- © 2002 Cambridge University Press
Melting of Torridonian arkoses, due to intrusion of shallow bodies of mafic melts on the Isle of Rum, was locally extensive, reaching up to 95 vol. %. Crystallization (to form granophyre) of these highly melted rocks was generally static in the vicinity of minor intrusions, although disruption of bedding is apparent in the aureole of the main mafic pluton. A distinctive millimetre- to centimetre-scale spherulitic texture developed during solidification of the partially melted arkose, in both the aureole of the main intrusion, and also in those of associated minor gabbro plugs. The spherulites are formed of radiating, fan-shaped intergrowths of plagioclase and quartz set in a matrix of quartz, plagioclase and K-feldspar. The importance of nucleation rates in determining texture development is demonstrated by a dominance of Na-rich compositions in the first feldspar to form, even in non-spherulitic rocks. It is suggested that the spherulites formed by the early growth of metastable plagioclase due to difficulties in nucleating feldspar.
Spherulitic textures are common in felsic igneous rocks, both extrusive and intrusive, and form during crystallization far from equilibrium, either directly from a melt or from a supercooled glass (e.g. Keith & Padden, 1963; Lofgren, 1971; Smith, Tremallo & Lofgren, 2001). They can be divided into two main types (e.g. Harker, 1909). Monomineralic spherulites are comprised of clusters of confocally radiating fibrous crystals which may show a range of morphologies, from a simple wheatsheaf to a completely spherical form. Intergrowths of two minerals are also known to form spherulitic textures, and perhaps the best known of these occur in granophyres. In this paper, I describe compositionally zoned spherulites developed in partially melted arkose from the aureole of the Rum Igneous Complex (Scottish Inner Hebrides). Previous accounts of these spherulitic rocks ascribed the texture to a variation in the extent of melting due to micro-scale differences in the concentration of phyllosilicate grains, with greater degrees of fluid-present melting in the centres of the spherulites (R. G. Greenwood, unpub. Ph.D. thesis, Univ. Glasgow, 1987; D. E. Kitchen, unpub. Ph.D. thesis, Queen’s Univ. Belfast, 1981). In this paper I show that the spherulites grew in almost completely melted arkose under conditions with inhibited feldspar nucleation.
2. Geological setting
The Central Igneous Complex of Rum (Fig. 1⇓) was intruded at 60.5 ± 0.08 Ma (Hamilton et al. 1998) at < 2 km depth into a ridge of Precambrian rocks flanked to the west and east by Mesozoic sedimentary basins. See Emeleus (1997) for a detailed description of the geology. The mafic intrusions forming the Layered Complex are almost all emplaced within either the quartzofeldspathic rocks of the Lewisian Gneiss (which is only locally exposed) or the feldspar-rich, Precambrian, Torridonian Sandstone.
Numerous Tertiary minor intrusions include at least 40 plugs of gabbro and peridotite, varying in diameter from 10 m to several hundreds of metres. The peridotite plugs were probably intruded as dense suspensions of olivine crystals in dyke-like fractures and pipe-like conduits emanating from the main magma chamber relatively late in the history of the igneous complex (Wadsworth, 1994). The gabbro plugs are thought to have fed major lava flows (Emeleus, 1997; Holness, 1999).
The layered mafic and ultramafic rocks of the Central Complex are in contact with Torridonian Sandstone at its northern and eastern margins (Fig. 1⇑). Anatexis of the Torridonian Sandstone occurs within 40–50 m of the contact and is best exposed in the east, with abundant intrusion breccia and pronounced disruption of bedding. Spherulitic textures are found in partially melted rocks at many localities along the eastern margin of the Central Complex (R. G. Greenwood, unpub. Ph.D. thesis, Univ. Glasgow, 1987). They are developed particularly in the relatively coarser grained metasediments, and evident on weathered surfaces as low-relief mounds up to 1 cm in diameter. This texture also occurs in the aureoles of many of the minor gabbro and peridotite plugs in the north of the island (Wadsworth, 1994; Holness, 1999; C. H. Emeleus, pers. comm. 1998).
