Date of Award

2009

Level of Access

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Earth Sciences

Advisor

Edward S. Grew

Second Committee Member

Christopher C. Gerbi

Third Committee Member

Martin G. Yates

Abstract

Pegmatites are present in many metamorphic complexes. Their distinctive mineral assemblages, especially when rich in boron, provide a distinct perspective into the pressure-temperature-chemical environment. Granulite-facies metasedimentary gneisses and associated anatectic pegmatites in Larsemann Hills, Antarctica are unusually enriched in B, and thus, provide a unique opportunity for the petrologic application of boron minerals. The seven pegmatites on which the thesis is based contain the borosilicate minerals prismatine, werdingite, boralsilite, grandidierite, dumortierite and tourmaline-group minerals. Six belong to the folded and discordant D2 and D3 generations and one belongs to the discordant and planar D4 generation. Research for the thesis included microstructural observations and chemical analyses with the electron microprobe analyses with an emphasis on tourmaline-group minerals. The first minerals to form in the D2 and D3 pegmatites were tourmaline in a graphic intergrowth with quartz, prismatine, plagioclase, K-feldspar and sillimanite. Werdingite, grandidierite and boralsilite soon followed. After a deformation event that fractured and allowed fluid infiltration secondary mineral growth occurred. This included tourmaline prisms, dumortierite, and andalusite. Tourmaline compositions evolve as crystallization proceeded in both the D2 and D3 and the D4 pegmatites resulting in an increase in X-site vacancy, a decrease in Ti content and a decrease in F at the W-site, changes consistent with decreasing temperatures. The D2-D3 pegmatites were emplaced at close to peak conditions, that is, the early-formed minerals could have crystallized at temperatures as high as 750-800 °C, P ~ 5 kbar, whereas the D4 pegmatite could have been emplaced relatively soon after. The D2-D3 and D4 pegmatites experienced the same evolution after initial crystallization, and crystallization continued until temperatures were below 600 °C, P ~ 3 kbar. The Larsemann Hills pegmatites show evidence that initial crystallization could have been relatively rapid due to chemical quenching, which was followed by more protracted episodes of complex replacement and secondary growth of minerals. The crystallization of the pegmatites occurred along a retrograde decompression P-T path similar to that of the host rocks.

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