Download Basaltic pillow mounds in the Vinalhaven intrusion

Journal of Volcanology and Geothermal Research 107 (2001) 171±184
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Basaltic pillow mounds in the Vinalhaven intrusion, Maine
R.A. Wiebe a,*, H. Frey b, D.P. Hawkins c
a
Department of Geosciences, Franklin and Marshall College, Lancaster, PA 17604, USA
Department of Geological Sciences, University of Michigan, Ann Arbor, MI 48109, USA
c
Department of Geology and Geography, Denison University, Granville, OH 45469, USA
b
Received 15 June 2000; accepted 14 August 2000
Abstract
The Vinalhaven intrusion is a dominantly granitic pluton of probable Devonian age, located on Vinalhaven Island and
adjacent islands, Maine. It consists of four main units: coarse-grained granite, ®ne-grained granite, a gabbro-diorite unit
consisting of interlayered ma®c, hybrid and granitic rocks, and a heterogeneous granitic unit. The gabbro-diorite unit occurs
along the south and east coast of the island as a sheet-like body, hundreds of meters to more than 1 km thick, that dips beneath
the central granitic units and rests on heterogeneous granitic rocks that form the base of the intrusion and are exposed on islands
to the southeast. Load-cast and pipe structures at the bases of ma®c sheets indicate that the gabbro-diorite unit represents a
sequence of basaltic injections that ponded on crystal-rich mush at the base of a silicic magma chamber and variably interacted
with overlying crystal-poor granitic magma. The pluton, therefore, represents a fossilized silicic magma chamber that was
periodically replenished by basaltic magma. Near the base of the gabbro-diorite unit, some basaltic injections produced large
mounds up to more than 10 m high and 100 m wide of tightly packed, meter-scale chilled basaltic pillows, tubes and sheets in a
granitic matrix. The mounds appear to represent ¯ow fronts of basaltic injections that entered and ponded on the ¯oor of a silicic
magma chamber. Although physical conditions differ signi®cantly, these plutonic pillow mounds appear to share many
characteristics with submarine pillow basalts and lava ¯ows. q 2001 Elsevier Science B.V. All rights reserved.
Keywords: basaltic pillow mounds; load-cast and pipe structures; basaltic injections; granite; magma chamber
1. Introduction
When ma®c magma intrudes into an existing chamber of silicic magma, the temperature and viscosity
contrasts between the two magmas commonly results
in commingling. Typical products of this commingling include both chilled, globular basaltic bodies in
granite, closely resembling pillows that form when
basalt is emplaced into water (Elwell et al., 1962;
Blake et al., 1965; Walker and Skelhorn, 1966), and
chilled sheets of microgabbro in a granitic matrix
* Corresponding author.
E-mail address: [email protected] (R.A. Wiebe).
(Wiebe, 1993a). These pillows and sheets tend to
form at the rheological interface between the resident,
crystal-poor silicic magma and the underlying, highly
viscous crystal-rich granite mush comprising the
chamber ¯oor (Wiebe, 1974; Wiebe and Collins,
1998). The resulting density inversion leads to the
development of ma®c load-casts, silicic ¯ame structures and silicic pipes that are closely analogous to
sedimentary structures that indicate way up. Successive infusions of ma®c magma into the silicic chamber
produce a depositional (stratigraphic) package of
rocks that records the sequence of events in the
magma chamber (e.g. replenishment, mixing, fractional crystallization); this depositional sequence
0377-0273/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved.
PII: S0377-027 3(00)00253-5
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provides an unambiguous record of the gradual
construction of the pluton beneath an active magma
chamber (Wiebe and Collins, 1998). Similar granitic
plutons with ma®c layers occur throughout the geologic record in a variety of tectonic settings (e.g. Wiebe,
1974, 1993b, 1994, 1999; Michael, 1991; Chapman
and Rhodes, 1992; Coleman et al., 1995; Sisson et al.,
1996; Wiebe and Collins, 1998).
The Vinalhaven intrusion is one of many SiluroDevonian plutons the coastal Maine that preserves
evidence for the interaction of ma®c and silicic magmas
in a silicic magma chamber. Unlike other coastal Maine
plutons, the Vinalhaven pluton also preserves unusual
accumulations or mounds of ma®c pillows. Excellent
coastal exposures of these pillow mounds are tens of
meters high and extend over areas as much as 100 m
in diameter. The mounds appear to represent the ¯ow
fronts of basaltic injections that moved into an unconsolidated transitional zone of crystal mush at the base of
a silicic magma chamber, between a solid ¯oor and a
crystal-poor interior. The purpose of this paper is to
describe these pillow mounds and discuss possible
mechanisms for their formation.