The present study concentrates on two representative exposures of spherulitic arkose. One is at Allt nam Bà (Fig. 1⇑) in the aureole of the Central Complex and the other is in the aureole of a minor gabbro plug exposed in the stream Allt Bealach Mhic Nèill in Kinloch Glen (Fig. 1⇑). Previous work on the latter established that the pressure of melting of the currently exposed rocks was 150 ± 50 bars (Holness, 1999). Although no published pressure estimate exists for the metamorphism of the main aureole at Allt nam Bà, it is likely to have been similar.
3. The Torridonian protolith
The samples collected as part of this study are from the Scresort Sandstone and the Allt Mòr na h-Uamha members of the Applecross Formation of the Torridon Group.
The Scresort Sandstone Member is a moderately well-sorted, medium- to coarse-grained, thickly bedded pink arkose containing 20 to 30 vol. % feldspar, of which the great majority is a perthitic alkali feldspar of variable but intermediate composition. There are also small quantities of generally albitic plagioclase and grains of conspicuously twinned microcline that are not visibly perthitic. Clastic muscovite occurs as scarce deformed grains up to 0.5 mm diameter, aligned with bedding. Multiple, thin (< 1 mm) layers of detrital magnetite, haematite ± epidote ± chlorite are common. The rock is cemented by a mixture of quartz overgrowths, turbid K-feldspar and an oxidized ferruginous clay now dominated by chlorite. Representative bulk rock analyses are given in Table 1⇓.
The Allt Mòr na h-Uamha Member directly underlies the Scresort Sandstone Member and consists of cyclically interbedded siltstones and fine- to medium-grained cross-bedded sandstones. This study concentrates on the latter which contain abundant chlorite-rich layers, commonly rich in magnetite and haematite grains. The feldspar component (up to 30 vol. %) is predominantly alkali feldspar (including visibly twinned microcline), with up to 3 vol. % plagioclase. Small, deformed muscovite flakes are common, as are detrital grains of apatite and epidote.
4. Textures in melted Torridonian arkose
4.a. Allt Bealach Mhic Nèill
This stream-bed locality exposes a complete section through the aureole of a 50 m diameter gabbro plug (described by Holness, 1999). The onset of melting in the country rock is visible in thin-section as the appearance of fine-grained granophyric intergrowths of quartz and feldspar. The correspondence of such granophyric texture to the previous presence of a melt phase is confirmed by the dominance of this texture (up to 95 vol. %) in rocks which show significant migmatitic disruption in other areas of the aureole (see next section). In outcrop the onset of melting is manifest as a whitening and hardening of the rock some 15 m from the contact. The melt proportion increases to about 75 vol. % within a few metres from this point and thereafter increases over 12 m to a maximum of ~ 95 vol. % at the contact (Holness, 1999). Limited mingling of the gabbroic magma and the partially melted country rock has produced a contact facies of intermediate composition. Melting was static, as shown by the preservation of millimetre-scale magnetite-rich layers even within rocks containing the maximum amount of melt (Holness, 1999). This lack of disruption is believed to be due to the very high melt viscosity.
Where the amount of melt exceeds about 80 vol. %, rock exposures show spherical growths with a concentric structure, set in an apparently uniform matrix. Spherulites furthest from the contact are small (2 mm diameter) and poorly defined, comprising a homogeneous white material set in a dark grey matrix (Fig. 2a⇓). At the contact with the gabbro they are larger (5–6 mm diameter) and diffuse, with a central dark grey portion surrounded by a pale 0.5 mm thick rim, set in a darker grey matrix. The most clearly defined spherulites occur in the central part of the partially melted zone, 9–10 m from the contact. In this region, they cluster in layers parallel to the undisturbed original bedding (Fig. 2b⇓).
Thin section observations show that melting began on grain boundaries between quartz and feldspar crystals. As the amount of melting increases the resultant granophyric rims become thicker, with eventual consumption of the feldspar. In samples with small amounts of restitic feldspar, the outermost 20 μm of each feldspar grain is optically clear in contrast to its turbid centre (Fig. 3a⇓). Electron microprobe analysis shows that these clear rims are invariably pure albite. Rounded and embayed grains, or polycrystalline aggregates, of quartz are also interpreted to be restitic. These latter grains have thin rims of highly turbid, fine-grained, granophyre (Fig. 3b⇓). Plate-shaped aggregates of polycrystalline quartz in the melt or attached to the surfaces of restitic quartz grains (Fig. 3c⇓, 4a⇓) are quartz paramorphs after tridymite.
