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Igneous petrology and volcanology

Two holes were drilled at Site U1377 on Hadar Guyot. In Holes U1377A and U1377B, 38.2 and 27.9 m of igneous rocks were penetrated after igneous basement was reached at 15.1 and 9.1 mbsf, respectively. The igneous sequence was divided into six lithologic units in Hole U1377A and 18 lithologic units in Hole U1377B. In both holes these units were grouped into a single stratigraphic unit (Unit III in each case; Fig. F8). To help achieve the paleomagnetic objectives of this expedition, each of the 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 T6. In this section we describe the igneous units in Hole U1377A and then the volcanic components in the sedimentary units of Hole U1377B, followed by each of the igneous units in Hole U1377B. Our interpretation of the entire igneous succession follows.

Hole U1377A

Lithologic and stratigraphic igneous units

Unit III
  • Interval: Sections 330-U1377A-3R-1, 0 cm, to 6R-3, 118 cm

  • Depth: 15.10–47.46 mbsf

  • Lithology: banded aphyric to highly olivine-phyric trachybasalt

  • Lithologic units: 1–6

The six lithologic units defined in Hole U1377A are dominantly aphyric lava flows with occasional olivine-phyric intervals. These lithologic units were combined to form stratigraphic Unit III. Strong flow banding, defined by variation in grain size and vesicle abundance, occurs throughout Unit III (Fig. F9), and in some intervals it causes the rocks to split into parallel-sided fragments (e.g., Fig. F9B). Strong flow banding is a common feature of intermediate alkaline volcanic rocks, and the intermediate composition of Unit III is confirmed by the only shipboard chemical analysis carried out on a rock sample from Site U1377 (Sample 330-U1377A-3R-2, 2–4 cm). Because this sample contains 55 wt% SiO2 and 3 wt% MgO (see “Geochemistry”) despite being moderately olivine-phyric, we applied the name “trachybasalt” to all igneous units from Hole U1377A (and also Hole U1377B). Postcruise chemical analysis will allow more rigorous identification of the igneous rock types.

Lithologic Unit 1 is 1.48 m thick and is composed of aphyric trachybasalt with vesicle bands that run vertically through the core. Its upper contact with the sedimentary cover (stratigraphic Units I and II; see “Sedimentology”) and its lower contact with lithologic Unit 2 were not recovered. Lithologic Unit 2 is a 0.17 m thick interval of moderately olivine-phyric trachybasalt from which the sample providing the trachybasalt composition was taken (see “Geochemistry”). The lower boundary of Unit 2 was not recovered. Lithologic Unit 3 was inferred to make up the bulk (20.36 m) of the igneous succession recovered from Hole U1377A, although we cannot be certain of this because of low core recovery (28%). Unit 3 is composed of aphyric trachybasalt, except for two thin intervals that contain altered olivine phenocrysts. One of these intervals is 13 cm thick and highly olivine-phyric, and the other is 15 cm thick and moderately olivine-phyric. Centimeter- to subcentimeter-scale flow banding occurs throughout the unit. Grain size varies from aphanitic to fine grained, and vesicle abundance ranges from 0% to 25% between adjacent layers (Fig. F9A, F9B). Lithologic Unit 4 is distinct from Unit 3 in being sparsely olivine-phyric. Only 35 cm was recovered, and this includes neither the upper nor the lower contact. The 1.99 m thick lithologic Unit 5 is visually distinctive in its abundance (20%) of vesicles. It is moderately olivine-phyric and also contains 0.5% augite phenocrysts. Lithologic Unit 6 makes up the lower 26 cm of Hole U1377A and consists of poorly recovered aphyric trachybasalt rubble.

Hole U1377B

Basaltic clasts in sedimentary Unit II

Two types of basalt clasts were observed in the conglomerate of stratigraphic Unit II in Hole U1377B (see “Sedimentology”) on the basis of the appearance of the larger clasts in hand specimen. Both types differ in appearance from the trachybasalt of Unit III:

  • Type 1: highly olivine-phyric basalt that is brown-orange, with 10% olivine phenocrysts (altered; maximum size = 5 mm, modal size = 2 mm) and 2% augite phenocrysts (maximum size = 4 mm, modal size = 2 mm). The groundmass is fine grained, with no vesicles.

  • Type 2: aphyric basalt that is brown-gray and contains no phenocrysts. The groundmass is fine grained, with 1% vesicles (low sphericity, subangular).

Lithologic and stratigraphic igneous units

Unit III (aphyric to plagioclase[±olivine]-phyric trachybasalt)
  • Interval: Sections 330-U1377B-2R-1, 0 cm, to 5R-4, 18 cm

  • Depth: 9.10–31.98 mbsf

  • Lithologic units: 1–18

The igneous succession in Hole U1377B consists of only one stratigraphic unit, which is dominantly aphyric with a 3.78 m olivine-plagioclase-phyric and plagioclase-phyric interval in the middle of the recovered section (Fig. F8). As with Hole U1377A, the rocks in Hole U1377B also exhibit flow banding and are therefore also likely to be trachybasalt, even though we have no thin sections or chemical analyses. Postcruise petrographic and analytical studies will allow us to apply more rigorous names to the rock types.

