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摘要
Kissomlyo 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 geo chemical 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. Typel 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 (68.4-813 Cr#) and appear in the antecrystic cores of type2b olivines. Based on the integrated analysis of olivines and their spinet 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.
- 出版日期2015-8-15
