Downloaded from http://sp.lyellcollection.org/ by guest on October 2, 2021 Physical geology of high-level magmatic systems: introduction CHRISTOPH BREITKREUZ l & NICK PETFORD 2 ~Institut fiir Allgemeine Geologie, Bernhard-von-Cotta-Str. 209599 Freiberg, Germany (e-mail: cbreit@geo, tu-freiberg, de) 2Geodynamics and Crustal Processes Group, Kingston University, Surrey KT1 2EE, UK Despite their wide occurrence and structural .... Laccoliths (Corry 1988) importance for the development of the upper ---- Plutons and batholiths ---- RMA for plutons and batholiths continental crust, the physical geology of high- RMA for laccoliths level dykes, sills and laccoliths (so-called minor ----- Mesoscale intrusions (Greenland) - - - Minor intrusions (Skye) intrusions) has not received the level of detailed C ...... Batholiths / attention that it deserves. Factors determining ............ Minor sheets (Elba) J. the final emplacement level of subvolcanic intru- ~-t sions are complex, and depend upon a range of physical parameters, including magma driving pressure, the local (and regional) stress field, and the physical properties (viscosity and density) of the intruding material (Breitkreuz -4 -3 -2 -1 0 1 2 3 et al. 2002). SiO2-poor magmas rise through Log mean L (km) tabloid or ring-shaped dykes, acting as feeder systems for Hawaiian to strombolian eruptions Fig. 1. Summary diagram showing the scaling or for their phreatomagmatic to subaquatic relationships between minor intrusions, sills, equivalents. The ascent of silica-rich magmas laccoliths and plutons (from McCaffrey & Cruden leads to explosive eruptions, extrusion of lava or 2002). The data suggest a genetic growth law linking emplacement of subvolcanic stocks and laccol- each individual geometry over a length scale of iths. The main reason for this variation in several orders of magnitude. However, the one- emplacement style appears to be the initial size-fits-all power-law relationship originally used to explain these data (McCaffrey & Petford 1997) volatile content of the rising magma (e.g. Eichel- appears not to hold over the entire range of natural berger et al. 1986). Despite this, and as shown in length-scales presented by igneous intrusions of this volume, the resulting emplacement geome- roughly six orders of magnitude. Instead, the tries are surprisingly limited in range, suggesting available data suggest an open S-curve with the that interactions between magma pressures and power-law slopes tangential to the overall shape. local (and regional) stress fields act to minimize the degree of freedom available for space creation, irrespective of initial composition. Interaction between magmas and sediments is in some detail. Field descriptions of laccoliths an important process in high-level intrusive and models for their emplacement reach back to complexes, and a number of papers address this the classic work of Gilbert (1877). Although topic. In the field, the distinction between sub- analogue and numerical modelling of laccolith volcanic intrusions and lavas, and even some emplacement and host-rock theology is quite high-grade rheomorphic ignimbrites, is not advanced (e.g. Corry 1988; Jackson & Pollard always clear cut, especially in the case of ancient 1988; Roman-Berdiel et al. 1995), internal pro- units exposed in limited outcrop or in drill cores. cesses and their controlling parameters still In particular, the distinction between very require further investigation. For example, shallow-level intrusions and subaerial or sub- many large laccoliths and sills cause remarkably aquatic lavas can be made very difficult, due to little thermal overprint on the host rock. their textural similarity (Orth & McPhie 2003), Emplacement and cooling textures that develop and careful analysis and modelling of rock tex- in the magmatic body and at its margins, such as tures remains an important task. flow foliation, vesiculation, brecciation, crystal- One high-level intrusion type in particular - lization and jointing, are still poorly understood laccoliths - serves as an important link between processes in this context. Given that high-level lava complexes and plutons (Fig. 1), and several intrusions can also act as reservoirs for hydro- papers in this volume deal with this relationship carbons, a better understanding of factors that From: BREITK~EUZ,C. & PEXFORD,N. (eds) 2004. Physical Geology of High-Level Magmatic Systems. Geological Society, London, Special Publications, 234, 1-4. 