Abstract

Batholiths of the Coast Plutonic Complex, in the area of Harrison Lake, British Columbia, are interpreted to have formed by horizontal sheeting and vertical inflation. Zoned ovoid batholiths exhibit sheeted margins as thick as 3 km. Shallow floors are indicated by moderately to gently inward dipping magmatic foliations, the observed trace of floor contacts across topography, and a published deep seismic reflection transect. Mineralogic aureoles under the floors preserve a paragenetic record of increasing pressure during aureole crystallization. Initial pressures of <3.0 kilobars are indicated by an early assemblage of andalusite + biotite. Garnet thermobarometry of late-stage aureole assemblages that overprint the andalusite indicates peak pressures of 5–6.5 kilobars, equating to an added load more than 7–12 km thick. The mode of emplacement of the ovoid batholiths is interpreted from these features to be a sequential process of (1) initial intrusion of a relatively shallow horizontal sheeted complex fed by a dike system, followed by (2) inflation vertically by multiple injections or ballooning of the original sheets to accommodate a high volume of magma. The floor of the initial sheets was pushed down and rotated into steeper dips at the pluton margins as the bottom sagged and the magma body assumed a bowl or tub shape. In this process, magma ascended via fractures, and space was made for the plutons primarily by downward displacement of country rock and concomitant depression of the Moho. U-Pb zircon dating reveals crystallization periods of as much as 8 m.y. for individual batholiths, a likely consequence of construction by episodic pulses of magma. This model finds accordance with observations of many workers in other orogenic belts.

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