74 How are Plutons Made? Physical and Chemical Records of Pluton Construction and Evolution (Posters)

This session integrates structural and petrologic views of plutonic systems, exploring the crustal response to pluton formation, and processes that control the physical and chemical characteristics of pluton-host rock systems from initial emplacement to solidification.

Sunday, 19 October 2025
8:00 AM - 5:30 PM
HBGCC, Hall 1

Mafic rocks of the Seventymile ophiolite and their tectonic implications for transitional melting regimes

Paper number: 74-5
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

The Seventymile ophiolite in eastern Alaska, and correlative rocks of the Slide Mountain terrane in Yukon and British Columbia, represent oceanic crust and upper mantle fragments of the Devonian to Permian Slide Mountain Ocean (SMO) that separated allochthonous continental fragments from western Laurentia. Late Permian-early Triassic plate motion reversal resulted in closure of the SMO and collision of the allochthonous Yukon Tanana terrane (YTT) and parautochthonous North America (NA). It also led to obduction of Seventymile thrust sheets containing mafic/ultramafic rocks overlying and/or imbricated with supracrustal YTT and NA rocks. Closure of the SMO also culminated in the Klondike orogeny, which produced the orogenic gold of the Klondike District. New geochemistry and U-Pb zircon + titanite ages help constrain SMO tectonics immediately prior to the Klondike orogeny.

The Seventymile ophiolite forms klippen of variably serpentinized peridotite, largely greenschist facies mafic rocks, and minor oceanic sedimentary rocks (argillite, chert, limestone, and metasandstone). Mafic components include isotropic intrusions in peridotite and more deformed greenstones in klippen structurally underlying, and in shear zone contact with, ultramafic klippen. Compositional and isotopic data for ultramafic-hosted intrusions show evidence for decompression melting of a weakly subduction-modified mantle source similar to sources of depleted mid-ocean ridge basalt (N-MORB) or back-arc basin basalt (BABB). Seventymile greenstones are more heterogeneous, ranging from subduction-unmodified N-MORB to enriched mid-ocean ridge basalt (E-MORB) and ocean-island basalt (OIB). Locally, a klippe of variably pillowed greenstone has continental arc affinity not observed in other Seventymile bodies. If all three Seventymile mafic units are contemporaneous, obduction associated with the SMO closure resulted in stacking of ophiolitic packages representing distinct tectonomagmatic settings across the transition from pericontinental, to epicontinental, to distal ocean back-arc. Ultramafic-hosted intrusions and MORB-like greenstones are similar to SMO rocks of the Campbell Range Formation (CRfm) in eastern Yukon. CRfm rocks show ~ 430 km of dextral separation across the Tintina fault providing a newly viable piercing point.

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

doi: 10.1130/abs/2025AM-412341

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