OXYGEN ISOTOPES OF IGNEOUS ZIRCONS AND THE CRUSTAL EVOLUTION OF THE ORIENTAL BORBOREMA PROVINCE, NE BRAZIL

O-isotope compositions of zircon from Neoproterozoic granitoids, Borborema province, NE Brazil, have been used for testing the hypothesis of terrane accretion. The São Rafael pluton (Seridó domain – SD) has an avg. δ¹⁸O value of 5.98 ± 0.16‰ (1 std dev; n = 10). In the Transverse Zone domain (TZD) high-K calc-alkalic granitoids from the Cachoeirinha belt have avg. δ¹⁸O of 9.29 ± 0.34‰ (n=10), while calc-alkalic granodiorites have avg. value of 10.01 ± 0.2‰ (n= 11). High-K calc-alkalic granitoids from the Alto Pajeú belt present slightly lower δ¹⁸O values, averaging 9.13 ± 0.09 (n = 8). Plutons of the Alto Moxotó (AM) belt, however, show lower δ¹⁸O values: a high-K calc-alkalic granodiorite has avg. value of 8.30 ± 0.12 (n = 3), while a shoshonitic pluton shows avg. value of 7.38 ± 0.18‰ (n = 5). Values of δ¹⁸O for a peraluminous granitoid in the Pernambuco-Alagoas domain (PAD) are intermediate between those for the other domains, averaging 6.88 ± 0.13 (n=2). Plutons with the highest δ¹⁸O values present less negativeεNd values and youngest tDM (0.6Ga) ages, implying important differences in magma source, and are compatible with higher supracrustal input with time. The São Rafael pluton (SD) shows negative εNd(0.6Ga) values (avg. -19.6) and tDM ages of ca. 2.8 Ga, and coupled with lower δ¹⁸O values indicate that magma formed by remelting of Paleoproterozoic lower continental crust. Plutons in the TZD have younger tDM ages (avg. 1.4 Ga), and εNd ~ -2, compatible with a significant high δ¹⁸O component from altered ocean crust, with a mixture of Neoproterozoic mantle-derived material with magma derived from melting of Paleoproterozoic high δ¹⁸O continental crust. However, plutons of the AM belt present more negative and εNd (avg -12.4) and older tDM (1.89 Ga). The studied pluton of the PAD exhibits variable eNd values (-4.3 to -17), and tDM (1.5 to 2.0 Ga), attesting to substantial supracrustal input to the source (Paleoproterozoic lower crust with input of buried supracrustal high δ¹⁸O rocks).

These data show that the three domains have diverse crustal evolution histories; data do not favor assembly of terranes in the Transverse Zone domain. On-going radiogenic and O-isotope analysis of a larger number of granitic plutons from different belts will shed light on the crustal evolution and anatomy of the domains and belts of this province.