THE NATURE AND TIMING OF PLUTONISM ACROSS THE COAST MOUNTAINS BATHOLITH, STIKINE RIVER AREA: PART 1

The Coast Mountains batholith (CMB) is the largest and youngest Jurassic-Paleogene magmatic arc of the North American Cordillera. Thus, the CMB is critical for understanding growth of the continental crust and all aspects of the Cordilleran margin. We present major and trace element compositions, and zircon U-Pb ages for plutons along a transect across the CMB near the Stikine River, British Columbia and Southeastern Alaska. Zircon U-Pb ages and pluton rock types indicate 3 age and compositional groups from west to east. A large group of ca. 92 Ma plutons (Admiralty-Revillagigedo) intruded Gravina belt metasediments west of the Coast shear zone (CSZ): a prominent high strain zone which extends at least 1,400 km parallel to the CMB. These plutons range from diorite to granodiorite and are characterized by hornblende, pyroxene, biotite, plagioclase, and lesser amounts of quartz. They have silica contents between 58 and 68%, alkali contents of ~7%, Sr/Y = 34 to 98, and La/Yb = 6 to 12. East of the CSZ, a ~20 km wide belt of 60 to 90 Ma plutons intruded medium- to high-grade metamorphic rocks of probable Yukon-Tanana terrane and rocks of unknown affinity, forming the CMB core. These plutons range from quartz diorite to granodiorite and tonalite and are characterized by hornblende, pyroxene, biotite, plagioclase, and quartz. They have silica contents between 55 and 72%, and total alkali of ≤10%, Sr/Y = 18 to 79, and La/Yb = 8 to 22. East of these mostly deformed plutons, a belt of c. 55 Ma plutons intruded along the boundary between core of the batholith and low grade metavolcanic rocks of the Stikine terrane. These plutons range from granodiorite to granite and are characterized by hornblende, biotite, plagioclase, K feldspar and quartz. They have silica contents between 55 and 72%, and total alkali ≤8%. Notably, Sr/Y ratios vary from 27 to 126, with one outlier at 228 (N=8), and La/Yb ratios range from 3 to 39, with one outlier at 96. All plutons have flat/absent Eu anomalies (Eu/Eu* = 0.88 to 1.2). Significantly, at least 70% of the plutons and all but one of the ca. 55 Ma group, exhibit Sr/Y ratios of ~50 or higher and Y <13 ppm. We tentatively infer that elevated Sr/Y and La/Yb ratios, their increase in the younger plutons, particularly those synchronous with a Paleogene flare-up documented in the CMB to the south, may reflect crustal thickening.