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doi:10.2204/iodp.proc.330.104.2012 Igneous petrology and volcanologyA total of 37.9 m of igneous rocks was penetrated in Hole U1373A. These rocks include 6.1 m of autobrecciated lava flows, which make up Unit II of the sedimentary cover, and 31.8 m of igneous basement from the base of the sedimentary succession at 33.9 mbsf to the bottom of the hole at 65.7 mbsf. Sedimentary stratigraphic Units I and III are conglomerate and breccia composed largely of pebble- to boulder-size basaltic clasts in a sandy matrix (see “Sedimentology”). The basaltic clast types are described below. Stratigraphic Unit II is made up of highly brecciated basalt with some massive intervals and was divided into five lithologic units. These units were defined on the basis of clast type and phenocryst abundance and range in thickness from 43 cm to 2.75 m (Fig. F11). The recovered igneous basement section (below 33.91 mbsf) was divided into 10 additional lithologic units, giving a total of 15 igneous lithologic units (numbered 1–15) ranging in thickness from 24 cm to 22.17 m. The 10 basement lithologic units are grouped into four stratigraphic units (Units IV–VII) ranging in thickness from 1.48 to 22.86 m (Fig. F11). 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, 2012a). A summary of the lithologic units and their ISCIs is given in Table T5. Basaltic clasts in sedimentary Units I and IIIThe larger basaltic clasts within the conglomerate and breccia were divided into 12 types on the basis of their appearance in hand specimen. Each clast type is described below, and the sedimentologic subunit(s) in which each clast type was found is also noted (see “Sedimentology”):
Although many of the clast types were found throughout most of the breccia and conglomerate sedimentary sequence in Units I and III, no individual clast type was found to occur in all of the subunits. The most common were the Type 2 aphyric basalt clasts and the distinctive porphyritic Type 7 highly olivine-pyroxene-plagioclase-phyric basalt clasts. Although similar in appearance to the basalt that makes up the two Unit IV flows at the top of the igneous basement, clast Type 7 differs slightly in that it contains plagioclase phenocrysts. A thin section photomicrograph taken from a particularly unaltered example of a Type 7 clast from Subunit IB is shown in Figure F12. One olivine crystal in this thin section shows well-developed strain lamellae (Fig. F13), suggesting that this crystal was entrained from a deformed cumulate pile beneath the volcano. Most of the clasts are rounded to subangular and have clearly been transported and abraded. Some of the Type 2 aphyric basalt clasts in Subunit IB, however, have lobate, fluidal, or amoeboid outlines, often with delicate protrusions (Fig. F14), and therefore could not have traveled far from their in situ emplacement position. Some of the Type 2 clasts closely resemble the fluidal clasts typical of some peperites (Skilling et al., 2002), and the significance of this will be discussed later. Lithologic and stratigraphic igneous unitsUnit II
Unit II is composed mostly of clast-supported basaltic breccia with some intervals of massive basalt. The breccia comprises angular clasts (typically ~50 mm in size), interstitial basaltic sand, and carbonate cement. The clasts frequently fit together as jigsaw-fit breccia (Fig. F15), showing that the breccia formed more or less in situ. Two intervals of grain-supported polylithic breccia are interbedded with the volcanic breccia units (see “Sedimentology”). These intervals have more rounded and weathered clasts and are interpreted as sedimentary units. The basalt forming the breccia varies from aphyric to moderately olivine- and olivine-augite-phyric, but phenocryst type and abundance are fairly constant over discrete intervals. Phenocryst type and abundance were used as the principal criteria for dividing Unit II into five igneous lithologic units, although lithologic Unit 2 and the upper 17 cm of lithologic Unit 4 are polylithic and have therefore been included with the sedimentary rocks (Fig. F11; Table T5; see also “Sedimentology”). Unit IV
Unit IV is 2.75 m thick and comprises two highly olivine-augite-phyric basalt flows that contain 7% olivine and 4% augite phenocrysts within nonvesicular groundmass (Fig. F16). The two flows are separated by a 10 cm thick interval of volcanic breccia that may separate two lithologically identical lobes of the same eruptive unit. The top of Unit IV is in contact with the sedimentary cover (see “Sedimentology”) at Section 330-U1373A-7R-1, 121 cm, but the contact between Unit IV and underlying Unit V was not recovered. This lower boundary was inferred from the disappearance of the olivine and augite phenocrysts between Sections 330-U1373A-7R-4 and 7R-5. Core recovery in Unit IV was almost complete. Unit V
Unit V is a 1.48 m package of thin aphyric basalt lava lobes. The upper lithologic Unit 8 is 24 cm thick and vesicular and contains 0.5% olivine phenocrysts. It has a thin (~1 cm) sediment layer at its top and is separated from the underlying unit by another thin (~1 cm) layer of brown sediment. Lithologic Unit 9 is 63 cm thick and vesicular and contains 0.5% olivine phenocrysts. It has a 2 cm thick sediment interval at its base. Lithologic Unit 10, at the base of Unit V, is peperitic and vesicular and contains 1% olivine phenocrysts. The lower boundary of Unit V is defined by a thin red oxidized layer, which separates these aphyric flows from the sparsely phyric flows below. The presence of the oxidized flow top implies subaerial eruption. Core recovery across Unit V was excellent (>100%). Unit VI
