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doi:10.2204/iodp.proc.309312.103.2006

Appendix: igneous petrology unit descriptions

Sheeted dike complex

Unit 1256D-66: sparsely plagioclase-phyric to aphyric fine-grained basalt

Basalts of this unit are fine grained with local coarser patches (Fig. F206). Contacts with adjacent units were not recovered.

Typical textures are hypocrystalline intergranular to intersertal with former glassy interstitial material completely altered, mainly to chlorite (Fig. F213A, F213B). Plagioclase is cloudy and moderately altered. Clinopyroxene is moderately altered to cryptocrystalline, brownish, often fibrous material with significant areas of unaltered prismatic primary clinopyroxene. Occasional (<0.5%) 2–3 mm glomerocrysts of clinopyroxene crystals with associated plagioclase are present throughout the unit (Fig. F213C, F213D). Both clinopyroxene and plagioclase contain melt inclusions, now recrystallized to brownish cryptocrystalline masses (Fig. F213E, F213F). Olivine is rare, occurring in two (1 and 2) of the three thin sections from this unit as millimeter-sized grains in clinopyroxene glomerocrysts (Fig. F213G, F213H).

The metamorphic overprint in the intersertal textures corresponds to Type 3 (as described in “Downhole metamorphic evolution”).

Unit 1256D-67: mixed aphanitic and fine-grained basalts

This unit consists of out-of-place, mixed cryptocrystalline and fine-grained lithologies from Units 1256D-66 and 68 disturbed during drilling.

Unit 1256D-68: aphyric fine-grained basalt

Fine-grained basalts of this unit are characterized by visible fine-grained acicular plagioclase. It is intruded along a lower subvertical contact by Unit 1256D-69 (Fig. F216).

Typical textures are hypocrystalline intersertal to variolitic. Formerly glassy material, interstitial to the plagioclase-clinopyroxene framework, is completely altered, mainly to chlorite (Figs. F214, F215). Plagioclase commonly occurs as clusters of radiating acicular crystals, some with cylindrical hollow cores. These clusters typically form graphophyric intergrowths with clinopyroxene. They are present in virtually all microcrystalline and fine-grained sheeted dike units. Clinopyroxene is partially altered to cryptocrystalline brownish material, as in Unit 1256D-66, and glomerocrysts are no longer present.

The alteration behavior of plagioclase and clinopyroxene is similar to that of Unit 1256D-66, and the metamorphic overprint in the intersertal textures corresponds to Type 3.

Unit 1256D-69: aphyric cryptocrystalline to fine-grained basalt

This unit appears to be a complete cooling unit, grading downhole from a cryptocrystalline upper chilled margin, through a fine-grained center, to a cryptocrystalline lower margin. The upper contact is intrusive into Unit 1256D-68, but no lower contact was recovered. Sparse microphenocrysts of euhedral clinopyroxene and clinopyroxene-plagioclase clots are visible in hand specimen, mostly within the cryptocrystalline margin (Fig. F216).

The coarsening in grain size from the chilled margin to the dike center is illustrated in Figure F217A–F217C. The texture changes from variolitic to intersertal, and average plagioclase grain size increases from 0.1 to 0.6 mm. The dike interior has essentially the same petrographic features as Unit 1256D-68, with mainly hypocrystalline intersertal to variolitic textures (Fig. F217B, F217C) and common clusters of radiating acicular plagioclase crystals (Fig. F217B–F217D).

The alteration of plagioclase and clinopyroxene is unchanged from above, and the intersertal metamorphic overprint remains Type 3.

Unit 1256D-70: aphyric microcrystalline to fine-grained basalt

Basalts of this unit are generally microcrystalline, grading to occasional fine-grained patches. They are distinguished from those of Unit 1256D-69 by the presence of abundant acicular plagioclase, visible in hand specimen (Fig. F218). Contacts with adjacent units were not recovered.

Typical textures are hypocrystalline intersertal with variolitic domains, and the mineralogy and alteration are similar in most respects to those of Unit 1256D-69. A distinguishing characteristic of Unit 1256D-70 is an abundance of acicular plagioclase that occurs as branching bundles of acicular to fibrous crystals, commonly intergrown with prismatic or acicular dendritic clinopyroxene (Fig. F219). In variolitic domains, plagioclase occurs in radiating clusters of acicular crystals with cylindrical hollow centers.

Clinopyroxene is more altered than in the shallower units. The dominant alteration product is now a brownish, dusty, cryptocrystalline material containing abundant tiny anhedral oxide grains. Formerly glassy interstitial material is completely altered, mainly to chlorite plus oxide.

The metamorphic overprint observed in the intersertal textures of this unit is intermediate between Types 3 and 4.

Unit 1256D-71: brecciated cryptocrystalline basalt

This short unit consists of several pieces of cryptocrystalline aphyric basalt. It contains an intrusive contact and was formed at a complex brecciated dike margin (Fig. F200). Adjacent to intrusive contacts, the rock is cryptocrystalline with no discernible texture; away from the contact, the texture grades over a few millimeters to spherulitic (Fig. F220).

Very sparse clinopyroxene and plagioclase microphenocrysts are present close to the margin. There is an internal contact within this unit, but contacts with the adjacent units were not recovered.

Texturally distinct zones within this unit are (1) aphyric cryptocrystalline basalt, (2) chilled margin of the same zone, and (3) brecciated hydrothermally altered cryptocrystalline basalt along the intrusive contact (Fig. F220A). The spherulitic outer part of the chilled margin (left side of Fig. F220) grades inward into a more variolitic region. Acicular microlites of plagioclase aligned parallel to the chilled margin record flow parallel to the chilled margin. In another part of the chilled margin (Fig. F220B), corner flow around a localized 90° bend in the dike contact has caused elongation of a number of spherulites, suggesting that viscous magma flow was ongoing as crystallization began.

Unit 1256D-72: aphyric microcrystalline to fine-grained basalt

In this unit, grain size increases downhole from microcrystalline to fine grained, suggesting a transition from dike margin to dike interior. However, neither of the contacts with adjacent units was recovered, and the presumed gradation back to a finer grained lower margin is also absent from the core.

