Emplacement mechanisms of plutons

Five different mechanisms have been proposed for the emplacement of plutons and batholiths:

- Stoping

- Regional tectonic processes - Diapirism

- Ballooning - Dyking

During stoping, blocks from the enveloping country rocks break off and sink through the magma chamber until either the magma crystallizes, the floor of the chamber or a point is reached where the magma has the same density as the

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block. This mechanism is unlikely to produce enough space for the final emplacement of the magma since it only describes a transfer of material in the magma chamber itself. The xenolith will require the exact amount of space in the magma chamber as it has made by breaking off from e.g. the roof of the magma chamber. Hence stoping can not make sufficient space for a magma chamber to emplace, it rather is a mechanism to produce an ascent path for the magma or assists other space-making processes since it only moves material inside the magma chamber itself.

Regional tectonic processes may play an important role in the space making process for ascending magmas because this describes a mechanism that moves material with respect to a far field reference frame. The movement may, for example, form a pull-apart basin with magma filling the actual pull-apart structure (Paterson, 1996).

For the recognition of a diapir or a ballooning pluton the position of structures in the granitoid body is of importance since structures in and around the intrusion are different depending on the position (see Fig. 1.1). For example near the roof of a diapir a shallow, outward dipping foliation in the enveloping rocks is expected while near the equator of the body a nearly vertical dip of foliation should develop (Clemens et al., 1997; see Fig. 1.1). In the following section the structures of these bodies will be described from the roof of the intrusion down to the floor region.

Pluton diapirism describes both, ascent and emplacement of the pluton. Per definition a diapir is a roughly tear drop-shaped body which has to rise at least one body diameter before its final emplacement (Paterson & Vernon, 1995). This ascent is usually described as forceful because the overlying strata are broken up to give way to the ascending magma. This ascent mechanism produces some unique structures in the diapir itself as well as in the surrounding country rocks. As postulated by Clemens et al. (1997) or Bateman (1984), within the roof-area a radial, gently outward dipping lineation should occur. In the enveloping rocks, narrow shear zones develop during intrusion that dip gently away from the intrusion center. Near the equator of the diapir the enveloping rocks are deformed in a way to produce structures pointing to an upward movement of the magma (steep lineations, prolate strain ellipsoids) and steeply inclined to even vertical shear zones should develop showing diapir-up shear sense in the contact area (Clemens et al., 1997). Prolate strain ellipsoids develop near the contact area

where the viscosity contrast with the enveloping rocks is more effective than in the center of the pluton. Therefore a strain gradient towards the center of the diapir should develop (Clemens et al., 1997, Bateman, 1984). Near the diapir floor a zone of high-temperature shearing with steep inward dip may develop, enveloping layered rocks should be deformed to build rim-synclines (Clemens et al., 1997). An example of a diapir is the Tenpeak pluton in the north American Cordillera Miller &

Paterson, 1999). Structures in the enveloping rocks (fold-axes and subhorizontal mineral lineation) swing to a down-dip orientation near the contact area indicating downward flow of the host rocks during upward movement of the magma. In the granitoid body itself, pluton-side-up indicators support diapiric ascent and emplacement of the pluton (Miller & Paterson, 1999).

A ballooning granitoid body is inflated at the point of its origin and should not show any signs of upward movement (Paterson, 1996). The formation of the (large) magma chamber is explained by ascent of a liquid crystal mush up to a level of neutral buoyancy which is the final site of emplacement. The balloon is gradually expanded with ongoing magma injection. Therefore, the enveloping rocks should show signs of pure flattening strain with finite strain ellipsoids paralleling the outer margin of the pluton at any level of the pluton (Ramsay, 1989). This implies vertical oblate strain ellipsoids in the equator area of the balloon and gently outward dipping, oblate strain ellipsoids in the roof area. In the balloon itself a concentric, margin parallel foliation should develop that decreases in intensity towards the center of the pluton (Clemens et al., 1997, Holder, 1981). A radial lineation may be developed near the center of the intrusion pointing towards the contact area which displays the direction of inflation (Clemens et al., 1997). One of the best known examples for a ballooning pluton is the Ardara pluton in the northwest of Ireland (Siegesmund & Becker, 2000; Molyneux & Hutton, 1999;

Holder, 1981; Pitcher & Berger, 1972). Structures in the enveloping sediments (e.g. tightening of folds, thrusting, stacking) point to radial expansion of the pluton;

the concentric foliation in the pluton shows a decreasing distinctness towards the center and a strain gradient has been measured that decreases from the outside of the pluton towards the center. All these are features which can clearly be explained by emplacement of the pluton through ballooning. A continuum exists between the definition of diapiric and ballooning emplacement of granitoid magma.

If magma ascends half of its body diameter it is per definition neither a diapir

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(should rise at least one body diameter) nor a balloon (should not rise at all).

Furthermore, structural markers in a granitoid body as well as in the enveloping rocks can show great conformities (folding/stacking of sediments; concentric, margin parallel foliation etc.) if the emplacement mechanism is in between a true balloon and a true diapir. In this work the mere descriptive terms ballooning and diapirism will be used pointing to a true balloon or a true diapir respectively.

