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doi:10.2204/iodp.proc.335.103.2012

Igneous petrology

Hole 1256D was deepened from 1507.1 to 1521.6 mbsf during Expedition 335. The first core was Core 335-1256D-235R, which continues the numerical sequence from Expedition 312 (Core 312-1256D-234R). Lithologic units (Table T4; Fig. F6) were defined based on criteria observable in hand specimen, including mineralogy, texture, and grain size (see “Igneous petrology” in the “Methods” chapter [Expedition 335 Scientists, 2012b]). The principal objective of the unit designations was to ensure that individual cooling and/or intrusive units are recognized and retrievable from the database. One new unit was identified during Expedition 335 (Unit 1256D-96). In addition, rocks drilled during Expedition 312 were redescribed.

The material recovered from the junk baskets during many of the cleaning and fishing runs (Runs 11–15 and 17–22), in addition to sand and gravel, contained cuttings, gravel, and cobbles (up to 20 cm × 10 cm × 11 cm, or 4.5 kg) (Tables T5, T6). For each run, material larger than ~1 cm was grouped by lithology and texture and subsequently weighed. Rocks of special interest were given a letter code (e.g., Sample 335-1256D-Run13-RCJB-Rock A; see Table T1 in the “Methods” chapter [Expedition 335 Scientists, 2012b]) and formally described using the procedures for core description.

Redescription of plutonic section recovered during Expedition 312

During Expedition 335, the plutonic section recovered during Expedition 312 was redescribed; the updated igneous stratigraphy is listed in Table T4 and illustrated in Figure F6. The new descriptions followed the original unit and subunit structure (Teagle, Alt, Umino, Miyashita, Banerjee, Wilson, and the Expedition 309/312 Scientists, 2006), and the Expedition 312 and 335 unit boundaries agreed, with two exceptions. The first exception is the definition of a new subunit (Unit 1256D-88B), which comprises a crosscutting coarse-grained oxide gabbro. The second change is in the lower part of Unit 1256D-88 and in Unit 1256D-89 (as defined during Expedition 312), where the Expedition 335 Igneous Petrology team identified significant olivine. As a result, the Unit 1256D-88/89 boundary was moved from 1450.80 mbsf (Section 335-1256D-223R-2, 0 cm) to 1439.66 mbsf (Section 335-1256D-221R-1, 6 cm), where the first olivine gabbro (olivine ≥ 5%) occurs, to create a single unit of olivine gabbro (Unit 1256D-89). Within this unit, two series of olivine gabbro were recognized (Unit 1256D-89A, 11.8 m thick, and Unit 1256D-89C, 2.8 m thick) that range from olivine poor at the top to olivine rich at the base (Figs. F6, F7). The two subunits are separated by a sharp modal contact, where the olivine mode drops abruptly from ~20% to 2% (Fig. F8). Below the olivine-rich (~20%) base of the second olivine gabbro series (Unit 1256D-89C), a single piece of relatively olivine poor (~4%) gabbro was recovered (Section 312-1256D-224R-1). Because of the significant drop in olivine content, this was designated as a separate subunit (1256D-89D). Unit 1256D-89B, which comprises two ~15 cm intervals of intrusive oxide gabbro and oxide quartz diorite at 1451.2 and 1451.6 mbsf, was retained from Expedition 312.

The modal mineralogy and textures of the Expedition 312 plutonic rocks were also redescribed during Expedition 335. The only significant difference in modal mineralogy is the recognition during Expedition 335 of large amounts of olivine, leading to the definition of olivine gabbro Units 1256D-89A and 89C, as described above. In addition, there are slight differences in oxide and orthopyroxene modes, with estimates made for these phases during Expedition 335 generally higher than those made during Expedition 312. Gabbro 2 is principally made up of two units (1256D-92A and 1256D-93), which were identified as orthopyroxene-bearing gabbro during Expedition 312. However, the orthopyroxene estimates made during Expedition 335 exceeded 5% (Fig. F7), and these units, and Gabbro 2 in general, have been renamed gabbronorite. The modal estimates made during Expedition 335 are in good agreement with the thin section modes described during Expedition 312, particularly with respect to the higher olivine (Unit 1256D-89) and orthopyroxene (Gabbro 2; Units 1256D-92A and 1256D-93) contents.

