Isotopic Constraints (U, Th, Pb, Sr, Ar) on the Timing of Magma Generation, Storage and Eruption of a Late-Pleistocene Subvolcanic Granite, Alid Volcanic Center, Eritrea

Isotopic analyses demonstrate that a shallow granophyric intrusion from the Alid volcanic center (AVC) was generated, intruded and crystallized over a 20,000-year period in the latest Pleistocene. The granophyre is not exposed, but was ejected as unmelted blocks within a ~1 km³ pyroclastic flow deposit around 15 ka and is a subvolcanic equivalent of the erupted rhyolitic pumice (Lowenstern et al., 1997: J Petrol 12, p. 1707-1721). The rock contains < ~ 1-mm-sized phenocrysts of Na-sanidine, quartz, ferroaugite, Fe-rich biotite (replacing pyroxene), and magnetite. Accessory phases include zircon, apatite, pyrrhotite, fluorite and rare chevkinite. The groundmass is a micrographic intergrowth of Na-sanidine and quartz. We interpret the silicic magma to have intruded subsequent to eruption of a basaltic lava flow at 36 +/- 9 ka (Ar-Ar age); the lava was strongly tilted by structural doming associated with the intrusion. The granophyre was completely crystallized by the time of the final pyroclastic flow eruption that brought the intrusive blocks to the surface. From the pumice in this deposit, the weighted mean Ar-Ar age of two splits of Na-sanidine (density > 2.59) is 15.2+/- 5.8 ka (all errors are 2 σ ). Two other splits with lower density (thus higher in Na) yielded ages older than 24 ka, and may retain some excess Ar. Thus, the time between intrusion and complete crystallization for the granophyre was < ~20,000 years. The ⁸⁷Sr/⁸⁶Sr ratio ( 0.7047) of the granophyre is similar to those of basalts erupted from the AVC and far less radiogenic than most basement granites and schists that form escarpments bordering the Danakil Depression and are found as unmelted lithic xenoliths in lavas and tuffs of the AVC. Pb isotopes also rule out significant assimilation of Precambrian basement during genesis of the young granophyre. Similarly zircon grains, analyzed with the Stanford-USGS SHRIMP-RG, show little evidence for inheritance, with only a single 760 Ma zircon (U-Pb age) that was also petrographically different from the 130 other zircons in the mount. The other zircons yielded SHRIMP ²³⁸U²³⁰Th disequilibrium ages of < 65 ka, with no discernible difference between ages of cores and rims. The weighted mean ²³⁸U²³⁰Th age of 23 analyzed zircons is 24.7 +/- 3.3 ka. The whole-rock U-Th isotopic values of the granophyre, by TIMS, show extreme enrichment of ²³⁰Th ([²³⁰Th /²³²Th] = 1.064; [²³⁸U /²³²Th] = 0.602; Th/U = 5.04). On an activity ratio diagram, the whole-rock value is essentially colinear with clinopyroxene, Na-sanidine, apatite and magnetite separates, forming an apparent isochron of 21.5 +/- 2.4 ka, similar to the zircon data (both SHRIMP and multiple-grain TIMS). The ferroaugite separate is more Th-enriched than the whole-rock, probably due to rare chevkinite inclusions. Isotopic and trace-element data indicate melt generation by partial melting of Quaternary mafic rocks, accompanied by minor (< 5%) assimilation of older basement. All mineral phases in the granophyre appear to have precipitated from the melt between about 36 and 15 ka, after intrusion into the shallow crust. >http://wrgis.wr.usgs.gov/docs/geologic/jlwnstrn/alid/ Alidpage.html</a>