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Igneous petrology and volcanology

In Hole U1376A, 140.9 m of igneous rock was penetrated after igneous basement was entered at 41.93 mbsf. The igneous sequence was divided into 42 lithologic units, which have been grouped into two stratigraphic units (Fig. F10). 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 T5. In this section we first describe the volcanic components of the sedimentary units, then describe each of the igneous units, and finally interpret the entire igneous succession.

Stratigraphic sedimentary units

Unit I (volcanic sandstone and breccia)

Unit I comprises four subunits of volcanic sandstone and breccia (see “Sedimentology”), representing the products of a phase of volcanism that postdates the erosion and submergence of Burton Guyot. These deposits are separated from earlier volcanic rocks by a 15 m thick unit (Subunit IIA) of white algal limestone (see “Sedimentology”). Clasts from Unit I breccia provide us with the only material from a posterosional or rejuvenation phase of magmatism recovered during Expedition 330. Unit I is mostly composed of fragments of basalt (Fig. F11), but some layers are dominated by angular fragments of glass with subordinate grains of plagioclase, augite, altered olivine, hornblende, and rare biotite (see “Sedimentology”). Basaltic clasts in Thin Section 234 (Sample 330-U1376A-1R-4, 39–42 cm) from Subunit IC contain anhedral crystals of orthopyroxene surrounded by coronas composed of tiny clinopyroxene crystals (Fig. F12). The orthopyroxene crystals are most likely xenocrysts derived from disrupted mantle xenoliths. Small (up to 15 mm) aggregates of olivine and pyroxene, which may in fact be intact mantle xenoliths, were noted in Subunits IA and IC.

Basaltic clasts in sedimentary Subunit IIB

The larger basaltic clasts within the conglomerate of Subunit IIB (see “Sedimentology”) were divided into four types on the basis of their appearance in hand specimen. Each clast type is described below:

  • Type 1: sparsely olivine-phyric basalt that is medium gray, with 1.5% olivine phenocrysts (altered to iddingsite and carbonate; maximum size = 4 mm, modal size = 2 mm). The groundmass is aphanitic, with 0.5% vesicles (elongate, subangular; 100% filled with carbonate and green clay) and veins (filled with green clay and carbonate).

  • Type 2: highly olivine-phyric basalt that is brownish gray, with 10% olivine phenocrysts (slightly altered to iddingsite and carbonate; maximum size = 7 mm, modal size = 2 mm) and small amounts of native copper. The groundmass is fine grained, with 6% vesicles (low sphericity, angular; filled with carbonate) and veins (filled with carbonate).

  • Type 3: moderately olivine-phyric basalt that is reddish gray, with 2% olivine phenocrysts (moderately altered to green clay, carbonate, and iddingsite; maximum size = 2 mm, modal size = 1.5 mm). The groundmass is fine grained, with 1% vesicles (moderate sphericity to elongate, rounded; filled with carbonate and green clay) and veins (filled with carbonate).Type 4: aphyric basalt that is reddish gray, with no phenocrysts. The groundmass is fine grained, with 10%–30% vesicles (low to elongate sphericity, subrounded [3% filled]; blue coating) and veins (filled with carbonate).

Lithologic and stratigraphic igneous units

Unit III

  • Interval: Sections 330-U1376A-5R-3, 122 cm, to 14R-4, 47 cm

  • Depth: 41.93–110.14 mbsf

  • Lithology: moderately to highly olivine-augite-phyric basalt and basalt breccia

