Magmatic development of an intra-oceanic arc: High-precision U-Pb zircon and whole-rock isotopic analyses from the accreted Talkeetna arc, south-central Alaska

Abstract

The accreted Talkeetna arc, south-central Alaska, is an archetypal example of an intraoceanic arc crustal section. Arc-related units include all levels of a lithospheric column, from residual mantle harzburgites to subaerial volcanic rocks, and provide a rare opportunity to study intrusive arc processes directly. We present the fi rst high-precision U-Pb zircon ages and an extensive new data set of 143 Nd/ 144 Nd and 87 Sr/ 86 Sr isotopic analyses from Talkeetna arc plutonic rocks. These data provide new insight into the timing and extent of Talkeetna arc magmatism, the tectonic development of the arc, and the role of preexisting crustal material in the generation of arc magmas. New analyses from the exposed arc crustal section in the Chugach Mountains indicate that the Talkeetna arc began to develop as a juvenile [ε Nd (t) = 6.0-7.8 and 87 Sr/ 86 Sr int = 0.703379-0.703951] intra-oceanic arc between 202.1 and 181.4 Ma. This initial arc plutonism was followed ca. 180 Ma by a northward shift in the arc magmatic axis and generation of a large plutonic suite in the Talkeetna Mountains. Plutons from the eastern Talkeetna Mountains yield U-Pb zircon ages of 177.5-168.9 Ma and are isotopically similar to the Chugach Mountains intrusions [ε Nd (t) = 5.6-7.2 and 87 Sr/ 86 Sr int = 0.703383-0.703624]. However, plutons from the western Talkeetna Mountains batholith have more evolved initial isotopic ratios [ε Nd (t) = 4.0-5.5 and 87 Sr/ 86 Sr int = 0.703656-0.706252] and contain inherited xenocrystic Carboniferous-Triassic zircons. These data are interpreted to represent assimilation of adjacent Wrangellia crust into arc magmas and require amalgamation of the Talkeetna arc with the Wrangellia terrane by ca. 153 Ma. As a whole, the combined U-Pb zircon and isotopic data from the Chugach and Talkeetna Mountains indicate that the main volume of Talkeetna arc magmas formed with little or no involvement of preexisting crustal material. These observations justify the use of the Talkeetna arc as a type section for intrusive intra-oceanic arc crust.

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

“…As such, the data presented here, combined with existing data from Rioux et al [2007], provide new insights into the temporal evolution of the low-velocity crust. The U-Pb zircon ages from the arc record ∼60 Ma of arc magmatism from 213 to 153 Ma.…”

“…In addition, the distribution of ages suggests that the evolved magmas were Figure 7. Radiogenic isotope data from Talkeetna arc plutonic rocks analyzed in this study and by Rioux et al [2007]. Modern arc fields based on data from McCulloch and Perfit [1981], Ewart and Hawkesworth [1987], Woodhead [1989], Pearce et al [1995], Ewart et al [1998], Taylor and Nesbitt [1998], and Leat et al [2003] were recalculated to be consistent with the standard and normalization values in this study.…”

“…In addition to the CA-TIMS analyses, we used laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb zircon dating to study the systematics of complex samples. These analyses used analytical and data reduction procedures described by Hacker et al [2006a] and Rioux et al [2007].…”

“…[8] Whole-rock samples for isotopic and trace element analyses were powdered, spiked with 149 Sm −150 Nd and 87 Rb −84 Sr tracers, and dissolved in the clean laboratory facilities at Boston University following the procedures outlined by Rioux et al [2007]. Resulting sample solutions were evenly divided for trace element and isotopic analyses.…”

“…Resulting sample solutions were evenly divided for trace element and isotopic analyses. Trace element solutions were converted to HNO 3 , diluted, and analyzed on the VG PQIIXS quadrupole ICP-MS at Boston University, following the procedures outlined by Kelley et al [2003] Rioux et al [2007]. Major element analyses were carried out on fused samples by inductively coupled plasma-atomic emission spectrometry (ICP-AES) at Boston University, following the procedure outlined by Kelley et al [2003].…”