Abstract Kissomlyó is a small-volume Pliocene alkaline basaltic eruptive centre located
in the monogenetic Little Hungarian Plain Volcanic Field (western Pannonian Basin).
It consists of a sequence of pyroclastic and effusive eruptive units: early tuff ring
(unit1), pillow and columnar jointed lava (unit2), spatter cone (unit3). The tuff
ring sequence is overlain by a unit of lacustrine sediments which suggests a significant
time gap in the volcanic activity between the tuff ring formation and the emplacement
of the lava flow. High-resolution investigation of mineral textures, zoning and chemistry
as well as whole-rock geochemical analyses were performed on stratigraphically controlled
samples in order to characterize the magmas represented by the distinct eruptive units
and to reveal the evolution of the deep magmatic system. Based on the bulk rock geochemistry,
compositionally similar magmas erupted to the surface during the entire volcanic activity.
However, olivine crystals show diverse textures, zoning patterns and compositions
reflecting various deep-seated magmatic processes. Five different olivine types occur
in the samples. Type1 olivines represent the phenocryst sensu stricto phases, i.e.,
crystallised in situ from the host magma. The other olivine types show evidence for
textural and compositional disequilibrium reflecting single crystals consisting of
distinct portions having different origins. Type2a and type2b olivines have antecrystic
cores which are derived from two distinct primitive magmas based on the different
compositions of their spinel inclusions. Type4 olivines show reverse zoning whose
low-Fo cores represent antecrysts from more evolved magmas. The cores of type3 and
type5 olivines are xenocrysts originated from the subcontinental lithospheric mantle.
These xenocrysts are surrounded by high-Fo or low-Fo growth zones suggesting that
olivine xenocryst incorporation occurred at different levels and stages of magma evolution.
Olivine-hosted spinel inclusions show three distinct compositional groups. Group1
spinels are very Al-rich (0–0.22 Cr#) and coexist with the antecrystic cores of type2a
olivines, group2 spinels have 44.5–62.3 Cr#s and occur in the phenocryst s.s. (type
1) olivines, while group3 spinels are very rich in Cr (68.4–81.3 Cr#) and appear in
the antecrystic cores of type2b olivines. Based on the integrated analysis of olivines
and their spinel inclusions four magmatic environments were involved into the evolution
of the magmatic system. These crystals bear evidence of various petrogenetic processes
playing role in the formation of the erupted magma batches: fractional crystallization,
olivine (+ spinel) recycling, xenocryst incorporation, magma recharge and interaction
of multiple small magma packets in a multi-level magmatic system. Clinopyroxene-melt
thermobarometry yields an average pressure of 6.6 ± 0.9 kbar corresponding to a depth
of about 25 km, implying that the main level of final clinopyroxene fractionation
could have occurred around the Moho (in the lowermost crust). This study shows that
high-resolution mineral-scale analyses carried out through monogenetic sequences provide
a unique, more detailed insight into the evolution of these “simple” magmatic systems
as crystal growth stratigraphy and compositions yield direct evidence for various
petrogenetic processes which are usually obscured in the whole-rock geochemistry.
