IGNEOUS PETROLOGY

Site 1189 is located at the high-temperature Roman Ruins hydrothermal site within the PACMANUS hydrothermal field on Pual Ridge. Core was recovered from two drill holes. Hole 1189A was placed in a shallow trough stretching ~5 m across between a group of active chimneys encrusted by fauna and was established to investigate the style of hydrothermal alteration underlying this high-temperature vent field. The hole was cored with the RCB system to a depth of 125.8 mbsf, with an average recovery of 6.8%.

The rocks recovered from Hole 1189A included very little material with significant sulfide mineralization. Partly for this reason, Hole 1189B was placed higher on the mound ~30 m northeast, again among the active high-temperature smokers at the Roman Ruins hydrothermal site. Hole 1189B was RCB cored from 31.0 to 206.0 mbsf with an average recovery of 7.8%. Rocks from the upper part of Hole 1189B include samples of semimassive sulfide and stockwork veining, whereas those from the lower part represent altered volcanic rocks with many similarities to those from Hole 1189A.

This chapter contains documentation of the igneous-rock types and igneous features observed in core from Holes 1189A and 1189B based upon hand specimen and thin section descriptions and supplemented with the results of XRD analyses. In the following sections, the two holes are described in turn.

Hole 1189A

The first core recovered from the uppermost part of Hole 1189A consists of unaltered, black, moderately vesicular aphyric dacite with a glassy to microcrystalline groundmass (Unit 1). The felsic composition of the unit has been confirmed by measurements of the refractive index of the glass, which indicates a SiO2 content of 66 wt%, and by shipboard chemical analysis (see "Geochemistry").

Below the first core (Core 193-1189A-1R), alteration intensity increases rapidly. Unit 2 in Core 193-1189A-2R is lithologically very similar to Unit 1 except for a gradual change in the color of the groundmass from dark gray to white with depth, indicating increasing pervasive bleaching (see "Hydrothermal Alteration"). The vesicles in Unit 2 are lined or partially filled by silica, anhydrite, and minor pyrite.

Below Unit 2, alteration is complete and identification of primary lithology is commonly difficult. However, remnant volcanic features including vesicles, flow banding, and perlitic groundmass textures have been recognized in most units.

Several units show fragmental textures that have been logged as hydrothermal breccia because conclusive evidence for a volcaniclastic origin (e.g., grading, layering, rounding, and polymict composition) is lacking. These units typically contain flow-banded clasts (Units 9, 11, 13, and 16), and locally, textural evidence indicates clast movement prior to, or as a result of, hydrothermal activity. Some units contain perlitic fragments that consisted of volcanic glass prior to alteration. Clasts with well-preserved vesicles are absent in hydrothermal breccia units.

Volcaniclastic breccias have been identified in the lower portion of Hole 1189A. The altered and mineralized Unit 21 contains abundant tube pumice and nonvesicular aphyric volcanic clasts with angular, blocky to shardlike shapes. Unit 23 is a polymict volcaniclastic breccia with aphyric clasts showing substantial differences in alteration mineralogy. These units mark paleoseafloor positions in the Pual Ridge stratigraphy.

Units with well-preserved vesicles typically show pervasive bleaching and/or silicification along quartz veins. Any brecciation associated with fracturing during alteration generated perfect jigsaw-fit textures. These features indicate that these units originally were coherent parts of lava flows that locally developed pseudoclastic textures caused by hydrothermal activity.

Hole 1189A intersected a silica-Fe-oxide-pyrite-rich jasperoidal rock with altered volcanic clasts (Unit 14) and sulfide-rich breccia (Units 18 and 21). These units are described in more detail in "Sulfide and Oxide Petrology".

A full description of the various units, including lithologic characteristics and alteration mineral assemblages, is provided in Table T2. A graphic log of Hole 1189A summarizes the distribution of units and their lithologic and alteration characteristics (Fig. F2). Detailed hand specimen descriptions of individual pieces and thin-section descriptions are available (see "Site 1189 Thin Sections").

Unit Summaries

The following is a brief summary of the lithologic units from Hole 1189A that have either well-preserved or remnant igneous features. Note that Units 3, 14, and 18 have no such features.

Unit 1 is black, aphyric, moderately vesicular, microlite-bearing, glassy dacite. Most of the vesicles are flattened as a result of flow during eruption and cooling. It is similar in appearance to fresh rhyodacite recovered from the top of Hole 1188A at the Snowcap hydrothermal site, except for the lack of phenocrysts.

Unit 2 is lithologically similar to Unit 1 but shows a gradual increase in pervasive silica-clay bleaching, which is reflected by a change in the color of the groundmass from gray to light gray to white with increasing depth.

Unit 3 is a single piece of a coarse-grained quartz-sulfate vein.

Unit 4 is a hydrothermal breccia with completely altered, green, angular perlitic volcanic clasts with a coarse, white to gray anhydrite-silica-pyrite stockwork matrix (Fig. F3). Some clasts have minor (1-2 vol%) vesicles (<1-1 mm across), which may be filled by anhydrite and/or traces of pyrite. The perlitic texture of some clasts is locally well preserved, indicating that the clasts were originally glassy volcanic fragments (Fig. F4).

Unit 5 is a highly to completely bleached, aphyric, moderately vesicular volcanic rock similar to Unit 2. Vesicles may be lined or filled by silica, zeolite, or anhydrite (Fig. F5). Locally, these minerals grow preferentially on the footwall side of the vesicles, indicating that they may be used as geopetal structures.

Unit 6 is a single piece of hydrothermal breccia, which consists of white, sulfate-rich volcanic clasts in a bluish green, clay-rich quartz-anhydrite-pyrite stockwork matrix.

Unit 7 is a pervasively bleached, light gray, sparsely vesicular volcanic rock with abundant anhydrite-quartz-pyrite veins and partially quartz-filled vesicles. Jigsaw breccia textures are prominent and clearly related to fracturing and vein formation.

