IGNEOUS PETROLOGY

Site 1188 is located at the low-temperature diffuse-flow Snowcap hydrothermal site within the PACMANUS hydrothermal field on Pual Ridge. Core was recovered from two drill holes. Hole 1188A was cored with the RCB system to a depth of 211.6 mbsf. Recovery averaged 10.4%. Hole 1188F, located ~30 m away from Hole 1188A, was drilled to 218.0 mbsf, where coring commenced with the ADCB to a total depth of 386.7 mbsf. The 168.7-m cored interval produced 30.89 m of core, corresponding to a recovery rate of 18.3%, which is considerably higher than that of the RCB system.

The descriptions of the igneous rock types and igneous features of altered rocks as observed in core from Holes 1188A and 1188F are based upon hand specimen and thin-section descriptions supplemented with the results of XRD analyses.

The upper part of Hole 1188A (down to ~40 mbsf) consists of fresh to slightly (or moderately) altered, black rhyodacite that has been divided into three units based on groundmass texture and phenocryst abundance. The first unit (Unit 1) is moderately plagioclase-clinopyroxene-phyric and moderately vesicular with a glassy to microlitic groundmass. Measurements of the RI on fresh glass indicate an SiO₂ content of ~72 wt% (according to a calibration of this method using volcanic glass of andesitic to rhyodacitic composition from the Pual Ridge) (see Fig. F3 in the "Explanatory Notes" chapter). This unit appears to extend to a depth of 34 mbsf. However, recovery was <1% in this unaltered part of the sequence, so more than one flow might be represented in Unit 1. Aphyric rhyodacite underlies Unit 1 and can be divided into a unit with a perlitic groundmass texture (Unit 2) and a moderately vesicular unit (Unit 3) with a glassy to microlitic groundmass. A rhyodacitic composition was again indicated by the results of RI measurements on Unit 3.

Alteration intensity increases sharply at a depth of ~40 mbsf, and the definition of individual units was governed by an integrated approach of recognition of primary textural features of the rocks and changes in alteration mineralogy and texture. Some units show fragmental textures that have been described as jigsaw breccia or pseudoclastic textures. These are interpreted as resulting primarily from alteration outward from fractures and/or brecciation during hydrothermal activity. However, in several units there is unequivocal evidence that at least some fragments have moved or rotated, which may indicate that the units are primary volcaniclastic rocks (hyaloclastite?). In other units, alteration is pervasive, leading to almost homogeneous replacement of the groundmass by silica-sulfate-clay alteration referred to as "bleaching" because the groundmass is light gray to white (see "Hydrothermal Alteration"). The primary vesicles are usually preserved in bleached rocks but commonly are lined or filled by alteration minerals.

The primary composition of the altered volcanic units can generally not be determined petrographically, because phenocrysts or their pseudomorphs are absent or unrecognizable except in thin section. Most lithologic units of Hole 1188A are identified as "volcanic rock" because remnant volcanic textures including vesicles, phenocrysts, perlite, and flow banding are common; however, because of the effect of hydrothermal alteration, the composition of the igneous precursor is difficult to constrain. It is likely that the rocks represent altered equivalents of aphyric dacite or rhyodacite equivalent to Units 2 and 3 because a gradual increase in alteration intensity has been observed (see "Hydrothermal Alteration"). Geochemical data, especially the immobile element ratio Zr/TiO₂, provide independent evidence to substantiate this interpretation (see "Geochemistry").

An interval with volcaniclastic pebble breccia (Units 12 and 14) separates the upper volcanic rocks (Units 1 to 11) from a lower part consisting of variably altered volcanic rocks (Units 16 to 22). Units 23 through 26, recovered from the lower part of the drill hole, possess no remnant volcanic textures and were logged using descriptive terminology purely based on alteration mineralogy.

A full description of the various units, including lithologic characteristics and alteration mineral assemblages, is provided in Table T2. A graphic log of Hole 1188A summarizes the distribution of the units and their lithologic and alteration characteristics (Fig. F4). The modal proportions of groundmass, vesicles, and phenocrysts in Units 1, 17, 18, and 22 were determined by point counting (Table T3).

Unit Summaries

The following is a brief summary of the lithologic units from Hole 1188A that have either well-preserved or remnant igneous features. Note that Units 15, 25, and 26 have no such features.

Unit 1 is black, moderately plagioclase-clinopyroxene phyric, moderately vesicular, microlite-bearing, glassy rhyodacite (Fig. F5). It is more vitreous at the top than any other lithologic unit from Hole 1188A. It contains ~72 wt% SiO₂ based upon RI measurements. Most of the vesicles are flattened as a result of flow during eruption and cooling. Thin sections show that, in addition to euhedral and subhedral plagioclase and clinopyroxene phenocrysts (generally <1.5 mm long), there are abundant smaller (to ~0.2 mm long) euhedral magnetite phenocrysts. These are commonly enclosed partially or wholly by both plagioclase and clinopyroxene. The groundmass bears numerous microlites, which include acicular transparent plagioclase, granular pyroxene, and very small (2-10 µm diameter) opaque minerals. Skeletal plagioclase in the groundmass and among the phenocrysts is rare. Some plagioclase phenocrysts are zoned, including a broad oscillatory style of zoning. Olivine from olivine-plagioclase clots (xenocrysts?) in Pieces 2 and 3 of Section 193-1188A-3R-1 is Fo₉₂ based upon RI measurements.

