Saskia Erdmann

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Intermediate Diploma (Geology), Technical University Berlin, 2000
Diploma Degree (Exploration Geology and Petrology), Technical University Berlin, 2002
 
 

Ph. D. Thesis

Country-rock Contamination and Assimilation in the South Mountain Batholith

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The South Mountain Batholith (SMB) of southern Nova Scotia is a large composite, peraluminous granitoid complex, intruded dominantly into metapsammitic to metapelitic rocks of the Meguma Group. Field and textural relations reveal evidence for contamination of the SMB by rocks of the Meguma Group in the form of xenoliths, xenocrysts, and former partial melt, and assimilation of the country-rock material through a combination of fracturing, dispersal, partial melting, dissolution, and ion exchange reactions. Assimilation of the metapsammitic rocks released orthoxenocrysts of quartz, biotite, and plagioclase, and up to 50 vol% of partial melt into the SMB magmas, whereas assimilation of the metapelitic rocks released paraxenocrystic garnet and cordierite, and up to 80 vol% of partial melt. In the SMB rocks remote from contacts with Meguma Group rocks, textural relations, chemical compositions, and/or zoning patterns permit the identification of xenocrystic garnet and cordierite with confidence. On the other hand, deciphering the origin of any given quartz, biotite, and plagioclase crystal in the SMB has a considerable uncertainty, because textures and compositions of country rock-derived and fine-grained cognate magmatic crystals are similar. Despite these difficulties, assuming that all suspect crystals are true xenocrysts, and that all xenocrysts present in a given rock are physically detectable, xenocrysts make up <= 4 vol% in the marginal rocks and <= 3 vol% in the more central rocks of the SMB. Using ratios of xenocrysts to partial melt determined in melting experiments employing rocks of the Meguma Group, the abundance of xenocrysts may suggest that <= 10 vol% of physically invisible, complementary former country-rock partial melt is present in the SMB rocks, and together, <= 16 and <= 8 vol% of Meguma Group country-rock material appears to occur in the marginal and more central rocks of the SMB, respectively. However, existing whole-rock isotopic data indicate that at least the most evolved, and typically the more central, rocks of the SMB may consist of up to 56 vol% of additional country rock-derived partial melt, which may have been added to the SMB magmas through the assimilation of Meguma Group rocks, or by a Meguma Group source contribution. Energy-constrained numerical modelling of the assimilation of Meguma Group rocks in the SMB magmas shows that all former country-rock partial melt present in the exposed SMB rocks may be explained by country-rock assimilation through partial melting, if the exposed rocks were selectively contaminated, but cannot rule out that the former Meguma Group partial melt added to the SMB as part of the source until further whole-rock and mineral isotopic analyses are available.

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Pages: 72
Supervisor: Barrie Clarke