THE UPPER CRUSTAL EVOLUTION OF AN ERUPTED BATHOLITH: EVIDENCE FROM CRYSTAL-SCALE RB/SR ISOTOPIC HETEROGENEITIES IN THE FISH CANYON MAGMATIC SYSTEM, COLORADO

Integrated textural, mineralogical, and compositional observations on the 5,000 km³ Fish Canyon Tuff suggest that the eruption evacuated a silicic crystal mush (>45 vol% crystals) of batholithic dimensions from its shallow crustal reservoir (~8-10 km depth) following new influx of hot magma from below that reactivated the mush (by partial dissolution of feldspars and quartz while hydrous phases were crystallizing). Although the magma is well-known for its whole-rock homogeneity (68 ± 1 wt% SiO₂), heterogeneities are omnipresent at the mineral scale. Data presented here reports Rb/Sr isotopic analyses of single crystals (sanidine, plagioclase, biotite, hornblende, apatite, titanite) and sampling by micromilling of selected zones within glass, sanidine and plagioclase crystals document widespread isotopic disequilibrium at many scales. High and variable ⁸⁷Sr/⁸⁶Sr_i values for euhedral biotite grains cannot be explained by any model involving closed-system radiogenic ingrowth, and they are difficult to rationalize unless much of this radiogenic Sr has been introduced at a late stage via assimilation of local Proterozoic crust. Hornblende is the only phase that approaches isotopic equilibrium with the surrounding melt, but the melt (glass) was isotopically heterogeneous at the millimetre scale, and was therefore apparently contaminated with radiogenic Sr shortly prior to eruption. The other mineral phases (plagioclase, sanidine, titanite, and apatite) have significantly lower ⁸⁷Sr/⁸⁶Sr_i values than glass values (as much as 0.0005). Such isotopic disequilibrium implies that feldspars, titanite and apatite are antecrysts that crystallized from less radiogenic melt compositions at earlier stages of magma evolution, whereas highly radiogenic biotite xenocrysts and the development of isotopic heterogeneity in matrix melt glass appear to coincide with the final stage of the evolution of the Fish Canyon magma body in the upper crust. Assimilation and blending of phenocrysts, antecrysts and xenocrysts reflects chamber-wide, low Reynolds number convection that occurred within the last ~10,000 years before eruption, but the mechanics of such a stirring event in a viscous silicic magma are still poorly understood.