Munazzam Ali Mahar1*, Terry L Pavlis1, John R Bowman2, Walter K Conrad3, Philip C. 5. Goodell1. 6. 1 Department of Geological Sciences, University of Texas ...
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Early Cretaceous ridge subduction beneath southern Alaska: Insights from zircon U–Pb geochronology, hafnium and oxygen isotopic composition of the Western Chugach Tonalite-Trondhjemite Suite
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Munazzam Ali Mahar1*, Terry L Pavlis1, John R Bowman2, Walter K Conrad3, Philip C
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Goodell1
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1 Department of Geological Sciences, University of Texas at El Paso, El Paso, TX 79968
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2 Department of Geology and Geophysics, the University of Utah, 115 S 1460 E, Salt
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Lake City, UT 84112-0102
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3 Knight Piésold and Co. 1999 Broadway, Suite 600, Denver, Colorado, 80202
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*Correspondent Author
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ABSTRACT
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New studies of forearc tonalite-trondhjemite plutons in southern Alaska provide further
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support for an Early Cretaceous intra-oceanic ridge subduction along the northern
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Cordilleran margin. The geological setting together with the timing and geochemistry of
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the plutons strongly suggest the origin of these plutons can be related to a ridge
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subduction event in the Early Cretaceous. The plutons were emplaced along the Border
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Ranges Fault System, the structural boundary between forearc and arc assemblages in
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southern Alaska. The plutons were emplaced late in the ductile deformational history of
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an amphibolite to greenschist facies shear zone that developed in a previously accreted
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mélange at this arc-forearc boundary. New U–Pb zircon dates reported here indicate the
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five main plutons are indistinguishable in age at ~123Ma. The zircon hafnium (εHf (t) >
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+12 to +20) and oxygen (δ18O = 4.6 to 6.0 ‰) isotope compositions strongly suggest
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these plutons were derived from a typical depleted mantle source. These Hf-O isotope
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compositions coupled with the absence of zircon inheritance indicate a minimal
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contribution from partial melting of either subducting slab or preexisting evolved Jurassic
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crust. The restricted timing (~123 Ma), zircon Hf-O isotopic compositions, and the
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whole-rock major and trace element geochemistry of these plutons strongly support their
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origin by anatexis of a garnet-free amphibolitic source derived from a mafic rock of
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MORB composition, unaffected by seawater interactions, and without significant
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contribution of continental crust (sediment) components. Therefore, we suggest that
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during a restricted time period from 126 to 123 Ma, a “slab window” opened up when a
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ridge segment was subducted underneath the forearc region. By analogy with younger
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sites of ridge subduction, we infer that decompressing asthenosphere melted lower
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oceanic crustal metagabbros that presumably had been underplated by earlier phases of
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the ridge interaction. The magma thus generated differentiated and was emplaced as near-
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trench plutons. Our results suggest that these Alaska forearc plutons represent a primary,
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relatively unmodified magma type that may be diagnostic of magmas generated during
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ridge subduction. Because ridge subduction represents a potential modern analog to
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Archean subduction, comparison to compositionally similar tonalite-trondhjemite suites
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and oceanic plagiogranites is informative for general models of granitoid petrogenesis.
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The mantle-like Hf-O isotope compositions of zircons from these forearc tonalite plutons
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in southern Alaska are comparable to those of the oceanic plagiogranites, suggesting
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partial melting of similar sources such as depleted mantle and unaltered oceanic crust.
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These forearc tonalite plutons are systematically depleted in REE compared to the
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suprasubduction zone ophiolite-related plagiogranites produced by extreme fractional
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crystallization. However, the REE patterns of these Alaska tonalities exhibit near-
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complete overlap with those observed in ophiolite plagiogranites produced by partial
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melting of metamorphosed oceanic crust. A number of Archean TTG rocks have
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chondritic Hf and mantle-like oxygen isotopic compositions coupled with the REE
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pattern similar to that observed in experimental melts produced from garnet-absent
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amphibolite. These similarities suggest that some Archean TTG rocks may have been
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produced by partial melting of garnet free amphibolite underplates, possibly in a similar
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ridge subduction tectonic setting.
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INTRODUCTION
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The concept of ridge subduction was recognized early in the development of plate
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tectonic theory (e.g., Delong et al., 1977; Marshak and Karig, 1977) based in part on the
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recognition that all subduction zones ultimately interact with a spreading ridge at some
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point in the history of any ocean basin. Different aspects of this concept have been
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discussed in GSA Special Paper 371 (e.g., Sisson et al., 2003; Bradley et al., 2003;
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Groome et al., 2003) yet surprisingly few well-documented examples of ridge subduction
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are known, and characteristic signatures of the process remain elusive (e.g., Sisson et al.,
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2003).
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A fundamental process in ridge subduction is the production of a “slab window” that
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widens successively as the spreading plate boundary moves beneath the subduction
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megathrust (e.g., Thorkelson and Taylor, 1989; Thorkelson, 1996; Thorkelson and
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Breitsprecher, 2005). As the slab window develops, asthenosphere should rise toward the
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subduction interface, transferring heat to the refrigerated forearc as well as generating
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decompression melts that can rise into the forearc system (Thorkelson and Taylor, 1989).
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