TEMPORAL EVOLUTION OF MID-CRUSTAL SHEETED INTRUSIONS: EVIDENCE FROM U-PB GEOCHRONOLOGY OF THE TENPEAK, ENTIAT AND SEVEN-FINGERED JACK INTRUSIVE SUITES, NORTH CASCADES, WA

Recent refinements of geochronologic techniques make it possible to describe the magmatic and thermal evolution of continental magmatic arcs with high-precision temporal constraints. Specific issues to be resolved include the duration, tempo, and rates at which individual intrusions are constructed. These data provide a test for models of arc construction based on field relationships and thermal modeling. At the southern end of the Coast Plutonic Complex, several elongate, internally-sheeted intrusions in the North Cascades form a natural laboratory for studying batholith construction. We use high-precision U-Pb geochronology to document the construction of three intrusive systems, the 7-9 kbar Tenpeak, ca. 7 kbar Seven-Fingered-Jack (7FJ) and ca. 7 kbar Entiat intrusive suites. The Tenpeak intrusion was emplaced from 92.4-89.7 Ma with <0.5 m.y. time intervals between magma pulses. Texturally and compositionally distinct phases form well-defined internal magmatic contacts that suggest only limited mixing between pulses. U-Pb titanite dates indicate that the oldest phases cooled below the solidus <1 m.y. after emplacement. Prior to this study, the 7FJ and Entiat suites were assumed to be coeval. Models for their construction involved intrusion of thin magma sheets which then formed “preheated pathways” for the development of a more homogeneous magma chamber. Zircon from five thin tonalite and diorite sheets at the NW tip (7FJ suite) yields ca. 90-92 Ma dates. In contrast, coarse-grained tonalite and fine-grained diorite from the SE end of the body (Entiat suite) yield ca. 71-73 Ma zircon dates. This ca. 20 m.y. time lag indicates that the emplacement of the Entiat suite is temporally unrelated to the emplacement of the 7FJ suite. Three samples from the 7FJ suite yielded dates that indicate a 2-3 m.y. time span of intrusion suggesting that perhaps early-formed sheets were intruded by, and partially mixed with, slightly younger sheets. The range of dates from each intrusion suggests that all three suites were emplaced over a 2-3 m.y. duration. This time span is much longer than predicted by dike emplacement models. Although this study focuses on the timescales of batholith construction and heat transfer through a narrow slice of a single arc, the results form a basis for which magma flux through other arcs can be compared.