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doi:10.2204/iodp.proc.330.103.2012 Igneous petrology and volcanologyHole U1372A was drilled to 232.9 mbsf. Igneous basement was encountered at 45.67 mbsf, giving a 187.3 m basement section. Igneous rocks were also found as clasts in the breccia and conglomerate of stratigraphic Unit II, which overlies the igneous basement rocks (see “Sedimentology”). The igneous basement was divided into 81 lithologic units, numbered 1–81 and ranging in thickness from a few centimeters to 16.7 m. To help achieve the paleomagnetic objectives of this expedition, we determined the in situ confidence index (ISCI) for each lava cooling unit by following the procedures described in “Igneous petrology and volcanology” in the “Methods” chapter (Expedition 330 Scientists, 2012). A summary of the lithologic units and their ISCIs is given in Table T6. Lithologic units were grouped into 15 stratigraphic units that range in thickness from 2.03 to 33.46 m and are numbered III–XVII (Fig. F18). In this section we describe briefly the types of igneous clasts found in stratigraphic Unit II (sedimentary breccia and conglomerate). We then give a description of each of the igneous stratigraphic units (III–XVII) and provide an interpretation of the entire igneous succession. Basaltic clasts in sedimentary Unit IIStratigraphic Unit II comprises breccia and conglomerate composed largely of basaltic clasts ranging from gravel to boulder size (see “Sedimentology”). Seven types of basaltic clasts were defined on the basis of petrography. The defining characteristics of these seven clast types are summarized below:
Lithologic and stratigraphic igneous unitsUnit III
The base of Unit II is preserved at Section 330-U1372A-8R-3, 63 cm (Fig. F19), where the contact seems to show mingling between the first igneous body (Unit III; lithologic Unit 1) and overlying sediment, although the evidence for a peperitic texture is somewhat equivocal here. Unit III consists of five cooling units of aphyric to sparsely olivine-phyric basalt with thicknesses ranging from 1 to 3.9 m. These cooling units are probably flow lobes from the same eruption. The rocks have peperitic textures in which basalt is mingled with sediment on a centimeter scale (Fig. F20A). The basalt component frequently has fluidal or lobate contacts with the sediment (Fig. F20A). Mingled intervals alternate with intervals of massive basalt. The composition of the sediment incorporated into the peperites ranges between an azoic micritic limestone and a medium-grained basalt sandstone with volcanic glass fragments. No bioclasts were encountered in the sediment, and it cannot be matched to any of the lithologies encountered in Units I or II (see “Sedimentology”). Unit IV
Unit IV is a single flow, 9.7 m thick, distinguished from adjacent units by a higher abundance of olivine phenocrysts. The upper 2 m section of the flow shows extensive development of a peperitic texture through mingling with sediment (Fig. F20B). Olivine phenocrysts are completely altered in this upper part of the flow, but below Section 330-U1372A-9R-5, 43 cm, the lava flow becomes massive and the olivine phenocrysts are much less altered (Fig. F21). From Section 9R-6, 125 cm, to the base of the flow the olivine phenocrysts are again altered, and the more abundant vesicle and void spaces in this part of the flow are partially filled by calcite. Unit V
Unit V consists of four flows that have scoriaceous instead of peperitic tops and bottoms. Thin oxidized layers within the scoriaceous material define the top of some of the flows (lithologic Unit 9 at Section 330-U1372A-10R-5, 62 cm, and lithologic Unit 10 at Section 10R-7, 16 cm). The flows have more massive interiors and are composed of aphyric to moderately olivine-phyric basalt. The lack of peperite, together with the lower olivine abundance, distinguishes these flows from those of Unit IV. Unit VI
