The pluton is a term used to describe any igneous body emplaced beneath the Earth's surface and crystallized where it is located--a plutonic body.In: Encyclopedia of Geology (Second Edition), 2021

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2.3 Plutons

Platons are bulbous masses that are commonly associated with the subduction of plates beneath strings of volcanoes.Hundreds of plutons may nestle together in batholiths, intimately intermingled or even piercing one another. .Although the individual plutons within these batholiths are horizontally flattened and, to some degree, sheetlike, they have much smaller aspect ratios (2-20) than sills and dikes.Instead of being injected like sills and dikes, plutons rise in the form of sluggish thunderheads, eventually losing buoyancy at the leading edge, slowing down, then spreading as the lower reaches ascend.The body inflates in place, just as other body parts that start as necks and expand into plutons.Even though plutons can have enormous areas (hundreds of square kilometers), they are often as large as spheres of diameters of 2-10 kilometers.

Figure 25.11. Field relationships of tonalite–trondhjemite–granodiorite in the Stolzburg block. (A). Typical pavement of homogeneous trondhjemite of the Theespruit Pluton (11a on Fig. 25.10). (B) At the same locality, close-up showing the homogeneous and pristine igneous texture. (C and D) In narrow zones around the Stolzburg pluton, field relations become more complex. (C) Several generations of dykes transecting the common coarse-grained trondhjemite in a quarry near the R541 turn-off (11c on Fig. 25.10). (D) A dark, fine-grained dyke in the ordinary coarse trondhjemites (11d on Fig. 25.10, i.e., close to a locality where a sample was dated at c.3.2 Ga; Schoene et al., 2008). (E) An intrusive breccia of the Theespruit Pluton in Onverwacht amphibolites at Elukwatini (11e on Fig. 25.10); also see Fig. 25.13. The box in photo (E) highlights a trondhjemitic phase similar to sample BAR-11-08 of François (2014), dated at 3450 ± 40 Ma at this locality. (F) Igneous layering in the Doornhoek Pluton (11f on Fig. 25.10).

Both the Theespruit and Stolzburg plutons are intrusive into adjacent greenstone belt (Onverwacht Group) lithology. They cut across both stratigraphic contacts and an older foliation in tightly folded, steeply dipping, and steeply lineated lowermost parts of the BGB. For example, the northern part of the Theespruit Pluton shows a sharply truncating contact against folded greenstone lithology (Fig. 25.10; Kisters and Anhaeusser, 1995a; Diener et al., 2005). The TTG plutons contain angular xenoliths of amphibolites and schists near their contacts, and greenstone lithologies contain intrusive breccias, pointing to the originally relatively high-level emplacement of the TTGs (Anhaeusser, 1984; Kisters and Anhaeusser, 1995a). The original igneous contacts are now variably deformed and range from pristine, well-developed intrusive breccias (Figs. 25.11E, 25.13), to deformed intrusive breccias containing strongly prolate (steeply plunging) strains, to pervasively transposed contacts invaded by aplitic veins (Van Kranendonk, 2011). Although subtle, the original, probably sheeted, nature of the trondhjemites can still be discerned in places (Kisters and Anhaeusser, 1995a).

Emplacement ages of Stolzburg block TTG (Fig. 25.6) are c.3450 Ma for the Stolzburg Pluton and 3440 Ma for the Theespruit Pluton. Two ages from trondhjemites to the south of the Theespruit Pluton (BA118 and BA151; Kröner et al., 2016) are also in the same range. Younger ages of c.3380 Ma were found by Armstrong et al. (1990) in a smaller, marginal, phase of the Theespruit Pluton (sample BG4-86, Fig. 25.10). Ages from both plutons show a narrow peak at 3440–3450 Ma and then a long “tail” with ages extending to 3430 Ma or younger (Fig. 25.6), suggestive of a lead loss event affecting c.3450 Ma igneous zircons.