Although they appear fine grained in hand specimen, under the microscope the rocks of this unit display predominantly microcrystalline variolitic textures with common cryptocrystalline domains. Plagioclase and minor clinopyroxene phenocrysts form clots in the finer grained upper part of the unit (Fig. F221). In variolitic domains, branching or radiating aggregates of acicular to fibrous plagioclase are intergrown with dendritic clinopyroxene.

Clinopyroxene is strongly altered to dusty brown cryptocrystalline masses, sometimes with a fibrous structure, containing abundant tiny anhedral oxide (magnetite?) grains. Former interstitial glass is difficult to distinguish, as it is strongly altered to brown cryptocrystalline masses, chlorite, and oxide. The metamorphic overprint is in an intermediate state between Types 3 and 4.

Unit 1256D-73: aphyric cryptocrystalline to fine-grained basalt

This unit consists of small broken pieces from an interval of poor recovery. Grain size is predominantly microcrystalline with some fine-grained pieces. All the pieces are lithologically similar, with the exception of a single cryptocrystalline basalt piece that includes an intrusive igneous contact (~1290.5 mbsf; interval 312-1256D-179R-1 [Pieces 1 and 2, 0–9 cm]). These observations are consistent with the drill remaining within a single dike but sampling different grain size regions. Upper and lower unit contacts were not recovered.

Typical textures are hypocrystalline intersertal with common variolitic domains (Fig. F222); plagioclase commonly has cylindrical hollow cores. Graphophyric intergrowths of plagioclase and clinopyroxene are also present (Fig. F222B, F222C). These textures can be interpreted as products of crystallization in undercooled melts. Similar textural details are present throughout the sheeted dike section, but they are exceptionally well developed in Unit 1256D-73.

In this unit, fibrous actinolite appears as the dominant product of the alteration of clinopyroxene and former mesostasis. A second generation of magnetite also becomes prominent as tiny oxide grains dispersed through the products of clinopyroxene alteration. These changes are characteristic of metamorphic textural Type 4.

Unit 1256D-74: aphyric cryptocrystalline to microcrystalline basalt

This unit is finer in grain size than Unit 1256D-73. It consists predominantly of microcrystalline aphyric basalt with very sparse, 0.1 mm tabular plagioclase grading to cryptocrystalline, presumably approaching the unit boundaries, which were not recovered. We infer that this is a thin grain size–graded cooling unit.

Typical textures are mostly hypocrystalline variolitic with common cryptocrystalline domains (Fig. F223). Plagioclase and clinopyroxene are commonly arranged in fine variolitic fans, bundles, or radiating aggregates (Fig. F223A, F223B). Plagioclase is also present as sparse microphenocrysts (Fig. F223C).

Fibrous actinolite and associated disseminated fine magnetite continue as the dominant alteration product of clinopyroxene and interstitial glassy material, typical of metamorphic textural Type 4.

Unit 1256D-75: aphyric cryptocrystalline to fine-grained basalt

Unit 1256D-75 is a single cooling unit. In contrast to Unit 1256D-74, this unit contains acicular plagioclase. It is aphyric and predominantly microcrystalline, grading to cryptocrystalline near the unit margins and to fine grained near the center. There is a possible small fragment of an upper intrusive contact in Sample 312-1256D-186R-1 (Piece 1, 0–2 cm). Unit 1256D-75 is intruded by Unit 1256D-76 at a subvertical contact recovered near 1325 mbsf (interval 312-1256D-187R-1 [Pieces 13 and 17–20, 55–85 cm]).

The basalts from Unit 1256D-75 typically have hypocrystalline variolitic textures, commonly with cryptocrystalline domains (Fig. F224A).

The intensity of the alteration overprint is greater than it is in Unit 1256D-74, with higher abundances of both fibrous actinolite and tiny secondary magnetite grains. Texture is transitional between Types 4 and 5.

Unit 1256D-75b is a narrow (~1 cm) kinked, cryptocrystalline basalt dike (Fig. F225). Diffuse cryptocrystalline banding at the chilled margins grades inward to spherulitic texture. Both aligned, bladed plagioclase microphenocrysts and elongated deformed spherulites record flow parallel to the chilled margin (Fig. F224B). Small detached wallrock fragments are also aligned parallel to the margins (Fig. F224C).

Unit 1256D-76: aphyric cryptocrystalline to microcrystalline basalt

Basalt from Unit 1256D-76 is predominantly cryptocrystalline. Grain size grades downhole to microcrystalline and perhaps back to cryptocrystalline in the deepest pieces. It is intruded by cryptocrystalline basalt of Unit 1256D-75 (Fig. F226). The lower unit boundary was not recovered.

The intersertal to variolitic textures of this unit resemble those from Unit 1256D-75. Plagioclase typically forms skeletal radiating clusters of acicular crystals and sometimes graphophyric intergrowth with clinopyroxene. The metamorphic overprint continues to increase, and in this unit it is beginning to obscure primary textures; it is not clear whether glass was initially present. The same is true for all basalts of the following units.

Both clinopyroxene and the original interstitial material are commonly completely overgrown by fibrous actinolite, with abundant secondary tiny magnetite grains. The metamorphic overprint corresponds to texture Type 5.

Unit 1256D-77: aphyric fine-grained basalt

Unit 1256D-77 consists of predominantly fine-grained basalt with grain size increasing to almost medium grained near the unit center at 1334.5 mbsf (interval 312-1256D-189R-1 [Pieces 9–11, 55–90 cm]) and then grading to microcrystalline at the lower unit boundary near 1343.5 mbsf (interval 191R-1 [Pieces 1 and 2, 0–11 cm]).

Textures are typically intergranular with a well-developed framework of hollow plagioclase prisms and interstitial clinopyroxene. Variolitic (Fig. AF1A) and subophitic (Fig. AF1B) patches are common.

A strong alteration overprint corresponds to texture Type 5. Both clinopyroxene and former interstitial material are completely replaced by fibrous actinolite. In certain distinct patches, primary clinopyroxenes appear to have been fragmented into tiny granular clusters within masses of fibrous actinolite (Fig. AF1C). This may represent the first occurrence downhole of secondary clinopyroxene (see later discussion).