Unit 8 is a light green-gray pervasively green silica-chlorite altered, moderately vesicular volcanic rock. The vesicles are generally at least partially filled by silica, which can also be observed in the groundmass, and the rock is veined by quartz-anhydrite-pyrite.

Unit 9 is a completely altered hydrothermal breccia with flow-banded volcanic clasts showing patchy to pervasive multiphase alteration (Fig. F6). The volcanic fragments are bleached but locally preserve the flow banding. This alteration is overprinted by gray-green chlorite-smectite alteration, and the last stage was quartz-pyrite veining locally forming a stockwork matrix.

Unit 10 is pervasively silica-clay altered, variably vesicular volcanic rock that has been overprinted by silicification, which is concentrated in halos along quartz-pyrite veins.

Unit 11 is a hydrothermal breccia with bluish green, silica-chlorite altered, flow-banded volcanic fragments that are cemented by fine-grained, gray silica. Textural evidence indicates that flow-banded fragments have been rotated relative to each other.

Unit 12 is pervasively silicified, sparsely vesicular volcanic rock, essentially identical to Unit 10.

Unit 13 is a hydrothermal breccia with silica-chlorite altered, flow-banded clasts in a fine-grained, gray silica stockwork matrix. This unit is very similar to Unit 11 and also shows textural evidence for relative movement of volcanic fragments.

Unit 15 is a white-tan massive volcanic rock with a fine network of silica-pyrite veins. Some pieces show relict perlitic groundmass texture in hand specimen. Minor plagioclase phenocrysts (<1 vol%) in a trachytic groundmass have been observed in thin section. The unit is intensely veined and fractured (Fig. F7). In addition, two specimens (Samples 193-1189A-8R-1 [Pieces 4 and 17]) each contain a black patch, both a few centimeters across, that exhibit an unusual coarse bladed texture. These patches may represent xenoliths of quenched mafic rock or an unusual alteration style (see below).

Unit 16 is a hydrothermal breccia with green, silica-clay altered, flow-banded volcanic fragments in a stockwork matrix of quartz and pyrite veins, which have silicified halos. Textural evidence indicates that some flow-banded fragments have been rotated and/or moved relative to each other (Figs. F8, F9). This unit is very similar to Units 11 and 13.

Unit 17 is a completely bleached, moderately vesicular volcanic rock.

Unit 19 is a completely altered and veined hydrothermal breccia with abundant jigsaw-fit textures (Fig. F10). In thin section, this unit is slightly plagioclase-phyric with a trachytic groundmass, and the veins consist of quartz-anhydrite-pyrite.

Unit 20 is a completely silicified, moderately vesicular volcanic rock. Vesicles are typically lined with quartz and pyrite. Remnant microlites in the groundmass are altered to illite and are set in a chlorite or clay matrix.

Unit 21 consists of a single piece of semimassive sulfide with abundant, white to light green, silica- and/or chlorite-altered, aphyric volcanic clasts (Fig. F11). They have angular (blocky, wispy, or polygonal) shapes and maximum diameters are in the range of 0.5-1 cm. In thin section, many of these clasts show laminar, fibrous textures, which are interpreted as the remnants of tube vesicles sufficiently abundant to warrant calling them tube pumice (Fig. F12). In addition, there are nonvesicular volcanic clasts that are about equally abundant as the tube pumice clasts. Sulfide-poor domains show a densely packed, clast-supported texture indicating that this unit is a volcaniclastic breccia or hycloclastite (Fig. F13).

Unit 22 is a pervasively bleached, moderately vesicular volcanic rock with patchy silicification. Vesicles are commonly lined with pyrite crystals.

Unit 23 is a polymict volcaniclastic breccia with aphyric, nonvesicular volcanic clasts distinguished by different styles of alteration. Green clasts are pervasively clay altered, whereas gray clasts are silicified or bleached.

Unit 24 is a pervasively bleached, moderately vesicular volcanic rock showing patchy silicification.

Volcanic Textures

Primary volcanic features have been recognized in many units of Hole 1189A, despite the hydrothermal alteration. These include vesicles, phenocrysts, perlitic texture, flow banding, and tube pumice clasts. Point counts of the thin sections of several fresh and altered volcanic rocks, which have identifiable phenocrysts and vesicles (or remnant vesicles), are given in Table T3.

Vesicles

Vesicles are common in the rock units from Hole 1189A. They are present in the fresh dacite of Unit 1 as well as in many of the altered volcanic rock units (including Units 2, 5, 7, 8, 10, 12, 15, 17, 20, 22, 23, and 24). The vesicularity of individual specimens varies from 0 to 20 vol%. The vesicle size varies from as small as several tenths of a millimeter in diameter to several centimeters (maximum dimension) in large, stretched or coalesced, vesicles. Commonly, the majority of vesicles are aligned, oblate, and tube shaped, 1 or 2 mm across, and 1 or 2 cm long. These reflect stretching and/or flattening of originally spherical vesicles in the lava as it flowed and cooled. An extreme example of stretched vesicles is represented by pumiceous volcanic clasts with abundant tube vesicles in Unit 21 (Fig. F12).

Vesicles are commonly lined with secondary minerals or completely filled to form amygdules. In one notable example, vesicles are decorated with small (0.1-0.2 mm) blocky zeolite crystals and crystal clusters (Fig. F5). Most of these zeolite crystals are on the same side of the vesicles and may define a geopetal reference. Similar features have been observed in oriented pieces from Hole 1191A, suggesting that the zeolites preferentially occupy the floor or footwall of individual vesicles.

Phenocrysts

All of the rocks sampled from Hole 1189A are aphyric, with no phenocrysts noted in hand specimen. One thin section of the fresh aphyric dacite of Unit 1 in Section 193-1189A-1R-1 is completely aphyric, whereas a thin section of bleached aphyric volcanic rock from Unit 5 in Section 193-1189A-3R-1 has just one phenocryst of plagioclase and one altered phenocryst of clinopyroxene. Thin sections also reveal very scarce phenocrysts in Units 10, 12, and 15 (Table T3).

