Soutenance mémoire
Tectonic and geodynamic settings of magmatism
Magmatic rock is one of the most important components of the continental crust, and records the processes of crust growth and recycling. Therefore, the study of magmatism, especially in the large zones or provinces where occurred numerous magmatism, can help to understand not only the magma evolution but also the crust evolution.
The entire magma evolution system, including the magma generation, segregation, transport and emplacement (Fig. 1-1), is usually controlled by the geodynamic and regional tectonic settings (Maniar and Piccoli, 1989; Pearce and Peate, 1995; Wilson, 2007). Previous studies have principally classified the magmatism intro two groups, according to the geodynamic and tectonic settings. One concerns the magmatism developed at an active plate margin setting, and another is the magmatism developed in an intraplate setting (Fig. 1-2; Wilson, 2007) and different features of these two groups of magmatism will be recorded due to the different natures of magma and overlying crust.
Magmatism developed at an active plate margin setting
Magmatism yielded at an active plate margin mainly divided into two settings, one is the plate subduction related and another is plate rifting related. The magmatism under the subduction setting, e.g., island arc magmatism, may be produces by the partial melting of the thickened lower crust during the collapse period, which usually developed at an continental-continental subduction setting (e.g. Himalaya magma belt; Inger and Harris, 1993), or the melting of the lower crust assisted by the thermal and fluid addition from the enriched mantle wedge, which is modified by the material contribution from the footwall side of the subduction (e.g. Tatsumi et al., 1986). Therefore, the source of the felsic magma is derived from the lower crust with small magma addition from mantle due to the limitation in magma mixing, but mafic rocks, which is derived from mantle, are usually observed (Sparks and Marshall 1986; Defant and Drummond, 1990). In the plate rifting setting, the mafic and felsic usually co-exist, and are characterised by the bimodal feature. The emplacements of magma under a plate margin setting usually are along the plate margin, and the long axis of a single pluton is of the parallel with the plate margin (e.g. Aspden et al., 1987).
Magmatism under an intraplate setting
The mechanism of magmatism developed at an intraplate setting is more complicated than that in the plate margin one (Wilson, 2007). This magmatism may driven by the mantle convection or lithosphere extension, the magma derived from the mantle will directly intrude into the crust to form intrusive rocks or directly to the surface to form the extrusive rocks. For example, Halle Volcanic Complex (e.g. Romer et al., 2001) the oceanic island basalt (e.g. Hawaii hot spot; Richards et al., 1989) and flood basalt in intracontinental (e.g. Emeishan flood basalt; Xu et al., 2004). Magma emplacement rate of this extrusive magma is usually faster that of the intrusive one, and the distribution of igneous bodies mainly depends on the location of the magma supply channel, the rheology and the rigidity of crust.
Overview of the magma emplacement mechanisms
Introduction of the emplacement mechanism
Magma emplacement is one of stages of the magma evolution system, including the genesis, segregation, ascent, emplacement and/or eruption (Fig. 1-1), and is essential for the understanding of the continental crust evolution (Pitcher, 1979; Barley et al., 1997; Petford et al., 2000; Annen and Sparks, 2002; Barboni et al., 2015; Annen et al., 2015 and references therein). According to the different settings of final magma emplacement, the magmatic rocks can be divided into two basic types: intrusive and extrusive (Taylor and McLennan, 1995). The emplacement process of these intrusive bodies, especially in those regions with large volume of magma emplaced at a shallow crustal level, will change the thermal state of the country rocks and yield the magma-related mineral deposit (Hedenquist and Lowenstern, 1994; Thompson et al., 1999). Therefore, the study of the magma emplacement of the intrusive rocks is essential. During the emplacement, a variety of features of the process have been recorded in the pluton and its country rocks, which reflect the interaction between the hot magma and its country rocks as well as the creation of the space for magma emplacement. Previous studies found that the features of the intrusive bodies are not only controlled by the composition, temperature and pressure of the magma itself, but also strongly affected by the rheological conditions of the crust, synmagmatic regional tectonic and geodynamic setting (Castro, 1987; Hutton, 1988; Moyen et al., 2003; de Saint-Blanquat et al., 2006; Glazner and Bartley, 2006; Caricchi, et al., 2007; Wei et al., 2014). Consequently, the deciphering of the magma emplacement process, i.e. the rate of magma ascent (Mourtada-Bonnefoi and Laporte, 2004; Humphreys et al., 2008; Rutherford, 2008), the style of magma channel (Clemens and Mawer, 1992; Paterson, 2009), the depth of emplacement (Moyen et al., 2003; Menand, 2011 and references therein), the deformation and thermal conditions of the granite and its country rocks (Paterson et al., 1989; de Saint-Blanquat et al., 2001, 2006; Žák et al., 2007; Byerly et al., 2017) and the geometry of the pluton (Cruden, 1998; O’Driscoll et al., 2006; Stevenson et al., 2007; Mathieu et al., 2008; Cruden et al., 2017), can quantify these factors and give hints of further studies on both deep and shallow process of the crust evolution. In this part, we compile the general characteristics of magma emplacement mechanisms and the relative possible interpretations (Fig. 13).