  • Lithologic units: 1–17

Unit III is the uppermost stratigraphic unit in the igneous basement and was encountered at 41.93 mbsf, where the sedimentary conglomerate of Subunit IIB sits unconformably on its inclined, eroded, upper surface. Unit III was defined on the basis of its dominant olivine-augite phenocryst assemblage. The uppermost three lithologic units form a distinctive 4.82 m thick interval composed mostly of moderately to highly olivine-augite-phyric hyaloclastite basalt breccia with a central interval of massive basalt (lithologic Unit 2), which we interpret as a pillow or lobate lava flow. The breccia units contain abundant unaltered glass and clasts of altered basalt. Lithologic Unit 3 is heterolithic breccia containing, in addition to olivine-augite-phyric basalt clasts, some clasts in which augite is absent from the phenocryst assemblage. This interval grades into a 25.46 m thick interval of moderately olivine-augite-phyric massive basalt units separated by thin units of hyaloclastite breccia (lithologic Units 4–14). Lithologic Units 4–6 comprise rounded or lobate bodies of basalt that have chilled margins and are surrounded by thin zones of red clay containing spalled chips of altered glass (Fig. F13). These bodies of basalt are interpreted as pillows, most of which appear to be in situ. Lithologic Unit 5 is a 1.1 m thick pillow or lobate lava flow. Lithologic Units 7–14 are composed of breccia with intervals of massive basalt that may be lobate flows, pillows, or large pillow fragments. Lithologic Units 8, 10, and 12 are thin lobate flows or large fragments of olivine-phyric basalt. Olivine phenocrysts seen in Thin Section 253 (Sample 330-U1376A-6R-6W, 128–131 cm) from lithologic Unit 12 have abundant small glassy inclusions (<50 µm in diameter), some containing shrinkage bubbles (Fig. F14). The pillows become increasingly fragmented downhole through lithologic Units 13 and 14, and the proportion and size of basaltic clasts then increase until the basalt becomes entirely massive in lithologic Unit 15. The lower part of lithologic Unit 14 is probably, therefore, the brecciated flow top of lithologic Unit 15.

Lithologic Unit 15 is a 33.11 m thick massive lava flow composed of highly olivine-augite-phyric basalt that is remarkably uniform throughout. It has 10% unaltered olivine phenocrysts as large as 10 mm and 4% augite phenocrysts as large as 8 mm (Fig. F15). The lowest 29 cm of this lava flow is fractured, and one of the fractures is filled with hyaloclastite breccia from underlying lithologic Unit 16, whereas hyaloclastite in the uppermost 30 cm of lithologic Unit 16 is baked and noticeably darker than that below (Fig. F16A). The baked layer has a much higher magnetic susceptibility than either the overlying lava flow or the underlying hyaloclastite (see “Physical properties” and “Paleomagnetism”). Lithologic Unit 16 does not contain pillows but has small angular clasts similar to those in the breccia intervals above the ~33 m thick lava flow (lithologic Unit 15). Many of these clasts are augite-free and may have been derived from Unit IV. Unit III ends with a 2.07 m thick highly olivine-augite-phyric basalt lava flow (lithologic Unit 17) that contains 10% unaltered olivine and 2% augite phenocrysts.

Unit IV

  • Interval: Sections 330-U1376A-14R-4, 47 cm, to 23R-6, 75 cm

  • Depth: 110.14–180.32 mbsf

  • Lithology: moderately to highly olivine-phyric basalt and breccia with aphyric basalt

  • Lithologic units: 18–42

Unit IV is distinguished from Unit III by the disappearance of augite phenocrysts and is dominated by moderately to highly olivine-phyric basalt breccia. An 8 cm thick interval of aphyric basalt separates the highly olivine-augite-phyric lava flow at the bottom of Unit III from the moderately olivine-phyric breccia of Unit IV. The thin interval of aphyric basalt (lithologic Unit 18) is interpreted as a fragment of basalt derived from the first of two intrusive bodies that we interpret as dikes (see “Intrusive sheets”). The top of Unit IV is interpreted to represent an erosional contact, as discussed in “Interpretation of the igneous succession at Site U1376.”

