麻豆传媒

The South Mountain Batholith, a peraluminous granitic complex, ranges in composition from biotite granodiorite to muscovite-topaz leucogranite. Leucogranitic rocks form minor and late components of the batholith. The Lake Lewis Leucogranite belongs to the late-stage magmatic evolution of the batholith, and this thesis presents a new textural and chemical investigation of its white micas. Texturally the white micas form two groups: 1) primary micas are coarse-grained, dimensionally compatible with other minerals, euhedral to subhedral with well-defined, sharp boundaries, and have muscovite-phengite epitaxial overgrowths; and 2) secondary micas are coarse-grained, anhedral, mechanically deformed (bent, kinked, and sheared), altered with fluorite inclusions, and have ragged edges. Chemically, these groups correspond mainly to muscovite-phengite, and protolithionite, respectively. Lithium may constitute an essential component required for complete calculation of the structural formulae and classification of the white micas, and the application of regression equations based on F and SiO² appear valid for the estimation of the lithium contents. Classifications of the micas, without calculated lithium using cation proportions, and with calculated lithium using the Mg-Li - Fe+Mn+Ti-AlVI diagram, produce nearly identical nomenclature. Distinct oscillatory zoning of muscovite(K²Al⁴[Si⁶Al²O²⁰](OH, F)⁴)-phengite(K²FeAl³[Si⁷AlO²⁰](OH, F)⁴) in some of the undeformed micas is consistent with build-up and release of fluid pressure in the late stages of crystallization of the Lake Lewis Leucogranite. Textural and chemical evidence support the primary magmatic nature of the undeformed white micas, and the secondary hydrothermal nature of the deformed micas. Chemical substitution in the primary magmatic white micas is a combination of Tschermak substitution (Mg, Fe²+)VI,SiIV * AlIV,AlVI and biotitic substitution 2/3 AlVI, 1/3_VI * (Fe²+)VI, where _ stands for a vacant octahedral site, exchange mechanisms. Chemical substitution in the secondary hydrothermal magmatic white micas produces zinnwaldite (K²Fe³-1Li1-3(Al, Fe)²[Si⁵-7Al³-1O²⁰](F, OH)⁴). Texturally, the plagioclase and K-feldspar are euhedral and unaltered, and appear to be primary magmatic phases. Chemically, the plagioclase compositions are An0-5 and the K-feldspars are Or85-100. In high-fluorine magmas, complexing of calcium with fluorine may force the crystallization of nearly pure albite from the magma. The primary nature of the feldspars is consistent with the deduced primary nature of the white micas. Magmatic fluids expelled from the Lake Lewis Leucogranite probably carried Ca + F + metals that formed mineralized greisens, such as the nearby Walker Moly deposit. The epitaxially zoned muscovite-phengite grains have implications for tracing the magmatic evolution of the late leucogranite magma, and for the reliability of ⁴⁰Ar/³⁹Ar dating.

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Pages: 159
Supervisors: D. Barrie Clarke