TIMESCALES OF INCREMENTAL PLUTON GROWTH: THEORY AND A FIELD-BASED TEST

Many granitic plutons have tabular to wedge shaped cross-sectional geometries whereby their vertical thickness scales with horizontal width according to an empirical power law. Some plutons are also constructed by the addition of discrete pulses of either similar composition (cryptic layering/zoning) or variable composition (petrological layering/zoning). If each pulse addition constitutes a discrete dike-sill event, then estimated pulse delivery rates are similar in magnitude to observed magma chamber inflation and volcanic eruption rates (0.1 – 1000 m³ /s). Using the empirical scaling law, minimum pluton growth times are estimated as a function of intrusion width, number of pulses, and the time gap between pulses. For example, a 10km wide tabular pluton made up of 200 pulses delivered at 1 m³ /s will take ~6000 yr to form if pulse addition is separated by a few days to years. If pulses are separated by time intervals >> longer than the individual pulse addition time (~30 yr) the total time required for pluton growth is ~6 Myr. We are testing these hypothetical filling times by precise U-Pb zircon geochronology on well defined, gently inclined km-scale compositional layers in granitic plutons of the Mezozoic Coastal Cordillera batholith, N. Chile. The plutons were emplaced by fault-assisted floor depression, and in addition to containing mappable km-scale compositional units are also characterized by cryptic m- to dm-scale layering. In each case U-Pb ages indicate that the plutons grew over a period of ca. 3 Myr, and successive compositional units are separated by intervals of 1 to 2 Myr. Ages determined from the top and bottom of a single compositional unit in one pluton separate within analytical error and constrain that the filling/crystallization time of the layer is <600 kyr. Two of the plutons grew by successive top-down addition of magma, with abrupt changes in incoming magma composition every 1 – 2 Myr. This field-based test indicates that although pulse addition and crystallization times may be geologically rapid, individual plutons likely grow over a few millions of years because of a significant time lag between the addition of pulses. The timescale of pulse delivery to a pluton is likely limited by processes in the magma source region, such as melt-accumulation and melt extraction rates.