Unit 2 is an altered aphyric volcanic unit with well-preserved perlitic texture (Fig. F6). This indicates that it consisted of volcanic glass prior to alteration, which experienced solidification hydration. In parts, the rock shows a pseudoclastic texture because it has been altered preferentially along, and extending outward from, the perlitic cracks and away from irregular fractures.

Unit 3 is a slightly to moderately altered, moderately vesicular volcanic rock without any phenocrysts visible in hand specimen.

Unit 4 is a completely altered, bleached, aphyric, moderately vesicular volcanic rock with intervals showing remnant perlitic texture, flow banding, and/or fracturing.

Unit 5 is a completely altered, fractured volcanic rock with pseudoclastic texture. Fragments show remnant perlitic texture that is pseudomorphed by the alteration assemblage (Fig. F7).

Unit 6 is a completely altered, fractured, nonvesicular volcanic rock with incipient pseudoclastic texture. Fragments show flow banding that is pseudomorphed by the alteration assemblage (Fig. F8).

Unit 7 consists of a light gray to white bleached rock whose protolith was a sparsely to moderately vesicular aphyric volcanic rock (Fig. F9). Vesicles are typically millimeter scale in diameter and commonly are stretched into tubular shapes as long as 1 cm.

Unit 8 is a completely altered fractured volcanic rock with a pseudoclastic texture. Fragments show remnant perlitic texture that is pseudomorphed by the alteration assemblage.

Unit 9 is a bleached, sparsely vesicular volcanic rock that is very similar in appearance to Unit 7.

Unit 10 is a completely altered, fractured, intermittently perlitic and flow-banded volcanic rock. Flow banding is defined by microspherulitic bands and entirely devitrified bands. Folded flow banding has been recognized locally (Fig. F10).

Unit 11 is another completely altered, pervasively bleached volcanic unit with sparsely vesicular patches.

Unit 12 is a completely altered, volcaniclastic, grain-supported, granule to pebble breccia with aphyric white and light gray clasts. The white clasts are completely bleached, and the light gray clasts are partially silicified. Locally, gray clasts show preserved vesicles (1%-3%; <1 mm diameter). The matrix consists of silica ± pyrite.

Unit 13 is a pervasively bleached, fractured volcanic rock with possible remnant flow banding and vesicles in some pieces (Fig. F11).

Unit 14 is a completely altered volcaniclastic, granule to pebble breccia (Fig. F12). This unit is indistinguishable from Unit 12.

Unit 16 is an intensely bleached and silicified, sparsely vesicular volcanic rock.

Unit 17 is an intensely silicified, sparsely vesicular volcanic rock, some pieces of which show traces of primary volcanic banding.

Unit 18 is a silicified, flow-banded volcanic rock (Fig. F13). Despite the alteration, a remnant trachytic or pilotaxitic alignment of groundmass microlites, visible in thin section, is the key to recognizing this unit as a coherent volcanic rock rather than volcaniclastic rock. Furthermore, rare plagioclase phenocrysts are evenly distributed in the groundmass rather than concentrated in particular layers. Minor, small clasts (<1 cm diameter) are interpreted as xenoliths within the flow.

Unit 19 is a silicified, bleached volcanic rock with rarely preserved perlitic texture, traces of vesicles, and some intervals of hydrothermal brecciation with silica veinlets.

Unit 20 is a silicified and bleached sparsely vesicular volcanic rock with some remnant perlitic texture.

Unit 21 is a silicified volcanic rock with a trace of vesicles and with some pieces exhibiting remnant microperlitic texture.

Unit 22 is a dark gray, moderately altered, moderately vesicular, aphyric volcanic rock. The groundmass contains ~30 modal% euhedral plagioclase microlites and rare clinopyroxene microcrysts. By comparison with fresh aphyric rhyodacite in the upper part of the hole (Unit 3), it can be inferred that this unit is also of felsic composition (dacite?).

Unit 23 is an intensely silicified, weakly to moderately chloritic magnetite-bearing volcanic rock.

Unit 24 is a distinctive dark green silicified unit with hairline silica-filled fractures and ovoid sulfate spots that may represent altered/filled vesicles.

Volcanic Textures

Primary volcanic features have been recognized in many units of Hole 1188A despite the frequency of moderate to complete alteration. In particular, primary vesicles, phenocrysts, perlitic texture, spherulitic texture, and flow banding have been observed. 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 1188A, including the fresh rhyodacite of Unit 1 (Fig. F5) as well as many of the altered volcanic rock units. Some specimens that exhibit the complete bleaching-type alteration nonetheless perfectly preserve empty vesicles (Fig. F9). The vesicularity of individual specimens varies from a trace to 15 vol% (except for one piece from Unit 1 that was sampled in Section 193-1188A-7R-1, which has 30 vol% vesicles). 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. Most 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 of originally spherical vesicles in the lava as it flowed and cooled. Other specimens contain a population of highly flattened vesicles, which can result from compaction or shearing of originally spherical vesicles in the lava as it flowed.