The Unit IV breccia commonly has a hyaloclastite matrix with moderately altered to unaltered glass shards and abundant large (up to 30 cm) olivine-phyric basalt clasts (Fig. F17). There is no clear evidence that any of these clasts are in situ pillows, although they commonly have glassy margins and may be pillow fragments. The lower 2.6 m of the 17.35 m thick interval of hyaloclastite breccia at the top of Unit IV (lithologic Units 19 and 21) is composed of exceptionally fresh basaltic glass shards cemented by only a thin film of clay minerals (Fig. F18). The poorly recovered interval from Sections 330-U1376A-17R-4, 44 cm, to 19R-1, 0 cm (130.00–144.40 mbsf), marks a downhole change in the breccia clasts from monolithic to heterolithic. Breccia forming lithologic Units 26, 30, 34, and 36 (134.80–166.50 mbsf) contains five different clast types, in contrast to other breccia units. These clast types were distinguished by their phenocryst assemblage and vesicularity. Types 1 and 4 are composed, respectively, of nonvesicular and highly vesicular (20%) highly olivine-phyric basalt that has a similar phenocryst content to the hyaloclastite matrix. Types 2 and 3 are composed of nonvesicular and highly vesicular (10%–20%) aphyric basalt, respectively. Type 5, restricted to one occurrence in lithologic Unit 36, is represented by a single moderately augite-olivine-phyric basalt clast—one of only two occurrences of augite phenocrysts in Unit IV, the second being in lithologic Unit 42, the lowermost unit recovered at this site. The first occurrence of heterolithic breccia was observed in Core 330-U1376A-18R in lithologic Unit 26, which had by far the poorest recovery (11%) of all cores at this site. The recovered material consists of 15 separate 2–14 cm fragments and a large number of smaller fragments of highly vesicular basalt, some of which are yellowish or red and oxidized. Downhole logging (see “Downhole logging”) shows that the unrecovered intervals in Cores 17R and 18R are occupied mostly by breccia. This interval was left uncolored in the stratigraphic column (Fig. F10) because clast types could not be determined through macroscopic examination.

The ~40 m interval from Sections 330-U1376A-17R-2, 93 cm, to 22R-3, 82 cm (lithologic Units 22–37; 127.57–167.23 mbsf), which includes intervals of heterolithic breccia, also contains six aphyric basalt bodies (lithologic Units 22, 24, 27, 31, 35, and 37) as thick as 1.6 m. These basalt bodies are highly vesicular (5%–15%) and are interpreted as lobate flows or lava fragments. Lithologic Unit 29 is a 1.79 m thick massive basalt flow that contains no vesicles and varies from aphyric to highly olivine-phyric (0%–30% olivine phenocrysts) over intervals of 25 cm. Olivine phenocrysts are concentrated toward the base of the flow, although there are aphyric intervals in its center. Two other olivine-phyric basalt bodies are fragmented (lithologic Unit 33) or thin (lithologic Unit 39) and lack clear evidence to determine whether they are larger clasts within the breccia or lobate flows.

Drilling ended at 182.8 mbsf in the 0.56 m thick lithologic Unit 42, which consists of isolated fragments of moderately olivine-augite-phyric basalt, some with adhering breccia selvages.

Intrusive sheets

Lithologic Units 18, 20, and 41 in Unit IV are parts of intrusive sheets composed of fine-grained aphyric basalt. The 8 cm thick lithologic Unit 18 sits between stratigraphic Units III and IV and is highly fractured, making its contacts difficult to observe and interpret (Fig. F16D). It contains thin trains of very small vesicles and closely resembles the chilled margin of another intrusive sheet 1.78 m downhole (lithologic Unit 20). These vesicle trains are truncated upward at the base of lithologic Unit 17, a 2.07 m thick basalt lava flow that defines the bottom of Unit III, and downward at what looks like the broken lower edge of lithologic Unit 18. It seems likely, therefore, that lithologic Unit 18 is a detached fragment of lithologic Unit 20, which was exposed nearby at an erosional surface at the top of Unit IV. If so, the boundary between Units III and IV could represent a significant time gap between eruptive phases. The 1.78 m interval of breccia that separates lithologic Units 18 and 20 contains two intervals (330-U1376A-15R-1, 8–15 cm, and 15R-1, 79–87 cm) that are partly occupied by the margin of an aphyric basalt intrusion. The chilled margin of lithologic Unit 20 runs parallel to the edge of the core and is brecciated in places (Fig. F19). Vesicle trains run parallel to the sheet margins and are aligned vertically in the center of the sheet. This suggests that the sheet is near vertical and is therefore a dike.

Lithologic Unit 41 is very similar in appearance to lithologic Unit 20 and has a steeply dipping upper contact with a well-preserved glassy margin. The lower contact was not recovered. Bands of vesicles run vertically through the unit and parallel to the margins, which wrap around clasts within the breccia. This unit is heavily fractured in places, with vertical veins (see “Structural geology”) that may be the result of contraction during cooling. All of these features suggest that this sheet is also a dike.

