Leucocratic and Gabbroic Xenoliths from Hualalai Volcano, Hawai'i

Abstract

A diverse range of crustal xenoliths is hosted in young alkali basalt lavas and scoria deposits (erupted $3-5 ka) at the summit of Hual a alai. Leucocratic xenoliths, including monzodiorites, diorites and syenogabbros, are distinctive among Hawaiian plutonic rocks in having alkali feldspar, apatite, zircon and biotite, and evolved mineral compositions (e.g. albitic feldspar, clinopyroxene Mgnumber 67-78). Fine-grained diorites and monzodiorites are plutonic equivalents of mugearite lavas, which are unknown at Hual a alai. These xenoliths appear to represent melt compositions falling along a liquid line of descent leading to trachyte-a magma type which erupted from Hual a alai as a prodigious lava flow and scoria cone at $114 ka. Inferred fractionating assemblages, MELTS modeling, pyroxene geobarometry and whole-rock norms all point to formation of the parent rocks of the leucocratic xenoliths at $3-7 kbar pressure. This depth constraint on xenolith formation, coupled with a demonstrated affinity to hypersthenenormative basalt and petrologic links between the xenoliths and the trachyte, suggests that the shift from shield to post-shield magmatism at Hual a alai was accompanied by significant deepening of the active magma reservoir and a gradual transition from tholeiitic to alkalic magmas. Subsequent differentiation of transitional basalts by fractional crystallization was apparently both extreme-culminating in >5. 5 km 3 of trachyte-and rapid, at !2. 75 • 10 6 m 3 magma crystallized/year.

Forty years of TTG research

“…Finally, basalts at the base of a thick plateau-type pile are unlikely to become hydrated; therefore, the resulting melts will, in general, not be TTG (or even I-type granites). This is illustrated by the fact that the typical felsic magma in any oceanic plateau is syenitic rather than I-type (Cousens et al, 2003;Gagnevin et al, 2003;Giret, 1990;Marsh et al, 1991;Martin et al, 2008;Shamberger and Hammer, 2006). Consequently, melting of a thick basaltic plateau can account for the origin of some TTG-like rocks (high HREE, or "low pressure" TTGs s.l.…”

Rift zones and magma plumbing system of Piton de la Fournaise volcano: How do they differ from Hawaii and Etna?

“…The interpretations proposed for Hawaiian volcanoes to explain the shift from shield to post-shield magmatism invoke a decrease in the amount of partial melting of the oceanic lithosphere Frey, 1983, 1985;Chen et al, 1991), a progressive decrease in partial melting (Feigenson et al, 1983), or a significant deepening of the active magma reservoir (Shamberger and Hammer, 2006). For Piton de la Fournaise, the coeval activity of the "post-shield" and "shield" type rift zones and their link with a single transport path make unlikely a major influence of the degree of partial melting in the synchronous development of the two magma types.…”

Geochemistry of mafic lavas from Sivas, Turkey and the evolution of Anatolian lithosphere

“…The basaltic and related evolved samples contain 5.6-10.6 wt.% MgO and 46.9-53.3 wt.% SiO 2 ; most basaltic lavas share a narrow compositional range, with MgO contents between 9.5-10.0 wt.%. These basalts have normative olivine (3-18 vol.%) and diopside (8-23 vol.%) in addition to minor nepheline (<6 vol.%) or hypersthene (<13 vol.%), and the basaltic trachyandesites contain normative quartz.The basanites are compositionally more primitive (10.9-13.7 wt.% MgO, 42.7-43.0 wt.% SiO 2 ), and are characterized by 17-29 vol.% normative nepheline and up to 5 vol.% normative leucite.These mafic lavas have among the highest MgO contents recorded in young lavas across the Anatolian plate, and their bulk compositions plot in the field defined by experimental alkaline melts at 20-30 kbar(Sack et al, 1987;Shamberger and Hammer, 2006; Fig. 5).…”