The c.3.45 Ga plutons are mostly high-silica trondhjemites defining a high-Sr, low-Y, high-Sr/Y series. The compositions are tightly clustered and define good trends in the diagram of Moyen et al. (2017) (Fig. 25.5). These plutons have the most homogeneous and igneous-looking compositions of the BGGT. Small, but consistent, variations allow individual plutons, or even components of plutons, to be identified. At least three main phases can be identified in the Stolzburg Pluton, having slightly different field appearances (e.g., coarse- vs. medium-grained: Fig. 25.10) and defining very slightly different geochemical series in a ΔSr–ΔNC diagram (Fig. 25.5). The boundaries between the phases are gradational, with no sharp contacts or breccia textures. This may suggest that the different phases correspond to successively emplaced magma batches intruding a partially molten mush.

In the Theespruit Pluton, where only one phase has been identified, the trondhjemite is somewhat less sodic and straddles the boundary with tonalite. Stolzburg block TTGs cover a range of SiO2 values, from c. 67%–74%, and some diorites are present close to the center of the pluton (Anhaeusser, 2010). In map view, this variation in silica content defines a concentric chemical zonation (Fig. 25.12), with the core of the pluton being lower in silica and bordered by higher silica margins. This zonation is elongated along the long axis of the pluton. The small Honingklip Pluton, to the west of the Theespruit Pluton, geochemically resembles the Theespruit Pluton more than the Stolzburg Pluton.

Parts of the plutons show prolate solid-state fabrics evidenced by the presence of a steep stretching lineation (and lack of clear foliation), locally observed in the cores of some plutons and affecting the intrusive breccias along the pluton margins (Fig. 25.13). To better define the timing of this constrictional deformation, two samples were dated from an intrusive breccia on the western side of the Theespruit Pluton, near Elukwatini village (Fig. 25.13; see Appendix 1 for methods and results). From this locality, the deformed coarse-grained trondhjemite that created the intrusive breccia was dated at 3450 ± 40 Ma (François, 2014). Two undeformed, to syntectonic, dykes that cut across the deformed intrusive breccia give identical ages of 3383 ± 11 Ma (BL13/11) and (one zircon only) 3388 ± 38 Ma (BL13/12) (Fig. 25.13). This constrains at least some of the deformation at between 3450 and 3380 Ma.

Granite plutons of Proterozoic age are found intermittently throughout the TBSZ, with the most important plutons forming in Kanigiri, Podili, and Vinukonda.Vinukonda lies on the western margin of the TBSZ, while Kanigiri and Podili plutons are spatially connected in the central part of the zone.Detailed descriptions of each pluton are given below. .Its northern part displays ductile shear zones signifying intense deformation.NSB metabasic rocks in the central part of KGP show sharp contacts.The pluton contains fluorite-bearing quartzofeldspathic veins.The geochemical and petroleum studies of KGP reveal that molybdenite and rare metals are present there.

The Kanigiri biotite granite and Podili alkali granite plutons are emplaced along the contact zone between Udayagiri Group of upper NSB toward the west (chlorite schist, agglomerate tuffs and intercalated quartzite) and the sheared granite towards the east. Enclaves of rock units of older NSB are widely noticed across Kanigiri-Podili granite plutons (PdGPs), while deformed basement biotite granite gneiss are exposed intermittently as low-lying outcrops along the western margin of TBSZ.

