Kathleen Clark

B. Sc. Honours Thesis

Trace element geochemistry of biotite from the Scrag Lake and New Ross plutons of the South Mountain Batholith, Nova Scotia, Canada: Implications for magma differentiation

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The South Mountain Batholith (SMB) is a 7300 km2, peraluminous, felsic intrusion occurring in SW Nova Scotia. The batholith consists of 13 plutons emplaced in two different phases between ~385 (Phase 1) and ~368 Ma (Phase 2). Previous research revealed that samples from Phase 2 plutons show significant incompatible trace element variability in biotite compared to Phase 1 plutons. Since Phase 2 plutons tend to be more geochemically evolved, it is not clear if this difference arises due to sampling bias or is an intrinsic property of the second stage of batholith emplacement. Therefore, the goal of this work is to better characterize biotite compositions across a broad compositional range of representative Phase 1 and Phase 2 plutons. Here I present the results of analyses of samples from the Phase 1 Scrag Lake pluton (SGP) and the Phase 2 New Ross pluton (NRP). An electron microprobe and laser ablation ICP-MS have been used to collect major and trace element spot analyses and compositional maps on biotite from a suite of 5 samples from each pluton covering a compositional range of ~68 to ~75 wt% SiO2. Data for 34 trace elements were obtained, and 18 of these trace elements were used to create trace element maps of 10 different biotite grains. Incompatible trace elements show increases from core to rim within these biotites. For similarly-sized grains and similar whole-rock wt% SiO2, Phase 1 samples show within grain variation from 10s – 100s g/g, while variation within phase 2 biotites is 10s – 1000s g/g. This indicates that the more extreme extent of incompatible trace element variation is an intrinsic property of Phase 2 plutons. Compatible trace elements show opposite trends, with decreases from core to rim. Phase 1 samples show higher concentrations of compatible trace elements, up to 1000 times more than in Phase 2 samples. Some less evolved SGP samples also show oscillatory zoning in Ba and Ga, suggesting that the crystallization and melting of K-feldspar affected these trace element concentrations. Comparing data to simple fractional crystallization models shows that there are trends in the more evolved samples that cannot be explained by fractional crystallization alone, suggesting a non-magmatic process such as hydrothermal activity affected Phase 2 samples. The modeling also shows that Phase 2 biotites have crystallized from more evolved melts than Phase 1 biotites, which explains why there is a difference in the trace element concentrations in biotite between the two phases.

Key words: South Mountain Batholith, Biotite, Trace Elements, LA ICP-MS Mapping, Magma Differentiation
Pages: 65

Supervisor: James Brenan