Formation of tabular plutons - results and implications of centrifuge modelling (original) (raw)
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Experiments simulating surface deformation induced by pluton emplacement
We present centrifuge experiments to study the surface deformation induced by shallow pluton emplacement in a rheologically stratified crust. Sand simulates the topmost brittle crust; plastilina and denser silicone represent more and less competent crustal portions, respectively; lighter silicone simulates a buoyant intrusion. In the models, density differences affect the rate of intrusion but not their evolution or shape, whereas viscosity and strength stratifications control both the shape and rate of the intrusions. With a higher viscosity contrast (10 2-4) between the intrusion and the embedding media, the rise of the lighter silicone induces a laccolith-like intrusion, responsible for doming and thinning of the overburden; an apical depression may form, inducing silicone extrusion. Conversely, with a lower (10 1) viscosity contrast, the overburden and the intrusion exhibit a lens-shaped form, with a broad central depression bordered by an upward flexure towards the periphery. A sag in the floor of the intrusion is commonly observed; no silicone extrusion occurred. The intrusion is a hybrid between a laccolith and a lopolith. The comparison with nature (1) confirms roof uplift as an important means of accommodating space during pluton emplacement and (2) suggests that, where roof uplift plays a major role, pluton emplacement can induce a well-correlated sequence of events at surface: doming, the development of a depression and extrusion.
The diagenetic destruction of porosity in plutoniclastic
The Miocene Diligencia Formation of southern California was deposited in an alluvial/ fluvial depositional system developed within the Diligencia basin. This basin formed as an extensional half-graben during latest Oligocene-Early Miocene crustal extension. This continental halfgraben was superposed on an Eocene fault-controlled submarine canyon, in which the Maniobra Formation was deposited. The arkosic composition of the sandstone framework would predict a high-quality reservoir with preserved primary porosity or at least important intergranular volume (IGV). However, inspection of sandstone samples indicates that diagenesis was intense, as several diagenetic processes drastically reduced the reservoir quality. Dominant diagenetic processes were compaction and cementation. Mechanical compaction, expressed as intense deformation of ductile grains and chemical compaction in the form of pressure solution, reduced the IGV of sandstone at the bases of the Maniobra and Diligencia formations to low levels (10-20%). In the upper parts of both units, early diagenetic cements partially inhibited compaction, maintaining IGV values close to 35%. Several mineral phases constitute cements responsible for occlusion of primary porosity. Many of these phases also replaced framework components and early cements. The more important cements are: (1) quartz and K-feldspar, which appear mainly as grain overgrowths, although quartz mosaics have been observed at the base of the Maniobra Formation; (2) carbonates, such as ankerite, dolomite and calcite in diverse textures and with significant occluding character; (3) phyllosilicates, such as kaolin, that developed mainly in Diligencia sandstone as early pore fillings, and locally as illite coats around detrital grains; and (4) other mineral phases, such as Fe-oxides and fluorite, which occur exclusively at the base of the Maniobra Formation, exhibiting aggressive textures against framework grains and older cements. The chronology of diagenetic processes includes marine early diagenesis (eodiagenesis) for Maniobra sandstone, characterized by K-feldspar, ankerite and dolomite cements. Continental early diagenesis is identified in Diligencia sandstone, manifested by the presence of smectite (replaced by illite), kaolin and Fe-oxide cements. Mesodiagenetic processes are similar in both formations. In addition, hydrothermal phases (fluorite and quartz mosaics) are identified at the base of the Maniobra Formation. Diagenetic mineral associations suggest palaeotemperatures above 130 8C. The geotectonic scenario in which diagenesis occurred explains these post-depositional processes. During latest Oligocene-Early Miocene crustal extension, the Diligencia basin developed in an area of high heat flow, as expressed in the eruption of interbedded basaltic-andesitic lavas. High heat flow favoured compaction and cementation, accelerating these diagenetic processes during a relative short time interval. Hydrothermal fluxes produced mineral phases that contributed to the destruction of a potentially good reservoir by intense diagenesis.