2. Regional geologic setting
Vinalhaven Island (128 km 2) is located in southern
Penobscot Bay, about 15 km east of Rockland, Maine.
The plutonic rocks on Vinalhaven belong to the
Coastal Maine Magmatic Province (Hogan and
Sinha, 1989), an association of more than 100 granitic
and ma®c plutons, formerly known as the Bays of
Maine Complex (Chapman, 1962). These plutons
intruded parallel to NE trending fault-bounded
terranes from the Late Silurian to Early Carboniferous
(Hogan and Sinha, 1989). The terranes consist primarily of pre-Devonian metavolcanic and metasedimentary rocks and are thought to be microplates of
continental crust that accreted onto the North American craton during the Acadian Orogeny (Hogan and
Sinha, 1989). Hogan and Sinha (1989) proposed that
post-accretion magmatism in the Coastal Maine
Magmatic province is related to rifting in a transtensional environment where crustal extension and lithospheric thinning allowed for the emplacement of
ma®c magma at various crustal levels. A subsequent
shift to a transpressional regime is thought to have
trapped ma®c magma at the base of the crust and
induced partial melting of the overlying granitic
crust. Gravity studies suggest that most granitic
plutons are only a few km thick, and lie above ma®c
rocks (Hodge et al., 1982).
3. Geology of the Vinalhaven pluton
The Vinalhaven pluton was emplaced into early
Paleozoic, deformed, low grade schists of the Calderwood Formation and into essentially undeformed,
Silurian volcanic rocks which may contain some eruptive products contemporaneous with the intrusion
(Mitchell and Rhodes, 1989; Hawkins and Wiebe,
1999) (Fig. 1). Brookins (1976) produced a Rb±Sr
isochron of 361 ^ 7 Ma for the granitic rocks. Contact
metamorphism is well developed in country rocks,
and there is no evidence of regional deformation or
metamorphism subsequent to cooling of the intrusion.
The pluton consists of four main units (Fig. 1). The
most extensive unit is coarse-grained granite that
forms the center of the island and is in contact along
its northern (upper) margin with country rock. A ®negrained granite (unit 2) forms a small inner core of the
intrusion that sharply cuts the gabbro-diorite unit (unit
3) and locally appears to mix with the coarse-grained
granite (unit 1). The gabbro-diorite unit consists of
interlayered ma®c, hybrid, and granitic rocks and
forms a curved, sheet-like body, hundreds of meters
to more than 1 km thick, that dips 10±308 to the north
and west beneath the granite. Evidence for commingling of ma®c and granitic magmas occurs along the
contact of the coarse-grained granite with the top of
the gabbro-diorite unit (Fig. 1). Extensive blocks of
country rock, identical to the country rock in the
northern part of the island, occur in association with
the gabbro-diorite unit (Fig. 1). Metamorphism of
pelitic blocks in this unit suggests pressures of
between 1 and 2 kb (Porter et al., 1999). The
gabbro-diorite unit rests on heterogeneous granitic
rocks (unit 4) that range in composition from granite
to quartz diorite. These heterogeneous granitic rocks
occur locally along the coast and on small islands to
the southeast; they appear to represent the lowest
exposed part of the pluton in contact with underlying
country rocks that occur in islands further to the
southeast (Fig. 1). Steeply dipping basaltic dikes,
R.A. Wiebe et al. / Journal of Volcanology and Geothermal Research 107 (2001) 171±184
173
Fig. 1. Geologic map and cross-section of the Vinalhaven intrusion. Map modi®ed from Mitchell and Rhodes (1989).
1±3 m thick, occur widely in the gabbro-diorite unit,
but are very scarce in the overlying granite.
Much of the gabbro-diorite map unit consists of
layers, each of which has a chilled gabbroic base
against underlying more felsic rocks. Above each
chilled base, ma®c rocks may form all or part of a
layer which may be less than one to more than
100 m thick. Where the ma®c sheet is less than a
few meters thick, it commonly has a chilled upper
margin against a more felsic, overlying layer similar
to that beneath the sheet. Where a ma®c sheet is
thicker than a few meters, it generally grades upward
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R.A. Wiebe et al. / Journal of Volcanology and Geothermal Research 107 (2001) 171±184
to dioritic and granitic rocks, and the resulting layer,
although a cumulate, displays evidence for incomplete mixing between ma®c and felsic magmas. Widespread load-cast and pipe structures at the bases of
these layers indicate that the gabbro-diorite unit represents a sequence of basaltic injections that ponded at
the base of a silicic magma chamber (resting on granitic to dioritic crystal mush) and variably interacted
with overlying granitic magma (Wiebe, 1993a). Similar layers (termed macrorhythmic units) in the nearby
Pleasant Bay and Cadillac Mountain intrusions are
described in more detail (Wiebe, 1993b, 1994).
