Width Of Intrusion Research Articles

Orientation of Tabular Mafic Intrusions Controls Convective Vigour and Crystallization Style

The microstructure in basaltic dykes is significantly different from that in sills and lava lakes of the same bulk composition. For a given width of intrusion (or depth of lava lake), vertical tabular bodies are coarser grained than horizontal bodies, with an invariant plagioclase shape across the intrusion. When comparing samples from sills and dykes for which the average grain size is the same, the dyke samples contain fewer small grains and fewer large grains than the sill samples. In contrast, the variation of median clinopyroxene-plagioclase-plagioclase dihedral angles in dykes correlates precisely with that observed in sills and is a function of the rate of diffusive heat loss. These patterns can be accounted for if the early stages of crystallization in dykes primarily involve the growth of isolated grains suspended in a well-mixed convecting magma, with the final stage (during which dihedral angles form) occurring in a crystal-rich static magma in which heat loss is primarily diffusive. In contrast, crystallization in sills occurs predominantly in marginal solidification fronts, suggesting that any convective motions are insufficient to entrain crystals from the marginal mushy layers and to keep them suspended while they grow. An exception to this general pattern is provided by members of the Mull SolitDykes, which propagated 100-1000 km SE from the Mull Palaeogene Igneous Centre, Scotland, through the shallow crust. These dykes, where sampled >100 km from Mull, have a microstructure indistinguishable from that of a sill of comparable thickness. We suggest that sufficient nucleation and crystallization occurred in these dykes to increase the viscosity sufficiently to damp convection once unidirectional flow had ceased.

A large scale investigation into changes in coal quality caused by dolerite dykes in Secunda, South Africa-implications for the use of proximate analysis on a working mine

The coalfields of South Africa contain numerous dolerite intrusions, which affected the quality of the surrounding coal through thermal processes, commonly believed to be controlled by the size of the magmatic body. Data gathered from a working coalfield in Secunda, South Africa, suggest that the relationship between intrusive sills and coal is complex and factors other than intrusion width must be considered in relation to the contact metamorphic effect. The study area contains multiple dolerite intrusions of Karoo age, of which three intrusions occur as sills intruded close to the main coal seam of the. A large database (>8000 boreholes) of coal quality data was used to investigate the presence or absence of a change in coal quality relative to dolerite proximity. Reduction in coal quality was defined using three proximate analysis values, namely the ash, volatile content and dry ash free volatile (DAFV) as defined in the coal industry. The resultant investigation showed no correlation between the position and thickness of the dolerites, and changes in coal quality as measured by proximate analysis. In the absence of a linear relationship between coal quality and dolerite proximity, two processes are proposed to explain the absence of the contact metamorphic effects expected from previous studies. Firstly dolerite emplacement dynamics may influence the size of the metamorphic aureole produced by an intrusion, invalidating intrusion size as a measure of thermal output. Secondly, hydrothermal fluids mobilised by the dolerite intrusions, either from the country rock or the intrusion itself may percolate through the coal and act as the metamorphic agent responsible for changing coal quality, by dissolving the volatile and semi-volatile components of the coal and transporting them to other locations. These two processes are sufficient to explain the lack of a clear “metamorphic effect” related to the dolerite intrusions. However, the perceived lack of a clear correlation between the coal quality parameters and the metamorphic effects associated with dolerite intrusion may also reflect the inadequacies of proximate analysis techniques in quantifying geological processes within the coal.

Influence of the temperature dependence of thermal parameters of heat conduction models on the reconstruction of thermal history of igneous-intrusion-bearing basins

