Introduction

Concentrically expanded plutons (CEPs) are a common igneous feature. Paterson and Vernon (1995) defined CEPs as plutons with a distinct zoning either with a mafic rim phase getting more felsic towards their centers (normal zoning) or vice versa with a felsic rim and a mafic core (reverse zoning). Commonly flattened enclaves are found near the margins of the plutons. Structures in the immediate country rocks are deflected into parallelism with the pluton margins and ductile deformation is a common feature in the contact aureole. In both the pluton and the host rock intensity of the foliation increases toward the pluton/host rock contact.

Generally, two intrusion mechanisms are proposed for CEPs. The first proposed process combines dyking as an ascent mechanism with ballooning as the emplacement process sensu strictu (e.g. Petford 1996). According to this idea CEPs are constructed by numerous dykes which feed the same magma chamber often using the same conduit as pathway for the magma. Dyking is defined as the upward movement of a fluid phase through the elastic fracturing of rocks driven by the high magma pressure and buoyancy (Lister and Kerr 1991). Dykes usually propagate perpendicular to the least principal stress and follow zones of weakness in the rocks they travel through. They form a conduit with a large lateral extent relative to its thickness (Spera 1980). The emplacement of multiple dyke-fed magma batches to the same magma chamber leads to magma chamber expansion or ”ballooning“ (e.g. Molyneux and Hutton 2000): host rocks of the pluton deform ductilely and the magma chamber inflates radially and relatively symmetrical as a result of the intrusion pressure of the intruding magma (Ramsay 1989, Paterson and Vernon 1995).

Diapirism is the upwelling of relative mobile material (e.g. magma) through overlying rocks (Van den Eekhout et al. 1986) and has also been proposed as a mechanism for the ascent and emplacement of CEPs (e.g. Miller and Paterson 1999). The driving force of diapirism is buoyancy. Diapiric plutons are generally regarded as the result of the buoyant rise of an elliptically shaped body of magma. Multiple bodies of magma intruding subsequentely as diapirs into the same region / space are also referred to as nested diapirs (Paterson and Vernon 1995). They can be described as diapir-in-diapir structures, where earlier magma bodies are intruded by subsequent diapirs of possibly different composition.

Natural examples of CEPs include the Devonian Ardara pluton (Fig. 1) in Ireland that intruded into a depth of ca. 7-9 km (Kerrick 1987). The pluton is normally zoned from quartzmonzodiorite at the rim through tonalite to granodiorite in the core. The magmatic foliation crosscuts internal contacts. It is parallel with the pluton / host rock interface in some parts of the pluton and discordant in others. Country rock markers such as bedding planes and pre-intrusive faults are deflected into parallelism with the contact only close to the pluton. At the southwestern margin a ductile shear zone is developed which incorporates both the host rock and the pluton, which is elongated along the shear zone (Paterson and Vernon 1995).

Figure 1. a) Simplified geologic map of the Ardara pluton (redrawn from Paterson and Vernon 1995). b) Cross section through the Ardara pluton along line A-B in Fig. 1a, based on data from Paterson and Vernon (1995) and Pitcher and Berger (1972). Colours from the legend in Fig. 1a, country rock markers as thin solid lines, magmatic foliation as thin dashed lines. Intensity of the magmatic foliation is indicated by the spacing between the lines. (Select image for enlargement)

The Permian (Bultitude and Champion 1992) Cannibal Creek pluton (Fig. 2) in Australia was emplaced into a depth of 7 to 9 km (Bateman 1985a). The pluton is not zoned, but consists of granite with K-Feldspar megacrysts. The granite is crosscut by megacrystic granodioritic to equigranular granitic ring dykes. Moreover, equiranular microgranitic dikes encircle the pluton almost completely (Bateman 1985b). A magmatic and perfectly contact parallel magmatic foliation is developed within the pluton. Country rock markers such as bedding and a bedding parallel foliation are deflected towards parallelism with the contact in almost the entire aureole. The pluton has discordant contacts with the host rocks only at its northern and southern ends (Paterson and Vernon 1995).

Figure 2. a) Simplified geologic map of the Cannibal Creek pluton (redrawn from Paterson and Vernon 1995). b) Cross section through the Cannibal Creek pluton along line A-B in Fig. 2a, based on data from Paterson and Vernon (1995), Paterson (1988) and Bateman (1985a). Colours from the legend in Fig. 2a, country rock markers as thin solid lines, magmatic foliation as thin dashed lines. Intensity of the magmatic foliation is indicated by the spacing between the lines.(Select image for enlargement)