Texturally, many of the gabbros were described during Expedition 312 as being poikilitic. Although some oikocrysts were recognized by the Expedition 335 Igneous Petrology team, the gabbros were deemed to be predominantly subophitic.

Cores recovered during Expedition 335

The four cores recovered during Expedition 335 (Cores 335-1256D-235R, 236R, 238R, and 239R) comprise predominantly fine grained aphyric basalt with granoblastic textures. The degree of recrystallization is variable, ranging from strong (in which plagioclase generally retains lath shapes) to complete. This textural variation is the basis for dividing Unit 1256D-96 into Units 96A (strongly granoblastic) and 96B (completely granoblastic) (Table T4).

Unit 1256D-96A

In Unit 1256D-96A, plagioclase forms an isotropic framework of lath-shaped crystals (as long as 1 mm), which is interpreted as relict intergranular texture (Fig. F9A). The interstices between the plagioclase laths are filled with equant to prismatic clinopyroxene and orthopyroxene, along with roundish oxide grains. There are numerous ~1–30 µm inclusions of Fe-Ti oxide within both types of pyroxene.

Unit 1256D-96B

The granoblastic basalts of Unit 1256D-96B are characterized by complete recrystallization of the matrix but contain relics of former lath-shaped plagioclase microphenocrysts, as long as 2 mm, that have survived the granoblastic overprint (Fig. F9B). Bands of elongated, ~50 µm sized clinopyroxene, orthopyroxene, and oxide grains in the core of these former microphenocrysts are interpreted as metamorphosed groundmass or glass inclusions originally in the central part of the microphenocrysts. Such inclusions are commonly present within (micro)phenocrysts within unmetamorphosed basalt and dolerites higher up in the hole (e.g., figs. F207 and F212 in Expedition 309/312 Scientists, 2006).

Unit 1256D-96C

Three pieces of leucocratic rock were recovered, which together define Unit 1256D-96C. The first (Section 335-1256D-235R-1 [Piece 5]) comprises oxide quartz diorite with a centimeter-wide tonalite vein (Fig. F10). The contact relationships with the granoblastic basalt were not recovered. Microscopic observations show that the contact between both leucocratic rocks is sutured. The tonalite is fine grained and granular, with clinopyroxene as the principal mafic phase, and 2% Fe-Ti oxides. A 3 mm grain of euhedral, primary magmatic amphibole associated with clinopyroxene suggests high water activities during the formation of this rock (Fig. F11). Apatite and zircon are accessory phases, forming needles as long as 1 mm. Many of the quartz grains host abundant fluid inclusions.

The oxide quartz diorite hosting the tonalite vein is fine grained and granular, with clinopyroxene and amphibole as mafic phases. Clinopyroxene shows marked twinning, contains many oxide inclusions, and ranges from a prismatic to a poikilitic habit, enclosing plagioclase. Amphiboles, although strongly altered, can still be recognized as a primary magmatic phase by their euhedral (Fig. F12) and poikilitic habits. Locally, amphibole is associated with apatite, titanite, zircon, and Fe-Ti oxides, representing typical late-stage igneous assemblages. Plagioclase is strongly zoned, and some crystals show relict cores.

The second piece (Section 335-1256D-235R-1 [Piece 7]; Fig. F10) is fine-grained, granular, oxide-rich (±10%) quartz diorite. The contact relationships with the granoblastic basalt were not recovered.

The third leucocratic rock (within Section 335-1256D-236R-1 [Piece 1]) is in sharp planar contact with cryptocrystalline granoblastic basalt. It is a strongly altered, microcrystalline, granular rock composed of dusty altered plagioclase intergrown with aggregates of fibrous to prismatic needlelike actinolite (Fig. F13). The extinction angles of the plagioclase vary between 0° and 20°, consistent with a high albite component (<40 mol% An). Given its high proportion of plagioclase (85%), this rock has been classified as albitite. Former small ilmenite grains have been converted to “leucoxene” (mixture of ilmenite, titanite, and other ilmenite replacement phases). Minor epidote is present, locally occurring as poikiloblasts enclosing granular plagioclase. Matrix plagioclase is granular, with a similar texture as the granoblastic basalt, and relict plagioclase microphenocrysts occur locally. The absence of quartz, the relict granoblastic texture, and the presence of relict microphenocrysts suggest that this rock is a strongly altered granoblastic basalt.

