Post-deformational impregnation of depleted MORB in Nain lherzolite (Central Iran)

Abstract

Plagioclase lherzolites of Nain mélange, Iran, show peculiar textures that indicate melt impregnation: (1) droplet or bleb -like grains of plagioclase distributed in the peridotite matrix, (2) plagioclase -bearing clinopyroxenite seams, and (3) trails of plagioclase crosscutting pyroxene porphyroclasts. The textural characteristics show post -deformational igneous formation of plagioclase, and possibly, associated clinopyroxene, from the impregnating melt. The melt has precipitated the clinopyroxenite seams and chemically modified all the peridotite minerals. Highly refractory compositions of the precipitated minerals suggest involvement of a highly depleted MORB -like melt. The melt was an increment of partial melt produced by 8% to 10% fractional melting from the MORB source. This is in contrast to the involvement of ordinary MORB in melt impregnation in abyssal plagioclase peridotites. Integration of increments of mantle partial melts to form MORB was possibly incomplete in the very incipient mid -ocean ridge as in the short -lived Nain back -arc basin.

Evolution of lithospheric mantle in the north of Nain‐Baft oceanic crust (Neo‐Tethyan ophiolite of Ashin, Central Iran)

“…One of the processes supposed for plagioclase occurrence in peridotites is crystallization from trapped melt, where the lithospheric mantle undergone decompression in solidus status (e.g. Church & Stevens, ; Dick et al, ; Menzies & Allen, ; Pirnia, Arai, & Torabi, ; Tartarotti, Susini, Nimis, & Ottolini, ; Rampone, Romairone, Abouchami, Piccardo, & Hofmann, , etc.) above the ‘cold’ thermal boundary layer (TBL), in the upper part of the mantle column in spreading ridges (Niu, ).…”

“…Shirdashtzadeh, Torabi, & Arai, ; Torabi, Shirdashtzadeh, Arai, & Koepke, , etc.). Pirnia et al () introduced some plagioclase‐bearing lherzolites in Nain ophiolite formed by melt impregnations. Later, Pirnia, Saccani, and Arai () investigated another group of Nain plagioclase‐bearing lherzolites (PlLz) that were formed by the subsolidus recrystallization of spinel lherzolites (SpLz) under plagioclase facies conditions during ‘mantle uprising’ from asthenospheric to lithospheric levels in response to continental rifting and lithospheric extension.…”

“…Torabi, Shirdashtzadeh, Arai, & Koepke, 2011, etc.). Pirnia et al (2010) introduced some plagioclase-bearing lherzolites in Nain ophiolite formed by melt impregnations. Later, Pirnia, Saccani, and Arai (2018) investigated another group of Nain plagioclase-F I G U R E 1 (a) Distribution of the main ophiolites in Iran including the study area of Ashin ophiolite (Km: Kermanshah; Ny: Neyriz; Om: Oman; Bf: Baft ophiolites; ages are from: Shirdashtzadeh, Kachovich, Aitchison, & Samadi, 2015;Pirnia et al, 2019); (b) Geological map of the Ashin ophiolitic mélange (after Sharkovski, Susov, & Krivyakin, 1984); (c) a field view of peridotites in the study area (PLSpLz: plagioclase-bearing spinel lherzolite; SpLz: plagioclase-free spinel lherzolite; Hz: harzburgite; Du: dunite) bearing lherzolites (PlLz) that were formed by the subsolidus recrystallization of spinel lherzolites (SpLz) under plagioclase facies conditions during 'mantle uprising' from asthenospheric to lithospheric levels in response to continental rifting and lithospheric extension.…”