I chose a set of previously well-described igneous bodies (sills and a lava lake) with approximately the same magma composition (to avoid any effects of significantly different mineral mode or liquid line of descent), but widely varying size (and thus crystallization times), to explore the effects of cooling rate on plagioclase grain shape. The sills examined in this study include those that were the subject of earlier work linking dihedral angle variation with cooling timescales (the Traigh Bhàn na Sgùrra, Whin, Portal Peak and Basement Sills, Holness et al. (2012); and the Skaergaard Campsite dykes, Holness et al. (2013)). For each of the chosen bodies, the country rock was likely to have been cool since intrusion was generally shallow. Plagioclase is thought to have grown in situ in most of the bodies studied here, although the Basement Sill may have been filled with an initially plagioclase (and orthopyroxene)-phyric magma (Bedard et al. 2007). However, our Basement Sill samples contained no very large plagioclase grains so it is likely that plagioclase grain shape in our sample suite reflects controls by cooling and crystallization in situ.
The Traigh Bhàn na Sgùrra Sill
The Traigh Bhàn na Sgùrra Sill, on the Isle of Mull (Inner Hebrides), ranges from 2.5 m to ~8 m thick (Holness and Humphreys 2003). Flow through the sill was localized into the wider portions (c.f. Bruce and Huppert 1989), which are characterized by a wide thermal aureole and the absence of a well-developed chilled margin. We sampled the sill in two places, one where it is 3.5 m thick and where flow is inferred to have been continuous for some time (samples with prefix ROM48—this is Traverse Z of Holness and Humphreys (2003) at UK grid reference NM 42289 18542, or 56.289098°N 6.1661259°E), and the other where the sill is 3.1 m thick and where the flow duration is inferred to have been short, leading to a well-developed chilled margin and a negligible thermal aureole (samples with prefix ROM43—this is Traverse X of Holness and Humphreys (2003) at UK grid reference NM 41348 18248 or 56.285952°N 6.1809986°E).
The sill is a basaltic andesite and shows no significant stratigraphic variation in major element composition, with SiO2 of 54–55 wt%, MgO of 5.6–6.2 wt% and total Fe (FeO + Fe2O3) of 10–11 wt% (Preston 1996; a single representative bulk composition (labelled SOB1) is presented by Preston et al. 1998). Groundmass plagioclase has a compositional range from core to rim of An66–56 (Preston et al. 1998). Phenocrysts are rare and the silica-rich mesostasis comprises altered glass containing dendritic crystals of oxides. Holness and Humphreys (2003) documented weak flow alignment of plagioclase in four of the Traigh Bhàn na Sgùrra samples we examined (ROM43-190, -170, -80 and -BASE).
The Whin Sill
The Whin Sill crosses northern England from Newcastle to Carlisle. The samples described here are from the Throckley borehole (UK grid reference NZ 14557 67617, or 55.002913°N 1.7739581°E) where it is 38.56 m thick. It comprises tabular plagioclase, with interstitial Ca-rich pyroxenes and minor Fe–Ti oxides, rare orthopyroxene phenocrysts, and a silicic mesostasis of graphic intergrowths of quartz and alkali feldspar. Plagioclase core compositions vary from An74 to An55, with little systematic variation with height (Figure 4 of Dunham and Strasser-King (1981)). Bulk rock major and trace element concentrations of the samples examined here are presented by Dunham and Strasser-King (1981) and demonstrate a general compositional uniformity, with SiO2 between 49 and 50 wt%. MgO exhibits a C-shaped profile, with 6 wt% in the marginal 3–4 m and 5 wt% in the central portion. Total Fe (FeO + Fe2O3) is 13.5 wt% at the base, decreasing to 12.6 wt% by 5 m stratigraphic height, with a subsequent increase to 13.4 wt% at the top of the sill.
The variation of average plagioclase grain size (from Dunham and Strasser-King (1981) and assumed to be the average long axis length) is greatest within a few metres of the margins and is approximately symmetrical across the sill. In contrast to the other bodies examined as part of this study, plagioclase grains in the Whin samples tend to cluster in groups of parallel grains with their (010) faces in contact, suggestive of synneusis and agglomeration.
