Table T4. Igneous unit description for rocks cored in Hole 1256D during Expeditions 335 and 312.
|
Upper contact |
|
Lithology |
Upper unit boundary |
Lower unit oundary |
Description |
|
|---|---|---|---|---|---|---|---|
Depth (mbsf) |
Core, section, interval (cm) | ||||||
| Rocks drilled during Expedition 312—relogged during Expedition 335: | |||||||
| 81 | 1406.62 | 213R-1, 52 | 4.60 | Medium-grained orthopyroxene-bearing oxide gabbro | Intrusive margins in Section 213R-1 (Piece 13) | Intrusive margin in Section 214R-1 (Piece 9) | Mixed gabbro and altered dike material. The gabbro occurs in two dikes, 5 and 40 mm thick. This gabbro has an intrusive contact with the altered basalt host. There is a 0.5 mm wide band of oxides on the margin of the intrusion but little change in grain size in the gabbro toward the margin. The gabbro appears to have a broadly equigranular grain size distribution and granular texture. |
| 82 | 1411.22 | 214R-1, 32 | 0.66 | Medium-grained oxide quartz-diorite | Intrusive margins in Section 213R-1 (Piece 9) | Intrusive margin with Unit 84 in Section 214R-1 (Piece 26) | Narrow (<1 m) intrusion into Units 81 and 83. Primary minerals include abundant quartz (20%–25%), abundant interstitial Fe-Ti oxides (>20%), and a primary mafic phase that was probably hornblende. It has a seriate grain size distribution and granular texture. |
| 83 | 1411.66 | 214R-1, 76 | 0.36 | Medium-grained orthopyroxene-bearing oxide gabbro | Intrusive margin with Unit 82 in Section 214R-1 (Piece 15) | Not recovered | This oxide gabbro is characterized by a patchy texture, with dark patches diffusely invaded by subordinate leucocratic patches. It has a seriate grain size distribution and subophitic texture. |
| 84 | 1412.24 | 214R-1, 134 | 0.43 | Medium-grained orthopyroxene-bearing oxide gabbro | Intruded by Unit 82 in Piece 26 of Section 214R-1 | Not recovered | Unit 84 is very similar to Unit 83 and may indeed be part of the same magmatic body. Both are intruded by Unit 82; because they are not contiguous in the core they are defined as separate units. Locally, a poikilitic texture is developed. |
| 85 | 1412.67 | 214R-2, 32 | 4.81 | Medium-grained orthopyroxene- and olivine-bearing oxide gabbro | Not recovered | Not recovered | Similar to Unit 83, Unit 85 is characterized by the development of a distinct patchy texture, where 1 cm diameter roughly spherical dark patches are separated by 1–5 mm wide zones of leucocratic material. The dark domains include clinopyroxene oikocrysts 1 cm or more in diameter, and strands of the leucocratic network domains are 1–5 mm wide. It has an inequigranular seriate grain size distribution and subophitic to poikilitic texture. |
| 86 | 1417.48 | 215R-2, 35 | 1.77 | Medium-grained disseminated oxide gabbro | Gradational upper boundary based on textural change | Lower boundary based on further textural change | This unit is part of the same intrusion as Unit 85 and is defined on the basis of a gradual textural change from patchy in Unit 85 to more equigranular with subophitic to ophitic pyroxenes in Unit 86. Toward the base of the section, a number of large, elongate clinpyroxene crystals are present, with long axes as long as 25 mm. The unit contains a patch of amphibole-bearing oxide gabbro with a diffuse margin. |
| 87 | 1419.25 | 216R-1, 135 | 2.79 | Medium-grained orthopyroxene-bearing gabbro | Based on textural change between Pieces 25 and 26 | Appearance of coarse-grained and pale plagioclase patches from Section 217R-1 (Piece 11) downward | Unit 87 may be part of the same igneous body as Unit 86, but there is a marked change in texture, becoming poikilitic, containing oikocrysts as large as 30 mm. |
| 88 | 1422.04 | 217R-1, 43.5 | 17.63 | Medium-grained olivine-bearing disseminated oxide gabbro to olivine-bearing oxide gabbro | Appearance of coarse-grained and pale plagioclase patches from Section 217R-1 (Piece 11) downward | First appearance of olivine gabbro, Section 221R-1 | Unit 88 is varitextured, being characterized by diffuse centimeter-scale coarse-grained patches that are highlighted by higher oxide mineral contents and by the absence of poikilitic clinopyroxene. Boundaries between the coarser and finer regions appear to be igneous but vary in nature, some being very diffuse and, in other cases, fairly sharp, occurring over a couple of millimeters or less. Unit 88 is also distinguished by the appearance of scattered olivine as small, highly altered interstitial grains with dark, oxide-rich alteration halos. It is crosscut by a coarse-grained oxide gabbro in Section 220R-1 (Unit 88B). |
| 89 | 1439.66 | 221R-1, 6 | 19.24 | Medium-grained orthopyroxene-bearing olivine gabbro | First appearance of olivine gabbro, Section 221R-1 | Not recovered | Equigranular, subophitic olivine gabbro, occurring in three sequences (Units 89A, 89C, and 89D). Olivine mode is variable, ranging from <5% to 20%, with the highest concentrations at the base of Units 89A and 89C. Some coarser grained, locally poikilitic patches occur throughout. The unit is cut by a pair of narrow, coarse-grained, seriate oxide gabbro and oxide quartz diorite dikes and patches (Unit 89B), mineralogically similar to the diffuse network domains of Units 83–87. |