Results that can not clearly be assigned to one of these mechanisms will be marked.

Fig. 1.1: Emplacement of granitoid bodies and their related structures. a) ballooning pluton. 1) Zone of structural and thermal weakness below the magma body; 2 and 4) Through the expansion of the nearly circular body sedimentary structures are folded; 3) Spherical expansion of the magmatic body can lead to a decreasing distinctness of magmatic structures towards the center of the pluton.

The center of the pluton is usually isotropic; 5) The outer rim may show a zone of high temperature solid state deformation when replenishing of the magma chamber is pulsed; 6) Measured strain ellipses show a gradual increase of oblateness towards the contact area.

b) diapiric emplacement (taken from Clemens et al., 1997) 1) Tail of the diapir reaches deep into the crust; 2) Surrounding rocks and sediments are deflected upwards due to the destructive upward movement of the magma; 3) A steep lineation may develop in the center of the pluton; 4) Sediments in the contact area show a margin parallel foliation and a radial lineation indicating of-the-dome sliding of strata; 5) A lineation pointing away from the center of the intrusion may be developed related to magmatic flow fabric; 6) The outer rim of the intrusion may show a zone of extensively deformed rocks of both, granitoid and surrounding rocks; 7) Measured strain ellipses indicate flattening strain as well as upward movement of the magma (prolate strain ellipsoids).

a) b)

Dyking emplacement of magma more describes the ascent of magma than the actual emplacement. During dyking, magma ascends via open cracks or fractures (Paterson, 1996) into the area of final emplacement with the main driving force for the magma being buoyancy. The final emplacement of the pluton is either as a dyke, a sill or a laccolith (see Fig. 1.2). A common feature of this emplacement mechanism is the great lateral extent versus limited thickness of the finally constructed body. The driving force of magma is usually its buoyancy;

overpressuring of magma may also occur but needs a constant open pathway between the overpressured source and final site of the dyke (Paterson, 1996).

Fig. 1.2: Laccolith-like emplacement of magma fed by a dyke-like conduit. 1) Feeder dyke; 2) Surrounding rocks are only deformed in the contact area of laccolithic bodies and more than one laccolith may form from the same feeder dyke; 3) Surrounding rocks outside the direct contact area remain undisturbed; 4) Laccolithic bodies may form from a set of magma pulses giving rise to a layered, sheet-like intrusion; 5) Rocks on top of the intrusion may be lifted; 6) Due to the uplift of the surrounding rocks a central graben may form.

Fracture propagation and therefore magma ascent during dyking may stop or change direction at the level of neutral buoyancy, if a very ductile zone is intersected, stress barriers (e.g. large horizontal compressive stresses) are encountered or a free slipping surface is reached (Paterson, 1996). A well known feature of (small) natural dykes is the marked absence of contact metamorphic zones. This indicates that the cooling rate is fast which in turn implies a fast magma ascent rate necessary to keep the ascent path open for further magma injection (Cory, 1998, Paterson, 1996). During ascent, Marsh (1982) predicts that

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xenoliths are carried up with the ascending magma since they do not have enough time to settle or sink down to lower levels of the dyke.

Experiments performed by Roman-Berdiel et al. (1995) on the laccolith-like emplacement of magmas showed a linear relationship between the shape (thickness) of the intrusion and the thickness of the overburden (which can be correlated with the intrusion depth) (see. Fig. 1.3). According to the amount of intruded material they found a fluent transition between sill-like forms of the intrusion to a lens-shaped laccolith and finally a bell-shaped laccolith.

Fig. 1.3: Experiments performed by Roman-Berdiel et al. (1995) on the laccolith emplacement revealed a linear connection between the diameter and the thickness of a laccolith and the thickness of the overburden (intrusion depth). Experiment parameters have been chosen so that 1cm in the experiment is 1km in nature.

The formation of the different intrusions can be correlated with the thickness of the overlying burden and hence with the intrusion depth. The greater the total thickness of overburden the more the intrusion tends to form thin sills rather than thick laccoliths. According to the rheology of the overburden basically two different types of laccoliths exists, punched laccoliths and domed laccoliths (Cory, 1998;

Roman-Berdiel et al., 1995). Punched laccoliths only show small deformation of their overburden but large-scale shear zones develop at or near the periphery of the intrusion site. Field characteristics of a punched laccolith are a flat top, steep sides and peripheral faults. This type of laccolith forms when the overburden has an elastic-plastic behavior (Cory, 1998). Domed laccoliths do not show the peripheral faults; overlying strata are continuous over the laccolith and may be strongly deformed. If doming continues, a graben may form in the overburden.

According to Cory (1998) a continuum exists of possible shapes between these two end members.

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13 12 11 10 9

1 2 3 4 5 6 14

Thickness of brittle overburden (cm)

Diameter of laccolith (cm)

2.4 2.2 2 1.8 1.6 1.4 1.2

2.5 3 3.5 4 4.5 5 5.5 6

Thickness of laccolith (cm)

Thickness of brittle overburden (cm)