ISOTOPIC CONSTRAINTS ON SOURCE RESERVOIRS FOR CRETACEOUS MAGMATISM WITHIN THE WRANGELLIA TERRANE

An isolated window into Cretaceous age magma intruded into Wrangellia terrane basement is preserved near Nabesna, AK (White Mountain). Although the total area of exposed granitoid at White Mountain is relatively small (~ 1 km²), substantial textural, chemical and isotopic complexities exist. The main granitoid suite consists of six surficially isolated bodies all of which are calc-alkaline and metaluminous, ranging in composition from hornblende-biotite quartz diorite to biotite granodiorite. Three ⁴⁰Ar/³⁹Ar analyses provide cooling ages between 113.3 ± 1.3 and 117.38 ± 0.54 Ma, suggesting at least two pulses of magmatism are represented in the granitoid suite. Sub-samples from one granitoid hand specimen exhibit textural and geochemical evidence for mixing at the low MgO end of the compositional spectrum. Although the granitoid suite exhibits a narrow range in whole rock isotopic compositions (ε_{Nd(115 Ma)} 7.2 to 9.1 and ⁸⁷Sr/⁸⁶Sr_(i) 0.7032 to 0.7043) suggesting open system differentiation, these values do not require an appreciable role for ancient, evolved continental lithosphere in the White Mountain magmatic system(s). Neodymium and the least radiogenic Sr isotopic compositions of the granitoids are similar to those observed in modern Aleutian volcanic arc and the most isotopically primitive Wrangell Volcanic Field rocks. These isotopic compositions provide no evidence for a significant role of an isotopically evolved component in the petrogenesis of the granitoid suite. Rather, the possible source materials responsible for the production of these magmas are limited to the depleted mantle wedge above the downgoing slab ± isotopically primitive lithosphere. The significance of this interpretation lies in the age of the White Mountain granitoid suite and our present understanding of the tectonic architecture of southern Alaska during the mid-Cretaceous, in that arc related magmatic activity occurring within the Wrangellia composite terrane at ~115 Ma, represented locally by the White Mountain granitoid suite, was not sourced or significantly modified by isotopically evolved cratonic lithosphere. This conclusion provides evidence that mid-Cretaceous magmatism in this region was generated in either an intra-oceanic island arc or “immature” (proto-) continental arc tectonic setting.