Unit VI consists of 10 lithologic units comprising 8 lava flow and 2 volcanic breccia units. These units can be distinguished from the olivine-phyric unit above by their generally aphyric nature. The boundary is defined by a chilled margin at the base of Unit V and the presence of scoriaceous material at the top of Unit VI. The first flow becomes massive from Sections 330-U1372A-11R-3, 63 cm, to 11R-7, 9 cm, with an olivine-rich (interval 11R-7, 0–9 cm) and then scoriaceous (interval 11R-7, 9–19 cm) base. This scoriaceous material includes a 2 cm thick layer of recrystallized limestone at interval 11R-7, 17–19 cm (Fig. F22), implying intermittent deposition of sediment on the flow tops at this stage. From lithologic Units 11–16, many of the flows have scoriaceous tops and more massive aphyric basalt interiors. Lithologic Unit 17 is a 19 cm thick breccia with a completely altered glassy matrix and is the highest stratigraphic level from which hyaloclastite was recovered. This approximately coincides with a significant decrease in core recovery (Fig. F18). Below this breccia is a thin vesicular aphyric basalt lava flow, followed by a thicker unit of breccia with an altered hyaloclastite matrix and aphyric basalt clasts (Fig. F23) in interval 330-U1372A-13R-3, 0–44 cm (lithologic Unit 19). The lithologic unit below is another moderately vesicular to massive aphyric basalt lava flow that marks the base of Unit VI. Unit VII
The Unit VI/VII boundary was not recovered. The uppermost lithologic Unit 21 is distinguished from the overlying unit by an increase in the proportion of olivine phenocrysts. A second highly olivine-phyric lava flow occurs at the bottom of Unit VII (lithologic Unit 23). Between these two lava flows is a 36 cm thick interval of volcanic breccia (lithologic Unit 22) consisting of sparsely olivine-phyric basalt clasts (up to 50 mm in size) in an altered glassy matrix. Unit VIII
The base of Unit VII was not recovered, and so Unit VIII was defined by another reduction in the abundance of olivine phenocrysts. This unit begins at Section 330-U1372A-14R-3, 15 cm, with a 13 cm thick unit of aphyric basalt breccia on top of a 16.65 m thick aphyric basalt lava flow. If the core recovered over this interval represents a single lava flow, this would be the thickest flow recorded at Site U1372. However, recovery is low over this interval, so Unit VIII could comprise more than one flow. The unit has a vesicular top (10% vesicles in the interval 330-U1372A-14R-3, 15–74 cm), and from Section 15R-1, 85 cm, to its base it becomes massive with occasional vesicular patches. Unit IX
The top of Unit IX was not recovered but is distinguished from the flow above because many of the larger fragments have curved surfaces and some have what may be radial vesicle trains (Fig. F24), suggesting that they may be fragments of lava pillows. If so, this is the first appearance of pillow lava in Hole U1372A. Some of the fragments have attached pieces of brecciated matrix containing altered glass. Clasts within this brecciated matrix are as large as 20 mm, whereas the larger pillow fragments are as large as 160 mm. Unit X
Unit X could be a 7.43 m thick lava flow or a sequence of pillow fragments, but recovery is too low to be certain. This interval was recorded as a distinct lithologic unit because there is no evidence of volcanic breccia between or on the surfaces of recovered rock pieces in the core, even though it is composed of aphyric basalt similar to the units above and below. Unit X could be interpreted as a sheet flow. Neither the upper nor the lower contact of the unit was recovered. In intervals 330-U1372A-16R-3, 21–27.5 cm, and 16R-4, 0–9 cm, there are more vesicular areas, with 10%–15% vesicles compared to an overall <2% vesicularity for the rest of the unit. Unit XI
Unit XI is a 2.88 m thick, poorly sorted unit of volcaniclastic breccia with cobble-size aphyric basalt clasts. The large number of individual lithologic units in this stratigraphic unit reflects its heterogeneity. The breccia consists of 40% aphyric basalt gravel in a dark green matrix of altered volcanic glass. The basalt fragments are commonly 0.5–20 mm in size, moderate to low in sphericity, and angular. Larger (40–500 mm) aphyric basalt clasts often have alteration rims around their edges. They are sparsely microporphyritic, with microphenocrysts of plagioclase and pyroxene in varying abundances throughout the dominantly aphyric unit. The base of this unit becomes moderately plagioclase-phyric in places (Samples 330-U1372A-17R-2W, 4–10 cm [Thin Section 45]; 17R-2W, 46–48 cm [Thin Section 46]; and 17R-2W, 135–136 cm [Thin Section 47]). Recovery of this unit was very good (82%). However, the upper and lower contacts of Unit XI were not recovered but are inferred from a change in lithology. Contacts between the individual lithologic units were rarely recovered because the larger fragments of basalt tend to become detached from their matrix. Those that were recovered are usually either sharp (broken pillows; Fig. F25A) or separated from the matrix by a zone of spalled pillow fragments (pillow margins; Fig. F25B). Unit XII