The Portal Peak Sill
The Portal Peak Sill in the Queen Alexander Range, Antarctica (83.83°S 165.60°E), is part of the Ferrar Dolerite Group and is a sub-alkalic low-Ti quartz dolerite. The suite of samples described here were collected from a 129-m section through the sill. The main phases are plagioclase, augite and pigeonite, with a granophyric mesostasis. No plagioclase compositional data are available. The sill composition is essentially uniform with height (Hergt et al. 1989; Faure and Mensing 2010). The principal exception is the lower chilled margin, which has low concentrations of SiO2, MgO, Na2O and CaO, while Al2O3 and H2O + are unusually high. In the main body of the sill, the concentration of SiO2 decreases up-section from about 54 to 52 wt% at about 100 m stratigraphic height. In the upper 30 m, the SiO2 concentration increases towards 54 wt%. The concentration of MgO ranges from about 6 to 7 wt%. The average plagioclase grain size varies asymmetrically, with the coarsest region near the top of the sill (Holness et al. 2012).
The Koffiefontein Sill
The sedimentary rocks of the Karoo Supergroup, South Africa, host a large number of dolerite sills formed during the Mesozoic breakup of Gondwanaland, penecontemporaneous with the Ferrar Group of Antarctica (a summary description of this group of sills is provided by Cawthorn 2012). A borehole drilled in 1998 by Afriore Pty. Ltd., about 20 km southwest of Gariep Dam in the Northern Cape Province, contains a dolerite body 290 m thick. This body contains two younger intrusions, recognizable by their chilled margins. The original sill, known as the Koffiefontein Sill, is 189 m thick (Slement 2010; Cawthorn 2012). The chilled margins on the two subsequent intrusions are consistent with their intrusion being sufficiently later than that of the original to have had no effect on the crystallization history of the earliest sill.
Geochemical analysis of 36 samples through the Koffiefontein Sill (Slement 2010) reveals a smooth, but subtle M-shaped profile (using the terminology of Latypov and Egorova 2013). SiO2 varies between 50 and 56 wt%. MgO increases from 8 wt% at the base to 13 wt% at 24 m stratigraphic height, then decreases smoothly to 4 wt% by 160 m stratigraphic height. MgO then increases again to 9 wt% before a marginal reversal in the topmost few metres. These variations in bulk rock MgO are associated with variations in the concentration of olivine phenocrysts. No plagioclase compositional data are available.
The Basement Sill
The Basement Sill is the stratigraphically lowest member of the four Ferrar Dolerite Group sills exposed in the McMurdo Dry Valleys of Antarctica. In contrast to the three upper sills, which have doleritic to sub-ophitic microstructures, the microstructure of the Basement Sill is more similar to that of cumulate rocks, with a coarse-grained websterite central region under- and overlain by gabbronorite (Bedard et al. 2007). On the basis of a detailed microstructural and geochemical study, Bedard et al. (2007) suggested the sill formed from a slurry of orthopyroxene and plagioclase grains with subsequent migration and unmixing of crystals and liquid.
We examined samples from West Bull Pass (77.50°S 161.83°E). The upper 9 samples of the suite were collected along a 1.86-km East/West traverse through the sill outcrop, whereas the lower 6 were collected along a NW/SE traverse close to the basal outcrop of the sill (A. Charrier, pers. commun. 2011). Previous discussions of microstructural variation in this sample set were based on the assumption that the sill is 266 m thick in this locality (Holness et al. 2012). However, the sample set described by Bedard et al. (2007) was collected along essentially the same East/West traverse as the upper 9 samples studied here (locations given in their Supplementary Appendix A1.1) and, using altitude information and the dip of the sill, Bedard et al. (2007) report a thickness of 362 m. We accordingly adjusted the stratigraphic heights of our samples to give an overall sill thickness of 362 m.
Groundmass plagioclase varies in composition from An85 to An69 (Figure 16 of Bedard et al. 2007). In those samples containing abundant large phenocrysts of orthopyroxene, the plagioclase is generally finer-grained than in the Portal Peak and Whin Sills. Much of the variation of average plagioclase grain size occurs at the margins, and the coarsest grain size occurs near the top of the sill (Holness et al. 2012). The marginal chill zones have a granular microstructure.
