On the occurrence and implications of Jurassic primary continental boninite-like melts in the Zagros orogen

Ultramafic rocks, ranging from pyroxenites to hornblendites, are associated with granitoids of the Aligoodarz intrusive complex in the central Sanandaj–Sirjan Zone, representing the Mesozoic continental arc segment of the Zagros orogen. As inferred from the ultramafic whole rock composition and the most primitive clinopyroxene composition in pyroxenites, the geochemical signature of primary melt is significantly different from that of the continental arc basalts. In particular, primary melt is characterized by extremely low concentrations of incompatible elements and high concentrations of Mg and refractory elements typical of boninites. Amphibole is a late crystallizing mineral in these rocks and is in textural and chemical disequilibrium with olivine + orthopyroxene + clinopyroxene. Amphibole crystallized from a liquid underwent differentiation through a process of melt-rock reaction. In particular, early differentiated boninitic cumulates reacted with later melts with a strong crustal signature similar to Aligoodarz granodiorite. U-Pb zircon geochronology from ultramafic rocks and surrounding quartz-diorite yield similar ages and indicate that they are coeval with Aligoodarz granitoids (ca. 165–170 Ma). However, the occurrence of a marked negative Eu anomaly in zircon from the ultramafic rocks, which is absent in the boninitic primary melt, indicates that zircons crystallized from the infiltrating melt and in turn date the timing of melt infiltration. The interaction between ultramafic cumulates and infiltrated melt has generated a new liquid compositionally similar to high-Mg andesites and to the quartz-diorites hosting the ultramafic cumulates. The scenario that better account for the genesis of boninitic melts in the Sanandaj–Sirjan Zone is partial melting of a depleted mantle wedge in response to the onset of NeoTethys subduction. According to this hypothesis, middle Jurassic calc-alkaline magmatism in the Sanandaj–Sirjan Zone represents the mature stage of arc magmatism postdating boninite generation by about 10 Ma