Document Type : Original Research Paper

Authors

• Gh. A. Moafpourian ¹
• M. Pourmoafi ¹
• M. Vosoughi Abedini ²
• M. H. Emami ³
• M. R. Jannessari ³
• S.w. Parman ⁴

¹ Shahid Beheshti University, Faculty of Earth Sciences, Department of Geology, Tehran, Iran.

² Shahid Beheshti University, Faculty of Earth Sciences, Department of Geology, Tehran, Iran

³ Geological Survey of Iran, Tehran, Iran.

⁴ Durham University, Department of Earth science, Durham, England.

Abstract

Scattered outcrops of ultramafic – mafic rocks and interbeded sedimentary layers in the North of Fariman – Torbat-e- Jam lineament are exposed mainly at the west of Mashhad, east of Fariman and north of Torbat-e- Jam in Agh- Darband area. Our studies on ultramafic – mafic rocks have revealed several fields and petrographic evidences which demonstrate their volcanic to subvolcanic origin. Moreover, observation of various disequilibrium textures including the presence of glass, elongate and hopper pyroxene and olivine, occurring as pyroxene spinifex and olivine microspinifex textures in mafic and ultramafic rocks, can just be explained on the basis of their volcanic origin. On the basis of geochemical criteria, defined by IUGS for high MgO ultramafic and mafic volcanic rocks, these rocks are classified as komatiitic, picritic and basaltic types. Essential geochemical contrasts, including various ratios of incompatible immobile trace elements and different patterns of REE and spider diagrams, are too high to be explained on the basis of various degrees of partial melting of a uniform mantle source or AFC processes. It is thus possible to consider the role of a heterogeneous mantle source and/or dynamic melting in petrogenesis of these rocks. Furthermore to the cited heterogeneity which can be explained by mantle plume assumption, position of Fariman’s samples in the diagrams of (Nb/Th)_N versus (Nb/La)_N and  Nb/Th versus Zr/Y which mainly plot at the OPB area, and Zr/Y versus Nb/Y which plot on or above ∆Nb line, coincide with the same assumption. Here we have argued that disequilibrium textures had been formed by undercooling of superheated melts, originated from an ascending mantle plume source, and rapid growth of crystals in such a melts. This arguement is favored with regard to high Fo contents of olivines which make necessary equilibrium of these crystals to high MgO contents and hot melts. We have estimated the MgO content and temperature of parent melt as much as 23.74% and 1470-1535^ºC respectively based on maximum Fo content of olivine crystal in a komatiitic sample. These values are too high to correspond to a subduction or even a normal MORB related environments; however, similar to a hot spot tectonic setting.             

Keywords

• Ultramafic – Mafic
• Komatiite
• Disequilibrium texture
• Mantle plume

20.1001.1.10237429.1388.18.72.3.1