ZIRCON U-PB AGES AND PETROLOGIC EVOLUTION OF THE ENGLISH PEAK GRANITIC PLUTON: JURASSIC CRUSTAL GROWTH IN NORTHWESTERN CALIFORNIA (Invited Presentation)

In the central Klamaths, the English Peak plutonic complex (EPC) invaded the faulted contact between the outboard Eastern Hayfork and inboard North Fork terranes of the Western Paleozoic and Triassic Belt (WtrPz). This complex consists of two small, ~1–2 km diameter, relatively mafic plutons peripheral to the younger, larger, ~10–15 km diameter English Peak zoned granitic pluton. EPC magmas were mantle-derived and reflect temporary residence-mixing at various depths in the overlying crust, with initial storage and modification near the Moho, and uppermost crustal emplacement at 5–10 km depths. Phase assemblages suggest pre-emplacement magma storage at ~20–25 km for the early plutons, versus ~15–20 km for samples from the larger zoned granitic pluton. We obtained zircon U-Pb results from 7 samples in the complex via l LA-ICP-MS. The 172.3 ± 2.0 Ma Uncles Creek and 166.9 ± 1.6 Ma Heiney Bar satellitic plutons range from gabbro–quartz diorite to granodiorite. The main English Peak pluton consists of an early stage of gabbro–tonalite (3 samples; 160.4 ± 1.1 Ma, 158.1 ± 1.1 Ma, and 158.0 ± 1.2 Ma), and a late stage (2 samples; 156.3 ± 1.3 Ma and 155.3 ± 1.2 Ma) passing inward from tonalite through granodiorite to a central zone of granite. The 172 Ma age of the Uncles Creek pluton makes it coeval with Middle Jurassic Western Hayfork arc magmatism. The Heiney Bar and main English Peak igneous ages overlap some of the oldest and youngest components, respectively, of the Middle to Late Jurassic Wooley Creek plutonic suite. This complex is an illuminating example of gradual intermediate-to-felsic modification of the upper crust in the Klamaths. Inherited zircon ages of 172 Ma in 2 EPC samples indicate potential Middle Jurassic crustal sources or contaminants. Geochronologic correlation of the EPC with geologic histories of other Klamath terranes provides fresh insights for understanding spatial and temporal elements of Middle to Late Jurassic arc magmatism in the Klamath sector of the Cordilleran margin. This igneous activity provides instructive examples of processes whereby accreted ophiolitic basement terranes were modified and incorporated into the evolving Jurassic continental crust prior to earliest Cretaceous onset of westward transport of the stack of Klamath allochthons relative to the active Jura-Cretaceous Sierran arc.