Unit VI is 5.17 m thick and made up of three sparsely olivine-phyric basalt lava flows separated by oxidized layers. Each lithologic unit includes peperitic intervals. The upper flow (lithologic Unit 11) is peperitic throughout its 91 cm thickness and has a red oxidized top. The larger pods of green coarse sandstone show squeezing features and compaction swales. The basalt is highly vesicular and contains 1% olivine and 0.5% plagioclase phenocrysts. The middle flow (lithologic Unit 12) is 1.97 m thick and is also peperitic with a vesicular core. Lithologic Unit 13 is a 2.29 m thick peperitic lava flow that has 0.5% plagioclase and 0.5% pyroxene microphenocrysts that occur as glomerocrysts. It also contains sparse olivine microphenocrysts, which distinguishes it from the aphyric flow below and justifies its inclusion in Unit VI. This unit has a vesicular core and a scoriaceous and peperitic top and base. Its bottom contact, and thus the bottom of Unit VI, was not recovered but was inferred from the disappearance of olivine microphenocrysts and a return to an aphyric texture. Unit VI has excellent (96% average) core recovery. Unit VII
Unit VII is 22.4 m thick and consists of two aphyric basalt lava flows, both of which are peperitic. Lithologic Unit 14 is a thin (67 cm recovered) unit and is peperitic in its uppermost 32 cm, whereas the lower part of the flow is vesicular and has a distinct horizontal foliation. Lithologic Unit 15 is at least 21.7 m thick. Drilling stopped within this flow, so its overall thickness is unknown. The upper 22 cm of the flow is peperitic and composed of approximately equal amounts of green coarse sand mingled with vesicular aphyric basalt (Fig. F17). This peperitic interval grades into a 113 cm thick zone with aligned vesicles, which in turn grades into massive basalt that extends through the rest of Unit VII. Across interval 330-U1373A-13R-1, 28–38 cm, there is a moderately olivine-pyroxene-plagioclase-phyric patch with 1.5–2 mm phenocrysts. Core recovery in this unit was very good (87% average). Interpretation of the igneous successionThe igneous rocks encountered in Hole U1373A are interpreted in chronological order (oldest first, from the bottom of the hole). The inflated aphyric sheet flow of lithologic Unit 15, which makes up most of Unit VII, is the lowest unit in the drilled succession. The peperitic top of this flow (Fig. F17) suggests that it initially flowed into and mingled with wet sediment. A continued supply of magma resulted in periodic inflation of the flow, evidence for which is provided by vesicular patches and trails (including pipe vesicles) at several intervals throughout the unit. The presence of peperite in the top half of the overlying, much smaller flow might imply continued sedimentation after emplacement of the larger lava flow. Alternatively, the upper flow might simply be a smaller lobe formed during the same eruption. The subsequent flows that form Unit VI and the lower part of Unit V are also peperitic, in some cases almost entirely so, suggesting that sedimentation persisted for a while at Site U1373. Following the last occurrence of peperite in the lower part of Unit V, the lava flows become more massive and have oxidized tops, suggesting that the upper part of Unit V and all of Unit IV were likely erupted and emplaced under fully subaerial conditions. A sedimentary succession of breccia and conglomerate (Unit III) follows the volcanic rocks of Units VII–IV. The breccia is interpreted as a lahar deposit and the conglomerate as fluvial or intertidal deposits (see “Sedimentology”). This coarse clastic sedimentation was interrupted by the emplacement of autobrecciated lava flows forming Unit II. These flows are almost entirely brecciated, but the fragments appear to fit together in places and give the breccia a jigsaw-fit texture (Fig. F15). This is a common feature of blocky peperites and is widely inferred to reflect in situ quench fragmentation (e.g., Kokelaar, 1982). Busby-Spera and White (1987) suggest that the development of peperite with blocky clasts is favored by emplacement of magma into sediment with coarse grain size, high permeability, and poor sorting, all of which are features of the Unit III conglomerate. By contrast, fluidal, globular, and lobate peperite textures imply ductile fragmentation in situations where magma and sediment are separated by thin films of superheated water vapor (Skilling et al., 2002). This style of peperite formation is favored when magma is emplaced into fine-grained, well-sorted sediment with low porosity, such as micrite (Busby-Spera and White, 1987). The peperites at the top of Unit VII (Fig. F17) and in the uppermost lava flows in Hole U1372A (see Fig. F20 in the “Site U1372” chapter [Expedition 330 Scientists, 2012b]) are of this type. Uppermost stratigraphic Unit I is again composed of conglomerate with basalt clasts, some of which have lobate margins with delicate protrusions and therefore cannot have been transported far from their source. They may indicate syndepositional, peperitic interaction of lava and sediment, implying a late phase of volcanism contemporaneous with the formation of Unit I. We conclude that the volcanological features of the igneous rocks in Hole U1373A suggest lava flowing into an area where water and water-saturated sediments were present but which was not fully submarine. Lava flows forming the upper part of Unit V and all of Unit IV have oxidized tops and show no evidence for water-lava or water–wet sediment interaction. Emplacement of lava flows in an intertidal or fluvial environment provides a plausible scenario that is entirely consistent with sedimentologic observations (see “Sedimentology”). The absence of thick volcaniclastic deposits at Site U1373 (in contrast to Site U1372) supports the scenario that the site of lava effusion, if not emplacement, was mostly subaerial throughout the time interval represented by the Site U1373 cores. The thick, lowermost Unit VII is aphyric, whereas the two overlying units are aphyric or sparsely olivine-phyric. In contrast, Unit IV is a distinctive highly olivine-augite-phyric basalt. Groundmass augite and the rims of augite phenocrysts in Unit IV, and in the highly olivine-augite phyric clasts in the overlying conglomerate, have the distinctive purple color of titanaugite. The presence of titanaugite and the olivine-titanaugite phenocryst assemblage are characteristic of alkalic basalts. |
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