic margin of lithologic Unit 124), 126, and 128.

Unit XIX

  • Interval: Sections 330-U1374A-63R-4, 0 cm, to 73R-1, 81 cm

  • Depth: 445.02–521.81 mbsf

  • Lithology: aphyric to moderately plagioclase-phyric basalt and breccia

  • Lithologic units: 135–148

The base of a 0.86 m thick massive plagioclase-phyric basalt fragment or lava lobe at the bottom of Section 330-U1374A-63R-3 is taken as the upper boundary of Unit XIX because this is the last occurrence downhole of a significant interval of plagioclase-phyric basalt and because angular aphyric basalt clasts become more abundant than lobate plagioclase-phyric clasts around this depth. The last plagioclase-phyric basalt clast was found at Section 63R-7, 90 cm (449.8 mbsf), and thereafter the breccia is composed entirely of aphyric basalt clasts. The transition from the lobate plagioclase-phyric clasts of Unit XVIII to the angular aphyric clasts of Unit XIX occurs over an interval of 13.5 m.

The upper part of Unit XIX is composed of angular aphyric basalt clasts in a matrix of altered hyaloclastite. From Sections 330-U1374A-65R-4, 103 cm (465.23 mbsf), to 66R-1, 22 cm (470.12 mbsf), the hyaloclastite matrix is progressively replaced by calcite cement and the breccia grades into an interval with large jigsaw-fit clasts of aphyric basalt. This interval may represent the remains of an autobrecciated lava flow. Hyaloclastite matrix returns below this interval; the breccia in the lower part of Unit XIX ranges from clast- to matrix-supported, and in two intervals (69R-1, 0 cm, to 69R-4, 28 cm; and 71R-2, 71 cm, to 72R-1, 55 cm) it becomes sufficiently fine grained to be described as vitric-lithic sandstone and gravel-size breccia. The frothy clasts of altered basaltic glass noted in the lower part of Unit XVII and the upper part of Unit XVIII are found again in the lower part of Unit XIX (Sections 66R-1, 22 cm [470.12 mbsf], to 72R-1, 55 cm [518.45 mbsf]). An example of hyaloclastite breccia from the lower part of Unit XIX is shown in Figure F23D. Within Unit XIX five intrusive sheets of aphyric basalt were recovered (lithologic Units 136, 139, 140, 146, and 148). Drilling at Site U1374 terminated in the last of these intrusive sheets at 522 mbsf.

Intrusive sheets

A maximum of 13 intrusive sheets were intersected in interval 330-U1374A-52R-1, 0 cm, to 73R-1, 81 cm (lower part of Unit XVI and Units XVII, XVIII, and XIX). These sheets are composed almost entirely of fine-grained aphyric basalt, although discrete patches of phenocrysts were observed in a few of them. The uppermost sheet drilled (lithologic Unit 103) contains scattered plagioclase phenocrysts in the 1.89 m of interval 52R-1, 0 cm, to 52R-2, 52 cm. Lithologic Unit 126 contains a 13 cm interval (60R-2, 105–128 cm) of sparsely plagioclase-phyric basalt, and in lithologic Unit 146 there is a 12.5 cm interval (71R-2, 23.5–36 cm) of altered olivine phenocrysts (<1%). Some observed contacts dip at 50°–60°, but others are subvertical to vertical. The sheets have bands of small vesicles aligned parallel to their margins, and this allows the orientation of sheets to be assessed in cases where the margin was not recovered. The number of observed and inferred vertical contacts suggests that the sheets are dikes. In two cases the margins are peperitic, and in one of these the peperite is separated from a second intrusion by a thin sliver of green breccia. Examples of these contact relations are shown in Figure F29. The sheets are composed of fine-grained aphyric basalt with rather altered plagioclase laths and titanaugite prisms. Some samples contain partly resorbed microphenocrysts of plagioclase. The mineralogy of the sheets suggests that the parent magma was moderately to strongly alkaline.

