USING U-PB AGES OF DETRITAL ZIRCONS TO SOURCE TEMPER SANDS IN ANCIENT CERAMICS: A CASE STUDY FROM THE SOUTHWEST PACIFIC

By methodology now common for sedimentary tectonics, we use U-Pb ages of detrital zircons to source temper sand in an ancient ceramic assemblage. Most potsherds dating to ~500-0 BCE from intertidal sites in Roviana Lagoon of the New Georgia Group in the Solomon Islands contain local volcanic sand as temper, but the temper in a subordinate subset of sherds is granitic sand for which no local origin is known. Suspicion that the exotic sherds derived from granitic Lizard Island off the Queensland coast is disproved by disparate U-Pb ages of zircons in Roviana sherds and Lizard sands, but a close match in the U-Pb ages of Roviana zircons and zircons in granitic sand from Muyuw (Woodlark) Island in the Solomon Sea ~500 km west of Roviana Lagoon documents the origin of the exotic Roviana ceramics. U-Pb analyses of zircons from six Roviana sherds with the granitic sand temper yield all Miocene ages, dominantly 11-18 Ma. Ages of grains in materials from Lizard Island are significantly older, eliminating this island as a potential origin for the temper in the Roviana sherds. A sample of Lizard Island granitic bedrock has three main age peaks at 135 Ma, 230 Ma, and 300 Ma. Analyses of two sands from the island also yield these age peaks, though most grains in the sands have ages of 125-140 Ma and 300-315 Ma. A third Lizard Island sand contains predominantly 230-235 Ma grains. Ages of grains from two combined sherds from Lizard Island correlate well with the ages of the bedrock and two of the sands, implying that the temper in these sherds is likely derived from material on the island. Unlike the ages of the Lizard Island materials, the ages of grains in a granitic sand sample from Muyuw Island are a very close match with the ages of grains in the Roviana sherds. All grains in the Muyuw sand are 10-16 Ma, overlapping almost exactly with the majority of grains in the Roviana sherds. Given the strong similarity between the detrital zircon signals of the Roviana sherds and the Muyuw sand, it is highly likely that the temper in the Roviana sherds was derived from sands of Muyuw Island. The U-Pb zircon approach for sourcing temper sand should be effective in all cases where the ages of alternate sources for detrital zircon differ or where comparative data for potential temper sands can be obtained.