Growth and maturation of a mid- to shallow-crustal intrusive complex, North Cascades, Washington

Studies of plutons indicate that they are the result of a complex interplay of magmatic processes occurring during magma generation, ascent, and emplacement. A critical tool for deciphering these processes is high-precision geochronology, which can help determine the timing and rates of magmatism in the crust. We conducted a field and U-Pb geochronological study of the Cretaceous Black Peak intrusive complex in the North Cascades of Washington State to investigate magmatism at a detailed scale and to refine estimates of plutonic construction rates. High-precision chemical abrasion–thermal ionization mass spectrometry (CA-TIMS) U-Pb geochronology was carried out on 31 samples from five mapped intrusive phases. Field relations in the Black Peak intrusive complex show intrusive contacts that vary from sharp to gradational. Whole-rock Sm/Nd, zircon oxygen isotopes, and zircon trace elements were obtained on subsets of representative samples. The U-Pb geochronology from the Black Peak intrusive complex documents batholith intrusion over 4.5 m.y. and suggests that magmatism was semicontinuous for a minimum of 3.5 m.y. Individual samples display age dispersion in single-zircon dates that ranges from ∼105 yr to several 106 yr, with a general increase in the age range for younger samples. Whole-rock eNd and zircon δ18O for all Black Peak intrusive complex samples indicate that magmas were derived from mantle and crustal sources and that all magmas were isotopically homogenized prior to zircon saturation. Ti-in-zircon temperatures from zircon cores are generally above calculated zircon saturation temperatures, which suggests that most Black Peak intrusive complex magmas were zircon undersaturated in the melt source region. A range of thicknesses was considered, and a thickness of ∼10 km for the Black Peak intrusive complex gives an average intrusion rate of ∼1.1 ×10–3 km3/yr, which is high enough to sustain a magma reservoir in the shallow crust. The field evidence and long overall duration of intrusion are incompatible with the entire Black Peak intrusive complex being molten at any one time, but the larger, more compositionally homogeneous domains in the Black Peak intrusive complex are likely the solidified remnants of mushy magma bodies with ∼105 yr durations. These data suggest that the Black Peak intrusive complex may have remained “mushy” for long periods of time (105 yr) and may indicate that the spread in dates within individual samples is best interpreted as either antecrystic recycling and/or protracted autocrystic growth.