T102 The Virtue of Fieldwork in Volcanology, Sedimentology, Structural Geology, and Tectonics (Posters): A Session to Honor Cathy Busby, MGPV Distinguished Geological Career Award Recipient of 2020

Monday, 10 October 2022
9:00 AM - 1:00 PM
Exhibit Hall F

EVIDENCE FOR A POST 1 MA YAKUTAT SLAB TEAR: GEOLOGICAL, GEOCHRONOLOGICAL, AND GEOCHEMICAL CONSTRAINTS ON THE MACLAREN RIVER VOLCANIC FIELD, SOUTH-CENTRAL ALASKA (USA)

Paper number: T102-4
Program: Technical Sessions
Time: 9:00 AM – 1:00 PM

Authors

Abstract

The Yakutat slab is an oceanic plateau that has subducted beneath North America since 30 Ma, resulting in the archetypal example of flat slab subduction/slab edge arc magmatism (e.g., the Wrangell Arc). Yakutat subduction generated a ~400 km region between the Aleutian and Wrangell Arcs that was considered to be amagmatic since ~25 Ma (the Denali gap). Three newly identified volcanoes within the Denali gap (e.g., the Maclaren River volcanic field or MRVF) are the focus of this investigation. These volcanoes lie above a recently scattered-wave-imaged slab tear that is located west of the Wrangell Arc and separates the shallow dipping western Yakutat slab (~20°) and the steeper dipping eastern segment (~45°) under the Wrangell arc. Lava from the first volcano is a subalkaline basaltic andesite with an 40Ar/39Ar age = 940 ± 27 ka. Tephra and lavas from the second volcano are alkali basalt to shoshonite-like with Sr (~2300-3100 ppm) and Ba (~2700-3200 ppm) enrichments, high Ni (308-331 ppm) and Cr (442-501 ppm), but with Mg# between 67-73, suggestive of primary mantle melts. Groundmass from 4 separate units yielded 40Ar/39Ar ages between 424.5 and 420.5 ka. A lava sample from volcano 2 yielded an age of 423.2 ± 2.4 ka. Fine-grained lavas are exposed at a small (~1 km2) third volcano located ~9 km south of volcano 2. These rocks are transitional basaltic trachyandesite or trachyandesite, have ol + plag + cpx + opx + amph, and show evidence of crustal interaction (plag + qtz xenocrysts). Rocks from volcano 3 display a slab melt affinity (e.g., adakite), with lower incompatible element abundances than other MRVF rocks (e.g., Sr and Ba < 1000 ppm; Sr/Y = 64-73; La/Yb = 21-22) and Mg# = 67-69. Lava from volcano 3 lava yielded an age of 422.7 ± 6.7 ka, demonstrating that volcano 2 and 3 erupted coevally. All rocks display LILE enrichments and HFSE depletions consistent with subduction. However, their location, S-N alignment along a ~20 km transect, and volcano 3’s slab melt affinity are consistent with magma production in a subduction-affected environment along a slab tear. Our study provides geological evidence that conclusively demonstrates the existence of a Yakutat slab tear. We suggest that this tear formed ca. ~1 Ma, coincident with jamming of the Wrangell Arc trench and brought on by subduction of the thickest (~30 km) part of the Yakutat slab.

Geological Society of America Abstracts with Programs. Vol. 54, No. 6

doi: 10.1130/abs/2022AM-377236

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