Anorthosites, granulites and the supercontinent cycle

The pattern of occurrence of the massif-type anorthosites of the Proterozoic on a Rodinia reconstruction suggests that the geneses of the anorthosites, the associated granulites, and their chief repository- the Grenville age mobile belt may be inter-related. This relationship has been examined within the broad geodynamic and spatial-temporal framework of the Proterozoic supercontinent. All over the Grenville age mobile belt, approximately within 1500 - 1000 Ma, two successive, back-to-back episodes can be recognized. A continent-continent collisional episode with massive sedimentation, great crustal shortening and thickening with large folds and nappes, metamorphism and calcalkaline magmatism, and accretion of juvenile crust was followed within little over a hundred million years or so by an extensional episode, beginning with large scale mantle-derived basic magma invasion, ponding and differentiation at the base of the thickened orogen, anorthosite diapirism and pervasive thermal overprinting of the lower and middle crust producing granulite belts. Cooling, unroofing and erosion of the orogen coincided with the later stages of the extension, which ended with splitting of the supercontinent. We have argued that the first episode marks the phase of closing in and amalgamation of the converging continental lithospheric plates and the second episode represents the phase of reversal, rifting, plate separation and drifting away of the post-split continental blocks. We believe, the supercontinental closing and opening cycles provide (a) realistic force fields and appropriate spatial-temporal and thermal-tectonic framework for the origin of the Proterozoic massif-type anorthosites and the associated granulites and (b) an explanation for the spatially overlapping but apparently conflicting evidence of both collisional and extensional tectonic signatures in the Grenville age orogenic belts.