PdGP, located to the south of Podili town, represents a deformed leucocratic alkaligranite pluton. The PdGP occurs in the form of hills in an apparent continuation with KGP and its host rocks of quartz–mica schist, chlorite schist and intercalated quartzites of NSB. Enclaves of chlorite schist and meta-acid volcanics of NSB are also preserved within the pluton. The presence of an undeformed pyroxene–amphibole syenite body in the northern part of PdGP is a striking feature. Tourmaline-bearing quartz veins traverse the pluton in the western part while blue quartz is conspicuous in the southern part. The deformational fabric trending N–S to NNW–SSE is well developed throughout the pluton and is relatively more deformed along the margins. The interlying region between the Podili and KGPs is occupied by the dominant presence of quartz–chlorite schist and quartzites of NSB. Intensely deformed hornblende–biotite gneiss with NNE–SSW deformational fabrics is well exposed to the east of Kanigiri–Podili plutons. A volcanic plug composed of rhyodacite is reported to the east of the PdGP. Enclaves of the rhyodacite are noticed in the central part of the PdGP, indicating that the volcanic plug is possibly a part of the preexisting suite of lithounits belonging to the older Archean NSB.

A small, semielliptical body of pyroxene–amphibole syenite occurs in the central part of northern part of PdGP. It is relatively undeformed, coarse-grained, massive, mesocratic and essentially composed of microperthite, plagioclase, and amphibole with subordinate quartz, biotite, and clinopyroxene while sphene, chlorite, monazite, apatite, carbonates, and ilmenite are observed as accessory minerals. The field observations, distinct mineralogy, and chemical characteristics suggest that both KGP and PdGP are deformed along the margins and were emplaced along the TBSZ in a late-orogenic phase close to the vicinity of a possible collision boundary zone. The chemistry of these granites shows that they are crystallized from a fluorine saturated magma derived from the partial melting of enriched continental crust along the TBSZ.

All the granite plutons including the KGP and PdGP are intensely deformed particularly along the margins, while the development of crude foliation is observed in the central parts. Petrographically, a majority of these granites vary from alkali feldspar granite to granite. The field observations, mineralogical association, and chemical characteristics suggest that the emplacement of these granite plutons was restricted to TBSZ possibly during the late-orogenic to anorogenic tectonic setting close to the vicinity of a collision boundary zone (Sesha Sai, 2013). Although both KGP and PdGP are spatially coexisting, coeval (Mesoproterozoic), and ferroan in nature, they are distinct in their mineralogical characteristics. The PdGP is riebeckite–arfvedsonite–biotite bearing hypersolvus granite with higher Na2O/K2O ratio, while the KGP is essentially a subsolvus two-feldspar biotite granite with lower Na2O/K2O ratio. Fluorite is a conspicuous accessory mineral in both the plutons. The chemistry of both the plutons shows typical characteristic features of anorogenic A-type granites. Rb–Sr dating yielded an isochron age of 1120±25 Ma for Kanigiri granite (Gupta, Pandey, Chabria, Banerjee, & Jayaram, 1984), while Mesoproterozoic age of 1.33 Ga was attributed to the plagiogranite of Kanigiri ophiolite mélange (KOM) (Dharma Rao, Santosh, & Yuan, 2011) that occurs in the vicinity.

Vinukonda granite pluton (VGP) is located in the close vicinity of the Eastern Cuddapah thrust and immediately to the southwest of Vinukonda town. The VGP is leucocratic, medium to coarse–grained alkali feldspar granite in the form of a hill range (6 skmx 2 km) trending NW–SE with intense deformational fabrics along the margins. The VGP intruded the metamorphosed and strongly deformed granitic epidote–biotite gneisses, which were recrystallized during epidote–amphibolite facies metamorphism. The VGP consists of a medium to coarse–grained and weakly porphyritic leucocratic meta-granite with multigrain biotite blotches of up to two cm length that impart a spotted appearance. Widespread titanite–epidote amphibolite layers within the VGP are interpreted as metamorphosed basaltic dykes that intruded the plutonic precursors of the gneisses (Dobmeier, Lütke, Hammerschmidt, & Mezger, 2006). A supracrustal rock unit of magnetite–garnet–biotite schist (25×3 m) also occurs within the granitic gneisses. Fluorite is a common accessory phase in the white mica-bearing biotite–plagioclase–quartz–K–feldspar meta-granite. Accessory phases include apatite, magnetite, titanite, and zircon. Broadly, the foliations in the pluton show WNW–SSE trends. A shear zone with mylonitic fabrics occurs along the eastern margin of the pluton separating the spotted meta-granite and a medium-grained grayish meta-granite. The two-mica character of VGP indicates its subsolvus nature. Fluorite is noticed as conspicuous accessory mineral in the host alkali feldspar granite (Sesha Sai, 2013). The zircons from VGP yielded an age of 1590 Ma, which is interpreted as the emplacement age of the VGP (Dobmeier et al., 2006). The time of emplacement of the granitic precursor could be coeval with the emplacement of calc–alkaline plutons in the TBSZ (Ongole domain) between 1720 and 1704 Ma.

