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.
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.
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.
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.