0305-8719/04/$15.00 © The Geological Society of London 2004. Downloaded from http://sp.lyellcollection.org/ by guest on October 2, 2021 2 C. BREITKREUZ & N. PETFORD there be a preferred tectonic setting for laccolith J . Batholiths emplacement? More provocatively, are lacco- liths and high-level sills forming today, and what Plutons is the nature of the relationship (if any) between high-level intrusions and volcanic activity? Pre- sumably, laccolith textures represent a frozen-in early stage of cooling plutons. To this end, Henry et al. (1997) have coined the term 'lacco-caldera', and suggested that many caldera-feeding, supracrustal magma chambers might have a laccolith geometry. Geophysical and in particu- lar geodetic (interferometric) surveys in mag- matically active zones like the Andes may help to Log mean L (kin) provide new insight into these and related prob- Fig. 2. Synthesis of Figure 1, indicating the possible lems. It is hoped that with the publication of this relationship between sills, laccoliths, plutons and volume, a consensus will emerge that will help to batholiths for typical intrusion geometries at advance our understanding of the governing different scales (after McCaffrey & Cruden 2002). In factors controlling the emplacement of high- this model, plutons (and, by implication, pulsed level intrusions in the continental crust, and their batholiths), grow mostly via vertical inflation, but the geotectonic implications. final length/thickness ratio is arrived at via a more This volume contains 16 papers that cover a complex (S-type) path (Cruden & McCaffrey 2001). wide spectrum of topics relating to the physical geology of high-level magmatic systems. The structure of the book has been grouped broadly contribute to the permeability and transport into three general themes: regional studies and properties of sills and laccoliths may be crucial magma sediment interaction, field constraints for future exploration and production from non- on the emplacement of laccoliths, and sills in traditional oil and gas reservoirs (Petford & sedimentary basins. McCaffrey 2003). Indeed, several of the papers The geological complexity found in deposi- in this book deal with aspects of sill geometry tional, subvolume environments is documented and subsurface visualization from an industry by Awdankiewicz in a study of the Late Palaeo- perspective. zoic Intra-Sudetic Basin, southern Poland. Recent studies of laccolith dimensions from Breitkreuz & Mock describe multi-vent lacco- the Henry Mountains Group, Utah, and other lith systems from the Permo-Carboniferous of regions, show a power-law distribution of the central Europe, and suggest that they formed form N(~_T) = kT D, where N = number of intru- during transtensional basin development related sions equal to or greater than a thickness T, and to dextral strike-slip tectonics. It appears that D is the power-law exponent. Significantly, the laccolith systems grow from multi-feeder similar power-law relationships have also been systems that require the existence of a large found in the measured dimensions of granitic lower- to mid-crustal magma chamber. plutons, suggesting a genetic link between sills, Martin & Nemeth address the topic of laccoliths and plutons. Despite uncertainty in magma-wet sediment interaction in their study the exact values of the exponent D (Cruden & of the small Neogene Sfig-hegy volcanic McCaffrey 2001), an increasing number of complex, western Hungary. Here, intense inter- studies appear to support the power-law growth action and mixing of lava with the host tephras model, over at least part of the emplacement led to peperite formation along the outer rim of history corresponding to a finite length-scale a lava lake. Fluidization of wet tephra allowed (Fig. 1). It will be interesting to see how this basaltic magma to invade and mix with model develops as more data on intrusion phreatomagmatic tephra. dimensions are gathered (Fig. 2). Awdankiewicz et al. discuss the origin of Compilations like the one of Corry (1988) on emplacement textures in Late Palaeozoic the Tertiary laccolith complexes of Utah reveal andesite sills of the Flechtingen-Rol31au Block, the importance of subvolcanic bodies in the mag- Germany, which intruded into a 100-m thick matic continental systems throughout Earth's sequence of lacustrine to alluvial sediments. The history. They also hint that some geotectonic set- resulting intrusive complex, considered as a lava tings such as intra-continental transtension in by previous authors, comprises a range of struc- Late Palaeozoic Europe appear to favour the for- tures, including domes, sills, dykes and failed sills mation of sill and laccolith complexes. Could of varying lateral thickness. Magma-sediment Downloaded from http://sp.lyellcollection.org/ by guest on October 2, 2021 PHYSICAL GEOLOGY OF HIGH-LEVEL MAGMATIC SYSTEMS 3 interactions include quench-clastic brecciation how the emplacement rate can be constrained and post-emplacement hydrothermal breccias. by a combination of textural studies of the Machowiak