Sm-Nd isotopic compositions as a proxy for magmatic processes during the Neoproterozoic of the southern Brazilian shield

摘要

Combined analyses of Nd isotopes from a wide range of Neoarchaean-Cretaceous igneous rocks provides a proxy to study magmatic processes and the evolution of the lithosphere. The main igneous associations include the Neoproterozoic granitoids from the southern Brazilian shield, which were formed during two tectonothermal events of the Brasiliano cycle: the Sao Gabriel accretionary orogeny (900-700 Ma) and the Dom Feliciano collisional orogeny (660-550 Ma). Rocks related to the formation of the Sao Gabriel arc (900-700 Ma) mainly have a depleted juvenile signature. For the Neoproterozoic collisional event, the petrogenetic discussion focuses on two old crustal segments and three types of mantle components. However, no depleted juvenile material was involved in the formation of the Dom Feliciano collisional belt (800-550 Ma), which implies an ensialic environment for the Dom Feliciano orogeny. In the western Neoproterozoic foreland, records of a Neoarchaean lower crust predominate, whereas a Paleoproterozoic crust does in the eastern Dom Feliciano belt. The western foreland includes two amalgamated geotectonic domains, the Sao Gabriel arc and Taquarembo block. In the collisional belt, the old crust was intensely reworked during the Sao Gabriel event. In addition to the Neoproterozoic subduction-processed subcontinental lithosphere (Sao Gariel arc), we recognize two old enriched mantle components, which also are identified in the Paleoproterozoic intraplute tholeiites from Uruguay and the Cretaceous potassic suites from eastern Paraguay. One end member displays the prominent influence of Trans-Amazonian (2.3-2.0 Ga) or older subduction events, whereas the other can be interpreted as a reenrichment of the first during the latest Trans-Amazonian collisional or younger events. This reenriched mantle is documented in late Neoproterozoic suites from the western foreland (605-550 Ma) and younger suites from the eastern collisional belt (600-580 Ma). The other enriched mantle component with an old subduction signature, however, appears only in older rocks of the collisional belt (800-600 Ma). The participation of the subduction-related Brasiliano mantle as an end member of binary mixing occurred in some early Neoproterozoic suites (605-580 Ma) from the western foreland, but the contribution of the Neoarchaean lower crust increased near the late igneous event (575-550 Ma).