The spherulitic texture is obvious in hand-specimen, but more obscure in thin-section. All areas contain restitic quartz and quartz paramorphs after tridymite with no relationship between the amount of restitic material and the positions of the spherulite centres. The spherulites are characterized by a central, highly turbid core formed of clusters of sheaf-like radiating microgranophyric intergrowths 0.1 to 0.2 mm long. The individual components of the intergrowths are ~ 1 μm thick. Although the sheaves themselves radiate from the spherulite centre, under crossed polars it is apparent that the feldspar component of the intergrowth, the continuous phase, does not have a radiating structure. This contrasts with true monomineralic spherulites which are made of individual fibrils with non-crystallographic branching (e.g. Keith & Padden, 1963; Smith, Tremallo & Lofgren, 2001). The outer rim of the spherulites, corresponding to the lighter rings on weathered rock surfaces (Fig. 2b⇑) is also made of sheaf-like quartz–feldspar intergrowths. The inter-spherulitic matrix is comprised of coarse-grained intergrowths of quartz and turbid feldspar.
The partially melted samples are highly porous (Fig. 4b⇑), with no obvious porosity differences between the turbid spherulite cores and their clear rims. Comparison of density with that of the protolith suggests an increase of porosity of ~ 4 vol. %, probably due to feldspar dissolution during the high-temperature hydrothermal circulation that also caused the feldspar turbidity (I. Parsons, pers. comm. 2000).
Partially melted arkose within ~ 10 cm of the contact contains pores or drusy cavities up to 1 cm across, lined with prismatic quartz crystals. Sub-millimetre scale pores are also present within the inter-spherulitic matrix, concentrating in the regions between three or more spherulites. In contrast to the smaller pores attributed to dissolution, the large prismatic quartz crystals suggest that this porosity is primarily resulting from fluid saturation and expulsion from the melt during solidification.
4.b. Allt nam Bà
Partially melted rocks of the well-bedded Allt Mòr na h-Uamha Member in the aureole of the Central Complex are exposed in low-lying outcrops (10 m ×10 m) at Allt nam Bà. Disaggregation and loss of sedimentary structures occurs within a short distance of the melt-in isograd, pointing to melt movement probably related to nearby syn-metamorphic faults (Emeleus, 1997). Restitic blocks within the disaggregated material contain up to 50 vol. % melt, demonstrating that high volumes of melt were required for melt mobilization.
Rock outcrops interpreted to be close to the onset of melting have developed a spherulitic texture which results in a pustular appearance on weathered surfaces. The interspherulite matrix in these rocks is enriched in biotite (Table 2⇓) and the preferential weathering of the mica-rich component results in their characteristic appearance. In places, spherulites become gradually more prominent over a distance of several tens of centimeteres. Elsewhere a sharp, but irregular, boundary divides spherulitic from non-spherulitic areas (Fig. 5⇓). There is a poorly defined general increase in spherulite size as the contact is approached from 0.5 cm to a maximum of 1.5 cm. No spatial clustering of spherulites occurs on scales < 10 cm, in contrast to the frequently observed planar concentrations of spherulites at Allt Bealach Mhic Nèill.
In thin-section, faint colour differences occur between the spherulite centres (5 mm wide), rims (~ 1 mm thick) and the matrix. The spherulites are composed of confocally radiating sheaf-like microgranophyric intergrowths with a colony size similar to that at Allt Bealach Mhic Nèill, although the thickness of individual components within each colony is larger at ~ 5 μm. Albite twinning is sometimes visible in the optically continuous feldspar component of the intergrowths. Tridymite plates are generally smaller that those at Allt Bealach Mhic Nèill, are less well defined, and have undulose edges (Fig. 6a⇓) suggestive of recrystallization to reduce the high surface energy associated with the platy habit. The restitic quartz grains are smaller, more rounded in shape, and less abundant, possibly due to the finer-grained protolith. Abundant euhedral grains of orthopyroxene, variably replaced by retrograde chlorite, are scattered evenly throughout the rock (Table 2⇑). Dendritic or plate-like ilmenite crystals are also present (Fig. 6b⇓), together with restitic detrital apatite and zircon.