Unit 1256D-96D

Some of the granoblastic basalt (Section 335-1256D-239R-1 [Pieces 5, 7, and 10]) contains small (<3 cm × 1.5 cm) irregular patches of oxide diorite (Fig. F14), and these have been designated as Unit 1256D-96D. The contact between the oxide gabbro and granoblastic basalt ranges from sutured to gradational and is irregular. The oxide gabbro is medium grained and equigranular, has a granular texture, and contains ~7% Fe-Ti oxides, as well as subhedral, elongate amphibole crystals (~10%).

Rocks recovered during cleaning operations in Hole 1256D

A range of rock types was recovered during hole cleaning and fishing operation Runs 11–15 and 17–22 (Tables T5, T6). These are described below in order of abundance (by weight).

Granoblastic basalt

By far the most abundant rock type (95.3 wt%; Table T5) recovered is cryptocrystalline to fine-grained (<0.1–0.5 mm), generally aphyric basalt with a distinct granoblastic texture. Although all samples have granoblastic textures, the degree of recrystallization varies, with strongly granoblastic rocks dominant (92.9 wt%) over completely granoblastic (7.1 wt%) rocks.

Although the granoblastic basalts recovered during the cleaning operations are strongly recrystallized, some primary magmatic features are preserved. Sample 335-1256D-Run12-RCJB-Rock S has a sharp, planar, apparently igneous contact between a light-colored, cryptocrystalline, completely granular, plagioclase phyric domain and a darker, fine-grained, strongly granular and aphyric domain (Fig. F15). This is interpreted as a dike/dike contact. Although the fine-grained basalt does not show any significant variation in mineralogy and grain size, the cryptocrystalline basalt displays a 1 mm wide zone parallel to the contact, where the grain size is slightly smaller and the modal composition is slightly different from the interior of this domain (more clino- and orthopyroxene; less plagioclase and Fe-Ti oxide). This contact is therefore interpreted as a chilled margin of the cryptocrystalline plagioclase-phyric basalt against the fine-grained aphyric basalt, now granoblastically overprinted. A second magmatic feature preserved in some samples is plagioclase phenocrysts. The cores of these phenocrysts are commonly preserved, whereas the rims are recrystallized along with the granoblastic matrix. The contact between the relict cores and the recrystallized rims is commonly marked by granular crystals of clinopyroxene and Fe-Ti oxides. A higher extinction angle in the cores (Fig. F16) relative to the rims suggests that the cores have higher anorthite contents (following the Michel-Levy method). In addition, an increase in extinction angle throughout the rim suggests that the rims are reversely zoned. Similar reverse zoning is present in plagioclase within the granoblastic matrix, suggesting that the reverse zoning is metamorphic in origin.

Intrusions in the granoblastic basalt

Approximately half of the described granoblastic basalts contain a minor amount of evolved plutonic material. These evolved rocks occur in two styles: (1) igneous veins and dikelets and (2) igneous patches. The igneous patches are as large as 5 cm × 3 cm and are composed of oxide gabbro, diorite, and tonalite (Fig. F17). Some of the patches show elongate crystals growing inward from the contact with the granoblastic basalt, indicating that they are intrusive. Others, however, have more gradational contacts, and these may be either intrusions or segregations.

The plutonic patches, dikelets, and veins are generally medium grained and granular, and grain size distributions range from equigranular to seriate and poikilitic. Their magmatic textures (lack of recrystallization, subhedral to euhedral shapes of several phases, and poikilitic textures) contrast strongly with the granoblastic textures of the host rocks. Fe-Ti oxide contents are variable, ranging from 2% to 20%. Locally, the veins are observed to be offshoots of the igneous patches (Figs. F17, F18) and dikelets (Fig. F19). This observation indicates that the igneous veins, dikelets, and patches form part of the same network and could mark a single generation of intrusion of evolved melts into the granoblastic basalt. The igneous textures of the intrusive rocks demonstrate that intrusion occurred after the granoblastic recrystallization of the host rock. However, the latter was still hot during intrusion, as indicated by the sutured contacts and the diffuse impregnation of the granoblastic basalts by the veins (see below).