Unit 1256D-78: aphyric microcrystalline to fine-grained basalt

The aphyric basalts of Unit 1256D-78 are indistinguishable in hand specimen from those of Unit 1256D-77. We defined a unit boundary for two reasons: (1) Unit 1256D-77 decreases in grain size in both directions from the center of Section 312-1256D-188R-1, suggesting that this is a single cooling unit, and (2) Sections 191R-1 through 199R-1 recovered only a small number of drill-rounded fragments, which may not be in place. Unit 1256D-78 basalts are predominantly microcrystalline, grading to fine grained near 1358.4 mbsf (interval 312-1256D-194R-1 [Pieces 10–14, 40–60 cm]). Cryptocrystalline material near 1358.0 mbsf (interval 312-1256D-194R-1 [Pieces 3 and 4, 7–14 cm]) may represent the proximity of the core to a chilled margin, but microcrystalline basalts of Unit 1256D-78 continue below this depth. Despite the textural similarity of Units 1256D-77 and 78, geochemical variation between and within these units suggests that at least three separate intrusions were sampled across Units 1256D-77 and 78 (Fig. F201). Unit 1256D-78 encompasses a marked downhole increase in the intensity and grade of alteration. Primary textural and structural features (mostly intersertal to variolitic textures) (Fig. F207A) continue to be widely recognizable, but original clinopyroxene and interstitial features are increasingly obscured by dense fibrous actinolite with secondary tiny magnetite grains (Fig. F207A). Elsewhere, brownish inclusion-free amphibole, probably similar to hornblende in composition, occurs for the first time. In some places it forms poikiloblastic clusters.

A key feature of this unit is the appearance of granoblastic patches dominated by secondary clinopyroxene. This type of clinopyroxene, which is clear and relatively free of cleavage or inclusions, is probably diopsidic in composition. It occurs as small anhedral grains intergrown with secondary plagioclase associated with relatively large recrystallized oxide grains (Fig. F207B, F207C). In adjacent actinolitic areas, relict prismatic clinopyroxene is pseudomorphed by actinolitic amphibole containing abundant disseminated tiny oxide grains (Fig. F207B; lower left). Rare, very small anhedral orthopyroxene occurs in some secondary clinopyroxene domains.

The metamorphic paragenesis orthopyroxene-clinopyroxene, which normally implies granulite-facies conditions, was seen (subsequently confirmed) as an indication of posthydration thermal input from later intrusions. The metamorphic overprint increases from Type 5 to Type 7 through this unit, indicating a remarkably steep thermal gradient.

Unit 1256D-79: mixed aphyric fine-grained basalt and leucocratic rock fragments

Unit 1256D-79 includes all the geological material collected from junk baskets during hole-cleaning operations. The majority of samples are basalts from a range of depths throughout the hole (Fig. AF2A). These are not described here.

Only Sample 312-1256D-201G-1 (Piece 2, 19–40 cm; Thin Section 40) shows textural and mineralogical features (appearance of brown hornblende and microgranular clinopyroxene) consistent with those of Unit 1256D-78 and with the bottom of the hole at the time the junk basket material was obtained.

Of particular interest, however, are numerous fine-grained felsic rock fragments picked from the gravel sieve fraction of a junk basket. Several of these pieces were mounted and thin sectioned. It is probable that these fragments are derived from leucocratic intrusions into the dike section, similar to Unit 1256D-80b. They have holocrystalline subhedral equigranular textures, and the primary mineralogy is plagioclase, green hornblende, and quartz (Fig. AF2B). They are heavily hydrothermally altered with a strong cataclastic overprint, leading in some cases to foliated cataclasites with a marked alignment of disrupted oxide grains (Fig. AF2C).

Unit 1256D-80: aphyric cryptocrystalline basalt

Unit 1256D-80 spans a wide interval of very low recovery throughout which the recovered samples are surprisingly uniform in hand specimen. This uniformity may partly reflect the extensive metamorphic recrystallization that has largely obscured the primary textures. At 1406.6 mbsf (interval 312-1256D-213R-1 [Piece 12, 44–51 cm]), a narrow (~5 mm) medium-grained oxide gabbro dike has no chilled margin. A similar dike at 1406.7 mbsf (interval 312-1256D-213R-1 [Piece 13, 53–60 cm]) contains a lithologically similar, irregularly shaped dike. The top of the thicker lower dike defines the top of the plutonic section and the beginning of the uppermost gabbro unit.

Most basalts from Unit 1256D-80 have completely recrystallized under amphibolite-facies conditions. Primary textural and structural features (intergranular to intersertal textures with variolitic domains) are preserved in some places. Typically, granular plagioclase, tabular anhedral green actinolitic hornblende, granular anhedral colorless clinopyroxene, and larger subhedral oxide grains form microcrystalline equigranular domains in the groundmass (Fig. F209A). Amphibole forms occasional poikiloblastic clusters enclosing plagioclase and oxide. In secondary clinopyroxene-rich domains, clinopyroxene seems in many cases to have been recrystallized from actinolite that had previously replaced primary magmatic clinopyroxene (Fig. F209A). Rarely, poikiloblastic clinopyroxene coexists with carbonate (Fig. F209B), possibly suggesting that CO2 activity was locally high during the final metamorphism, although the carbonate may also have formed from later lower temperature hydrothermal solutions. Orthopyroxene occurs in two modes: around some metamorphic veins it occurs as patchy clots, and in some microgranular domains anhedral orthopyroxene coexists with clinopyroxene (Fig. F209C). In the first instance, orthopyroxene appears to be related to recent or precursory hydrothermal alteration. In the second, it appears that the metamorphic overprint has reached granulite-facies conditions.

Unit 1256D-80b: trondhjemite dike

Unit 1256D-80b is a 5–20 mm wide magmatic dike of microcrystalline trondhjemite that intrudes basalt from Unit 1256D-80a near 1404.3 mbsf (interval 312-1256D-212R-1 [Pieces 5 and 6, 20–28 cm]). It has a leucocratic appearance and is parallel-sided with sharp contacts and no chilled margins (Fig. F227, F228C).The host rock is significantly recrystallized, with microgranular domains dominated by secondary anhedral clinopyroxene.