Unit III is made up of 18 lithologic units, the first of which (lithologic Unit 1) is a 10.11 m thick aphyric trachybasalt flow with occasional banding (Fig. F9C), defined by grain size (aphanitic to fine grained) and vesicle abundance (0%–20%), overlying the remainder of the lithologic units comprising a sequence of trachybasalt pillows. Toward the base of lithologic Unit 1 is a 6 cm thick interval of moderately olivine-phyric trachybasalt (interval 330-U1377B-4R-1, 12–18 cm), below which rocks become more veined and finally brecciated. The boundary between lithologic Units 1 and 2 was not recovered. Lithologic Unit 2 is defined by the appearance of olivine and plagioclase phenocrysts, with the plagioclase sometimes occurring in glomerocrysts with augite. The base of lithologic Unit 2 is defined by a curved glassy margin. Lithologic Units 3, 4, and 5 are thin intervals (0.12, 0.07, and 0.12 m thick, respectively) of plagioclase-phyric trachybasalt, with small amounts of augite phenocrysts and also olivine microphenocrysts. Lithologic Units 3 and 4 are separated by curved glassy margins (Fig. F10). Lithologic Units 4 and 5 have curved lower glassy margins, but contacts with their respective units below were not recovered. Lithologic Unit 6 is 1.18 m thick and composed of vesicular (10%), sparsely plagioclase-phyric trachybasalt. The unit becomes increasingly veined toward its base, and the point where it breaks up into breccia defines the top of lithologic Unit 7, which has the same lithology as the overlying unit. Lithologic Unit 8 is a complicated interval with sparsely plagioclase-phyric trachybasalt clasts in an aphyric matrix that appears to have intruded into the breccia and is glassy in its upper part (Fig. F11). The 1.39 m thick lithologic Unit 9 has a curved glassy base, although its contact with lithologic Unit 10, a breccia composed of aphyric trachybasalt clasts, was not recovered. The boundary between lithologic Units 10 and 11 was also not recovered. Lithologic Units 11, 12, 13, and 14 are composed of aphyric trachybasalt, and all are <1 m thick and separated from each other by curved glassy margins. The lower boundary of lithologic Unit 14 was not recovered. Lithologic Unit 15 is an interval of brecciated aphyric trachybasalt, whereas lithologic Unit 16 is more massive aphyric trachybasalt. Lithologic Unit 17 is 51 cm thick and composed of aphyric trachybasalt with bands of vesicles. Its upper contact consists of a narrow glassy body that appears to protrude into lithologic Unit 16 (Fig. F12). The boundary between lithologic Units 17 and 18 is another curved glassy margin across which the lithology does not change. Drilling ended at 37.0 mbsf, 1.44 m into lithologic Unit 18.

Interpretation of the igneous succession at Site U1377

The igneous successions cored in Holes U1377A and U1377B are broadly similar in lithology. Both consist largely of aphyric trachybasalt with occasional olivine-rich bands, and Hole U1377B contains intervals with plagioclase-augite glomerocrysts. The presence of flow banding in all of the Hole U1377A succession and in the upper part of Hole U1377B suggests that these parts of the successions formed as massive lava flows or smaller lobate flows. There is no evidence in these flows that allows us to determine whether they were erupted in a submarine or subaerial environment. The lower part of the succession in Hole U1377B, however, consists of much smaller (7 cm to 2.08 m) individual cooling units with well-preserved curved glassy margins (Fig. F10), suggesting that they are small lobate flows or lava pillows emplaced in a submarine environment. A curious feature of these margins is that, in several instances, glass between adjacent pillows connects with the more massive interior of a pillow below (Figs. F11, F12). It appears that, while in the still molten interior of a pillow, lava broke out as a protrusion that filled the space between overlying pillows. Alternatively, magma may have been injected into a stack of pillows; however, the similarity in appearance between the injected and pillow trachybasalt suggests that both were part of the same eruptive event. The presence of glassy pillow margins that are distinct from the glass in the protrusion (e.g., Fig. F10) shows that the pillows must have had glassy crusts when lava from below protruded into the space between them. In one case, fragments of the pillows themselves are incorporated into the protrusion (Fig. F11). The observation of three examples of magma apparently intruding upward into the spaces between overlying pillows suggests that this was a common phenomenon at Site U1377.

Time constraints limited the shipboard petrographic and geochemical investigation of the lithologic units at Site U1377, so we have to infer the rock composition from visual inspection of the core. However, it seems likely that the magma represented by the rocks recovered at this site was generally alkalic and intermediate in composition. If postexpedition petrographic and analytical studies confirm this, then the rocks recovered at Site U1377 will have the most evolved composition of all the rocks drilled during Expedition 330.