DOI: 10.5327/Z2317-48892013000400002 FAST TRACK COMMUNICATIONS Guyana: the Lost Hadean crust of South America? Guiana: a crosta Hadeana perdida da América do Sul? Serge Nadeau1*, Wei Chen2, Jimmy Reece1, Deokumar Lachhman1, Randy Ault1, Maria Telma Lins Faraco3, Lêda Maria Fraga4, Nelson Joaquim Reis5, Léandro Menezes Betiollo6 ABSTRACT: A Hadean zircon xenocryst with a U-Pb zircon age RESUMO: Um xenocristal de zircão Hadeano com idade U-Pb de of 4,219 ± 19 Ma, along with several zircon xenocrysts of Archean 4,219 ± 19 Ma, ao lado de muitos outros xenocristais de idade Arqueana age (ca. 2,510 to 3,811 Ma) were found in a rock of the Paleopro- (ca. 2,510 a 3,811 Ma) foi encontrado em rocha da Formação Iwokra- terozoic Iwokrama Formation, which includes felsic volcanics and ma, de idade Paleoproterozóica, que inclui vulcânicas félsicas e intrusões co-magmatic granitic intrusions. It demonstrates the existence of graníticas co-magmáticas. Ele demonstra a existência de uma subjacen- an underlying “Lost Hadean Crust”, representing the oldest crustal te “Crosta Hadeana Perdida” como componente crustal mais antiga do component of the Guiana Shield. Detrital zircons of late Archean Escudo da Guianas. Zircões detríticos de idade Arqueana tardia, até age, up to 2,700 Ma, are also present in the high-grade rocks of the 2700 Ma, também estão presentes nas rochas de alto-grau do Complexo Kanuku Complex, located to the south, and may be derived from Kanuku, que ocorre mais a sul, e podem ser derivadas do mesmo bloco the same Hadean-Archean crustal block. crustal Hadeano-Arqueano. KEYWORDS: Guyana; Hadean; Archean; zircon; U-Pb. Palavras-chave: Guiana; Hadeana; Arqueana; zircão; U-Pb. INTRODUCTION GEOLOGY During a joint geological and geodiversity mapping Guyana is part of the Guiana Shield that includes also rocks program performed by the geological surveys of Guyana of Suriname and French Guiana and some areas of Colombia, and Brazil, ca. 95 rocks were collected in Guyana for petro- Venezuela and Brazil (Gibbs & Barron 1993). The oldest U-Pb graphic, chemical and age dating studies along their common ages of rocks in Guyana (ca. 2,000 – 2,245 Ma) occur in green- border, but these data will be presented elsewhere. Of these stone belts of the Barama-Mazaruni Supergroup in the north- samples, 18 rocks taken from Southern Guyana were dated western part of the country (area 1 of Fig.1 inset). In Southern by Laser ICP-MS, and about 450 zircon U-Pb age determi- Guyana, a limited number of rocks of the Iwokrama Formation nations were made and are summarized here. and Southern Guyana Granite Complex have been dated more 1Guyana Geology and Mines Commission, Georgetown, Guyana. E-mail: [email protected]; [email protected]; [email protected]; [email protected] 2State Key Laboratory of Mineral Deposit Research, Nanjing, China. E-mail: [email protected] 3Geological Survey of Brazil – CPRM-Belém, Brazil. E-mail: [email protected] 4Geological Survey of Brazil – CPRM-Rio de Janeiro, Brazil. E-mail: [email protected] 5Geological Survey of Brazil – CPRM-Manaus, Brazil. E-mail: [email protected] 6Geological Survey of Brazil – CPRM-Boa Vista, Brazil. E-mail: [email