oligocene partial melting in the takab metamorphic complex, nw iran: evidence from in situ u-pb geochronology

u-pb ages obtained on zircons from the investigated migmatites place new constraints on the evolution of the crustal rocks in the takab area. shrimp u-pb dating of inherited zircon in the melanosome parts of mafic migmatites from the takab complex gives a discordant age with an upper intercept of 2961±72 ma (mswd=0.5) and a lower intercept of 24.2±5.7 ma (mswd=1.7). in the concordia diagram, the melt-grown zircons yield a more precise age of 25.67±0.74 ma (mswd=1.7), similar to the lower intercept age of the discordia. at least part of the scatter along the concordia for the new zircon generations may be real, caused by an extended period of melting and crystallization, in which case the oldest age of c. 29 ma dates the onset of partial melting. the oligocene ages for the younger zircons show the importance of partial melting in these mafic rocks. the typical euhedral to subhedral shapes are features related to crystallization and resorption of the grains due to in situ melting. these processes were most likely related to the generation of the tertiary granitoids of the takab complex.