| 90 | 1458.90 | 225R-1, 0 | 24.28 | Microcrystalline–fine aphyric basalt | Not recovered | Not recovered | Unit 90 consists of fine-grained (meta)basalts very similar to those at the base of the sheeted dike interval. The igneous texture appears to have been overprinted by a granular metamorphic texture cut by several thin gabbroic and leucocratic dikes. At its lower boundary, Unit 90 is intruded by gabbronorite from Gabbro 2. Approaching the contact, secondary granular orthopyroxene appears to progressively replace clinopyroxene, suggesting that orthopyroxene formed by prograde reactions that were more intense in the boundary zone. Units 90B–90F are small narrow dikes of medium-grained quartz diorite (Units 90B, 90D, and 90E) and fine-grained tonalite (Units 90C and 90F). |
| 91 | 1483.18 | 230R-1, 18 | 1.06 | Medium-grained oxide gabbro | Significant increase in grain size in Section 230R-1 (Piece 5) | Disappearance of basaltic xenoliths in gabbro from Section 230R-1 (Piece 12) | This complex marginal unit contains medium-grained oxide gabbro, fine-grained basaltic fragments, and a coarser oxide gabbro (commonly altered) in Pieces 5, 6, and 8, adjacent to the margin. In Piece 5, this coarse gabbro cuts a large basaltic fragment, which is likely part of Unit 90A. The intrusion of gabbro into the finer grained dike material is confirmed by the crosscutting relationships. Fine-grained basaltic fragments of varying size and shape are found within this unit, and the end of the unit is defined by their disappearance. In Pieces 7 and 8, a zone of dark, fine-grained material is present. Basaltic fragments are slightly pink, implying the presence of orthopyroxene in the groundmass (cf. dike rocks in Unit 90A). The interval 49–69 cm contains abundant 1–2 mm magnetite grains and has extremely high magnetic susceptibility. Unit 91B is a 10 mm wide disseminated oxide gabbro vein that cuts Unit 91A at a high angle. |
| 92 | 1484.24 | 230R-1, 131 | 9.94 | Fine–medium gabbronorite | Disappearance of basaltic xenoliths in gabbro from Section 230R-1 (Piece 12) | Not recovered | Unit 92 is an equigranular to seriate, dominantly subophitic disseminated oxide gabbronorite. It is characterized by the presence of centimeter-sized, diffuse, equidimensional patches of coarser grain size and higher oxide content. Olivine occurs in small abundance. It is intruded by veins of diorite (Unit 92B), disseminated oxide quartz diorite (Unit 92C), and oxide diorite (Unit 92D). |
| 93 | 1494.18 | 232R-2, 20 | 0.77 | Fine–medium disseminated oxide gabbronorite | Appearance of basaltic enclaves in disseminated oxide gabbronorite of Unit 92A | Sharp contact with basalt in Piece 9 | This marginal unit is very similar to Unit 91, described at the upper contact of this Gabbro 2, containing disseminated oxide gabbronorite with fine-grained basaltic fragments (Unit 93A) and a coarser disseminated oxide gabbronorite adjacent to the lower margin (Unit 93B). The lower contact in Piece 9A is fairly sharp and sutured and shows no decrease in the gabbro grain size toward the contact. |
| 94 | 1494.95 | 232R-2, 97 | 7.64 | Microcrystalline–fine aphyric basalt | Intrusive contact with gabbronorite in Piece 9 | Not recovered | Unit 94 is orthopyroxene-rich, cryptocrystalline to fine-grained rock of basaltic composition. It is extensively recrystallized with well-developed granoblastic textures, but in some areas, especially away from its contact with Unit 93, it retains an igneous intergranular texture. It is cut by a medium-grained oxide-rich diffuse vein. |
| 95 | 1502.59 | 234R-1, 9 | 4.51 | Aphyric basalt | Not recovered | Not recovered | Cryptocrystalline aphyric basalt, which is distinguished from Unit 94 by its lighter gray color and finer grain size. It is also distinct from the other units of dike screens in its relatively low metamorphic grade and degree of alteration. |
| Rocks drilled during Expedition 335: | |||||||
| 96 | 1507.10 | 235R-1, 0 | 13.10 | Aphyric basalt | Not recovered | Not recovered | This unit is predominantly composed of fine-grained granoblastic basalt. The first 12.5 cm (Section 235R-1, Piece 1) is strongly recrystallized (Unit 96A), whereas the granoblastic rocks in the rest of the unit are completely recrystallized (Unit 96B). The unit contains small amounts of leucocratic rocks (tonalite, quartz diorite, and albitite; Unit 96C). Some of the granoblastic rocks contain centimeter-sized patches of oxide diorite (Unit 96D). |
This table is available in ASCII and in Microsoft Excel format (see 103_T04.XLS in CHAPTER_103 in TABLES in “Supplementary material”).
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