Igneous rocks of the Highwood Mountains, Montana: Part I The laccoliths

Hurlbut, C. S.

doi:10.1130/gsab-50-1043

Cited by 55 publications

(43 citation statements)

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“…Transport and sorting in igneous systems have long been recognized ever sine the initial work on Shonkin Sag when Hurlbut (1939) realized that all the extensive crystal fractionation in this thin 70 m laccolith was due to massive settling of the 35 vol% phenocrysts present upon emplacement (e.g., Marsh et al 1991;Marsh, 1996). Later work on many sills showed similar effects, but there has always been the over-riding urge to explain all such variations by the effects of layered precipitation through phase equilibria.…”

Section: Granular Processesmentioning

confidence: 99%

On some fundamentals of igneous petrology

Marsh

2013

Contrib Mineral Petrol

134

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The age-old process of crystal fractionation leading to the diversity of the igneous rocks and Earth itself is an exceedingly well-understood chemical process in magmatism and physical chemistry. But the broader physical aspects of this and related processes have proven elusive on many fronts, especially in its relation to the spatial variations in rock composition, texture, and macroscopic features like layering. Magmatic systems, be they volcanic, dikes, sills, or plutons, are generally analyzed with a problem at hand and an end result in mind. The processes invoked to solve these problems, which are most often purely chemical, are often unique to each problem with few if any general principles emerging that are central to understanding the wider perspective of magmatic processes and problems. An attempt is made at the outset to provide a list of inviolate Magmatic First Principles that are relevant to analyzing most magmatic problems. These involve: initial conditions; critical crystallinity; solidification fronts; transport and emplacement fluxes; phenocrysts, xenocrysts, primocrysts; crystal size; layering and crystal sorting; thermal convection; magmatic processes are physical. Along with these principles, two reference magmatic systems are suggested where the initial conditions and outcome are unequivocal: the Sudbury impact melt sheet and the Hawaiian lava lakes. Sudbury formed in *5 min by superheated magma crystallized to a near uniform sequence, while the tiny lava lakes, formed of crystal-laden slurries, form a highly differentiated layered sequence. The major difference is in the initial conditions of formation, especially the nature of the input materials. The challenge is to construct and analyze magmatic systems (i.e., magma chambers, sills, dikes, and lavas) using these reference end members and the suggested principles. The Hawaiian 500,000 year volcanic record exhibits what can be expected as input materials, namely a highly varied output of magma of an overall composition reflecting the abundance of entrained olivine primocrysts. The provenance of these crystals is varied, and within any single sample, the population may be highly heterogeneous in composition from crystal to crystal, yet the overall pattern of chemical fractionation is exceedingly regular and well defined. If similar inputs go to form large intrusions, these systems will undoubtedly be dominated by crystal-rich slurries, which provide a vast set of physical processes promoting exotic layering and, at the same time, given the effects of annealing and continued crystal growth, a final chemical record adhering to all the time-honored effects of crystal fractionation. The long assumed initial condition of instantaneously emplaced crystal-free magmas cannot reasonably produce the observed rock records.

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Section: Granular Processesmentioning

confidence: 99%

On some fundamentals of igneous petrology

Marsh

2013

Contrib Mineral Petrol

134

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“…On a more detailed basis, using the classification given by Sorensen (1974), rocks with < 10~o plagioclase, 0-5~ quartz, and 0-10~ feldspathoids may be called shonkinites or alkali melasyenites. However, when compared with the type shonkinites (Hurlbut and Griggs, 1939;Nash and Wilkinson, 1970) these rocks differ slightly, since the olivine is more forsteritic.…”

Section: Petrographymentioning

confidence: 89%

Mineral chemistry of coexisting phases from shonkinitic rocks, Salem, Tamil Nadu, India

Friend

Janardhan

1984

Mineral. mag.

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ABSTRACT.Gabbroic-textured shonkinitic rocks of unknown age cut deformed Archaean dunites and peridotites in the area immediately north of Salem. Magnesite formation post-dates their intrusion. The location of these rocks and the magnesite is thought to be controlled by the intersection of two major basement lineaments. A sequence of crystallization of the minerals and rocks from early undersaturated to late oversaturated rocks is established. Compositions of the coexisting mineral phases are reported here for the first time. Clinopyroxenes, which are relatively unevolved, commonly coexist with olivine (chrysolite) and sanidine. Bronzite, often iutergrown with magnetite and green spinel, is occasionally present. In the most undersaturated rocks nepheline-sanidine fingerprint intergrowths occur, whilst perthitic sanidine, albitic plagioclase, and quartz are present in over-saturated rocks. Late in the crystallization sequence, in oversaturated rocks, PH20 appears to have increased allowing the formation of amphibole as well as clinopyroxene.