“Pluton” is used for any intrusion, regardless of its shape, size, or composition. Many special names were coined in the 1900s for intrusions of particular shape and/or relationship with enclosing rocks, but most have fallen into disuse, either because of scarcity of examples or because they are recognized as variants of other, more common types. These common types include dikes (dykes in the UK), sills, lopoliths, laccoliths, cone sheets, ring dikes and bell-jar intrusions, funnel-shaped intrusions, batholiths, stocks, and plugs (Fig. 7).


Some of the most common types of intrusive bodies are dikes and sills. Both are tabular, parallel-sided bodies that are very much thinner than their lateral extent. Most are a few to a few hundred meters thick. When exposed by erosion they may extend for tens to hundreds of kilometers in extent. The difference between the two types is relationship to their host rocks. Like the banks or walls created to prevent flooding which cut across a landscape and which the intrusion type is named after, dikes crosscut bedding and mineral alignment structures within the country rock. Sills, on the other hand, are concordant bodies which generally lie between bedding planes within the country rocks. Consequently most dikes are vertical or steeply inclined, whereas sills are horizontal or of low inclination. Both bodies are normally emplaced by dilation of the country rocks induced by excess pressure of the magma, but faulting may sometimes be involved. Mapping of an area commonly reveals tens to many hundreds of dikes in a parallel array, a dike swarm. Some dikes display evidence of the movement of several magma batches through them, with later ones cutting earlier ones; these form multiple dikes. So-called sheeted dike complexes, consisting of a vast number of such dikes, characterize much of the oceanic crust. These form at mid-ocean ridges and act as feeders to the overlying lava flows that erupt in the axial rift valley.

As concordant bodies, laccoliths and lopoliths are variants of sills. Laccoliths are lens-shaped and normally 1–2 km at the thickest. They have a planar base but a domed upper surface, above which the country rocks are arched up. On the other hand, lopoliths have a saucer form, implying sagging of the underlying rocks under the weight of emplaced rock; most are several kilometers thick and can be very extensive in area, covering thousands of square kilometers, as in the case of the Bushveld Complex, South Africa. Laccoliths and lopoliths can be produced from the amalgamation of sills and have normally been fed by several dikes which, unable to rise higher, spread their magma laterally along bedding planes and coalesce.

Cone sheets, ring dikes, and funnel intrusions are all discordant bodies. A cone sheet is a thin dike (from less than 1 meter to several meters) with the form of a downward-pointing cone, causing it to display a circular outcrop pattern (Fig. 7). The diameter of the cone sheet may vary from several hundreds of meters to several kilometers. It is usual for large numbers of such sheets to be concentrically arranged. The apex of the cones is considered to be located at the top of a former magma chamber. Each sheet is formed by overpressure of magma in the chamber, causing fracturing of the overlying rocks and forcing magma into the fracture.

Ring dikes are also circular in outcrop, reflecting their upward-pointing, truncated conical form in three dimensions. They are typically inclined outward at a steep angle. Dikes vary in thickness from meters to hundreds of meters and their diameters range from several kilometers to several tens of kilometers. Ring dikes are commonly surmounted by a bell-jar intrusion, which is effectively a disk-shaped sill. The ring-dike plus bell-jar combination results from the vertical subsidence into an underpressured magma chamber of the block of country rock at the center of a ring dike. As this so-called cauldron subsidence proceeds, the resulting space is filled by magma displaced from the chamber. If the ring fracture penetrates to the earth's surface, a circular crater known as a caldera is formed and magma erupts within the crater (Fig. 7).