The Makaopuhi prehistoric lava lake
Makaopuhi crater occurs on the east rift zone of Kilauea volcano, Hawaii, and comprises two partially overlapping collapse pits. The larger and older east pit crater is filled with olivine tholeiite basalt to form a roughly elliptical prehistoric lava lake that was originally ~1,000 m across the widest diameter. The subsequent collapse of the west pit sliced about 170 m from the widest diameter, displaying a complete transect through the lake where it is 68.6 m deep. The younger west crater was subsequently filled to a depth of 100 m by an eruption in 1965, partially obscuring the section through the prehistoric lake.
Moore and Evans (1967) provide bulk compositions, together with descriptions of the mineralogy and microstructure (including average long and short axes of plagioclase grains observed in thin section) of a suite of samples collected through the entire exposed thickness of the prehistoric lava lake before it was partially obscured by the later eruption. Further, mineral compositional data are provided by Evans and Moore (1968). We used the samples collected and described by Moore and Evans (1967) for the present study.
Bulk rock SiO2 contents are in the range 46–53 wt%, with MgO of 6.6–19.4 wt% (Table 1 of Moore and Evans (1967)). Stratigraphic variation of bulk rock composition is primarily the result of settling of olivine phenocrysts: the originally erupted lava contained ~7 vol% olivine phenocrysts which sank to form an olivine-rich zone 23 m above the floor of the lake and an olivine-depleted zone between 30.5 and 39 m from the floor (Moore and Evans 1967).
The main minerals present are olivine, augite, pigeonite, feldspar, ilmenite, magnetite and apatite. The groundmass plagioclase is strongly zoned throughout, with maximum anorthite compositions ranging from An79, in the centre of the lake, to An68, near the floor (Evans and Moore 1968). Brown glass and cristobalite are common near the upper middle of the lake, and large poikilitic orthopyroxenes occur near its base.
The Skaergaard Campsite dykes: a test case
The Tertiary Skaergaard intrusion of East Greenland coast comprises approximately 280 km3 of basaltic magma intruded at the shallow crustal unconformity between Precambrian gneisses and overlying Tertiary flood basalts into a fault-bounded magma chamber (Nielsen 2004) formed at the extending continental margin. The magma crystallized as a closed system (Wager and Deer 1939). The Skaergaard intrusion is cut by two generations of steeply dipping Tertiary dykes that form part of the coastal dyke swarm of East Greenland (Brooks and Nielsen 1978; Nielsen 1978), the earlier of which comprises predominantly north–south trending basaltic and doleritic dykes (Nielsen 1978). Just north of Homestead Bay, close to the Skaergaard Peninsula (an area which has become informally known as the “campsite”) are two well-exposed dykes from this generation (at 68.167°N 31.720°W). One is a 6-m wide composite dyke known as the Campsite Dyke (Jakobsen et al. 2010, following Irvine et al. 1998), and the other (the plagioclase-phyric Dyke) is 6.5 m wide and contains numerous cm-scale phenocrysts of plagioclase. Core compositions of plagioclase in the plagioclase-phyric Dyke and in the central part of the Campsite Dyke are in the range An74-76, Jakobsen et al. (2010). The two dykes have similar bulk compositions, with SiO2 of 46–48 wt% and MgO of 6.3–7.6 wt% (full bulk compositional data are provided by Jakobsen et al. 2010).
These dykes are of particular importance as the Campsite Dyke contains abundant cumulate xenoliths that have been linked to the underlying parts of the Skaergaard intrusion (Jakobsen et al. 2010; Holness et al. 2013). Thus, the timing of dyke emplacement is of paramount importance for establishing these xenoliths as indicators of the contemporaneous chemical and microstructural state of the underlying plumbing system.
The relative age of the two dykes cannot be determined from field relations as no cross-cutting relationships are exposed (Irvine et al. 1998). Furthermore, both dykes have chilled margins and there is no field evidence that can be used to date dyke injection relative to the cooling history of the host. However, Holness et al. (2013) argued, using dihedral angle data, that the Campsite Dyke formed when the Skaergaard intrusion was still hot (920–970 °C), whereas the host intrusion had cooled substantially more, below 670 °C, at the time the plagioclase-phyric Dyke intruded. Accordingly, one might expect the plagioclase grain shape in the more slowly cooled Campsite Dyke to be different to that in the more rapidly cooled plagioclase-phyric Dyke. Four samples were examined, two collected from the central part of each dyke, to test this hypothesis.