Temporal evolution of the Angavo and related shear zones in Gondwana: Constraints from LA-MC-ICP-MS U–Pb zircon ages of granitoids and gneiss from central Madagascar

Abstract

Granitoids of Neoproterozoic age are well exposed in the Precambrian basement rocks of Madagascar, in the vicinity of and within the Neoproterozoic dextral Angavo shear zone (ASZ). Pre-kinematic granitoids, a syn-kinematic granite, and a granitic gneiss have been dated using Laser Ablation Multicollector Inductively Coupled Plasma Mass Spectrometry (LA-MC-ICP-MS). One pre-kinematic granitoid yields a weighted mean 206Pb/ 238U igneous zircon crystallization age of 823.6 ± 6 Ma, and a second a 756.6 ± 9 Ma igneous crystallization age and a 563 ± 11 Ma metamorphic age. A syn-kinematic granite gives a weighted mean 206Pb/ 238U zircon age of 549.8 ± 4 Ma, which points to the time of movement in the ASZ. A granitic gneiss yields a weighted mean 206Pb/ 238U protolith age of 732 ± 13 Ma and records a 558.8 ± 4 Ma metamorphic age. Accordingly, the ASZ was active and associated with the high-temperature metamorphism of the Angavo-Nondiana belt during the Ediacaran. The late Proterozoic ages of igneous intrusions recorded by the granites show that pre-tectonic granites were sheared in the ASZ, and recorded granulite–amphibolite facies metamorphic events. These U–Pb data suggest a link between the ASZ and the Achankovil shear zone in India, which were adjacent in several proposed Gondwana reconstructions.