Unit XII comprises lithologic Unit 46, a 6.46 m thick unit of volcaniclastic breccia, and lithologic Unit 47, a 3.28 m thick interval of vitric-lithic volcanic sand. The breccia contains 60% basalt gravel in a matrix of hyaloclasts. The basalt gravel particles are 4 mm in size, low in sphericity, and subangular. There is a distinct lack of cobble-size clasts. This unit is moderately well sorted, although the upper 9 cm shows a coarsening-upward trend and becomes poorly sorted at its top. This unit is conspicuously more phenocryst-rich than the other breccia units. It is moderately to highly plagioclase-augite phyric, although it appears aphyric in parts. Augite is commonly present as microphenocrysts (see Fig. F26, which shows a glomerocryst composed of augite microphenocrysts and plagioclase phenocrysts), and plagioclase phenocrysts frequently contain melt inclusions (Fig. F27). Unit XII marks the stratigraphically highest occurrence of abundant unaltered glass in the hyaloclastite matrix in lithologic Unit 46 (Fig. F28). The interval of vitric-lithic volcanic sandstone (lithologic Unit 47) is a grain-supported sandstone of volcanic glass, a high proportion of which is unaltered, with rare gravel-size breccia layers. This moderately well sorted hyaloclastite unit contains glass shards that are 1 mm in size, low in sphericity, and very angular. Although this unit appears aphyric at the macroscopic scale, Thin Sections 56 and 57 (Samples 330-U1372A-19R-1W, 32–38 cm, and 19R-3W, 44–47 cm, respectively) show that it varies from aphyric to moderately plagioclase-phyric. The upper and lower contacts of Unit XII were not recovered. These boundaries were inferred from a change in lithology. The boundary between lithologic Units 46 and 47 is gradational, showing a general coarsening-upward sequence of volcanic sandstone in Unit 47 to volcanic gravel-size breccia in Unit 46. Across Unit XII the core recovery was high (82%). Unit XIII
Unit XIII is a 3.38 m thick, poorly sorted unit of volcaniclastic basalt breccia with cobble-size aphyric basalt clasts. The grain size is bimodal, with a groundmass of glassy shards enclosing angular basaltic fragments as large as 70 mm. This glass-rich breccia (lithologic Units 48, 50, and 52) is interbedded with lithologic units (as thick as 2.21 m) of aphyric basalt blocks. The upper and lower contacts of Unit XIII were not recovered but were inferred from a change in lithology, particularly the lack of phenocrysts in this unit compared to that above and the presence of glomerocrysts in the unit below. As in Unit XI, the contacts between the alternating lithologic units of breccia and basalt are rarely preserved, even though core recovery across this unit was good (65%). Unit XIV
Unit XIV is a 27.6 m thick sequence of poorly sorted aphyric and moderately plagioclase-phyric basalt breccia and cobble-size clasts. The breccia comprises 80% aphyric to moderately plagioclase-phyric basalt gravel in a dark green altered hyaloclastite matrix. The basalt gravel is 2–10 mm in grain size, moderate to low in sphericity, and angular, and the aphyric to moderately plagioclase-phyric basalt clasts are 30–50 mm in size. Within the breccia are several units, up to 1.7 m thick, of aphyric to moderately plagioclase-phyric basalt. Unit XIV is distinguished from the other volcaniclastic units through the presence of abundant small (typically 2 mm) glomerocrysts of plagioclase and augite microphenocrysts (with rare olivine). The augite microphenocrysts sometimes show the sector zoning characteristic of titanaugite (Fig. F29). The base of this unit is marked by a 16 cm thick layer of vitric-lithic sand. The upper and lower contacts of Unit XIV were not recovered but were inferred from a change in lithology, particularly the lack of distinctive glomerocrysts. As with Units XI and XII, the contacts between the alternating lithologic units of breccia and basalt were rarely preserved. Core recovery in this unit was poor (29%). Unit XV