The interspherulite matrix has two distinct textural types, both of which may occur in a single thin-section. One is coarsely crystalline (Fig. 6c⇑), with large sieve-textured biotite grains, rounded quartz, turbid alkali feldspar and lacking paramorphs after tridymite. The other (Fig. 6d⇑) is composed of coarse granophyric textured feldspar and quartz with tridymite plates. In contrast to the Allt Bealach Mhic Nèill rocks, the porosity of these rocks is < 1 vol. %, perhaps related to a lesser amount of feldspar turbidity and hydrothermal alteration.
Three non-spherulitic samples were examined as control specimens. Each rock contains distinct tabular and subhedral grains of plagioclase in contrast to the anhedral plagioclase invariably intergrown with quartz of the spherulitic samples.
5. Small-scale compositional variation in spherulitic rocks
5.a. Electron microprobe analyses
An electron microprobe investigation was made of partially melted arkose from the two spherulitic localities and of non-spherulitic rocks for comparison. Analyses of fine-grained intergrowths and large feldspar grains were made using a defocused ~ 100 μm diameter beam. Compositional variation across spherulites was investigated using a spot diameter of up to 100 μm, with a 100 μm spacing between analyses. Traverses across spherulites were chosen to minimize intersections with restitic quartz and tridymite paramorphs. See Table 2⇑ for representative analyses.
The broad beam analyses are assumed to be representative of the composition of the melt phase, although the silica component of the analyses may be rather higher than that of the melt due to the almost unavoidable inclusion of tridymite paramorphs. A CIPW norm was calculated for each inferred melt composition (Table 2⇑). The most abundant elements in each analysis are Si, Al, Na and K, with minor Ca, Mg and Fe in the samples from Allt nam Bà. The Mg and Fe reside entirely in orthopyroxene and chlorite, with the rest of the analysed region comprising a mixture of feldspars and quartz. Consequently the averaged compositions minus Mg and Fe were recalculated both as simple ratios of Na, Ca and K, and also as mixtures of quartz, plagioclase and orthoclase to compare measured compositions with phase relations in the Qtz–Ab–Or–An system.
5.a.1. Samples with restitic feldspar
Granophyric intergrowths (interpreted as solidified melt) in these rocks, which all come from Allt Bealach Mhic Nèill, are found only on inter-phase grain boundaries. The compositions of inferred melt in sample R91 (see Fig. 3a,b⇑), averaged over a 30 μm diameter area, together with analyses of adjacent feldspar grains, were re-calculated for graphical representation in the quartz–albite–orthoclase system at low pressures (Fig. 7⇓). Most of the averaged melt compositions lie close to the 0.5 kbar cotectic of Tuttle & Bowen (1958) and the restitic feldspar grains are plausible sources for the immediately adjacent melt.
Slightly more sodic melt compositions (Na/(Na+K) ~ 0.45) occur adjacent to restitic material, with a reduction to Na/(Na+K) ~ 0.32 farthest from the restitic grains (Table 2⇑). This effect was investigated using a smaller spot size (~ 3 μm), despite the questionable validity of such a procedure for analysis of a hydrothermally altered polyphase intergrowth. These more detailed analyses demonstrate that the turbid granophyre adjacent to restitic quartz (Fig. 3b⇑) has Na/(Na+K) ~ 0.80.
5.a.2. Spherulitic samples
Samples from both localities were examined. Traverses made across individual spherulites, using a spot diameter of 100 μm and a spot spacing of 100 μm, are 30 μm, recalculated to Qtz–Ab–Or and shown with the 0.5 kbar cotectic of Tuttle & Bowen (1958). The tie-lines join the compositions of restitic feldspar with the adjacent granophyric intergrowth interpreted to be solidified melt. displayed in Figure 8⇓. Representative averaged compositional analyses, together with the CIPW norms, are given in Table 2⇑. Note that these analyses are averaged over large areas and have a higher silica content than the last melt to crystallize would have done due to the unavoidable presence of tridymite paramorphs.
