Intermediate to felsic middle crust in the accreted Talkeetna arc, the Alaska Peninsula and Kodiak Island, Alaska: An analogue for low‐velocity middle crust in modern arcs

Abstract

Seismic profiles of several modern arcs have identified thick, low‐velocity midcrustal layers (Vp = 6.0–6.5 km/s) that are interpreted to represent intermediate to felsic plutonic crust. The presence of this silicic crust is surprising given the mafic composition of most primitive mantle melts and could have important implications for the chemical evolution and bulk composition of arcs. However, direct studies of the middle crust are limited by the restricted plutonic exposures in modern arcs. The accreted Talkeetna arc, south central Alaska, exposes a faulted crustal section from residual subarc mantle to subaerial volcanic rocks of a Jurassic intraoceanic arc and is an ideal place to study the intrusive middle crust. Previous research on the arc, which has provided insight into a range of arc processes, has principally focused on western exposures of the arc in the Chugach Mountains. We present new U‐Pb zircon dates, radiogenic isotope data, and whole‐rock geochemical analyses that provide the first high‐precision data on large intermediate to felsic plutonic exposures on Kodiak Island and the Alaska Peninsula. A single chemical abrasion–thermal ionization mass spectrometry analysis from the Afognak pluton yielded an age of 212.87 ± 0.19 Ma, indicating that the plutonic exposures on Kodiak Island represent the earliest preserved record of Talkeetna arc magmatism. Nine new dates from the extensive Jurassic batholith on the Alaska Peninsula range from 183.5 to 164.1 Ma and require a northward shift in the Talkeetna arc magmatic axis following initial emplacement of the Kodiak plutons, paralleling the development of arc magmatism in the Chugach and Talkeetna mountains. Radiogenic isotope data from the Alaska Peninsula and the Kodiak archipelago range from ɛNd(t) = 5.2 to 9.0 and 87Sr/86Srint = 0.703515 to 0.703947 and are similar to age‐corrected data from modern intraoceanic arcs, suggesting that the evolved Alaska Peninsula plutons formed by extensive differentiation of arc basalts with little or no involvement of preexisting crustal material. The whole‐rock geochemical data and calculated seismic velocities suggest that the Alaska Peninsula represents an analogue for the low‐velocity middle crust observed in modern arcs. The continuous temporal record and extensive exposure of intermediate to felsic plutonic rocks in the Talkeetna arc indicate that evolved magmas are generated by repetitive or steady state processes and play a fundamental role in the growth and evolution of intraoceanic arcs.

Thermochronology of the Talkeetna intraoceanic arc of Alaska: Ar/Ar, U‐Th/He, Sm‐Nd, and Lu‐Hf dating

“…[11] U-Pb zircon ages reported in this paper are presumed to be crystallization ages, as discussed by Rioux et al [2007Rioux et al [ , 2010. The 40 Ar/ 39 Ar ages are presumed to be cooling ages because all of the dates reported in this paper are from plutonic rocks with high crystallization temperatures; two exceptions to this are the (sub)volcanic rocks 1709C06c and 89CNT08, for which the hornblende ages likely approximate the crystallization age.…”

“…Geochronology of the Talkeetna arc along the Alaska Peninsula. U-Pb zircon ages are from Rioux et al [2010]; other data are from this study. See Figure 1 for explanation of symbols and colors.…”

“…[24] U-Pb zircon ages show that the Talkeetna arc in the Alaska Peninsula crystallized over the interval from 183 Ma to 164 Ma [Rioux et al, 2010], similar to the 177-159 Ma magmatism in the Talkeetna Mountains; two ages from the Kodiak archipelago are significantly older, at 213 Ma and 217 Ma [Rioux et al, 2010]. Hornblende 40 Ar/ 39 Ar ages are on average 5-10 Myr younger than the U-Pb zircon ages, ranging from 171.7 ± 3.4 Ma to 157.8 ± 2.4 Ma, again similar to the Talkeetna Mountains.…”

“…The most recent 40 Ar/ 39 Ar study, by Drake and Layer [2001], reported eleven Jurassic to Cretaceous cooling ages from the northern Talkeetna Mountains. The intrusive history of the Talkeetna arc has been illuminated in some detail by the chemical abrasion U-Pb TIMS work of Rioux et al [2007Rioux et al [ , 2010. To these data sets, we add Sm-Nd and Lu-Hf dating of garnets from the base of the arc, 40 Ar/ 39 Ar hornblende and mica ages from all levels of the arc plutonic section, and U-Th/He ages of the arc plutonic section (Table 1 and auxiliary material).…”

“…Exposures in the Chugach and Talkeetna mountains ( Figure 1) include all the types of rocks that comprise a magmatic arc, volcaniclastic rocks (the Talkeetna Formation), hypabyssal and plutonic rocks, and mantle peridotite [Burns, 1985;DeBari and Coleman, 1989;Plafker et al, 1989;Kelemen et al, 2003b;Mehl et al, 2003;Clift et al, 2005;Greene et al, 2006;Rioux et al, 2010]. Using the thickness of the coeval volcanic section [Clift et al, 2005] and thermobarometry on igneous and metamorphic rocks at various levels within the arc, Hacker et al [2008] reconstructed the pseudostratigraphy of the arc as: 5 km of volcanic rocks, 4-14 km of felsic plutons, 9-16 km of mafic plutons, and 4-10 km of basal garnet gabbronorite.…”