The McKinley Sequence of granitic rocks: A key element in the accretionary history of southern Alaska

Abstract

The McKinley sequence of granitic rocks consists of several discrete plutons in the central Alaska Range. Most of these plutons crop out south of the Denali fault system (DFS) in the Talkeetna quadrangle. Plutons of the McKinley sequence largely intrude deformed upper Meszoic flysch between the DFS and the northern edges of Wrangellia and the Peninsular terrane, which jointly make up the Talkeetna superterrane. The average K-Ar age of biotite from nine granites of the McKinley sequence is 57.3 Ma; Rb-Sr data for whole rock samples indicate that the McKinley sequence cannot be older than 60 Ma. A selected suite of 20 samples of granite and granodiorite range in SiO 2 from 65.9 to 77.6%. All 20 samples are corundum normative, and 18 are moderately peraluminous. Initial 87 Sr/ 86 Sr ratios range from 0.7054 to 0.7085. The σ 18 O values range from +11.2 to +14.6‰. These high and variable Sr isotopic ratios, peraluminous nature, rare earth element patterns, and high σ 18 O values suggest that granitic rocks of the McKinley sequence crystallized from hybrid magmas produced by assimilation of sedimentary rocks by a mantle-derived melt. Mesozoic flysch is the likely source of the crustal component of the hybrid magmas. Geologic evidence suggests that the Talkeetna superterrane collided with stable Alaska after Early Cretaceous time. The flysch basin, lying south of stable Alaska, was closed by northward movement of the Talkeetna superterrane; maximum age for basin closure and terrane accretion is middle Cretaceous (Cenomanian). Paleomagnetic evidence indicates that all terranes north of the DFS have been part of stable Alaska since the Paleocene and that northward movement of Wrangellia was completed by 50 Ma. Granitic rocks of the McKinley sequence may be products of terrane accretion; the granitic rocks crystallized from hybrid magmas produced during terrane collision and deformation of the flysch basin. Isotopic ages of the McKinley sequence establish the time of final accretion of the Talkeetna superterrane as Paleocene.