Journal of Structural Geology, 2006
Magnetic fabric patterns and microstructures of granitic rocks provide evidence of structural modifications as a result of the coalescence of two plutons, Alpa Corral and Talita, during the construction of a large granitic batholith, Cerro Aspero, in the Sierras Pampeanas of Córdoba (Argentina). The Alpa Corral granite is a small and nearly circular pluton, while the larger Talita pluton displays a marked N-S elongation. The NW quadrangle of the Alpa Corral pluton underwent a deformation resulting in foliation trajectories that outline a crescent moon pattern whose inner arc is concordant with the Alpa Corral/Talita contact. The highest values for the magnetic susceptibility and its anisotropy are concentrated in the coalescence zone between both plutons. The magmatic and sub-magmatic nature of the microstructures demonstrates that such a deformation was caused by the southward overriding of the largest Talita pluton while the Alpa Corral pluton was not yet fully crystallized. Rb-Sr geochronology indicates that this process took place 369G9 Ma ago, long after the Famatinian (Ordovician to Early Devonian) deformation of the country rocks, which confirms a synmagmatic, rather than a regional, deformational event. q
The production of chemically stratified and adcumulate plutonic igneous rocks
1996
We review recent advances on the physical principles of crystallization in multicomponent systems, and use them to provide a framework for interpreting petrological and geochemical observations from igneous intrusions. The thermal structure of crystallizing boundary layers imposes strong constraints on the chemical and mineralogical compositions of the solid that can form from a given melt. The thermal problem is largely independent of the chemical composition of the melt, and sets the course of crystallization. A key problem to understand is the temperature of the solidification front (which we take to mean that point at which the last drop of liquid solidifies) particularly in the geologically relevant case in which the temperature at the cold boundary is below the eutectic temperature. Focussing on the solidification front rather than on the liquidus is a valuable perspective. Adcumulus growth requires specific conditions and much can be learned from trying to understand how these can develop from given starting conditions. We discuss the physical reasons and field evidence Ibr the existence of mushy layers, where solid fraction and temperature vary by large amounts. In such regions of the magma chamber, thermodynamic equilibrium is nearly achieved locally and, for a given temperature, this specifies the composition of the interstitial melt. Thus, in a magma chamber, the whole liquid line of descent is present simultaneously. Compositional convection is likely to set in, and this exchange between the interior of the mushy layer and the main reservoir leads to a chemically stratified solid, and to adcumulus growth. The contribution of crystal settling to the floor cumulates is evaluated as a function of the magnitude of convective heat flux through the roof. It is shown that crystal settling is unlikely to overwhelm in-situ nucleation and growth at the floor.
Tectonophysics, 1999
Granitic intrusions typically have large aspect ratios, with an horizontal major axis about 4-8 times the vertical one. Combined structural and geophysical (gravity) data allows distinction between two main types of plutons. Flat-floored plutons (the most represented) are rather thin (3-4 km) and extend in every horizontal direction with a gently dipping floor toward several root zones. These contrast with the thick (>10 km) wedge-shaped plutons, more elongated in one direction with a few root zones. We interpret the shape of flat-floored plutons as the result of a switch in the stress pattern caused by the emplacement of magma. Magma is preferentially emplaced into the plane (¦ 1 -¦ 2 ) perpendicular to the least principal stress component (¦ 3 ). This plane is initially vertical, except for compressional conditions. This dilation causes a local re-organization of the stress field, by increasing the minor and intermediate principal stress components. When they overcome the lithostatic load, a drastic change in the orientation of the opening plane results, switching from vertical to horizontal. This constitutes a change from vertically-oriented, dike-shaped intrusions to sub-horizontal laccoliths. Crustal anisotropies (vertical faults or horizontal zones of different rheology) also contribute to modify the shape of the intrusions. The continual exchange of the intermediate stress components also explains the rounded shape of intrusions in strongly extended regions. Our analysis suggests that the concept of magmas rising to a level of neutral buoyancy is not applicable to many settings. Rather, we suggest that the feedback between displacements resulting from magma intrusion and the local stress pattern controls the geometry of magma emplacement.
