Geochemistry and isotopic constraints on the origin of the mesoproterozoic Rio Branco ‘anorogenic’ plutonic suite, SW of Amazonian craton, Brazil: high heat flow and crustal extension behind the Santa Helena arc?

Abstract

The Rio Branco plutonic suite (RBS) occurs in the southwestern Amazonian craton, crops out in an area of 1500 km 2, and is emplaced into the ca. 1.79 Ga Alto Jauru terrane (Rio Negro/Juruena geochronological province). The RBS comprises basic (gabbro, diabase, and basalt) and felsic (porphyritic and rapakivi granite) rocks. Hybrid rocks (monzosyenite) with rapakivi-like textures indicate commingling and mixing among the basic and felsic magmas. Silica contents range 45–47% in the basic rocks (metaluminous) and 69–71% in the felsic rocks (slightly peraluminous–metaluminous). Lithogeochemical investigation also indicates higher contents of K 2O, Rb, Zr, and Ba in felsic rocks, comparable with results reported elsewhere for rapakivi granites. Trace element discrimination diagrams indicate that the RBS felsic and basic rocks have within-plate signatures. In addition, the felsic rocks have strongly fractionated REE patterns that show marked negative Eu anomalies, probably due to plagioclase fractionation. The basic rocks are similarly LREE enriched but display flatter patterns, characteristic of weakly fractionated gabbros. Single-grain IDTIMS U–Pb analyses yield an upper intercept age of 1427±10 (MSWD=1.7) for magmatic zircon from a granophyre of the RBS. This age contrasts significantly with an upper intercept age of 1471±8 Ma (with a concordant 207Pb/ 206Pb age of 1471±18 Ma) obtained for zircon from a sample of the basic group. The latter rocks show positive ɛ Nd(1420) ranging from +1.2 to +1.9 ( T DM=1.86−1.82 Ga), which indicates mantle-derivation, whereas the felsic ones yield ɛ Nd(1420) values from +0.2 to −1.0 ( T DM=1.80−1.73 Ga), indicating some older crust in their source. The RBS is interpreted to have formed at 1.47–1.42 Ga from a mixture of mantle source and crustal-derived magma. We propose high heat flow and an extensional environment for the origin of the RBS as a response to the inboard Santa Helena arc (ca. 1.45–1.42 Ga) that developed at the southwestern margin of the Amazonian craton at approximately the same time.