Phenocrysts

Phenocrysts observable in hand specimens are exclusive to the unaltered rhyodacite (Unit 1) in the upper, relatively unaltered part of Hole 1188A and are absent from 34 mbsf to the end of the hole. Although rare phenocrysts reach 10 mm, most phenocrysts are generally <1-2 mm long. They comprise both elongate lath-shaped plagioclase and equant to stubby-prismatic clinopyroxene. One thin section from Unit 1 (Sample 193-1188A-3R-1 [Piece 1, 0-2 cm]) contained enough phenocrysts that allowed us to estimate the plagioclase composition by the Michel-Levy technique. The resulting value, probably accurate only to about ±5% absolute, is An₅₄. Thin-section analysis of Unit 1 rhyodacite also reveals a population of euhedral to anhedral magnetite microphenocrysts, which may be partly or completely enclosed in plagioclase.

Because slightly to moderately altered aphyric rhyodacite (Units 2 and 3) exists just below Unit 1, it can be inferred that the absence of large phenocrysts is a primary feature of the volcanic rocks and not necessarily caused by alteration. This is consistent with the XRD analyses that show there are significant amounts of fine-grained plagioclase in several moderately to completely altered units. Hence, any primary plagioclase phenocrysts in Units 2 and 3 should have survived hydrothermal alteration.

Plagioclase phenocrysts very similar to those in the unaltered Unit 1 are also observed in thin sections of some of the highly altered volcanic rocks deeper in the core. The plagioclase crystals preserve igneous oscillatory zoning and in places exhibit secondary overgrowths of possibly sodic plagioclase.

Perlitic Texture

Perlite is volcanic glass with arcuate and gently curved intersecting cracks that form in response to hydration. Perlitic texture is often preserved even in completely devitrified volcanic rocks and provides important evidence for the interpretation of their original composition. In Core 193-1188A-5R (Unit 2), pieces of altered, aphyric dacite(?) with well-preserved perlitic texture are present, indicating that this unit was originally glassy (Fig. F7). Furthermore, textural evidence indicates that alteration was focused along the perlitic cracks, and, hence, it is inferred that primary volcanic textures are an important control on alteration processes during hydrothermal activity (see below).

Spherulitic Texture

Well-preserved spherulites with radiating aggregates of quartz and feldspar have been recognized in thin sections of samples from Hole 1188A. This indicates that some units underwent a period of high-temperature devitrification prior to solidification, which is common in felsic lavas.

In general, if the melt is quickly chilled, it will solidify to quenched volcanic glass, whereas an equigranular to micropoikilitic microcrystalline groundmass is generated if solidification proceeds slowly enough to allow complete, homogeneous crystallization. However, depending on a variety of factors, including cooling rate, viscosity, chemical composition, and volatile content, undercooled felsic melts may experience a phase of high-temperature devitrification leading to the formation of spherulites instead of equigranular microcrysts (Lofgren, 1971). Individual spherulites commonly form spheres of radiating, fibrous quartz and feldspar crystals, with diameters ranging from >1 cm to <0.1 mm (microspherulites). They may coalesce and form completely devitrified domains within the lava.

The preserved spherulites that have been observed in thin section of samples from Unit 6 are ~1 mm in diameter and form groundmass domains consisting of coalesced radiating aggregates of quartz and feldspar (Fig. F14). Faint outlines of the spheres can be recognized under polarized light, and the radial arrangement of the microlite needles is apparent under crossed polarizers. Based on this observation, it can be inferred that parts of Unit 6 were devitrified prior to solidification, indicating relatively slow cooling rates.

In addition to these well-preserved spherulites, there are also microspherulites that typically form linear, coalesced necklacelike aggregates (Fig. F15). However, most of these structures have been altered and radiating crystal aggregates are rarely recognizable.

Flow Banding

Flow banding is a common feature of felsic lava flows. Individual bands are defined by substantial differences in microlites formed during cooling crystallization and/or differences in vesicularity. If cooling rates are low and the material remains sufficiently ductile, banded parts of the lava may become deformed and folded during flow.

Flow banding has been recognized in several altered units of Hole 1188A. In general, this primary volcanic texture is preserved in units that experienced patchy, multiphase alteration resulting in the formation of pseudoclastic textures and hydrothermal jigsaw breccia. Typically, it can be observed in the light gray, green silica-clay (GSC) altered (see "Hydrothermal Alteration") apparent clasts and is defined by alternating light and dark, linear to fibrous domains that are interpreted to reflect the primary flow banding of the lava (Fig. F8). This texture is particularly common in Unit 10, which also includes pieces of core showing isoclinal folding of flow banding (Fig. F10). In thin section it can be observed that the flow banding is defined by variable abundances of microspherulites (Fig. F16). Light gray bands consist of coalesced microspherulites, whereas dark gray bands consist of altered glass (very fine grained clay and silica) with isolated microspherulites.

Volcaniclastic Textures

Classification of the volcanic rocks recovered from Hole 1188A is hindered by their aphyric texture and the textural and mineralogical overprint of alteration. In particular, the discrimination of primary breccia (volcanic autobreccia, hyaloclastite, or sedimentary breccia) from alteration-related brecciation or formation of pseudoclastic textures ("hydrothermal breccia") is difficult. However, volcanic pebble breccia consisting of completely altered aphyric clasts has been recognized (Fig. F12).