Funnel-shaped intrusions are mostly occupied by basic and ultrabasic rocks. One of the best studied is the Skaergaard intrusion in east Greenland (see below, at the end of Section VI.A). This body has the shape of a champagne glass with two feeder pipes at the base. In some cases funnel intrusions have the long, linear form of a dike but are V-shaped in cross section and narrow downward. These intrusions are described as funnel dikes. Although uncommon, they can be very large; for example, the Great Dike in Zimbabwe is over 500 km long, several kilometers wide and up to 3 km thick; it contains at least 35,000 km3 of rock.

A batholith is the collective name for a group of plutons of various shapes, sizes, and rock types that have accumulated and intruded one another over a long interval of time. They typically form a linear belt up to hundreds of kilometers long and tens of kilometers wide, as in the case of the coastal batholith of Peru and the Sierra Nevada batholith of California. Most batholiths have an overall granitoid composition but can include gabbros and even scarce ultramafic rocks. The term is also used for a single, steep-sided, granitoid intrusion, circular-ovoid in plan, and of great vertical and areal extent (>100 km2, in outcrop area). Similar-shaped granitoid intrusions that are smaller than this are known as stocks.

Eroded volcanic landscapes are often characterized by upstanding hills and knolls formed of the hard, resistant plutonic rock that solidified inside a volcanic pipe, blocking further eruption of the volcano. These are referred to as plugs. The Castle Rock in the city of Edinburgh, Scotland, and the towering rock pillars of the Puy region of France are well-known examples.

Contiguous granitic plutons forming a 75-km-long north-northeast-trending belt 1100 km2 in area and averaging 15–20 km in width were termed the Kanzachaung batholith by UNDGSE (1978a), whilst a few smaller granitic intrusions lie to the east. The Mawgyi Volcanics and Mawlin Formation separate the batholith from the Pinhinga Plutonic Complex to the northeast. At its southern end the batholith disappears beneath sedimentary cover that continues southward for 155 km to the Monywa segment of the arc where Mesozoic magmatic rocks reemerge. Mineralization in and near the batholith is shown in Fig. 9.5.

The batholith consists predominantly of medium to coarse-grained granodiorite with less than 30% K-feldspar. In these rocks, biotite is usually the predominant mafic mineral and weathering creates a subdued relief. Fine-grained granodiorite often contains abundant hornblende with biotite and forms higher ground. Quartz diorite and diorite are much less abundant than granodiorite and occur as relatively small plutons. The greatest complexity of intrusions within the batholith is at Shangalon where several plutons ranging in composition from granodiorite to diorite occur within a 25-km2 area. The best natural exposures of the batholith are at the Mu Rocks or rapids on the Mu River.

Two large separate plutons close to the eastern margin of the batholith are the Peinnegon adamellite or quartz monzonite and Sadwin granodiorite. The Peinnegon pluton southwest of Shwedaung is a biotite-bearing rock with more K-feldspar than most of the main batholith. Small areas of granitic rock protruding from alluvium for some 25 km southwest of the Peinnegon pluton suggest that it is much larger than outcrops suggest.

The Pinhinga Plutonic Complex covers 250 km2 and lies 15 km north of the Kanzachaung batholith between latitudes 24°20′ and 24°40′N. A geological map, available only for the southern part of the Complex (UNDGSE, 1979a), shows intrusions of diorite, granodiorite, and biotite–muscovite and foliated garnet-bearing biotite–muscovite granite. Diorite also occurs in the south of the Complex, and a weakly foliated hornblende–biotite granodiorite in the west. The garnet-bearing intrusion comprises a foliated to gneissic coarse-grained biotite or biotite–muscovite granite with pink garnets and up to 40% K-feldspar; it includes small bodies of leucocratic foliated granite. K-feldspar-rich muscovite leucogranite lacking both mafic minerals and foliation crops out in the poorly known central-northern part of the Complex in an area rarely visited by geologists.