Perlitic Texture

Volcanic clasts with well-preserved perlitic texture are hosted within an anhydrite-silica-pyrite stockwork matrix in Unit 4 (hydrothermal breccia). This texture indicates that these originally glassy fragments were hydrated prior to hydrothermal alteration. In thin section, perlitic domains are arranged in a jigsaw-fit pattern and separated from each other by linear to irregular fractures (Fig. F4). This indicates that hydrothermal fluid flow was largely independent of the fracture pattern provided by the perlitic cracks and probably more focused along fractures generated during hydrothermal activity. In contrast to Site 1188, altered volcanic rocks showing pseudoclastic textures with round to lensoidal apparent clasts (perlite kernels?; cf. Figs. F6, F7, both in the "Site 1188" chapter) have not been observed in the recovered rocks from Hole 1189A.

Flow Banding

Flow-banded volcanic fragments are common in most units consisting of hydrothermal breccia in Hole 1189A. In general, these units have been overprinted by several stages of alteration, and remnant flow-banding textures are preserved as alternating white to light gray and gray-green linear to wavy domains (Figs. F8, F9). In thin section, flow-banded clasts consist of micro- to cryptocrystalline minerals with low birefringence (silica, sulfate, zeolites, fine clay?), and the dark domains are enriched in a dark mineral that is too fine to be resolved under the microscope (Fig. F14). The boundaries between light and dark groundmass are poorly defined, and in detail dark domains consist of irregular, black patches that are loosely aligned. We infer that hydrothermal alteration preserved the flow banding because of primary textural and/or mineralogical differences between the bands, whose original nature remains uncertain.

Volcaniclastic Textures

Fragmental textures are abundant in the altered volcanic rocks recovered from Hole 1189A. However, because of the textural and mineralogical overprint of alteration, it is difficult to discriminate between hydrothermal breccia and rocks that consisted of volcanic clasts prior to alteration. In the absence of observations regarding contact relationships or internal organization (such as grading) caused by limited recovery, most fragmental units with monomict composition have been logged as hydrothermal breccia. However, several hydrothermal breccia units contain flow-banded volcanic fragments and show textural evidence for relative movement of individual clasts (see below). Despite these limitations, two volcaniclastic breccia units have been recognized in Hole 1189A marking paleoseafloor positions.

Altered and Mineralized (Tube Pumice-Bearing) Volcaniclastic Breccia

Unit 21 is a semimassive sulfide with abundant silica and/or chlorite altered volcanic clasts. There are about equal proportions of nonvesicular clasts and tube pumice with abundant fibrous laminar textures representing the former tube vesicles (Fig. F12). These delicate features have been preserved by an early (diagenetic?) stage of alteration, which generated a fine, dark brown lining on the inner vesicle walls. The central parts of the vesicles are filled with the same material (silica and/or chlorite) that replaced the originally glassy groundmass. The volcaniclastic origin of this unit is indicated by the polymict composition of the clast population and the chaotic, densely packed, clast-supported texture in sulfide-poor domains. The angular, platy to wispy shapes of the volcanic clasts suggest that they were generated by quench fragmentation and deposited by settling or mass-flow processes not far from their source.

Polymict Volcaniclastic Breccia

Unit 23 consists of densely packed, aphyric, nonvesicular clasts that are clay-altered, silicified, or bleached. Because individual clasts show distinctive types of alteration, it is inferred that they were derived from different source areas and mixed together during mass flow transport.

Hydrothermal Breccia with Evidence for Clast Movement

Several units with abundant flow-banded volcanic fragments have been logged as hydrothermal breccias (Units 9, 11, 13, and 16). These units experienced several stages of pervasive to vein-controlled alteration causing complex mineralogical and textural modifications. In general, most remnant volcanic fragments are arranged in a jigsaw-fit pattern and, therefore, may have been generated during hydrothermal activity. However, textural observations indicate that at least some differential movement occurred in these units, because adjacent clasts show internal flow banding with random orientations (Figs. F8, F9). This observation may be interpreted as evidence for a volcaniclastic origin of these units or for brecciation and clast movement in void fractures during hydrothermal activity.

Hydrothermal Breccia with Pseudoclastic Textures

In several units logged as hydrothermal breccia, the volcanic fragments are separated by <<1-cm-wide veins and are arranged in a jigsaw-fit pattern (Figs. F7, F10). This indicates that this fragmental texture is related to fracturing and alteration of coherent volcanic rocks during hydrothermal activity generating pseudoclastic textures. In several units it has been observed that alteration (typically silicification) is concentrated in halos along the veins. Locally, remnants of groundmass encircled by these halos may be misinterpreted as angular to irregular clasts if not carefully examined (Fig. F10). Similar textures have also been commonly observed at Site 1188 (Snowcap hydrothermal site), suggesting that pseudoclastic textures are a common product of hydrothermal alteration processes.

Black Patches in Unit 15

Two black patches revealed after cutting core of Unit 15, each ~2-3 cm across, possess unusual textures. The patches are roughly equant and angular and appear to be xenoliths in a fine-grained volcanic unit with scant plagioclase phenocrysts. Their interiors contain long pseudomorphs (1-2 cm) after acicular crystals, whereas their rims comprise a millimeter-wide finer grained zone.

Thin sections show that the cores of the patches consist mainly of a chloritic groundmass hosting faint pseudomorphs of medium-grained acicular crystals, which are now altered to anhedral feldspar and/or quartz grains, and lineations of opaque material (now magnetite, hematite, and pyrite) (Fig. F15). There are also two medium-grained relict felsic phenocrysts in the thin section from Sample 193-1189A-8R-1 (Piece 17, 103-105 cm) (Fig. F16). The millimeter-scale rim zones of the patches contain slightly smaller altered acicular crystals and a groundmass of plumose, skeletal (hollow) feldspar crystals (Fig. F17). The microlites in the volcanic host material of Unit 15 conform to the edge of the patch (Fig. F17).