Thin section observations provide constraints on the nature and crystallization histories of the magmatic patches. Sample 335-1256D-Run11-EXJB (Thin Section 13) (Fig. F18) is a medium-grained quartz diorite with relatively high amounts of clinopyroxene and green to brownish hornblende as primary mafic minerals (both 27%); the oxide content is about 2%. Hornblende grows around clinopyroxene, implying a crystallization order of clinopyroxene before amphibole. The presence of zircon documents the highly evolved nature of the melt that crystallized this rock. Toward the contact with the granoblastic basalt, a ~2 mm wide zone is developed where the grain size of the quartz diorite decreases and a higher degree of alteration is recorded. The contact between both lithologies is sutured. Another magmatic patch (in Sample 335-1256D-Run13-RCJB-Rock A; Thin Section 31) is an oxide-rich (8%), fine-grained tonalite (Fig. F20). The primary mafic minerals are remarkably fresh clinopyroxene (20% of the primary mode) and green to brownish hornblende (17% of the primary mode). Similar to Sample 335-1256D-Run11-EXJB (Thin Section 13), prismatic clinopyroxenes are enclosed by amphibole. The abundance of quartz (30%), as well as the presence of several grains of apatite and zircon, implies that this tonalite patch crystallized from a highly evolved melt.

The igneous veins (~1–2 mm wide) and dikelets (<1.5 cm wide) are composed of oxide gabbro to quartz diorite. Contacts between the veins and the granoblastic basalt range from planar to irregular but are invariably sutured. Similar to the igneous patches, the association within the igneous veins of green to brownish hornblende—some of which overgrew pyroxene—quartz, Fe-Ti oxide, and the local occurrence of zircon and apatite points to an evolved melt composition. However, compared with the igneous patches, the veins contain a higher content of mafic minerals (as much as 83% in Sample 335-1256D-Run12-RCJB-Rock B), commonly including granular orthopyroxene (as much as 25% in Sample 335-1256D-Run12-RCJB-Rock D) (Fig. F21A). In addition, there are several textural differences. First, some of the veins contain clinopyroxene oikocrysts enclosing either granoblastic clinopyroxene and orthopyroxene (Sample 335-1256D-Run19-RCJB-Rock C) or plagioclase (Sample 335-1256D-Run12-RCJB-Rock D; Fig. F21B), and others (Samples 335-1256D-Run12-RCJB-Rock B and 335-1256D-Run12-RCJB-Rock J) contain plagioclase oikocrysts that enclose granular clinopyroxene (Fig. F21C). Second, one diorite dikelet (in Sample 335-1256D-Run19-RCJB-Rock C; Figs. F19, F22) has a fine-grained marginal facies containing relics of granoblastic pyroxenes. Third, many plagioclase grains, both granular matrix grains and chadacrysts, contain inclusions of micrometer-sized pyroxene (Fig. F21D). This is common in recrystallized plagioclase within the granoblastic basalt (see above). Overall, the abundance of mafic minerals (particularly granular orthopyroxene), the poikilitic texture, and the granular pyroxene inclusions suggest that the veins are impregnations of the granoblastic matrix by evolved melts, resulting in a hybrid rock composed of relict granoblastic matrix material (orthopyroxene and clinopyroxene chadacrysts; plagioclase with inclusions) and magmatic grains (plagioclase and clinopyroxene oikocrysts, amphibole, zircon, and apatite).

Gabbroic rocks

Gabbroic rocks form 2.9 wt% of the material recovered (Table T5). They have variable modes, ranging from disseminated oxide gabbro to orthopyroxene-bearing olivine gabbro. Irrespective of composition, the gabbroic rocks are medium grained, with grain size ranging from 0.5 to 7 mm (average = 3 mm). The grain size distribution ranges from seriate to equigranular. Oxide contents are generally low (0.5%–2%).

