Pietro Brotzu

Petrogenesis of coexisting SiO2-undersaturated to SiO2-oversaturated felsic igneous rocks: The alkaline complex of Itatiaia, southeastern Brazil

Abstract The Itatiaia alkaline complex is a Late Cretaceous intrusion (72 Myr) made up of felsic differentiates, with syenitic rocks dominant throughout and with presence of both nepheline- and quartz-rich varieties. Dykes with phonolitic or trachytic composition cross-cut the coarse-grained facies. The rocks are arranged concentrically, with the core of the complex being formed by SiO 2 -oversaturated syenites (with a small outcrop of granites), and are radially displaced by faults related to regional tectonic lineaments. The minerals show gradual but significant changes in composition (salitic and augitic to aegirine-rich pyroxenes, hastingsite and actinolite to richterite and arfvedsonite amphiboles, sodic plagioclase to orthoclase feldspars and so on) and the whole-rock trends are broadly consistent with fractional crystallization processes dominated by alkali feldspar removal. Sr-isotopic data indicate more radiogenic ratios for the SiO 2 -oversaturated rocks (0.7062-0.7067 against 0.7048-0.7054 for the SiO 2 -undersaturated syenites), consistent with small amounts of crustal input. The favored hypothesis for the petrogenesis of the different syenitic groups is the prolonged differentiation starting from differently SiO 2 -undersaturated mafic parental magmas (potassic alkali basalts to ankaratrites, present in the Late Cretaceous dyke swarms of the area), accompanied by variable crustal contamination prior to the final emplacement. The lack of carbonatite as a significant lithotype, the potassic affinity of the Itatiaia complex, and the relatively high Sr-isotopic ratios match the characteristics of the other complexes of the Rio de Janeiro-Sa˜o Paulo states coastline and confirm the ultimate derivation of these differentiated rocks from an enriched lithospheric mantle source.

Magmatism and fenitization in the Cretaceous potassium-alkaline-carbonatitic complex of Ipanema São Paulo State, Brazil

The Ipanema alkaline-carbonatitic complex is part of the Meso-Cenozoic alkaline magmatism located within the southeastern part of the Brazilian Platform. Drill-core and field sampling have indicated the occurrence of glimmerites, with subordinate shonkinites (mela-syenites), clinopyroxene-bearing glimmerites, diorites and syenites. The glimmerites are cross-cut by lamprophyric dykes and calciocarbonatites. Fenitization has deeply affected the country rocks, originating dioritic and syenitic rocks. The Ipanema rocks show a distinct potassic affinity. The initial Sr-Nd- isotopic composition of the Ipanema rocks (87Sr/86Sr = 0.70661–0.70754 and 143Nd/144Nd = 0.51169–0.51181) is similar to that of tholeiitic and potassium-rich-alkaline rocks of the Eastern Paraguay. Stable isotope data for the Ipanema calciocarbonatite suggest interaction with fluids at temperatures typical of hydrothermal stages, as hypothesized for other carbonatite complexes from southeastern Brazil. The chemical differences between the lamprophyre, glimmerites, carbonatites, apatitites and magnetitites, and the absence of marked REE enrichment in the evolved lithologies, all indicate that fractional crystallization and accumulus of liquidus phases in a magma reservoir, likely coupled with liquid immiscibility processes, may have played an important role in the genesis of the Ipanema rocks.

Petrogenesis of the Late Cretaceous tholeiitic magmatism in the passive margins of northeastern Madagascar

New chemical and Sr-Nd isotopic data on the Late Cretaceous mafic dike swarm intruding the Archean-Proterozoic crystalline basement in the Tamatave-Sainte Marie Island sector (northeast coast passive margin), and on lavas and dikes of the northeastern part of the Mahajanga sedimentary basin (passive margin after the opening of the Jurassic-Cretaceous Somali basin), allow better knowledge of the chemical variations observed in the northern part of the Madagascan igneous province. Two distinct basalt groups have been identified. Group 1 basalts have low light to heavy rare earth element (REE) ratios [(La/Yb) n = 2.2-2.9], low Zr/Y and Nb/Y (4-6 and 0.2-0.4, respectively), low ( 87 Sr/ 86 Sr) 88 (0.7034-0.7042), and high to moderate e Nd (88) (+5.1 to +1.5). Subgroup la comprises basalts with the same light to heavy REE ratios [(La/Yb)n = 2.7-3], Zr/Y and Nb/Y (4.5-5.8 and 0.2-0.3, respectively), and slightly high ( 87 Sr/ 86 Sr) 88 (0.7042-0.7048) at the same e Nd (88) (+5.4 to +4.4) of the group 1 basalts. Group 2 basalts have high light to heavy REE ratios [(LaNb) n = 5.3-7.8], high Zr/Y and Nb/Y (7-11 and 0.5-0.8, respectively), relatively high ( 87 Sr/8 6 Sr) 88 (0.7045-0.7057), and low e Nd (88) (+3.8 to +1). The basalts of the groups 1, la, and 2 cannot be linked by closed-system magma-differentiation processes, and require distinct mantle sources. The major and trace element variations of the Tamatave dikes of the group 1-la are compatible with moderate degrees of crystal fractionation (∼60%) from the least (MgO = 7.3 wt%) to the most evolved compositions (MgO = 4.2 wt%), involving the separation of plagioclase, augite, pigeonite, and minor oxides, perhaps accompanied by crustal contamination or differences in the 87 Sr/ 86 Sr ratios. The mantle sources of the group 1-la basalts seem to be located well within the spinel stability field, whereas a larger contribution of melts derived from garnet-bearing residual mantle is observed in the geochemistry and in the melting models of the group 2 basalts. The chemical and isotopic composition of both rock groups indicate their ultimate provenance from variably enriched lithospheric mantle sources; there is no clear evidence of a hotspot component like that found in the present-day lavas of the Marion-Prince Edward archipelago. The sources of this volcanism seem to be significantly similar to those of the Mahableshwar and Ambenali basalts of the later erupted Deccan Traps, located on formerly contiguous parts of the Gondwana lithosphere.