THE NATURE AND TIMING OF PLUTONISM ACROSS THE COAST MOUNTAINS BATHOLITH, STIKINE RIVER AREA: PART 2

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. Preliminary Nd isotope data for plutons along a transect across the CMB near the Stikine River, British Columbia and Southeastern Alaska. There are three age 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). These plutons intruded an east verging tectonic wedge during crustal thickening which resulted in kyanite after andalusite and metamorphic pressures up to ~10 kbar. East of the CSZ c. 20 km wide belt of 60 to 90 Ma plutons intruded medium- to high-grade metamorphic rocks of the Yukon-Tanana terrane. Crustal thickness of this segment is poorly constrained; however, rocks along the Skeena River preserve evidence for ~10 kbar metamorphism. East of these mostly deformed plutons, a belt of c. 55 Ma plutons intruded the along the boundary between core of the batholith and low grade metavolcanic rocks of the Stikine terrane. Sillimanite after andalusite in the metamorphic rocks indicates crustal thicknesses corresponding to pressures at or below the aluminum silicate triple point.

Preliminary Nd whole rock isotopic values for 12 plutons on the Stikine transect are mostly within 3 epsilon Nd (ε_Ndi) units of the chondritic uniform reservoir (CHUR). However, the ε_Ndi values vary from c. +3.5 to +2.2 in 95 to 75 Ma plutons and from +0.2 to -2.4 in 65 to 50 Ma plutons with a single outlier at -8.8. The results indicate little or no input from evolved crustal rocks during magmatism, and relatively little mantle depletion prior to CMB plutonism. ε_Ndi values and pluton ages change across the CSZ and core of the CMB with the core forming a transition zone between the high epsilon values on the west and mixed to lower values on the east. The distinctive e_Ndi decrease post 70 Ma is similar but more pronounced to those in the central batholith. We speculate that the decrease in ε_Ndi with time reflects increased melting of older more evolved rocks in the source region during increased magma flux in the arc between 70 and 50 Ma.