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Research Article| April 01, 2007 High-magnesian andesite from Mount Shasta: A product of magma mixing and contamination, not a primitive mantle melt Martin J. Streck; Martin J. Streck 1Department of Geology, Portland State University, Portland, Oregon 97207, USA Search for other works by this author on: GSW Google Scholar William P. Leeman; William P. Leeman 2National Science Foundation, Division of Earth Sciences, Arlington, Virginia 22230, USA Search for other works by this author on: GSW Google Scholar John Chesley John Chesley 3Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA Search for other works by this author on: GSW Google Scholar Geology (2007) 35 (4): 351–354. https://doi.org/10.1130/G23286A.1 Article history received: 11 Aug 2006 rev-recd: 22 Nov 2006 accepted: 27 Nov 2006 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Martin J. Streck, William P. Leeman, John Chesley; High-magnesian andesite from Mount Shasta: A product of magma mixing and contamination, not a primitive mantle melt. Geology 2007;; 35 (4): 351–354. doi: https://doi.org/10.1130/G23286A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract It has been proposed that high-Mg andesites (HMAs) from the Mount Shasta area may represent near-primary mantle melts, carrying signatures of slab melt interaction with the Cascadia mantle wedge. We present strong evidence that their formation involved mixing of dacitic and basaltic magmas and entrainment of ultramafic crystal material, and thus they cannot represent primitive magmas. The rocks contain (1) low-Mg# (65–72) clinopyroxene (cpx) and orthopyroxene (opx) phenocryst cores containing dacitic melt inclusions, and (2) high-Mg# opx and olivine xenocrysts, all of which are rimmed by euhedral overgrowths of cpx or opx similar in Mg# (87) to skeletal olivine phenocrysts. Textural relations indicate that ultramafic xenocrysts reacted with dacitic liquid, after which the contaminated magma mixed with basaltic liquid to produce a hybrid HMA bulk composition. High Mg, Cr, and Ni derive from the latter inputs, whereas high Sr/Y and overall adakite affinity is inherited from the dacite end member, which is arguably crustal in origin. We suggest that open system processes may be more important in the petrogenesis of HMAs than generally recognized, and that their magnesian compositions do not necessarily imply that they are primitive mantle melts. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.