Petrogenesis and U-Pb zircon geochronology of migmatitzation during Neo-Tethyan Jurassic magmatic arc extension: The Boroujerd example, western Iran

Abstract

The migmatites in the Boroujerd region, Western Iran are spatially associated with felsic and mafic magmatism. However, the genetic relationship between partial melting and magmatism in the region is unclear. Four metapelite migmatite samples (two melanosome and two leucosome) were selected for U-Pb zircon geochronology to determine the timing of anatexis. Major and trace element compositions of leucosome in the migmatites are consistent with being derived from biotite-breakdown partial melting of metapelites and not from the injection of granites associated with the Boroujerd igneous complex. Cores of zircon from metapelitic migmatite are inferred to be detrital in origin and yield ages of 320-250 Ma. The age of the detrital zircons is compatible with the timing of Paleozoic and Mesozoic magmatism in the Central Iranian micro-plate. The pelitic protolith of the migmatites in the Boroujerd region were probably originated from the Mesozoic continental margin of the Central Iranian microplate. Zircon rims and neoblastic grains yield ages of 220-210 Ma and 170-160 Ma. The geological significance of the older age population is unclear, but the younger population is similar to that of mafic and felsic intrusions in the area and is interpreted to record the general timing of high-temperature-low-pressure contact metamorphism and partial melting. We infer that the Middle Jurassic mafic and felsic magmatism was the principal heat source for the metamorphism and partial melting of the Boroujerd migmatites in response to the subduction of Neotethyan oceanic lithosphere.