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Age and tectonic setting of the Duke Island ultramafic intrusion, southeast Alaska

Saleeby, Jason B. (1992) Age and tectonic setting of the Duke Island ultramafic intrusion, southeast Alaska. Canadian Journal of Earth Sciences, 29 (3). pp. 506-522. ISSN 1480-3313. doi:10.1139/e92-044.

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Concordant U–Pb zircon ages on hornblende–plagioclase pegmatites and a related granophyre from marginal and roof zones of the Duke Island ultramafic intrusion, and from leucoquartz diorite internal to hornblende–clinopyroxenite of nearby Mary Island cluster between 108 and 111 Ma. Sm–Nd mineral and bulk-rock data from pegmatite and granophyre zircon samples and from wehrlites and clinopyroxenites of the main Duke Island cumulate sequence are consistent with the consanguinity of the ultramafic cumulates and the zircon-bearing feldspathic rocks, and with an Early Cretaceous igneous age for the intrusion. Wall rocks for the intrusion consist of Ordovician–Silurian amphibolite- and greenschist-grade metamorphosed plutonic, volcanic, and minor sedimentary rocks and crosscutting Late Triassic gabbro–diorite of the Alexander terrane.The Duke Island ultramafic intrusion formed as a northwest-trending elongate funnel-shaped stratiform body commensurate with extensional brittle–ductile faulting in its roof and adjacent wall rocks. Intrusion occurred in a basinal setting within the Gravina volcanic arc along the Cordilleran continental margin. During the latter phases of intrusive activity the Gravina basin began to close by regional thrust faulting, which in the Duke Island region was west-northwest directed. The intrusion may have still had local intercumulate liquid upon initiation of thrust faulting. It responded first by open folding with incipient crystal plasticity. Once completely solidified, the intrusion behaved like a large "augen" with ductile deformation concentrated along its margins and within quartzo-feldspathic and micaceous members of its wall-rock complex. Much of the intrusion's northern margin behaved as a ductile shear zone serving as a tear fault between thrust plates to the northeast and an oblique thrust complex that roots beneath the southwest margin of the intrusion. The intrusion may thus be tectonically transported relative to its original underpinnings. North-trending high-angle faults and parallel fracture cleavage also cut the intrusion in response to axial loading as a result of its initial elongate shape and orientation relative to the thrust kinematics.

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Additional Information:© 1992 Canadian Science Publishing. Revision accepted October 15, 1991. This work was supported by National Science Foundation grants EAR-8605386 and EAR-8803834, and the Alaskan Branch of the United States Geological Survey. Assistance in the field by G. E. Gehrels, C. M. Rubin, P. Sacker, and I. Saleeby; in Sm - Nd aspects of laboratory work by D. A. Pickett and J. G. Wasserburg; and in hand sorting of zircon, drafting, and manuscript preparation by C. Saleeby, J. Mayne, and J. Haskell are gratefully acknowledged. Relatively pristine samples of clinopyroxene-bearing cumulates for Sm - Nd isotopic determinations were kindly provided by T. N. Irvine. Critical reviews of this manuscript by R. G. Anderson, M. A. Lanphere, and G. T. Nixen were helpful.
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Caltech Division of Geological and Planetary Sciences5070
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Official Citation:Age and tectonic setting of the Duke Island ultramafic intrusion, southeast Alaska Jason B. Saleeby Canadian Journal of Earth Sciences, 1992, 29:506-522, 10.1139/e92-044
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:49328
Deposited By: Tony Diaz
Deposited On:08 Sep 2014 17:24
Last Modified:25 Aug 2022 18:38

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