Primary minerals are quartz, plagioclase, and amphibole, forming a fine-grained anhedral equigranular texture. Some quartz forms microgranophyric intergrowths with plagioclase, indicating that quartz is magmatic and not formed during alteration. Primary magmatic features are obscured by alteration. In particular, the original composition of the primary mafic mineral, now completely converted to actinolite, is not clear. Several partially obscured amphibole basal sections are present (Fig. F228A, F228B), suggesting that amphibole was the primary mafic mineral. This suggestion is consistent with the absence of relict clinopyroxene.

This dike is classified as trondhjemite because of its high modal quartz content (50 vol%) and relatively low mafic content (<10 vol%). Geochemical results show that this is the most differentiated rock sampled during Expedition 312.

The contact between the dike and the host basalt is clearly delineated in thin section (Fig. F228C) and is commonly marked by the development of fibrous actinolite in the host rock. Together with the absence of any grain-size gradation in the dike, these observations suggest that the vein was emplaced while the host basalt was still hot. Both lithologies must have been affected by late metamorphism, as both show the same style and grade of metamorphic overprint.

Plutonic section

Unit 1256D-81: intermixed medium-grained oxide gabbro and basalt

The first occurrence of gabbro in Hole 1256D is at ~1406.5 mbsf (interval 312-1256D-213R-1 [Pieces 12 and 13, 48–63 cm]). The gabbro occurs in two very small dikes; the upper is 5 mm thick (Piece 12) and the lower is 40 mm thick (Piece 13). The gabbro dikes have both upper and lower sharp intrusive contacts with metabasalt of Unit 1256D-80, and there is no change in gabbro grain size away from the contacts.

Below ~1410.9 mbsf, a 30 cm interval of rubble includes medium-grained gabbro pieces from Unit 1256D-81 intermixed with basalt from Unit 1256D-80. At 1411.2 mbsf (interval 312-1256D-214R-1 [Piece 9, 27–36]), an intrusive contact of Unit 1256D-82 into Unit 1256D-81 is preserved (Figs. F229, F230).

The medium-grained gabbro of Unit 1256D-81 is characterized by the first occurrence of relatively large (up to 8 mm) anhedral poikilitic clinopyroxenes (oikocrysts) enclosing unzoned prismatic plagioclase, a textural feature more typical of dolerites that recurs throughout the Expedition 312 gabbro interval. Outside the oikocrysts, plagioclase is distinctly larger and strongly zoned, suggesting that crystals grew from melt of varying composition after generation of the original doleritic texture.

Primary clinopyroxene has been strongly altered to actinolite and oxide, and there are some indications that primary amphibole was also present as a late-crystallizing phase. These indications include crystal shapes, diffuse amphibole cleavage, and inferred basal sections, but pervasive alteration has obscured any definitive evidence. Abundant interstitial and less common poikilitic Fe-Ti oxide indicates that the magma was iron rich.

Unit 1256D-82: quartz-rich oxide diorite

Unit 1256D-82 consists of an unusual rock type with mineralogical and compositional characteristics that do not appear to have been described previously. Based on thin section description, we identified it as an oxide-rich (~20 vol%) tonalite, but its chemical composition is that of an evolved FeTi basalt (49% SiO2, 4% MgO, 18% FeO, and 4% TiO2) and the rock name diorite seems more appropriate. It is light in color and medium grained with inequigranular seriate texture, including occasional coarse patches with grain sizes up to 10 mm (e.g., Sample 312-1256D-214R-1 [Piece 27, 139–140 cm). It is readily distinguishable from the gabbros of the surrounding Units 1256D-81, 83, and 84 by its pale color, visible quartz, and abundant white plagioclase (seen in thin section to be intergrown with quartz) and a common euhedral, elongate mafic phase, usually replaced by secondary amphibole (Figs. F229, F230).

Unit 1256D-82 intrudes Unit 1256D-81 as described above. It has a complex subvertical contact downhole with Units 1256D-83 and 84 (interval 312-1256D-214R-1, 71–143 cm) (Fig. F229). The contact with gabbro from Unit 1256D-81 is regular and characterized by an abrupt grain size increase from Unit 1256D-81 to 82 (Sample 312-1256D-214R-1 [Piece 9, 34–35 cm]) (Fig. F230). In places, elongate plagioclase and hornblende of the oxide diorite show a preferred orientation perpendicular to the contact, which can be interpreted as a comb structure consistent with the intrusion of Unit 1256D-82 into Unit 1256D-81. The absence of both low-temperature alteration and a grain-size gradient suggests that the diorite intruded while the gabbro was still hot.

The dominant primary minerals were abundant quartz (20%–25%), a primary mafic phase, probably hornblende (>20%), interstitial Fe-Ti oxides (<10%), and plagioclase, which occurs both as large subhedral tabular prisms with significant zoning and as granophyric intergrowths with quartz, suggesting eutectic crystallization (Fig. F231A). Primary magmatic features are obscured by pervasive alteration of the primary mafic phase(s) to actinolite and by nearly complete replacement of plagioclase.

The nature of the primary mafic phase(s), whether amphibole or clinopyroxene (or both), is of particular interest, as it provides direct evidence for the role of water in gabbro genesis. Evidence for a primary amphibole, probably a brownish green hornblende, includes (1) optically continuous, brownish green relict crystals displaying typical basal amphibole cleavage (Fig. F231B), (2) very small tabular (flaky) grains of brownish hornblende within actinolitic aggregates (Fig. F231C), (3) no relict clinopyroxene, and (4) elongate prismatic mafic crystals displaying simple twinning, more typical for amphibole than for clinopyroxene (Fig. F231D).

Unit 1256D-83: medium-grained disseminated oxide gabbro

This gabbro is distinguished from Unit 1256D-82 principally on the basis of color, with the plagioclase appearing darker on cut surfaces, and on the absence of quartz. Its contact with Unit 1256D-82 is diffuse over 1–2 mm, and the order of intrusion is unclear, although Unit 1256D-83 appears to have a smaller grain size than Unit 1256D-82 near the contact. Because of the irregular margins of Unit 1256D-82, Units 1256D-83 and 84 are not contiguous in the core (Fig. F229) and their relationship to one another cannot be determined.