The patches contain significant sulfide mineralization, and they are cut by quartz veins. This alteration style is similar to that observed in the host felsic volcanic rock of Unit 15. The groundmass of the patches also contains a minor pleochroic green to yellow mineral with high-birefringence resembling either well-crystallized celadonite/glauconite or amphibole (actinolite or hornblende).

Interpretation

The volcanic rocks near the top of Hole 1189A are fresh dacites based upon the SiO2 content of their glass. They are aphyric, containing only traces of small plagioclase and clinopyroxene phenocrysts. Altered volcanics deeper in the cored section are similar to the fresh ones in terms of their phenocryst content and are probably altered equivalents of dacite.

The dacites from Hole 1189A are distinguished from the rhyodacites of Hole 1188A by the silica concentration of their glasses and by the fact that the Hole 1188A rhyodacites are sparsely phyric, typically with 2%-3% plagioclase, 1% clinopyroxene, and 0.5% magnetite phenocrysts (see "Igneous Petrology" in the "Site 1188" chapter). This suggests that magma differentiation from 66 to 72 wt% SiO2 (i.e., from dacite to rhyodacite) may have been accomplished by low-pressure fractional crystallization of plagioclase, clinopyroxene, and magnetite deeper within the crust.

Black patches occurring within felsic volcanic host rock of Unit 15 in Core 193-1189A-8R probably represent quenched, originally mafic material. The patches possibly represent xenoliths of mafic material ripped off the chilled margin of a dike or sill somewhere deeper within or beneath Pual Ridge.

Hole 1189B

The first 31.0 mbsf at Hole 1189B was not cored. Instead, the hole was spudded directly into a high-temperature chimney field using a hammer-in casing system. Two sources of information are relevant to the lithostratigraphy of this upper part of the hole. First, the hammer drill encountered very hard rock at depths of 7-8 mbsf and again at 10-15 mbsf (see "Introduction"). These intervals probably correspond to volcanic rocks similar to the resistant volcanic units sampled at Sites 1190 and 1191 or near the top of Site 1188 (recovered in Hole 1188A). Second, the uppermost 6 m, the material between 8 and 10 mbsf, and that between 15 and ~30 mbsf, was relatively soft and nonresistant. The suspicion that these intervals represent, at least in part, massive sulfide deposits was reinforced when the hammer drill tool was retrieved and found to have sand-sized sulfide debris stuck inside.

The first rock recovered by coring is semimassive sulfide with minor altered volcanic clasts (Unit 1 in Core 193-1189B-1R-1). This was followed by a succession of completely altered coherent volcanic rock units alternating with various types of brecciated units (Fig. F18). From ~31 to 128 mbsf (curated depth), lithologies alternate between vesicular (or amygdaloidal) volcanic rocks (Units 2, 7, 9, 11, 13, and 15) and hydrothermal breccias (Units 3-6, 8, 10, 12, 14, 16, and 17). The breccias, particularly in Units 3, 4, and 5, are altered volcanic rocks cut by stockwork veins with a structure transitional to breccia. In fact, these units are distinguished primarily on the basis of the stockwork mineralogy rather than the original igneous protolith.

Unit 18 at 128 mbsf is a polymict volcaniclastic breccia and is underlain by a sequence of highly altered vesicular volcanic rocks alternating with flow-banded and perlitic volcanic rocks that exhibit, to varying degrees, both pseudoclastic textures and poorly sorted, ungraded, true clastic textures. The latter are typically manifested as subangular, grain-supported, poorly sorted breccias of flow-banded clasts, rotated with respect to each other and, in at least one case, intruded by coherent, flow-banded lava. These rocks are interpreted as in situ and/or resedimented autoclastic breccias that formed on the margin of lava flows during extrusive eruptions on the seafloor.

Volcaniclastic sandstone with sedimentary grading and petrographically diverse rock fragments forms Unit 31 at 186 mbsf, and polymict volcaniclastic breccia forms Unit 35 at 197 mbsf. These units provide further evidence of paleoseafloor surfaces sampled by the drill core at Site 1189.

A full description of the various units including lithologic characteristics and alteration mineral assemblages is provided in Table T4. A graphic log of Hole 1189B summarizes the distribution of units and their lithologic and alteration characteristics (Fig. F18). Detailed hand specimen descriptions of individual pieces and thin-section descriptions are available (see "Site 1189 Thin Sections").

Unit Summaries

Unit 1 is a semimassive sulfide with minor volcanic rock clasts. The matrix is fine-grained pyrite-chalcopyrite-anhydrite-quartz, whereas the sparsely vesicular volcanic clasts (2-3 mm in maximum dimension) are completely altered to clay, silica, and anhydrite.

Unit 2 is a completely altered (green clay and silica), moderately vesicular, aphyric volcanic rock with ~10% elongated and aligned vesicles that are lined by pyrite.

Units 3, 4, and 5 are matrix-supported hydrothermal breccias of completely altered (green clay and silica) volcanic clasts in a stockwork vein matrix. The angular to blocky clasts (up to 1 cm in maximum dimension) are generally arranged in a jigsaw-fit pattern, and some show preserved flow banding or perlitic groundmass texture. In Unit 3, the stockwork matrix consists of pyrite-anhydrite veins, which alternately cut and parallel the flow banding present in some clasts. In Unit 4, the vein matrix is pyrite-quartz, which forms a stockwork of semimassive sulfide. Unit 5 has a stockwork that is jasperoidal (pyrite + hematite + quartz), which is distinct from the otherwise similar Unit 4. The perlitic clasts in Unit 5 have minor vesicles and rare plagioclase phenocrysts, which are locally truncated at the clast margin.

Unit 6 is a small piece of vuggy massive sulfide, composed of 90% very fine grained euhedral pyrite, 9% quartz, 1% anhydrite, and traces of chalcopyrite and red-brown sphalerite. This is possibly a fragment of stockwork vein.

