Date of Award

Spring 1993

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Ocean & Earth Sciences

Program/Concentration

Geology

Committee Director

Francis Dudas

Committee Member

Carl F. Koch

Committee Member

Ali Nowroozi

Committee Member

James S. Beard

Call Number for Print

Special Collections LD4331.G4R64

Abstract

Up to 37% granophyre occurs in the center of a 150m-wide Mesozoic diabase dike near Farmville, Virginia. The exceptional abundance of granophyre compared with other Mesozoic diabase dikes suggests unusual petrogenetic processes are involved in formation of the dike. Petrographic analyses show that granophyre increases from 5-10% near the flanks of the dike to a maximum of 37% in the core of the dike, and that granophyre distribution is approximately symmetrical about the center of the dike. Major and trace element concentrations parallel variations in the modal abundance of granophyre, with SiO2 , Na2O and K2O increasing in the granophyre-rich portion of the dike, and MgO and Cao decreasing. Fe2O3* and Al2O3 remain fairly constant. There is a sharp break, not a gradation, between the granophyre-rich and granophyre-poor compositions.

The dike is a high-TiO2 high-iron quartz-normative tholeiite (average SiO2 = 53 wt.%), while the granophyric core is much more evolved (max. SiO2 = 58.4 wt.%). In situ differentiation is inconsistent with the sharp compositional break between components of the dike. Multiple injection of melts generated by differentiation of a parental high-TiO2 high-iron quartz-normative tholeiite magma is possible, but cannot easily account for the constant Fe2O3* and increasing TiO2 in the granophyre. Magma mixing of basaltic and felsic melts or contamination by crustal melts is also inconsistent with TiO2 and Fe2O3* patterns, and cannot explain the compositional gap. Fractional crystallization and multiple injection of melts related to different parental magmas (TiO2 = 1.2% and TiO2 = 1.7%) is the most probable explanation for mineralogical and chemical variation in the Farmville dike.

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DOI

10.25777/22yp-st17

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