1 Guyana Geology and Mines Commission, Georgetown, Guyana. E-mail: goldenserge@hotmail.com; jimmy@ggmc.gov.gy; deokumarlachmann@yahoo.com; randy_ault07@yahoo.com 2 State Key Laboratory of Mineral Deposit Research, Nanjing, China. E-mail: nmgchenwei@163.com 3 Geological Survey of Brazil – CPRM-Belém, Brazil. E-mail: telma.faraco@cprm.gov.br 4 Geological Survey of Brazil – CPRM-Rio de Janeiro, Brazil. E-mail: leda.fraga@cprm.gov.br 5 Geological Survey of Brazil – CPRM-Manaus, Brazil. E-mail: nelson.reis@cprm.gov.br 6 Geological Survey of Brazil – CPRM-Boa Vista, Brazil. E-mail: leandro.betiollo@cprm.gov.br *Corresponding author Manuscrito ID 30050. Recebido em: 25/10/2013. Aprovado em: 11/11/2013 ABSTRACT: A Hadean zircon xenocryst with a U-Pb zircon age of 4,219 ± 19 Ma, along with several zircon xenocrysts of Archean age (ca. 2,510 to 3,811 Ma) were found in a rock of the Paleopro- terozoic Iwokrama Formation, which includes felsic volcanics and co-magmatic granitic intrusions. It demonstrates the existence of an underlying “Lost Hadean Crust”, representing the oldest crustal component of the Guiana Shield. Detrital zircons of late Archean age, up to 2,700 Ma, are also present in the high-grade rocks of the Kanuku Complex, located to the south, and may be derived from the same Hadean-Archean crustal block. KeywoRdS: Guyana; Hadean; Archean; zircon; U-Pb. ReSUMo: Um xenocristal de zircão Hadeano com idade U-Pb de 4,219 ± 19 Ma, ao lado de muitos outros xenocristais de idade Arqueana (ca. 2,510 a 3,811 Ma) foi encontrado em rocha da Formação Iwokra- ma, de idade Paleoproterozóica, que inclui vulcânicas félsicas e intrusões graníticas co-magmáticas. Ele demonstra a existência de uma subjacen- te “Crosta Hadeana Perdida” como componente crustal mais antiga do Escudo da Guianas. Zircões detríticos de idade Arqueana tardia, até 2700 Ma, também estão presentes nas rochas de alto-grau do Complexo Kanuku, que ocorre mais a sul, e podem ser derivadas do mesmo bloco crustal Hadeano-Arqueano. Palavras-chave: Guiana; Hadeana; Arqueana; zircão; U-Pb. Guyana: the Lost Hadean crust of South America? Guiana: a crosta Hadeana perdida da América do Sul? Serge Nadeau 1 *, wei Chen 2 , Jimmy Reece 1 , deokumar Lachhman 1 , Randy Ault 1 , Maria Telma Lins Faraco 3 , Lêda Maria Fraga 4 , Nelson Joaquim Reis 5 , Léandro Menezes Betiollo 6 doI: 10.5327/Z2317-48892013000400002 FAST TRACK COMMUNICATIONS INTRodUCTIoN During a joint geological and geodiversity mapping program performed by the geological surveys of Guyana and Brazil, ca. 95 rocks were collected in Guyana for petro- graphic, chemical and age dating studies along their common border, but these data will be presented elsewhere. Of these samples, 18 rocks taken from Southern Guyana were dated by Laser ICP-MS, and about 450 zircon U-Pb age determi- nations were made and are summarized here. GeoLoGy Guyana is part of the Guiana Shield that includes also rocks of Suriname and French Guiana and some areas of Colombia, Venezuela and Brazil (Gibbs & Barron 1993). Te oldest U-Pb ages of rocks in Guyana (ca. 2,000 – 2,245 Ma) occur in green- stone belts of the Barama-Mazaruni Supergroup in the north- western part of the country (area 1 of Fig.1 inset). In Southern Guyana, a limited number of rocks of the Iwokrama Formation and Southern Guyana Granite Complex have been dated more Brazilian Journal of Geology, 43(4): 601-606, December 2013 601