Unit 7 is a completely altered, moderately amygdaloidal rock with soft clay-rich alteration and <1-mm-diameter quartz-filled amygdules.

Unit 8 is a hydrothermal breccia of completely altered, matrix-supported volcanic clasts in an anhydrite-pyrite-quartz stockwork vein matrix (Fig. F19). Rock clasts (up to 1 cm in maximum dimension) are generally green clay altered. Some are flow banded and some contain minor <1-mm-diameter quartz-filled amygdules. Unit 8 is very similar to Units 3, 4, and 5.

Unit 9 is a completely altered, slightly vesicular volcanic rock. The rock is completely altered (silica, clay, and sulfate bearing), with patchy silicification and both pyrite and anhydrite in vesicles.

Unit 10 is a hydrothermal breccia of completely altered volcanic clasts in a pyrite-quartz matrix. The altered fragments, up to 1 cm in size, are either green, soft, and clay-rich or tannish and silicified.

Unit 11 is a completely altered, silicified, moderately vesicular volcanic rock with between 1 and 8 vol% ovoid vesicles (up to 1 mm). Some vesicles are lined by quartz, which is overgrown by pyrite.

Unit 12 is a hydrothermal breccia of completely altered nonvesicular volcanic clasts, which locally show remnant flow banding, in a pyrite-quartz stockwork matrix. This unit is very similar to Unit 4.

Units 13 through 15 are coherent volcanic rocks. Unit 13 is a silica-clay altered, moderately amygdaloidal, aphyric volcanic rock (Fig. F20); Unit 14 is a silica-chlorite altered, spherulitic, and flow-banded aphyric volcanic rock (Fig. F21); and Unit 15 is a completely altered, silicified, moderately vesicular, aphyric volcanic rock.

Units 16 and 17 are hydrothermal breccias of completely altered volcanic rock clasts. In Unit 16, the 1-cm clasts are flow banded and exhibit random rotations with respect to each other. The breccia matrix is siliceous ± magnetite bearing. Unit 17 is distinguished by a jasperoidal matrix containing quartz and hematite ± magnetite.

Unit 18 is a polymict volcaniclastic breccia of completely altered, <1-cm volcanic clasts, which are green-clay altered (locally flow banded), white-clay altered, or gray and siliceous. The matrix is quartz ± pyrite and magnetite.

Unit 19 is highly altered, moderately vesicular, aphyric volcanic rock. Highly stretched, steeply oriented vesicles and abundant (25 to 40 vol%) plagioclase microlites, as seen in thin section, distinguish this unit. Furthermore, several black xenoliths (1-2 cm) have been observed.

Unit 20 is a volcaniclastic breccia with abundant, clay-altered volcanic clasts in a quartz-anhydrite ± pyrite matrix. Clasts are mainly perlitic or flow banded; however, locally amygdaloidal, trachytic, and pumiceous clasts have been observed in thin section.

Units 21 and 22 are aphyric volcanic rocks. Unit 21 is completely silica-clay altered, massive, and nonvesicular, whereas Unit 22 is silicified, mineralized, and sparsely vesicular.

Unit 23 is a volcaniclastic breccia of green- and white-clay altered, commonly flow-banded, volcanic clasts in a quartz-anhydrite matrix (Fig. F22). The clasts, commonly rotated with respect to each other, are in one specimen intruded by flow-banded lava (Fig. F23), which is interpreted as an exceptional example of a coherent lava intruding its own autoclastic breccia at the flow margin.

Unit 24 is a completely altered, highly silicified, sparsely vesicular, aphyric volcanic rock. Vesicles are elongated and lined with quartz and pyrite, followed by anhydrite.

Unit 25 is a poorly sorted, clast-supported, volcaniclastic breccia of completely altered (silicified and green- and white-clay altered), flow-banded volcanic rock. The unit is partly coherent and partly (auto)clastic with abundant rotated clasts (>5 cm in maximum dimension) (Fig. F24). Locally, alteration along, and outward from, a network of microfractures has overprinted the flow banding generating a nodular, pseudoclastic texture (Fig. F25). In one sample, flow banding wraps around a black, vesicular xenolith (Fig. F25).

Units 26 through 28 are variably vesicular, aphyric to sparsely plagioclase phyric volcanic rocks. Unit 26 is sparsely vesicular and sparsely porphyritic volcanic rock with up to 2 vol% of small (2 mm) plagioclase phenocrysts (laths or tabular to rounded shapes). It is silicified and contains a fine network of quartz (-pyrite-magnetite) veins that network imparts a pseudoclastic texture locally (Fig. F26). Unit 27 is very highly altered (silicified), moderately vesicular, and contains trace amounts of fresh plagioclase phenocrysts (up to 2 mm in maximum dimension, <1 vol%). Vesicles (up to 2 cm; round to lensoidal) are lined by quartz and anhydrite, and the groundmass has an alteration texture resembling hieroglyphics (Fig. F27). Unit 28 is intensely silicified and moderately vesicular aphyric volcanic rock. Vesicles are lined by quartz ± pyrite.

Unit 29 is a volcaniclastic breccia of completely altered (silicified and green-clay bearing), flow banded clasts. Some clasts show folded flow banding, and in one clast, flow banding is wrapped around a black xenolith.

Unit 30 is a silicified, flow-banded, aphyric volcanic rock. Flow banding is wrapped around a xenolith of black volcanic rock with a spinifex-like texture in one piece. Locally, fine, fresh plagioclase phenocrysts are present (up to 1 vol%).

Unit 31 is a graded volcaniclastic sandstone and represents a sedimentary unit deposited on a paleoseafloor. It contains a variety of volcanic fragments (generally <1 mm in maximum dimension) including porphyritic, perlitic, and glassy clasts, indicating that it was derived from a heterogeneous source area.

Unit 32 is a flow-banded and silicified aphyric volcanic rock with a fine network of quartz-pyrite veins.

