Les lamprophyres apparaissent sous forme de coulées de laves aphanitiques et de dykes, associés ou non à des édifices phréatomagmatiques (maars et diatrèmes, Fig. 1).

2.1 Coulées de lave

2.2 Intrusions

La pétrographie, plutôt que la géochimie, permet de classer les lamprophyres. Nous avons utilisé la terminologie proposée par Rock et al. [13].

3.1 Coulées de laves lamprophyriques

3.2 Dykes (lamprophyres lamproı̈tiques)

Une sélection représentative d'analyses chimiques (éléments majeurs et traces) des laves alcalines du volcan d'Afyon et des lamprophyres associés est présentée dans le Tableau 1.

4.1 Éléments majeurs

Dans le diagramme TAS (Fig. 2), les laves d'Afyon appartiennent à la série alcaline saturée en silice. Il s'agit essentiellement de trachyandésites et trachytes, alors que les lamprophyres sont globalement plus basiques. Les variations en fonction de la silice confirment les deux ensembles : saturé (trachytique essentiellement) et lamprophyrique (Fig. 3).

4.2 Éléments en traces

On note un net enrichissement en LILE, et en Zr, surtout pour les lamprophyres (Fig. 4). Les diagrammes multi-éléments montrent aussi la similarité des deux séries, nettement enrichies (Fig. 5). Nos données analytiques (en particulier la faible teneur en Nb et la teneur élevée en Zr) et les données de la littérature [7,10] montrent une affinité orogénique liée à une ancienne subduction pour les laves d'Afyon.

D'après la chimie, les deux séries semblent issues de la même source, et ont ensuite évolué séparément. La mise en place des lamprophyres d'Afyon coı̈ncide avec la fin de l'activité volcanique dans la région. Ils contiennent du phlogopite et de la leucite. Ce volcanisme s'est développé dans un bassin lacustre au Miocène moyen, en régime extensif. Les lamprophyres sont génétiquement associés à la série trachytique. Leurs caractères minéralogiques, pétrographiques et géochimiques, ainsi que le contexte extensif, nous amènent à proposer pour leur genèse la fusion partielle d'une source hydratée (manteau métasomatisé), avec des taux de fusion variable.

Lamprophyric lava flows are either interstratified with the sedimentary deposits, to the north-northwest of the volcano [8] or overly lacustrine deposits. Lamprophyres are dark grey or black coloured, fine-grained and aphyric. The grey lamprophyres exhibit more vesicles than the black ones. The black lavas are vitreous and exhibit thin laminae related to their fluidal texture.

Two kinds of intrusion were observed in the fields: maar-related intrusions and independent dykes (or dyke swarms).

2.2.1 Maar-related intrusions

2.2.2 Dykes

There are two groups of lamprophyres randomly distributed in the investigated area: lamproitic lamprophyres (LL) and alkaline lamprophyres (AL).

3.1.1 Lamproitic lamprophyres

3.1.2 Alkaline lamprophyres

We saw jumillite (orendite), verite and fitztroyite.

Jumillites (orendites) are composed of phlogopite, sanidine, diopside, olivine, leucite, and oxides ± volcanic glass. The phlogopite phenocrysts contain sagenite nets. Sanidine occurs as ophitic arrangements. Olivine is commonly altered to pilite pseudomorphs. The groundmass contains small, rounded leucites.

Verites contain olivine, diopside, leucite, phlogopite, ± sanidine, apatite, oxides and volcanic glass. The phenocrysts are olivine, diopside, rarely phlogopite, while leucite, sanidine, apatite, most of the phlogopite and oxides are found in the groundmass together with abundant glass.

Fitztroyites are very glassy and composed mainly of phlogopite and leucite. Apatite and oxides are scattered within the glassy groundmass.

25 samples from the central volcano and 19 samples of lamprophyres were analysed. The TAS (Total Alkalis-Silica) diagram (Fig. 2) shows a complete series between trachybasalt and trachyte, in a silica-saturated alkaline suite. The SiO₂ content of lamprophyres ranges between 49 and 62%. Although the fluctuation of mg∗ (magnesium number: MgO/MgO+Fe₂O₃, ranging from 0.57 to 0.31) versus SiO₂ suggests that all lamprophyres are affected by different degree of differentiation, it also indicates heterogeneous compositions of the starting magmas with their different ${\mathrm{mg}}^{*}$ for similar SiO₂ (for 55% SiO₂, ${\mathrm{mg}}^{*}$ ranges between 0.38 and 0.55). The lamprophyres are mainly ultrapotassic (K₂O/Na₂O: max. 11.37) or potassic (K₂O/Na₂O: min 0.64 with K₂O>2%).

The MgOSiO₂, TiO₂SiO₂, Al₂O₃SiO₂ and Fe₂O₃∗SiO₂ variation diagrams (Fig. 3) exhibit two distinct trends: the lamprophyre series, on the one hand, and other lavas, mainly trachytic, called here the trachytic series, on the other hand. The main differences between both series are related to TiO₂, Fe₂O₃∗ and Al₂O₃ contents. Correlation between TiO₂SiO₂ is weak (TiO₂ content is almost constant, about 2%) for the lamprophyre series, while TiO₂ decreases with increasing SiO₂ for the trachytic series. Based on the chemical analysis and the oxide–silica variations, it can be concluded that some lamprophyres constitute the more mafic terms of the trachytic series, completing each other to design a genetically related suite. This dominantly trachytic series probably evolved independently within the crust, in shallow magma chamber(s). In comparison to the trachytic series, the lamprophyric series exhibits lower Al₂O₃ contents that increase with increasing SiO₂ (Fig. 3). Moreover, K₂O contents of ultrapotassic lamprophyres mostly increase with increasing ${\mathrm{mg}}^{*}$, while the potassic ones have no correlations. Such enrichment was also observed in Mascota lavas, western Mexico, related to the presence of phlogopite within the partially melted source area [5].

A problem is that both series are silica-saturated and most lamprophyres exhibit normative quartz. Paradoxically, the observed minerals may be either leucite or olivine. This is probably linked to the rapid ascent of magma towards the surface, leaving leucite crystals no time to break off.

Fourteen samples from the trachytic series and 12 samples from the lamprophyres were analysed. All the samples are enriched in LILE (Large Ion Lithophile Elements) (Fig. 4). The trachytic series exhibits high Ba (1115–3218 ppm) and Sr (710–1522 ppm) contents, with variable Cr (9–249 ppm) and Ni (5–64 ppm) contents.

The lamprophyres however are much more enriched than the trachytic group, except for Ba, e.g., Rb contents of the lamprophyres were sometimes very high, ranging between 81 and 1413 ppm. Zr contents of lamprophyres (521–1371 ppm) are higher than in the trachytic series (Zr: 211–388 ppm) (Fig. 4). In addition, Nb contents of lamprophyric and trachytic series are low (<42 ppm) which is a characteristic feature of subduction-related volcanic rocks. Furthermore, lamprophyric Cr (172–760 ppm) and Ni (16–286 ppm) contents are also higher than in the trachytic series. A MORB normalized spidergram confirms both this general enrichment (Fig. 5) and also the similarity of both series.