The disseminated oxide gabbro, recovered predominantly as 1–5 cm platy chips during Run 11, has two textural domains, being subophitic in the finer grained parts and granular in the coarser grained parts (Fig. F23). In thin section, clinopyroxene shows symplectite-like intergrowth with a second clinopyroxene toward the rim (Fig. F24), perhaps due to a high-temperature reaction related to interaction between clinopyroxene and a melt. Similar features were observed in Gabbro 1 cores recovered during Expedition 312 (see “Gabbro 1: Units 1256D-81 through 89” in Expedition 309/312 Scientists, 2006; Koepke et al., 2011). Locally, amphibole forms a coherent interstitial network, suggesting it is of primary magmatic origin. It may contain fragments of relict clinopyroxene and locally replaces clinopyroxene, suggesting it formed by reaction. Thus, both clinopyroxene and amphibole record late-stage reaction of the gabbro with a melt; the presence of primary amphibole suggests this melt may have been hydrous.

The disseminated oxide olivine gabbronorite, recovered as chips during Run 11, has a subophitic to granular texture. In thin section (Sample 335-U1256D-Run11-EXJB; Thin Section 29), it contains 15% olivine with prismatic habit, which is in part enclosed by orthopyroxene. Olivine contains micrometer-sized wormy exsolutions, probably of Fe-Ti oxides (Fig. F25), which were also found in Gabbro 1 olivine (Sample 335(312)-1256D-223R-2, 65–67 cm [Piece 3]; Thin Section 1) during Expedition 335. Similar features were described in olivine from the Rum layered intrusion and explained as a result of reaction resulting from an increase in the oxidation state of the rock (“oxidation exsolution”; Nitsan, 1974). Clinopyroxene and orthopyroxene are prismatic and locally intergrown subophitically with plagioclase. The enclosed plagioclase is elongate and locally encloses trains of small clinopyroxene crystals (now altered to actinolite), suggestive of fast crystal growth. Similar features were observed in Gabbro 1 during Expedition 312. Green to brownish green amphibole locally forms an interstitial network, which is commonly associated with millimeter-sized oxide grains, implying a primary magmatic origin.

The disseminated oxide orthopyroxene-bearing olivine gabbro, recovered as a single, ~1 kg block (Sample 335-1256D-Run20-Rock C; Fig. F26), is medium grained, equigranular, and has a uniform subophitic texture. Microscopic features of this rock are very similar to the olivine gabbronorite described above (Sample 335-U1256D-Run11-EXJB; Thin Section 29), except that the orthopyroxene content is much lower (2%). Plagioclase forms a granular framework of tabular to elongated crystals, locally intergrown subophitically with clusters of anhedral clinopyroxene. Plagioclase commonly displays patchy zoning as a result of the presence of dusty ghost cores. Green to brownish hornblende locally formed interstitially. Olivine contains different types of inclusions: micrometer-sized wormy exsolutions of probably Fe-Ti oxides, trails of micrometer-sized Fe-Ti oxides, and ~100 µm sized roundish inclusions, some with radiating cracks, which may represent former melt inclusions (now filled with alteration products).

Basalt

Basaltic lava forms 1.3 wt% of the rocks recovered during cleaning and fishing (Table T5). The lava is aphyric and glassy to fine grained, and some pieces show an intersertal texture (Fig. F27). Although generally aphyric, some pieces contain <1% microphenocrysts of plagioclase and clinopyroxene.

Leucocratic rocks

Most runs recovered generally small (<5 cm) pieces of leucocratic rocks (0.3 wt% of the total recovery) (Table T5; Fig. F28). The leucocratic rocks have differing proportions of mafic minerals (5%–20%) but are all characterized by cryptocrystalline to fine grain size (typically ~0.1 mm), granular textures, and dominance of Fe-Ti oxides over mafic phases. Hence, they are grouped into one lithologic unit. A few pieces contain planar and sutured contacts between the leucocratic rocks and the granoblastic basalt, and one piece preserves a gradational contact between the two. In addition, two pieces (Sample 335-1256D-Run20-RCJB-Rocks D and E) contain contacts between leucocratic rocks and coarse-grained diorite, the latter being intrusive.

Microscopic observations show that the leucocratic rocks are very similar to the albitite recovered in Section 335-1256D-236R-1; like this sample (Thin Section 4), they are characterized by pervasive alteration, which has resulted in a dusty, cloudy appearance with a very low grade of crystallinity (Fig. F29A). They also consist mostly of plagioclase (85%–95%) and secondary minerals (greenish actinolite to hornblende, poikiloblastic epidote, secondary diopside, and titanite/oxide associations) and contain relict plagioclase microphenocrysts. In addition, the distribution and the shape of oxides in these rocks (Fig. F29B) are very similar to those observed in the granoblastic dikes. Hence, like part of Section 335-1256D-236R-1 (Piece 1), these leucocratic rocks are interpreted as strongly altered granoblastic basalt.

