Structural and tectonothermal evolution of the ultrahigh-temperature Bakhuis Granulite Belt, Guiana Shield, Surinam: Palaeoproterozoic to recent

Abstract

We constrain the multistage tectonic evolution of the Palaeoproterozoic UHT metamorphic (P ​= ​0.9–1.0 ​GPa, T >1000 ​°C, t ​= ​2088–2031 ​Ma) Bakhuis Granulite Belt (BGB) in Surinam on the Guiana Shield, using large- to small-scale structures, Al-in-hornblende thermobarometry and published fluid inclusion and zircon geochronological data. The BGB forms a narrow, NE–SW striking belt between two formerly connected, ~E–W oriented granite-greenstone belts, formed between converging Amazonian and West African continental masses prior to collision and Transamazonian orogeny. Inherited detrital zircon in BGB metasediments conforms agewise to Birimian zircon of West Africa and suggests derivation from the subsequently subducted African passive margin. Ultrahigh-temperature metamorphism may have followed slab break-off and asthenospheric heat advection. Peak metamorphic structures result from layer-parallel shearing and folding, reflecting initial transtensional exhumation of the subducted African margin after slab break-off. A second HT event involves intrusion, at ca. 0.49 ​GPa, of charnockites and metagabbros at 1993–1984 ​Ma and a layered anorthosite at 1980 ​Ma, after the BGB had already cooled to <400 ​°C. The event is related to northward subduction under the greenstone belts, along a new active margin to their south. A pronounced syntaxial bend in the new margin points northward towards the BGB and is likely the result of indentation by an anticlinorial flexural bulge of the subducting plate. Tearing of the subducting oceanic plate along this bulge explains why the charnockites are restricted to the BGB. The BGB subsequently experienced doming under an extensional detachment exposed in its southwestern border zone. Exhumation was focused in the BGB as a result of the flexural bulge in the subducting plate and localised heating of the overriding plate by charnockite magmatism. The present, straight NE–SW long-side boundaries of the BGB are superimposed mylonite zones, overprinted by pseudotachylites, previously dated at ca. 1200 Ma and 950 ​Ma, respectively. The 1200 ​Ma mylonites reflect transpressional popping-up of the BGB, caused by EW-directed intraplate principal compressive stresses from Grenvillian collision preserved under the eastern Andes. Further exhumation of the BGB involved the 950 ​Ma pseudotachylite decorated faulting, and Phanerozoic faulting along reactivated Meso- and Neoproterozoic lineaments.