USING COMBINED LASER ABLATION TRACE-ELEMENT AND HIGH-PRECISION CHEMICAL ABRASION-TIMS U-Pb ZIRCON ANALYSES TO DATE METAMORPHISM: A CASE STUDY FROM THE SAMAIL (OMAN-UAE) OPHIOLITE

Combined laser ablation trace-element and high-precision chemical abrasion-TIMS U-Pb zircon analyses provide a powerful tool for directly dating the timing of metamorphic processes. Here we present a case study from amphibolite- to granulite-facies (0.8–1.4 GPa, 700–900 ºC), garnet- and clinopyroxene-bearing metabasalt samples from the metamorphic sole at the base of the Samail (Oman-UAE) ophiolite. The timing of sole metamorphism has been controversial: previous U-Pb zircon dates range from ~96.2–94.8 Ma, while Lu-Hf garnet-whole rock isochron dates from the same localities range from 104–98 Ma, the latter of which has been used to suggest that zircon grew on the retrograde path. Our new laser ablation-ICP-MS analyses of zircon grains from sole garnet amphibolites along the length of the ophiolite show systematic trends of decreasing heavy rare earth element slope (HREE; Yb/Dy) with decreasing Yb concentration, while garnet-free samples exhibit uniform HREE slopes. A limited number of zircon grains also show core to rim zoning from high to low Yb/Dy. Trace-element data from all major rock forming minerals in a subset of the studied samples demonstrate that garnet is the only phase with Yb/Dy > 1, and is therefore the only mineral that could lead to the observed decrease in zircon Yb/Dy. Synchronous zircon and garnet growth are also texturally supported by the occurrence of zircon inclusions within unfractured garnet. Finally, increasing Ti-in-zircon temperatures with decreasing Yb/Dy in half of the studied samples further supports prograde zircon and garnet growth. CA-TIMS U-Pb dates from high and low Yb/Dy zircon identified by laser ablation (n = 158 zircon grains/fragments in 18 samples) bracket the timing of prograde garnet and zircon growth between 96.70 ± 0.09 and 95.16 ± 0.06 Ma. Given the textural and geochemical data, we argue that the zircon crystallized during dehydration melting and peritectic garnet formation in these rocks, and that the zircon dates provide the most precise and accurate constraints on the timing of prograde to peak metamorphism. These data highlight the power of high-precision zircon petrochronology for precisely dating key metamorphic reactions.