| Summary: | We present U-Pb geochronologic and Hf isotopic data from 29 plutonic samples within the Coast Mountain batholith, north-coastal British Columbia and southeast Alaska. Hf isotopic values do not correlate with age or variation in magmatic flux, but rather they increase systematically from west (ε_(Hf)[t] = +2 to +5) to east (ε_(Hf)[t] = +10 to +13) in response to changing country rock assemblages. By comparing our pluton Hf data with previously reported Nd-Sr and detrital zircon characteristics of associated country rocks, we identify three crustal domains in an area where crustal affinity is largely obscured by metamorphism and voluminous pluton intrusion: (1) a western domain, emplaced into continental-margin strata of the Banks Island assemblage; (2) a central domain, emplaced into the Alexander terrane; and (3) an eastern domain, underlain by the Stikine terrane and its inferred metamorphic equivalents. Between the interpreted Alexander and Stikine terranes, there is a zone of variable ε_(Hf)(t) (+2 to +13) that coincides with the suture zone separating inboard (Stikine and Yukon-Tanana) from outboard (Alexander and associated) terranes. This variation in ε_(Hf)(t) values apparently results from the structural imbrication of juvenile (Alexander and Stikine) and evolved (Yukon-Tanana) terranes along mid-Cretaceous thrust faults and the latest Cretaceous–early Tertiary Coast shear zone. Shifts in the Hf values of plutons across inferred terranes imply that they are separated at lower- to midcrustal levels by steep boundaries. Correlation between these Hf values and the isotopic character of exposed country rocks further implies the presence of those or similar rocks at magma-generation depths. © 2011 Geological Society of America. Manuscript received: 2 January 2011; revised manuscript received: 3 May 2011; manuscript accepted: 4 May 2010. This work was sponsored by National Science Foundation (NSF) award EAR-0309885 for support of the BATHOLITHS projects, and by EAR-0732436 for support of the Arizona ...
|
|---|