13 Cordilleran Tectonics from the Basin and Range to Alaska and the Arctic II: A Celebration of Elizabeth Miller’s Career (2018 GSA Structure and Tectonics Division Career Contribution Award)

Wednesday, 15 May 2019
1:30 PM - 3:30 PM
Room B117-119

IMPLICATIONS FOR LOWER JURASSIC ISLAND ARC MAGMATISM FROM IGNEOUS CLASTS IN THE LOWER CRETACEOUS CONGLOMERATE, NE YUKON-KOYUKUK BASIN, ALASKA

Paper number: 13-3
Program: Theme Sessions
Time: 2:10 PM – 2:30 PM

Authors

Abstract

The latest Jurassic–Early Cretaceous Brooks Range orogen is flanked by two basins: the Colville foreland basin to the north and the Yukon-Koyukuk Basin (YKB) to the south. While the former is well studied, less is known about the later. Tectonic interpretations of the YKB vary widely, from a syncollisional forearc basin to a post-accretionary hinterland basin. We present a geochronological, geochemical, isotopic pilot study of igneous clasts from Lower Cretaceous conglomerate exposed along the Alatna River. The data adds to the growing body of evidence documenting the existence of a late Triassic –early Jurassic island arc.

Samples were collected from the in the northeast YKB. Twenty clasts were analyzed for chemistry, seven for isotopes, and three yielded zircon for U-Pb (LA-ICPMS) concordia ages. The clasts represent tholeiitic to calc-alkaline magmas; depletion in high field strength elements (e.g. Nb, Ta) relative to LREE [(Nb/La)NMORB = 0.1-0.7], and enrichment in fluid mobile large ion lithophile elements (e.g. Sr, K, Rb and Ba; RbNMORB= 12-80), are consistent with subduction-related island arc chemistry.

On the basis of REE-patterns and age data, the KKB clasts can be divided into four compositional groups: Group 1 - tholeiitic (age unknown) with little to minor LREE enrichment relative to HREE [(La/Yb)NMORB= 2-3]; Group 2 - calc-alkaline (195 ± 1 Ma, diorite) with enriched LREE relative to HREE [(La/Yb) NMORB = 4-6] and depleted HREE [Yb/NMORB <1]; Group 3 - calc-alkaline (193 ± 2 Ma, granodiorite) with a more distinct LREE enrichment relative to HREE [(La/Yb) NMORB = 9]and a weak depletion in MREE relative to HREE [(Dy/Yb)NMORB = 0.9]; Group 4 - calc-alkaline (213 ± 2 Ma, diorite) with enriched LREE relative to HREE [(La/Yb) NMORB = 7] and a more pronounced Nb-Ta depletion than the other groups. Sm/Nd analyses indicate all groups have juvenile isotopic signatures similar to modern island arcs (ɛNd = +7.7 to +8.8).

Our ages are similar to those reported in O’Brien et al. (2017), which are generally older than previously reported for island arc rocks in the region, e.g.- the Koyukuk arc terrane (KAT) or the Brooks Range ophiolite (BRO). This confirms that clasts from the northeastern-most KKB were eroded from a late Triassic –early Jurassic island-arc.

Geological Society of America Abstracts with Programs. Vol. 51, No. 4

doi: 10.1130/abs/2019CD-329239

© Copyright 2019 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.