Volcaniclastic Breccia

Densely packed, clast-supported volcanic pebble breccia exists in Core 193-1188A-14R (Units 12 and 14; see Fig. F12). These units are completely altered; however, the original outlines of the clasts can still be recognized. The moderate sorting of this unit, and the slight rounding of the clasts, suggests that it is a resedimented volcaniclastic deposit. All of the clasts are aphyric, and it can be inferred that they were derived from a common, local source. The clasts could have been derived from quench fragmented parts of a lava flow (hyaloclastite) and redeposited as mass-flow units. However, they show color variations (light gray and white) because of the variable intensity of silicification and clay-sulfate alteration, which may reflect differences in hydrothermal alteration at the source area. These units are important because they define paleosurfaces in the stratigraphy of Pual Ridge.

Hydrothermal Breccia with Evidence for Clast Movement

In some units logged as hydrothermal breccia, textural evidence indicates that individual fragments were moved relative to each other. This is an important observation because many rocks with fragmental appearance in Hole 1188A are jigsaw breccia, which are clearly the product of fracture-controlled alteration, generating variably developed pseudoclastic textures (Figs. F6, F7, F8, F11) (see below). However, breccia units containing clasts with laminar textures, such as flow banding, showing variable, unsystematic orientations indicate that these clasts have been moved relative to each other (Fig. F17). This textural feature may indicate a volcaniclastic origin of the particular unit (resedimented hyaloclastite?) or, otherwise, may be the result of intense fracturing during which sufficient open space was generated to allow for clast movement.

Evidence for clast movement has also been observed on a centimeter to millimeter scale. A thin section of Unit 8 (Sample 193-1188A-9R-1 [Piece 7, 68-70 cm]) shows a clastic texture with angular, locally perlitic vitriclasts generally containing <<1% vesicles. In parts, these clasts are separated by linear veins that are probably related to fracturing during alteration, showing a strict jigsaw-fit arrangement (Fig. F18A). However, in other areas of the thin section, the clasts are arranged in an irregular manner indicating that individual fragments have been moved relative to each other (Fig. F18B). Furthermore, there are also clasts with laminar textures (elongate tube vesicles?) that are clearly not continued in adjacent clasts, indicating that they are out of place (Fig. F18C). These textural features may be the result of hydrothermal alteration and minor fracturing in a primary volcaniclastic rock or intense fracturing associated with alteration and accompanied by substantial movement of fragments in the void space within fractures generated during brecciation.

Hydrothermal Breccia with Pseudoclastic Textures

Remnant perlitic textures in slightly to moderately altered aphyric dacite (Core 193-1188A-5R [Pieces 7 and 8]) indicate that Unit 2 consisted of volcanic glass prior to alteration. The perlitic groundmass texture is best preserved in light gray siliceous domains (Fig. F6). However, large parts of the groundmass consist of interconnected networks of dark gray clay-rich alteration assemblages encircling and separating round to irregular light gray siliceous groundmass domains. Hence, the rocks show an apparent clastic texture.

It is inferred that fluid flow and alteration occurred preferentially along the previously existing perlitic cracks in the rock, which led to preferential replacement of the material in these fine fractures extending outward into the surrounding siliceous groundmass. Consequently, the light gray round to lensoidal central parts of the perlites became increasingly surrounded by dark gray clay alteration forming an apparent matrix (Figs. F6, F7). However, there are also linear or irregular fractures that encircle more blocky domains (Fig. F19). Hence, the apparent clastic texture is created entirely by alteration processes and represents an excellent example of the development of "pseudoclastic textures" in coherent felsic volcanic rocks caused by hydrothermal alteration. Such features are commonly observed in the hydrothermally altered footwall of ancient massive sulfide deposits on the continents hosted by felsic lavas (Allen, 1988).

The deeper parts of Site 1188 were investigated in Hole 1188F using diamond-drilling technology. The upper portion of this hole was stabilized by inserting a casing string down to a depth of ~200 mbsf. Core was obtained from 218.0 to 386.7 mbsf. Recovery rates for individual half cores (typically 4.6 m long) varied from 0% to >90%. The overall recovery for ADCB drilling in Hole 1188F was 18.3%, compared to the 10.4% recovery experienced in Hole 1188A, which was drilled with the RCB system.

In total, 45 units (Unit 27 to 72) have been identified in Hole 1188F based on changes in the alteration mineralogy, presence or absence of plagioclase phenocrysts, remnant spheroidal textures, presence and abundance of amygdules or vesicles, and locally developed fragmental textures. Many of these units have a curated thickness of <1 m. The upper portion of the hole, down to a depth of ~340 mbsf, consists of silicified volcanic rocks, which are aphyric or slightly porphyritic with altered plagioclase phenocrysts and amygdules of variable size (ranging from submillimeter to 10 mm) and composition (mainly quartz, anhydrite, and/or pyrite). Below 340 mbsf, magnetite-bearing units become increasingly prominent. Hydrothermal magnetite has been observed within vesicles, in siliceous halos along anhydrite-pyrite veins, and disseminated through the groundmass. Several units are porphyritic and contain fresh, unaltered plagioclase phenocrysts in a groundmass with abundant plagioclase microcrysts. Furthermore, several units have preserved vesicles that are only partially filled or lined with alteration minerals. These features indicate that alteration of the lower part occurred under significantly different conditions than in the upper part.

