Trondhjemite leucosomes generated by partial melting of a hornblende-gabbro (Alvand plutonic complex, Hamedan, NW Iran)

ABSTRACT

This work describes the petrogenesis of in situ and in-source trondhjemite leucosomes generated by low-degree partial melting of the Jurassic Cheshmeh-Ghasaban hornblende-gabbros in the northern Alvand batholith (Hamedan, NW Iran). These leucosomes occur in a metatexite migmatite as patches, net-structures, veinlets, and dikes at scales ranging from a few millimetres to a few metres. They have high SiO₂ (mean ≈ 78 wt%) and Na₂O (4–5 wt%) with low Al₂O₃ (<15 wt%), K₂O (<1.5 wt%), Sr (33–267 ppm), Eu (0.31–0.62 ppm) and heavy REE (<4 ppm) contents similar to typical low-Al₂O₃ trondhjemites. Batch-melting models indicate these leucosomes are the result of ≈1-2% partial melting of the hornblende-gabbro, whereas thermobarometric modelling constrains their petrogenesis in the field of hornblende hornfels-facies metamorphism (pressure ≈3.0–4.5 kbar and temperature ≈700-750°C). On a wider scale, our results document natural trondhjemites generated in the upper crust by amphibole-dehydration melting of a mafic source at temperature conditions close to the solidus, filling the existing gap of data in low-pressure (P < 5kbar) and low-temperature (T < 800°C) amphibole-dominated partial melting experiments. The genesis of these trondhjemites is then discussed with respect to the Mesozoic evolution of the Alvand plutonic complex.

Graphical Abstract

KEYWORDS:

• Partial Melting
• Metatexite
• Mafic migmatites
• Trondhjemites
• Sanandaj-Sirjan Zone
• Iran

Acknowledgments

We gratefully thank J. Connolly for the usefull advices during the manuscript preparation. We thank R. Oberhänsli for helping with BSE-SEM investigation. We are grateful to M. Miri for the assistance, and A. A. Sepahi for the helpful discussion during manuscript revision. FL gratefully acknowledges the grant to Department of Science, Roma Tre University (MIUR-Italy Dipartimenti di Eccellenza, ARTICOLO 1, COMMI 314 – 337 LEGGE 232/2016) and the PRIN2017 Project 20177BX42Z_001 (Intraplate deformation, magmatism and topographic evolution of a diffuse collisional belt: Insights into the geodynamics of the Arabia-Eurasia collisional zones) funded by MIUR-Italy. The constructive reviews of H. Azizi and two anonymous reviewers as well as the professional handling of the Editor R. J. Stern greatly contributed to improve the manuscript.

Disclosure statement

The authors declare that they have no competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Supplementary material

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Additional information

Funding

This research is part of the corresponding author Postdoc programme at the Roma Tre University. This research was supported by PRIN2017 Project 20177BX42Z_001 (Intraplate deformation, magmatism and topographic evolution of a diffuse collisional belt: Insights into the geodynamics of the Arabia-Eurasia collisional zones) funded by MIUR-Italy. The research was also supported by Shahid Chamran University of Ahvaz (grants no. EG98.44295 and SCU.EG98.44295);Ministero dell’Istruzione, dell’Università e della Ricerca [PRIN2017 Project 20177BX42Z_001];Shahid Chamran University of Ahvaz [EG98.44295,SCU.EG98.44295];