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

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, 90.7±1.3 and 91.3±1.1 Ma (host syenite), and 90.2±0.7 and 91.6±0.7 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.

针对伊朗西北部乌鲁米耶深成杂岩体(Urumieh plutonic complex)中的细粒镁铁质包体(microgranular mafic enclaves)及其寄主正长岩(host syenite)，本数据集包含野外观测数据、全岩地球化学(whole-rock geochemistry)、矿物化学(mineral chemistry)以及锆石U-Pb定年(zircon U-Pb dating)结果。研究表明，该类包体与寄主岩石形成时代一致、成因相关，且通过同步的岩浆混合-分异作用完成演化。地球化学特征显示二者均隶属于钾玄岩系列(shoshonitic series)，其源区可能源自俯冲环境(subduction setting)下富集的大陆下伏地幔(subcontinental mantle)源岩浆。对寄主岩石与包体代表性样品的离子探针(SHRIMP)锆石U-Pb定年结果显示，二者结晶年龄无显著差异，均为晚白垩世：寄主正长岩的定年结果为90.7±1.3 Ma与91.3±1.1 Ma，包体的定年结果为90.2±0.7 Ma与91.6±0.7 Ma。通过角闪石中铝(Al-in-amphibole)及角闪石-斜长石对(amphibole-plagioclase pair)估算的温压条件表明，相互作用的岩浆在相似的温压条件下于中地壳深度发生混合并结晶，平均温度区间为662~716°C，压力区间为2.2~2.8 kbar。混合型至镁铁质包体的产出以及被破坏的镁铁质岩脉(mafic dikes)指示，该杂岩体的结晶演化过程中存在长期活动的镁铁质源区，且经历了多期岩浆补给，同时岩浆体系内的黏度存在显著差异。从岩浆混合(magmatic mixing)到岩浆混杂(magmatic mingling)的转变取决于相互作用岩浆的结晶状态差异，尤以寄主岩浆的结晶状态最为关键，其对岩浆体系的流变性质具有强烈调控作用。当岩浆处于液-液状态时，混合作用通常会持续至完全均一化；随着寄主岩浆的聚合度与晶体加载量不断增加，岩浆间的相互作用逐渐从化学混合过渡为更偏向机械作用的混杂过程，依次形成分散分布的镁铁质包体、被破坏的镁铁质岩脉，若时间充足则最终形成同深成杂岩体的镁铁质岩脉(syn-plutonic mafic dikes)，构成完整的演化连续序列。