In addition to these well-de®ned layers, the gabbrodiorite unit also contains areas of tightly packed pillows
of chilled gabbroic rocks in a granitic to hybrid matrix.
Some of these areas of commingled rocks appear to be
the eroded remnants of mounds, up to more than 10 m
high and 100 m in diameter. Tightly packed, tabular
chilled bodies appear to de®ne structures that resemble
domal, deltaic, or trough-like accumulations of pillows.
Although pillows exposed on two-dimensional surfaces
typically appear to be isolated bodies, in many threedimensional exposures these pillow-like masses are
often elongate in one direction and can be seen to represent buds or branches from a tube-like mass. Most
pillow accumulations appear to grade in one direction
to massive gabbro.
4. Petrography and geochemistry
The globules or pillows of chilled ma®c material
closely resemble submarine basaltic pillows. They
vary greatly in shape and size, ranging from small
and nearly equant (5 cm) to large and elongate
(1 £ 5 m 2). Pillow margins are sharply chilled and
commonly have crenulate, convex-outward projections against the matrix. In thin-section, their textures
are characterized by skeletal and radiating clusters of
plagioclase with variable proportions of hornblende and
augite. The chilled margins are typically hornblenderich; some also have thin biotite-rich rims, re¯ecting
exchange of H2O and alkalis between the pillow
margin and the adjacent granite (Wiebe, 1973).
Towards the center of most pillows, grain size and
the amount of pyroxene increases, and the amount of
hornblende decreases. The ®ne-grained gabbroic rocks
of the pillow interiors contain much less hornblende
and consist primarily of thin lathes of normally zoned
(An75-20), euhedral plagioclase, equant olivine crystals
and anhedral augite. Plagioclase lathes commonly
occur in radiating clusters with augite exhibiting a
subophitic texture. Minor interstitial orthopyroxene,
biotite and/or hornblende, and opaque minerals are
also present. Subsolidus alteration products (e.g. chlorite, epidote) are scarce and absent in most rocks.
The granitic matrix between the pillows varies in
thickness, composition and texture. It is commonly
between 1 and 2 cm thick. Pillow margins are locally
molded around individual crystals in the matrix, indicating that the matrix was a mixture of crystal and
liquid (a cumulate) when the pillows formed. In the
lower parts of some mounds, matrix is commonly
absent between strongly chilled pillows, suggesting
that the matrix was readily mobilized and squeezed
out during ¯ow and compaction of the pillows.
The most common matrix is homogeneous, mediumgrained biotite granite with scarce hornblende-bearing
ma®c clots. This matrix grades locally to hornblenderich and heterogeneous granodiorite to diorite with
varying amounts of ma®c enclaves. Plagioclase crystals display weak normal zoning (An24-18) and are
commonly resorbed and corroded. Subequant alkali
feldspar crystals commonly have plagioclase rims.
Since the pillow margins are consistently sharp and
lack gradations to the matrix, the hybridization of
the matrix probably occurred prior to the emplacement of the ma®c pillows. Comparable gradations
between ma®c hybrid rocks and granite occur in
many of the thick hybrid layers in the gabbro-diorite
unit.
Thin (less than 1 cm thick), wispy veins or ª¯amesº
of leucocratic granite extend from the matrix into some
pillows. Their contacts with the interior of the ma®c
pillow typically become increasingly gradational
inward. Similar veins that are connected to larger equant
areas of granite within the pillows appear to represent
silicic magma that broke across chilled margins while
the pillows were still ¯owing and as cavities began to
form in the interior of the pillow. Some pillows are also
cut by thicker (.1 cm), sharply bounded, planar veins
of highly leucocratic granitic material that connect to the
matrix. These appear to have formed by residual granitic
liquid in the matrix that was ®lter-pressed from the
matrix somewhat later, when the pillow interiors were
solid enough to fracture.