Figure 8a⇑ shows the Na, K and Ca content along a traverse across a spherulite in sample R122, collected from Allt nam Bà. The average SiO2 content is ~ 80 wt %. In contrast, the Ca, Na and K contents show a distinctive pattern that can be correlated with the texture. The spherulite centre is dominated by a plagioclase feldspar (~An20), with no measurable K. Towards the spherulite margins the Ca content decreases over ~ 1 mm, with a corresponding slight increase in Na. At a distance of 500 μm from the edge of the spherulite Na/(Na+Ca+K) drops from ~ 0.80 to a value of ~ 0.40, accompanied by an antipathetic increase in K/(Na+Ca+K) from zero to ~ 0.60. In the inter-spherulitic matrix the composition averaged over 100 μm varies on a scale of ~ 800 μm. Na and Ca are coupled, and have an antipathetic relationship with respect to K. This is presumably because there are two feldspars present, one a K-rich alkali feldspar and the other oligoclase.
Figure 8b⇑ shows an analogous profile obtained across a pair of spherulites from Allt Bealach Mhic Nèill (sample R93). Again there is a strong partitioning of Na and Ca into the spherulite centres, corresponding to the highly turbid region, with almost all the Ca concentrated there. These centres are thus comprised of quartz and plagioclase. The yellowish margins to the turbid centres correspond to the region over which Na/(Na+Ca+K) decreases from ~ 0.9 to ~ 0.3, with a corresponding rise in K/(Na+Ca+K) from ~ 0 to ~ 0.65. As in sample R122, the matrix between the spherulites has a composition alternating between Na-rich and K-rich ‘end-members’ on a scale of ~ 500 μm. The amount of SiO2 across the traverse in all the average analyses remains at ~ 80 wt %.
5.a.3. Non-spherulitic samples with high melt proportions
All samples in this category came from Allt Bealach Mhic Nèill. The spatial pattern of compositional variations of the granophyric textured material is essentially identical to that of the inter-spherulitic matrix in the samples described above.
5.b. Back-Scattered Electron images
The spatial variations in the Na/K ratio were investigated on a finer scale using Back-Scattered Electron (BSE) imaging on a scanning electron microscope. Due to the high porosity of samples from Allt Bealach Mhic Nèill, BSE imaging was confined to sample R122 (Allt nam Bà).
A plagioclase-bearing spherulite centre is shown in Figure 9a⇓, and demonstrates the fine (micron) scale of the granophyric intergrowth. Towards the edge of this central region, there are large irregular pockets of K-feldspar (Fig. 9b⇓) containing plagioclase inclusions whose shape suggests simultaneous crystallization rather than exsolution (Fig. 9c⇓). At the edge of the Na-rich core, the feldspar is K-rich, with prominent lamellae (Fig. 9d⇓) or large irregular patches of Na-rich feldspar. In the inter-spherulite matrix two feldspars form an irregular intergrowth on a scale of < 10 μm, with sub-micron exsolution lamellae in the K-rich phase. The margins of restitic quartz grains have discontinuous rims of Na-rich feldspar.
6. Summary of textural observations
The spherulitic texture is comprised of a core of radiating intergrowths of plagioclase and quartz, with a rim containing two feldspars, as either large irregular pockets and patches or as discrete ?exsolution lamellae within large grains. The observed scale of possible exsolution is compatible with that expected for the aureole of a large intrusion (Brown & Parsons, 1984). The spherulites are surrounded by a coarser-grained matrix of inter-grown quartz, plagioclase and K-feldspar, the latter showing signs of exsolution of a plagioclase component. Restitic material is randomly distributed suggesting that during the early stages of crystallization the partially melted rock was homogeneous.
In common with other descriptions of spherulites (e.g. Smith, Tremallo & Lofgren, 2001) the spherulitic texture is interpreted as a consequence of limited nucleation of feldspar at relatively widely spaced centres, with subsequent growth of fan-like granophyric colonies, each nucleating on earlier colonies. The first feldspar to nucleate was invariably albitic plagioclase. The matrix crystallized last, with a cessation of fan-like growth once alkali feldspar began to crystallize. Plagioclase appears to have been the first feldspar to crystallize, even in non-spherulitic samples, forming Na-rich granophyre adjacent to restitic grains (Table 2⇑, Fig. 9⇑) and albitic rims on restitic feldspars.
