Emplacement of dikes, sills and crustal magma chambers at divergent plate boundaries
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Dikes in which two different liquids flowed simultaneously, usually called composite dikes, naturally fall into two classes depending on which lithology forms the contact with the country rock, and hence, which liquid was the first to enter the fracture. In these two kinds of dikes, the structures formed by the mingling of the two liquids differ. Dikes in which the more basic liquid entered first have contacts between the two lithologies that are nearly planar, and parallel to the dike walls, whereas the more basic lithology forms discrete pillows in those dikes in which the more silicic liquid entered first. Experiments indicate that these pillows probably form from a flow-front instability that develops when a liquid invades another of higher viscosity between two parallel rigid walls. We provide scalings for the critical flow rate for the onset of this instability, the time required for the instability to develop, and the wavelength that is selected. These scalings are consistent with field observations.
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The development of discrete volcanic centers reflects a focusing of magma ascending from the source region to the surface. We suggest that this organization occurs via mechanical interactions between magma chambers, volcanic edifices, and dikes and that the stresses generated by these features may localize crustal magma transport before the first eruption occurs. We develop a model for the focusing or “lensing” of rising dikes by magma chambers beneath a free surface, and we show that chambers strongly modulate dike focusing by volcanic edifices. We find that the combined mechanical effects of chambers, edifice loading, and dike propagation are strongly coupled. Chambers deeper than ∼20 km below the surface with magmatic overpressure in the range of 20–100 MPa should dominate dike focusing, while more shallow systems are affected by both edifice and chamber focusing.
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A review of geologic literature shows many important relationships between ore deposits and associated dikes and sills. Most significant are the structural controls by pre-ore dikes on ore localization. Dikes and sills commonly are competent, brecciated bodies that act as host rocks or as channelways for ore solutions. Incompetent dikes and sills serve as dams, ponding ore solutions at their contacts. Fracturing at dike contacts also creates channelways and loci for deposition; the nature of this fracturing is variable, and depends on relative competency and on the nature and direction of differential stress at the contact. Intersections of dikes and sills with fault zones, formation contacts, and other planar structures provide excellent sites for ore deposition. Primary features of dikes and sills, such as permeability, width, and configuration are sometimes important in localizing ore.Contemporaneous and post-ore dikes and sills in a few cases are important as indicators of the factors controlling movements of ore solutions. Post-ore injections may seriously dilute ore deposits, or may by heat action effect changes in mineral composition and distribution.Genetic relationships between dikes, sills, and ore are not well understood. Magmatic segregations commonly take the form of dikes and sills, and other gradational relationships between ore and igneous rock are occasionally noted. Dike injection is predominantly pre-ore, with lamprophyres and diabase apparently closely related in time to mineralization. Most dikes and sills associated with ore are of intermediate composition.
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The Koolau dome, forming the east half of the island of Oahu, is notably elongate, built about a linear rift zone in which numerous feeder dikes occur in a dike complex over 30 miles long. Scattered dikes and sills occur in the leeward parts of the dome, the concentration being progressively reduced with the distance away from the dike complex. There are secondary rift zones and subcomplexes with increased concentrations of dikes and sills, which trend at right angles to the main rift zone. The dikes and sills show three stages and patterns of columnar jointing which are related to the cooling history. Shallow intrusives are vesicular and banded but not columnar jointed. Dikes with a thickness of about 2 feet are preponderant to such an extent as to suggest that this is an optimum determined by the amount of lateral crowding-together of contracted lava formations which can be achieved by the pressures of invading lava.
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