Emplacement of shallow dikes and sills beneath a small basaltic volcanic center – The role of pre-existing structure (Paiute Ridge, southern Nevada, USA)
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Many sills and dikes intruded the southern half of the Chikuhō Coal Field in Kyushu. The mechanism of intrusion, petrology of the igneous rock, and thermal metamorphism, of the coal were studied in the Ōminé and Shimoyamada mines. Sills selectively intrude only thick seams of coking coal. Two modes of sill intrusion are apparent: 1) flows of fluid magma freely following the plane of bedding before solidification and, 2) forced intrusions of solidified andesite which pulverized the coal in the seam as it progressed. The sills originated from magma rising along steeply dipping tension cracks caused by pressure of the magma chamber, and now filled with dike rock. Areal folding and faulting preceded the intrusion of sills and dikes. The igneous rock is two-pyroxene andesite containing saponite pseudomorphs of olivine. Thermal metamorphism has resulted in variations of coal ranging from semi-anthracite to natural coke. M. Russell
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Woodson County in southeastern Kansas has been the scene of post-Douglas (Upper Pennsylvanian) intrusive igneous activity. Evidence of this is the presence of (1) an outcropping granite dike, (2) basic dikes or sills which were encountered by at least three drilled wells, and (3) hydrothermally metamorphosed rock in four separate bodies, one of which outcrops. The dikes and metamorphic rocks occur where the strata are folded into domes which are exceptionally sharp for Kansas. The writers suggest that the domal structure is due to the force of intrusion of magma into the deeper sedimentary rocks and that consequently these domes may properly be termed laccoliths.
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Abstract The degree and extent of crustal hydrothermal alteration related to the eruption of large igneous provinces is poorly known and not easily recognizable in the field. We here report a new δ 18 O dataset for dikes and lavas from the Columbia River Basalt Group (16–15 Ma) in the western USA, and document that dikes on average are 1–2‰ more depleted in δ 18 O than basalt flows. We show that this observation is best explained with the involvement of heated meteoric waters during their cooling in the crust. The largest 6–8‰ depletion is found around and inside a 10 m-thick feeder dike that intruded the 125 Ma Wallowa tonalitic batholith. This dike likely operated as a magma conduit for 4–7 years, based on the extent of heating and melting its host rocks. We show that this dike also created a hydrothermal system around its contacts extending up to 100 m into the surrounding bedrock. A model that considers (a) hydrothermal circulation around the dike, (b) magma flow and (c) oxygen isotope exchange rates, suggests that the hydrothermal system operated for ~150 years after the cessation of magma flow. In agreement with a previously published (U-Th)/He thermochronology profile, our model shows that rocks 100 m away from such a dike can be hydrothermally altered. Collectively, our sample set is the first documentation of the widespread hydrothermal alteration of the shallow crust caused by the intrusion of dikes and sills of the Columbia River Basalt Province. It is estimated that heating and hydrothermal alteration of sediments rich in organic matter and carbonates around the dikes and sills releases 18 Gt of greenhouse gases (CH 4 and CO 2 ). Furthermore, hydrothermal δ 18 O depletion of rocks around dikes covers 500–600 km 3 , which, when scaled to the total CRB province constitutes 31,000 km 3 of low-δ 18 O rocks. These volumes of crust depleted in δ 18 O are sufficient to explain the abundant low-δ 18 O magmas in eastern Oregon and western Idaho. This work also demonstrates that the width and magnitude of δ 18 O depletion around dikes can identify them as feeders. Given this, we here interpret Paleoproterozoic dikes in Karelia with the world’s lowest δ 18 O depletions (−27.8‰) as feeders to the coeval large igneous province aged 2.2–2.4 Ga that operated under the Snowball Earth glaciation conditions.
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The exceptional 3-d exposures of the mid-Tertiary intrusive sheet network on the southern margin of Mount Hillers, Henry Mountains, southern Utah, have undergone no syn- or post-emplacement deformation. The sills and dikes, which formed above the underlying Mount Hillers laccolith, therefore provide an ideal opportunity to study purely magmatic processes in a shallow crustal intrusive sheet network. For this study, field work and laboratory analysis were employed to constrain the timing, emplacement mechanisms, and internal flow characteristics of these sills and dikes. Detailed geologic mapping of cross-cutting relationships, in addition to qualitative textural analysis in the field, indicate that younger, relatively fine-grained dikes cross-cut older, relatively coarse-grained sills. Crystal size distribution, thin section petrography, and major and trace element geochemistry all suggest two distinct batches of magma (one coarse- and one fine-grained) were involved in the construction of the sill/dike complex. Field fabrics and anisotropy of magnetic susceptibility fabrics suggest complex internal flow of the intrusive sheets throughout the growth of the central intrusive igneous body. Field observations indicate that intruding magma exploited radial fractures and bedding planes in the sedimentary host rock. In addition, rigidity contrasts in the host rock were likely an important control on the stratigraphic level of sill emplacement and on intrusive sheet thickness. The proposed construction model for the intrusive sheet network consists of an initial phase of dike-fed sill emplacement in subhorizontal strata. During subsequent growth of the underlying main laccolithic body, which included uplift and rotation of the overburden, continued sill emplacement was followed by radial dike intrusion. This work provides insight into the growth and evolution of shallow crustal magmatic systems, such as those that underlie active volcanoes. Â
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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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