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Abstract

The Emerald Lake pluton, of the mid-Cretaceous Tombstone plutonic suite in Yukon is concentrically-zoned, from west to east and in order of relative age, and comprises four distinct mapped phases: augite syenite, hornblende quartz syenite, hornblende quartz monzonite, and biotite granite. U-Pb and Ar-Ar dates on major intrusive phases return statistically identical ages (93.1 Ma). The intrusion is also characterized by large miarolitic cavities distributed along the southern margin of the intrusion. They are spatially and temporally associated with a sheeted system of aplite dykes, quartz - K-feldspar pegmatites, and molybdenite - bismuthinite - gold - quartz veins. Miarolitic cavity and pegmatite formation is coeval with the main intrusive events, and were followed by continued pegmatite development, fracturing, and injection of aplite dykes and veins. Modal abundance, density and magnetic susceptibility, and major element geochemical parameters vary systematically from east to west and correlate with mapped intrusive phases. REE and trace element geochemistry indicate that major intrusive phases are cogenetic and derive from a homogeneous, crustally contaminated, source magma that retains an arc or sub-continental lithospheric mantle signature. The oxidation states of the major intrusive phases range between QFM + 3 and 10 % FeO(T), and evolve from more oxidized augite syenite to reduced biotite granite. Several samples appear relatively reduced by interaction with country rocks. Major element chemical variations within the augite syenite, hornblende quartz syenite, and biotite granite are consistent with sorting of K-feldspar phenocrysts alone, while compositional variability in the hornblende quartz monzonite is consistent with sorting of K - feldspar and mafic phases. Chemical variations between the major intrusive phases are inconsistent with the sorting of K-feldspar and mafic phases with the exception of the hornblende quartz syenite and hornblende quartz monzonite, indicating that 3 distinct pulses of magma, that cannot be related to each other by simple internal differentiation processes, formed the pluton. These pulses were emplaced sequentially (augite syenite, hornblende quartz syenite/monzonite, biotite granite) and rapidly, enabling significant chemical and textural interaction between intrusive phases.