7. Compositional controls on textural development
Any model for the formation of the spherulitic texture is based on the assumption that the observed spatial patterns of compositional variation described above are original and have not been modified since formation. The high porosity of the samples, especially those collected from Allt Bealach Mhic Neill, point to significant post-solidification hydrothermal alteration, and it is certainly possible that the observed compositional variations are not original. However, it is likely that such fluid circulation would act to homogenize, rather than amplify, original compositional variations and so the Ca- and Na-rich cores to the spherulites are most probably original.
A further point is that the observed textures support spherulite formation at high temperatures, consistent with formation during solidification of a hot, supercooled, liquid rather than devitrification at low temperatures. Evidence such as the cavities lined with euhedral quartz in the inter-spherulitic matrix at Allt Bealach Mhic Nèill, the concentration of coarse biotite in the inter-spherulitic matrix at Allt nam Bà and the generally coarse grain-size of the quartz–feldspar intergrowths in the inter-spherulitic matrix demonstrate high diffusivities and low nucleation rates associated with crystallization at high temperatures. Although it is well known that a significant under-cooling (of perhaps several hundreds of degrees: Smith, Tremallo & Lofgren, 2001) is required for the onset of spherulitic feldspar growth, the high temperatures required for melting at 150 bars mean that undercooling of 200 °C could still lead to crystallization at ~ 700 °C.
Sample R137 was collected from the area shown in Figure 5⇑. This sample has a spherulitic ‘front’ running through it, with a sharp contact between spherulitic and non-spherulitic portions. The bulk compositions for both the non-spherulitic and the spherulitic portions are given in Table 1⇑. These are indistinguishable, demonstrating there has been no bulk compositional control on order of crystallization and the development of spherulites. This is a general feature of these rocks, as can be demonstrated by considering a plausible equilibrium crystallization path.
The CIPW norms calculated from bulk compositions of samples containing no restitic feldspar (Table 1⇑) are dominated by quartz, orthoclase and albite. The normative anorthite content is generally low, with all but five samples (all from Allt nam Bà) containing <1 w % An. Normative Ab:An ranges from 3.7 to 55. With the exception of one analysis, more than ~ 95 % of the norm of partially melted arkose is comprised of Q, An, Ab and Or (Table 2⇑) and so the arkoses of Rum can be described using the Qtz–An–Ab–Or–H2O system, shown schematically in Figure 10⇓. Bulk rock compositions are plotted on the Qtz–Ab–Or face of this system in Figure 11a⇓.
The crystallization path expected for a rock solidifying under conditions close to equilibrium depends on the relative position of the bulk liquid composition, projected onto the Ab–An–Or face, compared to that of the polythermal solvus intersection and the projection of line cd, labelled PAQL (Fig. 11b⇑). For bulk compositions in field 1, the melt will initially crystallize plagioclase which is later joined by alkali feldspar. Some resorption of plagioclase would occur in rocks lying close to the polythermal solvus intersection. For bulk compositions in field 2, complete resorption of early plagioclase would occur and the solidified rock would contain a single (alkali) feldspar. In field 3, the completely solidified rock would contain quartz and two feldspars, the first to form being an alkali feldspar. In field 4 early plagioclase would be completely resorbed and the solidified rock would contain a single (alkali) feldspar. The true positions of the polythermal solvus intersection and the line PAQL are poorly known and are highly sensitive to H2O activity, with field 1 increasing at the expense of field 2 on the addition of H2O (Nekvasil, 1990). Hence it is possible only to surmise the expected crystallization history for the Rum rocks.
All spherulitic samples crystallized two feldspars, with initial formation of plagioclase spherulites, followed by simultaneous (non-spherulitic) crystallization of plagioclase and an alkali feldspar. Resorption of plagioclase, if it occurred, was incomplete. All spherulitic samples should thus lie in field 1 and non-spherulitic samples in fields 1 or 3. The CIPW normative compositions of samples containing no restitic feldspar are projected onto the Ab–An–Or face in Figure 11c⇑, giving an approximation of the melt composition. There is no evident correlation between composition and texture. Even given the uncertainty of the position of the field boundaries the samples most likely to fall in field 1 and exhibit the observed order of crystallization are non-spherulitic.
