Remarkable isotopic and trace element trends in potassic through sodic Cretaceous plutons of the Yukon‐Koyukuk Basin, Alaska, and the nature of the lithosphere beneath the Koyukuk terrane

Abstract

During the period from 110 to 80 m.y. ago, a 450‐km‐long magmatic belt was active along the northern margin of Yukon‐Koyukuk basin and on eastern Seward Peninsula. The plutons intruded Upper Jurassic(?) and Lower Cretaceous volcanic arc rocks and Cretaceous sedimentary rocks in Yukon‐Koyukuk basin and Proterozoic and lower Paleozoic continental rocks in Seward Peninsula. Within Yukon‐Koyukuk basin, the plutons vary in composition from calc‐alkalic plutons on the east to potassic and ultrapotassic alkalic plutons on the west. Plutons within Yukon‐Koyukuk basin were analyzed for trace element and isotopic compositions in order to discern their origin and the nature of the underling lithosphere. Farthest to the east, the calc‐alkalic rocks of Indian Mountain pluton are largely tonalite and sodic granodiorite, and have low Rb (average 82 ppm), high Sr (>600 ppm), high chondrite‐normalized (cn) Ce/Yb (16–37), low δ18O (+6.5 to +7.1), low initial 87Sr/86Sr (SIR) (0.704), and high initial 143Nd/144Nd (NIR) (0.5126). These rocks resemble those modelled elsewhere as partial melts and subsequent fractionates of basaltic or gabbroic metaigneous rocks, and may be products of melting in the deeper parts of the Late Jurassic(?) and Early Cretaceous volcanic arc. Farthest to the west, the two ultrapotassic bodies of Selawik and Inland Lake are high in Cs (up to 93 ppm), Rb (up to 997 ppm), Sr, Ba, Th, and light rare earth elements, have high (Ce/Yb)cn (30, 27), moderate to low δ18O (+8.4, +6.9), high SIR (0.712, 0.710), and moderate NIR (0.5121–0.5122). These rocks resemble rocks of Australia and elsewhere that were modelled as melts of continental mantle that had been previously enriched in large cations. This mantle may be Paleozoic or older. The farthest west alkalic pluton of Selawik Hills is largely monzonite, quartz monzonite, and granite; has moderate Rb (average 284 ppm), high Sr (>600 ppm), high (Ce/Yb)cn (15–25), moderate δ18O (+8.3 to +8.6), high SIR (0.708–0.712), and moderate NIR (0.5121–0.5122). These rocks may be the product of interaction of magma derived from old continental mantle and magma derived from old continental crust. Plutons between eastern and western extremes show completely gradational variations in the concentration of K and Rb and in the isotopic compositions of Sr, Nd, and O. These plutons probably originated either by melting in a mixed source composed of a Paleozoic or older continental section (mantle + crust) overlain by Mesozoic mafic arc rocks, or by mixing of ultrapotassic to potassic magmas from continental sources (mantle + crust), and tonalitic magmas from arc sources. We infer from these results that the northwest portion of Yukon‐Koyukuk basin is underlain by a substantial continental basement of Paleozoic or greater age. This basement probably thins out to the east. There is no geochemical evidence for continental basement east of about longitude 157°, or along a belt of at least 50 km width flanking Ruby Geanticline as far to the southwest as about longitude 161°. These areas are probably underlain by oceanic and Mesozoic arc rocks.