Browsing by Author "Narduzzi, Francesco"

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    Genesis and evolution of a Neoproterozoic magmatic arc: the cordilleran-type granitoids of the Araçuaí Belt, Brazil.
    (Stellenbosch : Stellenbosch University, 2018-12) Narduzzi, Francesco; Stevens, Gary; Farina, Federico; Lana, Cristiano de Carvalho; Nalini Jr., Herminio Arias; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    ENGLISH ABSTRACT: The Araçuaí orogen (SE Brazil) is one of the largest (350,000 km2) and long-lived (ca. 630 – 480 Ma) 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 the continental crust. This study investigates the field, textural, geochemical, geochronological and isotopic evolution of the pre-collisional Galiléia Batholith (ca. 15,000 km2) outcropping in the central 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 is characterized by the widespread occurrence of grossular-rich garnet and epidote. This is a rare mineral 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 white mica crystals (low-Si phengite), are magmatic. There is no difference in bulk rock major and trace element composition between the Galiléia and other garnet-free cordilleran-type granitoids worldwide. The uncommon garnet+epidote parageneses are related to the pressure, temperature and water content conditions of magma crystallization. Comparison with the mineral assemblages and mineral compositions obtained from crystallization experiments from garnet-bearing metaluminous calc-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 km depth). This shows that the Galiléia batholith was assembled in the lower crust during the accretionary/collisional stages of the Neoproterozoic Brasiliano Orogeny. However, the lifetime of deep magma chambers and the duration of magmatic activity in them remains a puzzle, contrary to young upper crustal magmatic systems. Despite being homogeneous with respect to mineralogy/texture and major/trace elements, all samples from the central part of the batholith record extreme variability in U-Pb magmatic ages from ca. 630 to 555 Ma. Trace element geochemistry 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, rather than a simple cooling following fractional crystallization at the level of magma emplacement. We interpret the age variability as the result of a long-lived, uninterrupted injection of compositionally similar magmas in the lower crust during the batholith assembling. These conditions kept the system above its solidus through the 80 Ma of zircon crystallization. Unradiogenic 176Hf/177Hf and 143Nd/144Nd isotopic values of the Galileía samples indicate no direct mixing with mantle-derived magmas. 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 Galiléia granitoids. Thus it is suggested that this giant batholith was assembled in an accretionary prism during the Brasiliano Orogenic stages. Eventually, it is likely that during the Brasiliano/Pan-African orogeny, accretionary prism, fore- and back–arc setting were sites of voluminous silicic magmatism and commonplaces for the stabilization of continental crust and its differentiation.