U-PB DATES FROM MADAGASCAR DETRITAL ZIRCONS CONSTRAIN TECTONIC MODELS FOR GONDWANA ASSEMBLY

Gondwana assembly at the end of the Precambrian is recorded by the crystalline basement rocks exposed in Madagascar, and boundaries between tectonic terranes have been proposed based on metamorphic grade, lithology, and structural style. However, metamorphic grade (amphibolite to granulite) hinders stratigraphic correlation, and also restricts efforts to test whether or not regions separated by shear zones are distinct terranes. In these difficult circumstances, SHRIMP U-Pb analysis of detrital zircons has proven hugely helpful. This approach has allowed us to determine the probable source areas for Paleoproterozoic Itremo Group quartzites, and to identify a previously unrecognised Neoproterozoic sequence, the Molo Group.

Most recently, we have tested whether the Betsileo Shear Zone (which separates the Itremo Group from unnamed quartzites and gneisses of the Antananarivo Block) is a fundamental terrane boundary. We used U-Pb ages from detrital zircons to see whether or not the Antananarivo Block quartzites were related to the Itremo Group. We found that the detrital zircon population spectra of the two sets of quartzites were very similar, with strong matching of both the major and minor populations in terms of age and proportion of grains. We conclude that the unnamed quartzites are structural outliers of the Itremo Group, and that the Betsileo Shear Zone is an intra-block fault, not a terrane boundary.

This approach works only when the pre-metamorphic context of the zircons is unambiguous; but interpreting protolith for upper-amphibolite and granulite-grade gneisses is often problematic. To be sure of getting detrital zircons, we therefore focus on quartzites. A perk of using granulite-grade quartzites for this sort of project is that metamorphic zircon overgrowths on the detrital cores are often thick enough to date using the SHRIMP, so we can retrieve the metamorphic age (556 ± 10 Ma) as well as the provenance signature.