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Granitic Magma Ascent and Emplacement: Neither Diapirism Nor Neutral Buoyancy

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Granitic Magma Ascent and Emplacement: Neither Diapirism Nor Neutral Buoyancy Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2021 Granitic magma ascent and emplacement: neither diapirism nor neutral buoyancy J. L. VIGNERESSE 1 & J. D. CLEMENS 2 1CREGU, UMR CNRS 7566 G2R, BP 23, 54501 Vandoeuvre C~dex, France (e-mail jean-louis, [email protected] 2School of Geological Sciences, CEESR, Kingston University, Penrhyn Road, Kingston-upon-Thames, KT1 2EE, UK Abstract: It is probable that granitic magma ascent does not result from the intrin- sic properties of the magmas. Within the uppermost crust, neither the reduced viscosity nor the density contrast between magma and surroundings are them- selves sufficient to induce either low-inertia flow (diapirism) or fracture-induced magma propagation (dyking). Igneous diapirism is intrinsically restricted to the lower, ductile crust. Dyking is therefore the most probable ascent mechanism for granitic magmas that reach shallow crustal levels. A neutral buoyancy level in the crust, at which magma ascent should stall, is never observed. This,is demon- strated by coeval emplacement of magmas with different compositions and densi- ties, and the negative gravity anomalies measured over many granitic plutons. We suggest that deformation, through strain partitioning, is necessary to magma ascent. Pluton formation is controlled by local structures and rock types rather than by intrinsic magma properties. As a result of its intermittent character, deformation (both local and regional) induces magma pulses, and this may have important consequences for the chemical homogeneity of intruded magmas. Ascent of granitic (leucogranitic to tonalitic) magma in the continental lithosphere is the most potent means of mass transfer and chemical segregation between the lower and upper crust. Granitic magmatism contributes to crustal recycling and, to a lesser extent, continental growth. In rapidly accreted crust, granitic rocks can occupy up to one-third of the volume of the middle crust (Meissner 1986). Together with shear zones, they are the most conspicuous effects of plate convergence and collision in orogenic zones. Consequently, they are a key element in understanding the evolution of Earth's mechanical workings. As most felsic magmas are emplaced in a liquid, or near-liquid state, at temperatures over 800~ a major heat source must be involved in their genesis. The existence of such a heat source also has consequences for the deformation regime of the crust. Com- monly, a large temperature rise in the lithosphere will also increase the capacity of crustal material to respond to stress (by deformation). Such a thermal anomaly will contribute to the dissipation of energy and thus buffer the temperature effects. The stress regime will be imposed externally by the tectonic processes operating at the time. As a reaction to a significant increase in crustal heat flow, the rheological capacity of rocks to sustain stresses decreases, and this activates deformation. Therefore, most departures from a 'normal' heat distribution in the lithosphere will trigger deformation (e.g. formation of extensional rift zones, or shear zones in convergent situations). From: VENDEVILLE,B., MART, Y. & VIGNERESSE,J.-L. (eds) Salt, Shale and Igneous Diapirs in and around Europe. Geological Society, London, Special Publications, 174, 1-19. 1-86239-066-5/00/$15.00 9The Geological Society of London 2000. Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2021 2 J. L. VIGNERESSE & J. D. CLEMENS Growing granitic intrusions are also sites at which chemical systems can be driven out of equilibrium. This occurs mainly because interactions between the incoming magma and upper-crustal rocks (commonly H20 saturated) will drive local hydro- thermal convection cells, generated to dissipate the excess heat from the cooling magma. The intense element circulation that this situation may trigger can lead to concentration and precipitation of economically interesting elements (e.g. Li, Be, Cu, Zn, Sn, W, Au, Pb, U). For the above reasons, granitic plutons have been intensively surveyed for their internal structure, mineralogy and chemical composition, as well as to constrain the physical conditions of magma generation and emplacement. An important issue in such studies is the time-scale of the whole process of granite generation (thermal diffusion, melting, segregation, ascent and emplacement; Clemens et al. 1997). Granitic plutons represent only the terminal stages of the process, and studies of granitic bodies are particularly informative on the final emplacement conditions. However, they usually provide little or no insight into the magma ascent mechan- ism(s). At the other end