8. Discussion and conclusions
The development of spherulites in the Rum arkoses is not linked to inferred melt composition. It results from early nucleation and growth of (probably metastable) plagioclase forming confocally radiating granophyric intergrowths with quartz, with coarse-grained, irregular intergrowths that formed once alkali feldspar had nucleated. Previous studies have shown that a large degree of undercooling, or departure from equilibrium, is necessary before crystals grow with the non-crystallographic branching required for true spherulite development. In the examples discussed here, although there is no evidence for non-crystallographic branching or fibre formation, crystallization far from equilibrium resulted in early plagioclase nucleation in preference to a more stable, but harder to nucleate, alkali feldspar. Spherulites formed from the resultant growth of confocally radiating clusters of granophyric intergrowths at widely dispersed nucleation sites.
Further support for the importance of nucleation in texture development is given by the presence of distinct tabular crystals of plagioclase in the three non-spherulitic samples from Allt nam Bà. The nucleation and growth characteristics of feldspar clearly differed between spherulitic and non-spherulitic rocks. The onset of spherulite development is controlled by the relative and absolute ease of nucleation of both plagioclase and alkali feldspar. In spherulitic rocks, plagioclase not only nucleates in preference to alkali feldspar (regardless of thermodynamic equilibrium), but also forms fan-like granophyric intergrowths. In non-spherulitic rocks feldspar nucleation occurred at many sites, with growth of euhedral plagioclase grains in preference to cotectic intergrowths. The difference between the two feldspar growth forms is noteworthy, as it is well known that alkali feldspar can form spherulites made of fibrous crystals (Lofgren, 1971; Fenn, 1977; Smith, Tremallo & Lofgren, 2001). However, in these rocks, the onset of alkali feldspar crystallization coincides with the cessation of spherulitic growth.
The importance of nucleation is clearly demonstrated by the concentration of spherulites in layers parallel to the (undisturbed) protolith bedding in the Allt Bealach Mhic Nèill aureole. This suggests that initial feldspar nucleation was heterogeneous and occurred at sites (such as restitic detrital grains) concentrated on distinct planes inherited from the protolith. This is consistent with experimental and field evidence for difficult nucleation of feldspars (Swanson, 1976; Fenn, 1977; Vernon, 1986).
The sharp division between spherulitic and homogeneous rock observed at Allt nam Bà suggests that whatever was controlling feldspar nucleation also changed abruptly at the boundary. Given that the thermal history could not change significantly on a metre-scale and bulk melt composition plays no role in texture development we are left with two possibilities for the controlling factor. One is the relative abundance of nucleation sites provided by restitic grains, and the other is the relative abundance of some cryptic component such as H2O.
The formation of planar concentrations of spherulites at Allt Bealach Mhic Nèill shows that the density of sites for heterogeneous nucleation is certainly important, although the disaggregation and disruption of protolith bedding at Allt nam Bà prevents this hypothesis being tested.
In common with Smith, Tremallo & Lofgren (2001), I suggest that the melt H2O content played an important role in texture development. It has been shown experimentally that nucleation rates can be greatly reduced by increasing the melt H2O content (Swanson, 1976; Fenn, 1977). This effect is accompanied by a reduction in the undercooling required for the nucleation rate to peak (Fenn, 1977). It is possible that subtle differences in melt H2O content, perhaps inherited from the protolith, may also have affected the amount of undercooling required for feldspar nucleation, with suppression of alkali feldspar nucleation.
In conclusion, the spherulitic texture developed at high temperatures during solidification of partially melted arkoses. Crystallization was dominated by the kinetics of nucleation, with early plagioclase forming granophyric intergrowths in spherulitic regions, and growing as euhedral laths in non-spherulitic regions. This resulted in a strong compositional zoning within spherulitic rocks, with most of the Ca and much of the Na concentrated into the spherulite centres.
The spherulitic arkoses of Allt Bealach Mhic Nèill were brought to my attention by C. H. Emeleus to whom I am grateful. Field assistance was provided by S. T. C. Siklos. Discussions of the importance of Ca on the crystallization history with J. D. Clemens and Ian Parsons greatly clarified my understanding of the subject, but any remaining misconceptions are my own. The manuscript benefited from the insightful comments of Ian Parsons, Richard Hinton, John Clemens and an anonymous reviewer. The samples collected for this study are housed in the Harker Collection in the Sedgwick Museum, University of Cambridge, and are accessible for examination on application to the Curator.
- Received February 14, 2002.
- Accepted July 15, 2002.