R.A. Wiebe et al. / Journal of Volcanology and Geothermal Research 107 (2001) 171±184
175
Table 1
Major element analyses of pillows, massive gabbro, and the matrix to pillows in three pillow mounds
Sample
Clayter's Beach
Pillows
4C
4D
4F
Massive
gabbro
Matrix
4A
4E
East Lane Island
West Lane Island
Pillow
Pillows
4G
32
43A
Massive
gabbro
Matrix
43B
43C
43D
43E
SiO2
TiO2
Al2O3
Fe2O3(T)
MnO
MgO
CaO
Na2O
K2 O
P2O5
LOI
48.80
1.75
15.89
11.38
0.18
7.35
10.19
2.77
0.57
0.15
0.14
48.08
1.32
16.78
10.49
0.17
8.51
11.40
2.82
0.18
0.11
0.17
52.70
1.53
16.08
10.09
0.16
6.42
9.06
3.10
1.16
0.15
0.27
51.70
1.09
16.75
9.25
0.15
7.66
9.96
2.81
0.85
0.10
0.22
69.02
0.78
14.01
4.89
0.08
1.31
2.26
3.19
4.18
0.13
0.72
69.72
0.44
14.32
3.28
0.04
0.65
1.52
3.37
5.14
0.11
0.58
50.04
2.46
15.11
12.09
0.20
5.66
8.79
3.15
1.22
0.42
0.26
48.71
1.94
15.18
11.72
0.19
7.28
10.93
2.94
0.44
0.29
0.48
49.04
1.96
15.27
11.66
0.19
6.87
10.43
3.12
0.54
0.29
0.26
49.47
1.39
14.94
10.4
0.18
7.39
10.58
2.95
0.65
0.23
0.81
49.21
1.34
12.72
10.08
0.19
9.43
12.95
2.12
0.48
0.17
1.11
71.35
0.47
13.92
3.14
0.04
0.92
1.95
3.61
3.97
0.12
0.68
Total
99.17
100.03
100.72
100.54
100.57
99.17
99.40
100.10
99.63
98.99
99.80
100.17
Representative major element analyses of gabbroic
rocks and granitic matrix from three pillow mounds
accurately re¯ect the bimodal character of the Vinalhaven plutonic rocks (Table 1). The compositions of the
chilled gabbros range from 48 to 53 wt.% SiO2 and from
5.7 to 8.5 wt.% MgO. The highly variable wt.% K2O
generally correlates with increasing wt.% SiO2,
although the wide K2O variation at constant SiO2 is
probably due to selective contamination (alkali
exchange) with the matrix. Gabbro with the lowest
wt.% K2O is compositionally similar to low-K, backarc tholeiites (Wilson, 1989). Higher concentrations of
CaO and MgO in one massive and coarser-grained
gabbroic sample from the west Lane Island mound
(Table 1) suggest some local accumulation of augite.
The three analyzed samples of matrix between
pillows are typical of the thicker and more homogenous portions of the matrix. They are somewhat lower
in SiO2 and enriched in CaO, MgO and FeO relative to
the main Vinalhaven granitic rocks. Whole-rock
analyses were not obtained for the relatively ma®c
and hybrid matrix because of its heterogeneity.
5. Field occurrences of the pillow mounds
Nearly all of the pillow mounds form rocky headlands along the coast near the base of the thick
gabbro-diorite unit (Fig. 1). Their occurrences suggest
that they are more resistant to erosion than either the
adjacent granitic rocks or the coarser-grained ma®c
rocks that make up much of the gabbro-diorite unit.
Intertidal exposures suggest that the mounds sit on
rocks ranging from granite to hybrid quartz diorite.
Many of the mounds appear to be contiguous with
thick sections of massive gabbro and probably grade
laterally to them. Orientations of elongate pillows
suggest that ¯ow directions in the mounds do not
generally coincide with the dips of ma®c sheets in
the gabbro-diorite unit (Fig. 1); hence, these accumulations appear to have developed local topography.
Stacks of sheets, tubes and pillows in larger mounds
commonly have dips that radiate outward toward the
margins of the mounds. All of these features indicate
that the ma®c sheets, tubes and pillows built up
mound-like bodies on granitic to hybrid crystal
mush that formed the ¯oor of the magma chamber
at the time it was invaded by basaltic magma. The
internal structure of four smaller but characteristic
mounds, are described below.
5.1. Clayter's Beach pillow mound
The exposed portion of the Clayter's Beach pillow
mound is about 25 £ 40 m in area, with at least 4 m of
relief (Fig. 2). About 8 m of massive gabbro at the SE
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Fig. 2.
R.A. Wiebe et al. / Journal of Volcanology and Geothermal Research 107 (2001) 171±184
177
Fig. 2. Clayter's Beach mound. (A) Overview of the mound in a section roughly parallel with the direction of ¯ow of sheets and tubes (to the
right). Note that the gentle dips on the left gradually steepen to the right. To the left, these sheets grade to massive gabbro; to the right, they
grade to disrupted pillows with larger amounts of matrix. Field of view approximately 25 m wide. (B) Detail of mound structure, just to the
right of center in A. Note that the lower bodies are pillow-like with convex-upward tops and bases that point downward, ®lling irregularities
between underlying pillows. At higher levels, the ma®c material is sheet-like with mostly ¯at, smooth upper surfaces and an irregular base that
®ts underlying pillows. Field of view about 3 m wide. (C) View of opposite side of mound with ¯ow to the left. Field of view about 6 m wide.
