Zircon U–Pb geochronology, major-trace elements and Sr–Nd isotope geochemistry of Mashhad granodiorites (NE Iran) and their mafic microgranular enclaves: evidence for magma mixing and mingling

ABSTRACT

Mashhad granitoids and associated mafic microgranular enclaves (MMEs), in NE Iran record late early Mesozoic magmatism, was related to the Palaeo-Tethys closure and Iran-Eurasia collision. These represent ideal rocks to explore magmatic processes associated with Late Triassic closure of the Palaeo-Tethyan ocean and post-collisional magmatism. In this study, new geochronological data, whole-rock geochemistry, and Sr–Nd isotope data are presented for Mashhad granitoids and MMEs. LA–ICP–MS U–Pb dating of zircon yields crystallization ages of 205.0 ± 1.3 Ma for the MMEs, indicating their formation during the Late Triassic. This age is similar to the host granitoids. Our results including the major and trace elements discrimination diagrams, in combination with field and petrographic observations (such as ellipsoidal MMEs with feldspar megacrysts, disequilibrium textures of plagioclase), as well as mineral chemistry, suggest that MMEs formed by mixing of mafic and felsic magmas. The host granodiorite is a felsic, high K calc-alkaline I-type granitoid, with SiO₂ = 67.5–69.4 wt%, high K₂O (2.4–4.2 wt%), and low Mg# (42.5–50.5). Normalized abundances of LREEs and LILEs are enriched relative to HREEs and HFSEs (e.g. Nb, Ti). Negative values of whole-rock εNd_(t) (−3 to −2.3) from granitoids indicate that the precursor magma was generated by partial melting of enriched lithospheric mantle with some contributions from old lower continental crust. In the MMEs, SiO₂ (53.4–58.2 wt%) is lower and Ni (3.9–49.7 ppm), Cr (0.8–93.9 ppm), Mg# (42.81–62.84), and εNd_(t) (−2.3 to +1.4) are higher than those in the host granodiorite, suggesting a greater contribution of mantle-derived mafic melts in the genesis of MMEs.

KEYWORDS:

• Mafic microgranular enclaves
• Mashhad granodiorite
• magma mixing
• Palaeo-Tethys ocean
• Iran

Highlights

• Mafic microgranular enclaves (MMEs) are commonly present in Mashhad granodiorite, Northeastern Iran

• The mafic and felsic magmas are derived from partial melting of an enriched lithospheric mantle metasomatized by slab fluid

• The Mashhad granodiorite and its MMEs are the result of the mixing between mafic and felsic magmas

Acknowledgments

This paper is part of PhD dissertation of the first author, supported financially by Université Clermont Auvergne, CNRS, IRD, OPGC, Laboratoire Magmas & Volcans, 63000 Clermont-Ferrand, France. Leon E. Long edited an earlier version of the manuscript to improve English usage and scientific content. We would like to thank H. Shafaii Moghadam for editorial handling of the paper as well as providing insightful scientific guidance, and F. Lucci and two anonymous reviewers for in depth comments, all of which improved this paper significantly.

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplemental material

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Additional information

Funding

This work was supported by the Université Clermont Auvergne, CNRS, IRD, OPGC, Laboratoire Magmas et Volcans, 63000 Clermont-Ferrand, France.