Rcvista Brasilcim dc Gcocien cias Julio Cezar Mendes et al. J 4( I ):79 -X6. marco de DIORITE AND HORNBLENDITE ENCLAVES AT SUMIDOURO, RJ, CENTRAL RIBEIRA BELT JULIO CEZAR MARIA DO CARMa nUSTAMANTEJUNHO & ANDRE GHIZI Abstract Dioriticami homblen ditic enclaves crop out in the southeastern Sumidouro region. Rio de Janeiro State. TIley arc enveloped hy tonaliticgneiss of the Rio Negro Complex. In the main occurrence. at Bardo de Aquino District, a set of rounded mcgacnclavcs areclosely related with the tonalitic gneissgroundmass. Petrographic andgeochcmical aspects (major, trace and rare earth clements) combined with fi eld relations. supporta cumulauc interpretationfor the hornbl enditc.This rock probabl y evolved by differ entiation process from the same parental magma of the diori tes. In turn. the tonalitic gneiss shows distinct petrographic and geochemical signature. sugges tinga differe ntsource . Krvwords : Diorites. Enclaves. Rio Negro Comp lex . Central Ribcira Belt Resume I, NClA VES 1)/: IJIOIiITO E 1/0 Ii NIIU ,N DH V EM SUMmOUIiO. IiJ. ClN'I'UIiA O /I/lII:lliA CENIli AL A sudeste de Sumidouro. rcgi fio scrraua do E. stado do Rio de Janeiro. OCOlTcm enc lave s dio rft icose hornbl cndfticos cnvolvidos por to nal ito- gnaissc do Complcxo Rio Negro. Na principa l ocorrencia cstudada . a do Distrito de Barile de Aquino. mcgacnclnvc s arrcdondado s csuto con ccmradose intimamcntc rclacionndos COIll a matriz tonalite gnaissica. Aspectos pctrograficos e gcoqufm icos (elementos maiores c traces. incluindoterms rams). juntamcntccomas rclacocs de campo. pcnnucm a lntcrprctacno do hornblcnditocomo um cumulado. fonnado prova vclmcnt c a partir do magmaque gcrou as rochas diorft icas. Por sua vcr; 0 tonalito gnaissc aprcscntou diferentcs assin.uurus pctrognificas e gcoqufmicas. 0 que indicalima origemde Ion tc diversa. l'alavras-cliavc: Diontos. Encl aves. Complcxo Rio N egro. Fai xa Ribeiracentral INTR OD UCTI ON Geologicalstudies focusing tonalitic 10dioritie rocks of the Rio Negro Unit (Matos ct al. 1980; Barbosa & Grossi Sad 1985) have been reviewed by'Tupinarnba (1999), who renamed it as Rio Negro Co mplex, This author has interpreted the complex as a Brasiliano pre-collisional magmatic arc ranging in age from 660 (0 600 Mil. This complex is located within the central Ribeira Belt. an important Neopro terozoic (Brasiliano/Pan-African cycle) unit that runs along the southeastern Brazilian coast. The aim of this paper is to present and interpret results from field, petrographicaland geochemical studies carried out in a small area of the Rio Negro Complex, ncar Sumidouro, Rio de Janeiro State. GEOLOG ICAL SETTING The mountain region oft he Rio de Janeiro state consists mainly of roc ks of the following units: Rio Negro Complex (to nali ric to granodioritic orthog neisses, migmati tes) , Serr a dos Orgao s batholith ( gra no dioriti c orthogncisscs). Leucogranit ic gneisses and minor amounts of metasedimentary rocks of the Parafba do Sui Group. Several brasiliano agc igneous rocks, ranging from granite to granodiorite, and rarely gabbros. intrude those units, The Rio Negro Complex was initially described as an association of granitic orthogncisscs and heterogeneous migmatites (Matos ct al . 1980; Barbosa & Grossi Sad 1985). Machado & Demange ( 19 94) con s id er ed the Ri o Ne gr o orth ogneisses as a Palacoproterozoic basement. Recent works have pointed out (he arc related nature of the Rio Negro Complex (Tupi namba 1999. Heilbrun ct at. 2CXX» ,Tupinamb.i(1999)interpretedsuch magmatism as a pre-collisional event which generated Cordilleran-type. calc- alkalic grunodioritic to tonalitic and dior itic rocks. A V-Pb zircon a geof634± 10 Mu was obtained for the typical tonalitic gneiss of this unit (Tupinamba 1999). Banded to homophonous leucogranite gneisses crosscut the Rio Negro rocks, yielded a PblPb age of ca. 600 Ma and were defined as an individual unit of pre to syn-collisional, S-type granites. by Tupinarnba (1999). The Semidos Orgaos batholith comprises a garnet-hornblende- biotite granitic to granodioriticgneiss sur ro unded by the Rio Negro orthogneiss country rocks. It yielded a 5C>0± 4 Ma zircon age (Tupinamba 1999), related to a syn- to late-collisional orogenic eve nt. It has a conspicuous foliation given by mafic aggregates and subordinated diktionic structures, The granitic to granodioritic plutons constitute a large late- to post-collisional granitic province that extends from southwest to northeast Rio de Janeiro State. Gcocbronogicul whole rock Rb-Sr and U-Pb in zircon data indicate ages ranging from 540 MOl t0 480 Ma (Trouw ct al. 2(00), Subordinate diorites in close association with the granitoids have also been reported (Junho 1993. Junho ct at. 1993, Men des et al. 2(02 ). The gabbroic bodies arc lenticular in shape. isotropic or fo liated and calc-alkalic to tho leiitic. They range from hornblende gabbro. pyroxene-hornblende gabbro to hornblenditcs and may have been emplaced durin g or late the 580 Ma. high-grade regional metamorphism (Tupinamba 1999;Junho et al. 1999). TIlE RIO NEGRO MAGMATIC ARC AT SUMIDOURO According to Tupinum ba (1999) . the Sumidouro area consists Dcpro . de Geologia - l(iE() . lJFIU , Cidudc Univcrsitriria. CEP 2194 1 ).900. llha do Fundao. Rio de Jau ciro -Rl . E-mails: julio@ge olog ia. ufrj. br. I11junho@geologia.ul"lj .hr. ghizi@acd.ufrj. br Revista Brasileira de Geoaenctes. Volume 34, 2004 79