protected] *Corresponding author Manuscrito ID 30050. Recebido em: 25/10/2013. Aprovado em: 11/11/2013 601 Brazilian Journal of Geology, 43(4): 601-606, December 2013 Guyana: the Lost Hadean crust of South America? than 30 years ago by the Rb-Sr and K-Ar methods. A Rb-Sr epoxy, and then polished for subsequent cathodolumines- age of 2,052 ± 50 Ma and a Nd model age of 2,201 Ma were cence observation at the State Key Laboratory for Mineral obtained for two granulite rocks of the Kanuku Complex Deposit Research, Nanjing, China. U−Th−Pb isotopes of (Gibbs & Barron 1993). An old K-Ar age of 2,392 ± 47 Ma zircon were analysed using an Agilent 7500 ICP−MS, cou- (previously given at 2,595 ± 125 Ma) was also obtained in the pled with a 213 nm wave−length laser microprobe also at the Makarapan Granite Intrusion of southern Guyana (Berrangé same laboratory. Detailed analytical procedures, precision 1977) but this K-Ar age has large uncertainty and needs fur- and accuracy are described in Xu et al. (2012). For quality ther confirmation by U-Pb dating. control, more than 135 U-Pb age determinations of zircon The main geological and geochronological units in grains of standard GEMOC GJ-1 yielded a weighted average Guyana are shown in Fig.1 inset, as well as the location of value of 608.3 ± 5.2 Ma (1 std., MSWD 0.79) which agrees the samples collected for age dating. Detailed geological with published results (609.7 ±1.8 Ma: Elhlou et al. 2007). descriptions of the main stratigraphic units of Guyana can be found in Gibbs and Barron (1993). RESULTS MeThODOlOGY The dated rocks belong to the three main stratigraphic units present in Southern Guyana (Fig. 1). Histograms of Zircons were separated by using conventional magnetic zircon ages for each stratigraphic unit are shown in Fig. 2. and heavy liquid separation methods, and then handpicked ■ The rocks of the Iwokrama Formation display the larg- under a binocular microscope. Grains were mounted in est range of ages due to zircon xenocrysts. These rocks 60ºW 58ºW 60ºW VENEZUELA Atlantic Ocean 8ºN 8ºN 4ºN 0 50 100 Kilometers GEORGETOWN 6ºN 6ºN 0 10 20 Kilometers SURINAME 4ºN 4ºN BRAZIL 2ºN 2ºN 60ºW 58ºW GUYANA GEOLOGY LEGEND PALEOPROTEROZOIC (RHYACIAN) 1 BARAMA-MAZARUNI SUPERGROUP: GREENSTONE BELT PALEOPROTEROZOIC (OROSIRIAN) 2 BARTICA ASSEMBLAGE 3 KANUKU COMPLEX: GNEISS, GRANULITE AND CHARNOCKITE 4 IWOKRAMA AND KUYUWINI FORMATION: FELSIC VOLCANIC AND SUBVOLCANIC GRANITE 5 MURUWA FORMATION: SANDSTONE AND MUDSTONE 6 YOUNGER GRANITE - SOUTHERN GUYANA GRANITE COMPLEX 7 RORAIMA SUPERGROUP: SANDSTONE, CONGLOMERATE, TUFF + AVANAVERO SUITE (MAFIC SILLS AND DYKES) AGE DATING SAMPLES MESOPROTEROZOIC (STENIAN) IWOKRAMA FORMATION 8 MURI MOUNTAIN SUITE: NEPHELINE SYENITE AND INFERRED CARBONATITE MESOZOIC (JURASSIC-CRETACEOUS) KANUKU COMPLEX 9 TAKUTU GRABEN: REWA GROUP - SANDSTONE, SILTSTONE, CARBONATE AND APOTERI VOLCANICS SOUTHERN GUYANA GRANITE COMPLEX CENOZOIC (NEOGENE, QUATERNARY) SAND, LATERITIC SOILS, LATERITE, BAUXITE Figure 1. Location of samples collected for age dating in the Iwokrama Formation (black, dots), the Kanuku Complex (black, diamond shaped) and the Southern Guyana Granite Complex (black, squares).The inset shows a simplified geological map of Guyana including the main geological and geochronological units and their respective age periods (modified from Heesterman & Nadeau, 2010). 