Unit XV is a 21.5 m thick, poorly sorted assemblage of volcaniclastic breccia with cobble-size clasts of aphyric basalt and larger units of aphyric basalt that may be fragments of pillow lavas or lava lobes. The breccia consists of 80% aphyric to moderately plagioclase-phyric basalt gravel in a moderately altered matrix of hyaloclastite material. The basalt gravel grain size is 5–10 mm, with angular fragments of moderate to low sphericity. The aphyric to moderately plagioclase-phyric basalt clasts are 30–60 mm in size. This breccia separates units of aphyric basalt as thick as 40 cm that may be lava pillows or lobes or fragments thereof. Lithologic Unit 68 is aphyric basalt with large feldspar laths in the groundmass. It is bounded by a glassy margin at its upper and lower contacts, a feature not seen in the other basalt units. Way-up structures, such as upwardly pointing pipe vesicles and diapirs of glass rising from the upper contact into the surrounding breccia, also occur in this lithologic unit. The upper and lower contacts of Unit XV were not recovered but were inferred from a change in lithology, particularly the lack of the distinctive glomerocrysts seen in Unit XIV and the olivine phenocrysts seen in Unit XVI. The contacts between individual breccia and basalt lithologic units were only recovered in lithologic Unit 68, as described above. Good core recovery (76%) in this unit can be attributed to cementation of the breccias by a pale blue clay mineral (see “Alteration petrology”). Unit XVI
Unit XVI is 33.46 m thick and consists of poorly sorted volcaniclastic breccia with cobble-size clasts of olivine-phyric basalt and units of moderately olivine-phyric basalt. The breccia consists of 80% hyaloclastite matrix with angular clasts of olivine-phyric basalt as large as 40 mm. The basalt sand particles are 1 mm in size and are angular with low sphericity. This breccia separates units of moderately olivine-phyric basalt that are of indeterminate thickness because of poor core recovery (38% across the whole unit). Thin sections of these basalt units show that they also have a significant plagioclase phenocryst content and are moderately olivine-plagioclase-phyric. A notable feature of the clasts and lava in this unit is the near absence of vesicles. The upper and lower contacts of Unit XVI were not recovered. The boundaries were inferred from a change in lithology, with this unit being defined by its abundant olivine phenocrysts, in contrast to all other volcaniclastic units. Contacts between the lithologic units of breccia and basalt were not recovered. Unit XVII
Unit XVII is a 4.25 m thick unit of moderately olivine-phyric basalt. The rocks also contain abundant microphenocrysts of plagioclase and augite (Fig. F30). The augite microphenocrysts distinguish this unit from the basaltic units in Unit XVI. The olivine phenocrysts are mostly unaltered and among the least altered recovered in Hole U1372A. The near absence of vesicles in Unit XVI, noted above, is also a feature of the Unit XVII lava flow. The upper contact of Unit XVII was not recovered, but the boundary was inferred from the increase in the abundance of olivine phenocrysts and the unit’s higher plagioclase and augite content. The lower boundary was not recovered because drilling ceased within this unit. Core recovery across this unit was very high (107%). Interpretation of the igneous successionThe igneous rocks encountered during drilling of Hole U1372A will be interpreted in chronological order (oldest first, from the bottom of the hole). The rocks reflect basaltic eruptions in a wide range of environments. The earliest part of the recovered succession is a single olivine-augite-plagioclase-phyric lava flow (Unit XVII). This lava flow is overlain by thick packages of volcaniclastic rocks (Units XVI–XI and IX), interpreted as hyaloclastite-dominated volcaniclastic breccia most likely erupted in a marine environment during the constructional phase of the seamount. Vesicles are virtually absent from the lowest lava flow and from the immediately overlying volcaniclastic