PETROGENESIS OF THE PALEOPROTEROZOIC BASALT-ANDESITE-RHYOLITE DYKE ASSOCIATION IN THE CARAJAS REGION, AMAZONIAN CRATON

Paleoproterozoic basaltic, andesitic and rhyolitic dykes crosscut the Archaean Carajas basement. Basalts are distinguished into a high and a low TiO2 group (HTi and LTi), each group consisting of geochemically distinct NE- and NW-trending swarms. The HTi dykes are evolved transit ional basalts having essentially EMORB-type geochemistry. The LTi basa lts are tholeiites (NE-trending swarm) and high-Al basalts (MY-trendin g swarm) displaying incompatible trace elements patterns with variably negative Nb anomaly, enrichment in Rb, Ba, K (LILE) and La, Ce and Nd (LREE) and positive Sr anomaly. With respect to orogenic analogues, a ndesites have lower Al2O3, CaO and Ni, higher FeO, LILE, LREE, Nb, Zr and Ti and negative Sr anomaly. Rhyolites have geochemical characteris tics comparable with those of A-type granites. At 1.8 Ga, Sr-87/Sr-86 ranges from 0.700 to 0.705 in the HTi basalts and from 0.700 to 0.704 in the LTi group. Andesites define an isochron of 1874 +/- 110 Ma (Sr- o = 0.7038 +/- 0.0010). Rhyolites from Southern and Northern Carajas d efine two isochrons of 1802 +/- 130 Ma (Sr-o = 0.7062 +/- 0.0046) and 1535 +/- 82 Ga (Sr-o = 0.7625) respectively, the younger date being in terpreted as resetting of the Rb-Sr isotopic system. We propose a petr ogenetic model relating LTi basalts with melting of lithospheric mantl e metasomatized by acid melts derived from incipient melting of eclogi tes, representing in turn the subsolidus product of basaltic batches t rapped in the mantle. The HTi basalts are explained by melting of the lithospheric mantle containing the complementary residual eclogite, An desite petrogenesis is consistent with crystal fractionation from a hi gh-Mg andesite parent derived from a mantle source more extensively me tasomatized by eclogite-derived melts. Rhyolite composition is consist ent with low melting degree of the basement rocks. The basalt-andesite -rhyolite dykes may represent the effects of crustal extension and arc hing in Carajas, which produced the anorogenic acid to intermediate ma gmatism (Uatuma group) and affecting a large part of the Amazon craton between 1.85 and 1.7 Ga. (C) 1998 Elsevier Science B.V. All rights re served.