Units 33 and 34 are completely altered hydrothermal breccia consisting of flow-banded volcanic rock with a quartz (-hematite-pyrite) matrix. The two units are nearly identical, except that Unit 33 contains fresh plagioclase phenocrysts (<1 vol%, up to 1 mm), whereas Unit 34 is aphyric and shows a prominent nodular pseudoclastic texture caused by alteration along fine veinlets.

Unit 35 is a completely altered (green clay and silica), poorly sorted, clast-supported polymict breccia with flow-banded, spherulitic, and perlitic clasts in a quartz (-hematite-pyrite) matrix. Clasts with folded flow banding truncated at the margin (Fig. F28) indicate abrasion of clasts during particle transport. This observation indicates that Unit 35 represents a resedimented volcaniclastic breccia deposit.

Unit 36 is a completely altered (green clay and silica) hydrothermal breccia consisting of flow-banded volcanic clasts with a quartz (-pyrite) matrix. Textural observations indicate that whereas most of the rock was fragmented in situ, some clasts have been rotated.

Volcanic Textures

Several primary volcanic features have been recognized from Hole 1189B. These include phenocrysts, vesicles and amygdules, perlite, flow banding, spherulites, and xenoliths. In addition, there are several types of clastic rocks (including monomict and polymict volcaniclastic breccia, volcaniclastic sandstone, and hydrothermal breccia) and units with pseudoclastic texture.

Phenocrysts

The dominant phenocryst type observed in rocks from Hole 1189B is plagioclase, as it is at Site 1188 and Hole 1189A. In Hole 1189B, plagioclase phenocrysts (<1-2 vol%; up to 2 mm) are present mainly below ~125 mbsf (Units 19, 21, 23, 26, 27, and 35). These units in the lower half of the hole are characteristically less altered than rocks in the upper part. However, there are also plagioclase phenocrysts (from trace to 2 vol%) in several altered clasts in the shallow stockwork Units 5 and 8 at around 80 and 90 mbsf.

The plagioclase phenocrysts reach ~2 mm in length and are typically lath-shaped with highly rounded edges (Fig. F29). This texture suggests that the crystals underwent a corrosive dissolution prior to eruption of the magma. In several cases, the rounded phenocrysts coexist with acicular phenocrysts that are less rounded and with microphenocrysts that are euhedral.

In addition to plagioclase, several rocks studied in thin section contain minor, altered clinopyroxene phenocrysts, partially or completely replaced by clay minerals (Fig. F30). Traces of clinopyroxene phenocrysts were noted in Units 19, 25, 26, and 27. Likewise, a very few magnetite phenocrysts were noted in Unit 26, where they reach 0.25 mm across and may be free-floating in groundmass or included inside plagioclase and clinopyroxene.

Vesicles and Amygdules

The maximum vesicularity observed in volcanic rocks from Hole 1189B is 20 vol% in Units 15 and 19, 15 vol% in Unit 27, and 10 vol% or less in Units 1-3, 9, 11, 13, 16, 18, 22-24, 26, and 30. Volcanic rocks exhibiting flow banding are generally not vesicular. Filled vesicles (amygdules) are common and are generally lined or filled by quartz (Fig. F31), followed frequently by pyrite, sphalerite (Fig. F32), or anhydrite.

Many vesicles and amygdules are nonspherical, reflecting stretching or flattening in response to plastic flow. The vesicles from Unit 19 are striking in this regard not only because they are highly stretched, but also because the stretching direction is uniformly steep (between 70° and 90°; see "Structural Geology"), indicating subvertical laminar flow of the lava. Consequently, Unit 19 may represent a magma feeder zone within a syngenetic lava dome or older volcanic rocks.

Perlite

Well-preserved to remnant perlitic texture, indicating that the volcanic groundmass consisted of volcanic glass that became hydrated prior to hydrothermal alteration, has been observed in Units 3, 5, 20, 21, 23, and 31. These units represent a spectrum of volcanic rock types including aphyric volcanic rock, hydrothermal breccia, volcaniclastic breccia, and volcaniclastic sandstone.

Remnant perlite is present in the altered groundmass of a completely altered, aphyric massive rock (Unit 21) where the arcuate cracks have been preserved despite the replacement of the glassy volcanic groundmass by quartz, chlorite, and clay minerals (Fig. F33).

Perlitic clasts are a common component of volcaniclastic breccia (Units 20 and 23). The originally glassy groundmass has been replaced dominantly by microcrystalline quartz, and the fine arcuate domains defining the perlitic cracks are occupied by chlorite and black cryptocrystalline material (Fig. F34). Perlitic clasts with very fine, stretched, and aligned chlorite amygdules are present in Unit 20 (Fig. F35). These clasts were initially glassy and vesicular and probably represent tube pumice fragments.

In hydrothermal breccia (Units 3 and 5), perlitic texture is preserved in the central parts of clay-rich volcanic clasts that are hosted in a stockwork matrix of variable mineralogy (jasperoidal in Fig. F36). In Unit 3, perlitic clasts are hosted in an anhydrite and gypsum-rich stockwork matrix. Textural evidence indicates that these clasts are locally consumed and replaced along their margins during hydrothermal alteration in this unit (Fig. F37).

In volcaniclastic sandstone (Unit 31), angular submillimeter perlitic clasts are also important components (see below).

Flow Banding

Flow banding is a common feature in many units of Hole 1189B (Units 3-5, 8, 10, 12, 14, 16-18, 20, 23, 25, and 29-36); however, it is best preserved in units below ~120 mbsf. Some of the flow-banded units appear to be coherent lava (Units 14, 30, and 32) (Fig. F21), whereas others are clearly of volcaniclastic origin (Units 18, 20, 23, 25, 29, and 35) (Figs. F22, F24). It seems most likely that there are gradational contacts between brecciated and nonbrecciated flow-banded rocks grading from coherent facies to brecciated facies with jigsaw-fit texture to chaotic, poorly sorted, monomict breccia facies. A synvolcanic intrusion of flow-banded lava into brecciated facies consisting of cogenetic flow-banded clasts has been observed in Unit 23 (Fig. F23). A mixture of gradational and synvolcanic intrusive contact relationships between coherent and clastic facies (autoclastic and/or hyaloclastic) is characteristic for felsic lavas and has been described from several submarine examples (see "Igneous Petrology" in the "Explanatory Notes" chapter).

