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Journal of Volcanology and Geothermal Research 107 (2001) 171±184 www.elsevier.nl/locate/jvolgeores 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)00253-5 172 R.A. Wiebe et al. / Journal of Volcanology and Geothermal Research 107 (2001) 171±184 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 174 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 176 R.A. Wiebe et al. / Journal of Volcanology and Geothermal Research 107 (2001) 171±184 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 178 R.A. Wiebe et al. / Journal of Volcanology and Geothermal Research 107 (2001) 171±184 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. R.A. Wiebe et al. / Journal of Volcanology and Geothermal Research 107 (2001) 171±184 179 180 R.A. Wiebe et al. / Journal of Volcanology and Geothermal Research 107 (2001) 171±184 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 R.A. Wiebe et al. / Journal of Volcanology and Geothermal Research 107 (2001) 171±184 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 182 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 184 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. References Ballard, R.D., Moore, J.G., 1977. Photographic Atlas of the MidAtlantic Ridge. Springer, New York, 114 pp. Basaltic Volcanism Study Project, 1981. 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