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 transitional basalts having essentially EMORB-type geochemistry. The LTi basalts are tholeiites (NE-trending swarm) and high-Al basalts (MY-trending 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, andesites have lower Al2O3, CaO and Ni, higher FeO, LILE, LREE, Nb, Zr and Ti and negative Sr anomaly. Rhyolites have geochemical characteristics 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 define 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 interpreted as resetting of the Rb-Sr isotopic system. We propose a petrogenetic model relating LTi basalts with melting of lithospheric mantle metasomatized by acid melts derived from incipient melting of eclogites, representing in turn the subsolidus product of basaltic batches trapped in the mantle. The HTi basalts are explained by melting of the lithospheric mantle containing the complementary residual eclogite, Andesite petrogenesis is consistent with crystal fractionation from a high-Mg andesite parent derived from a mantle source more extensively metasomatized by eclogite-derived melts. Rhyolite composition is consistent with low melting degree of the basement rocks. The basalt-andesite-rhyolite dykes may represent the effects of crustal extension and arching in Carajas, which produced the anorogenic acid to intermediate magmatism (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 reserved.