U-Pb zircon geochronology and geochemistry of Neoproterozoic granitoids of the Maevatanana area, Madagascar: implications for Neoproterozoic crustal extension of the Imorona-Itsindro Suite and subsequent lithospheric subduction

摘要

Voluminous Neoproterozoic granitoid sheets of the Imorona-Itsindro Suite are important components of exposed basement in west-central Madagascar. Here, we report precise new zircon U-Pb ages and whole-rock geochemistry for granitoids within the Maevatanana area of Madagascar. The new laser ablation inductively coupled plasma mass spectrometry zircon U-Pb dating undertaken during this study indicates that Antanimbary granitoid and Antasakoamamy granitoid were emplaced at 747 +/- 9Ma and 729 +/- 9-727 +/- 8Ma, respectively. Geochemically, the Antanimbary granitoids show poor Nb, Ta anomalies, pronounced positive Zr anomalies, and are K-rich (K2O/Na2O>1), but the Antasakoamamy granitoids are relatively depleted in Nb, Ta, show slightly negative Zr anomalies, and are Na-rich (Na2O/K2O>1). Both suites contain zircons with strongly negative epsilon Hf(t), indicating participation of much older (Palaeoproterozoic and Archaean) crust. Their geochemical characteristics, along with the use of various discrimination diagrams, reveals that crustal delamination and asthenospheric upwelling resulted in crustal extension of the region before ~747Ma, with subsequent lithospheric subduction and arc magmatism after 729-727Ma.