Journal of South American Earth Sciences, 2002
The Cerro Aspero batholith (CAB) (440 km 2) is one of many discordant granites emplaced during the Middle to Late Devonian, marking the closure of igneous activity in the southern part of the Sierras Pampeanas de Co Ârdoba, Argentina. The main feature of the CAB is the conspicuous circular shape of some of its plutons and internal structures, which were developed entirely under pressures lower than 2 kb. The aim of this work is to study the processes responsible for the emplacement of these plutons. The CAB is composed of a succession of three major plutons aligned in a NNW±SSE trend that intrude mainly into mylonites corresponding to an Early Paleozoic shear zone. The emplacement of the plutons was controlled by a set of early master fractures, with magma ascent by means of fracture propagation. The space for magma was created by a combination of tectonically created cavities, internal magma-related buoyancy, and heating and thermal fracture of the host rocks. Field and structural evidence suggests that stoping mechanisms were the most effective for the ®nal emplacement of magma. Thermal fracture and stoping were favored by the thermal contrast between granite and host rock and by the magma viscosity as reduced by the presence of¯uorite. The apparent circular shape of the plutons is polygonal in detail and was developed from several linear and curviplanar segments as a result of brittle fracturing of the host rocks, as well as by changes in the stress ®eld, from vertical to horizontal, related to magma arrival at shallow crustal levels.
Journal of Petrology, 2008
This study reassesses the development of compositional layering during the growth of granitic plutons, with emphasis on fractional crystallization and its interaction with both injection and inflationrelated deformation. The Dolbel batholith (SW Niger) consists of 14, kilometre-sized plutons emplaced by pulsed magma inputs. Each pluton has a coarse-grained core and a peripheral layered series. Rocks consist of albite (An 11 ), K-feldspar (Or 96^99 , Ab 1^4 ), quartz, edenite (X Mg ¼ 0Á37^0Á55), augite (X Mg ¼ 0Á65^0Á72) and accessories (apatite, titanite and Fe^Ti-oxides). Whole-rock compositions are metaluminous, sodic (K 2 O/Na 2 O ¼ 0Á49^0Á62) and iron-rich [FeO tot /(FeO tot þ MgO) ¼ 0Á65^0Á82]. The layering is present as size-graded and modally graded, sub-vertical, rhythmic units. Each unit is composed of three layers, which are, towards the interior: edenite AE plagioclase (C a/p ), edenite þ plagioclase þ augite þ quartz (C q ), and edenite þ plagioclase þ augite þ quartz þ K-feldspar (C k ). All phases except quartz show zoned microstructures consisting of external intercumulus overgrowths, a central section showing oscillatory zoning and, in the case of amphibole and titanite, complexly zoned cores. Ba and Sr contents of feldspars decrease towards the rims. Plagioclase crystal size distributions are similar in all units, suggesting that each unit experienced a similar thermal history. Edenite, characteristic of the basal C a/p layer, is the earliest phase to crystallize. Microtextures and phase diagrams suggest that edenite cores may have been brought up with magma batches at the site of emplacement and mechanically segregated along the crystallized wall, whereas outer zones of the same crystals formed in situ.The subsequent C q layers correspond to cotectic compositions in the Qz^Ab^Or phase diagram at P H2O ¼ 5 kbar. Each rhythmic unit may therefore correspond to a magma batch and their repetition to crystallization of recurrent magma recharges. Microtextures and chemical variations in major phases allow four main crystallization stages to be distinguished:
Journal of Structural Geology, 2004
In the French Massif Central, the Rouergue-Albigeois area consists of three tectonic units stacked during the Hercynian orogeny. The structural analysis of the units and particularly the quartz < c> axis, textural and structural observations in the syntectonic Pinet-type plutons allow us to determine the deformation history. A first thrusting event, D1, is responsible for a southwestward emplacement of the high-grade Lévézou nappe, above the Lower Gneiss Unit itself overlying the para-autochthonous micaschists. In Late Devonian-Early Carboniferous, this stack of nappes experienced a second event, D2, characterized by a top-to-the-NW emplacement under medium pressure/medium temperature conditions of the para-autochtonous unit upon the Lower Gneiss Unit. During D2, the Pinet-type plutons were deformed by a shearing consistent with the kinematics recorded in the country rocks. In contrast to previous works, which considered the Pinet-type plutons emplacement to be coeval with the D2 event, we argue that the emplacement of the plutons occurred during the top-to-the-SW D1 nappe stacking. This work emphasizes the importance of the quartz < c> axis and microstructure analysis for the identification of the early structural history in polyphase deformed terranes.