The foliated granites are probably the oldest granites in the Wuntho–Banmauk segment. Diorite is intruded by granodiorite but the age relationship between these and the unfoliated muscovite leucocratic granite is unclear. The Pinhinga Plutonic Complex intrudes the Mawgyi Volcanics. A small body of olivine basalt in the western part of the Complex is probably late Cenozoic in age.

Emplacement of the Kanzachaung Batholith into marine sedimentary and largely marine basaltic volcanic and volcanosedimentary rocks implies that the arc was not a geanticline before the early Upper Cretaceous and supports the absence of observed older I-type granitic rocks.

The existence of plutons demonstrates that granitic magma does not reach the surface, for a range of reasons such as (i) granitic magmas are too viscous and stall during ascent; (ii) they cool down as they rise and solidify before erupting; (iii) they reach a neutral buoyancy level or (iv) they are trapped by structures such as fault planes, regional stratification or strong layers (Clemens, 2012). Field and gravimetric surveys have shown that large plutons have shapes ranging between two end-members: (i) tabular bodies with steeper feeder zones (with a typical thickness, T, related to the horizontal size of the pluton, L, by T ≃ 0.12 × L0.88 McCaffrey and Petford, 1997), when regional stresses play a moderate role in pluton emplacement; (ii) Wedged-shaped with a deeper root (> 10 km) and walls steeply plunging inward, and a shape reflecting the local stress regime, for tectonically-controlled emplacement (Vigneresse, 2004) (Fig. 6A).

Fig. 6. (A) Cross sections of several tabular and wedge-shape granitic intrusions. A similar true scale (vertical = horizontal) is applied to all massifs. (B) Map of part of the state of Victoria in southeastern Australia showing the first vertical derivative of total magnetic intensity. The lighter the gray, the higher the magnetic susceptibility of the rocks. Magnetic anomalies reveal some of the internal structure of Devonian granitic plutons that intrude Cambrian and Ordovician low-grade metasediments. The internal magnetic fabric is interpreted as flow patterns within successive magma pulses. The stars and arrows respectively represent possible sites for magma upwelling and horizontal flow patterns within the pluton.

Journal of the Virtual ExplorerApplied Earth Science

C. Jaupart, J.-C. Mareschal, in Treatise on Geochemistry (Second Edition), 2014 Variations within a single pluton

It is possible for radioelement concentrations to vary quite a bit within a single pluton in both vertical and horizontal directions (Killeen and Heier, 1975b; Landstrom et al., 1980; Rogers et al., 1965).Several factors may account for these variations, including facies changes, a fundamental heterogeneity of the material, fluid migration, and late-stage alteration.For example, in the Bohus granite of Sweden, concentrations of the relatively immobile thorium differ by a factor of five over distances as small as a few tens of meters and as large as a few kilometers (Landstrom et al., 1980).

Jacques.J. Aupart, J.-C. Mareschal, on Geochemistry, 2003 Variations within a single pluton

In a single pluton, radioelement concentrations are quite variable both vertically and horizontally (Rogers et al., 1965; Killeen and Heier, 1975b; Landstrom et al., 1980).Variations may arise from a variety of causes, such as faulting, fundamental heterogeneity of the source material, fluid migration, and late-stage alteration.Bohus granite, Sweden, exhibits variation of thorium concentrations of about a factor of 5 within horizontal distances as small as a few tens of meters and as large as a few kilometers (Landstrom et al., 1980).

