Complex Reheating and Reactivation of Basal Cumulate in the Peach Spring Tuff (USA) Magma Body Revealed by Glass and Feldspar Textures and Compositions
Abstract
The 103 km3, 18.8 Ma Peach Spring Tuff (PST) erupted from Silver Creek caldera in the southern Black Mountains, AZ (Ferguson et al. 2013). Outflow is dominated by phenocryst-poor high-silica rhyolite (HSR)(fiamme: 6-12 % subhedral phenos, 74-75 wt % SiO2)(Pamukcu et al. 2013). HSR tuff is locally capped by crystal-rich trachyte (TR)(Warm Springs Zone (WSZ)). Fiamme in WSZ ( 20-23 % phenos, commonly rounded and embayed; 65-69 % SiO2) match intracaldera fiamme in texture and bulk composition (Pamukcu et al 2013; Foley et al. 2014).
Three glasses are distinguishable in PST fiamme: high-silica rhyolite (HSRg; 76-77 wt% SiO2); low-silica rhyolite (LSRg; 70-73); and trachyte (TRg; <69). Both LSRg and TRg are highly enriched in phenocryst-compatible trace elements (up to 4000 ppm Ba, 350 Sr, 870 Zr, 340 Ce) and have elevated zircon saturation-Ts ( 850 - 920 °C) compared to HSR ( 5 ppm Ba, 3 Sr, 120 Zr, 80 Ce; 770 °C). Only HSRg is found in HSR tuff. All WSZ unit fiamme contain LSRg; some have one or both of the other glasses, complexly intermingled and displaying sharp, μm scale contacts. Elemental compositions of feldspar phenocrysts enclosed by the three glass types support growth from distinct melts: TRg Ba & Sr uniformly very high (kspar 10000-50000, 500-3000 ppm respectively; plg both 1000-4000 ppm); LSRg kspar Ba & Sr 150-7500, 15-700 ppm, rim strongly enriched in Ba; plg Ba & Sr intermediate (400-1200, 300-700 ppm resp.); HSRg kspar Ba & Sr 30-200, 15-100 ppm, decreasing core to rim; plg low Ba & Sr, decreasing core to rim ( 500-10, 100-20 ppm resp.). Bulk fiamme compositions suggest that TR was complementary cumulate to extracted HSR, and phenocryst textures in TR document profound reheating (Foley, 2017). TRg and LSRg compositions and high zircon saturation Ts document extensive phenocryst dissolution; Rhyolite-MELTS and trace element modeling suggest 50% dissolution of original accumulated phenocrysts. Variation of feldspar compositions within single fiamme reveal complex growth histories at different melt conditions. Thick zones in feldspar and zircon reflecting growth in high-T, compatible-rich melts indicate these conditions were sustained following reheating event(s) to permit protracted growth, but sharp contacts between melts indicate final assembly and modification of magma took place during eruption.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.V33D0271F
- Keywords:
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- 1036 Magma chamber processes;
- GEOCHEMISTRYDE: 3653 Fluid flow;
- MINERALOGY AND PETROLOGYDE: 8439 Physics and chemistry of magma bodies;
- VOLCANOLOGY