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

Abstract

AbstractMashhad 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 Al2O3/TiO2 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 ƐNd (t) values ranging from +9.0 to +10.4 and initial 87Sr/86Sr ratios ranging from 0.7039 to 0.7072, indicating they were derived from a depleted mantle source. They have a total 187Re/188Os and 187Os/188Os 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 SiO2 (45.08–48.87 wt.%), MgO (7.28–11.51 wt.%), nearly flat REE pattern, and ƐNd (t) values ranging from +6.5 to +7.9 and initial 87Sr/86Sr 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.