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Mafic magmatism in the Bakhuis Granulite Belt (western Suriname): relationship with charnockite magmatism and UHT metamorphism

posted on 05.11.2015, 00:00 by Martijn Klaver, Emond W.F. de Roever, Antonia C.D. Thijssen, Wouter Bleeker, Ulf Söderlund, Kevin Chamberlain, Richard Ernst, Jasper Berndt, Armin Zeh

The Bakhuis Granulite Belt (BGB) is a metamorphic terrain within the Guiana Shield that experienced ultrahigh-temperature (UHT) metamorphism at 2.07–2.05 Ga. In the southwest of the BGB, the Kabalebo charnockites were emplaced at ca. 1.99 Ga and thus postdate UHT metamorphism by at least 60 Myr. Two generations of gabbroic intrusions have been recognized within the BGB, which could act as a heat source for the two UHT events. A younger generation of tholeiitic “Charlie” gabbros yields a baddeleyite U/Pb age of 1971 ± 15 Ma. The presence of a metamorphic overprint indicates that the hornblende-bearing “Moi–Moi” metagabbros predate the Charlie gabbros. Large zircons with complex zoning patterns are found in a Moi–Moi metagabbro sample. The main growth domains of these zircons give an age of 1984 ± 4 Ma, which is indistinguishable from the surrounding charnockites. Matching trace element and Hf isotope characteristics indicate that the complex zircons are derived from the charnockites. We argue that the emplacement of the metagabbros and charnockite magmatism were contemporaneous and that zircon grains from the charnockitic melt were mechanically transferred to the gabbroic bodies during magma mingling. The new ages for the gabbroic bodies in the BGB confirm that they are contemporaneous with, and the likely heat source for, charnockite magmatism, but that they are not associated with the 2.07–2.05 Ga UHT event. Furthermore, the new ages and recognition of the Moi–Moi metagabbros as an Alaskan-type complex provide the first direct evidence for late Transamazonian subduction zone magmatism in the Guiana Shield.