Elemér Pál-Molnár

Amphibole perspective to unravel pre-eruptive processes and conditions in volcanic plumbing systems beneath intermediate arc volcanoes: a case study from Ciomadul volcano (SE Carpathians)

Ciomadul is the youngest volcano in the Carpathian–Pannonian region produced crystal-rich high-K dacites that contain abundant amphibole phenocrysts. The amphiboles in the studied dacites are characterized by large variety of zoning patterns, textures, and a wide range of compositions (e.g., 6.4–15 wt% Al2O3, 79–821xa0ppm Sr) often in thin-section scale and even in single crystals. Two amphibole populations were observed in the dacite: low-Al hornblendes represent a cold (<800xa0°C) silicic crystal mush, whereas the high-Al pargasites crystallized in a hot (>900xa0°C) mafic magma. Amphibole thermobarometry suggests that the silicic crystal mush was stored in an upper crustal storage (~8–12xa0km). This was also the place where the erupted dacitic magma was formed during the remobilization of upper crustal silicic crystal mush body by hot mafic magma indicated by simple-zoned and composite amphiboles. This includes reheating (by ~200xa0°C) and partial remelting of different parts of the crystal mush followed by intensive crystallization of the second mineral population (including pargasites). Breakdown textures of amphiboles imply that they were formed by reheating in case of hornblendes, suggesting that pre-eruptive heating and mixing could take place within days or weeks before the eruption. The decompression rim of pargasites suggests around 12xa0days of magma ascent in the conduit. Several arc volcanoes produce mixed intermediate magmas with similar bimodal amphibole cargo as the Ciomadul, but in our dacite the two amphibole population can be found even in a single crystal (composite amphiboles). Our study indicates that high-Al pargasites form as a second generation in these magmas after the mafic replenishment into a silicic capture zone; thus, they cannot unambiguously indicate a deeper mafic storage zone beneath these volcanoes. The simple-zoned and composite amphiboles provide direct evidence that significant compositional variations of amphiboles do not necessarily mean variation in the pressure of crystallization even if the Al-tschermak substitution can be recognized, suggesting that amphibole barometers that consider only amphibole composition may often yield unrealistic pressure variation.

Geochemical implications for the magma origin of granitic rocks from the Ditrău Alkaline Massif (Eastern Carpathians, Romania)

In addition to a series of ultramafic to mafic and alkaline igneous rocks, a granite body also occurs in the Ditrău AlkalineMassif, Eastern Carpathians, Romania. We present and discuss mineral chemical data, and major and traceelement compositions of the granites in order to define their nature and origin and to determine the depth of the magmaemplacement. The granites consist of K-feldspar, albite to oligoclase and quartz accompanied by Ti-rich annite± calcic amphiboles. Depending on the amphibole content they are classified as less fractionated amphibole-bearingand amphibole-free varieties. Accessories include zircon, apatite, magnetite, ilmenite, and allanite or monazite.High Zr, Nb, Ga, Ce and Y content and Ga/Al and Fe/Mg ratios, together with low CaO, Sr and Ba contents and Y/Nbratios of 0.04-0.88 are consistent with A1-type granites and mantle differentiates correspond to an intra-plate environment.The Ditrău Alkaline Massif granites were emplaced at middle – upper crustal levels between 14 and 4 km depthas indicated by the calculated crystallization pressure of 370 ± 40 MPa and the stability limit of calcic amphiboles.

Mafic enclaves in peraluminous Variscan granitoid in the Battonya Unit from Southeast Hungary

Variscan granitoids occur in the southeastern part of the Tisza Mega Unit of Hungary. The presence of amphibole, calc-alkaline-type Mg-rich biotite in metaluminous basic enclaves, and muscovite and Fe-Al-biotite in peraluminous granitoids, suggests a mixed I-S-type origin. Two types of muscovite have been identifi ed: a primary euhedral to subhedral, Ti-Na-Al rich variety, crystallized after Fe-rich peraluminous biotite in the two-mica granite and in muscovite granite, and a secondary subhedral Si enriched and Mg-bearing, Ti-poor mica formed as a hydrothermal alteration product of feldspars, and is present in all rock types. Given the compositional continuum of “white micas”, we suggest that magmatic crystallization was followed by autometasomatic and hydrothermal activity, due to a water-rich liquid rapped in the rock during the final stages of magmatic activity. Based on the bulk composition of the prevailing rock-type, the abundance of primary muscovite, the majority of the granitoid magma crystallized from a watersaturated peraluminous melt for which the pressure was 490–600 MPa, the temperatures were 650–685 °C and the depth of the intrusion was a minimum of 15 km.