Unit 1256D-83 gabbro is predominantly medium grained with primary magmatic features mostly obscured by pervasive alteration; only one of three thin sections (Sample 312-1256D-214R-1 [Piece 17A, 94–95 cm]; Thin Section 67) retains some unaltered primary domains. The texture is subhedral inequigranular seriate with some ophitic patches. Anhedral clinopyroxene, heavily altered to actinolite, forms irregular disrupted poikilitic patches, in contrast to Unit 1256D-81 where clinopyroxene forms large subspherical oikocrysts.

As in Unit 1256D-81, there are two generations of primary plagioclase, now cloudy and heavily altered: relatively small chadacrysts are generally unzoned, but larger prisms outside the oikocrysts are strongly zoned. Primary hornblende crystallized in the interstices formed by the larger zoned plagioclases. This is evidenced by relict brown cores in actinolitic pseudomorphs (Fig. F232) and by typical amphibole basal sections. Some areas of fibrous actinolite may also represent former olivine and/or orthopyroxene. Finally, small amounts of quartz form interstitial granophyric intergrowths, indicating that quartz is a late magmatic phase and not an alteration product.

Unit 1256D-83 incorporates key petrographic features of both Unit 1256D-81 (clinopyroxene oikocrysts and two plagioclase types) and Unit 1256D-82 (magmatic hornblende and granophyric plagioclase/​quartz intergrowths). This strengthens an impression that the genesis of this gabbro series involves crystallization from melts of widely varying compositions, from relatively primitive basaltic melts to more evolved and hydrous magma.

Unit 1256D-84: medium-grained disseminated oxide gabbro

Unit 1256D-84 is essentially identical to Unit 1256D-83 and is likely part of the same magmatic body. However, as the two units are not contiguous in the core, their relationship to one another cannot be determined and they are treated as separate units.

Overall, this unit is petrographically almost identical to Units 1256D-83 and 85. Features worth noting include clinopyroxene oikocrysts that enclose an unusual form of plagioclase with parallel growth patterns and corroded, wormy appearance (Fig. F233A) and an unusually high abundance of oxides in the coarse-grained (network) domains. Oxides are generally interstitial but sometimes partially enclose plagioclase grains (Fig. F233B).

Two domains reflecting different stages of magmatic evolution are clearly defined in this unit, although they are present throughout Gabbro 1. Dark domains characterized by early clinopyroxene-plagioclase crystallization to form large oikocrysts are separated by coarser, lighter colored network domains that apparently formed later (Fig. F210). This process is more fully discussed in the next section.

Unit 1256D-85: medium-grained (patchy) orthopyroxene-bearing disseminated oxide gabbro

Unit 1256D-85 is very similar to Units 1256D-83 and 84, from which it is distinguished by the development of a patchy texture in which roughly spherical, ~1 cm diameter dark patches are separated by 1–5 mm wide leucocratic zones (Fig. F205). The dark patches are finer grained and more pyroxene rich than the coarser grained plagioclase-rich zones. The Unit 1256D-83/84 boundary appears to be gradational, and it is likely that the two units represent textural variants from a common magma source.

The patchiness observed in hand specimen is caused by the presence of dominant subspherical, anhedral clinopyroxene oikocrysts up to 1 cm in diameter with relatively small plagioclase chadacrysts (0.5 mm) (Fig. AF3A). The oikocrysts are separated by a network domain characterized by coarser medium-grained orthopyroxene- and hornblende-bearing oxide gabbro.

As in the overlying units, plagioclase is coarser (2 mm) and strongly zoned in the network domain. Symplectic quartz-plagioclase intergrowths are present along relict clinopyroxene rims (Fig. AF3B, AF3C), sometimes in intimate association with primary hornblende (Fig. AF3D), which also forms coronas around clinopyroxene oikocrysts. In one sample (Sample 312-1256D-215R-1 [Piece 17, 84–88 cm]; Thin Section 71), a few grains of strongly altered orthopyroxene are present within the network domain.

As in Unit 1256D-84 and other Gabbro 1 units, the two domains appear to record different stages of magmatic evolution. The first stage is characterized by clinopyroxene and early plagioclase crystallization to form large oikocrysts (Figs. F210, AF3E). Subsequently, a more evolved orthopyroxene- and amphibole-saturated magma invaded to create the network domains, producing hornblende reaction coronas and symplectic textures around the clinopyroxene oikocrysts. Rapid compositional changes during the network stage are recorded in the strongly zoned plagioclase of the network domains.

Unit 1256D-86: medium-grained disseminated oxide gabbro

This unit is part of the same intrusion as Unit 1256D-85 and is defined on the basis of a gradual textural change from the patchy texture of Unit 1256D-85 to a more equigranular texture with readily visible ophitic clinopyroxenes that defines Unit 1256D-86. Toward the base of the unit, a number of large, elongate clinopyroxene crystals are as long as 25 mm.

As seen in thin section, the principal difference between this unit and Units 1256D-83 through 85 is that clinopyroxene originally occurred as anhedral prisms. These are now largely replaced by green actinolite and surrounded by brownish green hornblende (Fig. F234). Large tabular plagioclase is strongly zoned. Hornblende appears to be a high-temperature variety that grew as interstitial coronas around clinopyroxene. No quartz or olivine was found.

Unit 1256D-86b: medium-grained oxide gabbro

Unit 1256D-86b is coarser than Unit 1256D-86a. It is also more strongly altered, with mafic phases mostly converted to actinolite and altered plagioclase appearing light-colored in hand specimen. It is in magmatic contact with Unit 1256D-86 in Sample 312-1256D-216R-1 (Piece 19, 88–91 cm; Thin Section 75). The development of comb texture on the coarser grained side of the contact suggests that the coarser Unit 1256D-86b is intrusive into Unit 1256D-86.

