HIGH-PRESSURE AMPHIBOLE IN JURASSIC ARC MAGMAS: INSIGHTS INTO DEEP CRUSTAL MAGMATIC PROCESSES
Unlike amphibole in upper-crustal plutons, which shows regular core-to-rim zoning (e.g., Barnes et al., 2016), complexly zoned amphibole in the garnet andesites lacks regular core-to-rim zoning in any element or ratio. For example, the highest T values were calculated for crystal cores and crystal terminations. Similarly, high Ti contents and high Gd/Lu ratios are found in both core zones and rims, in the same grains. These zoning patterns indicate that deep-crustal reservoirs of garnet andesite magmas were locations of repeated magma mixing. Mixing was initially at T higher than garnet stability but ultimately involved garnet-bearing and garnet-free magmas.
Preservation of the high-P assemblage indicates rapid upward magma migration, leaving the open question: would the high-P assemblage or the geochemical signature of garnet fractionation be preserved after long-term mid-crustal storage?
Barnes et al. (2016) Am. Min. 101, 328–342. Putirka, K. (2016) Am. Min. 101, 841–858.