In most units, flow banding is defined by fine, linear, alternating bands (generally <1 to 2 mm in width and continuous for as much as 5 cm) of pale gray and light green color. In Unit 14 (Fig. F21) it is apparent from hand specimen inspection that the light gray bands consist of aligned, small, white concentric spherules. The dark groundmass contains isolated spherules or nodular groups of spherules. This observation is confirmed by thin-section examination. The spherules (0.2 to 0.3 mm in diameter) consist of light gray to brown clay and have coalesced to form linear structures with bulbous outer margins (Fig. F38). The central part of these necklacelike domains consists of very fine grained quartz. In other units, the flow banding is defined by similar structures; however, individual spherules are <0.1 mm and can only be resolved in thin section (Fig. F39). The dark bands between these linear domains consist of chlorite and/or dark brown clay minerals and contain minor, isolated spherules.

This type of flow banding is best interpreted as a result of high-temperature devitrification of lava generating spherulites. However, they lack the typical, spherulitic internal texture of radiating quartz and feldspar needles. This might be caused by a subsequent recrystallization of the original material during alteration. The dark bands are interpreted to represent the remnant volcanic glass that was left between the devitrified bands. The glass has been completely altered to chlorite and other phyllosilicates.

Flow banding has also been observed in clasts of Unit 20, which, however, lack the banded texture defined by aligned and coalesced microspherulites. Instead, the clasts contain fibrous, wispy linear domains consisting of very fine grained black minerals (Fig. F40). Possibly, this type of banding is a result of Fe oxide precipitation in very fine, stretched vesicles during an early stage of alteration.

Spherulites

Well-preserved spherulites have been observed in several clasts of a polymict breccia (Unit 35). These consist of densely packed, radiating aggregates of quartz and feldspar, which are impinging on each other (Fig. F41). Individual spherulites have diameters between 0.2 and 0.3 mm and are locally replaced by patches of micropoikilitic quartz.

Xenoliths

Small centimeter-scale xenoliths have been noted infrequently in Units 11, 19, and 25-30. Some are completely altered soft green clay xenoliths of unknown origin. Others are dark gray, green, or black. The xenoliths are commonly rounded and exhibit a halo or reaction rim to the host rock in hand specimen. When the host rock is flow banded, the lamination wraps around the xenoliths. Some xenoliths have acicular crystals several millimeters long and arranged in plumose arrays, resembling a spinifex texture. Xenoliths similar to these were described from Site 1188.

In thin section, most xenoliths have fine-grained intergranular or intersertal textures, commonly with acicular, skeletal, plumose, or variolitic plagioclases suggesting a quench texture (Fig. F42). In one case, the variolitic plagioclase is concentrated at the margin of the xenolith and extends into the host volcanic matrix (Fig. F43). That same xenolith contains a talc(?) pseudomorph of a mafic phenocryst, possibly olivine, which has a euhedral brown spinel inclusion embedded within it.

The xenoliths are interpreted to represent fragments of rock entrained by moving magma or lava, either within the crustal plumbing system or during eruption at the seafloor, respectively. It may be significant that the unit with the most xenoliths (i.e., three xenoliths noted in the igneous logs) is Unit 19, which has the uniformly steeply oriented stretched vesicles and might be interpreted as a dike or part of a magma feeder system.

Clastic Textures

There are abundant units with fragmental textures in Hole 1189B, which represent a range from volcaniclastic breccia and sandstone to hydrothermal breccia and coherent volcanic rock with superimposed pseudoclastic texture caused by alteration. These features are similar to Hole 1189A; however, because of the lower intensity of hydrothermal alteration, discrimination of "true" clastic texture from alteration-related, apparent clastic texture was possible for most units in the lower part of Hole 1189B. Textural evidence for a volcaniclastic origin includes polymict composition (Units 18, 20, and 30), abrasion of clasts (truncated phenocrysts, vesicles, amygdules, and flow banding; Units 18, 20, 23, 25, and 35) and rotation of large (>2 cm) flow-banded clasts (Units 18, 20, 23, 25, 29, and 35).

Polymict Volcaniclastic Breccia

Distinctive clast types have been recognized in Units 18, 20, and 35 (Table T4), indicating heterogeneous sources for these deposits. Distinctive, aphyric siliceous and clay-rich clasts in Unit 18 indicate that clasts were derived from source areas that were not only already altered, but which possessed different styles of alteration. Unit 35 contains aphyric, perlitic, flow-banded, and spherulitic clasts, and Unit 20 consists of a variety of clasts including perlitic, flow-banded, trachytic, amygdaloidal, and pumiceous fragments. Whereas Units 18 and 35 may have been derived from a single lava flow with variable groundmass textures and different styles of alteration, it is evident that clasts in Unit 20 are derived from a variety of parent rocks. Nevertheless, it is beyond any doubt that these units must have been emplaced by sedimentary, mass-flow processes and, therefore, represent paleoseafloor positions.

Graded Volcaniclastic Sandstone

Unit 31 is a thinly laminated, graded volcaniclastic sandstone and contains a variety of angular volcanic fragments including glassy, perlitic, and porphyritic clasts (Fig. F44). The maximum particle size is ~3 mm at the base of individual laminae, rapidly grading to <0.5 mm in the upper parts. The grading, if it is normal (the piece is unoriented), is evidence for deposition of the unit from dilute mass flows rather than by suspension settling, and therefore, it probably represents the distal equivalent of polymictic breccia units.