The two lowest intrusive sheets in Hole U1374A (lithologic Units 146 and 148) have several physical characteristics that distinguish them from the other sheet units:

  1. They do not contain vesicles, whereas the other sheets have bands of small vesicles parallel to their margins.

  2. They are more massive and less altered than the other sheets.

  3. The lower contact of lithologic Unit 146 and the upper contact of lithologic Unit 148, which were both recovered, are more irregular than the contacts of the other sheets.

Downhole logging, however, shows that the upper contact of lithologic Unit 146 dips at 60° in the direction of 141°, whereas the lower contact dips at 43° toward 166° (see “Downhole logging”). The dips are similar to those observed in the intrusions higher in Hole U1374A and suggest that lithologic Unit 146 is also an intrusive feature. Lithologic Unit 148 occurred at the bottom of the hole, so logging was not possible; however, because this unit shares many physical features with lithologic Unit 146, it is also likely to be an intrusive sheet. More detailed postexpedition investigations will determine whether these two lithologic units are compositionally similar to the unequivocally intrusive sheets and whether any of the sheets can be matched to stratigraphic units higher in the eruptive sequence.

Interpretation of the igneous succession at Site U1374

The igneous rocks encountered during drilling of Hole U1374A on the western flank of Rigil Guyot will be interpreted in chronological order (oldest first, from the bottom of the hole). These rocks reflect basaltic eruptions that cover the interval from the late submarine constructional phase of the seamount to its emergence as an island. The drilled succession probably accumulated in a volcaniclastic apron around the seamount, similar to but larger than the extensive submarine volcaniclastic fans observed in a recent (2007) multibeam bathymetric survey around the emergent island of Surtsey, off the south coast of Iceland (Fig. F30) (Jakobsson et al., 2009).

The deepest part of the cored succession comprises a >76.8 m interval of volcanic breccia (Unit XIX), dominated in its lower part by hyaloclastite containing angular clasts of aphyric basalt and smaller fragments of frothy basaltic glass, now altered to green clay. Frothy basalt fragments imply that the melt was degassing and fragmenting on eruption and therefore that the magma was emplaced into water shallow enough for magmatic volatiles to be exsolved. The exact depth depends on the original volatile content of the magma, and the occurrence of volatile-induced fragmentation does not provide a useful limit on depth of emplacement because submarine volcaniclastic rocks are now known to form even in deep water (e.g., Head and Wilson, 2002; Clague et al., 2009). The first conglomerates formed in a subaerial environment are found ~400 m above the volcaniclastic rocks at the bottom of Hole U1374A, providing an estimate of the minimum water depth at which the lowest volcaniclastic rocks were emplaced. This can only be a minimum estimate because of the possibility of unrecognized erosion surfaces in the volcaniclastic succession. Postexpedition research will enable us to investigate the processes of degassing and fragmentation of the lava in Hole U1374A in more detail.

A gradational change in clast type from aphyric and blocky to plagioclase-phyric and lobate marks the transition to Unit XVIII. This change in clast morphology could reflect a change in the style of eruption and/or magma composition. Unit XVIII contains several intervals of massive basalt that could represent thin flow lobes. The frothy basaltic clasts noted in the lower part of Unit XIX return in the upper 3.8 m of Unit XVIII and the lower 12.1 m of the overlying Unit XVII, which is defined by a return to blocky, angular aphyric basalt clasts.

A second return to plagioclase-phyric basalt, now with the addition of augite and minor olivine phenocrysts, marks the beginning of Unit XVI, which, like Unit XVIII, is also composed of lobate clasts. Breccia in the lower part of the unit has a green hyaloclastite matrix, but this becomes less prominent uphole through the unit and is a minor component by 328 mbsf. This reduction in the abundance of green hyaloclastite is followed, 37 m farther upsection, by the first appearance of bioclasts in the thin (95 cm) sandy layer at the top of Unit XVI. The presence of bioclasts implies shallow-water conditions nearby and is taken to indicate shoaling of parts of the seamount at a distance from Site U1374 (see “Sedimentology”).

