Mafic and salic igneous xenoliths in Late Tertiary alkaline basalts: Fluid inclusion and mineralogical evidence for a deep-crustal magmatic reservoir in the Western Carpathians
Huraiová, Monika; Konecny, Patrik; Simon, Klaus; Hurai, Vratislav
European Journal of Mineralogy Volume 8 Number 5 (1996), p. 901 - 916
published: Oct 30, 1996
manuscript accepted: Mar 22, 1996
manuscript received: Sep 22, 1995
ArtNo. ESP147050805003, Price: 29.00 €
Abstract Explosive Late Tertiary alkaline basalts have ejected pyroxenite, hornblendite, anorthoclasite and quartzbearing tonalite-trondhjemite xenoliths in the northern margin of the Pannonian Basin. One tonahte sample contains primary CO2-H2O inclusions with up to 20 vol. % of aqueous phase. Remaining tonalites, anorthoclasites and mafic rocks trapped CO2 inclusions without detectable water. Except for hornblendites, the CO2 inclusions coexist with silicate melts. Additional, not yet detected volatiles are responsible for depression of the CO2 final melting temperature, which clustered mostly around -58°C in all rock-types. Maximum densities of the CO2 inclusions tend to decrease from pyroxenites (0.92 g/cm3), through anorthoclasites (0.89 g/cm3), to tonalites (0.85 g/cm3) and hornblendites (0.75-0.87 g/cm3), thus reflecting decreasing depth of formation. Equilibration PT conditions for clinopyroxenes ranged between 1220°C at 10 kbar and 1068°C at 4.3 kbar, with the 50 % of estimates projecting between 6.5-7.5 kbar. Postcumulus hornblende, coexisting with An53-64 plagioclase, crystallized at 928-1074°C. Most of the PT data for pyroxenites indicate depths between 23-26 km, close to recent Moho discontinuity (27-29 km), while those for tonalites and hornblendites reflect their middle crustal emplacement (15-20 km). Cumulate textures, varying modal composition and essentially CO2-dominated primary inclusions suggest formation of the xenoliths by fractional crystallization in a large reservoir of mantle-derived basaltic magmas. The cumulate origin of tonalites and anorthoclasites is corroborated by nearly identical whole-rock REE distribution, showing pronounced positive Eu anomaly and enrichment in LREE. Contrasting calc-alkaline and alkaline affinities of clinopyroxenes and hornblendes in selected mafic xenoliths might result from heterogeneity of the parental basaltic magma generated either in metasomatised lithospheric mantle or in up welled asthenosphere.