(D) Simpli®ed sketch of (C). Note how several sheet-like bodies appear to have overridden pillows. Arrows indicate probable ¯ow directions.
Some irregular lobes appear to represent buds from tubes or sheets. (E) Near the distal margin of the mound. Here, nearby pillows have shapes
which indicate that they were molded against each other; they appear to have moved and rotated into granite after they solidi®ed.
end of the mound grades into well-packed chilled
sheets and pillows which de®ne a delta-like structure.
The outcrop, thus, displays a transition from massive
gabbro to ¯attened, tubular ¯ow units to discrete
pillows in granite. Fig. 2A shows a section parallel
to the ¯ow direction (left to right), in which chilled
bodies have aspect ratios of from about 20:1 to 5:1 and
are typically separated by 1±2 cm of granitic matrix.
At the proximal end (left) the sheets dip gently (about
108) to the right; toward the distal end of the mound,
dips steepen upward to about 608 (Fig. 2A). Sheets
and pillows typically ®ll in irregularities below them
and have tops that are characteristically smooth and
relatively ¯at (Fig. 2B). Magma in some sheets
appears to have overridden previously solidi®ed
pillows in their paths (see arrows in Fig. 2D). On
subhorizontal outcrop surfaces, sheets and pillows
can be seen to be elongate down dip of the sheets
(see the foreground of Fig. 2C).
The distal (NW) end of the mound grades into a
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Fig. 3. East Lane Island mound. (A) View along the axis of ¯ow (away from the observer). The ¯attened pillow-like bodies dip away from the
observer. The thickness of the granitic matrix tends to decrease upward, and some larger subequant areas of granite occur near the base of the
mound. (B) A perpendicular view of the mound taken from around the left margin of view in (A). Flow direction is to the left. Basal pillows are
nearly horizontal and the dip increases upward relatively abruptly to about 758.
region of matrix-supported pillows which have
greater than 5 cm of matrix between chilled bodies
(Fig. 2E). The shapes of nearby pillows appear
capable of ®tting together like a jig-saw puzzle; this
®t suggests that the pillows were originally molded
more closely against each other. After they solidi®ed, these pillows may have been dislodged by
granitic matrix that was ®lter-pressed from between
Fig. 4. West Lane Island mound. (A) View of a steep section nearly perpendicular to the axis of a trough-like accumulation of pillows. Pillows
tend to be smaller, ¯atter and more tightly packed at the central base of the mound. (B) A close-up view of a vertical face roughly parallel to the
direction of ¯ow (to the left). (C) Simpli®ed sketch of (B). Interconnections between the pillows are clear, and two examples of budding can be
seen (see arrows). The tight packing and highly irregular shapes of the tubes suggest that they continued to ¯ow after the chilled margins were
formed, squeezing out granitic matrix and continuing to deform without fracturing.
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179
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Fig. 5. Norton Point mound. Oblique view of central portion of the mound. Outcrop surface in the foreground dips about 408 toward the viewer.
Although dip of sheets is nearly vertical, the character of pillow stacking indicates stratigraphic tops to the right. Outside of the ®eld of view (to
the right) the mound grades to matrix-supported, disrupted and broken pillows. See text for discussion of features.
underlying ma®c bodies or may have slumped off of
the steep front the mound. A few meters further to the
NW, this zone of matrix-supported pillows grades into
granite with scattered and smaller angular fragments
of pillows.
5.2. Pillow mound on the east side of Lane Island
This mound is exposed in an outcrop about 25 m
wide and 8 m high (Fig. 3). The steep N±S face of the
outcrop exposes tightly packed, highly ¯attened
pillows which de®ne an asymmetric, weakly
convex-upward or domal shape with tabular pillows
that dip to the west (Fig. 3A). The pillows in this
section of the mound are uniformly elongate with an
aspect ratio averaging about 7:1. A section at nearly
right angles to the N±S face, shows that the dips of the
pillows increase strongly upward from about 158 at
the base to about 758 near the top (Fig. 3B). Comparable pillows with steep attitudes occur for several
meters west of the steep outcrop face.
