Yi Ding

M. Sc. Thesis

AFM Minerals in the Halifax Pluton

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The Halifax Pluton is part of the South Mountain Batholith. It consists of six lithologies with various assemblages of AFM minerals: biotite granodiorite and biotite monzogranite contain biotite, and traces of muscovite and cordierite; coarse - grained leucomonzograntie and biotite + cordierite monzogranite contain biotite, muscovite, cordierite, and garnet; fine - grained leucomonzogranite contains biotite, muscovite, cordierite, garnet, and andalusite; and aplite contains biotite, muscovite, cordierite, and garnet.

All phases of the Halifax Pluton contain magmatic biotite the compositions of which are close to those of biotites from metasedimentary rocks. Magmatic muscovite exists in the Halifax Pluton providing evidence for a magma rich in H2O and with a high A/CNK to achieve the low solidus temperature for muscovite. Magmatic cordierite is characterized by its euhedral shape, low M/(F+M) (<0.5), Na2O>1%, and rims rich in the ratio M/(F+M). Both magmatic and metamorphic garnets exist in the Halifax Pluton. Magmatic garnet is euhedral or subhedral, shows reverse zoning, and has no inclusions or reaction rims. Skeletal garnet grains in an aplite provide an "instant picture" of garnet growth. Fine - grained leucomonzogranite contains two types of magmatic andalusite: one mantled by muscovite, and the other as inclusions in K-feldspar.

Variations in transition elements (Fe, Mn, Ti) and Mg form chemical zoning in the biotite, muscovite, cordierite, garnet, substitutions are sensitive to changes in environments: temperature, pressure, concentration in melt, oxygen fugacity, and water pressure.

In the AFM diagrams, different slopes or position of tie-lines joining pairs of AFM minerals may indicate different magmatic environments (T, P, X). The theoretical topologies in P-T space show the order of crystallization of the AFM minerals. Biotite or cordierite was always the first crystallizing mineral. The intersection of the muscovite breakdown curve with the granite solidus determines the minimum pressure (3 - 4.4 kb) for emplacement of the Halifax Pluton.

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Pages: 203
Supervisor:  D.Barrie Clarke