M. S. Principato

Petrography and mineral chemistry of carbonatites and mica-rich rocks from the Araxá complex (Alto Paranaíba Province, Brazil)

The Araxa complex (16 km 2 ) comprises carbonatites forming a central core and a complex network of concentric and radial dykes as well as small veins; additionally, it includes mica-rich rocks, phoscorites and lamprophyres. Fenites also occur and are represented by Proterozoic quartzites and schists of the Araxa Group. The petrographic study of 130 borehole samples indicates that the complex is basically made up by two rock-types, carbonatites and mica-rich rocks, and subordinately by a third unit of hybrid composition. Carbonatites range chemically in composition, the most abundant type being magnesiocarbonatites. Dolomite and calcite correspond to the chief constituents, but other carbonate phases, including the Ce-group RE minerals, are also recognized. Phosphates and oxides are widespread accessories whereas silicate minerals consist of olivine, clinopyroxene, mica and amphibole. Mica-rich rocks are represented by abundant glimmeritic rocks and scarce cumulitic phlogopite-, olivine- and diopside-bearing pyroxenites. Hybrid rocks mainly contain phlogopite and tetraferriphlogopite as cumulus and intercumulus phases, respectively; carbonate minerals may also be found. Chemical data indicate that the carbonatites are strongly enriched in REE and have lower contents of Nb, Zr, V, Cr, Ni and Rb compared to the mica-rich rocks. The higher K, Nb and Zr contents of the latter rocks are believed to be related to metasomatic processes (glimmeritization) of the pyroxenites. Similar REE patterns for carbonatites and mica-rich rocks seem to suggest that they are related to a single

Late Pleistocene–Holocene planktonic assemblages in three box-cores from the Mediterranean Ridge area (west–southwest of Crete): palaeoecological and palaeoceanographic reconstruction of sapropel S1 interval

Planktonic foraminiferal and calcareous nannofossil assemblages of three box-cores from the western–central part of the Mediterranean Ridge area were investigated and correlated. In particular we focused on the most significant faunal and floral signals recorded before, during and after sapropel S1 deposition. The interval preceding S1 is characterised by an increasing trend of the two planktonic foraminiferal species, Globorotalia inflata and Truncorotalia truncatulinoides, that are usually related to a well developed cold and deep mixed layer. This change is supported also by the rarity of warm water coccolithophorid species and by the presence of the deep dwelling species Florisphaera profunda. The beginning of S1 is marked by the disappearance of G. inflata and T. truncatulinoides and by the significant increase in abundance of Globigerinoides ruber, especially the rosea variety usually considered indicative of warmer conditions. Warm water species increase also within the coccolithophorid assemblage, while the upwelling species Reticulofenestra spp. suddenly decreases and remains low across the whole sapropel interval. The sedimentary expression of the end of anoxia is the upper boundary of the oxidised level observed above S1 in the three box-cores. Across this boundary we detect a small but well defined increase of Braarudosphaera bigelowii, the decrease of warm water coccolithophorid species and the increase of Reticulofenestra spp. This boundary is marked also by the reoccurrence of the planktonic foraminiferal species G. inflata and Neogloboquadrina pachyderma dextral and by an evident decrease of G. ruber var. rosea, indicating the end of the Holocene Climatic Optimum and the beginning of a cold and wet phase, probably corresponding to the Atlantic–Subboreal continental transition.