Glassy orthopyroxene granodiorites of the Pannonian Basin: tracers of ultra-high-temperature deep-crustal anatexis triggered by Tertiary basaltic volcanism

摘要

Glassy orthopyroxene granodiorite-tonalite (named pincinite after type locality) was described from basaltic lapilli tuffs of the Pliocene maar near Pincina village in the Slovakian part of the Pannonian Basin. Two pincinite types exhibit a qualitatively similar mineral composition (quartz, An_(20-55) plagioclase, intergranular silicic glass with orthopyroxene and ilmenite, +- K-feldspar), but strongly different redox potential and formation PT conditions. Peraluminous pincinite is reduced (6-7 percent of total iron as Fe~(3+) in corundum-normative intergranular dacitic glass) and contains ilmenite with 8-10 mol percent Fe_2O_3 and orthopyroxene dominated by ferrosilite. High-density (up to 0.85 g/ cm~3) primary CO_2 inclusions with minor H_2, CH_4, H_2S, CO and N_2 (< 2 mol percent total) are present in Qtz and Plg. Equilibrium PT conditions inferred from the intergranular Opx-Ilm-Glass assemblage and fluid density correspond to 1,170 +- 50 deg C, 5.6 +- 0.4 kbar, respectively. Metaluminous pincinite is more oxidised (25-27 percent of total iron as Fe~(3+) in diopside-normative intergranular glass of rhyolite-trachyte-dacite composition) and contains Fe_2O_3-rich ilmenite (17-29 mol percent) associated with enstatite. Fluid inclusions are composed of CO_2-H_2O mixtures with up to 38 mol percent H_2O. Raman spectroscopy revealed H_2S along with dominant CO_2 in the carbonic phase. Equilibrium PT parameters for the intergranular Opx-Ilm-Glass assemblage correspond to 740 +- 15 deg C, 2.8 +- 0.1 kbar, respectively. Reducing gas species (<2mol percent total) in the CO_2-inclusions of the peraluminous pincinite resulted from hydrogen diffusion due to fH_2 gradient imposed during decrease of redox potential from the log fO_2 values near QFM during Qtz + Plg growth, to QFM-2 incidental to the superimposed Opx + Ilm assemblage in the intergranular melt. The decrease in oxygen fugacity was recorded also in the metaluminous pincinite, where log fO_2 values changed from approx QFM + 2.6 to QFM + 0.4, but hydrogen diffusion did not occur. Absence of OH-bearing minerals, major and trace element abundances (e.g. SIGMA REE 300-320, Nb 55-57, Th 4-31, Zr 240-300 ppm, FeO_(tot)/MgO up to 11), and Sr-O isotope ratios in the pincinites are diagnostic of high-temperature anorogenic magmas originated by dehydration melting of biotite in quartz-feldspathoid crust (~(87)Sr/~(86)Sr> 0.705-0.706, delta~(18)O>9 per thousand V-SMOW) around alkali basalt reservoir in depths between 17 and 20 km, and around late stage derivatives of the basalt fractionation, intruding the crust up to depths of 10- 11 km. Low water activity in the pincinite parental melt was caused by CO_2-flux from the Tertiary basaltic reservoirs and intrusions. The anatexis leads to generation of a melt-depleted granulitic crust beneath the Pannonian Basin, and the pincinites are interpreted as equivalents of igneous charnockites and enderbites quenched at temperatures above solidus and unaffected by sub-solidus re-equilibration and metamorphic overprint.