Genesis and evolution of a Neoproterozoic magmatic arc: the cordilleran-type granitoids of the Araçuaí Belt, Brazil.

[摘要] ENGLISH ABSTRACT: The Araçuaí orogen (SE Brazil) is one of the largest (350,000 km2) and long-lived (ca. 630 – 480Ma) granitic province in the world. Its wide variety of granitoids recording mid- to lower crustal P -T conditions allow direct investigation of petrological processes occurring in the deepest part of thecontinental crust. This study investigates the field, textural, geochemical, geochronological andisotopic evolution of the pre-collisional Galiléia Batholith (ca. 15,000 km2) outcropping in thecentral part of the Araçuaí orogen. This weakly foliated Neoproterozoic (ca. 632–550 Ma),metaluminous to slightly metaluminous (ASI = 0.97–1.07), calc-alkaline granitoid body ischaracterized by the widespread occurrence of grossular-rich garnet and epidote. This is a raremineral association in Cordilleran-I-type granitoids and of special petrogenetic significance. Field,petrographic, and mineral chemistry evidence indicate that garnet, epidote, biotite as well as whitemica crystals (low-Si phengite), are magmatic. There is no difference in bulk rock major and traceelement composition between the Galiléia and other garnet-free cordilleran-type granitoidsworldwide. The uncommon garnet+epidote parageneses are related to the pressure, temperature andwater content conditions of magma crystallization. Comparison with the mineral assemblages andmineral compositions obtained from crystallization experiments from garnet-bearing metaluminouscalc-alkaline magmas, indicates that the supersolidus coexistence of grossular-rich garnet, epidote and white mica is consistent with crystallization at pressures greater than 0.8 GPa (above 25 kmdepth). This shows that the Galiléia batholith was assembled in the lower crust during theaccretionary/collisional stages of the Neoproterozoic Brasiliano Orogeny. However, the lifetime ofdeep magma chambers and the duration of magmatic activity in them remains a puzzle, contrary toyoung upper crustal magmatic systems. Despite being homogeneous with respect tomineralogy/texture and major/trace elements, all samples from the central part of the batholithrecord extreme variability in U-Pb magmatic ages from ca. 630 to 555 Ma. Trace elementgeochemistry and Hf isotopes from the igneous zircons – here interpreted as autocrysts (ca. 555 –560 Ma) and antecrysts (> 560 Ma) – are all consistent with an open-system crystallization, ratherthan a simple cooling following fractional crystallization at the level of magma emplacement. Weinterpret the age variability as the result of a long-lived, uninterrupted injection of compositionallysimilar magmas in the lower crust during the batholith assembling. These conditions kept thesystem above its solidus through the 80 Ma of zircon crystallization. Unradiogenic 176Hf/177Hf and143Nd/144Nd isotopic values of the Galileía samples indicate no direct mixing with mantle-derivedmagmas. This explains the scarcity of mafic products in the region. Mineral textural,geochronological and isotopic similarities with other younger and older granitic plutons constructed within accretionary / fore-arc settings, better explain the characteristics showed by the Galiléiagranitoids. Thus it is suggested that this giant batholith was assembled in an accretionary prismduring the Brasiliano Orogenic stages. Eventually, it is likely that during the Brasiliano/Pan-Africanorogeny, accretionary prism, fore- and back–arc setting were sites of voluminous silicic magmatismand commonplaces for the stabilization of continental crust and its differentiation.

[发布机构] Stellenbosch University