This study evaluates the rare earth element (REE) geochemistry in altered trachyandesitic ignimbrites, tuff and lava flows in the Hashtjin area by assessing chondrite-normalized REE patterns and Y/Ho geochemical ratios. Modifications in the REE patterns took place along altered fault zones that were affected by hypogene and supergene alterations. The precursor volcanic and pyroclastic rocks contain phenocrysts of plagioclase accompanied by augite, zircon, apatite and pyrite. Based on X-ray diffraction analysis, the main mineral assemblages of the altered units consist of kaolinite as the main clay mineral, SiO2 polymorphs (quartz and cristobalite) and anatase as a minor constituent. The chondrite-normalized REE patterns of argillic samples reveal fractionation of light REEs (LREEs) compared to heavy REEs (HREEs), together with a marked Eu anomaly and a weak W-type tetrad effect related to the weak non-charge radius control (CHARAC) behaviour of REEs and slightly higher Y/Ho and Zr/Hf ratios. The relationship between the Y/Ho and Zr/Hf ratios and recognizable T3 and T4 effects (tetrad effect) suggests that an increasing degree of water–rock interaction occurred during hypogene alteration processes by acidic hydrothermal fluids that were overprinted by supergene alteration. Water–rock interaction and adsorption by Mn-oxides and clay minerals are considered to have played important roles in determining the close to non-CHARAC behaviour of REEs during the argillic alteration of the pyroclastic rocks in the Hashtjin area.

In the Calatrava province of central Spain, numerous Quaternary pyroclastic vents have erupted carbonatite magmas carrying silicate melt fragments, mantle debris and megacrysts. Lava flows are rare. Maar and scoria deposits have carbonate matrices and pass into tuff sheets with carbonate contents >50%, which are spread widely away from the eruptive centres and constitute the most abundant form of effusive carbonate. Immense quantities of mantle debris are present in the erupted material. The tuffs have a distinctive fabric, which consists of a pale matrix carrying black silicate glass clasts that contain globules of immiscible carbonate and carbonate phenocrysts. There is evidence of similar volcanism in the Limagne province of central France and in other intra-continental provinces in Europe and Africa. About 500 vents have been identified in France and Spain: all the vents examined to date have erupted carbonatite magma. Such eruptions are not generally recognized in classical volcanology. As pyroclastic carbonatite was not previously recognized in Spain and France, a detailed examination of other mafic and ultramafic alkaline provinces, where research has traditionally concentrated on lava flows, is vital. For any search to be successful, evidence from the pyroclastic rocks will be required.

The studied gabbroic nodules occurring in the Petrazza pyroclastic rocks consist mainly of plagioclase (An95–87), olivine (Fo83-73) and clinopyroxene (Mg# 90–77), with subordinate opaques (Ti-magnetite) and amphibole (Mg-hastingsite), which constitute the cumulate minerals. Interstitial material has a relatively high, but variable, degree of vesicularity and consists of variable amounts of glass and quenched crystals of plagioclase (An71–55), amphibole, clinopyroxene and rare biotite, olivine and opaques. Silicate melt inclusions are abundant in the cumulate minerals, but complete homogenization to melt has been observed only in the inclusions occurring in clinopyroxene, where the temperatures of homogenization vary from 1134 to 1190°C. Microthermometric investigations of fluid inclusions and of the shrinkage bubble of the melt inclusions suggest that the magma contained CO2. The apparent scarcity of H2O indicates that this component was strongly partitioned into the magma at the time of crystallization of the investigated minerals; this is further supported by the occurrence of (1) daughter biotite- and amphibole-bearing inclusions which show that the H2O activity in the magma was sufficiently high to allow their crystallization, and (2) calcic plagioclase (An95–87) which can be crystallized from a high-alumina basaltic magma at pressure ≤2 kbar, temperatures in the range 1050–1100°C and in the presence of 3–4 wt.% of water (MELTS software simulations).

The composition of the melt inclusions suggests that the hosting plagioclase, olivine and clinopyroxene crystallized from slightly different batches of magma. The S content in the melt inclusions of clinopyroxene and olivine is high (up to 0.41 wt.%). The presence of Fe-Cu(-Ni)-rich blebs of sulphide in plagioclase, olivine, amphibole, and locally in the melt inclusions too, further supports the important role of sulphur in the primitive magmas of the investigated gabbros. Small differences in redox conditions or in the Fe content of the melts favoured S mobilization as sulphide.