Mineral chemistry and petrology of highly magnesian ultramafic cumulates from the Sarve-Abad (Sawlava) ophiolites (Kurdistan, NW Iran): New evidence for boninitic magmatism in intra-oceanic fore-arc setting in the Neo-Tethys between Arabia and Iran

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• 作者：Khalil Allahyari ; Emilio Saccani ; Bahman Rahimzadeh ; Ottavia Zeda
• 关键词：Ophiolite ; Ultramafic cumulates ; Chromitite ; Zagros belt ; Iran
• 刊名：Journal of Asian Earth Sciences
• 出版年：5 January, 2014
• 年：2014
• 卷：79
• 期：part_PA
• 页码：312-328
• 全文大小：5160 K

文摘

The Sarve-Abad (Sawlava) ophiolitic complex consists of several tectonically dismembered ophiolitic sequences. They are located along the Main Zagros Thrust Zone, which marks the ophiolitic suture between the Arabian and Sanandaj-Sirjan continental blocks. They represent a portion of the southern Neo-Tethyan oceanic lithosphere, which originally existed between the Arabian (to the south) and Eurasian (to the north) continental margins. The Sarve-Abad ophiolites include cumulitic lherzolites bearing minor dunite and chromitite lenses in places. The main rock-forming minerals in ultramafic cumulates are cumulus olivine and inter-cumulus clinopyroxene and orthopyroxene. Minor (<5%) chromian spinel occurs as both cumulus and inter-cumulus phases.

Cr#, Mg# and TiO₂, Cr₂O₃, and Al₂O₃ concentrations of chromian spinel from ultramafic cumulates and chromitites plot in the forearc and boninite spinel fields, respectively. Clinopyroxene has very high Mg# and low TiO₂ contents. Calculated TiO₂ and Al₂O₃ compositions and Mg# in the parental melt that was in equilibrium with chromian spinel and olivine are consistent with supra-subduction zone-type compositions. Whole-rock geochemistry of the ultramafic cumulates is characterized by very low incompatible element content and a general enrichment in Th with respect to Ta and Nb. Chondrite-normalized REE patterns show different trends with either (La/Sm)_N < 1 and (Sm/Yb)_N < 1 or (La/Sm)_N > 1 and (Sm/Yb)_N < 1 (U-shaped pattern). Both these patterns are compatible with boninitic-type parental melts. Accordingly, petrogenetic modeling using REE composition indicates that Sarve-Abad ultramafic cumulates may have formed by small degrees (5-15%) of fractional crystallization from typical boninitic melts characterized by either light REE/medium REE depletion or enrichment.

Mineral chemistry and whole-rock chemistry clearly indicate that the Sarve-Abad ultramafic cumulates and chromitites record an episode of boninitic magmatism that occurred within the southern Neo-Tethys Ocean during the Late Cretaceous. Boninitic melts in the Sarve-Abad ophiolites were formed by partial melting of depleted peridotite which made up the residual mantle after MORB-type melt extraction. This was subsequently enriched with light REE and large ion lithophile elements by subduction-derived fluids. It is therefore suggested that this boninitic magmatism was generated in the forearc sector of a short-lived intra-oceanic arc that was located southward with respect to the 鈥淎ndean-type鈥?subduction below the Sanandaj-Sirjan continental margin.