ZIRCON FROM THE ALID VOLCANIC CENTER, ERITREA: IMPLICATIONS FOR MAGMATIC EVOLUTION

The Alid volcanic center, Eritrea, is located on an active axis of the Red Sea rift system. Its most recent activity (~15 ka) was marked by pyroclastic flows of clinopyroxene-bearing rhyolite pumice that contain lithic granophyric blocks reported to be the intrusive equivalent of the rhyolite (Lowenstern et al 1997). Trace element compositions of zircons extracted from the granophyre constrain the evolution and assembly of the Alid magmatic system and suggest possible distinctive characteristics of zircons from spreading centers.

Zircons from the granophyre exhibit low and restricted Hf concentrations (7000–9000 ppm) that attest to limited zircon fractionation within the host magma(s). While Hf contents of all grain domains (centers, interiors, and edges) overlap, there is a strong trend of increasing Hf (indicative of increasing melt evolution) from the centers to the edges. The Eu anomaly systematically deepens with increasing Hf, consistent with fractionation of feldspars, while Th/U decreases and Yb/Nd increases with increasing Hf values, consistent with fractionation of chevkinite. Zircon centers typically have higher values of Th/U and Eu/Eu* as well as lower values of Yb/Nd than interiors and edges. Uniform Ti values among all grain domains indicate growth at similar temperatures. Most Alid zircons are weakly zoned or have simple zoning patterns with little evidence for a protracted or complex crystallization history, and previous in situ U-Th analyses document little variability in zircon ages (Lowenstern et al 2006).

Zircon elemental trends, zonation patterns, and crystallization ages suggest that the Alid granophyre crystallized quickly and experienced a simple evolution by fractional crystallization. Crustal assimilation, recycling of zircon grains, and magmatic interactions were limited. Furthermore, elemental signatures of zircons from the Alid granophyre are remarkably similar to those of zircons within felsic Icelandic lavas and pumice (Carley et al 2009), implying that zircons from silicic magmas in environments transitional between a continental and oceanic rift may have a unique compositional fingerprint. Lastly, we use the example of Alid to suggest that most “volcanic” zircons may actually crystallize within intrusive bodies at depth and become entrained in erupting magmas.