Physical and chemical interactions between coeval ‎magmas: a case study of mixing and mingling from the ‎Urumieh plutonic complex, NW Iran

Abstract

ABSTRACT Field observations, whole-rock geochemistry, mineral chemistry and zircon U-Pb dating results from microgranular mafic enclaves and their host syenite, Urumieh plutonic complex, NW Iran, show that the enclaves and host are coeval, genetically related and evolved by synchronous magma mixing–fractionation. Geochemically all belong to the shoshonitic series and probably originated from the same enriched subcontinental mantle-derived magma in a subduction setting. SHRIMPU-Pb dating of zircon from representative samples of host rocks and enclaves yields mutually indistinguishable late Cretaceous crystallization ages, 90.71.3 and 91.31.1 Ma (host syenite), and 90.20.7 and 91.60.7 Ma (enclaves). Pressure and temperature estimates from Al-in-amphibole and amphibole-plagioclase pairs from both host and enclaves suggest that the interacting magmas mixed and crystallized under similar P-T ‎conditions at middle crustal levels (average 662–716°C and 2.2–2.8 kbar). The occurrence of hybrid to mafic enclaves along with disrupted mafic dikes indicates a long-lived mafic source with ‎multiple replenishments throughout the crystallization history of the complex, and large variability in viscosity within the magmatic system. The transition from mixing to mingling depends on the different crystalline states of the interacting magmas, especially the host magma, which strongly influence rheological properties. In the liquid-liquid state, mixing ‎typically will continue until complete hybridization. With increasing polymerization and crystal load in the host magma, magmatic relationships change progressively from chemical mixing to more mechanical mixing (mingling), producing distributed mafic enclaves, then disrupted mafic dikes, then syn-plutonic mafic dikes as a continuum given sufficient time.