PUNCTUATED GROWTH OF A CA 90 MA SHALLOW-CRUSTAL INTRUSIVE COMPLEX, NORTH CASCADES WASHINGTON

Processes for the formation of intermediate to silicic upper crustal intrusions and intrusive complexes remain controversial after decades of discussion. Major questions include: are large volumes of magma commonly emplaced over relatively short intervals, and thus do vigorous magma chambers commonly form in the mid- to upper crust? Are plutons an amalgam of intrusions assembled rapidly and separated by significant time gaps, or are they constructed more steadily by small increments intruded over a long period of time? Until recently geochronologic techniques have lacked the precision required to determine how large, relatively homogeneous intrusive complexes are built and to resolve the small (<0.5%) differences in zircon dates between and within different domains of mapped plutons. CA-TIMS zircon geochronology allows a better understanding of the timing and processes of pluton growth. We present new data for the mid to shallow-crustal, ~91-87 Ma Black Peak intrusive complex in the North Cascades continental magmatic arc. High-precision geochronology has allowed us to resolve subtle differences in zircon dates from different parts of the complex. Our work suggests that the complex was built incrementally with distinct temporal gaps between intrusive phases. Careful mapping has revealed that different domains, defined in the field by variations in mineral modes, can be linked to temporal differences on the order of hundreds of thousands of years. Evidence for assimilation of older phases is documented by whole inherited zircon grains, as opposed to obvious core and rims, and is less common in the oldest units. Features typically associated with magma chamber processes are rare. However, a ~ 5 km² region preserves evidence for magmatic interactions between successive intrusive increments at ca 89 Ma over an interval of ~500 ka. The area is characterized by enclave swarms, gradational contacts, and zones of magma mixing, which are evidence for small, short-lived magma chambers or a conduit system; there is no evidence for a “sheeted-dike” style of construction. Overall, exposures of the Black Peak intrusive complex support a model in which relatively small volumes of magma (<5 km³) are emplaced, cool, and are intruded by later increments, generally without significant magmatic interaction or evidence of eruption.