3/6/2012 - Geochronologic and isotopic constraints on the petrogenesis of Coast Mountain Batholith, BC


12:00 PM - 1:00 PM
4460 Geology

Presented By:
Robinson Cecil - CSU-Northridge


This study uses U-Pb geochronologic and Hf isotopic data from plutonic rocks of the Jurassic – Eocene Coast Mountains batholith (CMB) of north-coastal British Columbia, as well as Paleozoic plutons from the Alexander terrane of SE Alaska, to track the source and depth of batholith genesis through time and space. Hafnium isotopic values do not correlate with age or with variations in magmatic flux, but instead are found to increase from west (?Hf(t)= +2 to +5) to east (?Hf(t) = +10 to +13). ?Hf(t) values from Paleozoic plutons of the Alexander terrane range from +9.6 to +14.3. Comparison of our Hf data with Nd-Sr and detrital zircon characteristics of batholithic country rocks places constraints on the crustal architecture of the CMB. The Hf, Nd-Sr, and detrital zircon signatures delineate three discrete crustal domains: (1) a western domain, emplaced into continental margin strata of the Banks Island assemblage, which has evolved Hf and Nd-Sr signatures and mainly Precambrian detrital zircons; (2) a central domain, emplaced into the southernmost Alexander terrane, which has intermediate Hf signatures, and (3) an eastern domain, underlain by the Stikine terrane and its inferred metamorphic equivalents, with juvenile Hf and Nd-Sr signatures. Between the Alexander and Stikine terranes is a zone of highly variable ?Hf(t) (+2 to +13) that coincides with mid-Cretaceous through early Tertiary structures (including the Coast shear zone). This large variation in ?Hf(t) apparently results from imbrication of the juvenile Alexander and Stikine terranes with evolved continental-margin rocks of the Yukon-Tanana terrane.

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