.As a result, the erosion of several kilometre-high magma bodies in areas with low relief will tend to lead to the preservation of steep marginal contacts.However, field observations of plutons in high relief areas as well as geophysical surveys indicate that many granitic plutons are in fact tabular bodies with horizontal dimensions far greater than their vertical extent (Fig. 2.11B) (e.g. Vigneresse, 1995; McCaffrey and Petford, 1997; Cruden, 2006; Cruden et al., 2018 and references therein).Aside from gently inclined roofs and floors, these tabular intrusions also feature internal layers and sheets with shallow dips, parallel to pluton roofs and floors (Fig. 2.12).

Fig. 2.12:.Roofs, floors, and layers within plutons.From the Owens Valley, a view of Split Mountain, Sierra Nevada, USA. .The full report is available at Bartley et al. (2012).The cliffs on the Lindenow Fiord, Greenland, expose a * 500 m thick Proterozoic granite sheet laid in gneissic country rocks (photo: courtesy of John Grocott).A cliff of the Proterozoic Graah Fjeld granite (*800m high), Greenland.Country rock gneisses characterize the intrusive sheet boundaries (photo: courtesy John Grocott; Grocott et al., 1999).The Chehueque pluton, Coastal Cordillera, Chile.La Pignetta mountain (∼2000 m) displays three distinct intrusive units of the Chehueque plutons, granite (Pgt), granodiorite (Pgd), and monzonite (Pmz).

Two types of pluton floor geometries are observed: funnel or wedge-shaped and flat, tablet-shaped (cf. Vigneresse et al., 1999; Cruden, 2006). Wedge-shaped plutons can be symmetric or asymmetric and typically have one or more root zones, defined by downward-tapering linear deep structures, becoming a narrow cylinder, in gravity models, interpreted to be feeder structures (e.g. Ameglio and Vigneresse, 1999). Their floors dip inward from very shallow angles, defining broad open funnel shapes to steep angles, defining carrot-like shapes. Tablet-shaped plutons are characterised by almost parallel roofs and floors and steep sides (Fig. 2.12A and B). Some plutons have both wedge- and tablet-shape characteristics.

Field examples of the nature and geometry of pluton floors are relatively uncommon. However, limited observations in Greenland, the North- and South American Cordillera and the Himalaya (Fig. 2.12; e.g. Hamilton and Myers, 1974; Le Fort, 1981; Scaillet et al., 1995; Hogan and Gilbert, 1997; Skarmeta and Castelli, 1997; Grocott et al., 1999; Michel et al., 2008; Bartley et al., 2012) are in general agreement with geophysical data.

Paterson et al. (1996) reviewed the characteristics of mid- to upper-crustal pluton roofs exposed in the Cordillera of North and South America, showing that they consistently have gentle dips to slightly domal morphologies and discordant contact relationships with pre-existing wall-rock structures. Emplacement-related ductile strain in the wall rocks is typically absent to poorly developed, and there is also little evidence that the roofs have been lifted above their pre-emplacement position. Minor amounts of stoped blocks occur beneath the roof, and stoping is a likely candidate for generating the jagged profiles of the roofs, although its role as a major space-making mechanism is debatable (cf. Section 2.3). Other authors report more compelling evidence for upward displacements of pluton roofs (e.g. Morgan et al., 1998; Benn et al., 1999; Grocott et al., 1999), particularly in shallower crustal settings.