The lower 186 m of the succession drilled in Hole U1374A is intruded by intrusive sheets of aphyric basalt that we interpret as dikes. We have no way of knowing at this stage how much higher in the succession these dikes extend because they were not encountered at higher levels and we cannot match their composition to any of the lava flows or breccia units that were drilled (see “Geochemistry”). They could have extended to levels higher than the present guyot surface, or they could have been truncated at a yet-unrecognized erosion surface higher in the succession than 336 mbsf.

The thin sandy layer at the top of Unit XVI is the first of five sedimentary intervals encountered in Hole U1374A, each of which marks the end of a distinctive magmatic episode (Units XVI, XV/XIV, XIII, XII, and X). Units XV and XIV are composed of basaltic breccia and represent the third cycle of aphyric to plagioclase-phyric basalt magmatism. Clasts vary from angular to lobate, and at least some are probably pillow fragments (Fig. F26). The green hyaloclastite matrix that was so prominent in the lower units is now completely absent. Instead, the breccia is clast supported and the voids between clasts are either open or filled with calcite and/or zeolite. The complete absence of massive basalt intervals that might represent in situ lava lobes suggests that these units could be a talus deposit composed of pillow debris derived from shallow-submarine eruptions higher up on the flanks of the emergent island.

Unit XIV ends with a thin (74 cm) sandy interval that marks a change in the character of the basalt from relatively evolved (plagioclase-phyric or aphyric) below to more magnesium-rich (olivine-phyric) above. Olivine dominates the phenocryst assemblage from this interval to the top of Hole U1374A. The basalt of Unit XIII is sparsely to moderately olivine-phyric, occasionally with plagioclase and augite phenocrysts (Fig. F25A, F25B) and rare euhedral pyroxene-magnetite intergrowths (Fig. F25C, F25D). The latter are most probably hornblende phenocrysts that have broken down through dehydration reactions. Hornblende is unlikely to have crystallized from the host magma, so these intergrowths provide evidence for mixing with a more evolved magma type not represented by any of the igneous rocks encountered in the two holes drilled on Rigil Guyot.

Unit XIII is mostly basaltic breccia, but, unlike the two preceding units, it has a large number of more massive intervals that have preserved a consistent magnetic inclination (see “Paleomagnetism”). The fragmentation of lava flows in a subaqueous environment through a process of contact-surface steam explosivity (Kokelaar, 1986) provides a plausible mechanism for the formation of this assemblage of breccia and more massive intervals. In this process, magma becomes coated with a thin film of superheated steam, which expands and collapses on a very short timescale. Kokelaar (1986) argues that the resulting stress waves “can result in magma fragmentation and produce blocky, pyramidal or splinter-like fragments.” A photograph of a typical example of Unit XIII breccia is given in Figure F23B. This breccia may have formed through the fragmentation of lava erupted into water, followed by transport of the fragments downslope to form a talus deposit. This could explain the lack of fine particles but not the presence of what we have described tentatively as lava lobes within Unit XIII, the largest of which preserves consistent magnetic orientations (see “Paleomagnetism”). A solution to this problem might be that the lava “lobes” are actually large blocks that were still hot enough at their final resting places to preserve a record of the ambient magnetic field.

Unit XIII ends with another thin (1.12 m) sandy horizon, which probably represents a hiatus in magmatic activity at this site. The next episode of volcanism produced Unit XII, a breccia deposit with two thin (~1 m) lava lobes and capped by a 2.7 m thick lava flow, all composed of olivine-augite-plagioclase-phyric basalt. Plagioclase phenocrysts in Unit XII are conspicuous in their large size (up to 2 cm) and disequilibrium textures. Many have rounded outlines, corroded margins, and sieve textures caused by incorporation of glassy groundmass into an interconnected network of aligned and angular inclusions (Fig. F24). Magma mixing provides a plausible explanation for these disequilibrium features. Shore-based studies will investigate this further.