(Oxide) diorite

Discrete pieces of diorite form a very small component (0.09 wt%) of the recovered rocks (Table T5). The diorite is generally medium grained, although some coarse-grained diorite occurs (Fig. F28C). It is equigranular with a granular texture and is characterized by the presence of subhedral to euhedral, elongate amphibole crystals, similar to the diorite dikelet that crosscuts the granoblastic basalt (Sample 335-1256D-Run19-RCJB-Rock C). The oxide content ranges from 1% to 7%.

Discussion

Origin of junk basket material

Several features of the recovered material provide constraints on its origin and significance. The basalt shows typical textures (e.g., intersertal) and alteration (e.g., celadonite and smectite) of basalt recovered from the extrusive section of Hole 1256D and is likely derived from the upper part of the hole. In contrast, many of the granoblastic basalt and gabbro pieces are chips, some as thin as 0.5 mm, suggesting they were broken off of the borehole wall by the bit during drilling, reaming, and fishing runs. Some of these gabbros (Sample 335-1256D-Run11-EXJB; Thin Section 12) are similar to rocks recovered during Expedition 312; they are characterized by finer grained subophitic to poikilitic gabbro domains and coarser grained oxide gabbro domains, the same domains that predominate in Gabbro 1 (Koepke et al., 2011). However, many other gabbros (including the large olivine gabbro block) are unlike those recovered during Expedition 312 in that they are characterized by relatively simple, single igneous domain textures, in contrast to Gabbro 1 and Gabbro 2. In addition, their clinopyroxene is unusually dark in color, giving the rock a distinct black-and-white appearance. These observations suggest that many of the recovered gabbros represent a unit that was not previously recovered in Hole 1256D. Hence, it is most likely that, with the exception of the basalt and some of the gabbro chips recovered during Run 11, most of the material recovered from the junk baskets originates from near the bottom of the hole, below the section recovered during Expedition 312 (1507 mbsf) (Teagle, Alt, Umino, Miyashita, Banerjee, Wilson, and the Expedition 309/312 Scientists, 2006).

Igneous geology of rocks recovered during Expedition 335

The rocks recovered during Expedition 335 represent a ~15 m interval of the transition zone from the upper to the lower crust. Several observations constrain the igneous geology of this interval. First is the relative abundance of the different rock types, with granoblastic basalts forming the vast majority of the rocks recovered. The second observation is that the mineralogy, granoblastic texture, and preservation of dike/dike contacts and phenocrysts suggest that the granoblastic basalts are sheeted dikes that have been recrystallized. The peak metamorphic assemblage of orthopyroxene-clinopyroxene-plagioclase without amphibole in the granoblastic basalts suggests recrystallization occurred at granulite facies temperatures. The third observation is that the granoblastic basalts have been intruded by a series of small-scale patches, veins, and dikelets of evolved rocks (predominantly oxide diorite and tonalite). In addition, numerous chips and a single block of gabbroic rock were recovered, the contact relationships of which cannot be established unambiguously. However, because most gabbroic rocks do not resemble gabbroic rocks recovered during Expedition 312 (see “Origin of junk basket material”), it is likely that least some of the gabbroic rocks occur intercalated with the granoblastic basalt, perhaps forming small intrusions. Although we cannot constrain the maximum size of the intrusions, the relatively low percentage of intrusive rocks in the junk basket material suggests that the plutonic rocks did not form large intrusive bodies. Rather, they are likely restricted to small (tens of centimeters?) bodies.

Overall, a picture emerges of a section of metamorphosed granoblastic sheeted dikes that underwent small-scale intrusion by both gabbroic and evolved plutonic rocks. This section is very similar to the dike screen recovered between Gabbro 1 and Gabbro 2 during Expedition 312 at 1458.9–1483.1 mbsf (Teagle, Alt, Umino, Miyashita, Banerjee, Wilson, and the Expedition 309/312 Scientists, 2006).

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