of the process, migmatites are commonly considered to preserve the initial stages of granitic magma generation (Brown 1994). However, mig- matites represent only a snapshot of the process during its development, and there is debate over the question of whether migmatites truly represent the outcomes of pro- cesses involved in the formation of most granitic magmas (the magma v. migma argu- ment). Certain types of migmatites may typify the phase of initial magma segregation from its source. In some cases, migmatitic bodies are accompanied by small anatectic granites derived from the same source rocks. There is also evidence that some weakly mobile, restite-rich granitic magmas are formed as diatexitic migmatites (e.g. Finger & Clemens 1995). However, the emplacement of such bodies is far from being akin to the emplacement of large, high-level granitic batholiths displaying features that indi- cate high liquid contents on initial emplacement. Clemens & Droop (1998) have given a broad treatment of the theoretical outcomes of various partial melting scenarios (in terms of fluid presence or absence, the occurrence of segregation and the nature of the metamorphic P-T path). Experiments on partial melting of common crustal rocks provide considerable insight into the nature and products of partial fusion. At present, most experiments are performed under constant pressure, temperature and compositional conditions, and do not address the role of stress in magma segregation. Although they have provided valuable structural insights, attempts at dynamic partial melting experi- ments (e.g. Rutter & Neumann 1995) have not produced the near-equilibrium melt compositions or rock textures observed in nature. This is largely due to the limitations on strain rates attainable on laboratory time-scales, and the consequent necessity of performing short experiments at very high temperatures. Additionally, because of slow diffusion rates, even fine-grained natural rocks do not reach chemical equili- brium on laboratory time-scales. Numerical and analogue experiments (Romfin- Berdiel et al. 1995; Benn et al. 1998; Barnichon et al. 1999) are useful, as their boundary conditions can be controlled and varied at will. Their limitation is essen- tially in their relative naivetJ and the relatively primitive state of our knowledge of the rheological properties of minerals and their aggregates under realistic geological conditions. The paradigms developed from these approaches have been widely accepted as the best representations available to explain granitic magma generation, extraction and Downloaded from http://sp.lyellcollection.org/ by guest on September 27, 2021 GRANITIC MAGMA ASCENT 3 emplacement. This has led to the adoption of apparently attractive but unverified concepts such as metasomatic granitization (generally regarded as totally unverified) or diapiric upwelling of magmas. Features that most such models share are that they have been derived from single disciplines (e.g. petrology, fluid mechanics or tec- tonics), and they do not take account of knowledge gained through other disciplines. Here we attempt to formulate a convergent analysis of the processes that lead to granite generation, ascent and emplacement, using constraints from different fields of study. Granitic magma ascent has served as a model for many other forceful empla- cement mechanisms (e.g. salt or mud) and this paper could provide useful constraints and concepts for understanding these phenomena. In what follows, we consider ascent and emplacement as being intimately linked, even if the principal strain direction differs (dominantly vertical v. dominantly hori- zontal magma movement). We first briefly review what can be observed in outcrops of granitic intrusions (shapes, volumes and physical characteristics). We then examine the major proposed mechanisms of magma ascent (low-inertia flow or diapirism and fracture-driven flow or dyking). During diapirism, the response of the surround- ing medium controls the ascent mechanism, and partly controls the volume emplaced in the case of dyking. We also examine the question of whether magma properties control ascent to a neutral buoyancy level, at which forces driving ascent will cease. We focus on the intrinsic limits of these two magma ascent models, and suggest that deformation (stress) actively drives granitic magma ascent. As stress is not tem- porally constant in the crust, granite emplacement is predicted to be episodic, which is reflected in the chemical evolution of the magmas. Subsurface shapes of granitic bodies The present shapes of granitic intrusions at depth reflect the emplacement modes of the magmas rather than their ascent. They do provide information on the geometrical factors that controlled emplacement. Seismic profiles reveal the 2D shapes of intru- sions, although the internal
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