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Fractional crystallization of high-K arc magmas: biotite- versus amphibole-dominated fractionation series in the Dariv Igneous Complex, Western Mongolia

Bucholz, Claire E. and Jagoutz, Oliver and Schmidt, Max W. and Sambuu, Oyungerel (2014) Fractional crystallization of high-K arc magmas: biotite- versus amphibole-dominated fractionation series in the Dariv Igneous Complex, Western Mongolia. Contributions to Mineralogy and Petrology, 168 (5). Art. No. 1072. ISSN 0010-7999. doi:10.1007/s00410-014-1072-9.

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Many studies have documented hydrous fractionation of calc-alkaline basalts producing tonalitic, granodioritic, and granitic melts, but the origin of more alkaline arc sequences dominated by high-K monzonitic suites has not been thoroughly investigated. This study presents results from a combined field, petrologic, and whole-rock geochemical study of a paleo-arc alkaline fractionation sequence from the Dariv Range of the Mongolian Altaids. The Dariv Igneous Complex of Western Mongolia is composed of a complete, moderately hydrous, alkaline fractionation sequence ranging from phlogopite-bearing ultramafic and mafic cumulates to quartz–monzonites to late-stage felsic (63–75 wt% SiO_2) dikes. A volumetrically subordinate more hydrous, amphibole-dominated fractionation sequence is also present and comprises amphibole (±phlogopite) clinopyroxenites, gabbros, and diorites. We present 168 whole-rock analyses for the biotite- and amphibole-dominated series. First, we constrain the liquid line of descent (LLD) of a primitive, alkaline arc melt characterized by biotite as the dominant hydrous phase through a fractionation model that incorporates the stepwise subtraction of cumulates of a fixed composition. The modeled LLD reproduces the geochemical trends observed in the “liquid-like” intrusives of the biotite series (quartz–monzonites and felsic dikes) and follows the water-undersaturated albite–orthoclase cotectic (at 0.2–0.5 GPa). Second, as distinct biotite- and amphibole-dominated fractionation series are observed, we investigate the controls on high-temperature biotite versus amphibole crystallization from hydrous arc melts. Analysis of a compilation of hydrous experimental starting materials and high-Mg basalts saturated in biotite and/or amphibole suggests that the degree of K enrichment controls whether biotite will crystallize as an early high-T phase, whereas the degree of water saturation is the dominant control of amphibole crystallization. Therefore, if a melt has the appropriate major-element composition for early biotite and amphibole crystallization, as is true of the high-Mg basalts from the Dariv Igneous Complex, the relative proximity of these two phases to the liquidus depends on the H_2O concentration in the melt. Third, we compare the modeled high-K LLD and whole-rock geochemistry of the Dariv Igneous Complex to the more common calc-alkaline trend. Biotite and K-feldspar fractionation in the alkaline arc series results in the moderation of K_2O/Na_2O values and LILE concentrations with increasing SiO_2 as compared to the more common calc-alkaline series characterized by amphibole and plagioclase crystallization and strong increases in K_2O/Na_2O values. Lastly, we suggest that common calc-alkaline parental melts involve addition of a moderate pressure, sodic, fluid-dominated slab component while more alkaline primitive melts characterized by early biotite saturation involve the addition of a high-pressure potassic sediment melt.

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Bucholz, Claire E.0000-0001-5332-8278
Additional Information:© 2014 Springer-Verlag Berlin Heidelberg. Received: 9 May 2014. Accepted: 30 September 2014. Published online: 8 November 2014. Communicated by J. Hoefs. We acknowledge Lydia Zehnder for help with whole-rock XRF analyses; Markus Wälle for LA–ICPMS support; Uyanga Bold and Lkhagva-Ochir Said for helping to organize fieldwork logistics; and Adam Bockelie, Yerenburged Munkhbold, and Eson Erdene for their assistance in the field. Reviews by Cin-Ty Lee and an anonymous reviewer helped to clarify ideas presented in this manuscript and are gratefully appreciated.
Issue or Number:5
Record Number:CaltechAUTHORS:20180112-092037923
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Official Citation:Bucholz, C.E., Jagoutz, O., Schmidt, M.W. et al. Contrib Mineral Petrol (2014) 168: 1072.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:84290
Deposited By: Ruth Sustaita
Deposited On:12 Jan 2018 17:45
Last Modified:15 Nov 2021 20:18

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