The lithologic characteristics of the units recognized in Hole 1188F are provided in Table T4, and the principal features are summarized in a graphic log (Fig. F20). The units are briefly described below.

Unit Summaries

The following is a brief summary of the general characteristics of the lithologic units from Hole 1188F.

Unit 27 is a silicified, aphyric, massive volcanic rock with fine (submillimeter) quartz amygdules.

Unit 28 is similar to Unit 27 but shows a distinctive green color, which has been related to a higher abundance of chlorite. Furthermore, cyclic siliceous banding, observed along anhydrite-pyrite veins, locally encircles differently altered kernels and gives rise to an apparent clastic texture of the rock (Fig. F21).

Unit 29 is a silicified, aphyric, massive volcanic rock identical to Unit 27.

Unit 30 is a brecciated volcanic rock with silicified fragments embedded in a light gray clay-rich matrix (Fig. F22). This texture, though, may be a drilling-induced characteristic of the ADCB system because most of the recovered pieces consist of rubble. The fine, soft, clay-rich material may be an artifact caused by the grinding.

Unit 31 consists of one small piece that shows a prominent spherulitic texture in hand specimen (Fig. F23).

Unit 32 is a silicified, aphyric volcanic rock.

Unit 33 consists of silicified fragments of aphyric volcanic rock embedded in and/or coated by soft, gray clay similar to Unit 30.

Unit 34 is a silicified, aphyric volcanic rock. If the fragmental texture of Units 30 and 33 are drilling artifacts, then Units 32 to 34 may represent the same lithologic unit.

Unit 35 is a porphyritic, sparsely vesicular silicified volcanic rock. It contains as much as 2 vol% plagioclase phenocrysts (as long as 3 mm; commonly replaced by clay), which are locally aligned.

Unit 36 is dominantly aphyric, silicified massive volcanic rock with intervals containing pyrite-filled flattened vesicles and rare relict plagioclase phenocrysts.

Unit 37 consists of a single piece with a clastic texture. It contains abundant, rectangular to irregularly shaped light gray domains (mainly anhydrite, clay, and quartz) embedded in a tan-gray fine-grained silicified matrix.

Unit 38 is a silicified, dominantly aphyric, volcanic rock that locally contains pyrite-filled flattened vesicles and rare pseudomorphed plagioclase phenocrysts. This unit is similar to Unit 36.

Unit 39 is a porphyritic volcanic rock with clay-altered plagioclase phenocrysts (<1-1 vol%; as long as 4 mm) and as much as 3 vol% round to elongate vesicles (ranging from <1 to 5 mm, the maximum dimension). Several pieces contain quartz-rich (±pyrite) patches, a few millimeters to a centimeter across, that are slightly rounded.

Unit 40 is a completely altered, aphyric volcanic rock with prominent spherulitic texture in hand specimen and is identical to Unit 31.

Unit 41 is a silicified volcanic rock with fresh and clay-altered plagioclase phenocrysts (<1 to 2 vol%; as long as 4 mm). Fine amygdules (<1 mm in diameter) consist of anhydrite and pyrite. This unit contains several pieces with 1- to 2-cm-wide fragmental zones where <1-cm siliceous clasts are hosted within light gray finer siliceous material (Fig. F24).

Unit 42 is a silicified aphyric volcanic rock with elongated vesicles filled or lined with pyrite-anhydrite or quartz.

Unit 43 is a silicified volcanic rock with white clay pseudomorphs of original plagioclase phenocrysts (<1-2 vol%; as long as 3 mm) and elongate vesicles (as long as 10 mm, the maximum dimension) that are lined or filled by anhydrite, pyrite, and/or quartz.

Unit 44 is a silicified, generally aphyric volcanic rock with relict flow banding and a faint clastic texture. Some pieces contain traces of white clay-altered plagioclase phenocrysts. The flow structures and fragmental textures of Unit 44 are defined by domains showing variable relative proportions of silica and clay (Fig. F25).

Unit 45 is a silicified, porphyritic, sparsely vesicular volcanic rock with as much as 3 vol% clay-altered plagioclase phenocrysts (as long as 3 mm), which are commonly replaced by bluish-white clay. The vesicles are filled by anhydrite and/or pyrite and are typically aligned, defining a flow structure. Locally, round to angular quartz-rich patches (a few millimeters to 3 cm across) are present (Fig. F26).

Unit 46 is a silicified, aphyric volcanic rock with rare vesicles ()partially filled by pyrite). It contains several 1- to 2-cm-wide clastic zones with <1-cm fragments in a fine, light gray matrix. Similar textures have been observed in Unit 41.

Unit 47 is a silicified, aphyric volcanic rock with rare, pyrite-filled vesicles and a distinctive greenish coloration inferred to reflect the occurrence of disseminated chlorite.

Unit 48 is a silicified, sparsely plagioclase-phyric volcanic rock, with minor, pyrite-filled vesicles.

Unit 49 is a silicified volcanic rock with a slight dark green color owing to the presence of chlorite. Locally, it contains rare plagioclase phenocrysts (some as large as 5 mm) and minor pyrite-filled vesicles. Round patches of quartz (as large as 2 cm in diameter) are present in several pieces (similar to Units 39 and 45).

