Widespread chemical weathering of Tertiary alkaline volcanic rocks in the vicinity of Trabzon (NE Turkey) has led to the formation of well developed reddish-brown saprolite. These saprolites are dominated by kaolin minerals (predominantly kaolinite), with minor quantities of halloysite, pyrophyllite, chlorite, smectite, hematite and illite. Other common minerals are opal-CT, quartz and occasional feldspar, clinopyroxene, hematite and Fe-Mn-Ti oxides. The significant degree of fracturing and fragmentation of primary igneous minerals within the pyroclastic units, and subsequent penetration by reactive fluids, resulted in partial chloritization of clinopyroxene, albitization of plagioclase and precipitation of Fe-Mn-Ti-oxides within fractures and dissolution voids. Kaolin minerals occur as spongy fibrous meshes surrounding partly dissolved volcanic glass and devitrified sub-rounded grains that may be relict allophane. Field observations and mineralogical-chemical characteristics indicate that the alteration of the Tertiary volcanic units of the Black Sea region was controlled by chemical weathering during circulation of oxic meteoric or mixed meteoric and marine waters. Intense fracturing and fragmentation of the pyroclastic rocks resulted in the formation of clay minerals in the upper saprolite level under acidic and largely oxidizing conditions. Incongruent dissolution was associated with the leaching and downward transport of Si, Na, K and Ca and the concentration of Al. In such an environment, early precipitation of halloysite from allophane was probable, and it gradually transformed into kaolinite during aging and progressive weathering.

Several examples of magma mixing exist within the undersaturated alkaline magma series of the Tertiary/Quaternary volcanics in the French Massif Central. This study describes magma mixing in the Puy Griou/Griounot area of the Cantal volcano (10-3 Ma). Petrographic evidence for injection of blebs of basic magma into phonolitic host magmas is abundant (cauliform inclusions, liquid-liquid contacts, vesiculation and chilling). Compositions of the inclusions are basic tephrite, whereas the hosts are miaskitic phonolites. Petrographic examination reveals the presence of disequilibrium mineralogical features (e.g. Mg-rich olivine in phonolites) and strong zoning in many clinopyroxenes. Transfer of phenocrysts between basic inclusions and phonolite hosts was common, and can be seen clearly in the wide range of compositions of clinopyroxene. Hornblende, magnetite and olivine were also transferred from inclusions to host.

Sr and Nd isotope data indicate that, unlike most other fractionated magmas of the region, phonolites which show evidence for magma mixing are uncontaminated by the continental crust and have isotopic ratios similar to local primitive basic magmas. This leads to the suggestion that the magma mixing event took place at great depth, rather than being a high-level phenomenon. The phonolites were thus generated by high-pressure fractional crystallisation of an earlier basanitic or tephritic parent, perhaps at upper-mantle depths. This conclusion may explain why some phonolites elsewhere in the world have entrained spinel Iherzolite mantle xenoliths.

Cretaceous, strongly alkaline mafic igneous provinces occur around the margins of the Ordovician to Cretaceous Paraná sedimentary basin of southern Brazil. The Serra do Bueno diatreme is situated in the southern portion of the largest of these alkaline provinces, the Alto Paranaíba Igneous Province in Minas Gerais. The well-exposed diatreme crops out close to the south-west surface limit of the São Francisco craton and is adjacent to several other poorly exposed ultramafic alkaline pipes, previously described variously as kimberlites (Barbosa, 1991) and lamproites (Ramsay and Tompkins, in press). The diatreme has two distinct facies: (1) a crater facies dominated by lapilli tuffs; and (2) a magmatic hypabyssal facies formed by a relatively fresh ultramafic (MgO = 15 wt.%) potassic (K2O/Na2O = ∼ 1.5) intrusion that contains xenoliths of meta-sediments, feldspathic gneiss and dunite. It is massive and porphyritic, with large olivine phenocrysts (Fo87) and smaller crystals of diopside (Ca50Mg44Fe6), phlogopite, perovskite, ilmenite and zeolites in a fine-grained groundmass that contains altered leucite and up to 20% devitrified glass. The Serra do Bueno intrusion shows a strong enrichment in light relative to heavy rare earth elements, with La/Yb of ∼85. Its initial 87Sr/86Sr (0.705176) and 143Nd/144Nd (0.512312) isotopic ratios are similar to those of other intrusions (e.g. Limeira 2) and lavas (e.g. Presidente Olegário) in the Alto Paranaíba Igneous Province. This suggests that these Late Cretaceous alkaline magmas were all derived from a similar source, predominantly within the subcontinental lithospheric mantle.

Laser 40Ar/39Ar analyses have yielded an isochron of 90 ± 4 Ma for the Serra do Bueno intrusion. This age is higher than the corresponding K/Ar bulk-rock age for the same sample but similar to K/Ar ages determined on mica separates from both intrusive and extrusive rocks in the Alto Paranaíba Igneous Province.