Unit XVI, which could imply (1) eruption in deep water, which inhibited degassing; (2) open degassing, resulting in completely degassed melts; or (3) low original volatile content in the magma. Shore-based studies may be able to resolve this issue. The lava flow and clasts in the overlying volcaniclastic unit are olivine-phyric, unlike clasts in the later volcaniclastic rocks, which are essentially olivine-free and also moderately vesicular. Recovery of the volcaniclastic rocks was highly variable and inversely proportional to the amount of massive basalt within them. Units XV and XII had almost 100% recovery because these units are composed of well-cemented hyaloclastite breccia with few large basalt clasts (Fig. F18). Recovery was poor where there were large fragments of basalt, usually with adhering or separate fragments of hyaloclastite breccia. No unequivocal pillows were recovered, but several intervals of basalt included fragments with curved surfaces, glassy selvages, and a hint of radial vesicle trains (Fig. F24). Poorly recovered intervals of massive basalt that lack hyaloclastite breccia fragments (Units X and VIII) likely represent sheet flows. The appearance of unequivocal lava flows (Units VII–V) and the last appearance of hyaloclastite (near the base of Unit VI) in the upper part of Hole U1372A mark a transition from submarine to subaerial volcanism (Fig. F18). This transition is also marked by a change in alteration minerals from green clays to red iron oxyhydroxides (see “Alteration petrology” and “Physical properties”). These subaerial lava flows provide only a short record of the emergence of an island at Site U1372 before it subsided beneath the ocean again. The igneous succession is overlain by conglomerates deposited in a shallow-marine environment (see “Sedimentology” and “Paleontology”). In addition, the presence of peperite in igneous Units IV and III implies that the lava flows were emplaced into soft sand and carbonate mud with which they mingled. Critically, the presence of peperite throughout Unit III and the upper part of Unit IV shows that the sediment was in a fluid or semifluid state at the time of eruption, implying contemporaneous sedimentation and volcanism. Core recovery was excellent through the uppermost 12 lithologic units (Units III–V and the upper part of Unit VI; Fig. F18), so a virtually complete section through the peperitic flows was recovered. Peperite most commonly forms when magma is intruded into wet sediment, but it can also form when lava flows into or over wet sediment (Skilling et al., 2002; Waichel et al., 2007). The sediment within the peperites does not appear to match that in the overlying sedimentary Unit II. This implies that the lava flows forming Units III and IV flowed over pockets of sand and carbonate mud accumulating on the tops of earlier flows before sedimentation changed to a coarse-clastic regime in a marine environment soon after the eruption of the Unit III lava flows (see “Sedimentology” and “Paleontology”). Thus, we see in Hole U1372A a magmatic record of the evolution of Canopus Guyot from its submarine constructional phase, through its emergence as an island, and terminating with its subsidence back into the ocean. Petrographically, the igneous rocks recovered in Hole U1372A are mildly alkalic to transitional in character, and this is confirmed by chemical analysis (see “Geochemistry”). Pyroxene phenocrysts (Unit XVII), or more commonly microphenocrysts, are always titaniferous (Fig. F29). The common phenocryst assemblages are olivine, olivine + plagioclase + augite (Fig. F30), and plagioclase + augite (Fig. F26). Olivine-augite-phyric basalt (ankaramite), which is characteristic of moderately to strongly alkaline suites, was not seen in Hole U1372A. Olivine is present in the groundmass in several of the upper series of lava flows but not in those from the lower part of the succession, implying an increase in alkalinity with time. Chemical analysis shows that basalt samples from the lowest units (Units XVI and XVII) plot slightly below the line dividing the alkalic and tholeiitic series of Hawaii (see “Geochemistry”), yet the presence of titanaugite shows that these rocks are not truly tholeiitic. |
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