Heat conduction models are important tools for reconstructing the thermal history of sedimentary basins affected by magmatic intrusions. Accurate thermal properties of the intrusion and its wall rocks are crucial for accurate predictions of thermal history. Although data on the thermal properties of rocks used in the models are not yet sufficient, we theoretically evaluated the influence of their temperature dependence on the reconstruction of the thermal history of wall rocks of a cooling intrusive sheet. In this study, due to their relatively adequate thermophysical data over wide temperature ranges, diabase, limestone, and Berea sandstone are assumed as the intrusion and the wall rocks in a heat conduction model. A linear interpolation algorithm is used to fit the temperature dependence of their thermal conductivity and specific heat based on experimental data. Simulations indicate that (a) the temperature dependence of these thermal properties can improve the estimation of the contact temperature ( Tc), (b) the deviation of Tc from a temperature-independent-coefficient heat conduction model can be as large as 102 °C for limestone and 61 °C for Berea sandstone, and (c) this is also the maximum deviation in the peak-temperature profile of wall rocks. However, its effect on the peak-temperature profile of wall rocks which is above 20 °C only occurs in a very narrow zone that is less than a half width of the intrusion away from the contact or a domain with the peak temperature above 500 °C. Furthermore, the temperature dependence can also significantly change the thermal-evolution history of wall rocks, lowering the cooling rate of magma intrusion and hence influencing the prediction of vitrinite reflectance ( Ro) based on the EASY%Ro model. Until a magma intrusion almost completely cools down, the maximum deviation in the predicted Ro profile of wall rocks from the temperature-independent-parameter heat conduction model can attain 0.47% for Berea sandstone at a location of about 150% of the intrusion width away from the contact and 0.62% for limestone at a location of about 92% of this distance.

THE GEOMETRY AND EMPLACEMENT MECHANICS OF A BUSHVELD COMPLEX PERIDOTITE BODY, SOUTH AFRICA

The Apiesdooringdraai peridotite is a syn-Bushveld Complex, dominantly harzburgitic intrusion of Lower Zone parentage. Detailed mapping and three-dimensional modeling shows the intrusion to comprise several discordant segments linked by concordant, sill-like segments. Deformation in the contact aureole is strongly partitioned adjacent to the discordant segments of the intrusion and is detectable at distances from the contact on the order of the intrusion width. In contrast, areas above and below the intrusion, and at greater distances from the discordant intrusive segments, are effectively undeformed. The geometry of structural features suggests that the peridotite is a variably structurally-evolved stepped-sill system that was emplaced as kilometre-scale magma fingers from the northwest to the southeast, a trend heavily utilised by the mafic-ultramafic magmas of the Bushveld Complex. Deformation in the country rock was affected by the lateral expansion of the magma fingers at high postulated strain rates, and linkage of offset fingers formed prominent steps in the intrusion. Emplacement-related country rock deformation is postdated by a shallow dipping foliation that records compaction and crustal loading during contact metamorphism.

Coal metamorphism by igneous intrusion in the Raton Basin, CO and NM: Implications for generation of volatiles

Abstract The Raton Basin contains a series of Tertiary mafic dikes and sills that have altered millions of tons of coal to natural coke and may have played a minor role in generating some of the large coalbed methane resources currently being exploited in this region. Four outcrops within the Raton Basin were selected to investigate local coal rank elevation and volatile generation due to intrusive activity. Coal and organic shale samples were collected on traverses across meter-scale dikes and sills, and were analyzed for vitrinite reflectance, total organic carbon, bulk carbon isotopic ratios, and coke petrography. Reflectance patterns in the contact aureoles of the dikes display very consistent concave-up patterns, with reflectance values returning to background within one intrusion width from the contact. Bulk coal samples collected across dikes display an increase in δ13C of approximately 1‰ approaching all contacts. Reflectance patterns within the contact aureoles of sills do not decrease significantly until one half of an intrusion width away from the contact, but then drop steeply to background values by one intrusion width away. Bulk coal samples display complex isotopic patterns approaching sills, with an initial increase in δ13C followed by a steep decrease of approximately 1.5‰ approaching the top contacts. The distinct isotopic and reflectance patterns for dikes and sills may indicate that the coal in the contact aureoles of the sills experienced a longer heating duration than the coal near the dike contacts. This longer heating duration was due to the low thermal conductivity of coal, which acts as a thermal insulator to the sills that intrude coal beds. This style of intrusion is commonly observed throughout the central portions of the Raton Basin. By contrast, dikes intrude a variety of country rock types, and are not insulated by coal. Heat may escape the contact zone through nearby sandstones or shales. Based on reflectance patterns, hydrothermal convection played a role in heat transfer away from dikes. Data from both the dike and sill contacts suggest that time plays a role in controlling peak vitrinite reflectance values, at least over the short heating durations (tens of years) expected for meter-scale intrusions. Decreasing total organic carbon values approaching all contacts suggests the generation and escape of volatile species such as methane. Isotopic values within the contacts of dikes are consistent with the loss of 12C-enriched methane. The isotopic pattern observed at the contacts of sills may be due to accumulation of 12C-rich pyrolytic carbon close to the contacts, which is supported by petrographic observations. Even though multiple, thin, sheet intrusions are insignificant for conductive heating on a basin-scale, methane generation by sheet intrusions was volumetrically important in the Raton Basin because sills preferentially intrude coal beds in this region.

