XENOLITHS, FOLIATIONS, AND FOLDS IN A BIG TANK FILLED BY MANY SMALL INCREMENTS

Detailed mapping of magmatic fabrics and xenoliths within the 98 Ma granodioritic Jackass Lakes pluton (JLP) provides clues to the nature of fabric formation, magma rheology, and pluton assembly. Xenolith fields within the JLP locally account for greater than 60% of the exposure. Xenoliths 1) range in size from mm to > 200 m long; 2) contain metamorphic foliations that are subparallel to magmatic foliations in the JLP; 3) occur either at high angles to the magmatic foliation in the JLP or are elongate parallel to the magmatic foliations; 4) have locally rotated in the pluton relative to regional structural trends in the host rocks; 5) are intruded by granodiorite dikes that are folded about axial planes subparallel to the magmatic foliations in the JLP and plastic foliations in the xenoliths; and 6) do not deflect magmatic foliations within the adjacent JLP - even where blocks can be shown to have rotated, there appears to be no change in the magmatic foliations around xenoliths. MELTS calculations based on major and trace element compositions, 3 wt. % H₂O yield a liquidus T of 983°C; effective viscosities from 0-40% crystals ranged from 10⁴ to 10⁷ Pa s. Magma densities ranged from 2230 – 2410 kg/m³; calculated density for meta-dacitic xenoliths are 2640 kg/m³ indicating that host rock xenoliths in the JLP were negatively buoyant and should have sunk steadily through the crystal-poor magma. Assuming an average block radius of 1 m, the densities given above, granodioritic magma viscosities of 10⁴ to 10⁷ Pa s, and using Stokes' settling laws for static, Newtonian melts, settling rates of 4.0 x 10^-3 to 1.5 x 10^-7 m/s are predicted for meta-dacitic xenoliths. To achieve slower settling rates (e.g. < 0.1 m/1000 years) in order to ‘trap' xenoliths and allow a magmatic foliation to form post-incorporation, requires significantly higher viscosities on the order of 10¹³ Pa s ( ~ 0.213 m/1000 years). We conclude that: 1) the JLP magma must have been crystal-rich (> 80% crystals) at the time of xenolith ‘capture' and thus at temperatures approaching the solidus; 2) magmatic fabric formation post-dates the incorporation of host rock xenoliths; 3) xenoliths were incorporated by a combination of both stoping and in situ diking; 4) regional east-west contraction occurred during pluton solidification.