Unit 1256D-87: medium-grained disseminated oxide gabbro

Unit 1256D-87 contains much larger clinopyroxene oikocrysts than those found in Unit 1256D-86, and this difference in grain size distinguishes these two units. The dominant textural feature in Unit 1256D-87 is the presence of large poikilitic clinopyroxene aggregates forming spherical patches as large as 20 mm. Each patch is composed of several anhedral oikocrysts, some of which may partially enclose opposite ends of a common plagioclase prism. Coronas of hornblende that generate vermicular (symplectite-like) reaction textures around clinopyroxene oikocrysts may be magmatic in origin. Similar coronas are present in Unit 1256D-85.

Plagioclase within the oikocrysts is generally isolated and unzoned, and the oikocryst domains are essentially oxide free. The network domain between the oikocrysts has a seriate texture in which plagioclase is more equant and strongly zoned, with interstitial amphibole and Fe-Ti oxides, as in Units 1256D-83 through 86. Both primary hornblende and clinopyroxene are altered to actinolite, and their primary relationships cannot be established.

Unit 1256D-88: medium-grained disseminated oxide gabbro

Unit 1256D-88 is defined by the first appearance downhole of diffuse, centimeter-scale patches of coarser grain size, highlighted by pale (altered) plagioclase, higher oxide mineral contents, and the absence of poikilitic clinopyroxene. Patches are scattered within a predominantly finer grained gabbro that continues from Unit 1256D-87 with the same characteristic large clinopyroxene oikocrysts. Coarser patches appear to be more strongly altered than the background gabbro because of the paler plagioclase. The boundaries of the patches vary from very diffuse (e.g., interval 312-1256D-217R-1 [Piece 16, 62–70 cm]) to relatively sharp (over 1–2 mm) (e.g., interval 217R-1 [Piece 22, 94–99 cm]).

On a microscopic scale, the second distinguishing feature of this unit is the occurrence of scattered olivine as small, highly altered interstitial grains with dark oxide-rich alteration halos. In Sample 312-1256D-220R-1 (Piece 9A, 52–58 cm) (Fig. F235), a primary contact between olivine and prismatic orthopyroxene implies that these phases crystallized at the same time.

Unit 1256D-88 is unusually heterogeneous, with a strong variation in grain size. For example, in Sample 312-1256D-220R-1 (Piece 4, 24–29 cm; Thin Section 83), plagioclase grain size ranges from 0.4 mm (as chadacrysts in clinopyroxene) to 7 mm in the coarse patches. Clinopyroxene morphology varies from anhedral, patchy oikocrysts to subhedral prismatic crystals.

Alteration effects include high-temperature hornblende mantling clinopyroxene and producing symplectic reaction rims. This hornblende most likely crystallized as a late magmatic phase and/or as a consequence of fluid interaction at magmatic temperatures. Minor orthopyroxene occurs as subhedral prismatic crystals and, rarely, as oikocrysts (or oikoblasts) enclosing plagioclase. In contrast to the rather scattered high-temperature metamorphic effects, low-temperature actinolitic alteration of the primary mafic phases is widespread and obscures many of the primary relationships.

Unit 1256D-89: medium- to coarse-grained olivine- and orthopyroxene-bearing oxide gabbro

Unit 1256D-89 is distinguished from Unit 1256D-88 by its smaller grain size and by high but variable olivine contents up to ~20%. Unit 1256D-89 gabbros resemble those from Unit 1256D-88 in overall mineralogy (other than olivine abundance), heterogeneity, and texture in the level of alteration of mafic phases to actinolitic amphibole.

Features that differ from Unit 1256D-88 include the following:

  • Olivine is common, sometimes intergrown with clinopyroxene or orthopyroxene (Fig. F236A, F236B). It is preserved as fresh cores within original subhedral prisms, surrounded by halos of oxides followed by chlorite.
  • Clinopyroxene is more common as prismatic grains than as oikocrysts.
  • Orthopyroxene is more abundant, reaching 5% or more in some areas.

Within this lowest unit of Gabbro 1, two unusual features are important. First, deep brown pargasitic amphibole surrounds interstitial oxide grains (Fig. F236C). This is a typical late-stage occurrence at some other oceanic gabbro locations (e.g., those cored during ODP Legs 176 and 153). It implies a high-temperature (850°–1000°C) late-magmatic or metamorphic process.

In some areas, plagioclase occurs in microgranular domains (0.005–0.02 mm) associated with similarly sized anhedral granular clinopyroxene containing numerous tiny oxide inclusions (Fig. F236D). Although they are completely recrystallized, some of these clusters appear to preserve the outlines of primary variolitic plagioclase. For example, in Figure F236D (Sample 312-1256D-223R-2 [Piece 1A, 57–60 cm]; Thin Section 93) the central region is similar to but more strongly recrystallized than a similar cluster in Figure F212F. The latter figure represents texture Type 7 in the granoblastic dike sequence. If this observation is correct, then the metamorphic grade of Unit 1256D-89 has progressed beyond that of the lowermost sheeted dike complex. An important implication of this observation is that parts of Unit 1256D-89 must have originally been basaltic dike material with a primary variolitic texture. After its original rapid igneous crystallization and cooling, this material has undergone contact metamorphism (with the heat source likely to be proximal gabbro bodies) and possibly partial melting.

This plagioclase in Figure F236D contains tiny crystallographically aligned oxide inclusions. Nearby olivines contain exsolution lamellae, probably of titanium oxides (Fig. F236A, F236B). These two features may result from annealing processes related to slow cooling or to reheating.

Unit 1256D-89b: coarse-grained oxide gabbro

Unit 1256D-89 is cut by a pair of coarser grained oxide gabbro dikes: one is 2 cm thick, and the other is 5 cm thick. The mineralogy of these dikes is similar to that of the network domains in Units 1256D-85 through 88. The contact is characterized by perpendicular growth (comb layering) of large clinopyroxene and plagioclase crystals (>1 cm long axis) from the margin into Unit 1256D-89b (Fig. F236E).

