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.

Juvenile crust and mantle sources for the Nasrand intrusive rocks, the central Urumieh–Dokhtar magmatic arc, Iran: Insights from elemental and isotopic geochemistry

“…Wholly, the geochemical characteristics indicate that the studied samples over the UDMA show a mixed origin composed of juvenile crust and mantle material as major components and lower old crustal material as a minor component (Figure 9e), resulting from basaltic underplating related to the Neo-Tethys subduction (see Deng et al, 2018;Kazemi et al, 2020;Sarjoughian et al, 2018Sarjoughian et al, , 2019Sarjoughian et al, , 2020Sepidbar et al, 2019). It is interesting that the UDMA intrusive rocks et al, 2016), maybe supported by slab break-off and melting at depth (e.g., Omrani et al, 2008).…”

“…This indicates that most of them are mainly derived from partial melting of juvenile mafic lower crust and mantle magma that could be contaminated with old crust (e.g., Jahn, Wu, & Chen, 2000;Wu et al, 2006). Based on the initial ratios of low to high 87 Sr/ 86 Sr (i) and positive to negative ϵNd(t), a mixture of two end members can be inferred for the UDMA intrusive rocks, that is, mixing depleted mantle and a juvenile lower crustal end members, which generated more evolved UDMA magmatism (e.g., Chekani Moghadam et al, 2018;Kazemi et al, 2019;Sarjoughian et al, 2018Sarjoughian et al, , 2019Shafaii Moghadam, Whitechurch, Rahgoshay, & Monsef, 2009;Yeganehfar, Ghorbani, Shinjo, & Ghaderi, 2013) 9e), mixing between mantle and juvenile crust-derived magmas can be the key role for UDMA magmatism. Furthermore, the intrusive rocks of the south-east of the UDMA have higher 87 Sr/ 86 Sr (i) , lower ϵNd (t) , and MgO values than the other parts of the UDMA, which indicates the higher crust component than the others parts.…”

“…On other hand, some previous studies have argued for different petrogenetic models for the origin of the UDMA. For example, Honarmand et al (2014) and Nouri et al (2018) suggested a mantle origin, whereas other researchers (e.g., Asadi, 2018; Deng et al, 2018; Sarjoughian et al, 2018, 2019, 2020) exhibit evidence for a juvenile crustal source. Meanwhile, some researchers have proposed the idea of transtensional volcanism for part of the Tertiary magmatic activities in the Zagros orogen (Ebrahimi, Esmaeili, & Aouizerat, 2017).…”

“…Other intrusive rocks in UDMA that have been compared with the other area include: North‐west of UDMA (NW): (1) Haji Abad granitoids from Kazemi, Kananian, Xiao, and Sarjoughian (2019); (2) Gheshlaghe‐Aftabrow intrusions from Kazemi, Kananian, Xiao, and Sarjoughian (2020); (3) Saveh magmatic complex from Nouri et al (2018). Center of UDMA: (4) Niyasar plutonic complex from Honarmand et al (2014); (5) Kuh‐e Dom granitoids from Kananian, Sarjoughian, Nadimi, Ahmadian, and Ling (2014); (6) Kajan subvolcanic rocks from Golkaram et al (2016); (7) Zafarghand complex from Sarjoughian, Lentz, Kananian, Ao, and Xiao (2018); (8) Marshenan granitoids from Sarjoughian, Azizi, Lentz, and Ling (2019); (9) Soheyle‐PaKuh granitoids from Sarjoughian et al (2020); (10) Nadoushan granitoids from Shahsavari Alavijeh, Rashidnejad‐Omran, Toksoy‐Koksal, Xu, and Ghalamghash (2019). Southeast of UDMA (SE): (11) KuhPanj porphyries from Asadi, Moore, and Zarasvandi (2014) and Asadi (2018); (12 and 13) Bezenjan‐Bardsir complex from Sepidbar, Ao, Palin, Li, and Zhang (2019); (14 and 15) Rabor‐Lalehzar from Chekani Moghadam, Tahmasbi, Ahmadi‐Khalaji, and Santos (2018) and (16) Sarduiyeh granitoids from Nazarinia et al (2018)…”

“…versus the initial87 Sr/ 86 Sr diagram for (a) NIR and (b) other intrusions of the UDMA. The variations of the isotopic composition in major Earth's reservoirs taken fromWhite (2013); observed Earth ofCaro and Bourdon (2010); UDMA Juvenile crust ofSarjoughian et al (2019); Neoproterozoic juvenile crust ofChen and Jahn (1998). Symbols as in Figure4TheNb/U and Ce/Pb ratios in mafic-intermediate rocks and dikes are relatively low (Avg: Nb/U = 29.28 and Ce/Pb = 3.68 and Nb/U = 18 and Ce/Pb = 4.87, respectively), indicating continental crustal contamination contributions to those mantle-derived melt composition.…”