Unit 50 is a silicified, slightly plagioclase-phyric volcanic rock. The unit is distinguished from Unit 49 by the absence of greenish colored clay in the groundmass of the rock. Distinct, fine (0.1-1 mm) dark spots are common, representing vesicles filled with silica (±pyrite).

Unit 51 is aphyric and shows a clastic texture with angular to rounded silicified fragments embedded in a dark gray quartz matrix (Fig. F27).

Unit 52 is a single piece of porphyritic volcanic rock with fresh plagioclase laths and fine titanomagnetite phenocrysts.

Unit 53 is aphyric and shows a clastic texture similar to Unit 51.

Unit 54 is a completely altered, aphyric, nonvesicular volcanic rock. Silicification is strong and pervasive, and alteration halos and clastic textures are present in some pieces.

Unit 55 is a completely altered, vuggy, clastic volcanic rock, most of which contains domains where the matrix is black and magnetite rich (Fig. F28). The domains are irregular in shape and may represent centimeter- to decimeter-scale brecciated zones in which magnetite was precipitated from hydrothermal fluids.

Unit 56 is composed of light gray, completely altered (silicified and clay altered) volcanic rocks with mottled and clastic textures.

Unit 57 is a silicified, aphyric volcanic rock with anhydrite veining and scattered fine quartz amygdules.

Unit 58 consists of one piece of magnetite-rich rock with a clastic texture.

Unit 59 is a strongly silicified, sparsely vesicular, locally flow-banded, aphyric volcanic rock with scattered fine quartz amygdules similar to Unit 57.

Unit 60 is a silicified, sparsely plagioclase-phyric, sparsely vesicular volcanic rock.

Unit 61 is a silicified, aphyric volcanic rock with fine, submillimeter amygdules.

Unit 62 is a silicified, sparsely plagioclase-phyric and sparsely vesicular volcanic rock.

Unit 63 is a silicified and locally magnetite-bearing, aphyric volcanic rock.

Unit 64 is a silicified, aphyric volcanic rock with fine, submillimeter amygdules.

Unit 65 is a silicified and locally magnetite-bearing, sparsely vesicular aphyric volcanic rock.

Unit 66 is a silicified, sparsely plagioclase-phyric volcanic rock with fine, submillimeter amygdules.

Unit 67 is a silicified, aphyric volcanic rock with a clastic texture.

Unit 68 is a silicified volcanic rock with trace amounts of fresh plagioclase phenocrysts and finely disseminated magnetite in the groundmass.

Unit 69 is a silicified, aphyric, sparsely amygdaloidal volcanic rock.

Unit 70 is a silicified, aphyric volcanic rock with abundant amygdules consisting mainly of anhydrite.

Unit 71 consists of a single piece of silicified aphyric, sparsely vesicular volcanic rock. The vesicles are elongate (as wide as 10 mm, the maximum diameter), aligned, and typically partially filled with quartz, pyrite, chlorite, and/or anhydrite.

Unit 72 is a silicified, sparsely vesicular volcanic rock with rare, well-preserved plagioclase phenocrysts and several quartz-rich patches as wide as 2 cm in diameter. Magnetite is common in the vesicles and disseminated in the groundmass. Prominent alteration halos along the anhydrite-pyrite veins are silica and magnetite rich.

Volcanic Textures

A number of primary volcanic features are common in the rock samples from Hole 1188F. The coherent volcanic rocks are either porphyritic or aphyric and typically amygdaloidal or vesicular. Fresh or altered plagioclase microlites are generally aligned, defining a trachytic groundmass texture. Elongate vesicles and phenocrysts also show a preferred orientation. Other features such as clastic, mottled, xenolithic, and flow-banded textures have been observed less frequently. The major alteration overprint consists of silicification and clay alteration, followed by anhydrite and pyrite-rich veins (see "Hydrothermal Alteration," "Sulfide and Oxide Petrology," and "Structural Geology"). Note that the silicification present in Hole 1188F involves only quartz, in contrast to the shallower Hole 1188A, where silicification involves cristobalite at the top giving way to quartz at depth.

Amygdules

Amygdules, or vesicles filled with secondary minerals, are present throughout Hole 1188F (Fig. F29; Table T5). Open vesicles are less abundant, yet are sporadic in core samples practically all the way to the bottom of the hole. Most amygdules are millimeter sized and give the hand specimens a spotted appearance. In some cases, amygdules are flattened and aligned, and when present, phenocrysts are aligned parallel to the same trend.

The most common amygdule-filling mineral assemblage is quartz + pyrite + anhydrite. The second most common mineral fill is any two of these three minerals or any of these minerals alone. Quartz is present as intergrown anhedral crystals, generally 0.25-0.5 mm across, forming a mosaic texture. Quartz grains exhibit undulose extinction and numerous secondary planes of fluid inclusions, giving the grains a "dirty" appearance. Pyrite and anhydrite (1 mm) are subhedral to euhedral.

Unusual amygdule filling assemblages include chlorite + chabazite(?) and pyrite + magnetite + clay. The chlorite-chabazite(?) is from Unit 55, where most amygdules are quartz filled. Other vugs are incompletely filled, with incomplete linings of radiating aggregates of chlorite and chabazite? (a clear, low-birefringence, low-RI zeolite with inclined extinction and rhombohedral cleavage).