Exposures near the southern base of the pillow
mound show that the matrix to the pillows grades
upward from hybrid diorite to coarse-grained granite
in less than 3 m. This compositional variation of the
matrix probably re¯ects an original compositional
gradient in the cumulate crystal mush, much like
that seen in the upper, hybrid portions of layers in
the gabbro-diroite unit. The thickness of the matrix
between pillows decreases upward from a few cm at
the base to less than a few mm at the top. Larger
equant areas of granite up to 50 cm in diameter
occur locally in the mound (Fig. 3); these areas may
be sections through channels where granitic crystal
mush ¯owed as it was squeezed out of the mound
by the accumulating pillows.
5.3. Pillow mound on the west side of Lane Island
This mound is exposed in an outcrop roughly
15 £ 15 m with about 7 m of relief (Fig. 4). Flattened
pillows de®ne a trough which is exposed on vertical
faces both normal (Fig. 4A) and parallel to the trough
axis (Fig. 4B and C). The pillows are more ¯attened
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and tightly packed at the central base of the trough
(aspect ratio about 9:1) and become larger and more
nearly equant towards the trough margins (aspect ratio
about 3:1) (Fig. 4A). The granitic matrix is less than
1 cm thick between the basal pillows and increases to
several centimeters in thickness towards the trough
margins. At higher levels in the section, pillows are
strongly molded against each other and typically have
upward pointing projections.
A section parallel to the trough axis (Fig. 4B and C)
shows tightly packed and highly irregular, elongate
chilled bodies that probably represent deformed
tubes comparable to those that fed the pillow-like
bodies in Fig. 4A. The terminations of the tubes and
the direction of budding indicate that ¯ow was to the
left. About 4 m to the right of Fig. 4B, the tubes grade
to massive gabbro.
The internal structure of this mound contrasts with
the ®rst two mounds. It has a trough-like form in
which the dips of the ¯attened pillows remain nearly
horizontal along the trough axis. The accumulation of
pillows, here, appears to occur within the axis of a
subhorizontal ¯ow unit.
5.4. Pillow mound at Norton Point
The pillow mound at Norton Point is exposed in an
elongate and narrow coastal section and truncated at
each end by granitic dikes. The central portion of the
mound consists mainly of tightly packed sheets and
pillow-like masses with dips that range from about 708
SW to nearly vertical (Fig. 5). Inland (NE), the chilled
ma®c bodies become much larger and more nearly
equant; further inland, the mound appears to grade
to massive gabbro. To the SW, intertidal exposures
include some areas of matrix-supported, fractured
ma®c bodies. These relations suggest that the
mound built in a southerly direction.
Small-scale features in the mound also indicate tops
to the south (to the right in Fig. 5). Several ma®c
sheets ®ll in irregularities to the left and have ¯at
smooth tops to the right. Small-scale ¯ame structures
occur mainly on the left (lower) margins of the sheets.
Immediately above (to the right of) sheet #1 is an area
of deformed pillows with a central zone of ¯attening
at high angles to the underlying sheet. These
deformed pillows apparently slumped on a steep
depositional surface before they became solid. As
181
they did solidify, they formed a ridge on an otherwise
smooth mound. Subsequent ma®c sheets appear to
onlap and eventually cover (see sheet #2) this irregular topography on the surface of the mound. This
sequence of relations demonstrates that the mound
was built up incrementally by separate ¯ows rather
than by movement of all lobes simultaneously.
One unusual and distinctive feature of this mound is
the common occurrence of granitic cores within ma®c
sheets and pillows. Thin zones of hybrid material
between the granite core and enclosing pillow suggest
that the pillow interiors were partially liquid when
granite entered. On the steep slopes of this mound,
the pillows may have continued to move downslope,
well after a nearly solid rind had formed and the
supply of basaltic magma had waned. While comparable ¯ow in subaerial and submarine settings would
probably produce open tubes in lava ¯ows or intrapillow cavities, here, the pillows were immersed in
granitic magma at a pressure of perhaps 1 kbar.
Such conditions probably precluded the formation of
a cavity. Instead, wherever the rind broke and the
pillow and tube continued to move, granitic magma
from the surrounding matrix would have ¯owed into
the interiors of the pillows. These granitic cores
appear to be common only in mounds where the
dips of sheets and pillows are unusually steep. The
steep slope of ¯ows may have been essential to permit
continuing movement for a longer time as solidi®cation occurred.