Mechanical analyses of the emplacement of laccoliths and lopoliths

Elastic deformations of host rocks during the emplacement within the Earth's crust of magmatic intrusions, such as laccoliths and lopoliths, are analyzed. The present analysis is built upon semianalytic elastic solutions for a pressurized horizontal crack buried between an overburden and a semi‐infinite base. The current work improves upon recent analyses by including several important features inherent to the development of laccoliths and lopoliths. First, both elongated intrusions (plane strain case) and circular intrusions (axisymmetric case) are described by the current models. Second, the effect of the difference in elastic moduli between the overburden and the substrate is considered. This difference in elastic moduli is characterized by one of the Dundurs parameters. Third, the stress intensity factor at the tip of the crack is assumed to be zero. Thus the stresses are finite, and the obtained laccolithic shapes are doubly hinged with horizontal slopes at the peripheries, as observed in the field. Fourth, unlike plate bending models, which are commonly used for major laccoliths, the current models describe a full range of ratios of intrusion width to overburden thickness. Numerical results are obtained for different combinations of the geometrical parameters, the material contrast and the driving pressure distributions. Graphs of these results confirm the plate bending model for large laccoliths, support Gilbert's concept pertaining to small laccoliths, permit the prediction of the sill‐laccolith transition, allow the analysis of the asymmetry in the vertical displacements above and below the intrusion, and lead to a possible mechanism of deep lopoliths emplacements. Some of the obtained results are compared to the available field data from the Henry Mountains.

The Drifting Confluence Zone

Alongshore migration of a western boundary current separation is investigated with a nonlinear inviscid reduced gravity model on an f plane. Separation due to a collision with an opposing current is considered. It is known that for such a case stationary collision and separation is possible only for boundary currents with “balanced” transports, that is, equal near-wall depths. The authors perturb this stationary solution with a small steplike variation of the opposing current transport and focus on the resulting time-dependent flow. Two different analytical methods to compute the migration rate are used. The first method involves integrated balances, and the second involves the path equation for the separated flow. Using the fist approach, it is found analytically that the flow consists of one current intruding into the area occupied by the other. After an initial adjustment period the intrusion becomes steadily propagating. The width of intrusion is much greater than the width of boundary currents, whereas the migration speed is much smaller than the speed of the currents. The speed of the opposing current intrusion into the main western boundary current is given approximately by the formula C ≈ (g′H)1/2(D21 − D22)2/4D3/22, where H is the undisturbed depth of the main poleward flowing current, D1 is its depth near the wall (nondimensionalized by H), and D2 is the opposing current near-wall (nondimensional) depth. Due to the nonlinearity of the process, the expression for the opposite speed (i.e., the main western boundary current intrusion into the opposing current) has a similar looking but different form. Using the second approach, the original initial value problem is reduced to a time-dependent path equation for the separated current. It is shown analytically that, as should be the case, in the limit t → ∞ the path equation solution is identical to the earlier solution for the steadily propagating intrusion. The migration process exhibits an hysteresis; that is, the progression of the separated currents differs from its corresponding regression. Application of the theory to the collision and separation of the Brazil and the Malvinas Currents is discussed. It is suggested that the observed migrations of the separation latitude may be caused by seasonal changes of the Malvinas Current transport.

