DISTINCTIVE LOW LU SIGNATURES IN A DATABASE OF U-PB DATED ZIRCONS FROM THE WORLD'S MAJOR RIVERS: WHAT DO THEY MEAN?

The advent of LA-ICP-MS technology employing an effectively small volume ablation cell allows routine collection of compositional data for 11 elements (P, Ti Y, Zr, Hf and 6 REE) while U-Pb dating the zircon in the same 60 second analysis. Whereas the age patterns for zircons from sedimentary rocks and river sands are used extensively in the provenance literature, the trace element database is tapped rarely with the exception of Belousova et al. (2002) and co-workers who established a trace element-based classification system to assign a detrital zircon to a rock type. The primary discriminator for zircons in the Belousova scheme built from examining 9 igneous rock types using electron microprobe and LA-ICP-MS data is a Lu content of 20.7 ppm. From our database of zircons from the World’s major rivers, we concur that Lu content is diagnostic. Based on 4575 concordant zircons from 48 rivers, and using a stoichiometric silica content of 32.77 wt% as an internal standard, the mean Lu content is 62 ppm, the median is 51 ppm and there is an obvious outlier tail defined at less than 21±1 ppm based on a cumulative probability plot, in very good agreement with Belousova et al. (2002) who assigned these low Lu zircons to an alkalic igneous source. Defining other trace element features that robustly accompany the low Lu content in our river database plus analyses of zircons from sedimentary rocks is maddeningly difficult. The main overall feature of these low Lu zircons is a generally depressed trace element content.

Low Lu zircons make up 10% of the 4575 zircons but they are not equally distributed through time. We examine two examples with abnormally large low Lu populations: the 1895±20 Ma zircons of Amur-Zeya river sands (Russia-China) and 583±10 Ma zircons of Early Paleozoic sediments and modern river sands from SE Australia and, based on cathodoluminescence, regional geology, the presence of rutile in the same sediments, and its trace element composition, we deduce that the dominant protoliths for low Lu zircon are high-grade metamorphic rocks associated with continent-continent collision.