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“…El ascenso de un fluido magmático hasta niveles altos y posiblemente frágiles de la corteza conlleva contrastes fuertes de resistencia entre la intrusión y el encajante. Estudios experimentales de intrusiones de tipo lacolítico han utilizado materiales bastante resistentes para modelizar la roca caja (Howe, 1901;McCarthy, 1925;Hurlbut y Griggs, 1939;Dixon y Simpson, 1987). Unicamente experimentos recientes han permitido situaciones en las que una deformación frágil de la roca caja puede acompañar al mecanismo de intrusión (Merle y Vendeville, 1992;Román-Berdiel et al, 1995).…”

Section: Dispositivo Experimental Y Materialesunclassified

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Berdiel¹,

Gapais

Brun

1997

Estud. geol.

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El emplazamiento de granitoides en zonas de desgarre de la corteza superior se ha modelizado inyectando un fluido newtoniano en un paquete de arena en el cual se intercala una capa dúctil de silicona que actúa como un nivel de despegue potencial en el cual el material inyectado puede expandirse. El régimen de desgarre se obtiene utilizando una placa basal rígida y móvil que desliza horizontalmente. Se ha analizado la influencia de diferentes perfiles reológicos de la corteza superior en la forma de la intrusión. Los experimentos muestran que: 1) las intrusiones son alargadas y su eje mayor toma la dirección principal de elongación asociada al régimen de desgarre, 2) las intrusiones presentan forma asimétrica en planta, con el desarrollo de una cola en el sentido de cizalla, 3) el régimen de desgarre permite localmente la subida del fluido inyectado a lo largo de las fallas formadas en la cobertera, 4) cuando el perfil reológico de la corteza es igual a ambos lados de la zona de cizalla, las intrusiones presentan forma de gota asimétrica, 5) cuando la viscosidad de los materiales que están por encima del canal de alimentación es superior a la de los materiales del otro lado del accidente principal, no aparece la forma en gota asimétrica.Elleucogranito de Lizio en la zona de cizalla surarmoricana (Sur de Bretaña, Francia) es un ejemplo de que la modelización analógica puede dar las claves para explicar la geometría de muchos granitos sintectónicos emplazados en zonas de desgarre.Palabras clave: Intrusión, lacolitos, desgarre, modelización analógica, granitoide, cadena hercí-nica. ABSTRACTGranite intrusion in the upper crust along a crustal-scale shear zone has been modelled by injecting a Newtonian fluid (low-viscosity silicone putty) into a sandpack containing a ductile layer of silicone putty which acted as a potential décollement level along which the injected material could spread. The strike-slip regime was obtained using a mobile rigid basal plate sliding horizontally. Boundary conditions were chosen in order to analyse the influence of different rheologic profiles of the upper crust on the pattem of the intrusion. Experiments showed that: 1) intrusions are elongate and their long axis tends to track the principal stretching direction associated with the strike-slip regime, 2) intrusions are asymmetric shaped in horizontal view, with the development of a sheared tail trailing behind the intrusion, 3) the strike slip environment allowed local rising of the injected fluid along faults formed in the overburden, 4) when the crustal rheologic profile is similar in both sides of the shear zone, intrusions are asymmetric drop shaped, 5) when the viscosity of materials aboye the feeding pipe is higher that viscosity of materials of the other side of the shear zone, the asymmetric drop shape not appears.Lizio leucogranite of the South Armorican Shear Zone (South Brittany, France) emphasize that our experiments can explain the geometry of many syntectonic' granites emplaced along strike-slip zones. They further shed s...

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Igneous rocks of the Highwood Mountains, Montana: Part I The laccoliths

Cited by 55 publications

References 0 publications

On some fundamentals of igneous petrology

On some fundamentals of igneous petrology

Mineral chemistry of coexisting phases from shonkinitic rocks, Salem, Tamil Nadu, India

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