Masters Theses

Date of Award

12-1984

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Harry Y. McSween

Committee Members

Lawrence A. Taylor, Otto C. Kopp

Abstract

The Farmington gabbro-metagabbro complex is located along the western margin of the Charlotte belt of North Carolina. At least twp or more intrusions occur within the complex. The Farmington gabbroic intrusion is postmetamorphic and contains plagioclase, hornblende, orthopyroxene, clinopyroxene, and olivine. Small amounts of magnetite, ilmenite, pyrite, pyrrhotite, pentlandite, hematite, hercynitic spinel, and biotite occur. Textures are hypidiomorphic granular to hypidiomorphic inequigranular. Four types of gabbroic rocks are present; hornblende gabbronite, olivine-hornblende gabbronorite, hornblende norite, and olivine-hornblende norite, with olivine-hornblende gabbronorite predominating. A limited range of progressive differentiation is indicated by restricted increases in FeO/(FeO +MgO) ratios for mafic silicates and low anorthite contents of plagioclases.

The metamorphosed gabbroic intrusion surrounds the postmetamorphic gabbro. Four types of metagabbro can be distinguished based on mineralogy and texture: metagabbro protoliths, which have retained their original mineralogy and igneous textures; type I rocks, having relict igneous textures and clinopyroxene and/or olivine pseudomorphs; type II metagabbros, having poorly preserved igneous textures and minerals; and type III metagabbros, exhibiting textural and compositional banding. Types II and III are dominated by plagioclase and amphibole. Other minerals such as clinopyroxene and epidote may occur along with titanite, magnetite, ilmenite, hematite, chlorite, biotite, and apatite. Type I rocks have substantially more clinopyroxene than type II and III samples, and metagabbro proliths contain orthoproxene and may contain olivine but no epidote. Hypidiomorphic granular to granoblastic textures occur.

A multiple intrusive history for the Farmington complex is suggested by xenoliths of metagabbro in gabbro, metagabbro xenoliths in metagabbro and lack alteration of the gabbro. The metagabbro is composed of two or more intrusions as indicated by olivine-bearing and olivine-free metagabbro prolith, two distinct groups of igneous amphiboles, metagabbro xenoliths in metagabbro, and variability of metamorphic grade. The mineralogic differences and gneissic appearance would also support the presence of a layered intrusion.

Metamorphism ranges from greenschist to greenschist-lower amphibolite transitional grade. Metamorphism resulted in alteration of anhydrous mafic minerals and recrystallization. Variability may result from localized availability of H2O, shearing, and repeated intrusion.

Gabbro-metagabbro complexes southwest of the study area are believed to have formed above an active subduction zone. Due to similarities and areal relationship with other complexes, the Farmington complex is suggested to have had the same origin. It has been suggested that these complexes represent "chimneys" formed by repeated intrusion which would heat the lithosphere and insulate successive magma bodies. The multiple intrusive nature of the Farmington complex would appear to support this model of emplacement.

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