Interpretation of the igneous succession at Site U1376

The basement section cored at Site U1376 on Burton Guyot comprises a succession of basaltic breccia, pillow lava, and massive lava flows. Two units were defined on the basis of phenocryst phases, with Unit IV mostly containing olivine phenocrysts and Unit III containing both olivine and augite phenocrysts. The deepest unit cored (lithologic Unit 42) is breccia with clasts containing both olivine and augite phenocrysts. This unit may be the top part of a third stratigraphic unit; however, given the very short interval (0.56 m) occupied by this unit, it is included in stratigraphic Unit IV.

The bottom 13.1 m of the succession comprises mostly olivine-phyric basalt breccia. An interval of highly vesicular aphyric basalt (166.5–167.2 mbsf) overlying this breccia heralds the arrival of a second aphyric magma type. The next 31.7 m of the succession comprises heterolithic breccia with olivine-phyric and aphyric basalt clasts and thin flows of aphyric basalt. This interval records a period when two types of magma were being erupted in the area at the same time. Lithologic Unit 26, the upper unit in this interval, consists of a number of highly vesicular and oxidized fragments of aphyric and olivine-phyric basalt and may provide evidence for a period of shallow-water or subaerial volcanism. It may be significant that this unit lies in the middle of a 12 m interval of very low core recovery (11% in Core 330-U1376A-18R) that may be occupied by friable and poorly cemented breccia, consistent with the results of downhole logging (see “Downhole logging”). Two thin flows of aphyric basalt separated by a 24 cm thick interval of olivine-phyric basalt breccia (lithologic Units 22–24) mark the last occurrence of aphyric basalt in the eruptive succession. The upper 17.35 m interval of Unit IV (lithologic Units 21 and 19) is composed of olivine-phyric hyaloclastite breccia containing a high proportion of unaltered glass.

Unit III records a change in magma composition from one crystallizing olivine to a slightly more evolved one crystallizing olivine and augite. The first significant appearance of olivine-augite-phyric basalt is marked by two massive lava flows, 2.07 and 33.11 m thick, respectively, separated by a 2.75 m interval of basalt breccia. The remainder of Unit III is composed of olivine-augite-phyric basalt breccia and pillow lava, except for three short intervals of olivine-phyric basalt in Core 330-U1376A-6R (Fig. F10). The presence of pillow lava high in the Unit III succession suggests that most, if not all, of the basement section was erupted in a marine environment.

Intrusive sheets (dikes) cut the whole of Unit IV and therefore postdate the emplacement of this unit. Although these dikes were not seen in Unit III, they may have extended through it and, if so, would represent the last magmatic event recorded in the basement section of Burton Guyot. However, the contact relations of the uppermost dike (lithologic Units 18 and 20) with the overlying olivine-augite-phyric lava flow (lithologic Unit 17) provide evidence for an erosional surface between Units III and IV, implying that the dikes were intruded before the emplacement of Unit III and then were exposed by erosion. There are no aphyric basalt units in Unit III, immediately above the upper observed limit of the dikes, and therefore these dikes cannot be linked directly to any of the recovered volcanic units. It is possible that the dikes fed lava flows stratigraphically above Unit III that have not been preserved. Alternatively, they may have fed aphyric lava flows similar to those in Unit IV, which were removed by erosion before the emplacement of Unit III. Postexpedition studies may be able to address this question.

A record of a posterosional or rejuvenation phase of magmatism at Site U1376 is provided by the volcanic sandstone and breccia of sedimentary Unit I. The composition of clasts in Unit I implies at least two magma types. First, some of the sandstone layers contain fragments of hornblende and biotite, suggesting late-stage eruption of more evolved magma than that represented by the basement succession. Second, Subunit IC contains olivine-pyroxene aggregates that may be mantle xenoliths, as supported by the occurrence of partly resorbed orthopyroxene xenocrysts in basalt clasts in Subunit IC.

The presence of olivine and augite phenocrysts in the basaltic basement section at Site U1376 and the complete absence of plagioclase phenocrysts suggest that the magmas were alkaline and more basic than those represented by most of the volcanic rocks drilled at Sites U1372, U1373, and U1374. Reaction coronas around orthopyroxene xenocrysts in basaltic clasts in sedimentary Unit I suggest that the rejuvenation stage magmas were strongly alkaline.