244-8 New geochronology and structural constraints on the evolution of the high-pressure belt of the Brooks Range, northern Alaska: the later years

Session: Subduction Zone Processes: Insights from Geology, Geochemistry, and Petrochronology

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Abstract:

The Brooks Range in northern Alaska formed from Jurassic-Cretaceous convergence between a passive margin and the oceanic Angayucham terrane. It is generally agreed that the early evolution involved ophiolite and metamorphic sole formation, and intra-oceanic thrusting, followed by partial subduction of the continental margin to produce high-pressure metamorphism recorded by early blueschist/eclogite assemblages in the Schist belt. Models for the subsequent evolution of the Brooks Range, however, strongly diverge, ranging from crustal extension beginning around 130 Ma to shortening until ~105 Ma. Much of the ambiguity is centered on uncertainties regarding the age and kinematic significance of the extensive greenschist facies overprint, and the lack of constraints on the thermal history.

To address these deficiencies, we performed Lu-Hf dating of garnet tied to mapping and structural analysis from a transect across the Schist belt along the Wild River. The dominant structural fabric (S_d) dips moderately southward, parallel to lithologic layering. Sd is often a crenulation foliation that transposed an earlier (S_d-1) fabric and is overgrown statically by albite porphyroblasts. Lithologic layering and S_d are commonly folded into open folds that have a locally developed axial planar foliation that may post-date and overlap with albite growth. Faults appear to imbricate sections within the Schist belt along the transect.

Metabasite with garnet overgrowing S_d defined by greenschist-facies minerals yielded a Lu-Hf garnet age of 113.5 ± 0.5 Ma, effectively dating the uppermost greenschist facies metamorphism that may overprint an earlier high-P metamorphism. We obtained a second Lu-Hf garnet age of 113.9 ± 0.4 Ma from an interlayered metapelite where garnet is synkinematic with S_d, reflecting continued deformation in the weaker schists. Additional textural relations with fabrics and pseudomorphs after glaucophane may suggest that glaucophane was stable during garnet growth. Thus, we tentatively suggest that that garnet in both samples grew in transitional blueschist-greenschist P-T conditions, with compositionally controlled glaucophane stability.

To explain the garnet ages, we discuss potential models involving protracted high-P metamorphism and thrusting and/or re-imbrication lasting into the Aptian. Our new ages and structural-textural observations, combined with a published 40Ar/39Ar white mica plateau age of 110 ± 0.4 Ma, and a new zircon fission-track age of ~105 ± 12 Ma data suggest extension-related rapid cooling/exhumation immediately following garnet growth.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-11008

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