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The Upper Crustal Evolution of a Large Silicic Magma Body: Evidence from Crystal-scale Rb Sr Isotopic Heterogeneities in the Fish Canyon Magmatic System, Colorado |
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| Authors: | Charlier B L A; Bachmann O; Davidson J P; Dungan M A; Morgan D J |
| Institution: | 1Department of Earth Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK 2Department of Earth Sciences, University of Durham, South Road, Durham DH1 3LE, UK 3Section Des Sciences De La Terre, Université De Genève, Rue Des Maraîchers 13, CH-1205 Geneva, Switzerland 4department of Earth and Space Sciences, University of Washington, Mailstop 351310, Seattle, WA 98195-1310, USA 5laboratoire Géodynamique Des Chaînes Alpines, UMR5025, Maison Des Géosciences, 1381 Rue De La Piscine, 38400 Saint Martin D’hères, France 6School of Earth and Environment, Earth Science Building, University of Leeds, Leeds LS2 9JT, UK |
| Abstract: | Batholith-sized bodies of crystal-rich magmatic ‘mush’are widely inferred to represent the hidden sources of manylarge-volume high-silica rhyolite eruptive units. Occasionallythese mush bodies are ejected along with their trapped interstitialliquid, forming the distinctive crystal-rich ignimbrites knownas ‘monotonous intermediates’. These ignimbritesare notable for their combination of high crystal contents (35–55%),dacitic bulk compositions with interstitial high-silica rhyoliticglass, and general lack of compositional zonation. The 5000km3 Fish Canyon Tuff is an archetypal eruption deposit of thistype, and is the largest known silicic eruption on Earth. Ejectafrom the Fish Canyon magmatic system are notable for the limitedcompositional variation that they define on the basis of whole-rockchemistry, whereas  45 vol. % crystals in a matrix of high-silicarhyolite glass together span a large range of mineral-scaleisotopic variability (microns to millimetres). Rb/Sr isotopicanalyses of single crystals (sanidine, plagioclase, biotite,hornblende, apatite, titanite) and sampling by micromillingof selected zones within glass plus sanidine and plagioclasecrystals document widespread isotopic disequilibrium at manyscales. High and variable 87Sr/86Sri values for euhedral biotitegrains cannot be explained by any model involving closed-systemradiogenic ingrowth, and they are difficult to rationalize unlessmuch of this radiogenic Sr has been introduced at a late stagevia assimilation of local Proterozoic crust. Hornblende is theonly phase that approaches isotopic equilibrium with the surroundingmelt, but the melt (glass) was isotopically heterogeneous atthe millimetre scale, and was therefore apparently contaminatedwith radiogenic Sr shortly prior to eruption. The other mineralphases (plagioclase, sanidine, titanite, and apatite) have significantlylower 87Sr/86Sri values than whole-rock values (as much as –0·0005).Such isotopic disequilibrium implies that feldspars, titaniteand apatite are antecrysts that crystallized from less radiogenicmelt compositions at earlier stages of magma evolution, whereashighly radiogenic biotite xenocrysts and the development ofisotopic heterogeneity in matrix melt glass appear to coincidewith the final stage of the evolution of the Fish Canyon magmabody in the upper crust. Integrated petrographic and geochemicalevidence is consistent with pre-eruptive thermal rejuvenationof a near-solidus mineral assemblage from 720 to 760°C (i.e.partial dissolution of feldspars + quartz while hornblende +titanite + biotite were crystallizing). Assimilation and blendingof phenocrysts, antecrysts and xenocrysts reflects chamber-wide,low Reynolds number convection that occurred within the last10 000 years before eruption. KEY WORDS: Fish Canyon Tuff; Rb–Sr isotopes; microsampling; magmatic processes; crystal mush |
| Keywords: | : Fish Canyon Tuff Rb-Sr isotopes microsampling magmatic processes crystal mush |
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