The volcaniclastic deposits change significantly from the blocky breccia of Unit XIII to more scoriaceous material in Unit XII (Fig. F23A). A likely explanation is that Unit XII was the product of hydrovolcanic eruptions resulting from the interaction of lava flows with either wet sediment or shallow water. In either case the deposits could have fallen from a subaerial tephra plume on the emergent island. The peperitic base and top of the capping lava flow (Fig. F21C) indicate interaction of magma with wet sediment.

The end of Unit XII magmatism is marked by a 6.6 m thick conglomerate unit (Unit XI) that is the first deposit with an unequivocally terrestrial provenance encountered upsection at Site U1374. This provides further evidence that Site U1374 was finally at or close to sea level (see “Sedimentology”). Unit X repeats the style of Unit XII volcanism, with its scoriaceous breccia and thin flow lobes capped by a lava flow with a peperitic base (Fig. F21B), all composed of aphyric basalt. The end of volcanism in Unit X is marked by another thicker (21 m) sedimentary deposit (Unit IX).

Large sieve-textured plagioclase phenocrysts return in Unit VIII, a hydrovolcanic deposit of moderately olivine-phyric basalt clasts. Plagioclase phenocrysts, though conspicuous, account for <1% of the total phenocrysts. Units VII, VI, V, and IV are, respectively, finely bedded volcanic sandstone, a 3.25 m thick lava flow, volcanic breccia, and a 9.3 m thick lava flow, all composed of aphyric basalt. The Unit V breccia contains jigsaw-fit intervals and is clearly peperitic at the top (Fig. F21A). It is probably the base of the thick Unit IV lava flow. The whole assemblage suggests eruption of basaltic magma into a wet but probably not submarine environment. The final volcanic event recorded at Site U1374 is represented by a thin (1.3 m) lava flow of olivine-augite-plagioclase-phyric basalt. One of the augite phenocrysts in this flow contains an inclusion of hornblende, although hornblende was not seen as a primary phenocryst phase in any of the rocks studied at Sites U1374 or U1373. This provides further evidence for the availability of hornblende-bearing magmas at the time of formation of the Site U1374 succession.

Four patterns emerge from our study of the volcanic basement rocks at Site U1374:

  1. Magmatism on the western flank of Rigil Guyot started in a clearly submarine environment and, in the time interval considered here, progressed to a shallow-marine and then subaerial environment. This progression is clearly seen in the various breccia types recovered at Site U1374 (Fig. F23), which range from green hyaloclastite breccia with frothy basaltic clasts (marine), to blocky breccia (shallower marine), to scoriaceous (near sea level or subaerial) basalt. The proportion of lava flows increases upsection, and peperite is only found in the upper units (Units III–XII). Note that the short succession drilled at Site U1373, nearer to the center of Rigil Guyot, was erupted entirely in a subaerial (but occasionally wet) environment.

  2. Distinct eruptive packages are separated by intervals of background sedimentation, five of which were identified at Site U1374. The thicknesses of these intervals increase dramatically when the seamount is inferred to have grown above sea level and erosion could proceed more rapidly (see “Sedimentology”).

  3. The phenocryst assemblage changes from plagioclase-dominated in the lower part of the succession (Units XIV–XIX) to olivine-dominated in the upper part (Units III–XIV), suggesting that the magmas became more basic with time.

  4. The magma erupted at Site U1374 was alkalic throughout the drilled interval, but there is some evidence that the later basalt flows were more alkalic than the earlier ones. Basalt from Units III and XII contains titanaugite, whereas the augite in Units XV and XVI is bottle green in hand specimen and greenish brown in thin section, suggesting Ti-poor ferroaugite. Shore-based studies will determine the exact composition of the pyroxenes.