Diapirism and ballooning
Diapirism (Fig. 1-3a and -3b) was redefined by Paterson and Vernon, (1995), which considered as the movement of the magma with or without pierce through the country rocks. While, the ballooning (Fig. 1-3b) is the in situ radial inflation of the magma chamber with successively displaced magma and bulk shortening of the country rock is needed to produce the space for magma emplacement (Bateman, 1984). In the diapirism and ballooning models, the buoyancy of magma is the major force that drives the transport and displacement of the magma (Roberts, 1970). Accordingly, the magma will emplace at the place where the magma buoyancy alone is not enough to break the overlain stiff crust (Vigneresse, 1995). Gradually accumulated magma with thermal energy partioning into the country rocks will heat the country rock and attenuate the rigidity of the country rock to facilitate the ductile deformation of the country rock (Burchardt et al., 2010 and references therein). According to Bateman, (1984), the internal force of magma chamber should be similar to or higher than the lithostatic pressure of the country rock to maintain the diapir and ballooning form. The increasing of the internal force of magma chamber may be due to the additional magma input or increasing of the crystallisation of the magma.
The features of these mechanisms are usually characterised by (1) spherical or ellipsoid shape, when the tectonic absent; (2) zonation of the pluton composition; (3) gradually outward increasing of the internal ductile deformation of the granite; (4) high-temperature thermal aureole, due to the in-situ cooling (ca. 650℃) and the dissipating of the pluton accommodated by the country rock; (5) deformed thermal aureole with steep lineation and deflected foliation parallel with the pluton margin, due to ductile shortening or sagging of the country rocks (Cruden, 1988; Paterson and Vernon, 1995) .
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Table des matières
Remerciments
Chapter 1. General introduction
1.1 State-of-the-art of magmatism: relationship with the tectonic-geodynamic settings and overview of magma emplacement mechanisms
1.1.1 Tectonic and geodynamic settings of magmatism
1.1.1.1 Magmatism developed at an active plate margin setting
1.1.1.2 Magmatism under an intraplate setting
1.1.2 Overview of the magma emplacement mechanisms
1.1.2.1 Introduction of the emplacement mechanism
1.1.2.2 Diapirism and ballooning
1.1.2.3 Dykes and sills
1.1.2.4 Laccolith and lopolith
1.1.2.5 Stoping
1.1.2.6 Syntectonic magma emplacement
1.2 Overview of the study
1.2.1 Research background
1.2.1.1 Mesozoic magmatism of the South China Block
1.2.1.2 Previous opinions on the tectonic and geodynamic settings of the Mesozoic
magmatism in South China
1.2.2 Research purpose and contents
1.2.3 Research design and methodology
1.2.4 Workload of the study
1.2.5 Major findings and innovations
Chapter 2. Regional geological setting
2.1. The components and basements of the South China Block
2.2. Tectonic framework and evolution of the South China Block
2.2.1 Pre-Triassic tectonic evolution of the South China Block
2.2.2 The Qinling-Dabie orogen
2.2.3 The Longmenshan thrust belt
2.2.4 The Indochina – South China Block collision
2.2.5 Triassic intracontinental deformation
2.2.6 Mesozoic magmatism in the South China Block
Chapter 3. Incremental emplacement of the late Jurassic mid-crustal, lopolith-like Qitianling pluton, South China, revealed by AMS and Bouguer gravity data
3.1 Introduction
3.2 Geological setting
2.1 The South China Block
2.2 The Qitianling pluton
3.3 Field observations and sampling
3.4 Methodology and results
4. 1. Microscopic observations
4. 2. Magnetic mineralogical analysis
4. 3. AMS parameters
4. 4. AMS results
4.5. Gravity modelling
3.5 Discussion
5.1. Origin of the magnetic fabrics
5.2. Emplacement into ductile crust
5.3. Sill- to lopolith-like emplacement
5.4. Incremental magma emplacement
5.4.1. Model 1-Large-sheet emplacement, in situ fractionation
5.4.2. Model 2- Small-sheet emplacement, deep-level fractionation
5.5. Magma feeder zone(s)
3.6. Conclusions
Chapter 4. The emplacement mechanism of the late Jurassic Shibei pluton in the Wuyishan area, South China Block
4.1 Introduction of the reserch
4.2 Geological setting of the Shibei pluton and its surrounding area
4.3 Field observations and sampling
4.4 Petrofabrics of the Shibei pluton
4.4.1 Petrographic features of the granite
4.4.2 AMS of the Shibei pluton
4.4.2.1 Magnetic mineral analysis
4.4.2.2 Magnetic fabric parameters
4.4.2.3 AMS results of the Shibei pluton
4.5 Gravity modelling
4.6 The emplacement mechanism of the Shibei pluton
4.6.1 The acquisition and the implication of the magnetic fabrics of the granite
4.6.1.1 The acquisition of the magnetic fabric
4.6.1.2 The implication of the magnetic fabrics
4.6.2 The construction of the Shibei pluton
4.6.2.1 The geometry of the Shibei pluton
4.6.2.2 The magma supply channel and space for emplacement
4.6.2.3 The mechanism of the magma emplacement
4.7 Conclusions
Chapter 5. General Conclusion
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