The Tisza and Dacia mega-units constitute a central part of the Alps-Carpathians-Dinarides
orogenic system. Polyphase medium-grade metamorphism observed in mineral assemblages
from the crystalline basement is often correlated with Variscan and pre-Variscan events.
However, a mid-Cretaceous Sm-Nd garnet age (103.6 +/- 1.8Ma) from the Apuseni Mountains
is at odds with this interpretation. Electron-microprobe U-Th-Pb dating of monazite
in samples from the Apuseni Mountains, the Rodna Mountains, as well as the Simleu
Silvaniei, Ticu and Preluca inselbergs revealed a complex pattern of Alpine and pre-Alpine
age clusters. Pre-Variscan and Variscan ages were obtained from the core of zoned
monazite grains and from samples that apparently escaped Alpine overprinting. Relic
monazite in the latter is often replaced by rhabdophane and/or surrounded by allanite
coronas. Permian to Early Triassic monazite ages correlate with the intrusion of granitic
melts and pegmatites. Early Cretaceous ages from rims of chemically zoned grains and
from monazite inclusions in garnet, biotite and staurolite represent newly formed
metamorphic grains that crystallized on the prograde path during Alpine metamorphism.
Petrographic observations of prograde allanite breakdown reactions, Sm-Nd garnet analyses
and thermobaric estimates (500-550 degrees C/5-8kbar) from parts of the Tisza and
Dacia mega-units constrain medium-grade conditions during Early Cretaceous times.
Exclusively mid-Cretaceous monazite ages from the inselbergs and the Rebra-Unit of
the Rodna Mountains, allow extending the Alpine prograde overprint across the Transylvanian
basin. Together with other studies from the basement of the Pannonian basin, this
implies that the Dacia Mega-Unit and parts of the Tisza Mega-Unit experienced a medium-grade
metamorphic overprint and synkinematic garnet-growth during late Early Cretaceous
times. The Alpine prograde medium-grade overprint is pronounced in the contact zone
between the Tisza and Dacia mega-units and forms a continuous belt with the Cretaceous
metamorphic imprint in the Eastern Alps, when back-rotated to its original position
during the Cretaceous.