Monomict Volcaniclastic Breccia

Units 23, 25, and 29 are monomict, clast-supported, poorly sorted breccia with flow-banded clasts. There is abundant evidence for rotation of large (>5 cm) clasts (Fig. F24), and a synvolcanic intrusion of coherent flow-banded lava into the clastic facies has been observed in Unit 23 (Fig. F23). Based on these textural characteristics, we interpret these units as the clastic facies of lava (autobreccia and/or hyaloclastite) formed on the outer margin of the flow.

Hydrothermal Breccia

Clastic units with monomict composition, matrix-supported texture, and prominent jigsaw-fit arrangement of clasts are interpreted as hydrothermal breccia. Transport and rotation of clasts are also consistent with hydrothermal brecciation, however, only to limited degree because of the spatial constraints. Units 3-5, 8, 10, 12, 16, 17, 33, 34, and 36 have been logged as hydrothermal breccia in Hole 1189B. Unit 26 is an intensely veined volcanic rock and contains one zone were volcanic fragments (as much as 1 cm) were incorporated in an exceptionally large fracture (Fig. F45). These clasts are rotated with respect to each other and more intensely altered than the groundmass of the surrounding volcanic rock. This zone is an exceptional example of hydrothermal brecciation.

Pseudoclastic Texture

Prominent apparent clastic textures, which are clearly related to hydrothermal alteration yet do not result in clast rotation or relative movement, have been observed in several units. Examples include Unit 26, which is intensely veined with some parts of the. volcanic rock showing a clastic texture with prominent jigsaw-fit arrangement of the fragments (Fig. F26). Also, alteration along and outward from an irregular network of fine veins has produced a nodular apparent clastic texture in Unit 25, which is overprinting the flow banding of the rock (Fig. F25). The hieroglyphic groundmass texture of Unit 27 may also be mistaken for a fine-grained, clastic texture because the white domains resemble shard-shaped fragments (Fig. F27). However, these domains are never in direct contact with each other and the abundant vesicles are unequivocal evidence that this unit is a coherent volcanic rock. It is inferred that this texture is caused by an incomplete pervasive alteration and that the white domains represent remnants of the unaltered (or less altered) volcanic groundmass.

Summary

Holes 1189A and 1189B provide a first-order understanding of the volcanic origin of the upper 200 m of Pual Ridge at the Roman Ruins high-temperature hydrothermal site. The only fresh rocks were recovered in the first core from Hole 1189A, which is located a short distance away from the focus of the high-temperature venting. These fresh rocks are dacite and are completely aphyric. All of the other samples from Site 1189 exhibit varying degrees of hydrothermal alteration, ranging from high (40%-80%) to complete (95%-100%). Nevertheless, abundant evidence for extrusive volcanic features shows that the whole sequence is either volcanic or volcaniclastic in origin.

Whereas the fresh dacite at the top of Hole 1189A is aphyric, many coherent volcanic units from both Hole 1189A and 1189B contain sparse phenocrysts, both fresh and replaced by secondary minerals. Plagioclase, or pseudomorphs thereof, represent the most common phenocryst type, whereas clinopyroxene (almost always replaced) and magnetite are only infrequently noted. Fresh plagioclases, especially in Hole 1189B, are characteristically rounded, suggesting a period of phenocryst instability and dissolution prior to or during eruption.

Many coherent volcanic units from Site 1189 are vesicular, with the degree of vesicularity ranging up to ~20 vol%. Hydrothermal alteration has commonly resulted in vesicle filling by secondary minerals, resulting in an amygdaloidal texture. Other coherent units contain spherulites, formed as a result of high-temperature devitrification of glass, and/or perlitic groundmass textures, formed during low-temperature hydration of volcanic glass.

One moderately vesicular rock unit from Hole 1189B (Unit 19, from ~138 mbsf) is noteworthy for its highly stretched vesicles, all of which have steep stretching orientations ranging from ~70° to 90°, indicative of subvertical flow. This unit represents ~10-12 m of coring and could have obtained its unusual oriented structure during eruption at the seafloor or alternatively by synvolcanic intrusive emplacement. The bounding lithologies (Units 18 and 20) are both polymict volcaniclastic breccias, which have been interpreted to represent paleoseafloor deposits. Consequently, one could envision a tectonically disrupted seafloor with a veneer of polymict breccia. A vesicular lava flow cascades over a 10-m fault scarp, and a new polymict breccia eventually develops on top of it. These units are subsequently covered by younger extrusive volcanic rocks. The alternative explanation requires a dikelike body with an average dip of 80° to have been sampled by the drill core. With a 12-m vertical extent, the calculated dike width would be 2.08 m. Unfortunately, with no contacts recovered, these alternatives can not be tested simply.

The lithologic succession at Hole 1189A is composed of alternating coherent volcanic rock units and brecciated units. One obvious paleoseafloor position, a polymict volcaniclastic breccia, was sampled at a depth of ~125 mbsf.

The lithologic succession at Hole 1189B is more complicated. The first 31 m was drilled, not cored, yet it is reasonably certain that there are massive sulfides at the seafloor, hard-rock units at 7-8 and 10-15 mbsf, and either soft, highly altered rocks or massive sulfides from 0-7, 8-10, and 15-31 mbsf. In the cored part of Hole 1189B, below 31 mbsf, the rocks recovered alternate between coherent volcanic units and brecciated units. Above ~70 mbsf, many of the breccias have stockworklike veining. Below 70 mbsf, and particularly below 137 mbsf (beginning with Unit 19), the volcanic rocks are, on average, less altered and more plagioclase rich. Paleoseafloor positions are evident at ~135, 150, 193, and possibly 82 mbsf. This spacing places a broad upper limit on the thickness of eruptive units at Pual Ridge of ~50 m. Thus, the ridge was not built up in a single or even two or three very thick eruptions. No lower limit to the thickness of individual eruptive units emerges from the core data because all of the paleoseafloor indicators may not have been sampled in these fairly low recovery drill holes.

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