N Shirdashtzadeh; G Torabi; R Samadi
Abstract
Some dark green fragments of amphibolites are found within skarns at the south of Nain Ophiolite at the northeast of Nain city. They are similar to ortho-amphibolites (metamorphosed basic rocks) of this ophiolite in hand specimen, but mineralogically they are composed of amphibole (magnesio-hornblende, ...
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Some dark green fragments of amphibolites are found within skarns at the south of Nain Ophiolite at the northeast of Nain city. They are similar to ortho-amphibolites (metamorphosed basic rocks) of this ophiolite in hand specimen, but mineralogically they are composed of amphibole (magnesio-hornblende, Mg#>0.95), clinopyroxene (diopside, Mg#~0.61), garnet (grossular – andradite, with Grs~63-87 And~12-35), quartz, and minor amount of calcite and wollastonite. Accessory minerals are including chlorite and prehnite, mostly filling the fractures. Field studies, petrography and mineral chemistry indicate that amphibolitic fragments mineralogically differ from the skarns and ortho-amphibolites of this ophiolite; so they can be considered as olistoliths with sedimentary origin (calcic marls of sea floor), turned into amphibolitic rocks (para-amphibolite) during the regional metamorphism at amphibolite - granulite facies and low oxygen fugacity.
N Shirdashtzadeh; Gh Torabi; R Samadi; T Meisel; S.N Hussain Bokhari
Abstract
Picrites are seen within the mantle peridotites in the Darreh Deh area (in the east of Nain ophiolitic mélange). Based on petrography and mineral chemistry, these picrites are composed of olivine (chrysolite), clinopyroxene (diopside) and orthopyroxene (enstatite), showing a cumulate texture, ...
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Picrites are seen within the mantle peridotites in the Darreh Deh area (in the east of Nain ophiolitic mélange). Based on petrography and mineral chemistry, these picrites are composed of olivine (chrysolite), clinopyroxene (diopside) and orthopyroxene (enstatite), showing a cumulate texture, and small amounts (less than 10 volume%) of plagioclase and chromian spinels as accessory minerals. Application of clinopyroxene geothermobarometry methods indicates the equilibrium temperatures of ~1040 to 1205 °C and pressure of 8.6 kbar for the clinopyroxene crystallization in the melt. Amphibole and talc crystallization and the results of amphibole thermobarometry (~675-700 °C, 4.8-6.5 kbar) indicates that these rocks have undergone metamorphismunder amphibolite facies in Jurassic, similar to their host mantle peridotites. Whole rock chemistry and chromian spinel composition confirm high partial melting degrees of mantle and progressive ascending melt/wall rock reactions in a suprasubduction zone in which tholeiitic melts were produced. During the production of this melt, picrites with cumulous texture formed within the mantle peridotites by precipitation of olivine, pyroxenes and minor amounts of intercumulous plagioclase crystals from the primitive melt.