6. Discussion
The four Vinalhaven pillow mounds have many
features in common. Each is a portion of a larger
structure. Each mound was likely fed by a single,
homogeneous pulse of basaltic magma because there
is little chemical variation within any mound. The
forms and sizes of pillows and tubes in each mound
are similar, and it is possible to recognize the connections between sheets, tubes and pillows. Three of the
mounds developed steeply dipping foreset surfaces,
while one appears to have been deposited in a subhorizontal channel. The granitic matrix to the Vinalhaven
pillows, a mixture of silicic liquid and small
(1±2 mm) crystals, apparently was readily mobilized
at the margins of the pillows. This mobilization is
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R.A. Wiebe et al. / Journal of Volcanology and Geothermal Research 107 (2001) 171±184
demonstrated by the common absence of matrix
between strongly chilled pillows in the lower portions
of some mounds.
6.1. Geologic setting of the pillow mounds
Field relations indicate that the pillow mounds
developed on or within a viscous, crystal-rich mush
of hybrid to granitic material, almost certainly at the
base of a crystal-poor silicic magma chamber. Basaltic dikes that occur widely in the lower portions of the
pluton have compositions identical to the pillows and
comparable dikes probably acted as feeders to the
ma®c sheets and pillows mounds. Basaltic magma
would have been able to rise as dikes through a
solid lower portion of the chamber, but would have
been forced to spread out at the base of the crystalpoor interior of the chamber. Most of the mounds
appear to grade in one direction to massive gabbro,
which probably solidi®ed from thick ¯ows that spread
across the chamber ¯oor. The mounds, which could
have been fed by an in¯ating ¯ow, may have developed at the margins of these thick ¯ows.
The compositions of both ma®c and silicic rocks in
the Vinalhaven granite are similar to those in several
other coastal Maine plutons that record the entry of
ma®c magma into a silicic chamber (Wiebe, 1993b,
1994; Wiebe and Ulrich, 1997). In these other intrusions, ma®c input typically resulted in spreading
extensive sheets (from less than a meter to several
tens of meters thick) onto a ¯oor of granitic to dioritic
crystal mush; accumulations with the steep depositional surfaces found on these mounds have not
been noted. The deposition of numerous, parallel
sheets of ma®c magma appears to re¯ect a relatively
sharp rheological transition between an effectively
solid ¯oor and a liquid interior.
The Vinalhaven pillow mounds appear to be
concentrated near the base of the gabbro-diorite unit
with a thick section of granite lying underneath. While
a slow rate of infusion appears essential for the formation of pillows (Grif®ths and Fink, 1992; Walker,
1992), we believe one additional factor that might
have favored the formation of pillow mounds here,
rather than the extensive thin sheets seen at higher
levels, was the existence of a very thick transition
zone (tens to hundreds of meters?) between an effectively solid, crystal-rich ¯oor and a crystal-poor interior.
Such a thick transition zone might exist at the base of
a silicic magma chamber that had been undisturbed by
ma®c replenishments for a relatively long period of
time. Subsequent ma®c replenishments may have
lowered the crystallinity of the lower portions of the
chamber and produced a more abrupt transition
between a solid ¯oor and liquid interior, permitting
ma®c replenishments to ¯ow much farther from the
source and leading to the formation of the more
common sheet-like macrorhythmic units.
6.2. Physical conditions during emplacement of ma®c
magma
The compositions of the chilled ma®c rocks are
equivalent to basaltic magmas with about
7±8 wt.% MgO. The uniform basaltic textures of
these rocks suggest that, at most, only a few percent
phenocrysts were present when they encountered the
granitic magmas. At pressures of about 1 kb, these
ma®c liquids probably had initial temperatures
between 1100 and 12008C (Wilson, 1989). The
granitic matrix between pillows has compositions
which approach that of water-saturated minimum
melt liquids at about 1 kbar (Tuttle and Bowen,
1958). Since it is likely that the granite was undersaturated in H2O, the temperature must have been
somewhat higher than that of the minimum. Both
feldspar and quartz crystals must have been present
at the base of the chamber when the ma®c magma
was emplaced because the chilled pillows commonly
have irregular margins which are molded around
individual quartz and feldspar crystals in the granite,
and because both quartz and feldspar occur sparsely
as xenocrysts within the ma®c pillows. The initial
temperature of resident granitic magma was, therefore, at the quartz-feldspar cotectic, probably close
to 7508C, and initial temperatures at the contacts
between the two magmas would have rapidly
approached about 9508C.
The subsequent change in temperature at these
margins would have depended on the subsequent
movements of the two magmas. If other ma®c lobes
quickly displaced the adjacent granitic matrix, the
initial temperature (9508C) at the margin of a lobe
may have stayed the same or risen for a short time.
