Geochemical Signatures of Neoproterozoic Granites and Granitoid-gneisses from Angavo Belt, Central Madagascar

Abstract:

The basement rocks of Madagascar record high-grade metamorphism, magmatism, and contractional and extensional structures that accompanied the collision of the eastern and western Gondwana segments of the supercontinent followed by its collapse.

In the eastern central Madagascar, granitoids dominate the landscape and occur in a large area near and within the N-S trending highly strained zone known as the Angavo Shear Zone or Angavo belt. The area is a key in understanding the evolution of basement of Madagascar and the reconstruction of the Gondwana supercontinent. These granitoids range from layered to massive and previously published U-Pb zircon dating yielded three distinctive Neoproterozoic magmatisms at 770 Ma to 820 Ma, ca. 660Ma, and ca 550Ma. However, it was unknown whether these ages represent distinct magmatic pulses or reflect a continuous granitoid emplacement. This work contributes to the knowledge of the Malagasy basement rocks and to explore and discuss the origin and petrotectonic evolution of the granites and granitoid-gneisses from this part of the Madagascar.

The rock samples are placed into three groups: group A and B for granitoid-gneisses and a third group for the granite layers. Group A and B are metaluminous and slightly peraluminous, respectively. All rocks have typical subduction zone calc-alkaline signatures. Group A is characterized by enrichment in large ion lithophile elements (LILE) but low U, LREE enrichment, depletion in the high field strength elements (HFSE). In contrast, Group B has REE patterns closely similar to Archean sediments. The granite layers show fractionated REE patterns in which HREE patterns show strong correlation with Zr abundances. Trends in major element variation diagrams and the enrichment of incompatible elements could be explained by simple fractional crystallization, while the overall geochemical signatures reflect either (1) melting of ancient crust or (2) crustal contamination of a more evolved magmas that are produced by differentiation of mantle-derived magma. Geochemical signatures of studied rocks reveal the contribution of old continental crust and the role of continental arc magmatism in the genesis of both granites and granitoid-gneisses of eastern central Madagascar.