Date of Award

Fall 1992

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Ocean & Earth Sciences

Program/Concentration

Geology

Committee Director

Francis Ö. Dudas

Committee Member

Ali Nowroozi

Committee Member

Joseph H. Rule

Call Number for Print

Special Collections LD4331.G4S64

Abstract

The Morefield Pegmatite is a planar, nearly vertical body situated in Grenville age gneisses and schists in the southeast Piedmont of Virginia. It consists of a mineralogically simple albite-quartz outer zone and a texturally and mineralogically complex inner zone of microcline-topaz-quartz, with trace amounts of rare-element minerals. Fluid inclusions in samples of quartz, topaz, beryl, spessartite and albite that span the length, width and depth of both zones can be classified by chemical composition into two main groups: H2O-rich solutions and CO2- H2O -rich fluids. The H2O -rich inclusions consist of H2O -rich liquid and H2O vapor, occur in microfractures (secondary fluid origin), have a fairly constant liquid:vapor ratio, almost never exceed 30 μm in diameter, and have mostly uniform, distinctive textures. Three fluid phase CO2-H2O inclusions consisting of an outer H2O-rich liquid, an intermediate CO2-rich liquid, and an inner CO2 vapor, occur in microfractures (also secondary in origin), and have widely variable textures and sizes. A third type of inclusion consists of two- and three-fluid phases and contains up to five solid, anisotropic daughter minerals. The two main types were found in all areas sampled, while the third type was only found in the inner zone.

Microthermometry indicates that the H2O-rich inclusions consist of salt solutions, with a range of NaCl equivalent of 0 to 13 wt%. CO2-H2O fluids consist of 40 to 95 % CO2 (by volume) and also contain H2O-salt solutions up to 10% NaCl equivalent (by weight). At least three different H2O -rich inclusion populations with unique salt contents and at least two different populations of CO2-H2O inclusions exist. Throughout the pegmatite, the CO2 phase is very pure with respect to CH4 and other volatiles.

The microthermometry data indicate that there is no distinct distribution of inclusion types according to mineralogic zone. Assuming igneous inclusion origin, preliminary estimates of maximum pressure and temperature of pegmatite formation are 2.8 kilobars and 550-600°C. However, several different fluid generating events are inferred from the variety of different inclusion populations, including at least three volatile/solute-rich pegmatite fluids, fluids related to post-pegmatite hydrothermal alteration, and possibly subsequent metamorphic fluid migration. Large amounts of secondary inclusions indicate that the fluids originally trapped may have evolved in a complex manner after the pegmatite crystallized. This, along with post-entrapment alteration of the third inclusion type, inferred to be the most closely related to the last pegmatitic fluid, severely limits petrogenetic constraints that can be derived from fluid inclusion data.

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DOI

10.25777/1h6j-ae84

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