FORMATION AND STORAGE CONDITIONS OF LARGE-VOLUME SILICIC MAGMAS FROM THE AFRO-ARABIAN LARGE IGNEOUS PROVINCE

There are several proposed models for the generation of silicic magmas in predominantly mafic continental large igneous provinces (LIPs), including fractional crystallization from a parent mafic magma, crustal anatexis, re-melting of mafic underplated material, and remobilization of crystal mush. Deciphering among these modes of magma generation is a complex task that requires extensive information on the textures, geochemistry, and mineralogy of the volcanic deposits and their change through time. The northern Yemen section of the Afro-Arabian large igneous province is characterized by voluminous (150 to >1,000 km³, with one eruption ~3,100 km³ dense rock equivalent) and laterally extensive deposits of ignimbrites, tuffs, and caldera collapse breccia. The largest sequential pyroclastic eruptions, accounting for a minimum total eruptive volume of ~8,600 km³ dense rock equivalent, occurred in 48 ± 34 kyr from Ar and U-Pb dating constraints. The huge volumes of magma erupted in quick succession in LIPs raises the questions of how these magmas are generated and the timescales over which these processes occur.

In order to investigate these questions, an array of feldspar, clinopyroxene, zircon, and Fe-Ti oxide thermometers and barometers were employed to determine the temperatures and pressures of crystallization across the suite of silicic pyroclastic units. Results indicate that most of the rhyolites formed at temperatures >800ºC with mineral and whole rock compositions consistent with extensive fractionation from two distinct mafic parent magmas. Furthermore, these rhyolites formed at pressures consistent with mid- to lower-crustal magma assembly. In contrast, one unit exhibits marked differences in mineralogy and whole rock compositions with lower crystallization temperatures and polybaric fractionation. These results are consistent with crustal anatexis as the primary generation mechanism for this unit. While the presence of xenocrysts (i.e., crystals that are unrelated to the magma system of interest) are expected for pyroclastic deposits, this study highlights the necessity for careful assessment of apparent mineral-melt equilibria prior to determining magma storage conditions.