Petrology of the Gracefield pluton.

Abstract

The Gracefield pluton is composed of two distinct syenite intrusions; both of which appear to have been intruded late in the Grenville tectonic and metamorphic history. They both exhibit a well developed concentric mineralogical zoning. They have shonkinitic cores which grade laterally into alkali syenite. The larger (earlier) intrusive is locally transitional to nepheline syenite along the south margin. The temperature of mineral equilibration of the nepheline-bearing syenite was determined to be 730°C. The temperature of the magma at the time of intrusion was somewhat higher. If the oxygen fugacity during the crystallization history of the early intrusive was close to that of the QFM buffer, the fugacity of water was between 302 and 1950 bars. The silica activity in the magma was low during crystallization. The two intrusives are mineralogically, chemically, and texturally similar. They also appear to have had similar crystallization histories. It is highly probable that the temperature of intrusion, oxygen and water fugacities, and silica activity in the later intrusive, were in the same range as those in the early intrusive. The mineralogical zoning in the intrusives is due to the operation of convection cells during the early parts of their crystallization histories. Shortly after emplacement of a largely liquid magma, salite, apatite, and minor amounts of magnetite and ilmenite started crystallizing along the walls of the magma chamber. These crystals were then concentrated in the lower part of the magma chamber by slow moving convection cells. This resulted in an alkalic magma in the upper part of the chamber, and a crystal mush in the lower part. After convection ceased, crystallization of the feldspars, barkevikite, biotite, most of the magnetite, and nepheline and calcite commenced in the larger intrusive.