J.Y. Bradshaw
D.L. Kimbrough
T. W. Stern
T. K. Bundtzen
Marti L. Miller
1991
<div class="col-lg-9 article__content"><div class="article__body show-references "><div class="hlFld-Abstract"><div class="abstractSection abstractInFull"><p>The Idono Complex of west-central Alaska is a fault-bounded,<span> </span><span class="NLM_tex-math"><img src="https://www.journals.uchicago.edu/na101/home/literatum/publisher/uchicago/journals/content/jg/1991/i30081111/629485/20180625/images/eqs/eq-00001.gif" alt="" data-mce-src="https://www.journals.uchicago.edu/na101/home/literatum/publisher/uchicago/journals/content/jg/1991/i30081111/629485/20180625/images/eqs/eq-00001.gif"></span><span> </span>fragment of Early Proterozoic continental crust surrounded by overlap assemblages and younger terranes accreted in Mesozoic time. It is composed of granitic to dioritic orthogneiss, amphibolite, and metasedimentary rocks. Trace element compositions of the granitoids and amphibolite suggest rock formation in a subduction-related volcanic arc terrain. Nine zircon fractions from three samples of granitoid orthogneiss define a U-Pb discordia line intersecting concordia at<span> </span><span class="NLM_tex-math"><img src="https://www.journals.uchicago.edu/na101/home/literatum/publisher/uchicago/journals/content/jg/1991/i30081111/629485/20180625/images/eqs/eq-00002.gif" alt="" data-mce-src="https://www.journals.uchicago.edu/na101/home/literatum/publisher/uchicago/journals/content/jg/1991/i30081111/629485/20180625/images/eqs/eq-00002.gif"></span><span> </span>and<span> </span><span class="NLM_tex-math"><img src="https://www.journals.uchicago.edu/na101/home/literatum/publisher/uchicago/journals/content/jg/1991/i30081111/629485/20180625/images/eqs/eq-00003.gif" alt="" data-mce-src="https://www.journals.uchicago.edu/na101/home/literatum/publisher/uchicago/journals/content/jg/1991/i30081111/629485/20180625/images/eqs/eq-00003.gif"></span>. The upper and lower intercepts are interpreted, respectively, as approximations of the time of granitoid crystallization and major episodic Pb-loss. The lower intercept age is similar to that indicated by some hornblende and biotite K-Ar ages. Other biotite, hornblende, and white mica K-Ar ages record later isotopic disturbance in Early Cretaceous time. Nd isotopic compositions at 2.06 Ga for tonalite orthogneiss (<span class="NLM_tex-math"><img src="https://www.journals.uchicago.edu/na101/home/literatum/publisher/uchicago/journals/content/jg/1991/i30081111/629485/20180625/images/eqs/eq-00004.gif" alt="" data-mce-src="https://www.journals.uchicago.edu/na101/home/literatum/publisher/uchicago/journals/content/jg/1991/i30081111/629485/20180625/images/eqs/eq-00004.gif"></span>) and amphibolite (<span class="NLM_tex-math"><img src="https://www.journals.uchicago.edu/na101/home/literatum/publisher/uchicago/journals/content/jg/1991/i30081111/629485/20180625/images/eqs/eq-00005.gif" alt="" data-mce-src="https://www.journals.uchicago.edu/na101/home/literatum/publisher/uchicago/journals/content/jg/1991/i30081111/629485/20180625/images/eqs/eq-00005.gif"></span><span> </span>and +2.3) indicate that Early Proterozoic crust formation in the Idono Complex involved significant additions of mantle-derived magma. These rocks yield<span> </span><span class="NLM_tex-math"><img src="https://www.journals.uchicago.edu/na101/home/literatum/publisher/uchicago/journals/content/jg/1991/i30081111/629485/20180625/images/eqs/eq-00006.gif" alt="" data-mce-src="https://www.journals.uchicago.edu/na101/home/literatum/publisher/uchicago/journals/content/jg/1991/i30081111/629485/20180625/images/eqs/eq-00006.gif"></span><span> </span>(depleted mantle) model ages of 2.0 to 2.1 Ga, similar to the U-Pb zircon upper intercept age. Involvement of Archean crust in the formation of at least some rocks of the Idono Complex is indicated by a granitic orthogneiss (<span class="NLM_tex-math"><img src="https://www.journals.uchicago.edu/na101/home/literatum/publisher/uchicago/journals/content/jg/1991/i30081111/629485/20180625/images/eqs/eq-00007.gif" alt="" data-mce-src="https://www.journals.uchicago.edu/na101/home/literatum/publisher/uchicago/journals/content/jg/1991/i30081111/629485/20180625/images/eqs/eq-00007.gif"></span>), which yields a<span> </span><span class="NLM_tex-math"><img src="https://www.journals.uchicago.edu/na101/home/literatum/publisher/uchicago/journals/content/jg/1991/i30081111/629485/20180625/images/eqs/eq-00008.gif" alt="" data-mce-src="https://www.journals.uchicago.edu/na101/home/literatum/publisher/uchicago/journals/content/jg/1991/i30081111/629485/20180625/images/eqs/eq-00008.gif"></span><span> </span>model age of 2.5 Ga. The ~2.06-Ga age of the Idono Complex is similar to that indicated for provenance(s) of widely distributed sedimentary rocks in western North America and may represent a fragment of the North American craton displaced by northward movement of the Pacific plate. At such, rocks of the Idono Complex may provide important insights into both Early Proterozoic evolution along the craton margin, and subsequent displacements.</p></div></div></div></div>
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10.1086/629485
en
University of Chicago Press
Isotopic evidence for early Proterozoic age of the Idono Complex, west-central Alaska
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