A NEWLY-RECOGNISED LATE NEOPROTEROZOIC METASEDIMENTARY SEQUENCE IN CENTRAL MADAGASCAR SUGGESTS TERRANE JUXTAPOSITION AT 560±7 MA DURING GONDWANA ASSEMBLY

Two distinct Proterozoic metasedimentary packages (Itremo Group and newly-recognised Molo sequence) are preserved in central Madagascar. The units are lithologically similar and are tectonically interleaved. Consequently, they are distinguishable only on the basis of their detrital zircon populations. Zircon populations in the younger Molo sequence and associated granitoid rocks are significant because they suggest that rocks exposed in central Madagascar represent two distinct crustal blocks, which were deformed and metamorphosed together at 560±7 Ma.

Detrital zircon populations define mutually exclusive age brackets for the two sequences. The Itremo Group, of probable Mesoproterozoic age, was deposited some time after 1722±40 Ma (youngest concordant detrital age) and before 795±8 Ma (age of oldest dated intrusive rocks). The Molo sequence contains detrital grains equivalent in age to the granites that intrude the older Itremo Group, and was therefore deposited in the Neoproterozoic, after 647±29 (youngest concordant detrital age), and before regional metamorphism at 560±7 Ma. In addition to their different ages, the two units have dramatically different provenance signatures. The Itremo Group has five distinct detrital zircon populations between 1875 and 2690 Ma in age (n=271). In contrast, concordant detrital ages in the Molo Sequence range from 741 to 1075 Ma (n=79), indicating a period of uplift and sedimentation post-dating Itremo Group deposition and intrusion.

The Molo sequence detrital grains resolve into four age populations with averages at approximately 640±20 Ma (10% of grains analysed), 830±10 Ma (45%), 950±10 Ma (35%) and 1065±15 Ma (10%). The presence of grains in the 1000-1100 Ma (Kibaran) range is highly significant, because no crystalline rocks of that age have been found in Madagascar although Kibaran-aged rocks are common in most Rodinia fragments. Possible basement to the Molo sequence is represented by associated 800 Ma granitoids that also have Kibaran inheritance, suggesting that these rocks together may represent a fundamentally different terrane than that which underlies the Itremo Group and most of central/northern Madagascar. We interpret the 560±7 Ma metamorphism to record the amalgamation of the two crustal blocks.