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

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; Rb_NMORB= 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.