Ijolite, Magnet Cove

Foidolites are coarse-grained intrusive igneous rock with a feldspathoid mineral content greater than 60%. On the APF diagram, these rocks plot in the corner closest to F. The term "foidolite" is too general and, wherever possible, should be replaced with a name that clarifies which of the feldspathoid minerals is predominant and whether other minerals are present in any significant amount. For example, nephelinolites are foidolites comprising mostly nepheline; they can be further categorized as melteigite, ijolite or urtite depending on the relative proportion of dark-colored minerals (most of all, clinopyroxene and biotite) in their composition.
The volcanic equivalents are termed foidite and phonolitic or tephritic foidites. According to the mineralogy can be divided into:

• Ferusite For Fergus, Montana, USA: A variety of foidolite containing essential clinopyroxene and 30-70% modal leucite (typically replaced by a mixture of nepheline, alkali feldspar, kalsilite and analcime).
• Ijolite For Iijoki, northern Finland: A variety of foidolite containing essential clinopyroxene (diopside, aegirine-augite or aegirine) and 30-70% modal nepheline.
• Italite For Italy: A variety of foidolite in which leucite is the predominant feldspathoid mineral and ferromagnesian silicates comprise 30 modal % or less of the rock.
• Melilitite For Melteig, Norway: A collective name for extrusive and shallow intrusive rocks containing > 10% modal melilite.
• Missourite For Missouri, USA: A variety of foidolite containing essential clinopyroxene and 10-30% modal leucite; olivine is also commonly present.
• Tawite For Tavajok, Lovozero Mts., Kola, northwestern Russia: A variety of foidolite in which sodalite is the predominant feldspathoid mineral.
• Urtite For Lujavr Urt, Lovozero Mts. Kola, northwestern Russia: A variety of foidolite containing > 70% modal nepheline; clinopyroxene, apatite, titanite, magnetite and perovskite are commonly present in subordinate amounts.

Magnet Cove

The Magnet Cove Complex occupies about 12 square km in the northeastern part of Hot Spring County of central Arkansas . The Magnet Cove complex was first mapped by J.F. Williams in 1891. A later study by Erickson and Blade involved remapping the area and included much petrographic and geochemical work on the igneous rocks. Their study was published in 1963 "Geochemistry and Petrology of the Alkalic Igneous Complex at Magnet Cove, Arkansas". The authors renamed many of the rock units using current terminology and also made a number of significant corrections in William’s early map.

The complex is composed of a series of post-Mississippian ring dikes that intruded folded and faulted Paleozoic shales, sandstone and chert of the Ouachita geosyncline. It has a core of ijolite and carbonatite, an intermediate ring of trachyte and phonolite, an outer ring of nepheline syenite, and two masses of jacupirangite located along its northern and western edges. Erickson and Blade conclude that the age sequence from oldest to youngest is phonolite and trachyte, jacupirangite, alkalic syenite, ijolite and carbonatite. The carbonatite forms at least three central bodies composed of white to light gray, coarse-grained calcite with scattered concentrations of apatite, monticellite, magnetite, perovskite, kimzeyite, biotite and pyrite.

From their comprehensive studies in the Magnet Cove area, Erickson and Blade (1963) concluded that the alkalic intrusive rocks of the complex were derived by fractional crystallization and differentiation of a residual, melanocratic phonolite magma rich in alkali, lime and volatiles. The parent for this residual fluid was inferred to be a regionally undersaturated olivine basalt magma. The carbonatite-forming fluids themselves were interpreted to be end stage, "pegmatite-like" differentiates of this process on the borderline between magmatic and hydrothermal solutions and highly charged with volatile components.

Geological map of Magnet Cove Igneous Complex. From William C. Kelly (1977)

Bibliography

• Bruce.E., Nesbitt & William C. Kelly (1977): Magmatic and Hydrothermal inclusions in Carbonatite of the Magnet Cove Complex, Arkansas. Contrib. Mineral. Petrol. 63, 271-294 (1977)

Photo
Phlogopite, andradite (brown) and altered nepheline (dusty). PPL image, 2x (Field of view = 7mm)	Phlogopite, andradite and altered nepheline. XPL image, 2x (Field of view = 7mm)	Phlogopite, biotite (green), andradite (brown) and altered nepheline (dusty). PPL image, 2x (Field of view = 7mm)
Phlogopite, biotite (green), andradite (brown) and altered nepheline (dusty). PPL image, 2x (Field of view = 7mm)	Phlogopite, green biotite, andradite, apatite and altered nepheline. PPL image, 2x (Field of view = 7mm)	Altered nepheline. PPL image, 2x (Field of view = 7mm)
Altered nepheline. XPL image, 2x (Field of view = 7mm)	Phlogopite, biotite, andradite and altered nepheline. XPL image, 2x (Field of view = 7mm)	Phlogopite, biotite (green), andradite (brown) and altered nepheline (dusty). PPL image, 2x (Field of view = 7mm)
Phlogopite, apatite, andradite (brown) and altered nepheline (dusty). PPL image, 2x (Field of view = 7mm)	Phlogopite, apatite, andradite (brown) and altered nepheline (dusty). PPL image, 2x (Field of view = 7mm)	Phlogopite, apatite and altered nepheline (dusty). PPL image, 2x (Field of view = 7mm)
Phlogopite, andradite (brown) and altered nepheline (dusty). PPL image, 2x (Field of view = 7mm)	Phlogopite, andradite and altered nepheline. XPL image, 2x (Field of view = 7mm)	biotite (green), andradite (brown) and altered nepheline (dusty). PPL image, 2x (Field of view = 7mm)
Phlogopite, andradite, apatite and altered nepheline. PPL image, 2x (Field of view = 7mm)	Phlogopite, andradite, apatite and altered nepheline. XPL image, 2x (Field of view = 7mm)	Phlogopite, green biotite, andradite, apatite and altered nepheline. PPL image, 2x (Field of view = 7mm)
Andradite enclosed in biotite. PPL image, 2x (Field of view = 7mm)	Phlogopite, andradite, altered nepheline. PPL image, 2x (Field of view = 7mm)