Abstract Macrocryst assemblages of porphyritic alkaline dikes in the Mantiqueira range (SE Brazil) are mainly composed of clinopyroxene and olivine with different origins. Based on petrographic features, mineral chemistry, and equilibrium relationships with the host liquid, those macrocrysts are classified as xenocrysts, antecrysts, and phenocrysts. Described xenocrysts are mantle olivine, Cr-diopside cores compatible with garnet-bearing mantle facies, green-core clinopyroxene cores compatible with lower crust, and enstatite cores mantled by clinopyroxene, all reported for the first time in this region. Two contrasting types of clinopyroxene antecrysts prevail among the macrocryst cores (both occurring in the same samples and presenting corrosion and sieve textures): primitive colorless diopside and more evolved green-core clinopyroxenes. In the studied rocks, green clinopyroxene zones mantling colorless diopside cores (and vice-versa) are also found. Diopside- and green-cores antecrysts have similar compositions to those from mafic and felsic alkaline melts, respectively. Phenocrysts are mainly related to Ti-augite overgrowths, mantling all other types. Mixing-model curves between mafic and felsic alkaline equilibrium liquids calculated from clinopyroxene antecrysts indicate a hybrid origin for the host matrix. The macrocryst populations of the studied dikes are indicative of a complex plumbing system, recording several processes of an open-system magmatic evolution.

ABSTRACT: Three series of alkaline dikes are characterized in a restricted area of the Mantiqueira Range: a weakly silica-undersaturated series (normative ne < 7), of potassic character, represented by basanite, tephrites, alkali basalts, basaltic trachyandesites, phonotephrites, tephriphonolites, and trachyandesites, an intermediate series (7 > normative ne > 13), of potassic character, consisting only of basanites and tephrites, and a strongly silica-undersaturated series (normative ne > 13), of sodic character, consisting of basanites, tephrites, phonotephrites, and phonolites. Ar-Ar data for biotite from a tephriphonolite dike yielded an average age of 85.4±0.4 Ma. All three series show similar evolution trends controlled mainly by early-fractionation of olivine, spinel and clinopyroxene and, secondly, fractionation of plagioclase and apatite. Different degrees of crustal assimilation are suggested for each series. Assimilation processes are responsible for the wide 87 Sr/ 86 Sr i (0.70447-0.71172) and 143 Nd/ 144 Nd i (0.51237-0.51244) ranges, for the increased K 2 O/Na 2 O, Ba/Sr and Rb/Sr ratios, and for the reduced Ce/Pb ratios. In general, a significant contribution from partially melted host rocks is associated to the weakly silica-undersaturated potassic series. A phlogopite-bearing carbonate-metasomatized heterogeneous peridotite source accounts for the main chemical variations in the least contaminated samples of each series. Geochemical signatures of the strongly sodic alkaline series indicate a greater contribution from carbonated pyroxene-rich veins in the mantle source.