The pyrite + magnetite + clay amygdules are present in Unit 65. This dark gray to black rock contains numerous vugs that are filled with a soft black magnetic material and a central pyrite crystal. Other vugs contain anhydrite. The soft black material is clay with inclusions of tiny bladed magnetite crystals.

Phenocrysts

Plagioclase is the only phenocryst type observed in Hole 1188F (Table T6). Fresh plagioclase is present in 13 of the 45 units identified. Plagioclase phenocrysts pseudomorphed by secondary minerals are recognized in 12 units, and 20 units are totally aphyric.

Fresh phenocrysts are euhedral and lath shaped, with both stubby laths and, more commonly, elongated laths occurring together. Zoning is sporadic, with a few phenocrysts that have rounded core zones overgrown by euhedral mantles. This feature suggests that the phenocryst at some point in its history became unstable and began to corrode, which is commonly interpreted to reflect a former magma mixing event. When fresh plagioclase laths are in a sample, the groundmass microcysts commonly are also at least partly fresh (Fig. F30). Both phenocrysts and microlites, though, may have corroded rims.

Most fresh phenocrysts are present in such small abundances that optical estimates of chemical composition are impossible. However, some determinations by the Michel-Levy technique (which gives a minimum for the anorthite content and is probably accurate to ±5% absolute) gave An₆₄, An₅₁, and An₆₂. These results all reflect labradorite compositions, similar to other determinations from fresh dacites sampled during Leg 193 (An₅₄ from Section 193-1188A-3R-1, An₅₉ from Section 193-1190A-1R-1, and An₅₀ from Section 193-1190B-2R-1).

Plagioclase phenocrysts are commonly recognized as pseudomorphs, with characteristic stubby lath shapes and similar sizes compared to the fresh phenocrysts (i.e., 0.5-2.0 mm; average = ~1 mm). Plagioclase is replaced, in order from highest to lowest frequency, by quartz, clay (usually illite), clay + halloysite, clay + anhydrite, quartz + anhydrite, quartz + clay, chlorite + illite, and chlorite + illite + anhydrite (Table T6). The identification of halloysite is tentative. This is a fine-grained micalike material with low birefringence and sweeping extinction. It has been described as "silica," probably because Hole 1188A contained an abundance of fine-grained cristobalite (i.e., silica) with low birefringence. However, as Hole 1188F is otherwise devoid of cristobalite in favor of quartz (with the exception of the spheroidal Unit 31; see below), it seems unlikely that cristobalite would persist as an alteration of plagioclase.

Spheroidal Nodules with Randomly Oriented Plagioclase Microlites

Hand specimens of Units 31 and 40 (each of which consists of only one small piece) show a prominent spherulitic texture with abundant isolated or coalesced, white, round aggregates (as wide as 1 mm in diameter). In thin section, these aggregates correspond to brown, round to irregular, coalesced domains with bulbous margins (Fig. F31A). They are rimmed by dark brown, clay-rich material, which also outlines individual spheroids in aggregated domains. Internally, they consist of randomly oriented, very fine grained plagioclase microlites (Fig. F31B, F31C). Generally, the characteristic radiating arrangement of feldspar needles, diagnostic for spherulites generated during high-temperature devitrification, is lacking. Only in rare cases have small, concentric dark brown zones with radiating feldspar crystal aggregates been observed in the core of the spheroids. The groundmass between the spheroids consists mainly of fine-grained cristobalite; however, some coarser-grained radiating feldspar crystal aggregates are locally attached to the outer margins of some spheroids (Fig. F31D).

Even though the textures observed are somewhat ambiguous, it is inferred that the spheroids represent a primary volcanic feature related to high-temperature devitrification of a lava. Plagioclase microlites are well preserved within the spheroids, but generally altered in the surrounding cristobalite-rich domains, suggesting that they were protected from hydrothermal alteration in devitrified domains.

Flow Banding

Remnant, poorly preserved flow banding has been locally recognized in Units 44 and 59. Laminar textures are defined by subtle variations in the proportions of quartz and clay minerals. In thin section, the contacts between these domains are gradational. It is inferred that minor differences between individual flow bands (e.g., microvesicles, microlites, and ratio of glassy to devitrified groundmass) are reflected in the changes in alteration mineralogy.

Quartz-Rich Patches

Gray, generally rounded siliceous patches are present in several units of Hole 1188F (e.g., Units 39, 45, 49, and 72). These are embedded within the altered volcanic groundmass and consist of relatively coarse, interlocking, anhedral quartz crystals forming a mosaic texture, which are locally accompanied by interstitial pyrite and minor anhydrite and chlorite. Contacts to the surrounding groundmass are generally sharp (Fig. F32).

The origin of these patches is enigmatic. Their mineralogical composition is similar to many amygdules, which, however, rarely exceed 1 or 2 mm in diameter, except for elongated vesicles/amygdules that may be up to 10 mm long and 2 mm wide. Hence, if these quartz-rich patches represent large amygdules, there would be a significant gap in the size distribution of the vesicles.

Large cavities known as lithophysae may form in felsic lavas during high-temperature devitrification as a result of diffusion processes associated with the formation of spherulitic aggregates of quartz and feldspar microcrysts. Lithophysae may be round, irregular, or star shaped and may reach diameters of several tens of centimeters. However, their outer margin invariably consists of fine quartz and feldspar needles. Such textures have not been observed around the quartz-rich patches, and therefore, it is unlikely that they represent lithophysae.