Thermal development of low‐pressure metamorphic belts: Results from two‐dimensional numerical models

We have used two‐dimensional numerical models and analytical solutions to heat flow equations to investigate the mechanisms and the spatial and temporal development of regionally extensive low‐pressure facies‐series metamorphism (LPM). Two‐dimensional models are necessary for describing the evolution of isotherms in the crust where lateral heat flow and local time‐dependent heat sources are important. The models demonstrate that felsic intrusions can produce regionally extensive LPM where their abundance through time in the upper crust exceeds approximately 50%. Evenly spaced intrusions elevate vertical metamorphic gradients in regions between intrusions by ∼5° to 10°C km−1 over background values at ∼40% abundance and by ∼15° to >30°C km−1 at ∼70% abundance. Other mechanisms, such as intrusion of mafic magmas in the lower crust, crustal extension, or aqueous fluid advection, cannot independently produce maximum temperatures in such regions. If coeval with intrusion, these other processes will contribute to maximum temperatures; however, the distribution of isotherms and the timing of metamorphism in the upper crust are governed by the shallow intrusions. Intrusion width, abundance, and composition, and timing of emplacement of nearby intrusions have the dominant influences on maximum temperatures between intrusions. For 10‐km‐wide intrusions, abundances necessary for production of regionally extensive LPM are ∼15% less for gabbros than for granites and ∼20% less when intrusions are emplaced simultaneously than when complete cooling occurs between intrusive events. Intrusion shape, height, convection (magmatic and hydrothermal), and recharge (open system magma chambers) have a significantly less effect on thermal evolution. The total area affected by intrusions depends on their final distribution, not the magmatic flux. In the development of low‐pressure metamorphic terranes with abundant intrusions, temperatures in the upper crust are significantly elevated only near recently emplaced intrusions. Consequently, the final distribution of metamorphic grades has little resemblance to the distribution of isotherms at any time—a fundamental difference with LPM in regions lacking abundant shallow intrusions, where metamorphism likely occurs concurrently over large regions. This model of the development of widespread LPM by the juxtaposition through time of local, short‐lived thermal maxima should be considered when interpreting temperature‐dependent phenomena (for example, deformation) in terranes with abundant intrusions.

A comparison of discrete and continuous intrusion models for the thermal structure of the plates

Summary. We have obtained the temperature solution for a thermal model of the plates in which material is emplaced at discrete intervals rather than spreading continuously. The time-averaged heat flux and integrated heat flux for this discrete instrusion model are finite everywhere, differing from that given by a one-dimensional cooling model within a few intrusion widths of the ridge axis. In the limit as the intrusion width and period of emplacement tend to zero the temperature solution away from the ridge axis becomes that given by a modified plate model where a thermal balance boundary condition instead of a constant temperature condition is applied at the ridge axis. The integrated heat loss from a limited age interval about the ridge differs negligibly from that given by the one-dimensional cooling model or the modified plate model provided the interval extends more than two intrusion widths from the ridge axis.

Mechanics of emplacement of basic intrusions

The sunken nature of many basic intrusions can be explained by simple models for flexure of the lithosphere subjected to loading by a magma. The lithosphere is divided into the strata above the magma which deforms as a stack of elastic plates, and the substratum below the magma which is modelled as an elastic plate overlying a weak fluid asthenosphere. Plane-strain plate theory is used to calculate shape and size of plutons. Significant mechanical parameters controlling the shape are: (1) depth of emplacement; (2) width of intrusion; (3) total lithospheric thickness; (4) magma density; (5) effective thickness of the overburden; and (6) magmatic pressure. In areas with lithospheric thickness of 50–100 km, basic intrusions emplaced in the upper crust (< 10 km) will be laccolithic whereas bowl-shape intrusions form in a thin lithosphere (< 50 km) or result from viscoelastic deformation subsequent to emplacement. For a thick lithosphere (100 km) emplacement of magma in the lower crust should result in thin sill-like plutons. If lithospheric thickness was 25 km and magma pressure about 1 kbar, emplacement of the Freetown basic complex at 15 km depth results in a maximum magma thickness of 10 km. The thickness of the model compares well with that determined from Bouguer gravity models. Calculated thickness from the elastic bending model for Umfraville, Thanet and Tudor gabbros, Ontario, yield values smaller than thickness determined by gravity. Progressive thickening of these plutons subsequent to emplacement may have caused the greater thickness, where the underburden, subjected to high temperatures, reacted viscoelastically. Dip of igneous layering towards the center of some intrusions may result from viscoelastic depression of the floor while the magma crystallized.