Mashhad komatiitic rocks in NE Iran: Origin and implications for the evolution of the Paleo‐Tethyan Ocean

• Published in: Geological Journal
• Main Authors: Li, Xingbo, Miao, Laicheng, Zhang, Fochin, Ghasemi, Habibollah, Zhu, Shuaiming, Yang, Shunhu
• Other Authors: Liu, Y., National Natural Science Foundation of China, Ministry of Science and Technology of the People's Republic of China
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2018
• Subjects: Iceland
• Online Access: http://dx.doi.org/10.1002/gj.3329; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1002%2Fgj.3329; https://onlinelibrary.wiley.com/doi/pdf/10.1002/gj.3329; https://onlinelibrary.wiley.com/doi/full-xml/10.1002/gj.3329

Abstract Mashhad ultramafic volcanic rocks, located within the Paleo‐Tethys suture zone in the north‐eastern Iran, are composed mainly of komatiitic rocks with interbedded picrobasalts. The Mashhad komatiitic rocks are characterized by low Al 2 O 3 /TiO 2 ratios (9.85–11.44) and depleted HREE patterns with slightly high (Gd/Yb) N ratios (1.48–1.74), similar to those of the Al‐depleted Barberton‐type komatiites. These rocks have Ɛ Nd (t) values ranging from +9.0 to +10.4 and initial 87 Sr/ 86 Sr ratios ranging from 0.7039 to 0.7072, indicating they were derived from a depleted mantle source. They have a total 187 Re/ 188 Os and 187 Os/ 188 Os ratios ranging from 0.278 to 72.254 and from 0.12741 to 0.14012, respectively, and display consistent PGE patterns similar to that of the primitive mantle. The picrobasalts have high contents of SiO 2 (45.08–48.87 wt.%), MgO (7.28–11.51 wt.%), nearly flat REE pattern, and Ɛ Nd (t) values ranging from +6.5 to +7.9 and initial 87 Sr/ 86 Sr ratios focused on 0.7040 and 0.7070, illustrating that they were derived from a relatively enriched source compared with the komatiitic rocks. The thermobarometric calculations suggest that the Mashhad komatiitic rocks were generated by high degree partial melting between 20% and 30% at 1,604–1,681°C and 3.20–4.65 Gpa, essentially consistent with those of the Hawaiian mantle plume (1,500–1,600°C). These new data demonstrate that the Mashhad komatiitic rocks are of a mantle plume origin, which might have caused the opening of the Paleo‐Tethyan Ocean, and the mantle plume was likely located beneath or nearby the mid‐ridge of the Paleo‐Tethyan Ocean when the Mashhad komatiitic lava erupted, a scenario resembling the present Iceland and Ontong Java.