On the other hand, if the lobe continued to ¯ow
through large volumes of cooler granitic magma, the
R.A. Wiebe et al. / Journal of Volcanology and Geothermal Research 107 (2001) 171±184
temperature at its margin would have soon reached
7508C, producing a rigid crust on the pillow. Although
the granitic magma would initially have been much
more viscous, crystallization of the basalt would soon
lead to viscosity reversals that explain the later injections of granitic matrix into variably solidi®ed ma®c
pillows.
At an equilibrium temperature of about 9508C, the
margin of the basalt would have been partially crystallized. The dominance of hornblende in the chilled
margins suggests, however, that H2O must have
diffused relatively rapidly into the basalt, lowering
its liquidus temperature and delaying some crystallization to lower temperatures than in an anhydrous
basalt.
6.3. Initiation and growth of the pillow mounds
The injection of relatively dense, low viscosity
basaltic magma into a chamber of high viscosity
silicic magma should occur as a viscous gravity
current that overrides a layer of granitic melt (Snyder
and Tait, 1995). Fluid dynamic experiments suggest,
further, that the ¯ow front of the basalt will develop
instabilities that produce ®ngers in the direction of
¯ow (Snyder and Tait, 1995). As a ma®c ®nger
extends into the granitic magma, cooling should
lead to the development of a partly crystallized rind
of increasing thickness; continued supply of new
magma should eventually lead to in¯ation, rupture
and the budding of new lobes that move out and override earlier ones. These may produce isolated pillows
or branching tubular structures. If the ¯ow rate is low
enough, the crystalline rind of a tube may become
rigid, so that new ®ngers ¯owing above it will tend
to ®ll in irregularities in the earlier ®ngers (tubes),
producing, in sections normal to the ¯ow, downward
points characteristic of pillow lavas that form in
water. Pillows in the Clayter's Beach mound (Fig. 2)
generally are of this type.
If the rate of ¯ow is higher, then new ®ngers may
override earlier ones that have had less time to cool
and, hence, are less crystallized and more readily
deformable. Here, the weight of the new tubes may
deform underlying ones and squeeze them into upward
projecting points between active tubes. Many of the
pillows in the central portions of the trough-like structure
in the west Lane Island mound (Fig. 4A) are of this type.
183
Pillows in this mound also increase in size upward.
The basal pillows may represent the toes of the initial
¯ow front, and subsequent ¯ows may have overridden
the initial ¯ow front, leading to progradation of the
mound. Pillow-like bodies toward the top of a vertical
section through the trough are, therefore, probably
sections of tubes that are increasingly distant from
the advancing ¯ow front (toward the viewer in
Fig. 4A).
The Clayter's Beach and east Lane Island mounds
show rapidly steepening dips in the direction of ¯ow
that apparently re¯ects the inability of magma to
travel far from its source at the initial low slopes
before crystallizing. As these lower lobes solidi®ed,
continued ¯ow of ma®c magma probably led to in¯ation of the ¯ow and outbreaks that fed tubes at higher
elevations and produced a steeper ¯ow front that
migrated in a manner analogous to foreset beds in a
delta (Jones and Nelson, 1970). The Norton Point
mound (Fig. 5) also appears to have been constructed
by adding sheets on a very steep slope; the steepness
and internal structure of this mound closely resemble
a section through foreset tubular pillows in a submarine ¯ow front drawn by W.B. Bryan (Basaltic Volcanism Study Project, 1981, Fig. 5.2.28b). Both the
Clayer's Beach and Norton Point mounds tend to
have smaller pillows and thinner sheets in the direction of growth as well as some angular fragments of
pillows at the very front of each mound. This transition within these plutonic pillow mounds appears to
be analogous to that seen in some submarine mounds
(Ballard and Moore, 1977).
7. Conclusions
Pillows mounds in the Vinalhaven granite appear to
be a newly recognized type of structure that can form
when basaltic magma invades and ¯ows across the
¯oor of a silicic magma chamber. The development
of these mounds probably requires a relatively slow
rate of infusion; ®eld relations also suggest that they
may have formed when the base of the silicic chamber
had a thick transitional zone of crystal-rich mush
between a solid ¯oor and a liquid interior. The occurrence of these mounds provides insights into the
physical state of the magma chamber and provides
compelling evidence, along with more common
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R.A. Wiebe et al. / Journal of Volcanology and Geothermal Research 107 (2001) 171±184
chilled ma®c sheets, that granitic plutons commonly
solidify by accumulation on an aggrading chamber
¯oor.
Acknowledgements
This research was supported by NSF Research
Grant EAR-9804879 to RAW. We thank reviewers
Kelsi Dadd, Wendell Duf®eld and James White for
their helpful comments.
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