Unit 1256D-90: dike screen

Unit 1256D-90a: fine-grained aphyric basalt

This unit marks a return to fine-grained basalts and corresponds to an interval of low recovery. All of the samples are fine grained, except for Piece 3 from Section 312-1256D-225R-1, which is cryptocrystalline. The contact with the overlying gabbroic Unit 1256D-89 was not sampled. The contact with lower gabbroic Unit 1256D-91 was recovered in Sample 312-1256D-230R-1 (Piece 4, 11–14 cm). Macroscopically, the samples are very similar to dike rocks sampled just above Gabbro 1 (for example, in Unit 1256D-80). The igneous texture has been overprinted, giving a slightly granular appearance. We interpret Unit 1256D-90 as a dike screen trapped between the younger intrusions of Gabbro 1 and Gabbro 2. Unit 1256D-90 basalts resemble dikes from Unit 1256D-80, from shallower depths in the presence of fractured cryptocrystalline dike margins, and in a slight downhole coarsening in Section 312-1256D-227R-1. Both the granular metamorphic overprint and the presence of numerous thin gabbroic and leucocratic dikes cutting Unit 1256D-90 are consistent with its proximity to younger intrusions.

In thin section, secondary granular orthopyroxene is seen to replace clinopyroxene, and approaching the contact with gabbroic Unit 1256D-91, there is an increase in the proportion of orthopyroxene relative to clinopyroxene (Fig. F238). At a distance of 30 mm from the contact, the rock contains equal proportions of orthopyroxene and clinopyroxene, but within 5 mm of the contact all of the pyroxene is granular orthopyroxene. This transition indicates that orthopyroxene formed from granular clinopyroxene by a prograde metamorphic reaction.

Units 1256D-90b through 90f: varied dikes intruding Unit 1256D-90

Unit 1256D-90 is cut by a number of small dikes, varying in thickness (1–10 cm), grain size, and composition. Units 1256D-90b, 90d, and 90e are medium-grained quartz gabbros that show no chilled margins and often appear to have acted as conduits for later hydrothermal fluids. Units 1256D-90c and 90f are fine-grained tonalities, again with no chilled margin and relatively sharp and straight contacts.

Unit 1256D-91: medium-grained gabbronorite

Unit 1256D-91 is the narrow upper marginal unit of Gabbro 2, a medium-grained orthopyroxene-bearing gabbro intrusion. It intrudes Unit 1256D-90a of the upper dike screen along a complex, subvertical contact (interval 312-1256D-230R-1, 15–24 cm) (Fig. F237). Along this contact, medium-grained gabbro invades the metabasalt screen, surrounding and detaching fragments of metabasalt, which appear slightly pink in hand specimen, reflecting an abundance of orthopyroxene in the groundmass. The lower boundary of Unit 1256D-91 is defined by the disappearance of these fragments. A narrow pegmatitic zone is present adjacent to the margin in some pieces (Sample 312-1256D-230R-1 [Pieces 5, 6, and 8, 15–53 cm]), including a 2–3 mm wide marginal zone in which elongate plagioclase has crystallized normal to the margin (comb texture). A dark-colored alteration zone is present in the basalt adjacent to contacts. In interval 312-1256D-230R-1, 49–69 cm, there are abundant 1–2 mm magnetite grains, and this interval has unusually high magnetic susceptibility.

Orthopyroxene is abundant, though irregularly distributed. Orthopyroxene-rich gabbronorite patches are common. For the most part, Unit 1256D-91 is characterized by fine-grained inequigranular to poikilitic textures. The predominant primary minerals are plagioclase, clinopyroxene, orthopyroxene, and oxide. In some places, high-grade granoblastic metamorphic textural domains also occur. The mode of occurrence (and formation) of orthopyroxene in these domains is of particular interest, as it seems to blur the distinction between magmatic and metamorphic processes. Examples include the following:

  • Large orthopyroxene grains contain tiny inclusions of granular clinopyroxene and appear to have formed in the solid state (Fig. F241A, F241B).
  • Typical crystalloblastic intergrowths include orthopyroxene and clinopyroxene and are similar to those described for Unit 1256D-89.
  • The proportion of orthopyroxene increases from both sides with proximity to gabbro/basalt contacts, suggesting two-way thermal and chemical exchange between the host rock and the intrusion (e.g., Sample 312-1256D-230R-1 [Piece 5, 15–18 cm]; Thin Section 106).
  • Orthopyroxene is commonly concentrated along veins or fractures, again suggesting a high-grade metamorphic and/or metasomatic process.
  • Abundant dusty spots are included in plagioclase.

The multiplicity of orthopyroxene occurrences, including magmatic orthopyroxene in gabbronorite, apparently metamorphic orthopyroxene in granoblastic domains within gabbronorite, and similar orthopyroxene along contact zones, both in the intruding gabbronorite and in the host metabasalt, emphasizes that within and beneath the dike screen there is an important interface where the distinction between magmatic and metamorphic processes is blurred.

Unit 1256D-92: medium- to fine-grained orthopyroxene-bearing gabbro

The top of Unit 1256D-92 is defined by the disappearance of fine-grained basaltic xenoliths. The medium- to fine-grained orthopyroxene-bearing gabbro is otherwise similar to that of Unit 1256D-91, with dominant subhedral, inequigranular seriate to poikilitic textures and significant textural heterogeneity. Olivine, which is absent from Unit 1256D-91, is present in small amounts but almost completely altered to chlorite, talc, and oxides. Interstitial Fe-Ti oxides are abundant.

As in Unit 1256D-91, Unit 1256D-92 gabbroic rocks include apparently metamorphic and magmatic textural features (e.g., Sample 312-1256D-232R-1 [Piece 5C, 97–100 cm]; Thin Section 113). For example, orthopyroxene occurs in two forms: (1) as large poikilitic crystals (up to 6 mm) with small included clinopyroxene and enclosing subhedral plagioclase (Fig. F240A, F240B) and (2) in association with clinopyroxene and secondary plagioclase in poikiloblastic aggregates. These aggregates are almost completely recrystallized, with typical metamorphic textures. When they are viewed in the context of our documented downhole metamorphic evolution, it is possible to perceive vestiges of original radiate plagioclase aggregates or textural frameworks (Fig. F240C, F240D). Brownish green hornblende, absent from Unit 1256D-91, occurs as crystals or clusters that are commonly poikiloblastic. Their brownish green color and their morphology indicate a high-temperature origin either from a late melt or water-rich fluid. Pale green actinolite forms abundant fibrous patches pervasively replacing mafic and interstitial material.

