Magma mingling and hybridization in the Kuh-e Dom pluton, Central Iran

Abstract

The upper Eocene Kuh-e Dom granitoids in Central Iran comprise mafic microgranular enclaves ranging from a few centimeters to meters in size. The spherical to ellipsoidal enclaves consist of diorite, quartz-diorite, monzodiorite and quartz-monzodiorite whereas the more felsic host intrusions mainly comprise monzogranite and granodiorite. Most enclaves show sharp contacts with the host granites and distinct chilled margins in the small enclaves indicate rapid cooling after entering the felsic host magma. Disequilibrium textures include: (1) poikilitic and antirapakivi textures in feldspar megacrysts, (2) ocelli quartz rimmed by fine-grained mineral aggregates, (3) rounded and corroded plagioclase, (4) patchy plagioclase, (5) mafic clots, (6) blade biotite, (7) small lath-shaped plagioclase in larger tabular plagioclase, (8) acicular apatite, and (9) crenulated and cuspate contacts between enclaves and host rocks. These textures indicate mingling/mixing of two different magmas. Microprobe data show variable plagioclase compositions ranging between oligoclase and andesine in granodiorite, and between andesine to labradorite in the enclaves with normal zoning and oscillatory zoning. Geochemically, the enclaves are Na-rich whereas the host rocks are K-rich. In addition, the enclaves are depleted in LREE and LILE and enriched in HREE and Ti, P, Nb and Y, relative to their host granodiorites. Average initial Nd–Sr isotopic compositions for the host rocks are εNd (47Ma)=−5.09 and Sri=0.7068, compared to enclave Sri=0.7060 and εNd (47Ma)=−2.89. Fractionation of both enclaves and host rocks from the same melt cannot explain these petrological, geochemical and isotopic differences. Instead, enclaves and host rocks more likely originated from two separate magmas, which subsequently mingled and mixed. The mafic enclaves are modified mantle-derived melts while the granodioritic host rocks are partial melts of lower continental crust. The mafic enclave magma was at near liquidus conditions when injected into the granitic host melt, and the Kuh-e Dom intrusion reflects a hybrid product from mingling and partial (incomplete) mixing of these two melts.