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GRANITE-TYPES IN NORTHEAST BRAZIL: CURRENT KNOWLEDGE

1986; Sociedade Brasileira de Geologia; Volume: 16; Issue: 1 Linguagem: Inglês

10.25249/0375-7536.19865472

2177-4382

Alcídes N. Sial,

Geochemistry and Geologic Mapping

Several granitoids in Northeast Brazil, including the main granite-tytes, mostly located within the Cachoeirinha-Salgueiro foldbelt (CSF), Pernambuco and Paraíba States, have been studied. Four groups of granitoids have been identified in this foldbelt with syn- to post-orogenic emplacement: a) calc-alkalic, b) potassie-calcalkalic, c) peralkalic, and d) of trondhjemitic affnities. Rock compositions vary from tonalites, granodiorites, quartz-syenites, syenites to minor granites. The calc-alkalic plutons (Conceíção-type), ilmenite-bearing, are strictly found piercing the Cachoeirinha low-grade metamorphics; the potassíc-calc-alkalic granitoids (Itaporanga-type) intruded the gneissmigmatite basement, adjacent to the northern boundary of the CSF while the peralkalic rocks (Tríunfo-type) form a magnetite-bearing syenitoid líne, roughly following the southern boundary of the CSF. or found as small stocks and dike swarms (e.g. Catingueira-type) intruding Cachoeirinha metasediments. Rocks with trondhjemitic affinities (Serrita-type) form the core of the two ring-complexes, next to Serrita, Pernambuco, intruding Salgueiro metasediments. whole-rock δ 18 O values most clearly correlate with types of host rocks. Granitoids in high-grade terranes show normal δ 18 O values (<+ 10 permilSMOW), as in Itaporanga, Serra da Lagoinha, Monte das Gameleiras (Paraíba), and Bodocó (Pernambuco). Conceíçâo-type granitoids dysplay high δ 18 O (+10 to +13 permilSMOW), while those which intruded the Salgueiro metasediments (Serrita-type) show δ 18 O values between 8 and 10 permilSMOW, same happeníng with most peralkalic bodíes, REE patterns for the calc-alkalic plutons are very LREE-enriched relative to HREE, with díscrete, but significative, negatíve Eu anomaly, compatible with combined fractionation of plagioclase and hornblend. The parental magma probably derived from an amphibíolítc source from depht not greather than 45 km.. The potassic-calc-alkalic bodies exhibit LREE-enriched signatures relative to HREE, lacking Eu anomaly. Slight differences between oxygen fugacity, crystallízation order, type, and amount of each fractionated phase during magma ascent have been responsible for the differences among these patterns. The peralkalic bodies (sílica over to satureted) exhibit very fractionated REE sígnatures, with Eu anomaly usually abjent. Over satured bodies show discret, positive Eu anomaly sometimes, while the satureted ones behave the opposíte way. The major geochemistry and REE are in agreement in the four groups and are consistent with the behavíour of the oxygen isotopes, which vary sympathetically with ΣREE and Si0 2 . Magmatic epidote, a high-pressure phase, in three of the groups of granitoids, suggests that these rocks crystallized at relatively great depth what demands further structural and petrological support.