Finally, it may be possible that the quartz-rich patches represent xenolithic material that became incorporated into the lava during its passage through the subsurface or picked up from the seafloor during flow. However, it is difficult to envisage a source rock with the required, almost monomineralic composition (quartz-rich vein material?).

Xenoliths

Locally, isolated, round to wispy patches (as wide as 3 cm in diameter) with fine-grained igneous internal textures have been observed. They have sharp or transitional contacts to the surrounding altered volcanic groundmass. Locally, microlites in the groundmass are aligned parallel to the margins, indicating that the crystallizing lava was plastically deformed around these patches (Fig. F33). Based on their composition and the textural relationship to the enclosing volcanic groundmass, they are interpreted as xenolithic fragments, which were incorporated into the lava in the subsurface or from the seafloor during flow.

Thin Clastic Zones

Individual pieces of some units contain 1- to 2-cm-wide bands with clastic textures (Fig. F24). These bands contain fragments (0.5 cm) of the adjacent coherent rock, hosted in a fine-grained siliceous matrix. These zones may be interpreted as parts of autoclastically fragmented lava generated during emplacement or as fractures where brecciation of the coherent rock occurred as a result of hydrothermal and/or tectonic activity.

Pseudoclastic Texture

Silicified halos (up to 2 cm wide) are a typical feature of the veins in Hole 1188F. Where such veins intersect each other at high angles, the halos merge and encircle round to irregularly shaped groundmass domains. These kernels of less altered volcanic groundmass superficially resemble fragments and the texture of the rock may be described as pseudoclastic (Fig. F21).

Clastic/Mottled Textures

In addition to volcanic rocks with isolated quartz-rich patches, xenolithic clasts, thin clastic zones, and pseudoclastic textures, there are several units in Hole 1188F that show somewhat enigmatic mottled or fragmental textures. They contain domains with contrasting mineralogical composition mainly defined by changes in the relative proportions of microcrystalline quartz and very fine grained clays.

In general, quartz-rich groundmass domains, representing apparent clasts, are embedded in the surrounding slightly more clay-rich groundmass representing the apparent matrix. The contacts between these domains can be sharp or gradational and the shapes of the apparent clasts vary from angular to irregular (Fig. F34A, F34B). Locally, it can be observed that remnant plagioclase microlites are transgressing the boundary between apparent matrix and apparent clasts (Fig. F34C). Textural evidence supporting a sedimentary or volcaniclastic origin (e.g., polymict composition, clast-supported texture, phenocrysts abraded at clast margin, and grading) has not been observed.

It is possible that these fragmental units represent autoclastic zones of lava flows where the original clastic texture, defined by fragments of variable grain size of the same composition, has been subsequently enhanced during hydrothermal alteration. Alternatively, it may be argued that the textures observed are pseudoclastic, arising entirely because of the incomplete, domainal, multiphase alteration of a coherent volcanic precursor during which siliceous groundmass domains became isolated within more altered, less siliceous groundmass.

Holes 1188A and 1188F combined provide a one-dimensional view to a depth of 386.7 mbsf of the lithologic architecture beneath the low-temperature diffuse Snowcap hydrothermal site located within the PACMANUS hydrothermal field of Pual Ridge. With the lone exception of fresh volcanic rocks sampled near the surface, all of the rocks are hydrothermally altered, most of them nearly completely. Nevertheless, the majority of the rocks are demonstrably volcanic in origin and contain abundant vestiges of primary igneous features such as phenocrysts, trachytic groundmass, vesicles, amygdules, and flow banding.

The shallow, fresh volcanic rocks are rhyodacitic in composition. They contain phenocrysts of plagioclase with the optical properties of labradorite, as well as less-abundant titanomagnetite microphenocrysts. Volcanic rocks encountered deeper at Site 1188 frequently exhibit either plagioclase phenocrysts or pseudomorphs thereof. The fresh plagioclase crystals in the deeper rocks also have the optical properties of labradorite. Furthermore, some of the deeper samples contain leucoxene pseudomorphs after titanomagnetite. Thus, there is evidence that the whole volcanic section sampled at Site 1188 is essentially the same, or very similar, rhyodacite or dacite.

Most of the rocks are either vesicular or amygdaloidal, with instances of open or incompletely filled vesicles persisting to some of the deepest units. The groundmass of all the igneous rocks is uniformly fine grained, and porphyritic rocks contain relatively few and relatively small plagioclase phenocrysts (generally only 1%-2% and 1-2 mm long). Thus, all of the rocks are interpreted as volcanic, there being no coarser-grained rocks or textures, such as diabasic ones, that might indicate a hypabyssal origin.

Occurrences of perlitic texture, spherulitic texture, flow banding, volcanic brecciation, and autobrecciation all attest to the success of the coring program in sampling various coherent and brecciated portions of the volcanic rocks that built up the upper 387 m of Pual Ridge. Occurrences of hydrothermal breccia and pseudoclastic textures illustrate details about the subsequent lithologic modifications that overprint the volcanic rocks when they are subjected to subseafloor hydrothermal activity.