602 Brazilian Journal of Geology, 43(4): 601-606, December 2013 Serge Nadeau et al. cluster at ca. 2,196 – 2,202 Ma, ca. 2,395 – 2,489 Ma, xenocrysts clustering at ca. 2,086 – 2,138 Ma and ca. ca. 2,852 – 2,949 Ma, ca. 3,701 – 3,778 Ma and 4,219 ± 2,261 – 2,297 Ma. 19 Ma (Fig. 2). Th e oldest value is from a zircon core with a concordant Hadean age 4,219 ± 19 Ma (1 std., 100% concordant). A second zircon core age determina- DISCUSSION AND CONCLUSIONS tion from the same crystal yielded a slightly discordant age of 4,210 Ma ± 19 (1std.). Th e rim area of the same The new U-Pb zircon ages in rocks from Southern zircon gave a discordant age of 3,733 ± 23 Ma. Th e crys- Guyana have implications in the understanding of the geo- tallization age of the youngest zircon populations from logical evolution of the Guiana Shield. the Iwokrama Formation (felsic volcanics and associated granite), that are > 95% concordant, range from 1,980 Southern Guyana Granite Complex to 1,991 Ma. Th ese ages are overlapping with the U-Pb Th e ages ranging between 1,925 and 1,984 Ma deter- zircon ages of 1,977 ± 8 Ma and 1,984 ± 7 Ma mea- mined for the Southern Guyana Granite Complex suggest that sured by ion probe in two rocks of the Surumu Group these rocks are post-Transamazonian (Main Transamazonian in Brazil, considered to be correlative with the Iwokrama orogeny 2.08 – 2.26 Ma, De Avelar et al. 2003) and are Formation (Reis et al. 2000). not related to the group of Younger Granite intrusions of ■ The metamorphic age of zircons from the Kanuku Transamazonian age that are present in northern Guyana Complex is obtained from one S-type granite dated as was previously concluded (Berrangé 1977: Gibbs & at 1,956 ± 10 Ma and from paragneiss, migmatite Barron 1993). Some of the granitic intrusions of this com- and granulite samples ranging from 1,956 ± 12 Ma to plex could be correlated with rocks of the Rio Urubu Suite 1,979 ± 14 Ma. Moreover, several paragneiss samples dated between 1,930 – 1,950 Ma and some intrusions from contain signifi cant proportions of detrital zircons with the Martins Pereira Suite in Brazil (Fraga et al. 1999 and older age values clustering at ca. 2,200 – 2,269 Ma, ca. references therein). 2,450 – 2,520 Ma and ca. 2,635 – 2,721 Ma. ■ Th e zircons of the granitic rocks of the Southern Guyana Kanuku Complex Granite Complex yielded ages between ca. 1,925 and Th e metamorphic ages between 1,956 and 1,979 Ma obtained 1,984 Ma. Th ese rocks contain only a few older zircon in rocks of the Kanuku Complex overlap with the age range SOUTHERN GUYANA GRANITE COMPLEX (n = 102) KANUKU COMPLEX (n = 192) IWOKRAMA FORMATION (n = 153) 90 80 70 60 HADEAN 50 4,218.9 ± 19 Ma Frequency 40 ARCHEAN 30 IWOKRAMA FORMATION (n = 153) 20 KANUKU COMPLEX (n = 192) 10 SOUTHERN GUYANA GRANITE COMPLEX (n = 102) 0 1500 1600 1700 1800 1900 2000 2100 2200 2300 2400 2500 2600 2700 2800 2900 3000 3100 3200 3300 3400 3500 3600 3700 3800 3900 4000 4100 4200 4300 4400 4500 PROTEROZOIC 207Pb/206Pb Age (Million years) Figure 2.