Relatively undisturbed roofs, sharp transitions to steeply-dipping walls and the presence of either sharp wall-rock contacts or narrow strain aureoles with evidence for country-rocks-down sense of shear relative to the pluton margin have been used by Paterson et al. (1996), Paterson and Miller (1998) and Miller and Paterson (1999) to argue that most space for emplacement of granites is due to downward transfer of country rock material. Although these authors favour mechanisms such as stoping or return-flow of country rock during diapiric ascent, downward displacement and rotation of wall-rock structural markers and fabrics towards the margins of intrusions in Greenland, Sweden and N. America suggests that floor subsidence may be an important alternative space-making process (Bridgwater et al., 1974; Cruden, 1998; Benn et al., 1999; Grocott et al., 1999; Brown and McClelland, 2000; Culshaw and Bhatnagar, 2001). Pluton-side-down shear sense indicators and roll-over of strata adjacent to some plutons have also been ascribed to late-stage sinking of cooling magma bodies (e.g. Glazner and Miller, 1997). Large-scale tilting of roof pendants and wall rocks in the Sierra Nevada and Boulder batholiths has also been attributed to down-drop of pluton floors during batholith growth and emplacement (Hamilton and Myers, 1974; Hamilton, 1988; Tobisch et al., 2001).

There is increasing evidence that many plutons, including those that are macroscopically homogeneous, are made up of many metre- to kilometre-scale sheets (Fig. 2.12C and D) (e.g. McCaffrey, 1992; Everitt et al., 1998; Cobbing, 1999; Grocott and Taylor, 2002; Coleman et al., 2004; Michel et al., 2008; Grocott et al., 2009; Cottam et al., 2010). Detailed textural observations of intrusions in Maine, SW Australia and South New Zealand suggest that initially sub-horizontal sheets steepen with time during growth of a pluton (Wiebe and Collins, 1998). This is supported by U-Pb studies in the Coast Plutonic Complex, where plutons are interpreted to have grown from the floor upward by stacking of sheets and gradual subsidence and distortion of their floors (Brown and Walker, 1993; Wiebe and Collins, 1998; Brown and McClelland, 2000). Other field and geochronological studies indicate that some tabular plutons are assembled by downward stacking of pulses (Fig. 2.12D) (Michel et al., 2008; Grocott et al., 2009; Leuthold et al., 2012).

Given the final volume of a pluton or sill, the filling time is a multiple of the input flux. For a body of the size of Skaergaard (~ 250 km3), for example, the filling time is ~ 17 years if all the magma produced at the ocean ridges (15 km3 year− 1) were directed to this location. For a Hawaiian rate (1 km3 year− 1), the filling time is longer, 250 years. There is no simple way to decide the actual filling time; U–Th and Po–Pb–Ra isotopic disequilibrium techniques may offer some information, but the context of the materials measured is often unclear. The only direct, firm physical constraint comes from rates of solidification. The rate of filling must be significantly greater than the rate of solidification. For these sheetlike systems, it is straightforward to show by scaling the heat equation (see eqn <1>) and including the effect of latent heat (e.g., Jaeger, 1968) that the solidification time (t) is well approximated by the simple formula

where L is the half-thickness of the sheet and K is the thermal diffusivity (e.g., ~ 10− 2 cm2 s− 1). The half-thickness of a sill or pluton can be estimated by noting that the aspect ratio (n) of sills is 100 or more, whereas that of plutons is about 10; there are of course large variances in these values, especially for plutons. Nevertheless, for a given volume (V) of magma, the half-thickness of the equivalent rectangular sheet of dimensions n2L × n2L × 2L, is given by L = (V/8n2)1/3. Under this approximation, for a given volume of magma, sills will be thinner than plutons and the solidification time of sills will be significantly smaller than for plutons. The competition in filling time and solidification time for a range of fluxes operating over the characteristic eruptive times found by Simkin (1993) is shown in Figure 19. In light of the earlier discussion of the controls of crystallinity on magma fluidity, the calculated time for solidification has been lessened by a factor of 10 to ensure that the body is sufficiently fluid to ensure reinjection without creating an internal chilled margin or also to ensure that the sequence of arrivals of magmatic parcels can mix to make a single body. From this constraint, the characteristic thickness of a sill is ~ 100 m and ~ 1000 m for a pluton, and the filling times are, respectively, about 10 and 300 years. These are geologically reasonable results, but the actual filling times may be much less. This sequence of events for sills will be revisited later when discussing the Ferrar Dolerites.