Unit 1256D-93: medium-grained gabbronorite containing basaltic xenoliths

Unit 1256D-93 is the narrow lower marginal unit of the Gabbro 2 medium-grained orthopyroxene-bearing gabbro. It is defined by the presence of rounded basalt xenoliths that appear to be derived from the adjoining Unit 1256D-94 and is similar to the marginal upper unit, Unit 1256D-91. An intrusive contact was recovered in only one small piece and is diffuse over a few millimeters. Unit 1256D-93 is predominantly medium grained, similar to Unit 1256D-92, with occasional irregular pegmatitic patches. Rounded, included, fine-grained basaltic fragments are pinkish in color, reflecting their high orthopyroxene contents.

This subinterval of Gabbro 2 is very inhomogeneous and incorporates both primary magmatic and metamorphic features. In coarse-grained Sample 312-1256D-232R-2 (Piece 9, 98–100 cm; Thin Section 116), cumulus plagioclase provides the first evidence of magma chamber processes in Hole 1256D (Fig. F242).

As in the rest of Gabbro 2, orthopyroxene appears to occur in both magmatic and metamorphic forms. An unusual form, characterized by abundant oxide inclusions, is present in Sample 312-1256D-232R-2 (Piece 2, 52–54 cm; Thin Section 115) (Fig. F243).

This unusual unit has a number of other notable features:

  • Quartz is present in granophyric intergrowths, filling interstices in the plagioclase framework of Sample 312-1256D-232R-2 (Piece 9, 98–100 cm; Thin Section 116) (Fig. F244). Plagioclase in contact with these intergrowths has corroded boundaries, suggesting that a late quartz saturated melt was not in equilibrium with the plagioclase.
  • Xenoliths in the orthopyroxene-bearing gabbros and gabbronorites may represent either (or both) xenoliths of basaltic dike material or cognate xenoliths of very fine gabbronorite (e.g., Fig. F246A).
  • Microcrystalline xenoliths have a typical granoblastic texture very similar to basalts from Unit 1256D-80a, close to Gabbro 1 (Fig. F245).

The origin of the fine-grained xenoliths and the adjacent Unit 1256D-94 are subject to debate, with the main issue being the mode of origin of orthopyroxene. One possibility is that a fine-grained actinolite-grade metabasalt was later reheated by the intruding gabbroic magma(s) to form, first, granoblastic secondary pyroxene domains and, later, orthopyroxene oikoblasts enclosing the granoblastic domains. A second possibility is that the orthopyroxene is mostly primary and that the fine-grained material was intruded as a micro- or fine-grained gabbronorite with a pronounced equigranular texture.

Unit 1256D-94: fine-grained aphyric metabasalt or gabbronorite

Unit 1256D-94 is an orthopyroxene-rich, recrystallized, cryptocrystalline to fine-grained rock of basaltic composition. It is extensively recrystallized with well-developed granoblastic textures, but in some areas it retains an igneous, intergranular texture (Figs. F246A, F247B). It is unclear whether this unit is a contact metamorphosed dike screen (metabasalt) or a fine-grained magmatic gabbronorite. The latter term has been applied to very similar lithologies from the root zone of the sheeted dike complex in the Oman ophiolite (Nicolas and Boudier, 1991; Boudier et al., 2000).

Unit 1256D-94 is intruded by gabbronorite of Unit 1256D-93 near 1494.4 mbsf, and a contact that is diffuse over a few millimeters is preserved in interval 312-1256D-232R-2 (Piece 9, 98–100 cm). Xenolithic fragments of similar cryptocrystalline, pinkish orthopyroxene-rich metabasalt are present in Unit 1256D-93 (Fig. F246A).

Primary minerals are plagioclase, clinopyroxene, orthopyroxene, and Fe-Ti oxide, typically forming a fine-grained intergranular texture with occasional poikilitic or poikiloblastic domains (Fig. F247A). This unit is distinguished from other Expedition 312 metabasalt rocks by the presence of these common, large (~2–5 mm) orthopyroxene crystals that poikilitically enclose small plagioclase prisms. Elsewhere, isolated orthopyroxene prisms within the groundmass appear to have predated the background actinolitic alteration and can be interpreted as primary. As for Unit 1256D-93, it seems that Unit 1256D-94 orthopyroxene is varied in form and may have formed by a mix of igneous and metamorphic processes. This orthopyroxene is associated with secondary clinopyroxene, which is typical of many rocks of the plutonic section.

In the lower part of the unit (Fig. F246B), large orthopyroxenes are absent and plagioclase is less dusty in appearance, suggesting that metamorphism was less intense away from the gabbro contact. Smaller prismatic orthopyroxene is still present, however, and it cannot be determined with certainty if its origin is igneous or metamorphic.

Unit 1256D-95: cryptocrystalline aphyric basalt

Unit 1256D-95 is distinguished from Unit 1256D-94 by its lighter gray color and finer grain size. Its contact with Unit 1256D-94 was not recovered, and its lower boundary is below the limit of Expedition 312 coring.

In thin section (Fig. F248), the texture is intergranular to radiate. Distinctive features include the presence of abundant pink clinopyroxene, possibly titanaugite, and relatively low primary oxide content. The unit is also distinct from all other basaltic rocks of the plutonic section in its relatively low metamorphic grade and degree of alteration, which are roughly equivalent to those of much shallower units around Unit 1256D-70 (see “Unit 1256D-70: aphyric microcrystalline to fine-grained basalt”). According to the textural classification used for the sheeted dike section, this unit is classified as texture Type 3 or 4. The low metamorphic grade requires a significant period of cooling between gabbro intrusion and associated high-temperature metamorphic event(s) and intrusion of this crosscutting, relatively enriched basalt dike or sill. It is therefore possible that Unit 1256D-95 was intruded off-axis.