Tracing remnants of Ediacaran S-type granitoids from beach placers in SE Madagascar

Abstract

Southeast Madagascar hosts several major deposits of beach placers, the provenance of which is little understood. To redress this imbalance, we present new laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) U-Pb ages, and trace elements of zircons and monazites, as well as Hf isotopes of zircons from heavy mineral beach sands at Taolagnaro (Fort Dauphin) in SE Madagascar. The ages of detrital zircon grains range from 650 Ma to 450 Ma and from 1900 Ma to 1700 Ma (one grain has an age of 2633 Ma), and detrital monazite ages range from 574 Ma to 484 Ma. The Ediacaran−Cambrian zircons (650−450 Ma) and monazites (574−484 Ma) record a major Pan-African orogenic event in SE Madagascar. Geologically, the 650−450 Ma ages correlate well with coeval granitic gneisses in southern Madagascar, and a few 1900−1700 Ma ages can be linked to putative Paleoproterozoic protoliths of metasedimentary rocks from the Anosyen Domain and from the Itremo Group of central Madagascar. The zircon mineral chemistry indicates that some of the grains (P>25 μmol g−1) were sourced from S-type granitic rocks. The monazite chemistry points toward a metamorphic provenance from garnet-bearing amphibolite facies rocks, such as migmatitic gneisses in southern Madagascar. The Hf isotopic compositions of detrital zircons indicate reworking of their probable Paleoproterozoic−Archean source rocks. We conclude that the beach sand zircons and monazites were largely sourced from the uplifted and eroded Anosyen and Androyan Domains in SE Madagascar, and with minor contributions from central Madagascar, which were all transported by rivers to the ambient ocean to the southeast.