Geochemical and isotopic evidence for magma mixing/mingling in the Marshenan intrusion: Implications for juvenile crust in the Urumieh–Dokhtar Magmatic Arc, Central Iran

Abstract

The 20.5 Ma Marshenan intrusion, situated to the north‐east of Isfahan City, comprises a small part of the Urumieh–Dokhtar Magmatic Arc (UDMA). According to the field and microscopic observations, the study area is composed of two major units: intermediate‐mafic and felsic magmatic rocks. The felsic units include spheroidal to ellipsoidal mafic microgranular enclaves ranging in size from a few millimetres to meters in size. These enclaves are composed of monzodiorite and gabbroic diorite, whereas the felsic and intermediate‐mafic rocks mainly consist of granite, granodiorite, diorite, and rarely gabbro on the basis of both mineralogical and chemical compositions. Enclaves are characterized by a microgranular texture and also reveal some special types of disequilibrium textures, for example, anti‐rapakivi, poikilitic K‐feldspar, skeletal amphibole, acicular apatite, sieve textures, spongy cellular textures in plagioclase, small lath‐shaped plagioclase in large plagioclase phenocrysts, large quartz phenocrysts in enclaves, and mafic clots. These features along with crenulated and occasionally diffuse contacts with the host granite and megacrysts of host feldspar in enclaves, implied that the enclaves have been injected as magmas to form globules into host granodioritic magma and underwent rapid cooling (quenching). They are mostly calc‐alkaline and metaluminous and show strong enrichment of LILEs and LREEs and depletion in HFSEs with negative Eu. The whole rock 87Sr/86Sr(i) and εNd(T) for the host intermediate‐mafic and felsic rocks vary from 0.70528 to 0.70555 and −0.7 to 1.7 and for the enclaves from 0.70526 to 0.70552 and −0.05 to 1.39. Sr–Nd isotope modelling results, in combination with young Nd model ages, indicate their “juvenile” character and suggest their derivation from source rocks or magmas separated from the upper mantle. It probably originates by partial melting of a juvenile lower crust (70–80%) with variable amounts of old lower crust (30–20%), involving mantle components. It is believed to have resulted from intense basaltic underplating and subsequent remelting that took place during the Early Miocene related to the Neo‐Tethys subduction processes. It resulted from a change in the geodynamic regime, including breakoff/rollback of the subducted slab and subsequent regional extension. Moreover, it suggests that new crustal growth during the Meso‐ to Neoproterozoic is an important mechanism in the early development of the central UDMA.